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Liftboats: Unique above the sea
Tuesday, 12 April 2011 16:47

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While most boats have to find shelter or tough it out in heavy weather, the lift boat simply jacks up her hull and avoids the waves. Conventional vessels are often defined by their hull length, but it is the length of the jacking legs that is the significant number for lift boats.

Having recently delivered a lift boat with massive 98-metre legs, naval architecture firm AK Suda is currently overseeing the construction of two of their latest designs.

The larger of the two has 91-metre lift legs. Like most vessels of this type the beam, 36 metres, exceeds a 2:3 ratio relative to the 46-metre length. The length of the 91-metre legs is double the vessel’s length. The boat has a 3.9-metre moulded depth to its nearly flat bottom.

Conventionally, there have been two approaches to liftboat designs. One is a scaled down version of the jackups. Some of these are being built in the Far East. Suda designs are not merely scaled down versions of jackups. They are “true liftboats”.

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Optimising designs allows them to provide low coat stable work platforms (that are liftboats). The reduced hull weight allows for the use of durable, low maintenance jacking systems that are, once again, not an extrapolation of jackups. The referenced liftboat is an example of this design philosophy at work. Vessels of their similar design have been operating in other areas, including Angola, Africa.

Built to service oil industry production platforms, the lift boat has approximately 950 square metres of working deck interrupted only by the two forward legs and a pair of cranes. This allows for a cargo block profile of 30 by six metres with a 15-metre width. The larger of the two cranes lifts up to 250 tonnes with a 40-metre boom. The smaller is a 40-tonne crane with a 21-metre boom. A four-level accommodation and navigation block shares the aft-end of the boat with the third leg. A heliport extends out from the stern. The arrangement with a compact accommodation block allows for a more efficient load distribution between the legs.

The SUDA 300-L3 model lift boat is propelled by a pair of EPA Tier 2 Cummins KTA38-M engines each delivering 635kW to nozzled propellers. These engines are grid-cooled and used only for propulsion while the hull is in the waters. A bow-thruster assists in positioning the vessel. Two Cummins-powered 625-kWe generators provide power for the hydraulic jacking engines, the cranes and general shipboard services. Two 250kW generators provide primary power and backup servce for the ship service load. A 99kW generator provides emergency power. Unlike some earlier lift boats, these engines are air cooled with deck-mounted radiators to negate the need for hoses being lowered to the sea for water cooling when the vessel is jacked.

When fully extended, the legs can lift the bottom of the boat about 81 metres off the sea bottom, with some variation depending on penetration of the foot pads in the sea bottom. When retracted for travel, the 7.5-square-metre pads at the bottom of the legs are retracted into pockets in the bottom of the hull. The money-making part of the liftboat legs is the amount of leg extension below the hull. This vessel will boast the highest leg extension as a ratio of its total leg length.

The SUDA 300-L3 model lift boat is currently under construction at Gulf Island Marine Fabricators in Houma, Louisiana, USA. Delivery is scheduled for mid-2012.

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