Lifetime extension of offshore wind farms

Abstract: Renewable energy sources are in strong demand as the need for clean energy is increasing, driven by the need to reach the environmental targets. Offshore wind power is becoming an attractive source as the technology matures and cost decreases. With time all wind farms age and when the time comes one needs to consider end of life alternatives. The objective with this Thesis is to assess lifetime extension of offshore wind farms, with a focus on reliability, availability and maintenance. As of today there are few offshore wind farms reaching their end of life but one needs to be prepared. From the literature review it is clear that there is somewhat limited documentation around lifetime extensions. The reason for lifetime extension is to increase the return on investment (ROI) of the original project, with a limited additional investment. With capital expenditure (CAPEX) being a large part of the overall project cost it is beneficial to achieve more operational years to divide the cost over, one also need to consider increased operations and maintenance (O&M) cost with aging assets. In this Thesis, a Reliability block diagram (RBD) model was developed in ReliaSoft BlockSim to calculate wind turbine failures, their associated downtime and O&M costs to create an overview of the future behaviour and cost. To assess the wind farm economics a financial model was built, based on output from the RBD model, to evaluate the levelized cost of energy (LCOE) and internal rate of return (IRR) for several cases of lifetime extension. A case study was done for a generic wind farm of 30 wind turbines, assumed to be built around 2010 using 3.6MW rated turbines, to illustrate a relevant case. Focus was on O&M cost, and it was assumed the structural integrity of the wind turbines would be maintained for the lifetime extension for up to 10 years.