Wind turbine installation vessels

Added to the archive in 2026 · Reference note, not an original Bennett Offshore page

The ship that started the specialised fleet was built in China for a British company that collapsed days after the ship finished its first contract. Mayflower Energy ordered the Mayflower Resolution in 2002 from the Shanhaiguan shipyard in Qinhuangdao, to a design by the Danish naval architects Knud E. Hansen. The budget was £20 million; the final bill, with a cable-laying remotely operated vehicle added, came to £53 million. The ship arrived at Falmouth in 2004 after a 66-day voyage, installed turbines at North Hoyle, and in March 2004 its owner's parent company went into administration. Managers bought the vessel for £12 million and kept it working as MPI Resolution. It is usually described as the first self-elevating turbine installation vessel in the world.

Bennett Offshore's late designs belong to the same family. The restored wind farm vessels page, the ORCA 3500-WIND and ORCA 4500-WIND pages and the ORCA 10000 page show how a liftboat design office adapted its work to offshore wind. This page sets out what these vessels have to do and how the Bennett numbers compare.

The job

Most offshore wind farms stand on fixed foundations in relatively shallow water; the Wikipedia overview puts the practical limit for fixed foundations at around 50 to 60 metres. The most common foundation is a monopile, a single large steel column driven into the seabed, often about six metres across in water up to 30 metres deep. Deeper sites use steel jackets, tripods or concrete gravity bases. On top of the foundation the installer sets a tower, then the nacelle with the generator, then the hub and blades.

Each of those lifts needs a crane on a platform that does not move with the waves, because the crane hook may be well over a hundred metres above the sea. A floating crane barge pitches and rolls. A jack-up stands still. That is why the Wikipedia article on wind turbine installation vessels says most of them are self-elevating jack-ups, and why Resolution had six legs that let it lift itself between 3 and 46 metres above the sea.

What makes a WTIV different from a drilling jack-up

The Wikipedia article on these vessels lists the features that set them apart from a drilling jack-up, which usually relies on tugs:

• Self-propulsion, so the vessel can relocate quickly between turbine positions inside the wind farm.
• A slender, ship-shaped hull for a quick turnaround, carrying several foundations or turbines on each trip.
• Azimuth thrusters that position the hull while the legs go down.
• Large cranes: some vessels can carry five modern turbines and lift 700 tonnes to 125 metres above the deck.

Some vessels skip the legs entirely and use dynamic positioning to hold the hull steady while a pile driver works on a foundation. Others carry a crane but no legs. Crane-less feeder vessels with motion compensation can shuttle components to an installer that stays in the field.

Bennett's wind designs in numbers

The restored particulars tables give a clear picture of the range.

The ORCA 3500-WIND is 76 metres long and 38 metres wide, with tubular legs 100 metres long and a maximum water depth of 50 metres. Its deck takes two complete 4 MW turbine sets or one 6 MW set. A 600-tonne leg-encircling main crane can lift components to towers 120 metres above mean waterline. Transit speed is 5 to 6 knots, accommodation 70 people, and the class notation includes DP2.

The ORCA 4500-WIND is wider, 44 metres on a length of 77.9 metres, on truss legs 100 metres long with a maximum water depth of 30 metres. It carries two 6 MW turbines weighing 2,400 tonnes in total with their lifting gear and framing, has about 1,300 square metres of clear deck and a bare-deck variable load of up to 4,500 tonnes. Accommodation rises to 120 people.

The ORCA 10000, originally the KFELS MPSEP, is the heavyweight: 115 metres between perpendiculars, 50 metres beam, four triangular truss legs 106 metres long and a free deck of 3,750 square metres. Its main Liebherr crane is listed at 1,200 tonnes at 25 metres, and the page says the unit had installed turbines and foundations heavier than 800 tonnes. Kongsberg DP II positioning and four 3,000 kW thrusters give it 10 knots.

Both ORCA wind units list the Keppel OTD500FV-B electric jacking system. The ORCA pages define bare-deck variable load as the variable load the hull can support with no superstructure fitted, meaning no quarters, cranes or helideck. Operating variable load, afloat and elevated, is the more realistic figure: 1,750 tonnes for the 3500-WIND and 2,700 tonnes for the 4500-WIND.

The size race

Bennett drew its wind units for 4 to 6 MW turbines. The industry kept going. By 2019 the next generation of vessels under construction could lift 3,000 to 5,000 tons to 160 metres, and analysts expected a shortage of vessels able to install turbines of 10 MW and more. The Wikipedia article on installation vessels counted 16 WTIVs in 2020, with 23 expected by 2023, seven of them able to handle the largest turbines. It quotes a vessel cost of around $335 million and a day rate of about $220,000.

In the United States the Jones Act adds a further constraint; the same article notes that it complicates the transfer of parts between ships and raises installation costs. Keppel began building the four-legged, Jones Act compliant Charybdis in Texas in late 2020 at a cost of $715 million, planned for the Revolution Wind and Sunrise Wind projects.

A sense of scale

Europe installed the first offshore wind farm, Vindeby in Denmark, in 1991. By December 2019 Hornsea One, 120 kilometres off the English coast in 22 to 73 metres of water, was fully commissioned at 1,218 MW and briefly the largest in the world; Hornsea Two overtook it in 2022. Construction of Hornsea One started in January 2018 and the last turbine went in by October 2019.

The jacking basics are on the jack-up page, and the soil risks that matter every time an installer moves to a new position are on the legs, spudcans and preload page. The liftboat page covers the smaller service units that maintain wind farms once they are built.

Next in the archive: Legs, spudcans and preload · Marine operating manuals explained · Restored page: Naval architecture