There are several potential choke points on the road to meeting the UK Government's aim of 33 GW offshore wind capacity by 2020. These include insufficient supply of wind turbines engineered for the offshore environment, lack of appropriate grid capacity and interconnections, skill shortages and the perennial issue of slow consent and planning processes.

One of the crucial issues is the marine supply chain. Transporting turbines of 3-5 MW to offshore locations, erecting, installing and maintaining them throughout their service lives, and finally dismantling them, calls for seafaring assets ranging from crane barges and tugs to jack-up rigs and fast service vessels.

Closely connected with this supply chain is the question of port facilities; it is rare to have a combination of sufficient space to park acres of turbine components and transporters, quayside facilities for loading, deep water for installation vessels and berths for fleets of service craft. Certain ports must also accommodate wind turbine assembly plants since the largest structures will have to be produced at the same place they are shipped from.

Transport and installation

In terms of floating assets, the most critical link in the supply chain is the need for transport and installation vessels (TIVs). It is common practice to adapt vessels chartered from the offshore oil and gas sector. But this source is unreliable since the vessels become scarce whenever oil prices rise and there is a growing need to decommission installations where oil and gas reserves have been fully exploited.

Moreover, the vessels are rarely optimised for a wind energy role and tend to be available only at the high charter rates that the oil and gas industry has become accustomed to.

So far, there has been a tendency to install offshore wind farms in relatively shallow water using jack-ups. These structures – either towed flat barge-like craft or boasting self-propelled ship-shaped hulls – set down legs to the sea bed and jack themselves up to provide a stable working platform.

As farms extend to deeper waters, the emphasis will shift to platforms that are permanently floating or semi-submersible. In the latter case, wave disturbance is minimised since the buoyant parts of the vessel are below the sea surface and the working platform is steadier. Vessels must be equipped with cranes tall and powerful enough to erect lofty turbine structures. These can either be bespoke or fitted permanently to the host vessel or temporary land-based structures.

Turbines of modest size can be erected on land and taken by vessels out to sea. They may be carried upright, which raises stability challenges, or horizontally, which requires a capacious deck.

For large turbines deployed in deeper waters where floating crane vessels must be used, the turbine and tower are normally erected offshore, piece by piece. In these cases, self-powered ship-shaped vessels with rotating cranes are in high demand because they offer good overall performance. Along with flat-bottomed barges, however, they can be prone to weather delays. Rotating cranes are also more expensive than non-rotating sheer leg cranes, which are widely used in shallower waters.

Heavy-lift TIVs tend to be large and complex so building one takes years rather than months. Unsurprisingly, investors require strong assurance of potential profit before commissioning new vessels. There is therefore a danger that availability might lag behind demand, causing vessel shortages as the pace of offshore installation increases. However, industry insiders are becoming more optimistic and believe the sheer scale of envisaged capacity will make wind a multi-billion dollar industry able to attract the necessary investment.

Peter Madigan, Head of Offshore Renewables at RenewableUK (formerly the British Wind Energy Association), points out that the UK's Round 3 activity alone is expected to add up to 25 GW to the country's installed wind energy capacity and that a portion of Round 2 has yet to be delivered.

“A few years ago there was much concern about potential supply chain shortages,” says Madigan. “Things have moved on since then and, particularly with Round 3 getting underway, investors can see the scale and commitment are there and are more likely to come forward with the necessary finance.”

A ship-shape selection

One of the first purpose-built TIVs was Mayflower Resolution, subsequently renamed MPI Resolution. This vessel was completed in China in 2003 and cost US$75 million. It is a self-propelled six-legged jack-up with a ship-shaped hull that enables it to travel to offshore locations at speeds of up to 10.5 knots. Operated by Yorkshire-based MPI Offshore, it has helped construct several wind farms including Barrow, Lynn and Inner Dowsing, Robin Rigg and Thanet.

Resolution is 130.5 m long and 38 m wide, boasts 3200 m² of cargo area and can carry three offshore wind turbines of up to 3.5 MW. Propelled by diesel-powered azimuth thrusters, it can be manoeuvred precisely into position with the help of a dynamic positioning system before being jacked up clear of the water.

A primary offshore electro-hydraulic crane can lift 300 tonnes at a 25.5 m radius and 50 tonnes at a 78 m radius, while a second crane can lift 50 tonnes at a maximum 35 m radius. Pumping around 9 tonnes of water between internal ballast tanks helps preserve stability while lifting. A remotely operated vehicle (ROV) used for laying cables is also on board. Resolution normally has a 34-strong crew and can accommodate up to 36 installation personnel.

This bespoke TIV has proved so useful that two more vessels have been ordered. MPI Adventure and MPI Discovery, both currently being built in China, are slightly larger than Resolution and will be even more capable. They are also based on self-propelled conventional ship hulls with jack-up legs, and each vessel will have a 1000 tonne lift crane compared to the 300 tonne crane on Resolution. They are expected to start working on London Array in 2011.

