This article is taken from the January/February issue of Renewable Energy Focus magazine. To register to receive a digital copy click here.
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.
Catamarans have become dominant because of the roll stability provided by their twin hulls and the large area of working deck that joins the hulls. Such craft have shown themselves capable of working in brisk weather conditions, including significant wave heights (Hs) of up to 1.5m.
But the WFSV sector now faces a stiff challenge. It now needs to cater for the more distant offshore environment where conditions are often rougher. Vessels able to operate in waves up to 2.5m Hs will be necessary to secure viable wind farm economics.
For destinations such as UK Round 3 sites, vessels need to be larger than the craft currently serving Rounds 1 and 2. They need greater carrying capacity, seaworthiness and steadiness during the critical moments when technicians and equipment are transferred between the waveborne vessel and the seabed-rooted turbine structure (new designs of turbine access systems to meet this need are also essential). Equally, the economic justification for a new vessel must rest on a combination of transit speed, fuel efficiency and ride comfort – the latter so that personnel can arrive on site ready for work rather than weary and seasick.
However, so far there is little consensus about what form the next generation of long-range offshore WFSVs will take. Currently, vessel operators and their naval architects are backing a range of different vessel concepts and are focusing strongly on hull form.
New solutions required
A typical hull form at present is that championed by South Boats, based on the Isle of Wight, off the UK's central south coast. When he founded the company 14 years ago, Clive Jefferies took a conventional semi-displacement, high-volume load carrying catamaran hull form but improved it with a novel bow in which a fine entry at water level flares outwards and upwards so that buoyancy increases progressively as the bow sits deeper in the water. This resists the tendency (seen in some other designs) for the bow to dig into a wave which can cause rapid deceleration and ‘broach’ as the vessel turns, out of control, sideways on to the waves.
The high freeboard (height of the hulls above the waterline) that gives the hulls ample interior volume also means that the connecting deck is high. This decreases the tendency of waves to slam against the underside of the deck between the hulls, another issue with early catamaran designs. Knuckles and chines located strategically along each inner hull further mitigate this as well as controlling spray.
South Boats vessels have established a strong reputation for seaworthiness and ability to maintain high transit speeds in turbulent conditions. However, with Round 3 and other far-offshore demands in prospect, company managers are aware that a higher level of challenge may require radical solutions. Recently, in conjunction with Seaspeed Marine Consulting, the company has carried out an extensive hydrodynamic study of hull forms so it can better optimise designs for particular routes, duties and sea conditions.
One possibility is to adopt the SWATH (small waterplane area, twin hull) concept.
SWATH vessels have limited vertical movement in rough seas, whether under way or when deployed against turbine towers. “We promote SWATH vessels for minimum movement,” says Nils Olschner, sales director commercial at vessel builder Abeking and Rasmussen (A&R). “Vertical accelerations in particular are four to five times lower than on a conventional monohull.”
A&R concedes that, being displacement craft, SWATHs may not be as fast as planing vessels of similar length. The sea-keeping qualities, however, are impressive, it says. Because these vessels are less prone to those motions that most powerfully induce seasickness, passengers travel in greater comfort and arrive at task locations fresh and ready to work.
The design has proved a hit. Natalie Beker, a 25m SWATH craft that has been operating for more than two years with BARD Engineering, has demonstrated its offshore merits at the BARD 1 wind farm off North Germany, and in the North Sea. “We retain deployment capability even in bad weather, guaranteeing the high availability of our offshore wind power station,” notes BARD Services CEO Jorg Fangmann.
The vessel is said to permit direct boarding in waves of 2.5m Hs. A wide beam of 13m enables personnel to step over from a full-width, well fendered rail-protected foredeck without a need for the craft to be clamped on to the tower or for an advanced non-contact access/boarding system.
Given its size and steadiness, Natalie Beker can be kept on station at a wind farm and its accommodation enables working squads to remain on board, so avoiding time and fuel consuming shuttles to and from shore. This ability will be more in demand, the shipbuilder believes, as O&M operations move ever further from support facilities ashore.
On the downside, as well as being limited to displacement hull speeds, SWATHs are more expensive than conventional catamarans. A&R has tackled this issue with a lower-cost alternative dubbed the SWASH. This small waterplane area single-hull variant features a single torpedo-shaped bulb below the surface, providing sufficient buoyancy to support, via a central member, the main deck and superstructure. The single-hull vessel is stabilised by outriggers. With its shallower draft, the SWASH can overcome water depth limitations in smaller harbours and at shallower wind farms.
In part 2: All weather capability, bow innovation and fuel efficiency.
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.