he Northumbrian port of Blyth is becoming established as a leading centre for wind energy development. T During 1992, nine 300 kW wind

turbines were installed along the harbour’s east pier, to create the UK’s first semi-offshore wind farm. This pioneering scheme has now been followed by the commissioning of the Blyth Offshore wind farm-a joint venture between Shell, Powergen Renewables, Nuon and Ameccomprising two 2 MW wind turbines located 1 km off the coast. Prior to this latest development, offshore wind farms have been restricted to more sheltered waters in the Baltic

or the IJsselmeer, and Blyth Offshore represents the world’s first multi-megawatt wind farm in serious seas. Blyth Harbour and Blyth Offshore were both developed by Hexham-based AMEC Border Wind.

The period between these two projects has seen a dramatic increase in the size of wind turbines, represented by the jump from 300 kW to 2 MW, along with marked improvements in reliability, allowing the maintenance interval to be increased from fortnightly to annually. These two factors, which significantly reduce installation and operating costs, combined with the difficulty in obtaining planning permission

1 Blyth Harbour wind farm

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2 Rock drill on the Wijslift crane barge. The drill is guided into position by a vertical conductor tube

for onshore sites, have stimulated a growing interest in offshore schemes. The success of Blyth Offshore, along with specific governmental support for offshore wind power via the Renewables Obligation (see page 25) could lead to around six 50 MW wind farms in UK waters in the next few years.

Origins The Border Wind developinent team started work on the offshore project in 1994. Initial design ideas were produced, costs obtained and the lengthy task-a reflection of the scheme’s novelty-of obtaining all the necessary permits initiated. At the same time, an application to the European Union for funding support under the Thermie scheme was successfully submitted.

A site on a submerged rockhead 1 km off the coast was selected. The wind resource at this location was known and a good grid connection was available.

Technical aspects Foundation design and installation were the most technically challenging aspects of the project, as, hitherto, no offshore wind farm had been built on a site where large breaking waves w-ould be experienced. The electrical infra- structure design was straightforward as the distance to shore was short and only two turbines were involved. The wind turbine was developed from a well tested family of large machines, but the design had to be checked to ensure compatibility with the foundation structure.

Design parameters The site’s wind regime was known from the records of the Blyth Harbour wind farm. In

general, the wind conditions are less extreme than those experienced on many upland wind farms, while the winds from the east, which have originated over the North Sea, tend to be ‘very smooth’, i.e. of near constant intensity.

Proximity to the shore meant that obtaining wave data for the site was more problematic. Deep sea waves break when they hit shallow water of a depth similar to the wave height. The water depth at the site is 6 m at the lowest state of the tide, and the tidal range is 5 m.The wave climate for the site was established by modifying the deepwater wave climate to account for shoaling, refraction and wave breaking.

To be sure of the ground conditions, a bore hole was drilled in the seabed at each turbine location and a rock core extracted. A coal seam was found in one of these cores-a not unexpected result given the area’s historical associations with coal mining. In the 196Os, Blyth was the largest coal exporting port in Europe.

Foundations The financial viability of offshore wind farms depends heavily on suitable foundation design. Blyth Offshore’s foundations accounted for approximately 25% of the total installed cost. This compares with 8 - 10% for typical onshore sites.

Three different foundation designs were examined: piled tripod, ballasted steel caisson and monopile. Given the ground conditions at Blyth - a submerged sandstone outcrop-a steel monopile cemented into the rock was selected as the most appropriate option. Initially the pile diameter was fixed at 2 m. However, during the project’s development the turbine size increased from 7 5 0 kW to 2 MW, necessitating an increase in the pile diameter to 3 5 m. Drilling a hole of this diameter in rock under the sea requires very special equipment. Fortunately, at the time the project was being planned, appropriate technology was just becoming available from the UK company Seacore.

The apparent simplicity of the foundations does not reflect the design effort required. The foundations, tower and the turbine interact dynamically when subject to cyclic loads from the rotor blades, wind and waves. The relationship between the wave and wind loading on such a system is complex. The environmental loading, along with the complications of extracting energy from the wind, posed a major challenge for the design team.

It might be expected that the extensive

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ltriowledge aiid experience of offshore foundation design within the offshore oil and gas industry would be readily transferable to the Blyth project. However, at Blyth, the coin- biiiatiori of a dynamically-sensitive structure, a highly complex wind load spectrum and mii-

linear wave loading present new challenges. The detailed design was carried out by LIC Engineering of Denmark.

Turbine When the project w-as initiated, the largest commercial machine was 7 5 0 kW As the process of obtaining all the permits to build the wind farin slowly progressed, the size of turbine available increased rapidly, stimulated by a number of EU-inspired developments. In 1998, it was decided to opt for 2MW units, as t h e would be more representative of the machines that will be installed over the next ten years.

A number of turbines were considered. All the major inaiiufacturers are' currently developing large offshore designs, but operational experience with large niacliiiies is limited. The inachine chosen was the Vestas V66, a 2 MW design with a 66 m diameter rotor-comparable

to the wing span of a Jumbo jet. The electrical generator used is a doubly-fed induction iiiachiiie with the rotor controlled through a variable-speed drive. This allows variable-speed operation, leading to improved energy capture and reduced loading on the drive-train.

