Final ReportSummary - SUBCTEST (Development of novel Non Destructive Testing (NDT) techniques and autonomous robots to be deployed by Remote Operating Vehicles (ROVs) for the sub-sea inspection of off
The oil and gas industry is vital to the European economy, both in terms of revenue earned and security of supplies. The total replacement of oil and gas with renewable sources of energy and nuclear power in the future is still unrealistic. Therefore, maintaining the infrastructure of the oil and gas industry; the process plant, pipelines etc, is of crucial importance, which is why the European Commission has contributed funds to this EUR 2 million project, to develop remote operated vehicles (ROVs) deployable non-destructive testing (NDT).
There is little if any routine NDT carried out on critical sub-sea welds in offshore assets because current NDT equipment and techniques do not currently lend themselves to be applied sub-sea below depths of about 25 - 30 m and those cases are exceptional and rely on divers applying visual inspection and electromagnetic techniques for surface weld examination. The operating oil companies, under the encouragement of the regulatory authorities, are committed to reducing hazardous diver operations and replacing them with ROVs.
However, current ROV deployments are hampered by the size and weight of NDT equipment. SUBCTEST set out with the aim of reducing these to a size and weight that could be deployed from smaller 'observation class' ROVs. This would extend the equipment's availability and make operation possible from ordinary supply vessels instead of from dedicated ROV ones. The principal objective of SUBCTEST has been therefore to provide an ROV deployable platform for three NDT methods, namely alternating current field measurement (ACFM), phased array ultrasonic testing (PAUT) and long range ultrasonic testing (LRUT). By way of achieving this, SUBCTEST has also had the objective of developing suitable NDT techniques for inspecting a variety of subsea structures, including 'jacket' braces and nodes, risers, flow lines and mooring chains.
For the nodes, the ACFM array technique will detect external surface cracks in the weld toes in one pass and the multi-skip PAUT technique will detect weld root defects without having to swivel the probe away from an axial orientation to which the probe shoe can be contoured. The creep wave PAUT technique will detect both internal and external surface breaking cracks simultaneously. For the flow lines, the LRUT technique will detect defects in welds at either end of a pipe spool form one test location anywhere along its length. For the touch-down point in risers, an LRUT technique will scan the surfaces of the riser for corrosion and fatigue cracking from one test location. For the mooring chains, the 'race track' intermediate range UT technique can screen chains for fatigue cracks.
Most engineering associated with equipment for ROV deployment can be described as 'agricultural', that is to say it is over-engineered, with massive components and crude control systems. The challenge in SUBCTEST was to reduce the mass of the manipulator that carried the ACFM, PAUT or LRUT transducers or sensors to a level that could be deployed by an observation class ROV. Considerable effort was expended in the design stage to keep the buoyancy of the equipment as high as possible, so that its submerged load was as low as possible. Novel designs saved considerable costs. Including for example, the use of a simple games controller to operate the LRUT manipulator.
For the ACFM manipulator a very simple design was settled upon, using a saddle that give 170° coverage around pipe circumference. As part of the SUBCTEST development, one of the partners developed a simulation of the ROV operation, when clamping the LRUT onto the brace of a jacket structure.
More details of the 'SUBCTEST' ROV deployable NDT platform can be found at http://ww.SUBCTEST.com
Project context and objectives:
There is currently little if any routine NDT carried out on critical sub-sea welds in offshore support structures because existing NDT equipment and techniques do not lend themselves to be easily applied sub-sea at depths greater than 25 - 30 m, and even those cases are exceptional and rely on divers applying visual inspection and electromagnetic techniques for surface weld examination. The operating oil companies, under the encouragement of the regulatory authorities, are committed to reducing diver operations because of safety issues (recent Norwegian case awarded EUR 3.7million compensation to 3 divers), shortage of divers and cost. Diver operations are both depth and time limited and there is now a clear case for ROV applied NDT techniques to be used in order to help prolong and extend the operational life of European oil and gas field reserves.
The SUBCTEST project benefited the participating high technology small and medium-sized enterprises (SMEs) that wanted to develop their existing NDT and robotic technologies and techniques into new offshore sub-sea inspection applications. It also benefited the lower technology participating SMEs that wanted to enhance their NDT technology base by becoming specialist providers of the service to offshore oil and gas operators. The SMEs drew on their in-house capability and also that of the participating research organisations, who supplied research and development (R&D) to the SMEs on a sub-contract basis and to a work-scope defined by the SMEs.
The ROV would be an observation class ROV that did not require a support ship, which is a necessary requisite of the much larger work-class ROVs that are normally used. The NDT could then be controlled from the platform topside and would deploy a robotic inspection head to carry out NDT of platform jacket critical support welds using LRUT and PAUT for volumetric weld inspection and/or ACFM in order to detect surface breaking fatigue cracks.
The project therefore set out with a number of technical objectives:
- to write a technical specification for a system that could deploy the three NDT techniques and find test-pieces on which to validate the NDT (work package (WP)1);
- to develop an ACFM sensor and technique that could be deployed by a robot around a node weld in a single pass, without the problems associated with current ACFM sensors when scanning complex weld geometries (WP2);
- to develop a PAUT technique for scanning around node welds without the need to raster scan over the elliptical surfaces and thus affect the beam steering (WP3);
- to enhance the resolution capabilities of LRUT that is currently used to detect corrosion in pipes so that it can be used to detect defects in pipe welds (WP4);
- to develop a robotic manipulator for deploying the NDT sensors from an observation class ROV (WP5;)
- to conduct laboratory and field trials with manipulator prototypes (WP6).
As well as these technical objectives, the SUBCTEST project also set out to achieve the following objectives:
- Economic objectives by way of improving the competitiveness of a group of European SMEs operating in the world-wide oil and gas industry by exploiting the technology developed within the project. The technology developed would also reduce the economic costs of failures in offshore structures.
- Social objectives by way of eliminating the risks of diver deployed inspection systems and catastrophic failures that lead to multiple deaths as happened in the Piper-Alpha disaster in the North Sea during 1988.
- Environmental objectives by reducing the risk of failures that might lead to oil spills with possible catastrophic effects on the North Sea environment.
Project results:
The project work was organised into a number of discrete WPs, each consisting of tasks that collectively achieve the specific WP objective(s). At the 12-month meeting, these were re-organised and a new description of work (DoW) issued.
The principal changes were the following:
- Abandoning the PAUT marinisation because of the high cost.
- Use the funds for the PAUT marinisation to investigate electro-magnetic acoustic transducer (EMAT) probe designs for LRUT of coated pipe. This was recommended because the extensive use of coated pipe restricted LRUT ranges with current piezo-electric transducers, and to make most effective use of partner ZUT's knowledge of EMAT development.
- Developing a separate LRUT manipulator from the ACFM one because the designs requirements are quite different.
- Introduce simulation of ROV deployment on an offshore platform instead of demonstration trials, because of the high cost and partner GRI's expertise in this area.
The revised project DoW was issued in May 2010. It divided the work into 6 WPs (excluding management and exploitation), covering 18 tasks.
An extension of twi-months was added to the project to complete the prototype trials after delays in integrating the parts of the LRUT manipulator and problems with customs in transporting the equipment into Norway. Unfortunately, this delayed the dock-side trials and these could not be started before the Norwegian winter set in. The dock-side trials were to be conducted by Norwegian partner Dacon. A number of parties have shown an interest in these trials outside of the SUBCTEST project, and it is expected these will be conducted shortly.
A more detailed description of these WPs follows.
WP1: Design and procure samples and specify system requirements
The WP had two objectives:
1. to design and produce a set of test specimens containing a variety of representative known defects, including worst case (i.e. most difficult to detect) in welds for ACFM, PAUT and LRUT development work;
2. produce equipment functional and technical specifications.
The two objectives were achieved through two tasks. At the kick-off meeting, the partners decided that concentrating the development work solely on complex weld nodes was too high risk and that other simpler applications should be included. Moreover, the decision by Dacon to offer their inspection ROV for the trials increased the potential scope of the project to small structures and components. The partners therefore decided to look at other simpler applications for ROV deployment, namely welds in sub-sea mooring chains and in flow lines and the critical 'touch-down' points in risers where the flow lines bend vertically upwards from the sea bed towards the production platform. Samples of these were included in the library of test samples put together at the start of the project and the specification adapted to meet this requirement.
The library of test samples was built up from those available at TWI of which there were two types:
1. Samples used for developing sensors and systems. These have to be small for use in the laboratory and contain mainly artificial discontinuities. Samples have been made using a range of notches to represent surface breaking cracks at various angles of tilt.
2. Samples or mock-ups used for demonstration trials and which should include real flaws or artificial ones that represent real flaws as closely as possible. Preliminary trials were conducted in TWI's diver training tank, where suitable test nodes are already available. TWI also had available mock-ups of steel catenary risers, chain links and subsea flow lines, both uncoated and coated with concrete (used to anchor pipes to the sea floor) and nodes.
The specification went through several iterations as the costs of the various options became apparent. The final specification covered ROV deployment of only two of the three NDT systems considered, ACFM and LRUT for inspection of riser and welds in flow-lines and nodes.
Also the specification was written so that the sensor systems and manipulator could be deployed from an ROV provided by SME partner Dacon. This necessitated the separation of a clamping type manipulator (LRUT) and scanning type manipulator (ACFM).
WP2: Develop sub-sea automated ACFM system and techniques
The objective of this WP was to deploy ACFM sensors and systems for complex subsea weld geometries. Typical welds include butt, fillet and node welds. Node welds occur when two cylindrical sections, of different sizes, meet at an angle. These welds are by far the most complex. To tackle this problem TSC created a 'toolkit' of probes, software and instrumentation.
The probes included a scanned array probe that could be moved along the weld in a single pass by the manipulator. They have the advantage of detection and sizing sensitivity, but must be precisely deployed by the manipulator. This was deemed to be the correct choice, as such manipulators are relatively simple.
An alternative 'pick-and-place' probe would allow a weld to be inspected in a number of discreet steps, and thus avoid any requirement for scanning by a manipulator. The benefit is that it is easier to deploy, the downside is that the data density is limited by the physical positions of the sensors in the probe and so sensitivity, particularly to crack length, is lower than for a scanned probe.
A prototype (non-marinised) probe was produced at the six-month stage of the project, which allowed the concept to be tested before manufacture, to ensure the additional requirement on the software and instrumentation could be achieved.
Previous experience of subsea probes suggests a scanned array with multiple sensor coils for this task.
Sensor compliance was achieved by producing a robust probe with a stainless steel nose that can be pushed against the weld by the manipulator. The structure of the weld consists of a vertical chord, connected to a secondary brace; this forms a complex shaped node weld. Both toes of the weld must be inspected, in addition to the crown of the weld; this would typically require more than one probe, which is problematic as it is difficult to swap out probes without bringing the ROV to the surface.
The changing geometry of the structure affects the magnetic field generated by the probe. This causes a problem, as the electronics of the ACFM system limit the acceptable variation of magnetic flux density from sensor-coil to sensor-coil by a factor of two. The solution was to design a single scanning array probe that could cope with the changing geometry and inspect both toes of the weld, plus cap.
Comsol multiphysics was used to create an FEA simulation of the problem in order to optimise the probe design. The distribution of the magnetic flux density on the lower section of the brace differs from that of the top section. It was decided to use a wedge shaped probe, sized to fit the tightest angle likely to be encountered during the project. The magnetic field-generation coils, or field coils, would then be optimised to provide a suitably distributed field for all angles of the inspection, while maintaining uniformity, acceptable to the electronics of the ACFM system.
Several designs were considered to achieve the probe specifications. They were:
- perpendicular rectangular coil design;
- variable diameter coil design;
- variable field winding design.
