Vessels currently employed to transfer personnel and equipment to and from offshore wind turbines are basically similar in concept. A consensus has evolved around the catamaran hull form because it offers a useful combination of speed, sea-keeping ability and a large, relatively steady working platform for critical transfer operations.
This form having become a de facto standard, it has seemed that the conventional 12-passenger (regulations drive this number) fast cat might be with us ‘for ever’, unchallenged by any other concept. But that is now changing. The trend for new wind farms, hosting powerful turbines (6MW plus), to be located ever further offshore is altering the economic seascape and encouraging new ideas. Crew transfer vessels designed to serve this sector must still meet the highly divergent requirements of rapid transit with a steady transfer platform once on site, but with a greater emphasis on transit speed.
Consequently, a new generation of craft that will emerge over the next five years may differ substantially from today’s ‘standard model’. Some of the latest concepts were aired at the recent Seawork exhibition and conference held in Southampton in June.
Early speakers, including Andy Page of Alicat/South Boats and Andy White of CTruk emphasised the merits of the twin-hull configuration that is now so well established. Both explained that their firms had investigated various hull forms, including trimarans, monohulls, air-supported craft etc, but had settled on the catamaran as being stable, manoeuvrable and with large decks. Waterjets were a favoured form of propulsion, particularly for near-shore farms, conferring respectable transit speeds and high static push when butted up to a turbine.
White pointed out that of 325 vessels ‘out there’ 92% are cats and suggested that all their owners could not be wrong! Guy Whitaker of Missionkraft later described contemporary catamaran designs as high cube, low weight and fast. Other speakers, however, argued that cats are not the only answer, saying the time had come for new and radical solutions. The top priority now, they asserted, must be high transit speed so that expensive technicians can be conveyed offshore fast, maximising working time on site and minimising non-productive hours spent sitting in a passenger cabin.
Back to monohulls
Some in the room believed that the changing nature of the offshore wind sector will suit the re-emergence of monohulls. There was a time, in the earliest days of offshore wind farms, when converted fishing boats, fast launches and other monohulls were the norm. Those that were semi-displacement craft and had planing surfaces gained the ascendancy and paved the way for catamaran successors that could likewise boost their speed by planing across the sea surface. In general, though, it takes less power to drive a single long, narrow hull through or over the water than two hulls that may typically be shorter and squatter. Consequently a monohull can potentially be faster, more fuel efficient or a combination of both.
However, opponents of monohull solutions have long argued that the wide, steady working deck afforded by a catamaran cannot be achieved on a monohull unless the vessel is large. Stephen Beadsmoore from the Wave Access Project outlined an answer to this which epitomises lateral thinking. There is no law of nature that says a crew transfer vessel must butt up to a turbine transition piece by the bow; instead it could reverse itself against the turbine. In that case, a suitable monohull could be shaped somewhat like a wedge, having a fine (sharp) wave-cleaving bow progressively broadening to a wide stern and after-deck. Placing the latter against the turbine and using reverse thrust to keep it there could provide the required steady transfer conditions, especially if the hull also incorporated sponsons or other means to enhance transverse (roll) stability.
Such a solution is evidenced by Tenacity, a 25m long monohull built by Coastal Charters of Whitehaven Cumbria, UK. Beadsmoore told Seawork delegates that this and similar craft should be able to achieve 40 to 50 knots, twice as fast as many existing 12-passenger catamarans, as well as delivering improved comfort and economical fuel consumption. Tenacity has an axe-shaped bow that extends both well above and well below the hull extent and is designed to slice through waves of considerable height. It is waterjet propelled and also air-assisted, the hull’s wetted surfaces being lubricated by air to reduce hydrodynamic drag. Aluminium was chosen as the main constructional material due to its low weight, corrosion resistance, recyclability and local availability of aluminium fabrication skills.
The project has benefited from a £90,000 Investing in Business grant from a Cumbrian funding initiative, part of a larger sum derived from nuclear partners to the county, and some additional monies from the Technology Strategy Board. While Tenacity, which can carry 12 passengers and three crew, currently undergoes sea trials, a manufacturing facility is being considered to build further craft.
