Offshore wind turbines are located in harsh environments with cyclic loads from wind, waves and current. These cyclic loads are transferred to the soil through the monopile foundation. As a result, pore pressure accumulation and degradation of the strength and stiffness may occur. This paper presents a consistent, rigorous and super-fast monopile design approach, which allows for timeefficient design. It is based on the NGI approach for cyclic loading in combination with a time-efficient 3D Finite Element Analysis (FEA) and spring calibration approach. The FEA program is used to calculate: the permanent rotation of the monopile at the end of the lifetime (Serviceability Limit State – SLS), the capacity under maximum loads (Ultimate Limit State – ULS) and the pile response during operation (Fatigue Limit State – FLS). The optimal monopile geometry that fulfills the design limit states is then determined and used to calibrate soil reaction curves. This paper presents the approach used in the different limit states, the time-efficient 3D FEA program, the cyclic accumulation, and the optimization methodology to calibrate soil-reaction springs. Finally, the main assumptions and simplifications in the design approach are discussed.
Rasmus T. Klinkvort, Hendrik Sturm, Ana M. Page, Youhu Zhang and Hans Petter Jostad
Keywords: offshore wind, monopile, design methodology, scripting
Publisher: DFI