Study on the Degradation of the p–y Response of Offshore Wind Monopiles Under Vortex-Induced Loading

Current-induced vortex shedding imposes repeated cross-flow loading on offshore wind monopiles, yet its effect on the lateral pile-soil response is not explicitly represented in conventional p-y formulations. This study used a validated three-dimensional ABAQUS model of a large diameter monopile in sand to evaluate static in-line drag, equivalent peak cross flow lift, and stabilized harmonic lift at representative elevations. Static and post-cyclic p-y curves were extracted after up to 104 cycles and compared with the American Petroleum Institute (API) sand relation. The in-line load governed the short-term displacement and bending-moment response, whereas repeated cross flow loading progressively reduced the mobilized soil resistance. After 104 cycles, the reduction reached approximately 6.0–12.5% at z = D/2 and 6.75–13.4% at z = D across the representative low and high load cases. The numerical curves also exhibited a lower initial soil reaction modulus than the API prediction within the investigated shallow zone. A modified p-y formulation was therefore developed by combining a nonlinear depth dependent reference modulus with a cycle dependent degradation coefficient. The formulation provides an engineering representation of VIV-related stiffness degradation for the investigated monopile sand system and loading range.