The variable field winding design was found to be the most successful. This consisted of four field coils wrapped around steel cores. All field coils are activated simultaneously. The number of field windings around each core is increased to strengthen the field away from the weld crown. This produces the correct distribution of field intensity required for the simultaneous inspection of the parent plate, heat affected zone and weld crown.
A prototype probe was produced, in order to check the operational characteristics. Once the numerical simulations were completed, it was decided to validate the model with a prototype before building the final probe. The sensor coils are placed in pairs. The horizontal coil measures the horizontal field, used to determine the depth of the defect. The vertical coil measures the vertical field, and is used to determine the location and length of the defect.
The final probe was successfully marinised and tested.
A second task in this WP was devoted to design and build instrumentation and software to support the ACFM probes. Existing diver instrumentation was repackaged to suit deployment on the autonomous vehicles. This required a reconfiguration to allow most of the instrumentation to be housed on a host ROV while the inspection head is manipulated independently by a subsystem (the manipulator). The ACFM software was developed to run within the complete system man-machine interface (MMI) environment.
TSC's standard U31 instrument was developed for diver use and uses RS485 communications through a traditional copper twisted pair umbilical. When the ACFM system is deployed by ROV the standard U31 umbilical cable cannot be used easily due to the necessity to run in parallel with the ROV umbilical. An ROV umbilical is typically several kilometres long and is limited in the number of copper conductors available. Instrumentation and video cameras are required to send their data via an optical fibre in the ROV umbilical as this optical fibre is capable of supporting several functions simultaneously. An instrument called a fibre optic multiplexer (or 'Mux' for short) is used to interface between instrumentation and the umbilical fibre. A Mux is required at topside and sub-sea ends of the umbilical. The ACFM system needs to be able to interface with the Mux without error.
The U31 RS485 interface is configured at 19 200 Baud, 8 data bits, 1 stop bit. It operates in half duplex mode with the topside unit as master. Checksum and acknowledge signals are generated by both topside and sub-sea units. A Focal 907 was selected as the test Mux unit as this is commonly used by ROV systems.
Initial trials showed that the proprietary 'comms' protocol used by the ACFM system was not compatible with the Mux due to timing and latency issues. This was investigated and the problem was found to be that the receiving end's acknowledge signal was being ignored by the Mux resulting in the transmitting end perpetually repeating its message in the hope that the receiving end would receive the message and send an acknowledge signal. A solution was engineered that involved the modification of the U31 system topside and sub-sea firmware, introducing delays to accommodate the 1.5 ms latency in the Mux unit as the comms direction changed from up-load to down-load and vice versa.
Reliable performance of the comms link was obtained without significant delay to the transport of the ACFM data. Optical fibre and copper umbilicals were used.
ROV systems generally have 12 V or 24 V power available for auxiliary systems. The U31 Instrumentation uses a dual 12 V power supply suited to its sensitive analogue measurement electronics. In order for the U31 to be easily interfaced to any ROV a power adapter has been developed that allows direct connection of the U31 system to typical ROV power supply systems. The adapter is suited for connection to 12 V or 24 V supplies without further modification. Appropriate filtering has been incorporated so that noise from the ROV systems does not interfere with the U31 instrument electronics. The adapter has been housed in a pressure vessel capable of withstanding the pressures at 2 km water depth.
The current U31 ACFM inspection system is capable of supporting array probes with up to 32 channels. Expanding the capability of the system to 128 channels would allow the inspection of a greater surface area. The greater surface area may allow the cap and both weld toes to be inspected at the same time increasing the efficiency of the robotic inspection system. In some circumstances a larger array may be more tolerant of positional errors, reducing the precision required of the manipulator or scanner.
The U31 electronics has been adapted to allow up to 128 channels. A new PCB has been made to facilitate connection of the additional signals to the array probe cable.
It is sometimes necessary to change the type of probe during the inspection of a complex geometry. It may be, for example, that most of a complex weld can be inspected with a probe designed for general weld inspection, but where areas are constricted due to a more acute angle between chord and brace, a smaller tight access probe needs to be used. If the probe change required the ROV to be brought to the surface to allow the probe to be changed manually this would add significant time and cost to the inspection operation. If the U31 could accommodate more than one probe connected at a time, then the ROV operator could just drop the current probe in to a tool basket and pick up the next probe to be used depending on the application.
A multi probe interface has been built to facilitate remote probe change out. Up to four probes can be connected at one time. The four probes can be either independent of each other or synchronised so that multiple probes can be scanned at the same time. The electronics for the probe select interface have been made small enough to allow marinisation in a small package such that it is easy to integrate in to the ROV tooling frame.
The SUBCTEST project required extension to existing ACFM software and development of both software and firmware specifically to support new features for SUBCTEST. Underpinning the ACFM software, probe files define the operational parameters, including deployment methods, environmental factors and instrument settings that determine how an ACFM system operates.
In order to achieve the requirements for SUBCTEST, significant enhancements were required to support a number of new features as follows:
- Increase the maximum channel range from 32 to 128
For scanned array probes, channel definitions were extended to include a greater number of channels in a single dimension. For pick-and-place probes the channel range was extended in two dimensions.
- Support the inclusion of inclinometers and their parameters, including RS485 ID's and resolutions
- Introduction of a new 'master' probe file structure. This significant development, allowed the creation of master probe files with common probe configurations, allowing multiple connected probes to be referenced by a master probe file.
Firmware programming changes inside the subsea instrument, topside and probe interface units were required for SUBCTEST to support changes for:
- protocol compatibility changes for 907 Mux fibre optic timings;
- full high-speed RS485 long umbilical twisted-pair comms;
- channel range increase from 32 to 128 (achieved by I2C programming for higher channel lines);
- multiple inclinometer devices, via new multi-drop 485 bus;
- multi-way probe interface via new multi-drop 485 bus.
A number of software enhancements were made to cater for the following:
- Increased channel range. For both pick-and-place and scanned array probes, the visual software interface has been modified to allow presentation and selection of greater numbers of columns and rows. This is apparent through both enhanced row visibility panels and higher resolution contour plots.
- Inclinometer support. When configured, the software will display live-update inclinometer readings.
- Multi-way probe interface. The software allows the user to identify and use the specific probe presently connected to a port of the multi-way probe interface, thereby allowing the ROV to deploy multiple probes from the tool basket.
TSC routinely manufacture subsea probes for the oil and gas industry, sometimes operable to a depth of 2000 m. Each of the SUBCTEST probes were constructed of glass-filled peak plastic, a tough plastic with little hydroscopic absorption and good dimensional stability. An O-ring is used to ensure no water ingress. Additional water-proofing and strength is achieved through the use of a potting compound, which is poured into the cavities of the probe. As this material dries it solidifies, sealing the electronics and turning the probe into something that is in effect a solid plastic body with no moving parts or air cavities.
This was tested at the National Oceanography Centre in Southampton. Hydrostatic testing was completed, and the pressure test proved the design down to 150 m for two hours. The probe was tested before and after the test, and was found to be functional each time.
WP3: Develop sub-sea phased array automated ultrasonic test system and techniques
The main objective for this WP was to develop new and novel NDT sensors and systems based on multi-element phased array probe design so that the techniques developed can be applied to the sub-sea welds as part of the SUBCTEST NDT platform.
The work was divided between three tasks:
In the first task PAUT techniques for node welds between inter-connecting tubulars in a subsea 'jacket' structure were investigated. These are a particular problem for sensors that scan, because of changes in geometry between the scan surfaces as the probe orbits the weld. The PAUT would be complementing the ACFM technique, which detects only surface breaking defects, because it is sensitive to sub-surface defects. The two might even be combined. The PAUT is however much slower and therefore for SUBCTEST, the application of PAUT was confined to detecting surface breaking cracks on the internal surface of platform tubular structure. At the six-month meeting, a PAUT technique that employs a creeping wave along the internal surface and external surfaces simultaneously was discusse.
This has the advantage of not requiring the sound beam to be over the crack, the crack can in effect be detected at a distance of several centimetres from the probe. Creep waves are normally generated using conventional monolithic transducers and use a wedge that refracts a compressional (L-wave) along the outer surface that is known as the 'creep wave'. This is accompanied by a shear wave (T-wave) that refracts at 33º to the surface. This T-wave generates an L-wave component along the internal surface, the I.D. creep wave and reflects T-waves that 'leak' from the creep wave as it propagates along the internal surface.
If the internal creep wave can be generated over a distance of several centimetres this would obviate the need to raster scan the PAUT probe. This would be a major advantage within SUBCTEST, since welds could be scanned linearly in one scan as is the case with the ACFM array probe.
However, the L-wave beam is always (except when it is vertical) accompanied by a T-wave. This gives rise to a number of other echo-signals from one defect.
Although this is a disadvantage in terms of imaging the defect, it has the advantage in improving sensitivity to the defects on a go no-go basis because it gives rise to multiple signals from one defect that is more likely to be detected. If the aim is only to screen for defects rather than size them, then such a multiple-skip technique might be an advantage. For this reason, it was considered for SUBCTEST.
In the process of investigating the creep wave technique it was found however that:
- shallow cracks could be distinguished from mid-wall cracks and these from deeper cracks on the basis of the presence or otherwise of three identifiable signals on the A-scan;
- surprisingly high amplitude signals could be detected from corner reflectors at quite long stand-off distances from the weld. This led to the development of a multi-skip technique.
An analysis of the multi-skip technique was made of successive 'skips' of the sound beam into the weld from the probe using a simple diagrammatic representation. Using a PAUT beam restricted to fixed angle of 58? and sweeping from side-to side (i.e. horizontal linear array) a girth weld in a 12' pipe could be tested at a range of about 1 m. This would be a major benefit for in-service inspections of welds, which are often coated and inaccessible to conventional NDT.
An investigation was also carried out on the inspection of node welds using the horizontal linear array. Node welds when tested with conventional monolithic transducers are scanned in an orbital manner, with the front of the probe parallel with the weld axis and therefore the sound beam perpendicular with any longitudinal flaw in the weld. When tested with a PAUT probe, however, the size of the probe 'footprint' gives rise to probe 'rocking' on the surface and loss of a steady beam profile with which to scan the weld. The probe can only be prevented from rocking by scanning with the probe shoe contoured to the curvature of the brace and therefore pointing parallel with the brace axis). The multi-skip technique was shown to have adequate sensitivity to the gross weld flaws in the pipe girth welds.
Finally, the PAUT probes were used to inspect mooring chain links, where the 33° transverse wave beam, as well as the creep waves, could detect fatigue cracks in the head of the chain link.
WP4: Develop sub-sea LRUT equipment and techniques
At the six-month meeting it was decided to investigate LRUT techniques for inspecting:
- riser touch down points
- flow line girth welds
- 'jacket' node welds
- mooring chain links.
The original DoW included an investigation of composite multi-layered actuators as a method of propagating high amplitude guide waves under the attenuative concrete coatings found in some sub-sea flow-lines. The actuators can deliver and support high force loads with minimal compliance, but at the cost of delivering only small motions. However, they could only be offered in compression wave mode generation, which is not appropriate for LRUT. Firstly, from the frequency sweep results, it was observed that the signal was very dispersive. In addition, there were several wave modes produced, making data analysis difficult. More importantly, actuators proved to be highly resonant i.e. there were different frequency components present when excited at a certain specific frequency.
As an alternative to using higher power multi-layered composite transducers to generate guided waves further through pipes that are coated with highly attenuative materials, the generation of specific low attenuation modes was investigated. It is known that shear horizontal (SH) wave modes 'leak' less from the pipe-wall into the coating than L-wave or T-wave. SH waves are best generated using electro-magnetic transducers (EMATs). The principal of EMATs is the generation of stress waves in the surface of a metal by the interaction of induced eddy-currents with a magnetic field. Experimental work showed how simple EMAT designs could be used to generate guided waves in pipe and plate. It was therefore decided to explore the possibilities using numerical models. At the 12-month meeting it was decided to amend the DoW to give research and technology development (RTD) performer ZUT, who have in-depth expertise in EMATs, a task to develop models for selecting the best EMAT configuration for generating SH-waves as an alternative to the stacked actuators.
