Tubular Pile Pull-out Testing program (TPPT) Completed

WHY

A floating offshore wind project may include several hundred anchor points, which may be driven piles. The wind turbine and environment cause both static and dynamic loads on the anchor points. The current best practices for driven anchor piles were developed in the 1980s and applied to permanently moored offshore oil and gas systems such as CALM buoys and manned FPSOs. However, load cases that include predominantly cyclic loads in full tension are not considered by the existing anchor pile design guidelines. Anchor pile design parameters are currently based on conservative guidelines. The soil strength parameter is based on the interpretation of the strength of each soil layer. New tests may prove the soil strength parameters for the pile pull-out capacity and show a lower effect of cyclic degradation, which will improve the static and cyclic pull-out loads. Correlating the collected measurement results with the pile pull-out capacity predictions is valuable for underpinning design practices for offshore renewable foundations.

The tests are particularly valuable for the design of future foundation anchor points for floating offshore wind. It will provide a better understanding of the degradation of the soil-pile interface under cyclic tensile loads and the correlation of this degradation with the results of the Unified Cone Penetration Tests (CPT) results. This will enable application of existing industry design methodology, such as the Unified CPT method, to the offshore renewable foundation concepts with an understanding of what factors need to be applied on static and cyclic capacity.

WHAT

We are continuing a programme for pile load compression tests on four 1.22m tubular piles, which the Port of Rotterdam carried out at the end of 2022. We will perform static and cyclic tensile tests using a novel fibre optical measurement system that can measure the water drainage in the pile-soil interface. These measurement results can be added to the CPT results to determine the actual soil strength in each soil layer during in-situ geotechnical research campaigns.

The proposed load regimes in the tensile test programme are based on the results from typical floating offshore wind foundation analyses, considering specified load and material factors. The tests can be performed with a closure of the pile top. This closure is expected to significantly impact the pile capacity under tensile loads, as the soil plug in the pile is likely to behave like a gravity weight.

The scope of the TPPT test program includes:

• Perform static pull tests at 20-30% of the ultimate load, both with open and closed pile tops.
• Perform dynamic pull tests at 20-25% of the ultimate load with 2000 load cycles, both with open and closed pile tops.
• Comparing the results with the results of the compression and tension tests in the literature.
• To investigate the impact of internal soil plug weight on the tensile holding capacity.
• Providing accurate data results to the research and academic geotechnical community.

EXPECTED RESULTS

This research will provide fundamental knowledge about the soil-pile interface under static and cyclic tensile loads. The test results will be input to certification bodies, like Lloyd’s Register, Bureau Veritas, ABS and DNV, which determine the guidelines for pile designs. A better understanding of the actual pile behaviour under these conditions can result in a reduction in safety factors. The test results are expected to have a major impact on the design guidelines and be able to significantly reduce the anchor pile size by an expected 20%. This could lead to significant cost reductions and reduction of CO2 footprint for offshore floating wind projects.