A simplified method for estimating the permanent accumulated rotation of an offshore wind turbine monopile throughout its design life

Abstract

The permanent accumulated rotation is of great importance to the design of monopile foundations for offshore wind turbines because it is critical to the final dimensions of the monopile as well as its cost. Although design specifications require that the permanent accumulated rotation meet tolerances, they do not provide engineers with an appropriate method to calculate this value. This paper proposes a simplified method for estimating the permanent accumulated rotation of a monopile throughout its design life. To establish this method, a series of 1-g model tests were conducted in medium-dense sand to investigate the accumulative tendency of monopile rotation under typhoon and non-typhoon conditions. The results showed that the total accumulated rotation was mainly caused by typhoon events, further indicating that the static rotation generated by the maximum load magnitude among typhoon load sequences may maintain a certain proportional relationship with it. Then, the procedure for determining the two parameters in the method was illustrated by an NREL 5 MW wind turbine mounted on a monopile at a water depth of 28 m in the northern South China Sea. This method can provide a convenient tool for calculating the permanent rotation of a monopile foundation throughout its design life.