SH waves can be generated by periodic permanent magnet (PPM) EMATs or meander coil EMATs.
The challenge was to generate models that combine electromagnetic waves with stress waves in a single executable model that is able to show their inter-reaction. TWI and ZUT have experience in using the Comsol multi-physics finite element model for solving various engineering and physics problems. The software developers were contacted and their help enlisted in this application, which was unique in their experience.
Preliminary signal analysis was introduced based on the ultrasonic wave's two-dimensional (2D) fast Fourier transform (FFT) to distinguish ultrasonic wave modes. Numerical models for both the PPM and Meander-coil EMATs were created using three-dimensional modules in Comsol multiphysics software. Solving the problem was divided into three parts: permanent magnets, coil and stress waves.
Static magnetic field produced by permanent magnets was calculated in AC/DC module (magnetostatics). The impulse current excited in the coil was broken into harmonics. Eddy currents induced in the specimen by the coil were calculated in AC/DC modules (induction currents) separately for each harmonic of the excitation current. Stresses and displacements in the specimen were calculated independently for every harmonic in structural mechanics modules (solid, stress-strain).
All harmonics were calculated in the frequency domain with the processes running in parallel, at the same time. This was a much more time-effective procedure than the traditional procedure using time-domain calculations.
Finally, the analysis was made in the presence of an artificial defect. The calculations were made for simplified 2D analysis as well as for full three-dimensional (3D) case. Typical A-scan were shown, the reflected wave showing the real position of the defect with good accuracy. In conclusion it was found that the Meander coil EMAT produced the better field.
To complete the transducer development for LRUT in SUBCTEST, specifically for the tight curvatures encountered on chain links, flexible macro-fibre-composite (MFC) transducers were investigated. MFCs have been developed primarily as pressure sensors and only TWI has investigated their use in NDT. A prototype tool of MFC transducers was built and successfully demonstrated on a mooring chain.
Although different types of transducer were investigated in the project, because of time constraints it was decided to proceed with marinisation of an LRUT tool using conventional piezo-electric ceramics.
The original specification for SUBCTEST called for transducers that were able to operate at a depth of 350 m below sea level equal to a pressure of 35 bar. With a 10 % safety margin, this equates to a total pressure of 38.5 bar or 558.25 psi. There was a design for water immersing transducers developed for the RISERTEST project (project partners TWI and Dacon) but the maximum operating depth of this design had previously not been established, although the specification had called for an operational depth of 14.5 m. It was therefore necessary to test this design for performance at the increased depths required for the SUBCTEST.
During the first reporting period, a transducer design was successfully tested at 150 bar. However, during this second reporting period, the company that supplied the basic transducer went out of business, so the transducers were no longer available for marinisation.
An alternative design of transducer was produced outside of this project for general LRUT use, which could be manufactured in-house. This design had to be adopted for marinisation in the SUBCTEST transducers. This led to significant delay in integrating the LRUT system for the trials. The new design used fewer components, because it used a non-conductive coating on the aluminium housing. The need for separate electrode plates for applying the correct polarity to the transducer plate was therefore obviated.
The transducer was coated with Sikiflix sealant that was simply scraped from the wear plate to provide ultrasound coupling with the test surface.
For generating guided waves, the transducers have to be mounted in rings, to provide a sufficient density of vibrations around the pipe circumference to prevent the propagation of flexural waves. On a 12' diameter pipe decided upon for the final trials, sufficient density of transducers in the ring to propagate axi-symmetric guided waves is achieved with 36 transducers. To propagate guided waves in one direction a second ring is needed that operates in anti-phase to the first one. This destructively interferes with the backward going pulse. Also, to increase the amplitude of the forward going pulse, a third ring in the front constructively interferes with the forward going pulse. Three rings were therefore incorporated in the SUBCTEST prototype, each with 36 transducers.
To modularise the system, so that the number of transducers can be increased or decreased according to the pipe diameter, the transducers are mounted in a holder with sealed connectors. Problems were encountered in some of the modules at pressures above 20 psi, causing loss of signal, but the effect was reversible, possibly due to air voids within the potting material inside the module, which would compress and hence distort the connection wires within the transducer.
With regard to the application development for the LRUT tool, work that was running in parallel with the transducer development, the inspection of riser 'touch-down' points did not warrant work as it would use existing LRUT techniques as they are applied to pipes above water.
The inspection of girth welds in sub-sea pipelines was identified by partners at the six-month meeting as a more difficult application for the SUBCTEST ROV applied LRUT. TWI has a number of welds in its LRUT test loop and these were tested with a view to finding a weld containing a weld flaw. One flaw was detected in a 12' pipe girth weld by means of its increased flexural content.
Precise positioning on the defect around the weld circumference was obtained by using the phased array algorithm on the transducers to focus on the weld. Rotating the focal spot through eight positions around the weld showed an increase in reflectivity from the weld between 90º and 180º from top-dead-centre. The signals are also displayed in a so-called A-map. The flaw was later confirmed to be an area of lack-of root penetration. The sensitivity of the LRUT technique to gross defects in girth welds, such as root crevice corrosion, at each end of a pipe spool was therefore demonstrated.
Most sub-sea pipelines are coated with concrete in order to anchor them to the sea floor. Concrete coatings are very attenuative of guided waves. It was for this reason that stacked attenuators and subsequently SH wave EMATs were investigated.
However, some surprising results were obtained when testing a coated 12' subsea pipe with conventional LRUT transducers placed on the inside of the pipe. A surprisingly low attenuation (1.3 - 1.6 dB/m) was found and this was attributed to the presence of an epoxy layer between the pipe wall and concrete coating. Also, by having the transducers on the inside surface of the pipe there was a greater amount of ultrasound energy going along that surface than along the outer surface where the coating was. This suggested that if the ultrasound energy could be concentrated on the inner surface of the pipe when testing from the outside surface, the test ranges in concrete coated pipe could be increased. This was one of the reasons for pursuing EMATs for guided wave generation as they provide some degree of control on the 'penetration' depth of the waves.
With regard to the application of LRUT to node welds, trials were conducted on a section of nozzle from a 12' branch. The test range was limited to less than 2 m, resulting in the signals occurring within the A-scan dead zone. Despite the high level noise, patterns could be seen in the A-map displays corresponding to the weld Apex, two sides and crutch.
However, it was generally decided that the LRUT technique would need to employ much higher frequencies to achieve adequate sensitivity to cracks and corrosion in node welds.
At the kick-off meeting, the partners decided to include the inspection of mooring chain links in SUBCTEST, because of its simplicity and because mooring chain failures were now a major concern among operators of floating production and oil storage vessels (FPSOs). The initial experimental work was carried out with a standard 4' torsional wave tool, but much better results were achieved with a special tool comprised of MFC transducers. Unique feature of LRUT on a chain link is that the guided waves will continue circulating around the ring, and there will be multiple signals each time the guided wave passes the ring.
However, the tool is not optimised for LRUT, where the tools have more than one ring in order to select wave modes and determine the direction of propagation of the waves. With a single ring, the interpretation of the A-scans from multiple modes propagating in either direction would be very difficult. Therefore, a way of collecting A-scans from a frequency sweep and stacking them in a pseudo-3D display using propriety Matlab software was developed. In this way, signal patterns from defective and defect-free chains could be distinguished.
WP5: Develop sub-sea NDT robot manipulators and vision system
Major changes were introduced into this package because:
- the development of the LRUT manipulator had to be separated from the ACFM one, because the clamp action of the LRUT transducers is different from the scanning action of the ACFM transducer;
- the project funds would not extend to performing trials with the ROV manipulators at an actual oil platform site and therefore a simulation would be carried out, since one of the project partners (GRI) had this expertise.
As a result of the abandonment of the phased array ultrasonic test (PAUT) development, the manipulator with the scanning action was restricted to the ACFM technique only. The manipulator developed has a kinematic structure with four degrees of freedom:
- one rotary motion around the pipe;
- one linear motion along the pipe;
- one linear motion in axis perpendicular to the pipe (passive - spring loaded);
- one angular motion in axis perpendicular to the pipe (passive - spring loaded).
Linear motion along the pipe is needed to bring the transducer up to a weld. The mechanical construction consists of a bracelet with two ring tracks (for rotary motion), carriage with DC motor, linear actuators equipped with a sensor and special mechanism to close the bracelet after ROV has placed the manipulator on the pipe.
Two types of drives were considered: hydraulic and electric. Hydraulic motors (for rotary motion) and hydraulic actuators are ideal in an underwater environment but are very heavy. Therefore, MAXON geared DC current motors with encoders were selected. Two drives were used; the first drive (MAXON No. 341514) to deliver the rotary motion, the second one (MAXON No. 227733) to deliver the linear motion. In order to avoid the leak and damage of the motors, all the drive units were closed in special aluminium waterproof containers with sealing. The first motor drives the carriage along the ring tracks, while the second is coupled with the linear screw drive.
Binding the braces with robot to the pipe is provided by a special construction of braces and additional electro-magnets. A set of electromagnets is used to close the clamp after it is attached to the pipe and generate a force, which holds the manipulator in position. The position of the carriage movement is limited by electromagnetic limiting switches and the operating angle is equal to 1600. A waterproof camera (connected with a LCD display) with lighting system allows observation of the position and orientation of the transducer which is mounted at the end of linear motion module.
Manipulator control system comprises of prefabricated metal box with installed motor control devices, relays, suppliers, and installed outside: buttons, joystick, connections and fan. What is more, an independent LCD display is also part of control system. For control motion of the manipulator units' joystick and four mono-stabile switches are used. Moreover, one bi-stable switch was used to turn on and off electro-magnets which were specially designed for stabilisation braces on the pipe. The joystick and buttons are located at the top surface of the box. At the front surface of the box are situated three lighting power switches, in the form of bi-stabile buttons, and appropriate for each circuit fuses. Main power switch, red one first from the right side, allows turning on and off circuit supplying whole control system of robot (230 V AC). Then, placed in the middle green switch, allow turning on and off internal 24 V DC circuit. The last switch, the green one placed on the left side is supplying internal 5 V DC circuit. External connections and fan for conditioning device into box are placed at the front surface, as well.
The main parts of the manipulator control system are EPOSes - dedicated units for control and supply drives purpose. In addition, EPOSes allow for communication between drives and external environment. Information about actual position of drive shaft is measured by integrated encoder and fed to adequate EPOS via dedicated ports.
In the second task, the manipulator with the clamping mechanism for the LRUT was built. It can be divided into four components.The bracelet is made up of LRUT transducers mounted in modules that surround the pipe inside a clamp. The clamp is opened and shut by one actuator. As with all the actuators, this is hydraulically driven, using environmentally friendly vegetable oil. When the clamp is closed, a bolt locks the arms together before the bladders behind the transducer modules are inflated with oil to push the transducers down onto the pipe surface. The bolt has an emergency disconnect and release system, so that the manipulator can be released in the event of a power failure.
A second actuator is able to rotate the clamp in order to attach to both vertical and horizontal pipes.
The frame surrounding the brace guides the manipulator so that it is aligned with the pipe. This is often difficult when the ROV is not steady in passing currents.
In the electronics pod, the pulser-receiver for the LRUT transducers with associated amplifiers is housed in one pod. PCB's from conventional LRUT equipment had to be modified to fit into a smaller space, necessary to meet buoyancy requirements. This was not a simple task, since high powered circuits for firing the transducers had to be kept away from the amplifier receiver circuits.
Pump and valve pods for the hydraulic system includes the bladders as well as the actuators. The valves for controlling these were incorporated into a pod, which for buoyancy reasons, was separated from another pod for the pump.
A novel feature of the system is the game controller used to control the bracelet. Its software was a major development for the project.
