Accessing offshore wind turbines for maintenance: calculating access probabilities, expected delays and the associated costs using a probabilistic approach

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There are ambitious plans in place for the expansion of offshore wind-power capacity in the EU and elsewhere. However, the cost of energy from offshore wind is much higher than that from land-based generation and anything between 15% and 30% of this cost is attributable to the cost of operation and maintenance (O&M). For exposed UK round three sites these costs could be higher still. The stochastic nature of the occurrence of faults, down-times due to adverse weather and sea-state and the associated losses in energy production, as well as vessel and personnel costs, all add to the potential risk to the finance of an offshore wind farm project. There is a clear need to estimate these effects and the risks associated with them when planning and financing a wind-farm. Key to all such calculations are the restrictions on safe access for maintenance associated with vessels and access methods and the consequent delays caused by adverse sea-state and weather. A computational approach has been developed at University of Strathclyde, based on an event tree and closed-form probabilistic calculations, enabling very fast estimates to be made of offshore access probabilities and expected delays using a simple spreadsheet. Examples are presented for calculations of accessibility. Turbine availability and loss of energy production are calculated based on given turbine component reliability data together with an agreed maintenance scheme. Direct maintenance cost and revenue lost due to down-time can also be calculated with suitable data on the costs of personnel, components, and vessel hire as well as electricity unit and ROC prices, and examples are given. Sensitivities to some of the key parameters are also presented.
Original languageEnglish
Number of pages12
Publication statusPublished - May 2012
Event42nd ESReDA Seminar on Risk and Reliability for Wind Energy and other Renewable Sources - Glasgow, United Kingdom
Duration: 14 May 201216 May 2012


Conference42nd ESReDA Seminar on Risk and Reliability for Wind Energy and other Renewable Sources
Country/TerritoryUnited Kingdom


  • offshore maintenance
  • access delay
  • sea-state
  • probability
  • wind turbines
  • access probabilities

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