Feature

New generation offshore wind vessels come on stream


George Marsh

Part 2. George Marsh continues his look at the latest designs of offshore wind farm service vessels coming on stream.

This article is taken from the January/February issue of Renewable Energy Focus magazine. To register to receive a digital copy click here.

In part 1: Scores of wind farm service vessels (WFSVs) are currently serving offshore wind projects relatively close to shore. Wind farm operators have progressed beyond using vessels of opportunity, such as adapted fishing craft, to bespoke WFSV designs. While a few contractors still use single-hulled vessels, a typical WFSV today is a catamaran up to 24m long carrying up to 12 passengers...read the rest of the article here.

All-weather capability

Some wind farm operators believe that SWATH (small waterplane area, twin hull) and SWASH (small waterplane area, single hull) type vessels are the key to all-weather capability and can contribute to making offshore wind farms economically competitive. Danish boatbuilder Danish Yachts has pitched in with its SeaStrider SWATH, emphasising its stability in a large seaway. Its ability to transfer between towers at speed in severe conditions also reduces downtime for maintenance teams, the company says.

The 25m long by 10.6m wide craft can carry 24 passengers and five crew at a service speed of 21 kts. It has four submerged hull ‘bulbs’, one at each corner, and a high deck clearance over the waves.

Stability can be enhanced by pumping ballast water between tanks. Another key feature of this craft is that it is constructed from carbon fibre composite. “We are pleased to be working with Danish Yachts in the build of this series of WFSVs, with their depth of experience and knowledge in innovative carbon construction.” says vessel owner Fredrik Odfjell of Offshore Wind Services BV.

Another SWATH proponent is Australia's Austal. This company, already well known for its Wind Express series of aluminium catamaran WFSVs, has now augmented its range with the Tri-SWATH concept. The Tri-SWATH craft combines the benefits of a SWATH hull form with proven trimaran technology to provide a highly stable fast vessel, Austal says. Tri-SWATH, it adds, can be pinned to a turbine in higher sea states than its competitors, providing safe step-off conditions. The trimaran platform also provides a large working deck area for equipment, freight and/or an access system.

Meantime, British naval architectural consultancy BMT Nigel Gee has produced what it describes as a semi-SWATH concept, a cross between a conventional displacement catamaran and a full-SWATH. The first of two 28m craft, built for Turbine Transfers by Sepers BV in the Netherlands, has SWATH-style buoyancy ‘torpedoes‘, but these are located only just below the waterline so that draft is a modest 1.35m. The craft's small waterplane area constrains vertical motions. The vessel can carry 12 technicians and three crew, plus 7.5 tonnes of deck cargo and an access system, at up to 28kts. The second vessel is due for delivery in 2013.

Bow Innovation

The bows of a WFSV must both contribute to vessel speed by cleaving through the waters efficiently, and provide stability for the part of the vessel that serves as a platform for transfer to and from wind turbine towers. Much can be done short of going down the SWATH route, by paying close attention to the design of the forward sections of the craft.

Wave piercing bows, pioneered for use on large fast catamaran ferries, have inclined surfaces that enable them to pass through waves rather than up and over them. These are effective in securing high transit speeds and ride comfort, but are less convenient for maneuvering bow on to turbine towers.

Dutch shipbuilder Damen Shipyard has introduced its axe-bow concept as its answer to rapid transit in rough seas. The concept was developed in collaboration with the Delft University of Technology. Last July, two twin-axe FCS 2610 type catamarans destined for SeaZip Offshore Services were launched at Damen's Singapore yard. Four such vessels are, in fact, already serving in the North Sea/German Bight areas.

Damen says its innovation provides “superior seakeeping” and speed maintenance in rough conditions. In the axe bow concept the vessel's bow fans out vertically in profile to resemble the head of an axe, its forward cutting edge slicing through the waves. Because there are no severe gradations in bow buoyancy and the waterplane area is consistently low, peak vertical accelerations are reduced by up to 75% compared with a conventional bow, the company claims.

