A Critical Review of Serviceability Limit State Requirements for Monopile Foundations of Offshore Wind Turbines

摘要

Objective/Scope: Offshore wind turbines (OWTs) are getting taller and heavier, and medium waterrndepths are utilized in recent and future offshore wind farms. The increasing turbine size brings with it thernincreasing size of monopile foundations according to current design guidelines. The increasing diameterrnof monopiles increases the costs of the foundation, that is material, manufacturing, transportation andrninstallation costs. In addition, it poses engineering challenges, as well as environmental problems due tornthe high levels of noise emitted by pile driving. The strictest requirement on the monopile diameter isrnimposed by Serviceability Limit State requirements, which include allowable maximum deflection,rnallowable maximum initial- and accumulated rotation at the pile head (mudline). These seem to derivernfrom manufacturer requirements for the safe operation of wind turbines and from visual concerns, andrnmay originate from onshore wind technology. However, no obvious reasons were found by the authors forrnthe strict requirements in offshore wind technology considering the additional costs they add. Therefore,rnthe objective of this paper is to critically review the SLS requirements of OWT monopile foundations andrninvestigate the possible reasons for the strict requirements. In addition, in light of the much higherrnallowable tilt prescribed for floating OWTs, alternative scenarios are investigated with adjusted SLSrnrequirements on pile head deflection and rotation.rnMethods, Procedures, Process: In order to confirm that the SLS requirements typically dominaterndesign with the current guidelines, a simple framework assuming a thin walled pile is given to comparernthe monopile dimensions required for the Ultimate, Fatigue and Serviceability Limit States. Possiblernreasons for the strict requirements are investigated, and using the presented simple framework, monopilerndimension requirements are recalculated for adjusted less strict SLS requirements.rnResults, Observations, and Conclusions: The SLS requirements are confirmed to give the highestrndimension requirements for monopiles. Slightly higher allowable tilt and deflections already showrnreduced dimension requirements for monopiles. The investigation into reasons for strict requirementsrnsuggest that a critical review of these requirements for design codes is necessary and would be beneficialrnin terms of costs and applicability of monopile foundations for large turbines and medium waters.rnNovel/Additive Information: The investigation reveals possible opportunities to review design guidelinesrnfor offshore wind turbine foundations. Adjusting the design requirements in offshore environmentsrnwith a cost-benefit approach in mind could save substantial costs in material, manufacturing, transportationrnand installation of monopiles as well as extend the applicability of this type of foundations.