Walk-to-Work vessel will master waves to provide safe and affordable access
Source: Maritime Holland
Author: Bruno Bouckaert
As the nearshore locations for offshore windfarms grow scarcer, new developments are situated ever farther from the coast. Larger turbines, stronger winds and the absence of advocacy groups make this a solid business proposal, with the latest concession granted without subsidised energy cost.
Damen Shipyards Group has built many Crew Supply Vessels, which are used extensively in the nearshore windfarms. Even though these vessels have a high service speed of 25 knots and can handle pretty rough weather, there comes a point (at about 30 miles from shore) when the amount of traveling hours, fuel costs and discomfort at sea makes it an inefficient way of bringing technicians to offshore wind turbines.
To secure its leading position in the offshore wind service sector, Damen knew it had to develop a solution for the wind farms located farther from the coast. Bibby WaveMaster 1 is the company’s first go at this. The ship was designed and built from the ground up with one main goal in mind: affordable access to offshore wind turbines. If shuttling technicians to and from the wind farms becomes unfeasible, there is only one solution: house them at sea. This has long been done in the offshore industry, with rotational crews staying for a few weeks on a platform. But as offshore windfarms have no large accommodation platforms and are spread over a large geographical area, the service technicians need to be housed on a ship.
This gave rise to the aptly named concept of the “Walk-to-Work vessel”: a ship which can house seven maintenance crews (of three people each), drop them off at wind turbines within three hours in the morning, then stay as a standby vessel, and pick them back up in the evening. Because the turbine engineers are working a large part of the day on a fixed structure and spend the night on a ship, seasickness is an important consideration. A worker who is seasick can’t work and is therefore an unproductive person occupying a spot in a very expensive logistical chain.
Without the involvement of a client initially, Damen’s naval architects had “carte blanche” to develop the most suitable ship for this purpose. Based on the hull design of the PSV3300, they optimised the ship by varying length, beam and draught to obtain minimal horizontal and vertical accelerations, taking into account typical North Sea weather and wave conditions. From client input, they knew that 80 per cent of operability is acceptable, which defined the limits for the DP-system, gangway system, etc. This means that the vessels should be able to continue operations in sea states with a significant wave height up to 3 metres. The optimization resulted in a vessel of 90 metres length-over-all with a relatively wide beam of 20 metres and a relatively shallow draught of 4 metres. A deep and narrow forefoot reduces the occurrence of slamming, which can be very annoying due to its loud noise and intermittent nature. As the ship does not sail long distances, the top speed was limited to 13 knots. When Bibby Marine Services actually ordered the vessel, the detailed design was started, leaning strongly on the experience of the client.
A study of the accommodation position showed that the mid-ship position is best for comfort. The vessel will spend a lot of time heading into the waves, and then the vertical accelerations are the most annoying. These are lowest amidships. The midship accommodation has an added benefit in that the profile of the vessel is neutral to the wind, which results in less steering corrections when sailing straight in a cross wind.
The interior layout was based on an analysis of workflows, ensuring efficient operations and a strict division between clean areas and working areas. As it’s important that the crew enjoys working on the vessel, the cabins have a high standard of finish with tasteful decoration. All but a few cabins have portholes on the ship’s sides. Those located on centerline have a vertical skylight, facing away from the helicopter deck. The designers made an effort to create a separate sleeping and living area in each cabin, by using local dividers.
The ship has a natural roll period of 11 seconds, which is quite a long wave period not encountered so often in the North Sea. A passive anti-roll U-tank is installed to further reduce the roll motions. A large model testing program was conducted at the MARIN research institute to determine the motions and the required power for Dynamic Positioning in various conditions. On the basis of these results, Damen gives a ‘connection guarantee’: at a significant wave height of 2.5 metres, the gangway will 96 per cent of the time be connected in a timespan of three hours. Based on the results of the tests at MARIN, a computer model was made allowing DP simulations to be carried out in all kinds of weather conditions. The ship is equipped with a DP transit mode, allowing transfer from one turbine to another in DP tracking mode, rather than going in and out of dynamic positioning mode each time.
The gangway installed on Bibby WaveMaster 1 is from Uptime (Norway). This is a motion-compensated gangway accounting for both the vertical and horizontal difference between ship and turbine. Its two degrees of freedom are the gangway angle and gangway extension. It has a maximum range of motion athwartships of four metres. When the amplitude of the motions goes over three metres, boarding procedures are interrupted and the traffic signal goes to red. Connection is usually made in ‘bumper mode’: the tip of the gangway is pressed against the wind turbine with a force of up to 800 kilogrammes. This force is compensated by the ship’s dynamic positioning system.
