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Installation vessels - ship-shape project planning

OFFSHORE: One of the most significant segments of the offshore wind supply chain is the fleet of vessels needed to build a project at sea. From a turbine installation vessel weighing up to 14,700 tonnes down to a 24-tonne crew transfer boat, deployment must be planned meticulously to avoid costly delays.

Critical in the process is the installation vessel — its availability dictates a project's construction schedule. Until a year ago their shortage was seen as a potential problem across Europe, but that is no longer the case.

"There has been a big sea change of activity in this area since the start of 2010 and the announcement of the successful bidders for UK Round Three zones," says Rhys Thomas, RenewableUK's supply chain policy officer. "A number of new vessels are now on order and being built." He points to at least 12 ships for turbine and foundation installation that are expected to be delivered over the next few years, most of them from the Far East.

Purpose-built

Unlike the early vessels, which were adapted to install offshore turbines, this new generation is purpose-built and ready for the next generation of larger turbines.

MPI Offshore, which operates the first-ever dedicated wind turbine installation ship, the Resolution, has ordered two more vessels from China, both able to handle up to 1,000 tonnes. The first will soon start installing foundations at the 630MW London Array project off the UK's east coast.

Norwegian shipping group Fred Olsen and UK's Seajacks each have two vessels on order from Lampress, based in the Arabian Gulf, the first of which is to arrive next year. Four more are on order for other companies.

This year German energy company RWE takes delivery of the first of two €100 million vessels from Korean firm Daewoo. RWE is the first developer to circumvent the installation-vessel queue and build its own ships. And Danish developer Dong Energy has taken matters in hand by acquiring Danish wind turbine installation vessel operator A2 Sea.

Most installation vessels can handle foundations as well as turbines but different strategies may be used for moving the foundations to site. At Vattenfall's Thanet wind farm off Kent, the installation vessels took 100 monopiles from nearby Ramsgate Harbour to site, says project director Ole Bigum Nielsen. But at the Ormonde project off Cumbria, the installation vessel remained on site while barges brought the jacket foundations. "It depends on what kind of installation vessel is available, the distance from site to harbour and whether the site is relatively sheltered or the conditions tough," he says.

Cable installation requires a different set of specialised ships for laying the inter-array cables between the turbines and the substation, and the export cables taking power ashore. Route clearance, trenching, cable-laying and burial may be undertaken by different vessels, although some can perform more than one task.

Additional vessels are needed for servicing the major ships, and there are support vessels and tugs for transporting components, equipment and crew transfer, diving services, guarding the site — and even for rescue. And before work begins on site, craft will be conducting geotechnical and environmental surveys.

Many vessels needed

At the height of constructing the 100-turbine Thanet wind farm, up to 25 vessels could be deployed at any one time, says Nielsen. "We were undertaking installation of foundations, turbines, our offshore substation and cables all at the same time. That means quite a lot of traffic of equipment, and especially of people, in and out of harbour." There would be several small crew transfer vessels at a time," he says.

And once the project is complete, such boats will be used for operations and maintenance (O&M). "Using a large crew-transfer boat means that the last crew might have to be kept waiting before they can be deployed, so there may be a need for small, fast craft," says Robin Redfern, offshore O&M analyst at GL Garrad Hassan.

As the sector builds bigger Round 3 wind zones further out to sea, the number of vessels employed will grow, says MPI Offshore chairman Paul Gibson. "By 2018 the North Sea and Irish Sea might see 250 such vessels in operation with 10 offshore accommodation units.

"Further offshore, the need for offshore accommodation will be paramount. You can't have technicians at Dogger Bank, for example, sit in vessels for six hours and then expect them to perform," says Gibson.

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