20 - Offshore wind turbine foundations – analysis and design

Abstract

Foundations for offshore wind farms involve significant technical challenges, including design requirements to withstand the harsh marine environment, prolonged impact under large wave loading and wind turbulence. These foundations are subjected to a combination of axial loads, low-amplitude cyclic lateral loads, bending and torsional moments generated by the offshore wind turbine (OWT) structure and various environmental factors. Monopiles are the most widely adopted foundation choice for OWT structures worldwide, in terms of ease of installation, economy and logistics. For monopiles, the applied loads and moments must be resisted by earth pressures mobilized in the surrounding soil, with an adequate factor of safety provided. Compared with the axial loading case, the cyclic lateral loads are considered governing for serviceability requirements. This chapter presents details on the environmental loading, geometric dimensions and geotechnical design considerations for OWT foundations, with a focus on monopile fondations. Existing methods for modelling soil under cyclic loading are reviewed, with emphasis on strain accumulation models that take into consideration the monopile–soil interaction. Inherent limitations and shortcomings of these models for the analysis/design of OWT monopile foundations are discussed, along with recommendations for future research needs.