The control software was developed between two personal computers (PCs) over a network connection and transferring control information between applications running on the PCs.
The application on the control PC (the client) monitored a gamepad that a user would operate in order to produce the desired movements and actions on the physical hardware attached to the server PC. The system would operate in real time with visual feedback in the client to inform the user of the status of the remote hardware. The client PC was a general purpose PC with a wired Ethernet port and a spare USB port for connecting a gamepad device. The gamepad was a standard PC/XBOX 360 controller.
The hydraulic system was controlled by relays on a TWI produced circuit board. The relays were driven via a line driver chip by digital outputs on the analogue / digital I/O board. The PC-104 allowed the boards to be stacked so that only a small physical mounting space was required.
The PC board was a fully-featured computer including graphics, USB and disk drive interfaces.
The hydraulic pump was controlled by a relay on the relay I/O board. Pressure sensing was by analogue to digital converters on the analogue / digital I/O board. The collar interlock switches were monitored by inputs on the relay I/O board.
The board ran Windows XP (service pack 3) from a 2.5' IDE disk drive.
The client and server applications communicated over an Ethernet connection via Windows sockets. This is simple standard method for networked devices to be able to initiate and terminate connections. It does not however provide any assistance with protocols being used between the connection end points. The connection is full-duplex meaning that both ends of the connection can be receiving and transmitting data at the same time.
The protocol implemented over the socket communications is based on extensible mark-up language (XML) documents. This was chosen for its ease of creation and decoding using industry standard parsing techniques.
Both client and server applications were developed using Embarcadero Delphi. This provides an implementation of the Pascal programming language along with an extensive set of development libraries.
In the third and final task, partner GRI used their DeepWorks software to create a simulation of the ROV deployment of the LRUT manipulator. The ROV model faithfully reproduces all the actuators to accurately replicate operations. The buoyancy and drag characteristics of the ROV with manipulator attached are included to give realistic movements in different currents.
The simulations are much more than just the video. They are in fact models in which the ROV pilot can operate in specially constructed scenarios. As such they can be used in training and for planning ROV operations.
WP6: Conduct Laboratory and field trials
Because of the need to split the LRUT manipulator development from the ACFM, it became evident that full field trials on both manipulators would not be possible with the resources available. It was therefore decided by the project group at the 12-month meeting to restrict the ACFM manipulator trials to a diver tank only, while taking only the LRUT manipulator trials through to full ROV deployment.
The ACFM manipulator trials were therefore to be conducted in the diver tank at TWI North. TWI North has a series of samples, originally developed for the ICON project. Each sample consists of a series of braces connected to a central chord. Each weld contains fatigue-like defects of varying sizes and depths. In order to test the node array probe, it was decided to compare it with one of TSC's standard ACFM probes. These probes have been used for decades throughout the oil and gas industry, and are known for the reliability and accuracy. The SUBCTEST node array probe in a single scan along the weld was found to detect all the defects, giving the same readings for length and depth as the standard weld probe doing multiple scans along the same weld. These trials were conducted dry, but as there are no moving parts on the probe, and the field is not affected by water, there is no reason why these results would be different if the trials had been conducted subsea.
For the underwater trials, the ACFM array probe and manipulator were tested TWI North's Diver training tankor the trials a 12 x 1 mm defect was cut into the toe of a node weld. The manipulator scanned the ACFM probe over the defect, before lowering into the tank, and the software used to successfully locate and size the defect. The defect was then increased in depth by a further 1 mm. This was also successfully detected and sized. The size of this defect is considerably smaller than the required defect size specified at the beginning of the project. A video was made of this operation.
The LRUT manipulator components were first assembled in the engineering hall at TWI on a mock-up of the Dacon ROV. This consisted of a tubular frame, designed to replicate the attachment points for the manipulator components.
Once assembled on the frame, a start could be made on installing the control software. On powering up the system, it was found that the power supply from the series of 12 V batteries was not capable of delivering an acceptable voltage and therefore caused the micro running the electronics pod unit to constantly reboot. Eventually, the power was stabilised, but then it caused an internal fault within the electronics pod that resulted in a test channel that dropped out to zero (flat line). The power supply was eventually replaced with a more stable one, but this and other problems with short circuiting in the hard drives and mechanical closing of the clamp led to delays in shipping the system out to Dacon for the field trails and a request for a project extension.
One major design problem that could not be resolved was a back pressure in the hydraulic system which prevented deflation of the bladder. For the purposes of the demonstration, a hand valve was added. In a future development, this problem would be rectified by separating the hydraulics powering the actuators from those filling the bladder.
The system was ultimately set up to take LRUT data from a short length of pipe, where the best data was collected at an ultrasound frequency of 36 kHz.
The whole LRUT manipulator, including the mock-up ROV frame and the test pipe were shipped over to Norway for trials at their works. Delays at customs when entering Norway, cut into deadlines. Consequently, the final trials could not be completed until the very end of the project.
At Dacon the manipulator and system modules were loaded onto their observation class ROV without any problems and a video made of its operation.
To collect data, the manipulator was clamped to the 12' pipe held vertically.The signals from the pipe ends were clearly detectable. A flat was ground into the pipe wall about 50 mm diameter and 3 mm deep and 30 mm from the pipe end. The pipe was retested and the signal from the machined flat noted.
For the immersion trials of the LRUT manipulator on the Dacon ROV, it was lowered into the tank and went through number of operations, which included swimming forward and clasping the pipe. A 10 min video, taken from the camera on board the ROV was made of these operations.
Although the trials were successful, a list of modifications needed to improve the design has been put forward, that are a reasonable basis for taking the manipulator through to the next technology readiness level (TRL6).
Unfortunately, adverse weather conditions prevented the dock-side trials taking place. Because of the long Norwegian winter, these cannot now take place until Spring 2011.
Potential impact:
The SUBCTEST project has raised considerable interest in the offshore oil industry. A press-release in TWI's own newsletter for its members in the oil and gas sector raised almost 50 enquiries in a week.
The trials of the LRUT manipulator on partner Dacon's ROV were successful but were limited to the diver tank because an early onset of winter prevented dock-side trials. However, an interest from Chevron may provide an opportunity to take the system for offshore trials in the Gulf of Mexico.
Although the trials of the ACFM manipulator were diver only, the NDT system has since been adapted for use from a work-class ROV.
SUBCTEST has therefore proved a viable platform for ROV deployable NDT systems that can be used to detect corrosion and cracks in tubular offshore structures including subsea flowlines, risers, the 'jacket' structures on which oil and gas platforms sit and chains used to moor floating offshore structures. SUBCTEST might also find application for inspecting offshore wind turbine structures.
The future of the SUBCTEST system may largely depend on the whether it can offer solutions to particular problems requiring ROV deployment of inspection devices. Crises occur from time to time in the oil industry where 'money becomes no object' and further development of system, for example to deploy functioning PAUT and the associated high costs of marinised cable connections are no longer a problem.
In terms of benefits, this project will benefit the SMEs but it will also increase safety and reliability of Europe's critical offshore industry. The United Kingdom (UK)'s HSE has recently issued reports severely critical of the structural integrity in the UK's offshore industry, citing ageing infrastructure and changing ownership from major oil companies to smaller 'independent' ones as reasons for concern. With SUBCTEST, operators will be able to deploy a range of NDT tools; LRUT to identify and locate defects over long distances followed by ACFM and potentially PAUT to properly assess and size those defects to determine their significance to continued fitness-for-service of the structure. This modular interchange ability from an ROV deployed system is extremely important as moves to reduce or even eliminate diver use progress.
Diver deployment is extremely hazardous and often the extreme pressures under which they work reduces the sensitivity of the test because proper analysis of test results in an 'office' environment, as is the case with ROV deployment is not possible. Often it is the case that defects when found cannot be remedied immediately and a monitoring regime needs to be instigated. Repeat inspections by an observation class ROV that can be deployed from a small boat or from the platform side will be much more cost effective than when using a work-class ROV with associated infrastructure, including a diving support vessel. Indeed, minor adaptations to the LRUT manipulator would allow it to be detached and left on the pipe for permanent installation with periodic visits by the ROV to collect data.
On completion of the project, the four SMEs, led by Dacon have come together to formulate a business plan to develop the SUBCTEST NDT platform. Dacon is building up its inspection capability offering bespoke solutions for underwater problems. GRI has become a part of Fugro, a major supplier offshore engineering with a fleet of ROVs and has shown an interest in taking the technology forward. I&T Nardoni see an avenue for exploiting the LRUT and ACFM techniques but in above ground applications such as processing plant, refineries and pipelines. Finally, Vermon wish to take the PAUT technique developed by the SUBCTEST project to a higher TRL in a future development project.
The business plan hinges on taking each of the SUBCTEST components to a higher TRL. The completed LRUT manipulator is still only a prototype at TRL 5 to 6. To take it to a fully functioning system that is can be offered commercially requires the building of a suitable engineered 'production' system, its proper validation and subsequent deployment in a fully operational environment offshore. The costs of offshore operations are extremely high and even if they can be met, operators will not tolerate any activities that interrupt oil and gas production. As mentioned earlier, the oil and gas industry is very 'reactive' and generally we must wait for a major incident requiring ROV deployment for an opportunity. The business plan therefore rests on developing a 'position' in the market, where SUBCTEST is known to offer a solution to an inspection problem. This is not an argument entirely accepted by the two large organisations in the project, the UK's HSE and Norway's PSA. Both these regulatory bodies see an immediate need for SUBCTEST deployment and will do their utmost to promote its further development.
However, the SMEs did agree to seek further funding to take SUBCTEST and its components to take them to higher TRLs. The most probable source would be the major oil companies, Shell in Houston and Chevron in California have already shown interest. There are also the main offshore contractors such as Fugro, Subsea7 and Technip. Fugro might seem the most promising, because they now own GRI but other fruitful discussions have been had with Technip, which currently has a concern in inspecting mooring chains on a North Sea floating production and oil storage facility.
The exploitable knowledge arising from the project is in the following areas:
- techniques and equipment for application of ACFM using array transducers for inspecting welds in complex geometries such as a welded node in an offshore jacket structure;
- marinisation of the ACFM equipment for deployment from a scanner sub-sea;
- techniques for application of PAUT for detecting surface breaking cracks on internal and external surfaces using so-called 'creep waves';
- techniques for application of PAUT for detecting surface breaking cracks on internal and external surfaces at test ranges up to a metre using so-called 'multi-skip' ultrasonics;
- techniques for application of LRUT for detecting root cracks and corrosion in the welds at each end of a pipe spool from one test location along the pipe spool;
- EMATs designs for use in LRUT transducers when testing pipe covered in highly attenuative coatings;
- manipulator for ACFM transducer for scans of node welds subsea;
- manipulator for LRUT transducers for deployment from a sub-sea observation class ROV for clamping to a pipe;
- data link and control software for LRUT manipulator.
The SMEs came together to formulate a road map to give a strategic vision beyond immediate plans for taking SUBCTEST inspection platform.
The 'whys' in the road map must include the business case for developing SUBCTEST. This is particularly important for SMEs, where cash flow is the over-riding factor. An attractive plan, taking SUBCTEST forward to an accepted future might be completely undermined by a shortage of cash and absence of running capital at almost anytime, when SMEs have to finance the development themselves. Collaboration with a resource-rich organisation is therefore essential.
For the 'whats', a hierarchy has been established with the SUBCTEST platform at the top, divided between the LRUT, ACFM and PAUT systems. This can be divided into further layers.
Each item in the hierarchy can be attributed a TRL. An item at one layer cannot exceed the item in the layer below with the lowest TRL. Discussions with the beneficiaries have attributed TRL levels to the exploitable knowledge resulting from SUBCTEST. The results of SUBCTEST fall around the middle, where applications engineering dominates and technology push must now become market pull. Other results from the SUBCTEST project have much lower TRLs. For example, the EMAT modelling is only at TRL 3.
