Analysis of offshore wind farms shows there is no lack of resource, with North, Irish and Baltic Sea wind farm capacity factors - the overall achieved output in relation to the project's plated capacity - rising to 50-60% in windy months and annual averages greater than 35% achievable. The main challenge is to ensure the availability of turbines in all operational wind speeds to take advantage of that resource. Offshore wind farms' availabilities achieved to date range from 90-98%. There is a clear improvement in availability following a wind farm commissioning but availability does decrease during periods of high resource. We know from onshore wind farm operations - where maintenance access is straightforward - that 75% of failures are minor and cause only 5% of downtime, whereas 25% of failures are major causing 95% of the downtime.
Offshore wind farms are primarily populated with turbines and technology developed from the onshore market. Access logistics mean that offshore downtimes are longer, exacerbated as we move further offshore, and the figures above suggest that minor failures offshore may cause extended downtimes. Yet, to ensure safety and limit logistics costs, maintenance also needs to be undertaken during periods of low resource and calm sea.
It is extended downtimes and lower availability that dominate the operational costs of installed offshore wind farms, and this area should receive more attention.
Large offshore wind farms are huge, remotely controlled assets with a high degree of monitoring that is largely designed to keep turbines within safe operating limits. So many offshore turbine interruptions may be for less-important alarms that could easily be reset on an onshore project, but that require logistically costly interventions when offshore. Better handling of minor failures can reduce extended offshore downtimes, as can better planning for major failures and coordination with appropriate weather conditions. This means making better predictive use of the monitoring data available from the plant.
First focus
Wind turbine failure analysis shows that the key areas to attend are: drive train pitch mechanisms, gearbox and power electronics; managing wind turbine monitoring alarms and signals to produce predictive information about these hotspots; accurate knowledge of upcoming weather conditions; then scheduling predictive maintenance based on this information.
Cost of energy from offshore wind farms can be slashed by focusing on unreliability hotspots, concentrated in pitch, drive train gearbox and power electronics, and by effective predictive maintenance of those parts planned during favourable weather to limit the logistic costs.
There is clear evidence from offshore wind farm data that operators and OEMs who focus on these issues are achieving higher wind farm capacity factors, greater availability and a better yield at lower logistic cost.
As the industry matures and we face more stringent return-on-capital demands, the current certification and health and safety-oriented wind farm operations approach needs to change to a combined certification, health and safety, and production-oriented approach.
Peter Tavner is an Emeritus Professor of Durham University and author of Offshore Wind Turbines, launched in September by the Institution of Engineering and Technology.
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