A second way to transfer crew – only in fair weather – is with tenders. A 30 knot composite boat from Tuco (Denmark) can be placed overboard with a sliding davit and can transport eight persons and some light cargo. Once in the water, the daughter craft can be boarded from the ladder on the transom, which has an identical landing structure as found on the wind turbine foundations. A second ladder can be placed on port and starboard side with a crane, allowing the ship to create a lee with less wind and waves.
A lot of design effort was put into the logistical chain onboard, providing a “stepless approach”. This is helped in part by having the heli-deck on the foredeck rather than on an aluminum raised dedicated platform, as seen on most offshore vessels. From the helideck, light cargo and luggage can be transported with a cart over the portside side deck to the elevator at the motion-compensated gangway, just behind the accommodation. This elevator runs to all six deck levels, including the cargo space under the aft deck. Containers with spare parts can be loaded from the shore and placed in the cargo area. They are mounted on lowerable twistlocks. Once the container is in position and its doors are opened, it is entirely lowered within a recess, ensuring access to the container without a step.
The ship and its turbine spare-part and fuel capacity are designed for missions up to 30 days. Initially only single cabins were provided, but to increase versatility in difficult market conditions, the passenger capacity was ramped up by turning half of the cabins to double cabins. This brings the total capacity to 90 persons in 30 single and 30 double cabins. About 20 to 24 of those are the ship’s crew, while the rest of them are technicians.
Due to its low top speed requirement, relatively small propulsion load and long periods spent in dynamic positioning mode, the choice for a diesel-electric propulsion plant was straightforward. Four Caterpillar gensets provide the power. With two larger (3516) and two smaller (C32) gensets, this configuration allows to always match very well the available power to the required power, avoiding chronically underloaded gensets, which typically belch black smoke. The gensets are mounted in a double-flexible way, to eliminate vibrations in the structure. The electrical system is entirely AC, with a voltage for the large consumers (thrusters) of 690 V. In the stern, the two azimuthing thrusters from Schottel have an Z-drive, with the E-motors mounted horizontally within the ship’s hull. In the bow are two tunnel thrusters and one retractable bow thruster. The retractable thruster allows for quieter DP operations, as long as the water is deep enough and can also act as a tunnel thruster. The ship has a Comfort Class 2 notation from the DNV-GL classification society.
One of the innovations incorporated in the ship has its roots in naval shipbuilding. In the construction of the Holland-Class patrol vessels, sister company Damen Schelde Naval Shipbuilding developed a system – called Jet-X-Box – to propel the exhaust gases further away from the ship. This has proven so successful that the exhaust stack on Bibby WaveMaster 1 is basically a copy of those on the Holland-Class ships. A large “booster” fan at the base of the casing, just above deck, can be switched on to create a significant draught upwards from the top of the casing, which carries the exhaust gases several meters higher. The system is normally not in use, but when inconvenience from exhaust gas is experienced on the gangway or at the wheelhouse, it can be switched on with the flick of a switch.
The crane onboard Bibby WaveMaster 1 is heave-compensated with a safe working load of 5 tons. For future use, Damen has engineered the structure for a crane with 24 tons SWL. The helicopter deck has a foam fire extinguishing system and lighting for nighttime operations. A Helicopter Monitoring System keeps track of wind, cloud cover and ship’s motions to assist the helicopter pilot.
While the vessel does not yet have a Ballast Water Treatment system installed, the space and systems are reserved for it. The same applies for exhaust aftertreatment. SCR boxes are provided, as well as the necessary Ureum tank capacity, but the pump and dosing system will be retrofitted when it becomes required.
As this is a fairly new class of ship, a battle of terminology is still ongoing. Shall we call these a Walk-to-Work Vessel, a Service Operations Vessel or an Accommodation Support Vessel? Nevertheless, the purpose is clear. While this innovation was brought to life by the offshore wind industry, it’s the more traditional Oil & Gas industry that saw the full potential of it. After a first job in the Galloper Windfield from September 2017 to March 2018, Bibby WaveMaster 1 will be chartered by Total E&P Nederland to work in the offshore gas industry. New offshore gas platforms in the Southern North Sea are often unmanned and need exactly the type of solution provided by the Bibby WaveMaster 1, which provides access at a much lower cost than helicopters and jack-up platforms.