Project website:
http:// subctest.com
Contact details:
Graham R. Edwards
Consultant
NDT Technology Group
TWI Ltd
Granta Park
Great Abington
Cambridge
CB21 6AL
UK
+44-012-23899050
Graham.edwards@twi.co.uk
There is little if any routine NDT carried out on critical sub-sea welds in offshore assets because current NDT equipment and techniques do not currently lend themselves to be applied sub-sea below depths of about 25 - 30 m and those cases are exceptional and rely on divers applying visual inspection and electromagnetic techniques for surface weld examination. The operating oil companies, under the encouragement of the regulatory authorities, are committed to reducing hazardous diver operations and replacing them with ROVs.
However, current ROV deployments are hampered by the size and weight of NDT equipment. SUBCTEST set out with the aim of reducing these to a size and weight that could be deployed from smaller 'observation class' ROVs. This would extend the equipment's availability and make operation possible from ordinary supply vessels instead of from dedicated ROV ones. The principal objective of SUBCTEST has been therefore to provide an ROV deployable platform for three NDT methods, namely alternating current field measurement (ACFM), phased array ultrasonic testing (PAUT) and long range ultrasonic testing (LRUT). By way of achieving this, SUBCTEST has also had the objective of developing suitable NDT techniques for inspecting a variety of subsea structures, including 'jacket' braces and nodes, risers, flow lines and mooring chains.
For the nodes, the ACFM array technique will detect external surface cracks in the weld toes in one pass and the multi-skip PAUT technique will detect weld root defects without having to swivel the probe away from an axial orientation to which the probe shoe can be contoured. The creep wave PAUT technique will detect both internal and external surface breaking cracks simultaneously. For the flow lines, the LRUT technique will detect defects in welds at either end of a pipe spool form one test location anywhere along its length. For the touch-down point in risers, an LRUT technique will scan the surfaces of the riser for corrosion and fatigue cracking from one test location. For the mooring chains, the 'race track' intermediate range UT technique can screen chains for fatigue cracks.
Most engineering associated with equipment for ROV deployment can be described as 'agricultural', that is to say it is over-engineered, with massive components and crude control systems. The challenge in SUBCTEST was to reduce the mass of the manipulator that carried the ACFM, PAUT or LRUT transducers or sensors to a level that could be deployed by an observation class ROV. Considerable effort was expended in the design stage to keep the buoyancy of the equipment as high as possible, so that its submerged load was as low as possible. Novel designs saved considerable costs. Including for example, the use of a simple games controller to operate the LRUT manipulator.
For the ACFM manipulator a very simple design was settled upon, using a saddle that give 170° coverage around pipe circumference. As part of the SUBCTEST development, one of the partners developed a simulation of the ROV operation, when clamping the LRUT onto the brace of a jacket structure.
More details of the 'SUBCTEST' ROV deployable NDT platform can be found at http://ww.SUBCTEST.com
Project context and objectives:
There is currently little if any routine NDT carried out on critical sub-sea welds in offshore support structures because existing NDT equipment and techniques do not lend themselves to be easily applied sub-sea at depths greater than 25 - 30 m, and even those cases are exceptional and rely on divers applying visual inspection and electromagnetic techniques for surface weld examination. The operating oil companies, under the encouragement of the regulatory authorities, are committed to reducing diver operations because of safety issues (recent Norwegian case awarded EUR 3.7million compensation to 3 divers), shortage of divers and cost. Diver operations are both depth and time limited and there is now a clear case for ROV applied NDT techniques to be used in order to help prolong and extend the operational life of European oil and gas field reserves.
The SUBCTEST project benefited the participating high technology small and medium-sized enterprises (SMEs) that wanted to develop their existing NDT and robotic technologies and techniques into new offshore sub-sea inspection applications. It also benefited the lower technology participating SMEs that wanted to enhance their NDT technology base by becoming specialist providers of the service to offshore oil and gas operators. The SMEs drew on their in-house capability and also that of the participating research organisations, who supplied research and development (R&D) to the SMEs on a sub-contract basis and to a work-scope defined by the SMEs.
The ROV would be an observation class ROV that did not require a support ship, which is a necessary requisite of the much larger work-class ROVs that are normally used. The NDT could then be controlled from the platform topside and would deploy a robotic inspection head to carry out NDT of platform jacket critical support welds using LRUT and PAUT for volumetric weld inspection and/or ACFM in order to detect surface breaking fatigue cracks.
The project therefore set out with a number of technical objectives:
- to write a technical specification for a system that could deploy the three NDT techniques and find test-pieces on which to validate the NDT (work package (WP)1);
- to develop an ACFM sensor and technique that could be deployed by a robot around a node weld in a single pass, without the problems associated with current ACFM sensors when scanning complex weld geometries (WP2);
- to develop a PAUT technique for scanning around node welds without the need to raster scan over the elliptical surfaces and thus affect the beam steering (WP3);
- to enhance the resolution capabilities of LRUT that is currently used to detect corrosion in pipes so that it can be used to detect defects in pipe welds (WP4);
- to develop a robotic manipulator for deploying the NDT sensors from an observation class ROV (WP5;)
- to conduct laboratory and field trials with manipulator prototypes (WP6).
As well as these technical objectives, the SUBCTEST project also set out to achieve the following objectives:
- Economic objectives by way of improving the competitiveness of a group of European SMEs operating in the world-wide oil and gas industry by exploiting the technology developed within the project. The technology developed would also reduce the economic costs of failures in offshore structures.
- Social objectives by way of eliminating the risks of diver deployed inspection systems and catastrophic failures that lead to multiple deaths as happened in the Piper-Alpha disaster in the North Sea during 1988.
- Environmental objectives by reducing the risk of failures that might lead to oil spills with possible catastrophic effects on the North Sea environment.
Project results:
The project work was organised into a number of discrete WPs, each consisting of tasks that collectively achieve the specific WP objective(s). At the 12-month meeting, these were re-organised and a new description of work (DoW) issued.
The principal changes were the following:
- Abandoning the PAUT marinisation because of the high cost.
- Use the funds for the PAUT marinisation to investigate electro-magnetic acoustic transducer (EMAT) probe designs for LRUT of coated pipe. This was recommended because the extensive use of coated pipe restricted LRUT ranges with current piezo-electric transducers, and to make most effective use of partner ZUT's knowledge of EMAT development.
- Developing a separate LRUT manipulator from the ACFM one because the designs requirements are quite different.
- Introduce simulation of ROV deployment on an offshore platform instead of demonstration trials, because of the high cost and partner GRI's expertise in this area.
The revised project DoW was issued in May 2010. It divided the work into 6 WPs (excluding management and exploitation), covering 18 tasks.
An extension of twi-months was added to the project to complete the prototype trials after delays in integrating the parts of the LRUT manipulator and problems with customs in transporting the equipment into Norway. Unfortunately, this delayed the dock-side trials and these could not be started before the Norwegian winter set in. The dock-side trials were to be conducted by Norwegian partner Dacon. A number of parties have shown an interest in these trials outside of the SUBCTEST project, and it is expected these will be conducted shortly.
A more detailed description of these WPs follows.
WP1: Design and procure samples and specify system requirements
The WP had two objectives:
1. to design and produce a set of test specimens containing a variety of representative known defects, including worst case (i.e. most difficult to detect) in welds for ACFM, PAUT and LRUT development work;
2. produce equipment functional and technical specifications.
The two objectives were achieved through two tasks. At the kick-off meeting, the partners decided that concentrating the development work solely on complex weld nodes was too high risk and that other simpler applications should be included. Moreover, the decision by Dacon to offer their inspection ROV for the trials increased the potential scope of the project to small structures and components. The partners therefore decided to look at other simpler applications for ROV deployment, namely welds in sub-sea mooring chains and in flow lines and the critical 'touch-down' points in risers where the flow lines bend vertically upwards from the sea bed towards the production platform. Samples of these were included in the library of test samples put together at the start of the project and the specification adapted to meet this requirement.
The library of test samples was built up from those available at TWI of which there were two types:
1. Samples used for developing sensors and systems. These have to be small for use in the laboratory and contain mainly artificial discontinuities. Samples have been made using a range of notches to represent surface breaking cracks at various angles of tilt.
2. Samples or mock-ups used for demonstration trials and which should include real flaws or artificial ones that represent real flaws as closely as possible. Preliminary trials were conducted in TWI's diver training tank, where suitable test nodes are already available. TWI also had available mock-ups of steel catenary risers, chain links and subsea flow lines, both uncoated and coated with concrete (used to anchor pipes to the sea floor) and nodes.
The specification went through several iterations as the costs of the various options became apparent. The final specification covered ROV deployment of only two of the three NDT systems considered, ACFM and LRUT for inspection of riser and welds in flow-lines and nodes.
Also the specification was written so that the sensor systems and manipulator could be deployed from an ROV provided by SME partner Dacon. This necessitated the separation of a clamping type manipulator (LRUT) and scanning type manipulator (ACFM).
WP2: Develop sub-sea automated ACFM system and techniques
The objective of this WP was to deploy ACFM sensors and systems for complex subsea weld geometries. Typical welds include butt, fillet and node welds. Node welds occur when two cylindrical sections, of different sizes, meet at an angle. These welds are by far the most complex. To tackle this problem TSC created a 'toolkit' of probes, software and instrumentation.
The probes included a scanned array probe that could be moved along the weld in a single pass by the manipulator. They have the advantage of detection and sizing sensitivity, but must be precisely deployed by the manipulator. This was deemed to be the correct choice, as such manipulators are relatively simple.
An alternative 'pick-and-place' probe would allow a weld to be inspected in a number of discreet steps, and thus avoid any requirement for scanning by a manipulator. The benefit is that it is easier to deploy, the downside is that the data density is limited by the physical positions of the sensors in the probe and so sensitivity, particularly to crack length, is lower than for a scanned probe.
A prototype (non-marinised) probe was produced at the six-month stage of the project, which allowed the concept to be tested before manufacture, to ensure the additional requirement on the software and instrumentation could be achieved.
Previous experience of subsea probes suggests a scanned array with multiple sensor coils for this task.
Sensor compliance was achieved by producing a robust probe with a stainless steel nose that can be pushed against the weld by the manipulator. The structure of the weld consists of a vertical chord, connected to a secondary brace; this forms a complex shaped node weld. Both toes of the weld must be inspected, in addition to the crown of the weld; this would typically require more than one probe, which is problematic as it is difficult to swap out probes without bringing the ROV to the surface.
The changing geometry of the structure affects the magnetic field generated by the probe. This causes a problem, as the electronics of the ACFM system limit the acceptable variation of magnetic flux density from sensor-coil to sensor-coil by a factor of two. The solution was to design a single scanning array probe that could cope with the changing geometry and inspect both toes of the weld, plus cap.
Comsol multiphysics was used to create an FEA simulation of the problem in order to optimise the probe design. The distribution of the magnetic flux density on the lower section of the brace differs from that of the top section. It was decided to use a wedge shaped probe, sized to fit the tightest angle likely to be encountered during the project. The magnetic field-generation coils, or field coils, would then be optimised to provide a suitably distributed field for all angles of the inspection, while maintaining uniformity, acceptable to the electronics of the ACFM system.
Several designs were considered to achieve the probe specifications. They were:
- perpendicular rectangular coil design;
- variable diameter coil design;
- variable field winding design.
The variable field winding design was found to be the most successful. This consisted of four field coils wrapped around steel cores. All field coils are activated simultaneously. The number of field windings around each core is increased to strengthen the field away from the weld crown. This produces the correct distribution of field intensity required for the simultaneous inspection of the parent plate, heat affected zone and weld crown.
A prototype probe was produced, in order to check the operational characteristics. Once the numerical simulations were completed, it was decided to validate the model with a prototype before building the final probe. The sensor coils are placed in pairs. The horizontal coil measures the horizontal field, used to determine the depth of the defect. The vertical coil measures the vertical field, and is used to determine the location and length of the defect.
