Vessels & Access: A to Z - A maritime guide to offshore wind

An alphabetic tour through some of the key companies, developments, innovations and challenges affecting vessels and other means of access to offshore wind projects, plus some of the organisations and agencies responsible for regulation.

A A2Sea

Established in 2000 when offshore wind was in its infancy, the Denmark-based offshore support firm has grown with the sector to become one of the foremost providers of vessels, crew and expertise for the transportation and installation of foundations and turbines. In 2009 the company was acquired by Europe's leading offshore wind developer, Dong Energy, and turbine manufacturer Siemens Wind Power took a minority stake shortly after.

The fleet's flagship, Sea Installer, a purpose-built jack-up vessel, has just been joined by the Sea Challenger, almost identical except that its crane has a lifting capacity of 900 tonnes - 100 more than its sister vessel. Sea Challenger will be working this summer at Dong's 210MW Westermost Rough project in the North Sea, the first large-scale offshore wind farm to use Siemens' new 6MW turbine.

B Bottleneck

One of the biggest problems looming for European offshore developers is the potential shortage of foundation and turbine installation vessels, especially those with the size and lifting capacity to handle the new generation of 5MW-plus turbines. Historically, the offshore wind sector often chartered vessels from the oil and gas industries, but there are likely to be few of those available in the second half of the decade, with a heavy programme of decommissioning and redevelopment of existing oil and gas fields under schedule. The effects of the bottleneck could be tempered by the growing number of bespoke installation vessels being commissioned and constructed (though they take three years to build), and the fact that several significant offshore projects have been cancelled recently. But the vessels required to manage the new supersize XL monopile foundations, in relatively deep waters far from shore, are in desperately short supply.

C Catamaran

The speed and weight advantages of the twin-hull design have made catamarans the offshore wind industry's design of choice for crew and equipment transfer. Most catamarans serving the sector are constructed from aluminium, which makes them less expensive than those made from composite materials. But the lighter composites provide the vessel with a greater payload, which makes them cheaper to operate. Typical of the latest designs entering service is EVOC22 (efficient versatile offshore catamaran, 22 metres in length) from CTruk, a firm based in Essex, south east England. The composite twin-hull design has a 7.6-metre beam, a 1.25-metre draft, and features CTruk's patented moveable wheelhouse and flexible deck pod system. This allows it to transfer up to 12 technicians in comfort, or to remove the passenger pod to create 72 square metres of deck space for equipment transfer. Power is provided by two 800hp turbocharged diesel engines, while its 24,000-litre fuel capacity gives it the range to service projects far from shore.

D Dynamic positioning

This allows vessels to operate when mooring or anchoring are not feasible. The computer-controlled system automatically maintains a vessel's position and heading using its own propellers and thrusters. Position reference sensors, together with wind and motion sensors and gyro compasses, provide information about the vessel's position, and the strength and direction of the forces affecting it. Dynamic positioning is widely used across the offshore sector through all stages of project planning and construction, in its simplest form of DP1 to the high end DP3 with back up and ability to withstand fire or flood. Offshore wind vessels use it for maritime research, surveying, diving support, cable-laying and, not least, turbine installation.

E Energy Institute

The UK's professional body for the energy industry. In January 2013 it announced plans to collaborate with the G9 Offshore Wind Health and Safety Association to develop a programme of good practice and guidance on health and safety issues. The G9 was established in 2010 by nine of the world's biggest offshore developers - Centrica, Dong, E.on, RWE Innogy, Scottish Power Renewables, SSE, Statkraft, Statoil and Vattenfall - with the aim of delivering improved standards. The institute's knowledge director Martin Maeso describes its role in the G9 collaboration as that of "honest broker", ensuring a common approach to the health and safety issues affecting the offshore sector. The first fruits of the collaboration have been the publication of good practice guides for marine operations, lifting operations, and working at height.

F Floatel

As offshore wind projects grow in size and complexity and move further out to sea, the need for large accommodation vessels, or "floatels", has grown rapidly. Installation and construction crews live aboard, usually adopting the two-week on/two-week off shift rota practised in the oil and gas industries. Turbines access is gained using daughter craft or heave-compensated access systems. Converted cruise ferries are widely used in the floatel role, but the extensive capacity of ro/ro ferries also makes them popular. Floatels can accommodate around 100 workers at a time, and their facilities range from conference and meeting rooms to swimming pools and lounge areas.

