Non-Local Effects of Support Structure Diameter on Wave Induced Fatigue Loads of Monopile-based Offshore Wind Turbines

Abstract

One of the main cost drivers of an offshore wind power plant are the support structures of the wind turbines, it is therefore of primary importance to optimize their design. Among the support structures available, the concept most widely adopted is the monopile-based support structure, whose design is often fatigue-driven. Offshore structures need to withstand the wave loads, that play a major role among the cyclic loads that excite the support structure. The design of the support structure’s geometry is of primary importance, since it determines the structural vibrations.
Therefore, this thesis aims to understand how, varying the diameter of the support structure, affects the wave induced fatigue loads acting on a monopile-based offshore wind turbine. A FE model was developed to represent the structural motion, where the Euler-Bernoulli beam theory was adopted. The linear wave theory was used, and the wave loads were computed according to the Morison equation.
The wave induced fatigue loads were calculated in frequency domain, assuming a narrow-banded response spectrum. A case study was provided by Siemens Gamesa RE, and the wind turbine was assumed in parked mode.

Two assignments were derived, to tackle the research question. First, a sensitivity analysis was
performed, to study the non-local effects on the wave induced loads, due to varying the diameter of the support structure. Then, an analytical optimization was applied to a simplified structure, aiming to find the diameter that minimizes the mass of the support structure, accounting for fatigue damage. The hypotheses of thin wall and deep water regime were assumed.

The results of the sensitivity analysis suggested that the non-local effects do not differ significantly from the local ones, and that to reduce the loads: it is beneficial to reduce the diameter at waterline, to increase it around mudline, while variations along the tower are quite irrelevant to this end. The analytical optimization was run for different load cases. Wave induced fatigue loads alone were first considered,
then a diameter-independent fatigue load was introduced. It was concluded that, accounting for resonant waves only, the smaller the diameter of the support structure, the better.