Advanced technology for monitoring storage tanks
Leakage from corroded storage tanks, especially their floors, is a significant source of environmental contamination. Current methods of inspecting and repairing ASTs are inadequate mainly because they are costly, time-consuming, and also involve the emptying and cleaning of the tank prior to inspection, posing a threat to worker safety. Additional limitations include that inspection scheduling is done on an equal time basis, neglecting the fact that the degradation rates vary across different tanks. Tanks that require the highest priority may therefore not be examined or rectified first. To overcome the above drawbacks of the current inspection methods, the EU-funded project SafeAST has developed novel long-range ultrasonic testing techniques for monitoring corrosion in storage tank floors. Reliable non-destructive evaluation technique Detecting corrosion in AST floors is critical, especially for tank operators in the oil and gas industry. The project consortium came up with a new concept that involves the use of long-range ultrasonic waves for improved monitoring of corrosion on selected in-service oil and gas storage tanks. “Due to their ability to propagate long distances, guided waves are used to monitor the entire tank floor area,” points out project manager Kamer Tuncbilek. The novel solution utilises an array of ultrasonic transducers that are mounted to the chime plate outside the tank wall to periodically collect data and compare it to baseline signals. The purpose of the guided wave method of monitoring is not to supplant the current regime, but to provide additional information about the condition of the tank floor in between internal inspections. Important details that are relevant to tank integrity maintenance include forewarning of degradation of regions of the floor that may require action to be taken before the next scheduled outage and a priority list as to which tanks are in most need of attention. A prediction of the extent of degradation enabling planning of inspection and repair activity is also available. In the event of no degradation detected, justification may be provided for extension of the interval between internal inspections. “The ultrasonic transducers are excited by the pulser-receiver electronics around the perimeter of the tank floor,” says Tuncbilek. As she explains, the sensors allow the transmission of ultrasonic signals from one side to the other, covering tank floors measuring up to 30 m in diameter. Data is then used to generate tomographic images of anomalies, which could be related to the thickness loss in the tank floor. Any change in the condition of the tank floor – for example a patch of corrosion – is reflected in the signals and compared to the baseline signal. Baseline noise – short-term signal variations due to slight differences in the tank temperature and fluid levels – is separated from changes in the physical condition of the floor. The system has been installed in two operational tanks in Europe and the capability of the technique has been demonstrated at two test sites in the United Kingdom. The demonstrations involved tank floors measuring 4 m and 8.5 m in diameter and the use of 48 equally spaced sensors around their circumferences. Project partners succeeded in detecting a circular defect of 50 mm in diameter as well as multiple clearly discerned defects of 70 mm in diameter. The proposed SafeAST solution is expected to contribute to improved detection of defects in ASTs, increased worker safety and lower inspection costs. The current regulation demands a full inspection of a tank every 10 years. However, especially for older tanks, their condition has to be monitored a lot more frequently. As Tuncbilek also points out, “Saving money without compromising quality and safety is the biggest goal of the oil and gas industry. The aim was to develop a method for monitoring storage tanks that will largely reduce contamination and disruption to the service by providing evidence for justification of extension of the interval between internal inspections if no degradation is detected.”
Keywords
SafeAST, storage tank, tank floor, corrosion, ultrasonic testing, ultrasonic transducers, atmospheric storage tank