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Offshore Wind

The Potential of Offshore Wind

The following is based on an article by David Milborrow
1st published in WindStats Newsletter magazine, December 1996.

Of all the renewable technologies, offshore wind has the potential to deliver the highest quantities of energy. However it is not competitive with onshore wind. Offshore wind energy has the added attraction that it has minimal environmental effects and, broadly speaking, the best resources are reasonably well located relative to the centres of electricity demand.

The European Wind Atlas, complied by the Risø National Laboratory, contains comprehensive information about wind speeds and resources.

Wind resources over open sea (more than 10 km offshore)
for five standard heights (ms-1
+ Wm-2)

  10m 25m 50m 100m 200m
> 8.0
> 600		 > 8.5
> 700		 > 9.0
> 800		 > 10.0
> 1100		 > 11.0
> 1500
7.0-8.0
350-600		 7.5-8.5
450-700		 8.0-9.0
600-800		 8.5-10.0
650-1100		 9.5-11.0
900-1500
6.0-7.0
250-300		 6.5-7.5
300-450		 7.0-8.0
400-600		 7.5-8.5
450-650		 8.0-9.5
600-900
4.5-6.0
100-250		 5.0-6.5
150-300		 5.5-7.0
200-400		 6.0-7.5
250-450		 6.5-8.0
300-600
< 4.5
< 100		 < 5.0
< 150		 < 5.5
< 200		 < 6.0
< 250		 < 6.5
< 300

The Offshore Resource

There is no doubt that the size of the offshore resource is impressive. A study by the CEGB carefully examined all the relevant offshore constraints and concluded that the energy potential was around 230 TWh per annum - roughly equal to total UK electricity supply at the time the study was carried out. This particular study excluded all the following areas from the resource assessment:

  • everything closer to the shore than 5 km
  • shipping lanes
  • military exercise areas
  • regions where dredging concessions existed
  • known dumping grounds for ammunition, explosives and other hazardous materials
  • any areas where the sea depth was less than 10m or greater than 50m

The last three constraints would be considered less valid in future years. The distance to the shoreline may depend upon the planning classification of the land in question and existing sea uses. Regions with dredging concessions may be considered, depending upon the depth of water and draught of vessels used. Technological development relating to the foundations upon which turbines stand mean that water depth can now be well under 10 metres. Part of the area of the proposed Scroby Sands wind farm is a sand bank at low tide.

Technical Issues

One of the attractions of offshore wind power is that wind speeds are generally higher offshore than on land. However, this is not the case in undulating countryside such as is found in the British Isles, Italy and Greece since here the winds in upland regions are enhanced, relative to ground level, by the altitude and by acceleration due to hill shape. In the UK, for example, onshore winds range up to around 9.5 m/s at hub height, whereas offshore winds at, say, 5 km from the shore are in the range 8.3-9 m/s. Offshore wind power is therefore most attractive in Denmark and the Netherlands where pressure on land is acute and where windy hill top sites are simply not available. In these areas offshore winds may be 0.5 to 1 m/s higher than onshore, depending, again on the distance offshore. It is therefore, entirely logical that the first offshore wind farms should be situated in Denmark.

Wind turbulence is lower offshore. This means that turbines are subject to less stresses from the wind, although this tends to be offset by the higher wind speeds and the need to design the structures against wave loads and wind/wave interactions.

The need for more expensive foundations and the need to 'marinise' the wind turbines, to protect them from the corrosive influence of salt spray, push up the costs of offshore wind energy relative to onshore. (Marinising the turbines typically adds about 1 - 2% to the cost). The cost of the cable connection may also be more expensive than for on-shore developments, although this is not always the case. The cost of grid connection at remote upland sites may be more expensive if grid reinforcement is required.

Operation and maintenance costs may be increased and there may be a risk of lower availability due to difficulties in obtaining access to the wind turbines during bad weather. However, this has not been borne out by experience at Vindeby and Tunø Knob, see below.

However, set against these actual and potential extra costs is a reduction in the civil engineering costs due to the absence of access roads.