Chantiers To Start Delivering Tankers
Chantiers de l'Atlantique, a subsidiary of GEC Alsthom, is building five methane carriers for Petronas Marine of Malaysia. The keel for the first of the five tankers was laid in September 1992. This tanker will be delivered in July 1994; delivery of the remaining four ships will be staggered between July 1994 and July 1997. The 130,000-cu.-m. liquefied natural gas (LNG) carriers have been constructed using automated manufacturing and assembly processes. Each LNG carrier has four tanks which are incorporated in the ship's metal structure.
Thermal insulation for the liquid methane cargo is provided by a double layer of plywood boxes filled with perlite, an insulating powder made of volcanic materials.
Gastightness is ensured by a 0.7- mm-thick membrane made of Invar, a steel and nickel alloy which has an extremely low coefficient of thermal expansion. For safety reasons, a second, identical membrane is placed between the two layers of boxes to ensure tightness in the event of a leak in the first membrane. To minimize the cost of assembly operations aboard the ships, components have been extensively standardized and widespread use has been made of prefabrication techniques. The construction of a carrier requires 50,000 plywood boxes, each measuring 1 m (3.3 ft.) by 1.2 m (3.9 ft.), which are produced in a fully automated, purpose- built workshop on site. In addition, the special Invar parts forming the tank corner structures are made in completely pre-fabricated 10 foot (3 m) long elements.
Special attention has been given to optimizing the supply of the many components installed aboard the ship. Materials are delivered by the erectors themselves using the "justin- time" method with the aid of a computer system.
One of the first operations to be carried out inside the tanks is to weld metal elements called coupler studs to the ship's double hull, working from data provided by a precision topographical survey. The studs anchor the first layer of boxes. Chantiers de l'Atlantique has developed a special device for this purpose. The Dromadec system comprises a viewing unit, an onboard computer, a stud positioning arm and a welding torch. Using the topographical data provided by a laser and a distance measuring device, the computer places each coupler at the desired position before welding it automatically to the double hull. Dromadec reportedly makes it possible to achieve the precision specifications set for assembling the tank's insulation elements, namely a +/- 0.9 mm positioning accuracy for the studs relative to the topographical data.
Chantiers de l'Atlantique and a number of specialist firms have worked together to develop machines to automate the welding of May, 1994 33 the membranes and achieve maximum quality. A single ship requires 90,000 m of resistance seam welding and 21,000 m of TIG welding.
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