Horns Rev revs up

On 18 March work began on the world's biggest offshore wind farm, the Horns Rev, in the North Sea 16km off the west coast of Denmark. The first stage is the installation of monopile foundations for the 80 massive wind turbines that are to be erected. Then the turbines themselves are carefully placed on top.

The 160MW wind farm will be operated by Elsam. Danish firm MT Højgaard is main contractor for the civil works and it has subcontracted the foundations installation to a new joint venture, Mammoet Van Oord Windmills BV. Shareholders in Mammoet Van Oord include Hovago Cranes and Marine Construct, as well as Mammoet and Van Oord ACZ. Mammoet and Hovago provide the crane expertise, Van Oord and Marine Construct are the offshore and marine construction experts.

Mammoet Van Oord is installing 80 monopiles, each weighing 165t and measuring 33m long.

For the first month or so Mammoet Van Oord had to rent equipment because construction of a special jack-up barge, called Jumping Jack, purpose-built for offshore wind turbine erection, fell behind schedule. Mammoet says the delay was caused by high waters in the River Waal where the slipway of the shipyards is located. Jumping Jack is expected to arrive on the project this month.

Erecting delicate structures at sea is very weather sensitive and so the project could not be delayed to wait for Jumping Jack to be ready. The foundations have to be installed by the end of July. So Mammoet Van Oord began work using two smaller jack-up barges, one equipped with a Manitowoc 4100 and the other equipped with Mammoet's own Demag CC 1400.

Favourable weather enabled Mammoet Van Oord to get ahead of schedule within the first few weeks, even though the jack-ups used at the start could only take one set of foundations per trip. Jumping Jack, in contrast, can transport 10 complete foundation sets per trip, and it can operate in twice the sea swell of the other barges.

Jumping Jack represents a E25m investment by its backers. It is fitted with the 1,200t capacity Manitowoc M1200 Ringer that Walter Wright Mammoet used to have in Singapore (and which featured on the cover of the March 1998 issue of Cranes Today). The barge is 91m long, 33m wide and 7m high. It has four legs, each 40m long, to lift the vessel out of the water to keep the platform steady on the seabed for crane operations. An hydraulic winching system allows the barge to raise and lower its legs while it has a load on the deck. This means that it can install one monopile and then move on to the the next installation site. It can carry loads up to 4,000t.

The contract for transporting and lifting the turbines themselves into place is held by another new company, A2Sea, also established specifically to erect offshore turbines. A2Sea, contracted to the turbine supplier Vestas, began work on 12 April. Its equipment is a regular container ship, the Ocean Hanne, to which have been added four tension-controlled legs with hydraulic winches to lift the vessel 1.5m clear of the water. Craneage is a 450t capacity lattice boom Demag CC 2500 (Craneship, Feb02, p49). Last month it ordered a second vessel for its fleet.

For future offshore projects A2Sea and Mammoet Van Oord are likely to compete head to head. The advantage of using a ship, like A2Sea, rather than a jack-up barge is that it can move quicker from port to offshore site, and between sites offshore. Jumping Jack, however, has the greater load capacity and can carry more on a single journey.