Wave-piercing technology

No longer is a vessel’s performance a question of its installed propulsion system alone. Today, the trend points in the direction of taking the total grip of a ship’s design including the hull, engines, propulsion system and many other components. The new Rolls-Royce UT 790 CD vessel design is proof of this. Its wavepiercing technology is a well-proven concept for high-speed catamarans and trimarans, and now with the new UT 790 CD vessel design, Rolls-Royce is bringing the benefits of this technology to the offshore market. 

“With the UT 790 CD, we can support offshore exploration and production further and deeper, while at the same time improving safety and minimizing the impact on the environment,” said Svein Kleven, chief design manager of offshore operations at Rolls-Royce. The UT 790 CD vessel is a deepwater, anchor-handling vessel. Anchorhandling operations in deepwater rely heavily on the vessel’s stability, so a main focus in the design process was to create an inherently stable vessel that also felt instinctively stable. The vessel meets all existing regulations from the Norwegian Maritime Directorate and all of the latest international regulations such as SOLAS 2009 and Special Purpose Vessel codes and regulations. It is able to run continuously at service speeds, regardless of the sea state, due to the hull piercing through the water rather than riding on top of the waves, resulting in a reduction of fuel  consumption and an improvement in crew comfort. 

“This technology eliminates slamming and allows for a smooth ride even in extreme weather conditions,” said Kleven. “At speeds of 14 knots and violent storm conditions (9 m significant high waves), tank tests have shown no water above forecastle deck level. In extreme wave heights, water will be visible at the forecastle long before the situation gets critical and safety margins can be maintained.”

The UT 790 CD vessel

The engine room for the UT 790 CD is located far astern and a wet exhaust system has been applied. The system, designed by the Norwegian company Mecmar, is cooled by seawater from a temperature above 300°C to approximately 60°C. The saturated and cooled exhaust leaves the vessel at sea level — leaving the distance from the engines to the exhaust outlet ducts to a minimum.

 “With the engines and exhaust outlets astern, there are no casings limiting the view from the bridge. The 360° view significantly improves safety and has been high on our customers’ wish list,” said Kleven. Placing the engine room to the rear of the ship provides a number of benefits, including a reduction in engine noise and an increase of spare room for winches at the front of the vessel. The UT 790 CD has the same fiber and rope capacity, operating with four secondary winches as a traditional anchor handler has with six. The vessel has the power and capacity to handle cable, chain and rope down to 2000 m and fiber rope down to 3000 m. With its winches mounted lower and its 23 m width, the vessel has a low center of gravity, which provides excellent stability. Even with the increased beam, tests show that there is less hull resistance than on a traditional Anchor Handling Tug Supply (AHTS) vessel of 20 m, thanks to the wave-piercing hull shape.

One of many new features on this vessel is the introduction of a three screw propulsion system, combining a center controllable pitch propeller (CPP) with two Azipull thrusters with nozzles. A Bergen diesel engine BT32-40W16P, rated 8000 kW, drives the CPP. Four Bergen diesel generator sets, C25:33L9A, each engine rated 2880 kW, power the Azipull thrusters and provide general electric power. Combining the thruster with a large single-screw CPP ensures superior power and maneuverability. The Azipull also creates less resistance in transit position when compared to conventional azimuths. The multi drive power electrical system gives high flexibility in different modes. It can transition from maximum power when operating in anchor-handling mode with both mechanical and electrical power engaged, to electrical-only in dynamic positioning or slow maneuvering mode, to mechanical-only in transit mode.

“This hybrid propulsion system optimizes fuel efficiency and substantially reduces emissions compared to conventional propulsion systems,” said Kleven, “and like all our Bergen engines, this engine meets Clean Design class rules without further exhaust clean-up. However, catalytic converters can be fitted to the generator sets, providing 90% NOx reductions.” With its special redundant drive solution, the forward azimuth thruster can be powered by two independent switchboards rather than just one, and the vessel can achieve the same DP-2 capability with its three thrusters as conventional solutions with four.

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