The driving work at Horns Rev were planned to start on September 2, 2001 but only few days before the barge crane, the Rambiz, should have departed from Rotterdam for the Reef, a strong low pressure was reported in the North Sea and the crane arrived with a fortnight's delay.

Phase 1 activities related to the pile driving:

• Positioning and installation of a pile driving "mould" on the seabed.

This mould consists of a number of welded steel profiles holding thirteen pile sleeves in position and supporting the J-tubes (tubes for 36 kV and 170 kV cables) to secure the individual position of the piles in relation to each other once the mould and J-tubes have been placed correctly.

The Rambiz is held in position by means of four anchors placed diagonally at maximum 350 metres' distance. During pile driving its position is changed by pulling/releasing the anchor chains.

• Erosion protection. As erosion protection, a 1-metre thick stone mattress is placed around the pile. The mattress consists of a filtration layer of granite stones at the bottom and at the top, a reinforcement layer of large granite stones. This is done by the stone dumping vessel HAM 601 (owned by the Dutch HBG).

• Pile driving through the pile sleeves by the Rambiz.

• Establishment of the boat landing arrangement as well as diving activities in connection with the fitting of the J-tubes.

All secondary piles for the substation structure have been driven. Pump wells, tripod piles for the 36 kV cables (including test pile in full depth for pile driving tests) as well as pile for 170 kV J-tube.

The three large piles for the substation foundation and six collision piles have not yet been driven.

Handling and driving of the first piles have been done be means of the crawler crane placed on the Rambiz' front deck between the main crane booms.

The piles for the substation foundation must be driven by the crane's main boom due to their size. This has produced new challenges because even small movements of the barge are amplified in the top of the boom causing the load (pipes, vibrators or pile driver) to move sideways, movements which cannot be compensated. As the piles are fixed at the pipe sleeved on the seabed it is very difficult to control the pile during driving since the top of the pile moves with the vessel's movements.

These movements also endanger the work of the diver who tries to guide the pile into the sleeve, which is made even more difficult as the visibility is typically approx. 0.5-1 metre and the currents are strong.

A driving sequence has more lifting operations:

• Lifting of the pile by two cranes (the Rambiz and the crawler crane) by which the pile is turned while in the air
• Dismantling of the lifting device and positioning of driving head including vibrating to approx. 12 metres' depth
• Replacement of vibration by 55-tonnes hydraulic pile driver. Driving until specified level has been reached

The driving has been put on hold from October 20 2001 due to weather conditions (wind and waves) and defect driving-equipment has delayed the driving processes. In the meantime, other installations are made which are less dependent on weather conditions. It is to the credit of the staff who works almost around the clock to finish the work as planned - it is not unusual for them to work 16-18 hours a day.

With the present weather forecasts of increasing wind speeds and change of wind direction from east to west, the driving of piles may continue into the month of November.

Latest news: The Rambiz with staff is currently demobilising and heading towards Esbjerg Harbour.

Large swarms of songbirds have found interest in following the construction work in connection with the driving of piles for Eltra's substation. These small birds are migrating along the west coast of Jutland. The migration route depends on the wind direction and in the days of easterly winds - which was when the pile driving took place - the birds came to the area. Many birds were exhausted and took a rest on the Rambiz and other vessels.

The workers were actually able to pick up the birds with their bare hands where they would heat themselves. To make the birds fly the workers had to push the birds into the air. Many birds died of exhaustion but others continued after having regained their strength. This is important information in relation to the environmental aspects during construction of the windfarm. Once the turbines have been erected, this may happen naturally again -  songbirds lying dead around the turbines. The deaths of these migrating birds are therefore not the result of the operation of the turbines themselves.

The Rambiz in Esbjerg Harbour

Test pile (45 metres) under preparation is turned in the air.

Driving equipment on the Rambiz' front deck.

Pile entering sleeve.

Diver onboard the Rambiz.

The erection of the offshore prototype wind turbine in Tjæreborg, Denmark - a turbine exactly like the one designed for Horns Rev - has been delayed. Its installation is expected in early December 2001.