The final probe was successfully marinised and tested.
A second task in this WP was devoted to design and build instrumentation and software to support the ACFM probes. Existing diver instrumentation was repackaged to suit deployment on the autonomous vehicles. This required a reconfiguration to allow most of the instrumentation to be housed on a host ROV while the inspection head is manipulated independently by a subsystem (the manipulator). The ACFM software was developed to run within the complete system man-machine interface (MMI) environment.
TSC's standard U31 instrument was developed for diver use and uses RS485 communications through a traditional copper twisted pair umbilical. When the ACFM system is deployed by ROV the standard U31 umbilical cable cannot be used easily due to the necessity to run in parallel with the ROV umbilical. An ROV umbilical is typically several kilometres long and is limited in the number of copper conductors available. Instrumentation and video cameras are required to send their data via an optical fibre in the ROV umbilical as this optical fibre is capable of supporting several functions simultaneously. An instrument called a fibre optic multiplexer (or 'Mux' for short) is used to interface between instrumentation and the umbilical fibre. A Mux is required at topside and sub-sea ends of the umbilical. The ACFM system needs to be able to interface with the Mux without error.
The U31 RS485 interface is configured at 19 200 Baud, 8 data bits, 1 stop bit. It operates in half duplex mode with the topside unit as master. Checksum and acknowledge signals are generated by both topside and sub-sea units. A Focal 907 was selected as the test Mux unit as this is commonly used by ROV systems.
Initial trials showed that the proprietary 'comms' protocol used by the ACFM system was not compatible with the Mux due to timing and latency issues. This was investigated and the problem was found to be that the receiving end's acknowledge signal was being ignored by the Mux resulting in the transmitting end perpetually repeating its message in the hope that the receiving end would receive the message and send an acknowledge signal. A solution was engineered that involved the modification of the U31 system topside and sub-sea firmware, introducing delays to accommodate the 1.5 ms latency in the Mux unit as the comms direction changed from up-load to down-load and vice versa.
Reliable performance of the comms link was obtained without significant delay to the transport of the ACFM data. Optical fibre and copper umbilicals were used.
ROV systems generally have 12 V or 24 V power available for auxiliary systems. The U31 Instrumentation uses a dual 12 V power supply suited to its sensitive analogue measurement electronics. In order for the U31 to be easily interfaced to any ROV a power adapter has been developed that allows direct connection of the U31 system to typical ROV power supply systems. The adapter is suited for connection to 12 V or 24 V supplies without further modification. Appropriate filtering has been incorporated so that noise from the ROV systems does not interfere with the U31 instrument electronics. The adapter has been housed in a pressure vessel capable of withstanding the pressures at 2 km water depth.
The current U31 ACFM inspection system is capable of supporting array probes with up to 32 channels. Expanding the capability of the system to 128 channels would allow the inspection of a greater surface area. The greater surface area may allow the cap and both weld toes to be inspected at the same time increasing the efficiency of the robotic inspection system. In some circumstances a larger array may be more tolerant of positional errors, reducing the precision required of the manipulator or scanner.
The U31 electronics has been adapted to allow up to 128 channels. A new PCB has been made to facilitate connection of the additional signals to the array probe cable.
It is sometimes necessary to change the type of probe during the inspection of a complex geometry. It may be, for example, that most of a complex weld can be inspected with a probe designed for general weld inspection, but where areas are constricted due to a more acute angle between chord and brace, a smaller tight access probe needs to be used. If the probe change required the ROV to be brought to the surface to allow the probe to be changed manually this would add significant time and cost to the inspection operation. If the U31 could accommodate more than one probe connected at a time, then the ROV operator could just drop the current probe in to a tool basket and pick up the next probe to be used depending on the application.
A multi probe interface has been built to facilitate remote probe change out. Up to four probes can be connected at one time. The four probes can be either independent of each other or synchronised so that multiple probes can be scanned at the same time. The electronics for the probe select interface have been made small enough to allow marinisation in a small package such that it is easy to integrate in to the ROV tooling frame.
The SUBCTEST project required extension to existing ACFM software and development of both software and firmware specifically to support new features for SUBCTEST. Underpinning the ACFM software, probe files define the operational parameters, including deployment methods, environmental factors and instrument settings that determine how an ACFM system operates.
In order to achieve the requirements for SUBCTEST, significant enhancements were required to support a number of new features as follows:
- Increase the maximum channel range from 32 to 128
For scanned array probes, channel definitions were extended to include a greater number of channels in a single dimension. For pick-and-place probes the channel range was extended in two dimensions.
- Support the inclusion of inclinometers and their parameters, including RS485 ID's and resolutions
- Introduction of a new 'master' probe file structure. This significant development, allowed the creation of master probe files with common probe configurations, allowing multiple connected probes to be referenced by a master probe file.
Firmware programming changes inside the subsea instrument, topside and probe interface units were required for SUBCTEST to support changes for:
- protocol compatibility changes for 907 Mux fibre optic timings;
- full high-speed RS485 long umbilical twisted-pair comms;
- channel range increase from 32 to 128 (achieved by I2C programming for higher channel lines);
- multiple inclinometer devices, via new multi-drop 485 bus;
- multi-way probe interface via new multi-drop 485 bus.
A number of software enhancements were made to cater for the following:
- Increased channel range. For both pick-and-place and scanned array probes, the visual software interface has been modified to allow presentation and selection of greater numbers of columns and rows. This is apparent through both enhanced row visibility panels and higher resolution contour plots.
- Inclinometer support. When configured, the software will display live-update inclinometer readings.
- Multi-way probe interface. The software allows the user to identify and use the specific probe presently connected to a port of the multi-way probe interface, thereby allowing the ROV to deploy multiple probes from the tool basket.
TSC routinely manufacture subsea probes for the oil and gas industry, sometimes operable to a depth of 2000 m. Each of the SUBCTEST probes were constructed of glass-filled peak plastic, a tough plastic with little hydroscopic absorption and good dimensional stability. An O-ring is used to ensure no water ingress. Additional water-proofing and strength is achieved through the use of a potting compound, which is poured into the cavities of the probe. As this material dries it solidifies, sealing the electronics and turning the probe into something that is in effect a solid plastic body with no moving parts or air cavities.
This was tested at the National Oceanography Centre in Southampton. Hydrostatic testing was completed, and the pressure test proved the design down to 150 m for two hours. The probe was tested before and after the test, and was found to be functional each time.
WP3: Develop sub-sea phased array automated ultrasonic test system and techniques
The main objective for this WP was to develop new and novel NDT sensors and systems based on multi-element phased array probe design so that the techniques developed can be applied to the sub-sea welds as part of the SUBCTEST NDT platform.
The work was divided between three tasks:
In the first task PAUT techniques for node welds between inter-connecting tubulars in a subsea 'jacket' structure were investigated. These are a particular problem for sensors that scan, because of changes in geometry between the scan surfaces as the probe orbits the weld. The PAUT would be complementing the ACFM technique, which detects only surface breaking defects, because it is sensitive to sub-surface defects. The two might even be combined. The PAUT is however much slower and therefore for SUBCTEST, the application of PAUT was confined to detecting surface breaking cracks on the internal surface of platform tubular structure. At the six-month meeting, a PAUT technique that employs a creeping wave along the internal surface and external surfaces simultaneously was discusse.
This has the advantage of not requiring the sound beam to be over the crack, the crack can in effect be detected at a distance of several centimetres from the probe. Creep waves are normally generated using conventional monolithic transducers and use a wedge that refracts a compressional (L-wave) along the outer surface that is known as the 'creep wave'. This is accompanied by a shear wave (T-wave) that refracts at 33º to the surface. This T-wave generates an L-wave component along the internal surface, the I.D. creep wave and reflects T-waves that 'leak' from the creep wave as it propagates along the internal surface.
If the internal creep wave can be generated over a distance of several centimetres this would obviate the need to raster scan the PAUT probe. This would be a major advantage within SUBCTEST, since welds could be scanned linearly in one scan as is the case with the ACFM array probe.
However, the L-wave beam is always (except when it is vertical) accompanied by a T-wave. This gives rise to a number of other echo-signals from one defect.
Although this is a disadvantage in terms of imaging the defect, it has the advantage in improving sensitivity to the defects on a go no-go basis because it gives rise to multiple signals from one defect that is more likely to be detected. If the aim is only to screen for defects rather than size them, then such a multiple-skip technique might be an advantage. For this reason, it was considered for SUBCTEST.
In the process of investigating the creep wave technique it was found however that:
- shallow cracks could be distinguished from mid-wall cracks and these from deeper cracks on the basis of the presence or otherwise of three identifiable signals on the A-scan;
- surprisingly high amplitude signals could be detected from corner reflectors at quite long stand-off distances from the weld. This led to the development of a multi-skip technique.
An analysis of the multi-skip technique was made of successive 'skips' of the sound beam into the weld from the probe using a simple diagrammatic representation. Using a PAUT beam restricted to fixed angle of 58? and sweeping from side-to side (i.e. horizontal linear array) a girth weld in a 12' pipe could be tested at a range of about 1 m. This would be a major benefit for in-service inspections of welds, which are often coated and inaccessible to conventional NDT.
An investigation was also carried out on the inspection of node welds using the horizontal linear array. Node welds when tested with conventional monolithic transducers are scanned in an orbital manner, with the front of the probe parallel with the weld axis and therefore the sound beam perpendicular with any longitudinal flaw in the weld. When tested with a PAUT probe, however, the size of the probe 'footprint' gives rise to probe 'rocking' on the surface and loss of a steady beam profile with which to scan the weld. The probe can only be prevented from rocking by scanning with the probe shoe contoured to the curvature of the brace and therefore pointing parallel with the brace axis). The multi-skip technique was shown to have adequate sensitivity to the gross weld flaws in the pipe girth welds.
Finally, the PAUT probes were used to inspect mooring chain links, where the 33° transverse wave beam, as well as the creep waves, could detect fatigue cracks in the head of the chain link.
WP4: Develop sub-sea LRUT equipment and techniques
At the six-month meeting it was decided to investigate LRUT techniques for inspecting:
- riser touch down points
- flow line girth welds
- 'jacket' node welds
- mooring chain links.
The original DoW included an investigation of composite multi-layered actuators as a method of propagating high amplitude guide waves under the attenuative concrete coatings found in some sub-sea flow-lines. The actuators can deliver and support high force loads with minimal compliance, but at the cost of delivering only small motions. However, they could only be offered in compression wave mode generation, which is not appropriate for LRUT. Firstly, from the frequency sweep results, it was observed that the signal was very dispersive. In addition, there were several wave modes produced, making data analysis difficult. More importantly, actuators proved to be highly resonant i.e. there were different frequency components present when excited at a certain specific frequency.
As an alternative to using higher power multi-layered composite transducers to generate guided waves further through pipes that are coated with highly attenuative materials, the generation of specific low attenuation modes was investigated. It is known that shear horizontal (SH) wave modes 'leak' less from the pipe-wall into the coating than L-wave or T-wave. SH waves are best generated using electro-magnetic transducers (EMATs). The principal of EMATs is the generation of stress waves in the surface of a metal by the interaction of induced eddy-currents with a magnetic field. Experimental work showed how simple EMAT designs could be used to generate guided waves in pipe and plate. It was therefore decided to explore the possibilities using numerical models. At the 12-month meeting it was decided to amend the DoW to give research and technology development (RTD) performer ZUT, who have in-depth expertise in EMATs, a task to develop models for selecting the best EMAT configuration for generating SH-waves as an alternative to the stacked actuators.
SH waves can be generated by periodic permanent magnet (PPM) EMATs or meander coil EMATs.
