Periodic Reporting for period 2 - SEAFLOWER (Strategies for the Exploitation of Anchors for FLoating Offshore Wind Energy Reaping)
Période du rapport: 2023-03-15 au 2024-03-14
In recent decades, Europe has seen the development of offshore wind farms supported by monopiles, steel piles suitable only for shallow waters. However, the advancement of floating technology offers a solution for harnessing wind resources in deeper waters, expanding the potential of offshore wind energy beyond traditional bottom-fixed solutions. Floating wind farms hold particular promise for regions like the Mediterranean with narrow continental shelves, where wind resources are abundant but inaccessible to conventional installations.
SEAFLOWER contributes to the discussion on Europe's future energy supply by focusing on the geotechnical aspects of floating offshore wind turbines. Anchoring systems, which secure floating structures to the seabed, constitute a significant portion of the capital expenditure for large-scale wind farms, up to 30%. Optimizing anchoring solutions could lead to substantial cost savings. While various anchoring methods have been tested successfully to moor pre-commercial wind turbines, not all are suitable for real wind farms spanning wide areas with multiple turbines.
The objective of SEAFLOWER is to analyse the behaviour of different anchor foundations through a numerical investigation. The procedure make use of metamodelling techniques to emulate the response of finite element models able to represent the anchors’ performances. A metamodel is a surrogate numerical model that is able to encode the response of a complex and computational expensive FE model. Once built, the metamodel can be easily exploited for the design. The procedure developed by SEAFLOWER particularly suits cost analyses and pilot design activities. This would contribute towards overcoming the actual technical barriers to the deployment of offshore wind in deep waters.
Driven pile anchoring solutions are pertinent to Tension Leg Platform (TLP) floating wind turbines, where the platform is tethered to the seabed by vertical tendons. The study analysed the pile’s ultimate capacity under pure pull-out load using a finite element solution that incorporated the installation effects simplistically. As for the metamodelling study, the Polynomial Chaos Expansion (PCE) method was employed to summarise the load-displacement response of the axially-loaded pile. The developed PCE showed able to replicate with good approximation the observed behaviour of some experimental evidences from literature.
For plate anchors, known for their efficiency and ease of installation, deterministic solutions assume homogeneous soil properties, overlooking variations across the deposit. This is significant for floating wind turbine projects covering large spatial extents. The study employed the Polynomial Chaos Expansion (PCE) to assess a plate anchor's holding capacity in randomly variable soil under general loading conditions.
The first investigation revealed uncertainties in quantifying the installation's impact on pile performance, prompting small-scale experiments at the Centre for Offshore Foundation Systems (COFS). Tests examined installation methods' influence on uplift capacity, especially under cyclic loading. Some tests also explored noise reduction during pile driving by applying suction to decrease effective stress at the skirt tip level. Preliminary results suggest this approach merits further investigation.
As floating wind developments are in the pre-commercial phase, simplified tool –like the metamodel- would aid the design stage, particularly during the preliminary design state of advancements where the designer has to face with limited details for using more sophisticated numerical models.
The interest on using these artificial intelligence tools in the offshore geotechnical industry was highlighted by the recent OSIG international conference organised by The Society for Underwater Technology at the Imperial College in London, where a keynote lecture was specifically dedicated to the topic.
After SEAFLOWER, some national and international collaborations were established with other researchers, with the scope of applying the developed methods on other case studies. The first results of the project were presented to a broader audience at the Researcher’s night held in Bologna (Italy) in 2022, while the final outcomes will be proposed at the upcoming event of September 2024.