Centrifuge modeling of the cyclic lateral behavior of large-diameter monopiles in soft clay: Effects of episodic cycling and reconsolidation

Large-diameter monopiles supporting offshore wind turbines (OWTs) are subjected to intermittent episodes of cycling and reconsolidation during the lifetime. The lateral soil-pile stiffness is likely to degrade during the cycling but tends to recover during the subsequent reconsolidation. The former effect has been widely acknowledged in monopile design, while the latter is less commonly recognized. This study aims to (a) investigate the effects of episodic cycling and reconsolidation on the evolution of cumulative displacement, stiffness and bending moment of large-diameter monopiles in soft clay, and to (b) analyze their consequences to the structural safety of the OWT (i.e., evolving natural frequency f(N) and thus likelihood of resonance). A series of centrifuge tests were performed to simulate laterally loaded monopiles subjected to multi-stage episodic cycling and reconsolidation. The experimental results show that the lateral soil-pile stiffness degrades during the initial cycling episode, but it entirely recovers during the subsequent reconsolidation to exceed the initial stiffness, with a percentage increase up to 50%. Consequently, the cumulative lateral pile-head displacement and maximum bending moment induced by the 3rd episode of cycling can be 63% and 15% smaller than that due to the 1st episode, respectively. The effects of episodic cycling and reconsolidation on the lateral stiffness, displacement and bending moment of the pile become more pronounced as the cyclic amplitude increases. Despite these observed beneficial effects, the stiffer lateral pile response after the reconsolidation would have increased fN of a typical 6 MW turbine founded on the monopile by up to 13% (i.e., from 0.206 to 0.232 Hz), forcing it to unfavorably approach the 3P frequency limit that could trigger resonance.