Consequences of introducing a cut-out wave height on a floating wind turbine concept

Abstract

This master's thesis has investigated the consequences of implementing a survival mode system on the oating wind turbine concept Tension-Leg-Buoy (TLB) which shuts down the turbine at the cut-out wave height Hs;cut-out. The purpose of this is to reduce loads on the turbine structure at extreme wave conditions. One possible system is to move the top mooring lines upwards to the root of the nacelle, which has the potential of reducing loads in both mooring lines and anchors. If this is achieved, the excess buoyancy can be reduced by making the oater smaller which again reduces the material cost of the turbine. The downside of such a system is that the some of the annual energy production from the wind turbine or wind farm will be lost since the turbine does not produce power in survival mode. This leads to a yearly income loss which in total must be lower than the cost reduction obtained from using less materials in the turbine components, in order for the system to reduce the total cost of energy delivered by the turbine. By using time series of meteorological data sets containing wind speed and signi cant wave height from measurement stations in the North and Norwegian Sea, the energy loss at di erent values of Hs;cut-out for all the sites has been found. If an energy loss of below 1% is accepted, the optimal cut-out wave height for the various sites obtains a value of between 8 and 9 metres (signi cant wave height). Scaling of the wave data has shown that Hs;cut-out could be lower for milder wave climates. Additionally, three di erent control strategies for the survival mode system have been developed. The simulations using these strategies indicate that a strategy combining measurements and wave forecasts has the best overall performance, but these results rely on relatively high accuracy in the forecasting of wave heights. Load simulations on the TLB have shown that there are still many challenges left to solve with the survival mode system using recon guration of mooring lines. There was no clear trend that the system is capable of reducing overall loads, which prevents any reduction in the mass of the oater. Nevertheless, the ndings regarding energy loss and control strategies, which has the been the main focus of this thesis, will anyhow be valid for any survival mode system. These results may be used further even though the current concept does not achieve the load reductions required to bring down the material cost of the turbine.