The delay is due to problems with getting the foundation approved. The foundation for this site is the existing foundation used for the demonstration turbine demolished this summer. The design of the foundation has to be approved by Germanischer Lloyd WindEnergie GmbH in Hamburg, Germany, to be certified according to "Teknisk grundlag for typegodkendelse og certificering af vindmøller i Danmark" (technical specifications for the approval and certification of turbines in Denmark).

A2SEA has been awarded the contract of sea transport and lifting by crane of the turbines onto the foundations at Horns Rev. A2SEA will supply two identical crane barges which today operate as container feeder vessels with the Orion company based in Copenhagen, Denmark. Each vessel is approx. 95 metres long, 22 metres wide and has a capacity of 276 standard 20-feet containers. The vessels will be fitted with tailor-made sponsons with jack-up legs on both sides of the vessels. The jack-up legs can be lowered and lifted by means of hydraulic wire winches.

On one of the sponsons a lattice crane will be fitted for the lifting of turbine components.

The reconstruction of the first vessel, Ocean Hanne, is ongoing at the Orskov Shipyard in Frederikshavn, Denmark and is expected to be complete by the end of November 2001. Later the vessel will undergo a number of on-site tests at Horns Rev.

The picture shows the rear deck of Ocean Hanne during construction of the two sponsons, which are the two unpainted steel boxes on each side of the vessel. The cranes shown in the picture are the vessel's standard container cranes. The main crane, which will be fitted on the structure seen on the sponson at the vessel's port, is currently in use for turbine erection in Germany.

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A number of tests of the working procedures and the materials have been included in the planning of the Horns Rev construction project in order to optimise the working routines and procedures. Great emphasis has been put on optimising each of the individual working procedures offshore so that each operation can be carried out as quickly as possible. The operations and routines are much more difficult and laborious once they are carried out in the North Sea.

Hence, procedures have been optimised for grouting the foundation and the transition piece of the turbine tower. Procedures for installing the J-tubes have also been carefully examined. The J-tube is a construction by which the cable from the turbine is led through the turbine tower and into the seabed.

One of the ways to limit the work offshore is to assemble the turbines as much as possible on land before transportation to the windfarm area. The turbines for Horns Rev will be assembled as much as possible and as many components as possible will be installed - only it should still be possible to handle the turbines by the equipment available for this offshore installation. This optimisation has resulted in a six-step turbine main installation process.

MT Hojgaard is in charge of the supply of foundation piles for the Horns Rev windfarm. The production of the foundations (200-tonnes mono-piles) is well under way at the premises of MT Hojgaard and their European sub-suppliers.

The tubes for the mono-piles, a 4,000 mm (diameter) tube driven into the seabed and the 4,240 mm transition piece, are produced by the Dutch SIF Group in Roermonde near Maastrich, Holland. SIF are specialised in the production of all kinds of tubes and pipes for the offshore business and are responsible for the welding of the tailor-made flange on which the Vestas turbine will be mounted when the time comes. The flanges are produced in several steps, the first step being the casting and rolling of the workpiece rolled to circular rings. This process is done in an Italian steel foundry in Vicenza between Verona and Venice. The workpieces are transported to Siegthaler in the Ruhr area, Germany. Here, the workpieces are processed and given the final dimensions before being shipped to SIF, Holland.

Once piles and transition pieces with flanges have been processed and completed at SIF's premises, the pieces are shipped on the river Maas to Antwerp where another sub-supplier, Rheden Steel, completes the pieces by installing the boat landing arrangement structure, J-tubes for cables, platforms and surface treatment, etc. The estimated time of arrival of the first pieces is late January 2002 at MT Hojgaard's stock at Esbjerg Harbour. The actual installation of the foundation piles will begin late March 2002 as scheduled.

Three machined flanges ready for boring of 140 boltholes (45 mm in diameter).

Boring of flange boltholes, a total of 140 holes, ø45 mm.

Pile and transition piece on barge ready for shipment.

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