Electrical infrastructure The turbines are connected to the Northern Electric grid by a submarine cable. The coiiductors are copper (cross section 70 nim) , the insulation is EPR (ethylene propylene resin) and the cable is protected by two layers of licavyweight arniouring aiid a polypropylene outer layer. Corninunication and coiitrol signals are carried by a fibre-optic bundle incorporated within the cable.

The cable does not follow a straight path between the turbiiies aiid the shore, as this would liave necessitated cutting a long trench tlirough rock on the foreshore. A less direct route avoids this costly requireineiit, although the filial cable leiigtli is increased to 1 '8kin. Across the foreshore the cable is buried in a trench aiid protected by split pipes. Under the 3 Monopole foundation

pile being held in ~~

sea it lies oii the sea bed protected, where position during grouting

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necessary, with sand bags or additional plastics jackets. Plastic bends are used to constrain and protect the cable where it enters foundations.

The generation voltage is 690V and the connection voltage 1 1 kV. The step-up transformers are located in the nacelle-as close as possible to the point of generation- thereby minimising the I’R losses associated with low- voltage, high-current rransmissim

A ilexible cable conducts the power down to ring-main units in the tower base, where the submarine cables are terminated.

Project aspects All the project’s partners have an interest in offshore wind generation, and see this project as an important step to full commercial exploitation. The partners were closely involved in the design of the scheme, including the development of the safety systems. and they are now actively engaged in the detailed

monitoring of its operational performance. The project cost iE4 million, partly covered by

a L700 000 grant from the EU under the Thermie scheme, with additional assistance from the DTI. The contract price for the electricity is 5 pence/kWh.

Installation Careful planning is vital for the installation of such large structures. The main installation craft comprised two Ainec Marine vessels, the Wijslift crane barge and the Atlas supply barge, and the installation began with the loading of the drilling equipment onto the crane barge at its base on the River Tyne. The holes were drilled, the monopole foundation piles lowered into place, and cementitious grout was then pumped into the annulus between the piles and the rock while the pile was held steady by the crane barge. It took about 12 hours for the grout to gain enough strength for the barge to release its

~

~~~ ~

4 The two turbines viewed from the shore

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grip. At one point, large breaking waves forced the suspension of the drilling operations. However, the crane barge was able to remain on station-jacked up out of harm’s way-and drilling was resumed when sea conditions improved.

The turbine components were then brought out on the Atlas barge and lifted into place. The positioning of the nacelle involved the heaviest and highest lift-requiring the crane barge to be jacked well clear of the water. Again, unseasonal weather was a problem, causing delays during the installation of the first turbine when equipment was being transferred from the floating Atlas barge to the static Wijslift crane barge. To avoid this problem on the second installation, the nacelle and the lower tower section were loaded onto the crane barge in port.

Once the turbines were installed, and the barges with their associated anchors had left

site, the cable laying could proceed. Laying the cable to the shore took about a day, while the link between the two turbines required just a few hours.

Initial operation Various weather delays extended the installation period to two months. The electrical connection was energised in mid-November, and the turbines commissioned over the following couple of weeks. During the 120 hour run-in period there were extended periods of operation at f d l power, proving all control systems. The official opening, by the Energy Minister Helen Liddell, took place on 7 December 2000. The turbines have survived all the recent storms with no ill effects.

The future of offshore wind This project demonstrates that offshore wind farms can be built off the UK. With the rest of Europe planning to build 12000MW of offshore wind plant by 2010, and the UK following suit with initial plans to build around six 5 0 MW plants, the industry is poised for take off At the inauguration of Blyth Offshore, the Crown Estate, which owns the coastal seabed from low water outwards, invited consortia to come forward with proposals for the development of offshore wind farms on sites of up to 10km’; the DTI is planning to support research into offshore wind generation under its New and Renewable Energy Programme, and the Climate Change Levy, reinforced by the recently announced Renewables Obligation, offers an incentive to invest -suddenly, the market for offshore wind is looking up.

The UK has the best offshore wind resource in Europe with a potential several times that of the entire UK demand. Offshore wind farms are less subject to site restrictions then their onshore counterparts, so they can be larger, offering good economies of scale and, potentially, lower generating costs. As such, they are an attractive prospect to energy suppliers with Renewable Obligations to fulfil.

The lessons of Blyth Offshore should lead to faster and cheaper installation of future offshore wind farms; hastening the day when offshore wind makes a significant contribution to the UK electricity supply.

0 IEE: 2001 Bill Grainger and Tom Thorogood are, respectively, Technical Director and Project Engineer with AMEC Border Wind, Haugh Lane Industrial Estate, Hexham, Northumberland NE46 3PU, UK (tel +44 (0)1434 601 224; fax +44 (0)1434 601 200; bill@bordwind. co.uk/tom,tliorogood@amec.corn).

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