UK-based Houlder’s marine design consultancy has unveiled what it describes as a windfarm support base vessel concept. Mike Simpson, director of the consultancy, said an offshore windfarm needs people to maintain it, but there is much debate about how to set up maintenance regimes for offshore windfarms that are up to 100km offshore. “It clearly makes for a more challenging commute,” he said.
“Simply mirroring the North Sea’s existing fixed accommodation platforms and helicopter transfers has been widely discounted on the grounds of cost effectiveness,” said Mr Simpson. “Crewing a single platform is a very different prospect to maintaining a field of multiple turbines geographically, practically and economically.”
Noting this, Houlder proposes an offshore support base (OSB) vessel that recreates the convenience of port facilities – but offshore. To do this requires a highly stable platform combining quality accommodation with flexible working spaces, multiple crew transfer methods and the ability to assist directly with operations. “Such a solution delivers the quickest and most cost effective access to turbines – with the flexibility to accommodate both routine maintenance and unplanned events. It keeps maintenance crews within the windfarm and is able to move them around within it,” said the company.
There are several competing concepts but the starting point has to be a highly stable platform with a large and versatile deck plan. Houlder has been able to turn to a well-known and proven design from its offshore oil and gas heritage to deliver this with high crew comfort and excellent operability.
The base vessel it has used is a dive support vessel, Orelia, which has a long history of providing stable, safe and highly operable diving support. “Its unique form provides superior seakeeping to that of a standard monohull vessel with motions comparable to semi-submersibles in North Sea conditions,” Houlder says.
Using a proven, highly stable design will significantly improve the comfort levels onboard for future windfarm maintenance crews. In typical North Sea crew transfer conditions, the Orelia hull design limits roll to +/- 1 degree. This means a straightforward walk-to-work personnel transfer system can provide direct turbine access without expensive, complex motion compensating technology. For further flexibility Houlder’s design also provides three docking stations for crew transfer vessels and a helipad to maximise deployment and crew transfer options.
Taking Dogger Bank as an example, it is intended that the vessel will stay in the windfarm continuously throughout the summer weather period. It will be resupplied by standard supply boats, with the crew and maintenance personnel being changed out as required. During winter periods, when crew transfer becomes more challenging, returning the mothership to port on a 28 days’ cycle proves more cost effective.
Houlder’s OSB concept can be configured to provide the required deck space, warehousing, container storage and craneage (30 tonnes safe working load) to manage specific project equipment and spares. It can further be reconfigured to support specific campaign work as required: transporting and storing multiple gearboxes, blades or generators to support lower specification lift barges for example.
The facilities onboard are based on the latest offshore oil and gas standards including private cabins, wet rooms and project offices. Home comforts include cinemas, gyms and games rooms. The design leaves significant scope for additional client requirements.
Mr Simpson said: “Houlder brings over 30 years’ experience of offshore operations to its OSB design. Key to success is carefully designing a hull that minimises vessel motions to provide a safe and stable platform for man and machine. From that, the rest falls into place.”
The vessel is 126m overall, with a breadth of 17.4m, moulded depth of 11.25m, draft of 5m and speed of 10 knots. It is designed to provide accommodation for up to 80 turbine technicians in single cabins and has a marine crew of 27, also in single cabins.