G Gangway

In the pioneering days of offshore development, wind engineers gained access to turbines from a vessel that would be connected to the transition piece near the turbine's access ladder, and kept as still as possible. It was difficult and potentially dangerous for the engineers, and there was always a risk of damage either to the vessel or turbine, especially in high wind and waves.

Motion-compensated, telescopic gangways made from lightweight aluminium, linking the vessel to the turbine platform have made this task much safer and easier. Engineers can now "walk to work" from vessel to turbine. Gangways can be specified with closed roofs to protect personnel from falling objects (ice, mainly), and with trolleys and pallet carriers for parts and equipment.

H Helicopters

By far the quickest way to transport an engineer from land to a faulty turbine at sea is by helicopter, especially far from shore and in rough seas. Of course, helicopter use is subject to weather conditions too: when visibility falls below three kilometres, the cloud base is below 200 metres, or the wind speed is above 60 knots.

Their use has been made a little easier of late with the new generation of 5MW-plus turbines generally being fitted with purpose-built hoisting platforms, although that still requires engineers and components to be winched down from the hovering aircraft.

I Irish Sea

Developers will learn a great deal more about the Irish Sea over the next few years as a number of large projects move from planning to construction. These include the Rhiannon wind farm developed by Dong and Centrica, which could provide the first 2.2GW of the Celtic Array's potential 4.2GW. Dong is also involved in the 600MW First Flight Wind project, off Northern Ireland's County Down coast. The Isle of Man, with British Crown dependency, sits smack in the middle of the Irish Sea and is also considering projects in its territorial waters. The Irish Sea has plenty to recommend it - good wind resources, shallow depths, easy access to large ports - but it is also home to a wide range of wildlife, including leatherback turtles and dolphins, while its estuaries are vital feeding grounds for migrating shorebirds.

J Jack-up vessels

Originally designed for the oil and gas industries as exploratory drilling platforms, jack-up vessels for the offshore wind sector are essentially a buoyant hull with four or six extendable legs that reach the sea floor and can raise the hull over the sea's surface. From this position they create the platform from which the foundation can be driven into the seabed, and the turbine installed on top. The jack-up rigs used by the oil and gas industries generally have to be towed to their locations, but offshore wind is better served by self-propelled vessels.

K Knud E Hansen

Headquartered near Copenhagen and established in 1937, Knud E Hansen is a ship design and naval architecture firm, now taking a keen interest in offshore wind. It was responsible for the design of Pacific Orca and Pacific Osprey, two self-propelled jack-up turbine installation vessels commissioned by marine services company Swire Blue Ocean. The vessels, recently used at the 398MW West of Duddon Sands UK project, feature six truss-type legs and can jack to a safe height of 22 metres above the sea surface based on a water depth of 75 metres. Both have a 1,200-tonne leg-mounted crane.

L Leg-stabilised crane vessel

Effectively a scaled-down jack-up vessel, suitable only for the installation of small, 2MW-type turbines in shallow waters, these vessels are something of a rarity in today's offshore sector. A2Sea ran two of this type, Sea Energy and Sea Power, which were both converted to operate in offshore wind and retrofitted with the necessary equipment. A2Sea has since sold Sea Energy, but Sea Power remains in service, though it is now more likely to be used in the operations and maintenance field.

M Maritime Coastguard Agency

Responsible for provision of maritime safety information to ships in UK waters, the Maritime Coastguard Agency is also a formal consultee in the consent process for all UK offshore renewable energy installation - wind and tidal. The agency issues technical guidance and advice on all aspects of navigation safety for offshore developers.

N North Sea

The high wind resources of the North Sea make it an attractive proposition for offshore developers. But progress has been slower than anticipated. The main problem for German developers, who are required by law to build 12 nautical miles from shore, is that this has forced them to construct their projects further from shore and in deeper water than was the case with the earlier offshore wind farms. This requires the use of specialist vessels, capable of operating in significant wave heights and often in very challenging weather. Several of the North Sea projects have also been delayed by the discovery of large amounts of unexploded munitions on the seabed.

O Operations and maintenance

A wide range of weird and wonderful vessels have served the operations and maintenance (O&M) needs of the offshore sector in the past, but as the sector matures, vessels are becoming more specialised and fit for purpose. A new generation of small jack-up barges, built specifically for O&M use and capable of operating in challenging weather, is now becoming available. A typical example is provided by DBB Jack-up Services, a Danish offshore service provider. Its new Wind Server vessel will be able to work at a significant wave height of 2.0-2.5 metres, rather than at the more recent industry norm of 1.5-metres. That will allow it to operate for potentially 320 days a year rather than 200, representing a sizeable cost saving.