The challenge was to generate models that combine electromagnetic waves with stress waves in a single executable model that is able to show their inter-reaction. TWI and ZUT have experience in using the Comsol multi-physics finite element model for solving various engineering and physics problems. The software developers were contacted and their help enlisted in this application, which was unique in their experience.
Preliminary signal analysis was introduced based on the ultrasonic wave's two-dimensional (2D) fast Fourier transform (FFT) to distinguish ultrasonic wave modes. Numerical models for both the PPM and Meander-coil EMATs were created using three-dimensional modules in Comsol multiphysics software. Solving the problem was divided into three parts: permanent magnets, coil and stress waves.
Static magnetic field produced by permanent magnets was calculated in AC/DC module (magnetostatics). The impulse current excited in the coil was broken into harmonics. Eddy currents induced in the specimen by the coil were calculated in AC/DC modules (induction currents) separately for each harmonic of the excitation current. Stresses and displacements in the specimen were calculated independently for every harmonic in structural mechanics modules (solid, stress-strain).
All harmonics were calculated in the frequency domain with the processes running in parallel, at the same time. This was a much more time-effective procedure than the traditional procedure using time-domain calculations.
Finally, the analysis was made in the presence of an artificial defect. The calculations were made for simplified 2D analysis as well as for full three-dimensional (3D) case. Typical A-scan were shown, the reflected wave showing the real position of the defect with good accuracy. In conclusion it was found that the Meander coil EMAT produced the better field.
To complete the transducer development for LRUT in SUBCTEST, specifically for the tight curvatures encountered on chain links, flexible macro-fibre-composite (MFC) transducers were investigated. MFCs have been developed primarily as pressure sensors and only TWI has investigated their use in NDT. A prototype tool of MFC transducers was built and successfully demonstrated on a mooring chain.
Although different types of transducer were investigated in the project, because of time constraints it was decided to proceed with marinisation of an LRUT tool using conventional piezo-electric ceramics.
The original specification for SUBCTEST called for transducers that were able to operate at a depth of 350 m below sea level equal to a pressure of 35 bar. With a 10 % safety margin, this equates to a total pressure of 38.5 bar or 558.25 psi. There was a design for water immersing transducers developed for the RISERTEST project (project partners TWI and Dacon) but the maximum operating depth of this design had previously not been established, although the specification had called for an operational depth of 14.5 m. It was therefore necessary to test this design for performance at the increased depths required for the SUBCTEST.
During the first reporting period, a transducer design was successfully tested at 150 bar. However, during this second reporting period, the company that supplied the basic transducer went out of business, so the transducers were no longer available for marinisation.
An alternative design of transducer was produced outside of this project for general LRUT use, which could be manufactured in-house. This design had to be adopted for marinisation in the SUBCTEST transducers. This led to significant delay in integrating the LRUT system for the trials. The new design used fewer components, because it used a non-conductive coating on the aluminium housing. The need for separate electrode plates for applying the correct polarity to the transducer plate was therefore obviated.
The transducer was coated with Sikiflix sealant that was simply scraped from the wear plate to provide ultrasound coupling with the test surface.
For generating guided waves, the transducers have to be mounted in rings, to provide a sufficient density of vibrations around the pipe circumference to prevent the propagation of flexural waves. On a 12' diameter pipe decided upon for the final trials, sufficient density of transducers in the ring to propagate axi-symmetric guided waves is achieved with 36 transducers. To propagate guided waves in one direction a second ring is needed that operates in anti-phase to the first one. This destructively interferes with the backward going pulse. Also, to increase the amplitude of the forward going pulse, a third ring in the front constructively interferes with the forward going pulse. Three rings were therefore incorporated in the SUBCTEST prototype, each with 36 transducers.
To modularise the system, so that the number of transducers can be increased or decreased according to the pipe diameter, the transducers are mounted in a holder with sealed connectors. Problems were encountered in some of the modules at pressures above 20 psi, causing loss of signal, but the effect was reversible, possibly due to air voids within the potting material inside the module, which would compress and hence distort the connection wires within the transducer.
With regard to the application development for the LRUT tool, work that was running in parallel with the transducer development, the inspection of riser 'touch-down' points did not warrant work as it would use existing LRUT techniques as they are applied to pipes above water.
The inspection of girth welds in sub-sea pipelines was identified by partners at the six-month meeting as a more difficult application for the SUBCTEST ROV applied LRUT. TWI has a number of welds in its LRUT test loop and these were tested with a view to finding a weld containing a weld flaw. One flaw was detected in a 12' pipe girth weld by means of its increased flexural content.
Precise positioning on the defect around the weld circumference was obtained by using the phased array algorithm on the transducers to focus on the weld. Rotating the focal spot through eight positions around the weld showed an increase in reflectivity from the weld between 90º and 180º from top-dead-centre. The signals are also displayed in a so-called A-map. The flaw was later confirmed to be an area of lack-of root penetration. The sensitivity of the LRUT technique to gross defects in girth welds, such as root crevice corrosion, at each end of a pipe spool was therefore demonstrated.
Most sub-sea pipelines are coated with concrete in order to anchor them to the sea floor. Concrete coatings are very attenuative of guided waves. It was for this reason that stacked attenuators and subsequently SH wave EMATs were investigated.
However, some surprising results were obtained when testing a coated 12' subsea pipe with conventional LRUT transducers placed on the inside of the pipe. A surprisingly low attenuation (1.3 - 1.6 dB/m) was found and this was attributed to the presence of an epoxy layer between the pipe wall and concrete coating. Also, by having the transducers on the inside surface of the pipe there was a greater amount of ultrasound energy going along that surface than along the outer surface where the coating was. This suggested that if the ultrasound energy could be concentrated on the inner surface of the pipe when testing from the outside surface, the test ranges in concrete coated pipe could be increased. This was one of the reasons for pursuing EMATs for guided wave generation as they provide some degree of control on the 'penetration' depth of the waves.
With regard to the application of LRUT to node welds, trials were conducted on a section of nozzle from a 12' branch. The test range was limited to less than 2 m, resulting in the signals occurring within the A-scan dead zone. Despite the high level noise, patterns could be seen in the A-map displays corresponding to the weld Apex, two sides and crutch.
However, it was generally decided that the LRUT technique would need to employ much higher frequencies to achieve adequate sensitivity to cracks and corrosion in node welds.
At the kick-off meeting, the partners decided to include the inspection of mooring chain links in SUBCTEST, because of its simplicity and because mooring chain failures were now a major concern among operators of floating production and oil storage vessels (FPSOs). The initial experimental work was carried out with a standard 4' torsional wave tool, but much better results were achieved with a special tool comprised of MFC transducers. Unique feature of LRUT on a chain link is that the guided waves will continue circulating around the ring, and there will be multiple signals each time the guided wave passes the ring.
However, the tool is not optimised for LRUT, where the tools have more than one ring in order to select wave modes and determine the direction of propagation of the waves. With a single ring, the interpretation of the A-scans from multiple modes propagating in either direction would be very difficult. Therefore, a way of collecting A-scans from a frequency sweep and stacking them in a pseudo-3D display using propriety Matlab software was developed. In this way, signal patterns from defective and defect-free chains could be distinguished.
WP5: Develop sub-sea NDT robot manipulators and vision system
Major changes were introduced into this package because:
- the development of the LRUT manipulator had to be separated from the ACFM one, because the clamp action of the LRUT transducers is different from the scanning action of the ACFM transducer;
- the project funds would not extend to performing trials with the ROV manipulators at an actual oil platform site and therefore a simulation would be carried out, since one of the project partners (GRI) had this expertise.
As a result of the abandonment of the phased array ultrasonic test (PAUT) development, the manipulator with the scanning action was restricted to the ACFM technique only. The manipulator developed has a kinematic structure with four degrees of freedom:
- one rotary motion around the pipe;
- one linear motion along the pipe;
- one linear motion in axis perpendicular to the pipe (passive - spring loaded);
- one angular motion in axis perpendicular to the pipe (passive - spring loaded).
Linear motion along the pipe is needed to bring the transducer up to a weld. The mechanical construction consists of a bracelet with two ring tracks (for rotary motion), carriage with DC motor, linear actuators equipped with a sensor and special mechanism to close the bracelet after ROV has placed the manipulator on the pipe.
Two types of drives were considered: hydraulic and electric. Hydraulic motors (for rotary motion) and hydraulic actuators are ideal in an underwater environment but are very heavy. Therefore, MAXON geared DC current motors with encoders were selected. Two drives were used; the first drive (MAXON No. 341514) to deliver the rotary motion, the second one (MAXON No. 227733) to deliver the linear motion. In order to avoid the leak and damage of the motors, all the drive units were closed in special aluminium waterproof containers with sealing. The first motor drives the carriage along the ring tracks, while the second is coupled with the linear screw drive.
Binding the braces with robot to the pipe is provided by a special construction of braces and additional electro-magnets. A set of electromagnets is used to close the clamp after it is attached to the pipe and generate a force, which holds the manipulator in position. The position of the carriage movement is limited by electromagnetic limiting switches and the operating angle is equal to 1600. A waterproof camera (connected with a LCD display) with lighting system allows observation of the position and orientation of the transducer which is mounted at the end of linear motion module.
Manipulator control system comprises of prefabricated metal box with installed motor control devices, relays, suppliers, and installed outside: buttons, joystick, connections and fan. What is more, an independent LCD display is also part of control system. For control motion of the manipulator units' joystick and four mono-stabile switches are used. Moreover, one bi-stable switch was used to turn on and off electro-magnets which were specially designed for stabilisation braces on the pipe. The joystick and buttons are located at the top surface of the box. At the front surface of the box are situated three lighting power switches, in the form of bi-stabile buttons, and appropriate for each circuit fuses. Main power switch, red one first from the right side, allows turning on and off circuit supplying whole control system of robot (230 V AC). Then, placed in the middle green switch, allow turning on and off internal 24 V DC circuit. The last switch, the green one placed on the left side is supplying internal 5 V DC circuit. External connections and fan for conditioning device into box are placed at the front surface, as well.
The main parts of the manipulator control system are EPOSes - dedicated units for control and supply drives purpose. In addition, EPOSes allow for communication between drives and external environment. Information about actual position of drive shaft is measured by integrated encoder and fed to adequate EPOS via dedicated ports.
In the second task, the manipulator with the clamping mechanism for the LRUT was built. It can be divided into four components.The bracelet is made up of LRUT transducers mounted in modules that surround the pipe inside a clamp. The clamp is opened and shut by one actuator. As with all the actuators, this is hydraulically driven, using environmentally friendly vegetable oil. When the clamp is closed, a bolt locks the arms together before the bladders behind the transducer modules are inflated with oil to push the transducers down onto the pipe surface. The bolt has an emergency disconnect and release system, so that the manipulator can be released in the event of a power failure.
A second actuator is able to rotate the clamp in order to attach to both vertical and horizontal pipes.
The frame surrounding the brace guides the manipulator so that it is aligned with the pipe. This is often difficult when the ROV is not steady in passing currents.
In the electronics pod, the pulser-receiver for the LRUT transducers with associated amplifiers is housed in one pod. PCB's from conventional LRUT equipment had to be modified to fit into a smaller space, necessary to meet buoyancy requirements. This was not a simple task, since high powered circuits for firing the transducers had to be kept away from the amplifier receiver circuits.
Pump and valve pods for the hydraulic system includes the bladders as well as the actuators. The valves for controlling these were incorporated into a pod, which for buoyancy reasons, was separated from another pod for the pump.
A novel feature of the system is the game controller used to control the bracelet. Its software was a major development for the project.
The control software was developed between two personal computers (PCs) over a network connection and transferring control information between applications running on the PCs.
The application on the control PC (the client) monitored a gamepad that a user would operate in order to produce the desired movements and actions on the physical hardware attached to the server PC. The system would operate in real time with visual feedback in the client to inform the user of the status of the remote hardware. The client PC was a general purpose PC with a wired Ethernet port and a spare USB port for connecting a gamepad device. The gamepad was a standard PC/XBOX 360 controller.