Cats with wings
A different approach, championed by Southampton, UK based designer Missionkraft, is to graft the hydrofoil principle onto a modified catamaran hull form. Conventional hydrofoils have been familiar around the world as fast ferries and other service craft, albeit mostly as monohulls. Hydrofoils are open to the objection that the foils and their struts are vulnerable to damage caused by objects in the water, either floating or fixed, a danger that might increase around wind turbine installations.
Missionkraft’s answer is to place adjustable wing-shaped foils between the keels of the twin hulls of a catamaran so that they are scarcely more vulnerable to damage than the hulls themselves. As the craft builds speed it rides up out of the water until the foils are supporting about 80% of its weight. This reduces hull resistance, enabling the vessel to travel faster or with less power. One of the firm’s designs, a 20m service vessel, would have a maximum speed of 35 knots when powered by twin 965hp diesel engines.
Missionkraft has also tried to improve ride comfort by widening the tunnel between the hulls so that the wave slamming often experienced there in many catamarans is reduced. It has also designed into the inner hull surfaces a series of steps which, like longitudinal strakes, further reduce slamming and ease the awkward roll motion to which catamarans are typically subject. As a result, this Technicraft design should maintain speed in rough conditions, combining all-weather performance with the ride comfort needed if technicians are to reach turbine sites fresh and ready to start work. This latter aspect, says the company, is a business-critical factor, the more so the longer the transit time.
Yet another concept is based on the SWATH (small waterplane area twin hull) principle. A true SWATH can be likened to a catamaran in which the two hulls are submerged and joined by sidewalls to the load-carrying deck, which is above the sea surface. Because the hulls ride below the agitated sea surface and the load deck proceeds above it, only the sidewalls experience the worst agitation and since these have only a small cross-section (waterplane area) for the waves to act upon, vessel motion is minimised. As a result, SWATHs are extremely stable when in position against a turbine, providing superior transfer conditions.
However, as one conference delegate with relevant experience put it, SWATHs are generally costly to build and more complex to operate than conventional service craft. They also tend to be slower, a considerable handicap for UK Round 3 and other wind farm developments that are well offshore. Other potential drawbacks include high draft and relatively high powering requirements. One way SWATHs can be characterised is to say they minimise motion and seasickness at the expense of requiring extra power – properties that are diametrically opposite to those of a conventional catamaran
One answer to reducing these opposing extremes is the semi-SWATH, a catamarans in which each of the twin hulls narrows at the waterplane and broadens into a more bulbous form under the water. A small waterplane area, and hence limited wave-induced motions, are thereby retained, but these vessels requires substantially less depth of water and drive power than a full SWATH. UK designer BMT Nigel Gee markets both its ModCat semi-SWATH, and the more extreme Semi-SWATH XSS. The latter is well on the way to being a full SWATH, though still something of a compromise. XSS designs are currently proving themselves in the offshore energy industry.
UK-based CTruk, claimed to be the second largest producer of crew transfer vessels for wind farm service, has launched a semi-SWATH based on a BMT Nigel-Gee design. It claims a combination of a catamaran’s load-carrying capability with a SWATH’s enhanced seakeeping for its 26m craft, which should achieve some 30 knots. CTruk has compensated for the extra power (over a catamaran) required to drive this type of vessel by building it in advanced composite, so minimising its weight. In addition, the resin infused structure is said to have fairer lines than an aluminium equivalent, so that hydrodynamic resistance is lower.
Riding on Air
Hydrofoils are an accepted means for enabling a craft to ride with its main body (hull) clear of the water, thus obliterating its wetted surface and hence hydrodynamic drag. But it is not the only method available. Another is to organise for a cushion of air to separate the craft from the sea surface and this is the principle behind the air cushion vehicles (hovercraft) that are well established where amphibious capability is required. Unfortunately, supporting the entire craft weight clear of the water carries penalties of high power requirement with complex drive arrangements and the need for flexible skirts to retain the air cushion in position beneath the vessel.
The so-called air supported vessel (ASV) concept avoids these complications by supporting only a proportion of the craft’s weight. Ulf Tudem from Norwegian firm Effect Ships International AS, a subsidiary of SES Europe AS, told his Seawork audience that up to 85% of a craft’s weight can be supported by an air cushion generated by axial fans within the craft, without the need for high-maintenance flexible skirts and multiple engines. He explained that an air supported vessel can have 40% less hull resistance than an equivalent conventional craft, enabling it to go faster for a given power while also providing a softer ride in a seaway.
