Foundation models for the dynamic response of offshore wind turbines

SYNOPSIS Due to the variety of excitation frequencies and the larger influence of the foundation on the wind turbine response, modelling of the dynamic behaviour of the foundation becomes a more pronounced issue for offshore wind turbines. This paper investigates the sensitivity of the support structure's natural frequency to variation in models for pile foundations of a monotower, tripod and lattice tower. The use of a stiffness matrix at mudline provides a significant reduction of complexity and results in acceptable loss of accuracy. Uncoupled springs and an effective fixity depth model are discouraged. For the tripod and lattice structure of this study lateral flexibility of the foundation is more important than axial flexibility. An important aspect of extreme- and fatigue loading of the support structure of an offshore wind energy converter (OWEC) is its dynamic response. Its dynamic behaviour differs in some important aspects from that of platforms for the offshore oil industry and of onshore wind energy converters. The natural frequency of an OWEC is wedged between different excitation frequencies, whereas the natural frequency of a platform for the offshore oil industry is usually designed to be above all main excitation frequencies. The geometry and dimensions of offshore foundations differ from typical onshore solutions, resulting particularly in a larger influence of foundation stiffness. Due to the variety of excitation frequencies and the larger influence of the foundation on the wind turbine response, modelling of the dynamic behaviour of the foundation becomes a more pronounced issue for offshore wind turbines. This paper investigates the sensitivity of the support structure's natural frequency to variation in models for pile foundations. The assessment is performed for a monotower, a tripod and a lattice tower. These support structures are designed for North Sea conditions with water depths between 15 and 25 m and for a 3 MW turbine. The monotower and lattice tower are based on the design solutions of the Opti-OWECS project (4). The tripod is designed by Heerema and was used in a study of the simulation of offshore wind turbines under stochastic loading (5). The main dimensions of the support structures are given in Table 1.