Numéro : 2388 - Year : 2001
CSL - A new system for LNG carrier tank insulation
Karim CHAPOT, GAZTRANSPORT ET TECHNIGAZ
CS1 is a new on board LNG containment system developed by Gaztransport and Technigaz. It has been developed from two existing techniques respectively Mark III and NO 96 and that makes a process which brings together the best of both techniques. The system uses on the one hand reinforced polyurethane foam insulation and on the other two membranes - the first one 0,7 mm thick made of invar (low thermal contraction coefficient metal and high nickel content), the second made of a composite aluminium-glass fibre called triplex.
The development of CS1 benefited from the spectacular improvement in calculation means and therefore differs from the development of earlier techniques. Finite elements method allowed for early optimisation of technical solutions so as to reach a better correlation, finite elements models have been used to accurately determine testing conditions. Finally, the finite elements method considerably reduced the duration of testing.
CS1 developing method also largely benefited from an in-depth knowledge of existing systems: accurate knowledge of materials such as foams, plywood, mastics and triplex as used in the Mark III technique, accurate knowledge of fatigue behaviour of invar as used in the NO 96 technique.
All in all CS1 benefited from an extensive know how gained from over 40 years of experience
Chosen models allowed to check the reactions of a very complete set of the most stressed areas: flat panels, 90° and 135° dihedrals, trihedrals, fish-bone pattern tank n° l, liquid dome, gas dome, pump tower support.
The fatigue behaviour of different junctions has been thoroughly studied: Tube-tube junction, trihedralstube junction, pump tower support-primary membrane junction, primary membrane-gas dome junction, primary membrane-liquid dome junction. Loading cases comply with the IMO code. The worst conditions are considered as the ship can bear a 40 year North Atlantic spectrum with maximum safety.
The system optimisation has been designed taking into account reliability and final cost. Thus the system has been rationalised to make assembling easier. It's highly prefabricated and strong points of former techniques have been built on: for a quick assembling: invar primary membrane, for a high prefabrication and reliable insulation: reinforced polyurethane foam panels.
The systematic fatigue behaviour study of the most stressed areas and more specifically the improvement of the welding details have enabled to considerably decrease the risk of failure.
Furthermore, the impact of a damaged configuration of a part on the whole system has been carefully studied in order to check that the two membranes really behave independently and that the failure of one doesn't generate that of the other.
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