With 7GW of offshore wind, offshore operation and maintenance (O&M) activity is becoming big business, and increasing steadily. The value of the O&M sector now stands at roughly EUR 500 million a year and a great deal of effort is going into keeping the costs down, so that the cost of energy from offshore wind farms is reduced. After a period around the end of the last decade when costs went up, more sophisticated O&M strategies, higher winds and bigger turbines are likely to contribute to lower costs in the future.
The term "operation and maintenance" does not have a precise definition. The work (and cost) of attending to routine maintenance, so as to keep offshore turbines running in all weathers, accounts for roughly one half of all the yearly costs associated with keeping a wind farm operating. Insurance costs are the second most expensive factor, accounting for 10-20% of all O&M costs. Other expensive costs are lease or rental charges, import power (needed to keep systems operational when a turbine is shut down), administration charges and use of system charges for the electricity network. The latter are not charged in all jurisdictions but in the UK account for roughly one third of all O&M costs.
When the first UK wind farms were built, the developers - as a condition of access to a capital grant - were mandated to provide yearly reports on the operation of their wind farms.
Those provided for the 90MW Kentish Flats project itemise the components of the O&M costs for the first three years of operation from 2006-8. Averaging these figures enables a picture of the different elements of O&M to be assembled (see chart below).
Maintenance costs can be broken down further into scheduled and unscheduled maintenance. Scheduled maintenance includes periodic checks on the mechanical and electrical components of a wind turbine, safety checks, inspections of firefighting equipment, together with similar checks on support vessels. Unscheduled maintenance, as the name implies, deals with any malfunctioning items within the wind farm. The latter covers items such as the widely reported gearbox replacements that have been necessary at some wind projects and the rectification work associated with the more recent problems with grouting at the interface between the turbine support tower and the monopile.
During the first three years of operation at Kentish Flats, variations in maintenance costs were modest, from a high of £1.375 million in 2007 to a low of £1.1 million in 2008. The breakdown between scheduled and unscheduled work was not disclosed, but these figures suggest that there were no serious issues with unscheduled maintenance.
An analysis published in 2013 by the National Renewable Energy Laboratory (NREL) in the US - Installation, Operation, and Maintenance Strategies to Reduce the Cost of Offshore Wind Energy - suggests that corrective or unscheduled maintenance costs are significantly higher than preventive or scheduled costs.
The goals of much current research are therefore directed towards reducing unscheduled maintenance costs. The work is invariably expensive, and the loss of electricity production while the maintenance is carried out can have an adverse effect on the profitability of an offshore wind project.
The first Danish wind farms in the early 1990s were sited in shallow waters and fairly close to the shore. Perhaps, unsurprisingly, O&M costs were only slightly higher than the onshore equivalents. Some 15 years later, when the first UK wind farms were installed, machine numbers and sizes had increased and were in more remote and exposed locations.
Some of the O&M costs reported by these projects were up to double those of the first Danish project at Vindeby. The UK farms reported a cost range of EUR 11.3-EUR 33/MWh, whereas the Danish wind farm reported EUR12/MWh.
There was a further upward drift during 2006-10, however, with both the UK Department of Energy and Climate Change (Decc) and the US's NREL reporting costs around EUR 25/MWh. NREL's costs were based on projections, whereas it may be assumed that the UK figures were based on confidential data reported to Decc. Cost projections increased again after 2010, possibly because both the UK and Germany were planning projects further offshore.
The gains are higher wind speeds, but the downside is a more challenging operating environment and higher costs of access. As a result of these pressures, costs are now in the range EUR 40-44/MWh, according to the UK trade association RenewableUK, a study by Fichtner/Prognos, and a paper by Roland Berger. The first two studies both suggest that these costs will fall by about 25% by 2020, to about EUR31/MWh.
The diagram above, right, shows the way that costs have moved in 20 years, according to a variety of sources. These must be taken on as a guide, as the various sources are not necessarily directly comparable, as the figures depend on the precise content of the estimates - and this is not always clear.
As the offshore wind industry is still evolving, it is difficult to separate out the influences of parameters such as turbine size, array size and distance to shore.
However, the general trend is for all of these to increase over the next few years. Distance to shore is a key parameter, particularly because the goal is to access higher wind speeds and the developers have concluded that the benefits - in terms of higher energy yields - outweigh the disadvantages of an increase in operation and maintenance costs.
The Prognos/Fichtner study suggests that beyond 50 kilometres from the nearest port, it is likely to be more economic to site the O&M base offshore rather than onshore. At this distance the use of helicopters rather than seagoing vessels is likely to be more economic. The same study suggests that the "norm" in terms of machine size will grow from 4MW to 8MW by 2023. The study by Berger suggests that moving from 3MW turbines to 6MW turbines will reduce O&M costs by 14%.
The search for higher wind speeds often leads to deeper waters and with it the possibility of using wind turbines on floating foundations. There is a consensus that this will lead to higher O&M costs, with NREL suggesting a 30% increase.
Pitching scheduled maintenance at the right level is vital. If it is inadequate, then the unscheduled maintenance costs are likely to be too high and energy losses will be unacceptable. One report that studied the failure rates of offshore wind turbine components revealed that minor failures account for 75% of all failures, but are responsible for only for 5% of overall downtime. Major failures represent only 25% of failures, but account for 95% of the downtime.
On the other hand, too much scheduled maintenance is costly and may produce marginal benefits in terms of increased availability.
It is expected that, as the industry matures the optimum balance will be found, leading towards lower cost offshore wind energy.