With her four cavernous membrane cargo tanks, the 69,000dwt SK Summit is one of the largest LNG carriers yet built. A few days after her delivery in August 1999 she commenced loading LNG in Qatar, entering service between Ras Laffan in Qatar and Pyeong Taek in South Korea, on an anticipated 25-year contract.
The membrane principle
Unlike the spherical tanks of a Kværner-Moss type LNG tanker, the prismatic tanks of a membrane LNG carrier are fully integrated into the hull. The boundaries of the prismatic tanks form a continuous inner hull, comprising double bottom, sides and deck; this achieves a hull structure well suited to absorb global loads as well as local loads from the cargo. The cargo containment system is fitted inside the tanks, between the inner hull and the liquid cargo.
The containment system is designed by Gaztransport and Technigaz (GTT). It is built up as a cryogenic liner directly supported by the ships double hull. The liner includes the primary (inner) and secondary LNG barriers forming two identical metallic membranes, and the primary and secondary insulation forming two independent insulation layers.
The metallic membranes are 0.7 mm thick and 500 mm wide, of 36% Invar nickel-steel alloy, offering a very low thermal contraction coefficient which makes the material well suited for the low temperature (163°C) to which it is exposed. These membranes act as dual barriers protecting the hull structure from exposure to the low-temperature cargo.
The insulation boxes, filled with expanded silicone-treated perlite, are internally strengthened to withstand high-impact pressures and to absorb the energy resulting from the liquid motions and pressure head. The boxes are independently and mechanically secured to the double hull by means of studs and couplers specially designed for their thermal insulation performance.
A challenge for Daewoo Heavy Industries
For DHI this contract meant not only a significant investment but also a great challenge. DHI, familiar with well-known technologies for traditional ship construction, had to develop new design and construction methods for the building of these more advanced LNG carriers. New welding techniques, procedures and machines had to be developed, and welders for machine and manual welding trained and qualified. Production facilities for Insulation Box fabrication were built inside the yard, housing all the equipment to produce the complete cargo containment system, which required more than 50,000 insulation boxes.
Close co-operation with GTT and DHI Quality Control was established by Det Norske Veritas at an early stage to agree on common procedures, acceptance criteria and tolerances. The outfitting, carried on in all tanks concurrently over 12 months, required a well-organised team of three surveyors to cover all the inspections. DNV had one dedicated surveyor to follow up the installation of all 50,000 boxes and more than 120 kilometres of weld inside the cargo tanks.
After successful sea and gas trials, the vessel was named SK Summit by Mrs Lee Hee-Ho, wife of Korean President Kim Dae-Jung. Present were some 350 guests including the chairmen of Daewoo and the Sun Kyung Group. Later she was selected as Ship of the Year, by three maritime magazines, proving DHI to be a competitive builder and DNV a respected classification society in the area of membrane LNG carriers.