NDT inspection of offshore wind turbine jacket foundations

Offshore wind represents a key renewable energy source for meeting the UK’s carbon neutrality target and for helping to tackle climate change. The pace of development for new wind farms is accelerating, with deeper water sites being increasingly sought to unlock the full market potential of European waters.

For these deeper water installations, foundation structures are moving away from traditional monopiles towards jacket-type lattices, with designs adapted from the oil and gas industry. These structures contain many more welded joints than monopiles, with over 1km of total weld length per jacket. The quality of the welds is critical to ensuring the long-term integrity of the structure, particularly given its harsh, underwater service environment with complex fatigue loading conditions.

A leading energy supplier and TWI member company approached TWI to assess the weld quality of their offshore wind turbine jacket structures. At this point, the jackets were fully welded and assembled, located onshore and awaiting installation. Any indications identified in the welds would be evaluated for repair and also used to inform an engineering critical assessment (ECA) to evaluate the long-term integrity of the structures.

Objectives

To develop a suitable non-destructive testing (NDT) technique for inspecting the jacket welds
To inspect the welds using a developed procedure, specifically designed for detecting indications within the jacket foundations
To re-inspect any significant indications and provide precise characterisation and sizing of these defects
To evaluate any identified indications for repair and to inform an engineering critical assessment (ECA)

Phased-array ultrasonic testing (PAUT) procedure development

Solution

TWI developed a phased array ultrasonic testing (PAUT) procedure to enable volumetric inspection of the welds. Given the welds’ complex design, with asymmetric “TKY” type joints and a variable weld preparation, careful consideration of the inspection methods was needed. Several of TWI’s Level III NDT experts helped develop the technique to ensure that coverage was sufficient for all joint configurations and accurate indication sizing was achievable.

Ten personnel were deployed on-site for eight weeks, conducting the NDT at height. The inspection was carried out in two stages, the first for detection. Then, for indications requiring further evaluation, different focussing methods were used for more accurate characterisation.

TWI reported the inspections in the client’s required format and provided further contextual information and decision support when requested. This decision support was able to draw on TWI’s other technology services to provide insight into structural integrity queries. TWI’s efficient turnaround of inspection reports was crucial to enabling the client to keep to their project timeline.

Conclusion

TWI provided valuable information on the weld quality of the wind turbine jacket foundations, prior to their installation. The ability to provide accurate sizing data of particular indications has allowed the ECA of the structures to move forward with confidence. This, in turn, will help the asset owner predict and manage the long term integrity of these structures over the coming decades.

Jack Lambert Senior Project Leader - NDT

Jack Lambert works in the R&D team within the Non-Destructive Testing section at TWI Cambridge. He specialises in ultrasonic testing (UT) and Phased Array UT, undertaking collaborative research, consultancy and site inspection projects. Jack also manages the operation of the 7-axis immersion tank, which is used for UT of varied large engineering components. Prior to joining TWI in 2017, he worked as a research scientist at the University of California, San Francisco, optimising medical Computed Tomography (CT) with the goal of reducing patient radiation dose. Before this Jack studied Aerospace Engineering (MEng 2008) and Materials Engineering (PhD 2012) at the University of Southampton. There, his industry-sponsored research explored fatigue damage mechanisms in wind turbine blade composites using micro-CT.

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