Why do we need offshore wind farms?

There are several factors which suggest the development of an offshore wind energy industry. The resource is extremely large, the energy costs, although initially higher than for onshore, are cheaper than other renewable technologies and the risks are low, as several demonstration projects elsewhere have shown.

Many people, while agreeing that wind turbines are a useful strategy, are not happy to see them in their area. This is the NIMBY principle – not in my back yard. Siting wind turbines at sea will reduce the constraints that can be found on land, such as the visual impact and planning challenges.

The UK is currently in a position, given the right incentive and initiative, to create a significant new offshore industry. As the field is relatively new, we could take the lead in offshore installation by utilising our existing highly skilled offshore workforce. Conventional energy generation companies are already diversifying their operations into renewable energy sectors and the offshore oil industry is in a unique position to exploit its highly skilled offshore workforce.

Will they replace onshore turbines?

Although the expansion of the wind energy industry offshore will help to meet government targets on renewable generation, they will not replace development onshore. The DTI has indicated that they anticipate wind power to play a major role in achieving the 2010 target of 10% or the total UK electricity supply from renewables technologies. 2.6% will come from new onshore developments while a further 1.8% will be met from the emerging offshore sector.

As Energy Minister Michael Meacher said at BWEA’s annual conference in September 1999, meeting the 10 per cent target will be tough: “It will require not only existing technologies – waste, onshore wind and landfill gas – but also offshore wind and energy crops”.

“Offshore wind is seen by many as the practical way of addressing concerns about impact on the landscape. Certainly, it is likely to be increasingly significant in the energy supply mix. But it must not be seen as an alternative to onshore wind – I believe we will need both”.

How big is the wind resource offshore?

The wind energy resource at sea is extremely large, and the UK has one of the highest in the world with over 33% of the total European potential. In fact, the UK’s offshore resource is equivalent to three times the UK’s electricity usage in 1998. Conservative calculations show that offshore wind alone could meet the 10% target by 2010.

Offshore wind speeds are generally higher than coastal wind speeds. Ten kilometres from the shore, speeds are typically around one metre per second higher. Given that the power output is a function of the cube of the wind speed, this is very good prognosis! There are large areas of North Sea with wind speeds above 8 m/s and turbulence is lower offshore. This reduces the fatigue loads, but wind/wave interact ions will have to be taken into account during design.

Where will they be built?

There is currently only one offshore project operating in the UK, 1km offshore of Blyth Harbour in Northumberland. This is a two 2MW turbine pilot project built as an extension to the existing waterfront project.

Areas identified as having a high potential for offshore development are on both east and west coasts of the country. Starting south of the Humber, continuing through Lincolnshire and Norfolk, then running down and around East Anglia to the Thames estuary, while on the other coast the best resources are mainly to the north-west, from Blackpool down to North Wales.
For a map of locations released in this first round of offshore developments, click here.

How deep will they be?

The turbines will generally be built in relatively shallow water – less than about 30 metres in depth.

How far out to sea will they be?

BWEA Best Practice Guidelines are currently being revised to include offshore considerations. Amongst these will be the recommendation that all offshore projects should be sited at least 5kms offshore, although there is a proposal for locations as close as 2kms offshore.

Is there a limit to how far out to sea these can be built?

Although it is possible to build structures in water deeper than 30m (for example the oil platforms in the North Sea), it is very expensive and would not be economically viable for offshore wind turbines.

Wind speeds tend to increase as you move offshore. This means that turbines built further offshore should capture more wind energy. Unfortunately, as the distance to land increases, the cost of building and maintaining the turbines and transmitting the power back to shore also increase sharply, limiting the distance out to sea at which offshore wind projects will be built.

How do you build an offshore wind farm?

There are no technical barriers to installing offshore turbines but the construction, delivery to site and assembly of such large machines will need specialist equipment, facilities at ports and careful timetabling to make sure that the possibilities of using calm weather windows are maximised.

Most developments will be installed on either gravity foundations or sited on steel monopiles. Gravity foundations are concrete structures which settle and are stabilised by sand or water and the turbine tower fits into them. Monopiles are long, steel tubes which are hammered, drilled or vibrated into the sea bed until secure and then platforms and towers are installed on .

How realistic are floating platforms?

Although it would be technically feasible to mount wind turbines on floating structures, studies have shown that it would be very expensive to do this. However, technical developments may make floating offshore wind farms economically feasible in the future.

Won’t all the metal bits rust away?

The metal parts of the turbine structures will be specially coated to protect them from corrosion.

How will the electricity get into our homes?

Underwater cables will transport the electricity from the turbines to a transformer located onshore. This will convert the voltage for transmission through the national grid through the local grid connection.

How much power will the turbines produce?

A typical offshore installation will utilise 2 megawatt or larger machines Higher wind speeds at sea mean an increased energy production, as energy output is a function of the cube of the wind speed. Each turbine would typically generate enough electricity each year to meet the needs of 1500 households while displacing in the region of 35,000 tonnes of carbon dioxide as it does so.
Should all the sites announced by the Crown Estate go-ahead, they will produce sufficient electricity to power over 1.1 million households – or every home in the Manchester postcode area.
For real project information visit the Blyth Offshore website.