The offshore access problem and turbine availability: probabilistic modelling of expected delays to repairs

Difficulty of access due to adverse sea-state can cause significant delays to operation and maintenance on offshore wind farms, resulting in lost energy production and increased cost of energy. This paper presents a method of expressing such delays in closed form, in terms of a probabilistic event tree and Weibull parameters of the ‘storm’ and ‘calm’ duration distributions. Coupled with turbine subsystem reliability statistics and threshold sea states for access, annual expected delays and turbine availability can be estimated using a simple spreadsheet, without lengthy Monte Carlo simulations, as well as their sensitivities to site, turbine and access parameters.