Dynamic analysis of shared anchor loads for a prototype floating offshore wind farm

Abstract

Floating offshore wind turbines are subjected to hydrodynamic and aerodynamic loads that affect the design criteria of the anchor and mooring systems. An anchor system with a well-designed shared anchor (SA) can help reduce the number of anchors in a wind farm leading to reduced cost of installation. This study presents the results of dynamic simulations in OrcaFlex of an idling IEA 15MW reference turbine and VolturnUS-S reference platform in the North Sea environment in deep water. It considers a three-turbine floating offshore wind farm as well as a reference single wind turbine configuration. The premise of this study is to analyse the anchor loads on a SA in the global coordinate system and compare them to the single anchor loads. Moreover, three different incoming wind-wave directions were considered each parallel to one of the mooring lines.

The numerical modelling of the loads on the anchors was conducted by applying Morison's equation and potential flow theory for hydrodynamic loads and blade element momentum theory for aerodynamic loads. The results show that compared to the single anchors in the wind farm the peak load on the SA can be reduced by up to 46% in the x-direction and 17% in the y-direction. It also shows that there is an increase in vertical force on the SA which is 134% higher than the peak vertical load of the single anchors. The result also shows that the incoming wind-wave (w-w) direction greatly impacts the anchor loads. The x-directional load of the SA is most reduced compared to the single anchors when subdued to wind and wave loads from 240 degrees (deg), whereas the y-directional load is the most reduced compared to the single anchor loads under 180 deg w-w direction. This is due to the x-directional load being much higher under 180 deg and the y-directional load being higher at 240 deg w-w direction. This highlights the importance of considering different wind directions for anchor system design. This study contributes to an improved understanding of the load distributions of floating wind farms with single and shared anchors. The outcomes of this study can be used to assist design of SA systems.