Consequently, an axe bow does not pitch violently or slam. However, because it has less resistance to vertical movement when the vessel is not under way, the concept is likely to suit craft which use a clamping mechanism to latch onto the turbine tower.

The axe bow also promotes speed and fuel economy by providing “the ultimate in fine entry”, which is maintained over the full height of the bow, reducing wave resistance by some 60%. This results, says Damen, in a 20% cut in fuel consumption. The first of two of its high-speed service vessels has gone to Marinco of Scotland for wind farm service.

Another approach to securing a smooth passage in rough seas is the active ride control system, in which horizontal fin appendages are continually adjusted in inclination to counter vessel motions whilst under way. The previously mentioned Austal Wind Express catamarans are noted for their use of this system.

Stretching this foil principle much further takes one to the hydrofoil, a type of craft which, once up to speed, skates along the top of the water on foils, using hydrodynamic lift to keep the body of the hull out of the water. This results in a drastic reduction in drag and higher speed. A recent order by Mainprize Offshore Ltd of Scarborough, UK for two Technicraft-designed offshore support vessels from the Buckie Shipyard in Scotland will result in two 24m foil-assisted catamarans with semi wave-piercing bows.

Fuel efficiency

As wind farms migrate further offshore, fuel efficiency moves progressively higher up the priority list. This accounts for an emerging trend towards the use of weight saving composite materials for primary hull and deck structures.

Companies like CTruk in the UK, Danish Yachts, Spain's Mercurio Plastics, Sweden's Kockums Shipbuilders and Fintry Marine have produced WFSVs in advanced composites. A proportion of craft made by South Boats and others are also produced in glass reinforced plastic, long a boatbuilding mainstay.

CTruk claims that its resin-infused composite craft are up to 40% lighter than its competitors, making them highly fuel efficient. This company has recently added a SWATH design to its more conventionally-hulled catamaran range.

Finally, it should be noted that the catamaran hull form, though dominant for WFSVs, does not have everything its own way. The trimaran alternative has already been mentioned, and there are certain operators that still favour the monohull.

For instance, French crewboat builder Chantiers Allais has entered the wind farm market with its Wind Farm Surfer, a 24m aluminium hulled craft that combines a fine entry forward and a rising chine to produce a comfortable ride in head seas. V-sections in the forward lower hull further cushion the ride while contributing to speed with semi-planing capability.

Access to wind towers is available from either the bow or from the side of the vessel. Speeds of 25 to 30 kts are claimed and a range of 230 miles with full load is consistent with operations to the more remote offshore wind farms that the future will bring.

Explanation of terms

  • A displacement hull is one that sits in the water and progresses by pushing water aside.
  • A planing hull has flat sloping bottom sections that enable it, once a certain speed is reached, to aquaplane over the sea surface. Because most of the hull is not immersed, a planing hull can travel faster than a displacement hull.
  • A semi-displacement hull/semi-planing hull, is a cross between the above two, having a modicum of flat sections in the hull bottom so that it has some planing capability and can escape the hydrodynamic speed limitations that apply to a full-displacement hull. Because these limitations are directly related to waterline length, the only other way to make a full-displacement hull potentially faster is to increase this length.
Waterplane area

Because waves occur at the interface between sea and air, the movement they impart to a hull depends closely on the hull's waterplane area. This is the area of a horizontal slice of the hull taken at water level. Higher and lower levels of the hull are less directly impacted by waves. A typical WFSV hull today has a high waterplane area due to it being voluminous enough for load carrying. This gives it a tendency to ride up and down with the waves.

A SWATH (small waterplane area, twin hull) type hull, on the other hand, is less prone because the vessel's buoyancy is concentrated into bulbous torpedo-like hulls that ride deep in the water below wave action and are joined to the main deck, which rides above the waves, by thin sidewalls. The waterplane area, where the sidewalls meet the water level, is small, so minimising response to wave action.

About George Marsh: Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.

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