First IPG FSO Installed Off Nigeria

Long renowned for efficiency and technological advances, Japan has released another technological first with the delivery and installation of what is billed as the world's first purpose- built steel LPG Floating Storage and Offloading Vessel (FSO). Dubbed Escravos, the unit started operations in Chevron Nigeria Limited/Nigerian National Petroleum Corp. oil field offshore Nigeria in July.

Built by IHI's Aichi Works, the permanently moored unit uses an external turret mooring system, and is designed to operate 20 years without drydocking. Rising up to meet a number of technological challenges, the new vessel features the IHI SPB (Self-supporting Prismatic shape IMO type B) LPG containment system.

This containment system was originally developed and proven reliable with LNG carriers built by IHI. In addition, the FSO features redundancy in major machinery and equipment, and a maintainability which was introduced to comply with the requirements of operating for unusually long, uninterrupted operations.

Escravos was ordered by Chevron Nigeria Limited in February 1994, and completed in October 1996. It is the core installation of the Escravos Gas Project now being constructed at the Escravos oil field located about 33 km east offshore Lagos, Nigeria.

The project was started in October 1992, for environmental reasons. At present, the associated gas coming from the subsea oil field of Escravos has been flared and discharged into the atmosphere.

But in recent years, with global environmental protection in high demand, this project was implemented to reduce flaring and recover gas more effectively. The FSO is a floating structure with the facilities to chill the gas after refining, to store it as LPG of near ambient pressure and low temperature, and to offload it onto an LPG tanker.

While the environmental conditions in Escravos are relatively mild, the structural design of the hull and cargo tank are built to a design basis according to environmental conditions of the North Atlantic Ocean. In addition, the design of the FSO is based on the rules for building and classing steel vessels of the American Bureau of Shipping (ABS).

For the fire protection and fire extinguishing system and helicopter deck, the rules of F-AMC and HELDK-SH of DNV are applied.

General Arrangements The Escravos hull has a very simplified shape to secure structural accuracy during construction and reduce hull steel weight, as well as to secure rolling motions in waves to satisfy allowable operating conditions of the machinery and equipment, and avoid interference with the mooring chains.

To enhance steady heading during towage and directional stability in order to avoid swinging when permanently moored, a pair of skegs is provided at the stern. The FSO is permanently moored at the bow using an external turret mooring system supplied by SOFEC of the U.S. and MODEC of Japan.

The accommodation house is arranged at the bow so as to keep it predominantly upwind of the cargo tanks. The cargo tank portion has a double bottom, and three cargo tanks are arranged. In addition, water ballast tanks are provided in the double bottom spaces. The offloading stations for offloading stored LPG to the tanker are arranged at three places, both sides and port of the stern, allowing the stations to be used selectively, so according to conditions. Two revolving deck cranes are provided at midship and stern to Japan's first jetfoil service, and today it operates a fleet of five diesel ferries and five jetfoils. The company's high-speed, reliable service has been so effective that in 1996, its only competition an airline — stopped flying to Sado Island. That year, Sado Kisen vessels carried 650,000 people, most of them tourists.

Sado Kisen's jetfoils, built by Boeing Corp., are powered by aviation- type Allison gas turbines. The units create a waterjet system that, according to Boeing, helps to decouple the vessel ^ ^^ from surface water, providing a smooth ride even in heavy seas. With foils extended, the vessels cruise at approximately 43 knots. The propulsion system operates on a waterjet principle, using pumps to discharge water at high pressures through nozzles ^ ^ ^ directed aft. Each vessel has two pumps which deliver 24,000 gallons per minute. Each pump is powered by a gas turbine system.

According to Sado Kisen engineers, the most critical challenge for a jetfoil engine lubrication system comes from accumulated sodium chloride crystallized from seawater, which causes oil-path plugging.

Analyzing used oil is the best way to monitor such accumulations, but detecting seawater in a used oil sample in extreme high temperature atmospheres is difficult. To solve the problem, Mobil handle the offloading hose. The cargo tank of the FSO is designed by the Leak Before Failure (LBF) concept in conformity with the self-supporting tank type B in IMO Gas Code. In fatigue design of the type B tank, the builder insisted that the actual structural construction tolerance be better than the tolerance assumed in the design.

Although the hull structure is not subjected to the type B by IMO Gas Code, it was evaluated in order to ensure reliability equivalent to that of the cargo tank.

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