Membrane Technology for Offshore LNG

Abstract

Abstract Environmental and financial issues will make LNG FPSOs and LNG FSRUs a reality. The technical specificity of the membrane technology, with an outstanding record on LNG carriers, make it particularly adequate for offshore applications. Cost effective and reliable it also offers a great operational flexibility. It has already found its first application in the LNG offshore industry : El Paso LNG Bridge? are being currently built in Korea. Introduction The membrane containment system is widely used on LNG carriers and it has been so for more than 30 years with an outstanding record for safety and reliability (two third of current new orders are placed with membranes). After reviewing some of the history of LNG, the nature of the membrane technology is described. Environmental and financial issues that will make LNG FPSOs and LNG FSRUs a reality are addressed. Issues such as the growing concern about the location of gas terminals, and of course the benefit of having a mobile unit transferable to any part of the globe. The technical specificity of the membrane make it particularly adequate for offshore applications:Large free open deck available for the installation of a liquefaction or a regasification plant.A containment system designed to withstand liquid motion and allows for all filling levels.Safety heightened by a strong and simple design of the hull structure, by a low windage area and by a continuous monitoring of membrane tightness.Light components allowing for on-site repairs & maintenance.Operation flexibility with no continuous cooling down required. Background The commercial use of Liquefied Natural Gas (LNG) was introduced in the 1960's to monetize natural gas reserves that were remote from the consumers. In order to transport this gas on long distances with no consumer on the way, the most cost effective way was to place it on ships. To maximize the amount transported, the cargo is liquefied to take advantage of the density increase (multiplied by nearly six hundred (x600)). The pressure at which natural gas remains in its liquid form at ambient temperature being unacceptable, LNG is stored and transported at ambient pressure and low temperature : -163°C. This low temperature is maintained by insulating the liquid from outside heat. Today this principle is applied both for land storages and for LNG vessels. The containment system of the LNG must therefore face two challenges : resist such low temperature and provide sufficient insulation to keep natural gas in its liquefied state. A third challenge is of course to do that in a cost-effective way. Today, three metallic materials are used that retain ductility at this temperature : aluminum, stainless steel and nickel steel. Two different approaches have been developed : self supporting tanks (the structure of the tank in contact with the LNG is able to resist by itself the pressure of the cargo column) and membrane tanks (the membrane contains the cargo but relies on the ship / building structure to resist the pressure).