vol 7, issue 10

Offshore wind - laying the foundations

22 October 2012
By Peter Tavner
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An offshore wind farm as viewed from a boat

The UK needs to develop a robust strategy for its offshore wind energy capture

Constructing an offshore wind turbine

Maintaining offshore turbines takes time, but availability will be improved by better handling of minor and major failures

The UK must begin planning an offshore wind strategy that ensures reliability, availability and longevity, says the author of the IET's new book 'Offshore Wind Turbines'.

The development of offshore wind power has become a pressing modern energy issue and the UK is taking a major role, with a significant deployment of turbines in UK waters. Two rounds of UK deployment have resulted in about 1,800MW installed capacity.

Round three is just beginning, with the first site in the Moray Firth, where a 1,500MW offshore wind power station is to be developed by Moray Offshore Renewables. This entered the planning phase in June of this year.

There are major issues surrounding the deployment of offshore wind power to realise a competitive cost of energy per megawatt hour against other sources, particularly fossil-fired, and these depend upon the turbine installation costs and their reliability, availability and longevity.

The Department of Energy and Climate Change recently issued its 'Offshore wind cost-reduction task-force report', which rightly focuses on the capital expenditure (CAPEX) issues of contractual, planning, supply-chain and finance issues necessary to reduce the cost of electricity from offshore wind.

However, there is another vital issue, the through-life operational expenditure (OPEX) of the offshore wind power stations rather than the CAPEX. The OPEX concerns the way we operate and maintain these costly assets, once built, which must be addressed by engineers.

Analysis of offshore wind farms shows that there is no lack of resource, with European offshore wind farm capacity factors rising to 50-60 per cent in windy months and achievable annual averages greater than 35 per cent.

Resource availability

The main challenge is ensuring the availability of turbines in all operational wind speeds to enable us to take advantage of that resource. Well-operated onshore wind farms regularly achieve 98 per cent availability, but offshore we are achieving only 90-95 per cent to date.

There is clear availability improvement in the first two years following commissioning of UK offshore wind farms, but availability falls during periods of high resource. In the winter availability sometimes falls below 90 per cent, particularly at higher wind speeds up to turbine cut-out at ~25m/s.

We know from onshore wind farm operations that 75 per cent of failures are minor and cause only 5 per cent of downtime, whereas 25 per cent of failures are major causing 95 per cent of the downtime, in an environment where maintenance access was straightforward. These percentages alter offshore as minor faults make a larger contribution to wind-farm downtime, because of the difficulties of access during windy, high sea-state conditions.

Large offshore wind-power station technology has similarities to conventional generation, offshore oil and gas and marine operations, but consists of unmanned, robotic power units operating 24/7, controlled from remote onshore control rooms at low manning levels. To keep these wind-power stations at operational availability requires innovation, excellent quality control and effective management.

"For a long time the mantra of the wind turbine industry has been bigger and bigger but now it has moved to better and better and this change marks a change in the areas of innovation," Andrew Garrad, the co-founder of the UK wind consultancy now called GL Garrad Hassan, explains.

To ensure safety and limit logistic costs offshore maintenance is undertaken during periods of low resource and calm sea, generally in the summer months or as the opportunity arises. 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 suggest that minor failures offshore are causing extended downtimes, such as only experienced onshore for major failures, such as gearbox or generator changes.

"The classical principle of wind-turbine control and monitoring is to ensure that the wind turbine is always in a safe state; this is not automatically the same as ensuring that the operating time is maximised," Henrik Stiesdal, the CTO of Siemens Wind Power, says. It is these control issues which are contributing to the deterioration of availability in offshore wind farms.

As the wind industry matures and we face more stringent demands on return on capital, the current certification- and health and safety (H&S)-oriented wind-farm operations approach needs to change to a combined certification - a combination of H&S and production-oriented approach, the latter being achieved by the steps described below.

Extended offshore downtimes will be reduced by better handling of minor and major failures, better planning for major failures and coordination with appropriate weather conditions and better predictive use of the monitoring data available from the plant. Wind turbine failure analysis shows that the key areas needing attention are drive-train pitch mechanisms, gearbox and power electronics; management of wind-turbine monitoring alarms and signals to produce predictive information about those hot-spots; accurate knowledge of upcoming weather conditions; and scheduling predictive maintenance based upon combining this information.

In addition offshore wind-farm costs of energy can be slashed by focusing on turbine unreliability hot-spots, concentrated in the pitch, drive-train gearbox and power electronics. Also by effective predictive maintenance of those parts planned during favourable weather to limit the logistic costs.

There is clear evidence from current 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.

The keys are information, prediction and planning. This is an experience well understood by North Sea oil-gas operators and needs to be learnt by engineers in the offshore wind industry.

Professor Peter Tavner is an Emeritus Professor of Durham University and President of the European Academy of Wind Energy. Formerly with operational experience in the UK generating industry and for the last 10 years working on the improvement of reliability and availability of wind power assets aimed at lowering the cost of energy. Professor Tavner is the author of 'Offshore Wind Turbines', just published by the IET. (www.theiet.org/books-offshore)

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