The current newbuilding market for offshore oil and gas operations support vessels is characterized by a wide array of different ship types, sizes, and applications. Northern European yards in particular saw a boom in orders toward the end of last year. These ships are for European, American, and Asian owners, and range from small supply vessels to some of the largest anchor handlers ever constructed.
One example of the latest version of anchor handlers and production support vessels currently under construction at the Flekkefjord Slipp og Maskinfabrikk in Norway is a Vik & Sandvik VS480 design anchor handler, owned by Solstad Offshore. The vessel has a total installed power of 14.72 MW and will be in service in the North Sea. Its propulsion system, comprising two Wartsila 16V32 main engines, two Wartsila SCH 95 gearboxes, two JC-Lips controllable pitch propellers in nozzles and a Winchmatic control system, was delivered as a package by Wartsila.
Long shaftline solutions
The design and construction of an AHT or PSV can often be an exercise in fitting as much equipment as possible into the smallest conceivable space as a result, characteristic of most offshore support vessels is the long shaftlines. Putting the engines as far forward as possible provides more space for the cargo hold and allows a large, open aft deck for carrying a multiple range of cargo.
Unfortunately, this also creates additional complications when designing the propulsion system. The long shafts traversing almost half the vessel length add weight and present access and space problems. They also make alignment whirling and torsional vibration calculations more complex.
Putting the reduction gear as far aft as possible and using high-speed intermediate shafts between the engines and gearboxes reduces the weight because the higher shaft speed requires a smaller diameter to transfer the same power. More importantly, putting the reduction gear closer to the propeller allows a simpler propeller servo systems than if the main propeller shaft ran through the entire vessel.
The oil distribution box can be mounted on the gearbox instead of directly on the shaft. With long propeller shafts, there is a risk that compression of the servo oil can lead to vibrations and fluctuations in the pitch control and feedback. For a ship that relies heavily on maneuvering accuracy, like an offshore vessel in dynamic positioning operation, this can prove critical.
Long shaftlines also create complications beyond the actual fitting of the shafts through several compartments. An offshore support vessel has a very characteristic profile - a high forecastle with the accommodation and control rooms, the machinery space, and a large low aft deck. This creates a flexible hull with a great degree of hull flexing that must somehow be absorbed by the relatively stiffer shafts. Greater movement can be allowed by installing flexible tube shafts as part of the shaftlines between the engine and gearbox.
Flexible operation
Although this new vessel is characterized as a conventional anchor handler, it actually has a semi-diesel electric arrangement. For low-load operation or low-speed steaming, the ship can operate on one engine and two propellers. Instead of running two engines on part load, one engine can then run at optimum load, leaving the other engine free for service or maintenance. This is achieved by operating one main engine normally, driving the propeller directly, while providing the same power to the shaft generator. The other shaft generator acts as an electric motor, driving the other propeller through the power take-off (PTO) shaft on the gearbox.
The gearboxes are equipped with two clutches that can be operated remotely - one main clutch between the input shaft and the pinion shaft and one clutch on the input shaft, which is used to disengage the engine when using the PTO function.
The engines are equipped with a Wartsila engine control system (WECS 7000) for local monitoring and operation, while the Winchmatic propulsion control system handles control of the controlled pitch propeller (CPP) and user interface at the maneuvering stations. Interfaces to the joystick, dynamic positioning, and power management systems are also handled through the Winchmatic. Although the ship is not specified with Fi-Fi equipment, the engines will be equipped with a free-end PTO should the owners wish to add Fi-Fi pumps later on.
Multi-purpose PSV
The other newbuilding at the Flekkefjord yard is a large and versatile platform supply vessel that is not intended for a specific charter, but which can be used for normal supply duties, piping, construction, or could be adapted for diving/remotely operated vehicle assignments. Again, a complete Wartsila propulsion system has been chosen based around Wartsila 32 engines.
This vessel is also a Vik & Sandvik design - the diesel-electric machinery arrangement comprises two Wartsila 8L32 engines and one Wartsila 1V32, with combined output of 12.88 MW. Propulsion will be provided by two JC-Lips controllable pitch propellers, driven by electric motors, through two Wartsila reduction gears.
The machinery arrangement provides flexibility in terms of the layout and the system operation. Each engine and generator set is mounted on a common bedplate, which is flexibly mounted to reduce onboard vibrations and increase crew comfort.
Since the power transmission is electrical, there is no need for high-speed intermediate shafts. This significantly simplifies the design and layout as the generator sets can be installed higher up and further forward in the ship than on a conventional platform supply vessel.
Another advantage of diesel-electric machinery is greater freedom in engine operations. Use of CCPs in combination with variable speed electro-motors further increases the flexibility. The operator can optimize the propellers' pitch and rpm setting for any service conditions, an important feature in a vessel with widely varying duties. CPPs are especially beneficial when operating in dynamic psoitioning mode, as the response is faster than with fixed pitch propellers.
Using design concepts initially developed for cruise vessels. Engineers have taken care in the propeller design to achieve a low noise level while maintaining high efficiency and avoiding harmful cavitation.
