Tried and tested concepts used on existing vessels have been refined by a leading Danish owner to make their crew transfer vessels even better suited to the needs of the offshore wind industry
Two highly specified crew transfer vessels designed and developed by Danish operator World Marine Offshore and built in Denmark by Assens shipyard will start work shortly with a well-known charterer.
The Danish vessel owner is known for its focus on high-specifaction vessels with superior seakeeping characteristics, but the ‘Inertia PM5’ design improves on the company’s already specialised fleet in a number of respects.
World Marine Offshore already operates vessels with advanced hullforms that combine the benefits of a trimaran and small waterplane area twin hull (SWATH) and has taken the tried and tested concept a step further on the latest vessels in its fleet.
As World Marine Offshore chartering and sales manager Jacob Lykke-Kjeldsen tells OWJ, the vessels were designed by the shipowner’s daughter company, Inertia ApS, and incorporate ‘lessons learned’ from operating the shipowner’s existing fleet in addition to feedback from the company’s clients. As such, they are World Marine Offshore’s ‘third generation’ of crew transfer vessels designs. All were designed with an emphasis on seakeeping, stability and 24/7 operation in mind.
The company states that the vessels will be able to undertake transfers in sea states up to a significant wave height of Hs 2.5 m. “This will guarantee our clients the best possible utilisation and offshore accessibility,” says Mr Lykke-Kjeldsen, noting that the Inertia PM5 vessels can transport 24 passengers in business class seats or alternatively accommodate 12 passengers in single cabins. The design will enable passengers and crew to remain offshore for up to 14 days.
The trimaran hull shape confers the ability to operate at high speed with reduced fuel consumption. This gives the new vessels a service speed of 25 knots and a 29-knot sprint speed. The SWATH feature ensures a high level of seaworthiness and stability when undertaking offshore transfers.
“When the vessels are at speed, the trimaran gives crew and passengers optimal comfort and reduces slamming,” says Mr Lykke-Kjeldsen, noting that the accommodation and wheelhouse are placed at the centre of the vessel to reduce heave and pitch and further improve comfort for passengers and crew. The vessels will also be equipped with a motion monitoring system that will enable the master of the CTVs to ensure windfarm technicians arrive on location in as short a time as possible and are well rested.
“When operating in transit mode the vessel has reduced resistance, which enables it to achieve higher speeds than a conventional design and reduce fuel consumption. When in SWATH mode it has excellent stability and seaworthiness,” Mr Lykke-Kjeldsen says.
Apart from the innovative hullform designed to reduce fuel consumption and emissions while ensuring they are stable and comfortable platforms for windfarm technicians to live on, the Inertia PM5 vessels also have a new, fuel efficient diesel-electric propulsion system with waterjet propulsors that is augmented by using a battery pack.
In addition, they have an upgraded version of World Marine Offshore’s active ‘Soft Bow,’ an active, impact-reducing system built into the hull of the vessel that reduces the load on a boat landing on an offshore wind turbine.
The company claimed earlier versions of the Soft Bow reduced the load imparted on a boat landing by around 50%. It is at the heart of the new vessels and the way they operate. At 3.6 m, the enhanced Soft Bow on the new vessels will be approximately twice as wide as that on the company’s second-generation vessels, and unlike the earlier Soft Bow designs, it incorporates not one but two hydraulic cylinders that cushion the impact on the vessel when it pushes up against a transition piece on a turbine. Mr Lykke-Kjeldsen says having two hydraulic cylinders and a Soft Bow twice the width of the old one means it is able to compensate for the movement of the vessel, whatever the conditions and has greater built-in redundancy.
The effect of the enhanced Soft Bow is such that the impact of the vessel on boat landings is very much reduced and the speed at which the vessel can approach a boat loading is greater while always ensuring the impact load remains within design criteria. It also incorporates a fully automatic system that logs all data relating to the vessel’s interaction with a turbine and is integrated to the vessel’s monitoring system. Designed to need little maintenance, it ensures safety by having the vessel remain in contact with the turbine at all times during transfers.
“In the new vessels we shifted away from controllable pitch propellers to waterjets,” Mr Lykke-Kjeldsen explains. “Waterjets give us several advantages and even more control as we approach a turbine. Having five Hamilton waterjets, two in the side hulls and three in the centre hull further enhances manoeuvrability and redundancy.
“When we are in transit, when we need maximum speed, all of the engines will be in operation, providing power, but in other modes of operation, we can operate on fewer engines and waterjets. That reduces fuel consumption, emissions and wear and tear on the engines.”
If required, power from the 45-kW battery pack can be fed into the propulsion system. At other times its main role will be to meet the hotel load on board, providing electricity. If required, the vessel could also be operated purely on battery power for short periods.
Compared to World Marine Offshore’s other trimarans, the new vessels also have several other upgrades. These include an active ride control system with active interceptors, active hydrofoil and two 100-kW bow thrusters for enhanced manoeuvrability.
The hydrofoil mounted at the bottom of the centre hull is adjustable, reduces motions and optimises the trim of the vessel. Having an active ride control system also reduces rolling and pitching motions. The system also includes automatic trim and list control, compensating for cross winds or uneven loading.
“The hydrofoil and the interceptors work together to optimise the trim of the vessel,” Mr Lykke-Kjeldsen explains. “Because the hydrofoil is adjustable, and its operation can be independently controlled to either side of the centre hull, it can be used to reduce rolling motions.” It can also provide 7 tonnes of lift to adjust the trim and compensate for cargo on the vessel’s 110-m2 foredeck.
Information provided by the company suggests that the electrically actuated interceptors provide a 23-48% reduction in roll and pitch in 1.5 m to 3 m waves, a fuel saving of around 7% and help to increase speed by 1.4 to 1.5 knots.
In addition to working as crew transfer units, World Marine Offshore said the vessels are also suitable for a range of other tasks, including undertaking surveys, deploying remotely operated vehicles, standby rescue, chase and guard duties.
Looking further ahead, Mr Lykke-Kjeldsen says World Marine Offshore is already working on a successor to the Inertia PM5. He told OWJ that the weight of a battery pack is such that the technology would probably need to evolve if the company and industry as a whole is to make more extensive use of it, but he does not rule out using other types of propulsion technology, such as hydrogen, as the industry evolves towards greener operations and zero emissions vessels.