Horns Revolution

The world's largest offshore wind farm yet, Horns Rev is 14-20 km from Blåvandshuk, the most westerly point of Denmark. It is currently undergoing commissioning after a highly successful construction phase, helped, it must be admitted, by relatively good weather over the summer.

Installation of the last of the eighty Vestas 2 MWe turbines was completed at midnight on 21 August, two weeks ahead of schedule, allowing final cable connections between the turbines to be completed by Nexans on 23 August. First electricity was generated on 29 July, with commissioning of the first turbine. At the time of writing it was hoped to have the farm in full operation before Christmas.

This would be an exceptional achievement considering the difficult working environment of the North Sea and the fact that the main cable was damaged in July and a replacement piece had to be inserted.

At the time of writing, electricians were sailing from turbine to turbine to finish the internal cable work inside each machine. Following the electricians were the commissioning teams who are preparing the turbines for power production.

Several teams were also testing and commissioning the farm's advanced remote monitoring and control system, which makes use of fibre optics in the power cabling. This is the first application of the VestasOnline system, which was developed in conjunction with the Horns Rev project and enables the large number of turbines to be operated "as a power plant." Signals are transmitted to Esbjergværket, Elsam's existing conventional central power station in Esbjerg, where technicians have access to detailed data for operation and maintenance.

Elsam is the owner and is responsible for the operation of the wind farm, while Eltra, the independent transmission system operator for Western Denmark, is responsible for the connection of the wind farm to the national onshore grid. In charge of project management and all engineering was Elsam subsidiary Tech-wise A/S.

Laying the foundations

The Horns Rev project uses the "monopile" concept. The foundation for each wind turbine is a steel cylinder with a diameter of about 4 m and a wall thickness of 5 cm. A hydraulic ram drives the steel cylinders into the seabed to a depth of some 25 m, with their tops above the sea surface. Due to strong currents and severe wave conditions at the site, a lot of attention has been devoted to scour protection, with a blanket of stones placed round the foot of the foundations.

A preassembled transition piece (complete with pre-installed features such as boat landing, cathodic protection, J-tube cable ducts, turbine tower flange, etc) is attached to the monopile, extending 9.5 m above the sea surface. The overlap between foundation and transition piece is 6m, with an 80 mm thick annulus. This annulus is filled with high performance grout (Ducorit D4). The turbine tower is bolted to the flange that forms the top rim of the transition piece.

The transition piece allows the towers to be adjusted to a fully vertical position even if the foundation is not completely level. The final transition piece was installed on 3 August.

Turbine installation

The Horns Rev project is believed to be the first time machines as large as the Vestas V80-2.0 MW have been installed so far out to sea. For the installation Vestas hired Danish offshore wind logistics specialist A2SEA and used an innovative concept originally sketched out in 1999 at the kitchen table of Kurt Thomsen, the company's founder. Two container ships were modified by fitting them each with a giant crane and four large submersible legs. The legs are lowered down to the seabed to stabilise the vessels during heavy lifting, allowing safe lifts to be done even in rough seas. But unlike the old jack-up barge system, the new crane ships do not need tugs and handling vessels to get them where they are needed. They can pick up the turbines and towers from the harbour, two complete sets per ship, take them out to sea and do the installation unaided. This ability of the crane ships to do the whole job by themselves has been a major contributor to the fast construction time at Horns Rev. On a good day it proved possible to install two entire turbines.

At the time of writing the two crane vessels were acting as hotel ships for the commissioning teams.

To meet the special demands of offshore operation, the V80-2.0 MW turbines have been modified slightly relative to their land-based counterparts. For example, the back of the nacelle has a heliohoist platform allowing service people to be lowered from helicopters. The nacelles are also equipped with their own tools and repair equipment, as well as facilities for emergency overnight accommodation of technicians should unexpected bad weather necessitate it.

Vestas V80-2.0 MW wind turbines are also to be deployed at the 60 MWe North Hoyle installation, the UK's first major offshore wind farm, 7 km off the Welsh coast. Turbine installation at North Hoyle is scheduled for the summer of 2003 and will be done by Mayflower Energy using their own purpose-built six legged vessel.

Grid connection

The Horns Rev wind turbines are connected via 36 kV submarine cables to an offshore transformer substation located near to the farm. After stepping-up to 150 kV, the power is conveyed to shore via a trenched submarine cable and is connected into the grid via an existing substation.

The offshore substation platform is of tripod construction with a steel building some 14 m above mean sea level. The platform accommodates the following: 36 kV switchgear; 36/150 kV transformer; 150 kV switchgear; control and instrumentation system, and communication unit; emergency diesel generator, with 2 x 50 tonnes of fuel; seawater-based fire extinguishing equipment; staff and service facilities; helipad; crawler crane; and man-over-board boat. It also hosts a web cam (hornsrev.dk).

Operation and maintenance

Attention will now shift to the operational aspects of Horns Rev. The wind turbines are designed for minimum servicing, but each wind turbine will need two annual service visits. Technicians will travel by boat, or alternatively be hoisted down from helicopters.

There will also be great interest in how the wind farm will interact with the Danish national grid, which must learn to live with a projected 21 per cent contribution from intermittent renewables by the end of 2003.

In addition to obtaining green certificates, Elsam is guaranteed a selling price of DKK 0.33 per kWh for power produced from Horns Rev over a fixed number of full-load hours, equivalent to about ten years of production. When this number of full-load hours has been reached, all subsidies will be discontinued, and power from Horns Rev will be sold at market prices. The green certificates will be available throughout the full service life of the wind turbines, and it is expected that they will be traded at DKK 0.10-0.27 per kWh.

These arrangements "will enable power from Horns Rev to be sold at the lowest price for renewable energy that has ever been seen in Denmark", says Elsam.

Back in 1998 Horns Rev was originally conceived of as one of five offshore Danish wind farms, with a combined installed capacity of 750 MW. In 2002 the newly-elected Danish government revised the plan downwards to include only two demonstration wind farms, Horns Rev and the 158 MWe Nysted farm (formerly known as Rødsand) in the Baltic. Foundation work has now started on Nysted, with turbine installation (72 Bonus 2.2 MWe machines) scheduled for 2003.