The Effect of Post-Storm Reconsolidation on the Response of Monopile-OWT Systems in Clay

Abstract Geotechnical processes related to offshore monopiles for supporting OWTs in soft clay involve intermittent episodic remoulding during storm events and reconsolidation during calm conditions. To assess the resulting response of monopiles, this study employs a three-dimensional finite element analysis. The methodology incorporates a kinematic hardening soil constitutive model within the critical state framework, calibrated against cyclic triaxial experiments and validated against the centrifuge test results on a 6m diameter monopile in soft clay subjected to multiple episodes of cyclic loading and reconsolidation. Thereafter, we investigate the performance of a reference 15MW OWT supported on a 10m diameter monopile in clay, subjected to three ten-minute storms of combined wind and wave loading representative of a 10-year return period, with reconsolidation periods between each storm. The results show a reduction in effective stress around the monopile during storms followed by recovery during reconsolidation, leading to a reduction in the level of monopile displacement accumulated in each subsequent storm event. This beneficial effect is overlooked in design practice, but can be anticipated through the approach presented here, which can form the basis for the validation of a digital twin that monitors the lifetime integrity of a monopile foundation.