Insights from in-situ pore pressure monitoring around a wind turbine monopile

Offshore wind turbine monopiles transfer large bending moments into the soil. Due to the large diameter of the foundation, a significant volume of soil is mobilised to absorb the incoming forces from wind, waves and current. If excess pore pressures are generated during load cycles, the long drainage path length could lead to accumulation of pore pressures and loss of strength and stiffness in the soil. During monopile design, undrained behaviour of the soil during cyclic loading is often assumed to conservatively assess the lateral pile resistance during storms. In-situ monitoring of pore pressures in the soil surrounding a monopile foundation allows the assumption of undrained behaviour to be checked for several episodes during the operational life of the foundation. A 3-year period of monitoring data was analysed for a North Sea monopile which included normal operation, storm episodes and rotor stops. Pore pressure and earth pressure sensors were positioned on the pile at two different depths in the most intensely loaded layer of surface sand. The data provides insight into the amount of excess pore pressure accumulation with storm loading not leading to significant accumulation of excess pore pressure. The only episodes showing a significant development of excess pore pressures were rotor stops with suction pressures being developed on the unloaded side of the monopile. The observed response suggests that significant pile-soil stiffness reductions in the top layer due to cyclic loading are not expected for the monitored location. The study shows the value of monitoring pore pressures in the soil mass surrounding offshore foundations for checking design assumptions.