A Pressure LNG System

Abstract The concept of carrying LNG under pressure is not new. Considerable test work on LNG stored at 1150 psig/–75°F and 200 psig/-175°F was carried out in the U.S.A. in the early 1960's; this work culminated in the conversion of the Liberty ship renamed ‘Sigalpha’ to carry between 820 MCF at 200 psig and 1150 MCF at 1150 psig of compressed LNG. The basic idea(1) which remains valid today, was to reduce substantially the size and cost of the liquefaction plant at the expense of increased storage costs; the net effect being a significant overall cost saving in the transportation cost of LNG. Unfortunately, the cost of the multi vessel, high pressure, storage which had, of course, to be installed not only at the liquefaction plant but also on the ships and at the reception terminal, became much more expensive than expected and eventually absorbed the entire cost savings obtained in the liquefaction process. The pressure concept thus went into ‘suspended animation’ until, in 1975, one of the original inventors of the system conceived a multi-lobe storage tank design which, at the somewhat lower pressure range of around 312-412 atmospheres, enables LNG to be processed, stored and transported at an extremely competitive cost. The multi-lobe design in essence eliminates all the major disadvantages of multi vessel storage whilst still enabling advantage to be taken of the reduced liquefaction plant size and cost – and it is this fact together with certain other characteristics relating to cargo handling and safety which mate the concept particularly suited to offshore projects.