Cyclic Loading of Offshore Wind Turbine Suction Bucket Foundations in Sand: The Importance of Loading Frequency-Paper

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The study investigates how cyclic loading frequency affects suction-bucket foundations for offshore wind turbines in sand, particularly when windward buckets experience tension. Such loading can cause permanent foundation movement and turbine tilt.<br /><br />Centrifuge tests at 100g examined a model bucket in dense sand under realistic, irregular vertical load histories. Researchers varied loading frequency, cyclic load amplitude, and average vertical stress, while measuring displacement, stiffness, and pore pressure. Monotonic tests also assessed the influence of water depth and cavitation on pullout resistance.<br /><br />The results show that foundation performance depends strongly on drainage conditions. Slow loading can allow drainage and lead to cumulative settlement or uplift; very fast loading can generate excess pore pressure, reducing stiffness and increasing settlement. Under the tested conditions, an intermediate frequency—around 0.6 Hz—often produced more favorable behavior, although the effects varied with load amplitude and average stress.<br /><br />Notably, under favorable drainage conditions and with sufficient resistance to cavitation, buckets sustained substantial two-way cyclic tension around zero average load with little permanent uplift. Some tests imposed tensile loads several times greater than the drained pullout capacity without causing large displacement.<br /><br />The findings suggest that accounting for loading frequency and pore-pressure dissipation could improve the design of suction-bucket foundations in sandy soils. Further analysis is needed to confirm the proposed frequency-dependent mechanisms and translate them into design methods.

Keywords

suction-bucket foundations

offshore wind turbines

cyclic loading frequency

sand

centrifuge tests

pore pressure

pullout resistance

foundation settlement

drainage conditions

cavitation