The hydraulic system was controlled by relays on a TWI produced circuit board. The relays were driven via a line driver chip by digital outputs on the analogue / digital I/O board. The PC-104 allowed the boards to be stacked so that only a small physical mounting space was required.
The PC board was a fully-featured computer including graphics, USB and disk drive interfaces.
The hydraulic pump was controlled by a relay on the relay I/O board. Pressure sensing was by analogue to digital converters on the analogue / digital I/O board. The collar interlock switches were monitored by inputs on the relay I/O board.
The board ran Windows XP (service pack 3) from a 2.5' IDE disk drive.
The client and server applications communicated over an Ethernet connection via Windows sockets. This is simple standard method for networked devices to be able to initiate and terminate connections. It does not however provide any assistance with protocols being used between the connection end points. The connection is full-duplex meaning that both ends of the connection can be receiving and transmitting data at the same time.
The protocol implemented over the socket communications is based on extensible mark-up language (XML) documents. This was chosen for its ease of creation and decoding using industry standard parsing techniques.
Both client and server applications were developed using Embarcadero Delphi. This provides an implementation of the Pascal programming language along with an extensive set of development libraries.
In the third and final task, partner GRI used their DeepWorks software to create a simulation of the ROV deployment of the LRUT manipulator. The ROV model faithfully reproduces all the actuators to accurately replicate operations. The buoyancy and drag characteristics of the ROV with manipulator attached are included to give realistic movements in different currents.
The simulations are much more than just the video. They are in fact models in which the ROV pilot can operate in specially constructed scenarios. As such they can be used in training and for planning ROV operations.
WP6: Conduct Laboratory and field trials
Because of the need to split the LRUT manipulator development from the ACFM, it became evident that full field trials on both manipulators would not be possible with the resources available. It was therefore decided by the project group at the 12-month meeting to restrict the ACFM manipulator trials to a diver tank only, while taking only the LRUT manipulator trials through to full ROV deployment.
The ACFM manipulator trials were therefore to be conducted in the diver tank at TWI North. TWI North has a series of samples, originally developed for the ICON project. Each sample consists of a series of braces connected to a central chord. Each weld contains fatigue-like defects of varying sizes and depths. In order to test the node array probe, it was decided to compare it with one of TSC's standard ACFM probes. These probes have been used for decades throughout the oil and gas industry, and are known for the reliability and accuracy. The SUBCTEST node array probe in a single scan along the weld was found to detect all the defects, giving the same readings for length and depth as the standard weld probe doing multiple scans along the same weld. These trials were conducted dry, but as there are no moving parts on the probe, and the field is not affected by water, there is no reason why these results would be different if the trials had been conducted subsea.
For the underwater trials, the ACFM array probe and manipulator were tested TWI North's Diver training tankor the trials a 12 x 1 mm defect was cut into the toe of a node weld. The manipulator scanned the ACFM probe over the defect, before lowering into the tank, and the software used to successfully locate and size the defect. The defect was then increased in depth by a further 1 mm. This was also successfully detected and sized. The size of this defect is considerably smaller than the required defect size specified at the beginning of the project. A video was made of this operation.
The LRUT manipulator components were first assembled in the engineering hall at TWI on a mock-up of the Dacon ROV. This consisted of a tubular frame, designed to replicate the attachment points for the manipulator components.
Once assembled on the frame, a start could be made on installing the control software. On powering up the system, it was found that the power supply from the series of 12 V batteries was not capable of delivering an acceptable voltage and therefore caused the micro running the electronics pod unit to constantly reboot. Eventually, the power was stabilised, but then it caused an internal fault within the electronics pod that resulted in a test channel that dropped out to zero (flat line). The power supply was eventually replaced with a more stable one, but this and other problems with short circuiting in the hard drives and mechanical closing of the clamp led to delays in shipping the system out to Dacon for the field trails and a request for a project extension.
One major design problem that could not be resolved was a back pressure in the hydraulic system which prevented deflation of the bladder. For the purposes of the demonstration, a hand valve was added. In a future development, this problem would be rectified by separating the hydraulics powering the actuators from those filling the bladder.
The system was ultimately set up to take LRUT data from a short length of pipe, where the best data was collected at an ultrasound frequency of 36 kHz.
The whole LRUT manipulator, including the mock-up ROV frame and the test pipe were shipped over to Norway for trials at their works. Delays at customs when entering Norway, cut into deadlines. Consequently, the final trials could not be completed until the very end of the project.
At Dacon the manipulator and system modules were loaded onto their observation class ROV without any problems and a video made of its operation.
To collect data, the manipulator was clamped to the 12' pipe held vertically.The signals from the pipe ends were clearly detectable. A flat was ground into the pipe wall about 50 mm diameter and 3 mm deep and 30 mm from the pipe end. The pipe was retested and the signal from the machined flat noted.
For the immersion trials of the LRUT manipulator on the Dacon ROV, it was lowered into the tank and went through number of operations, which included swimming forward and clasping the pipe. A 10 min video, taken from the camera on board the ROV was made of these operations.
Although the trials were successful, a list of modifications needed to improve the design has been put forward, that are a reasonable basis for taking the manipulator through to the next technology readiness level (TRL6).
Unfortunately, adverse weather conditions prevented the dock-side trials taking place. Because of the long Norwegian winter, these cannot now take place until Spring 2011.
Potential impact:
The SUBCTEST project has raised considerable interest in the offshore oil industry. A press-release in TWI's own newsletter for its members in the oil and gas sector raised almost 50 enquiries in a week.
The trials of the LRUT manipulator on partner Dacon's ROV were successful but were limited to the diver tank because an early onset of winter prevented dock-side trials. However, an interest from Chevron may provide an opportunity to take the system for offshore trials in the Gulf of Mexico.
Although the trials of the ACFM manipulator were diver only, the NDT system has since been adapted for use from a work-class ROV.
SUBCTEST has therefore proved a viable platform for ROV deployable NDT systems that can be used to detect corrosion and cracks in tubular offshore structures including subsea flowlines, risers, the 'jacket' structures on which oil and gas platforms sit and chains used to moor floating offshore structures. SUBCTEST might also find application for inspecting offshore wind turbine structures.
The future of the SUBCTEST system may largely depend on the whether it can offer solutions to particular problems requiring ROV deployment of inspection devices. Crises occur from time to time in the oil industry where 'money becomes no object' and further development of system, for example to deploy functioning PAUT and the associated high costs of marinised cable connections are no longer a problem.
In terms of benefits, this project will benefit the SMEs but it will also increase safety and reliability of Europe's critical offshore industry. The United Kingdom (UK)'s HSE has recently issued reports severely critical of the structural integrity in the UK's offshore industry, citing ageing infrastructure and changing ownership from major oil companies to smaller 'independent' ones as reasons for concern. With SUBCTEST, operators will be able to deploy a range of NDT tools; LRUT to identify and locate defects over long distances followed by ACFM and potentially PAUT to properly assess and size those defects to determine their significance to continued fitness-for-service of the structure. This modular interchange ability from an ROV deployed system is extremely important as moves to reduce or even eliminate diver use progress.
Diver deployment is extremely hazardous and often the extreme pressures under which they work reduces the sensitivity of the test because proper analysis of test results in an 'office' environment, as is the case with ROV deployment is not possible. Often it is the case that defects when found cannot be remedied immediately and a monitoring regime needs to be instigated. Repeat inspections by an observation class ROV that can be deployed from a small boat or from the platform side will be much more cost effective than when using a work-class ROV with associated infrastructure, including a diving support vessel. Indeed, minor adaptations to the LRUT manipulator would allow it to be detached and left on the pipe for permanent installation with periodic visits by the ROV to collect data.
On completion of the project, the four SMEs, led by Dacon have come together to formulate a business plan to develop the SUBCTEST NDT platform. Dacon is building up its inspection capability offering bespoke solutions for underwater problems. GRI has become a part of Fugro, a major supplier offshore engineering with a fleet of ROVs and has shown an interest in taking the technology forward. I&T Nardoni see an avenue for exploiting the LRUT and ACFM techniques but in above ground applications such as processing plant, refineries and pipelines. Finally, Vermon wish to take the PAUT technique developed by the SUBCTEST project to a higher TRL in a future development project.
The business plan hinges on taking each of the SUBCTEST components to a higher TRL. The completed LRUT manipulator is still only a prototype at TRL 5 to 6. To take it to a fully functioning system that is can be offered commercially requires the building of a suitable engineered 'production' system, its proper validation and subsequent deployment in a fully operational environment offshore. The costs of offshore operations are extremely high and even if they can be met, operators will not tolerate any activities that interrupt oil and gas production. As mentioned earlier, the oil and gas industry is very 'reactive' and generally we must wait for a major incident requiring ROV deployment for an opportunity. The business plan therefore rests on developing a 'position' in the market, where SUBCTEST is known to offer a solution to an inspection problem. This is not an argument entirely accepted by the two large organisations in the project, the UK's HSE and Norway's PSA. Both these regulatory bodies see an immediate need for SUBCTEST deployment and will do their utmost to promote its further development.
However, the SMEs did agree to seek further funding to take SUBCTEST and its components to take them to higher TRLs. The most probable source would be the major oil companies, Shell in Houston and Chevron in California have already shown interest. There are also the main offshore contractors such as Fugro, Subsea7 and Technip. Fugro might seem the most promising, because they now own GRI but other fruitful discussions have been had with Technip, which currently has a concern in inspecting mooring chains on a North Sea floating production and oil storage facility.
The exploitable knowledge arising from the project is in the following areas:
- techniques and equipment for application of ACFM using array transducers for inspecting welds in complex geometries such as a welded node in an offshore jacket structure;
- marinisation of the ACFM equipment for deployment from a scanner sub-sea;
- techniques for application of PAUT for detecting surface breaking cracks on internal and external surfaces using so-called 'creep waves';
- techniques for application of PAUT for detecting surface breaking cracks on internal and external surfaces at test ranges up to a metre using so-called 'multi-skip' ultrasonics;
- techniques for application of LRUT for detecting root cracks and corrosion in the welds at each end of a pipe spool from one test location along the pipe spool;
- EMATs designs for use in LRUT transducers when testing pipe covered in highly attenuative coatings;
- manipulator for ACFM transducer for scans of node welds subsea;
- manipulator for LRUT transducers for deployment from a sub-sea observation class ROV for clamping to a pipe;
- data link and control software for LRUT manipulator.
The SMEs came together to formulate a road map to give a strategic vision beyond immediate plans for taking SUBCTEST inspection platform.
The 'whys' in the road map must include the business case for developing SUBCTEST. This is particularly important for SMEs, where cash flow is the over-riding factor. An attractive plan, taking SUBCTEST forward to an accepted future might be completely undermined by a shortage of cash and absence of running capital at almost anytime, when SMEs have to finance the development themselves. Collaboration with a resource-rich organisation is therefore essential.
For the 'whats', a hierarchy has been established with the SUBCTEST platform at the top, divided between the LRUT, ACFM and PAUT systems. This can be divided into further layers.
Each item in the hierarchy can be attributed a TRL. An item at one layer cannot exceed the item in the layer below with the lowest TRL. Discussions with the beneficiaries have attributed TRL levels to the exploitable knowledge resulting from SUBCTEST. The results of SUBCTEST fall around the middle, where applications engineering dominates and technology push must now become market pull. Other results from the SUBCTEST project have much lower TRLs. For example, the EMAT modelling is only at TRL 3.
Project website:
http:// subctest.com
Contact details:
Graham R. Edwards
Consultant
NDT Technology Group
TWI Ltd
Granta Park
Great Abington
Cambridge
CB21 6AL
UK
+44-012-23899050
Graham.edwards@twi.co.uk