P Propulsion

The diesel-electric powertrain is the offshore wind sector's favoured propulsion system. In essence, it is a diesel engine connected to an electrical generator that powers electric traction motors, delivering the power through azimuth thrusters (propellers that can be rotated at any horizontal angle). It provides vessels with better manoeuvrability than a fixed propellor and rudder system, and is essential to the use of dynamic positioning systems. It also operates relatively quietly - an important consideration for offshore work in wildlife-sensitive locations.

Q Quality and qualifications

Concerns that offshore wind development is proceeding at a pace beyond the supply chain's ability to deliver safely have been raised by the UK government's Marine Accident Investigation Branch (MAIB). It recently reported on an offshore service catamaran that crashed into a UK Royal Air Force practice target because the captain accidentally changed the route while showing a trainee how to use navigational equipment. It later transpired the captain did not hold qualifications to operate the vessel in UK waters. MAIB also reported on an accident - on the same day in November 2012 - when a service vessel crashed head-on into a turbine tower, injuring all five of the vessel's occupants. The offshore wind industry says an increase in accidents is a natural consequence of the greater number of vessels operating in challenging environments.


Remotely operated vessels are widely used by the offshore sector for a variety of jobs, the most important of which is "trenching" - cutting into the seabed to allow interconnecting power cables to sink in up to three metres deep. Cable installation has been one of the most problematic areas of offshore wind development. It is estimated that up to 80% of the sector's insurance claims are prompted by damage to cables during the installation process.


Or small waterplane area twin hull vessels, these are catamarans refined to minimise the cross-section area of the hull at the sea's surface where wave energy is at its highest. Most of the displacement required to keep the vessel afloat is located well beneath the surface where it is less affected by wave action. The result is a craft with significant stability advantages especially at high speeds in high sea. However, SWATH designs are more expensive to build and have higher maintenance requirements.

T Turbine Transfers

One of the offshore sector's leading operators of fast crew vessels. Based on Anglesey in north Wales, the company's large fleet of high-speed catamarans, ranging in length from 12 to 20 metres, have been chartered by utilities, developers and turbine makers to work on projects in locations ranging from the Irish to Baltic Seas and including the vast London Array in the North Sea.

U Uncertainty

The commissioning of the vessels that the offshore wind sector requires for future projects depends entirely on the confidence the developers have that their projects will get the green light, and that the funds to finance the construction of these huge and complex developments will be available. That confidence is in short supply, particularly in the world's leading producer of offshore wind energy, the UK.

V Vestas

Designed and built by Vestas, the first turbine from joint venture MHI Vestas, the V184-8.0MW prototype, is now in testing. This size of turbine is the future challenge for vessel suppliers, who are currently having to adapt to the demands of bigger 5 to 6MW units, such as Siemens and Alstom. Areva and Gamesa are also looking at an 8MW as a joint venture, and Sinovel a 10MW turbine. The installation challenges of such turbines, where the nacelle, head and blades add up to 500 tonnes, are yet to be addressed.


Walk-to-work is the name Damen Shipyards gave to a new crew transfer and accommodation vessel, a one-ship alternative to a floatel with daughter vessels. Designed to provide 80% availability in wave heights up to three metres, the W2W accommodates a crew of 20 and 40 management and maintenance staff. It can stay at sea for up to 30 days, and features dynamic positioning and telescopic motion-compensated gangways.

X XL monopiles

The standard foundation for offshore turbines in shallow waters, the steel monopile was thought to have reached its limit at water depths of 30 metres. But new developments in plate steel manufacture have opened way for monopiles with diameters up to ten metres, which can be installed in waters of twice the depth. However, the heavy so-called XL and even XXL monopiles pose further challenges vessels manufacturers.

Y Yellow

The lower sections of offshore turbines are required to be painted in high-visibility yellow so they can be easily seen by shipping. The tower should be painted from the level of highest astronomical tide to 15 metres, or the height of the navigational lights.

Z Zaratan

A self-propelled jack-up vessel commissioned and operated by Seajacks, a leading UK-based service supplier to the offshore wind sector. Zaratan accommodates 90 people and an 800-tonne, leg-encircling crane. Designed for harsh North Sea conditions, it has been used since July 2013 on the 288MW Meerwind project in German waters, installing the final turbine blade in April.