Also contracted for London Array is the jack-up barge Sea Worker, which is operated by Danish offshore contractor A2SEA. Sea Worker has four 73 m legs, which enable it to work in water depths up to 40m. The vessel has a state of the art leg penetration system (LPS) and air-cooled axial jacking engines so it can operate on sites that dry out at low tide, when seawater cooling can no longer be used. A FAVCO offshore rotary crane provides lift.

Sea Worker recently completed an installation of Siemens 3.6 MW turbines at Gunfleet Sands. A sister A2SEA vessel, Sea Jack, has installed 100 monopiles and 14 transition pieces for Vattenfall's Thanet offshore wind farm and is to install Siemens turbines at the Great Gabbard farm. Thanks to its size and a Manitowoc Ringer crane, Sea Jack can handle large components for wind turbines up to 5 MW. It can undertake complete installations, including foundations.

Two other A2SEA vessels – Sea Power and Sea Energy – are four-legged jack-up barges that the company describes as ideal for installing turbines in shallow waters while providing a cost-effective solution for turbine servicing. A2SEA, which since its foundation in 2000 has become a leader in offshore wind farm installation, has a still larger vessel due to join its fleet soon. Sea Installer will have a high transit speed, enabling it to reach sites further offshore more quickly, and is designed to operate in challenging offshore conditions.

A2SEA, which is now owned by DONG Energy, has its own crew and says that control and precision in lifting heavy items are key to its operations. It also operates service vessels and has pooling arrangements for crane vessels, jack-up barges, cable repair vessels, crew boats and accommodation vessels to share its assets efficiently.

Accommodation vessels are generally associated with oil and gas operations but, according to Madigan, they may become pertinent to wind energy as activity moves further offshore into deeper waters:

“When installing heavy turbines well offshore, the style of operations might come to resemble that in the oil and gas sector with helicopter movements and people living out on site on accommodation barges,” he says.

Shuttle innovation at sea

Madigan also believes the specific requirements of offshore wind will give rise to innovative structural forms for turbines and foundations, and that TIV innovation will have to keep pace. Such innovation is already evident in Huisman's concept for a wind turbine ‘shuttle’ designed for safe operation in waves up to 3.5 m without the need for jacking.

The Huisman Shuttle achieves this by exploiting the SWATH (Small Waterplane Area Twin Hull) principle in which twin slender hulls located below the surface (away from wave influence) are connected to a working platform above the surface by sidewalls. SWATH limits vessel motions, particularly roll.

The Huisman Shuttle is very stable and can carry two assembled 5 MW turbines, complete with towers and rotors, in an upright position. Transporting turbines that are fully erected, commissioned and tested onshore greatly reduces offshore commissioning time, while the ability to maintain operations in choppy sea conditions minimises expensive downtime.

The shuttle is equipped with two variable-pitch propellers so it can achieve transit speeds of 14 knots and, once on site, saves more time by not having to be jacked. Huisman points out that such a vessel can be used for returning complete turbine installations to shore for maintenance, repair or upgrading, as well as for wind farm installation duties.

On site, a mooring system is used to connect the vessel to the turbine foundation, while a dynamic positioning system helps the mooring system keep forces within defined limits. Huisman applies active heave compensation to onboard cranes and other lifting devices to counter residual heave motions. Active longitudinal compensation is applied to a trolley clamp that holds the wind turbine to eliminate pitch and surge motions. At the base of the tower an active controlled horizontal movable X-Y clamp positions the lower part of the wind turbine above the foundation.

Huisman has not yet announced any sales of its shuttle system, although it continues to sell its turbine installation cranes. It has secured an important contract with Centrica and MPI Offshore to supply and fit a 600 tonne crane aboard MPI Resolution. Centrica intends to deploy the upgraded TIV to its 270 MW Lincs wind farm project in Lincolnshire.

Meanwhile many more new TIVs are being ordered. RWE Innogy recently placed an order for a self-propelled vessel from Daewoo Shipbuilding & Marine Engineering, with options for another two vessels. Fred Olsen Windcarrier has ordered two bespoke TIVs from UAE-based Lamprell PCL, due in 2012. Each of these will feature high-speed jacking, a dynamic positioning system, 12 kilotonne transit capability, accommodation for 80 personnel and an 800 tonne crane. Various shipbuilders are eyeing the TIV construction business and a few, including Aker Philadelphia Shipyard, have declared an intention to construct vessels.

Vessels for service

Another area where seagoing assets are increasingly bespoke is service vessels for transporting personnel and equipment to and from offshore sites. These vessels are used during installation and throughout the operation and maintenance period, so their key requirements are high transit speed, good seafaring abilities and platform stability. Acquisition and running costs are also important.

