Offshore wind’s influence on OSV design starts at the waterline

Everything starts with a good foundation and in the case of an offshore support vessel (OSV), it is an efficient hull. “Excellent motion characteristics and optimised operating costs with better cargo capacity are the aims of every OSV design,” said Great Waters Maritime managing director Ashik Subahani, speaking on new design concepts at the recent Offshore Support Journal, Middle East Conference. “To have that, you have to have an efficient hullform,” he said.

An efficient hullform has to be a proven in operation or verified by using computational fluid dynamic or model testing, he said. Among the most popular current OSV hull designs are conventional, bulbous, Ulstein Design’s inverted bow concept X-Bow, and Damen’s Axe Bow design.

Highlighting the last two hull designs, Mr Subahani said, “X-Bows offer better seakeeping performance and less slamming,” while Axe bows can provide excellent wave piercing. Ulstein’s X-Bow has a tapered foreship shape, reducing pitching and bow impact in severe seas. It has been applied to offshore multipurpose vessels, service operation vessels (SOVs), cruise ships and many other types of vessels.

Damen’s Axe Bow concept grew from co-operation between the Dutch shipbuilding group and Delft University of Technology, the Maritime Research Institute of the Netherlands and the US Coast Guard.

Damen said the patented hull form, which was used for high-speed Sea Axe fast crew suppliers, crew transfer vessels (CTVs) and patrol boats, was initially developed to improve the vessel operating profile and enhance crew safety and comfort. Compared to conventional OSVs, Damen said the Sea Axe vessels offer superior motion behaviour, while significantly lower resistance in the water, yielding a 20% reduction in fuel consumption and lower greenhouse gas emissions.

Operating in severe sea states

Mr Subahani observed most offshore windfarms are located in severe sea state conditions. This means seakeeping in connection with human habitability is an important consideration and crew and passenger comfort has to be designed in. When considering a vessel’s seakeeping, he said, the naval architect must take into account all of the phases of the crew transfer vessel or OSV’s operations – the transit phase, approach phase and transfer phase.

Mr Subahani said the transit phase covers seakeeping at forward speed, effect of current, speed loss, slamming, ‘green water’ effects – when the bow could become completely submerged – and shallow water effects.

The approach phase covers seakeeping at low and zero speeds, effects of currents and manoeuvrability.

The transfer phase covers seakeeping at zero speeds, effects of currents, interactions between the vessel and the wind turbine, wave shielding by the wind turbine, dynamic positioning accuracy and shallow-water effects.

The vessel must operate within a comfortable range for crew and provide a stable and safe platform for transferring personnel to wind turbines or platforms. This means the vessel designer “might have to perform alterations such as modifying the hullforms or weight distribution or add stabilisation systems, such as fins or gyro-systems,” said Mr Subahani. This also means special attention to reduced vibration and insulation for enhanced crew comfort.

Unlike the typical offshore support vessel, which could serve four or five platforms, a CTV or SOV might operate to scores of wind turbines, while accommodating dozens of technicians who are likely to be more at home on shore than they are at sea.

“When conducting offshore windfarm operations, accommodation for 50 or more special technicians on board is preferable since vessels may visit as many as 50 or 60 turbines,” noted Mr Subahani.

Other common features on board offshore energy vessels are dynamic positioning capability, laser positioning reference systems, a motion-compensated crane and gangway systems.