A Simplified Approach to Modeling Long-Term Monopile Serviceability with Finite Element Analysis

Abstract Monopile foundations for offshore wind turbines are subject to many cycles of large horizontal loads and overturning moments. This paper describes the foundation analysis of a wind turbine foundation typical of 3 to 5 MW units that have already been constructed in the southern North Sea and are being proposed offshore in the northeastern United States. The foundation consists of a 5.1 m diameter monopile driven 26 m below the seafloor into medium dense sand. The environmental loads were highly idealized, represented as a 6 MN vertical force and a 4 MN horizontal force applied 30 m above the seafloor. The effects of one cycle of monotonic loading and up to 107 cycles of long-term cyclic loading were computed. The analyses were performed using a widely available three-dimensional finite element program, incorporating an implicit method to model the soil modulus degradation and increased strain as a function of the number of loading cycles. The computations showed that the monopile displacements and rotations accumulate with additional cycles of loading, almost doubling after 107 cycles. This suggested that a monopile that rotates within tolerances after one cycle of load, approximately 0.25°, may accumulate unacceptably high deflections and rotations over its design life. The paper demonstrated the use of commercially available software as part of a method to model the response of a wind turbine monopile foundation to long-term lateral cyclic loading. Introduction A monopile is a large-diameter, open-ended pipe pile driven into the seafloor, connected to a wind turbine tower by means of a transition piece. The tower supports the turbine nacelle and the rotor. Monopiles must withstand the forces from wind, waves, ice, currents, and vessel impacts over the life of the turbine, which can be up to 50 years. The design of the monopiles must consider the response of the supporting soil to the long-term applied cyclic loadings. As such, an accurate method to calculate the lateral deflection and rotation of the pile due to cyclic loading, including the effects of changes in properties of the supporting soil, is needed. This study is concerned with the effects on offshore wind turbine foundations of high cycle, low amplitude loading that will induce small strains. It is assumed that the cyclic loading has low strain-amplitude, less than 10-3 (Whichtmann et al., 2010), with as many as 107 cycles over the lifetime of the offshore wind turbine (OWT), consistent with the observations by LeBlanc et al. (2010). Monopiles are generally installed in water depths of less than 30 m (Malhotra, 2010). At greater depths, they become excessively large and uneconomical. Critical elements of their design include the loadings and performance requirements or limit states. One of the serviceability limits conditions is that a monopile cannot exceed a permanent rotation typically of 0.5° at the mudline level over its design life (Malhotra, 2010).