In order to ensure the safety of the offshore wind turbines (OWTs), the fatigue damage assessment is a key factor at the design and maintenance stages. To evaluate the fatigue damage accurately, the coupled effects of wind-wave loads should be taken into account seriously. The purpose of this paper is to investigate the influences of the coupled wind-wave loads on the fatigue assessment. Both of a fixed tripod-type OWT and a floating OWT are chosen as the target structures. To study the coupled effects, 3 load cases are performed, which are wind load only; wave load only and combined wind and wave loads. The dynamic response under 3 loads cases are calculated respectively. The time histories of responses at failure-critical locations are also derived. The fatigue damages are assessed with the rain-flow counting method and Palmgren-Miner rule in time domain. Finally, by comparing the results of uncoupled and coupled load cases, the coupled effects of wind and wave loads on the fatigue damage are concluded.
Wind energy plays a very important role in renewable energy utilization. Due to limit of installation space for onshore wind turbines, offshore wind energy has a broad prospect of application. And some more advantages like the more stable and higher wind speed, less visual disturbance and noise promote the rapid development of offshore wind technology.
The support structure has been identified as a vital contribution to cost-effective installations (Seidel, 2007). It has been subjected to the environmental loads such as wind, wave, current, ice and so on during the long service years. As a result, the accumulated fatigue damages are unavoidable and have an important effect on the operation and maintenance of the offshore wind turbine (OWT) support structures. The assessment of fatigue damage is required to ensure that the structure will fulfill its intended function by industry standards in the design phase (DNV, 2013).
