A critical review of serviceability limit state requirements for monopile foundations of offshore wind turbines

Objective/Scope: Offshore wind turbines (OWTs) are getting taller and heavier, and medium water depths are utilized in recent and future offshore wind farms. The increasing turbine size brings with it the increasing size of monopile foundations according to current design guidelines. The increasing diameter of monopiles increases the costs of the foundation, that is material, manufacturing, transportation and installation costs. In addition, it poses engineering challenges, as well as environmental problems due to the high levels of noise emitted by pile driving. The strictest requirement on the monopile diameter is imposed by Serviceability Limit State requirements, which include allowable maximum deflection, allowable maximum initial- And accumulated rotation at the pile head (mudline). These seem to derive from manufacturer requirements for the safe operation of wind turbines and from visual concerns, and may originate from onshore wind technology. However, no obvious reasons were found by the authors for the strict requirements in offshore wind technology considering the additional costs they add. Therefore, the objective of this paper is to critically review the SLS requirements of OWT monopile foundations and investigate the possible reasons for the strict requirements. In addition, in light of the much higher allowable tilt prescribed for floating OWTs, alternative scenarios are investigated with adjusted SLS requirements on pile head deflection and rotation. Methods, Procedures, Process: In order to confirm that the SLS requirements typically dominate design with the current guidelines, a simple framework assuming a thin walled pile is given to compare the monopile dimensions required for the Ultimate, Fatigue and Serviceability Limit States. Possible reasons for the strict requirements are investigated, and using the presented simple framework, monopile dimension requirements are recalculated for adjusted less strict SLS requirements. Results, Observations, and Conclusions: The SLS requirements are confirmed to give the highest dimension requirements for monopiles. Slightly higher allowable tilt and deflections already show reduced dimension requirements for monopiles. The investigation into reasons for strict requirements suggest that a critical review of these requirements for design codes is necessary and would be beneficial in terms of costs and applicability of monopile foundations for large turbines and medium waters. Novel/Additive Information: The investigation reveals possible opportunities to review design guidelines for offshore wind turbine foundations. Adjusting the design requirements in offshore environments with a cost-benefit approach in mind could save substantial costs in material, manufacturing, transportation and installation of monopiles as well as extend the applicability of this type of foundations.