ESI’s solution has both monohull characteristics, in that the forward section comprises a sharp wave-piercing bow, and catamaran features since the hull broadens to a wide beam further aft and has an underbody that includes tunnels for containing entrained air. Lift fans amidships generate air, which is injected under modest pressure into the tunnels and other structural cavities. Thus the air cushion is contained by solid entities which project into the water, unlike on a hovercraft where a flexible skirt is required to seal the gap between craft and water. A carbonfibre sandwich structure ensures low vessel weight so that there is less for the air cushion to support.
So far Effect Ships have built, with European Union funding assistance, a 20m craft in a fast ferry format and are trialing this vessel. Another ASV, with outline dimensions 18m by 5.2m, is under construction in carbon sandwich at Tuco Marine in Denmark. Design speed of this craft is 40 knots maximum. According to the Norwegian company, an ASV really comes into its own at substantial speeds. Crossover in efficiency between an ASV and a state-of -the-art catamaran is said to be around 35 knots on a 40m scale. At 55 knots, the ASV will, designers claim, be 40% more efficient and so require 40% less installed power (propulsion and lift fan) than a conventional catamaran CTV.
Five-year trends
Judging from the tone of the Seawork discussions, the next five years will see considerable change offshore. Steady migration of wind farms ever further from land will bring a need for craft that are larger, faster and more all-weather capable. Ability to maintain operations in higher sea states will be vital given the high costs of technical personnel and their transportation. Increasing the working window in this way will help limit overall expenditure
Already, we see a trend for catamarans, the present leading configuration, to become larger as, for example, with the 26m by 10m FCS2610 craft built by Damen Shipyards Group and a similar length craft from CTruk. Careful consideration for passenger comfort includes such features as shock absorbing seats and even resilient mounting of entire passenger compartments.
High personnel costs will also drive increased transit speeds, elevating this attribute to a leading priority, trumping even fuel consumption. Thus foiling craft, monohull or catamaran; air lubricated and air supported craft, innovative wave piercing designs and other concepts outlined above will be built, augmenting the diversity of hull forms and vessels serving this sector. Long monohulls with stabilising vanes or internal weights to counter roll motion may come into their own.
Matching the larger scale of vessel that will emerge will be the need to carry more passengers. For this to happen, the present somewhat artificial 12-passenger barrier, predicated by the present High Speed Code (HSC) for vessel design, will have to be overcome. This can be facilitated by accepting that the technicians who service offshore wind installations are, as one Seawork delegate put it, ‘more than just passengers’, and should be regarded as a separate category for regulatory purposes. This is justified by the high maritime sensibility that these frequent voyagers develop along with the fact that they are aware of potential marine emergencies and trained to act safely if they occur. A modified HSC or an addendum to it could, it is suggested, allow for at least a doubling of the permitted number of these special-category passengers.
One could then envisage, say, a 24 ‘passenger’ craft (one such already exists in Germany) able to carry eight teams of three technicians each to the offshore location and place them on eight turbines in sequence. This would represent a major increase in productivity.
Another likely trend is that it will become more economical to provide overnight accommodation on site than to transport personnel to and from a shore base daily. This can be allowed for either by stationing dedicated accommodation vessels, as happens in the oil and gas industry, or by using larger transportation vessels that also have accommodation. For the latter case, SWATH characteristics would be useful in minimising sea-induced motion, and the slower transits inherent in this form would be compensated, economically, by the overnight capability. In some cases, the semi-SWATH compromise might be appropriate so that there is less sacrifice of speed.
In the future when there are wind farms in deep waters well away from land, we are likely to see a mix of vessel forms active in operating and maintaining their wind turbines, some of which may be floaters. At any given time we might see a light, carbon composite vessel riding in on foils or on a cushion of air, a SWATH or semi-SWATH competitor, a wave-piercing catamaran or monohull, an accommodation vessel and even a helicopter or two, the expense of the latter being justified by the rapid and comfortable transits they provide for high-cost ‘techies’.
ABOUT THE AUTHOR
George Marsh had an active career in electronics and systems engineering, including a spell as a flight test engineer. After working for a London-based technology PR company, he became a free-lance writer and moved to the Isle of Wight where he developed an interest in renewables. He has written for Renewable Energy Focus magazine almost from its inception in 2001.