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Vessel design adapts to changing demands

WORLDWIDE: As offshore wind farms move further from shore and increase in size, there is a growing demand for a new types of vessel that can withstand rough seas, cope with deep waters and fulfil several functions during installation and service.

Offshore wind farms in Europe are increasingly being built further from the coast and in deeper waters. Last year the average project in Europe was 23.4 kilometres from shore and set in waters almost 23 metres deep. Now the average project under construction is 33.2 kilometres from shore and in waters over 25 metres deep. This trend in turn has led wind-farm developers and operators to push the frontiers of vessel design and access logistics as they face growing challenging of moving equipment and people to new and often hostile locations.

Distance from shore and water depth do not always correlate. Deep waters can be found in some near-shore locations in Portugal, Spain, North America and Japan, while at sites far from shore in the southern North Sea - where new wind projects are being planned - the water remains relatively shallow, allowing standard fixed-foundation turbines to be used.

Regardless of depth, however, increased distance from shore means frequent trips back to port are no longer an option and access becomes far more dependent on the weather. For installation, far-shore operations mean a new breed of vessels.

The vessels required to construct a new wind project are diverse. In the past 12 months, vessel broker DSB Offshore has been chartering to wind farm sites: an anchor-handling tug supply vessel to support a cable-lay barge; a shoalbuster that can operate in shallow waters; two flat-top barges for moving large components such as turbine towers and blades; four crew-transfer vessels, a bird-survey vessel; and numerous multi-hulled catamarans to take technicians to turbines.

Single platform, multiple access

But with distance comes a growing school of thought that sees a role for single-platform vessels that provide support throughout the wind-farm cycle and can be deployed around the clock for weeks on end. These vessels, with their multiple access systems, aim to help developers and operators derive maximum revenue from their assets by keeping the turbines turning.

In anticipation of a demand, UK-based SeaEnergy Marine has designed two new vessels - a 76-metre ocean-going ship and a larger, 95-metre version with a helicopter deck on the bow - to meet the accessibility challenges of far-shore wind farms. The inverted hull, designed by Ulstein, aims to give better stability in rough seas and a stable platform for access by a heave-compensating gangway such as an Ampelmann. It can also be used to launch smaller daughter craft carried aboard the mothership. The bigger vessel features high-tech dynamic positioning - a navigation system to place and keep the vessel in a precise location - with accommodation for up to 60 people.

The vessel can stay on site far from its home port for weeks, or months with replenishment and crew changes, and can be equipped with options such as cement tanks, a workshop and spares, as well as fast-rescue craft and a crane.

At least two other European shipbuilders have similar far-shore multi-purpose vessels on the drawing board: German-based FRS and Danish Esvagt. The 72-metre FRS accommodation and support vessel addresses the rigours of far-shore and deep-water work locations with rough seas, with a flexible multi-purpose platform. It is capable of a cruising speed of 14.3 knots, has a winch zone for transfer via helicopter and crew transfer vessels.

The design offers flexible accommodation to cater for different needs throughout the various phases of construction - growing from 38 berths to around 50, by adding accommodation containers on the desk, for periods of high activity, such as during simultaneous scheduled maintenance.

Multi-purpose has a role

While some in the industry believe that giving up vessel specialisation will ultimately increase costs, a degree of multi-functionality seems unavoidable when vessels stay out at sea for long periods, at least to the extent of providing accommodation and berths for daughter vessels.

The size of future projects will also add to the logistical challenges, believes Ed Dudson, technical director at vessel designers BMT Nigel Gee. "The current fleet of boats serving offshore wind is perfect for working 20 miles out, taking 12 people at a time with a 20-metre load line," he says. "Moving from 200 turbine wind farms near the shore to 1,000 turbine wind farms far away means that the number of people involved in servicing them will rise dramatically. We could be seeing 100 people stationed permanently offshore, working two-week-on and two-week-off shifts."

Transfer in rough seas from the accommodation ship or platform onto smaller inter-turbine transfer vessels (ITVs) to reach the individual wind turbines is another challenge. "The ITVs themselves need to be both nimble and agile and able to cope with heavy seas," Dudson says. "Vessels that have better handling characteristics in rough seas tend to be heavier, and so slower."

Solutions for large sites

A fixed platform in the middle of a wind farm as large as Dogger Bank would be 80 kilometres away from the eastern and western edges of the project, a distance that could see a crew vessel cope with up to two hours travel in rough sea to get personnel to a site. Floating accommodation, on the other hand, would be able to move around the wind farm site according to the work schedule. This then provides shorter transfer times for the ITVs, which in turn means that they can be slower but more stable. The chances of personnel turning up at the turbine capable of putting in a day's work becomes increasingly more likely.

How does Dudson visualise the 2020 North Sea offshore wind fleet? "It will be diverse," he says. "Fast, large transport craft similar to cross-Channel catamarans operating on the route between the UK and France will take people and equipment out to accommodation platforms, jack-ups or motherships. Another suite of smaller vessels, ITVs, deployed either from cranes or vessel docks will work within the wind farm. This will be augmented with some air support."

Rob Grimmond, managing director of offshore design, installation, operations and maintenance company Offshore Marine Management sees advantages in working in deeper waters. "It opens up the supply chain and allows us to use deeper-draft vessels with better control than the shallow draft barges we have to use inshore," he says. "An eight-metre draft vessel tolerates more weather and is more stable."

In the German Bight in the southern North Sea developers and operators are already working comparatively far from shore. "In the German sector we are looking to get more technicians on fewer boats because of the distances involved," Grimmond adds. But the very large projects such as Dogger Bank might need a different approach. "We might find that having one mothership with accommodation in the middle of a large wind farm isn't enough," he says. "Problems often arise in different sectors simultaneously and the mothership could be a long way from where remedial work is needed. Better to have two boats with six crew, than one with 12," he suggests.

One option will not suit all far-shore sites but, on balance, Grimmond thinks fixed platforms are a good solution, perhaps two or three for a site. "Two weeks on, two weeks off will become the new catchphrase," he says. "Eventually people might stay out offshore even longer, perhaps for a month at a time."

JACKING UP HOW DEEP CAN YOU GO?

A key element in the installation of offshore wind projects is the jack-up vessel, a powered barge or ship with four legs that extend to the seabed at a desired location to provide a stable platform for operations. They can be kitted out to provide mechanical, pneumatic or wired services for site investigation or installation of the foundation or turbine.

But in addition to facing potential operational restrictions at far-shore sites through poor seabed conditions, powerful currents or extreme weather, will there be a limit beyond which jack-up vessels can no longer be used to aid installation as wind projects move into deeper waters?

Not around Europe, says a recent report by marine drilling consultants and contractors Fugro Seacore. According to the report, today's jack-up vessels used for supporting offshore wind can operate up to a depth of about 45 metres - well within the scope of current and planned offshore wind projects in European waters.

While anything deeper would be beyond the scope of jack-up ships, it would also be beyond the scope of current solid foundation technology. Floating turbines assembled in port, towed out to sea and moored to the sea bed would be the preferred option.

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