The hull form best able to meet these requirements is arguably the catamaran, which makes up the majority of new service vessels. A landing operation is typically carried out by presenting the heavily-fendered bow of a vessel to friction posts on the monopile and keeping it there with forward propulsion while personnel transfer to a landing platform.

Current experience indicates that, for many craft, this method is only viable in wave heights up to about one metre. As a result, various enhancements have been tried including transfer via a motion-compensated telescopic covered gangway so that the service vessel does not have to be in direct contact with the pile. Deploying such devices would, however, require larger vessels.

As in the offshore oil and gas sector, vessel designers now have to cater for greater offshore distances while providing increased comfort so workers do not arrive on site seasick and exhausted.

The WindCat Mk4, designed primarily by Dutch company Vripack and being built by Bloemsma & Van Breemen, is designed to carry 50 people up to 300 miles offshore and offers considerable stability in a seaway. It includes a lounge with computers and Wi-Fi to keep passengers occupied during lengthy transits, as well as sleeping cabins, a galley and showers. A reinforced flush foredeck can carry equipment such as generators and a 20 ft cargo container. Two MTU V8 engines provide a top speed in excess of 30 knots.

The vessel is expected to be launched in July for delivery to WindC at Workboats, a leading provider of offshore crew transfer vessels which expects to be able to maintain operations in wave heights up to 2 m with the new craft. A feature of this and other vessels in the company's fleet is a proprietary ‘step over’ system that enables personnel to step safely onto a turbine platform because vertical movement of the vessel against the structure has been eliminated.

Safety is a top priority for Joint Managing Director Neil Clarkson, who is proud of his company's role in refining the technology of offshore workboats. For example, three types of propulsion system are in use on its vessels – fixed pitch propellers, variable pitch propellers and water jets – as the company evaluates their suitability for different sites.

“Our fleet comprises 20 working vessels and we have 7 under construction,” says Clarkson. “It's a very modern fleet and we are constantly upgrading the design.”

Another significant provider of wind farm service vessels is MPI Workboats, a wholly owned subsidiary of MPI Offshore. This company also relies on fast catamarans, in this case built by Isle of Wight-based South Boats. It has three 15 m-long craft and one measuring 20 m, all made of aluminium and built in the past two years.

The largest craft, MPI Don Quixote, can carry up to 15 passengers and three crew around 500 miles at a speed of 25 knots. Propulsion is provided by two MAN diesels engines driving twin Ultra Dynamics water jets. Personnel disembark onto offshore structures via a bow platform, while a Palfinger folding knuckle boom crane is used to transfer equipment. According to South Boats' sales director Ben Coleman, it is possible to carry out transfers in wave heights in excess of 2 m.

Extensive electronic fittings in the vessel include GPS, full offshore-class VHF communications, gyrocompass, radar, chart plotter, autopilot, 3G wireless internet and locator beacons. Passengers seated in sprung crew seats travel in air-conditioned, acoustically isolated accommodation and have access to cooking facilities and en-suite cabins.

A design change in recent vessels has an enlarged foredeck so that a 2.8 tonne crane can be placed on it. This is used for loading equipment from quaysides onto the foredeck, from which it can be offloaded by cranes fitted in turbine nacelles. A deck storage area is incorporated in the superstructure.

A 12 m vessel recently built for operator Offshore Wind Power Marine Services (OWPMS) has been designed as a rapid response vessel able to replace rigid inflatable boats and other small craft that were not originally intended for the role. OWPMS operates its catamarans on wind farm projects including North Hoyle, Rhyl Flats, Gunfleet Sands and Thanet.

Other operators of South Boats' Wind Farm Support Vessels (WFSVs) include Holyhead Towing, Turbine Transfers (a Holyhead subsidiary) and E.ON Climate and Renewables. Growing demand has seen the Isle of Wight company expand its production capacity to around 16 WFSVs a year measuring between 12 m and 20 m in length.

Challenges and opportunities

Potential supply shortages are clearly a threat both inside and outside the UK. As a result, the European Wind Energy Association (EWEA) has joined forces with the European Shipyards Association to lobby the European Commission and European Investment Bank to support the building of new ships. They argue that US$7.87 billion of investment in new TIVs plus substructure handling and cable laying vessels are needed for the predicted growth in offshore wind energy.

“Eventually, we are likely to see 40,000 MW per year built offshore,” says Eddie O'Connor, Secretary of EWEA. “This will require 10 to 12 new heavy lift vessels for transporting foundations, towers, nacelles and blading systems, plus numerous service vessels. New ports will have to be built across Europe.”

On the other hand, the very fact that such high levels of investment are being discussed suggests that there are still there are major opportunities for the marine supply chain. It is up to the producers of vessels and equipment, as well as the maintainers and suppliers of associated services, to step up to the plate and seize them.
Vitae

About the author:

George Marsh is technology correspondent for Renewable Energy Focus magazine.