Sloshing In Membrane LNG Carriers And Its Consequences From a Designer’s Perspective

The worldwide utilisation of natural gas has led producing countries to look for transportation solutions. Marine transportation becomes economically feasible for long distances or when pipe laying becomes unpractical. In order to ship natural gas in sufficient quantities to make a complete energy supply project viable, it is liquefied at -163° C thereby reducing its volume by a factor of 600. Several containment technologies have been implemented, including SPB, Moss and Membrane. Contrary to its two main competitors, where the insulation is installed on the outer part of a self-supporting tank, the membrane systems (the only currently used designs on board LNG carriers are exclusively developed by Gaztransport & Technigaz) incorporate a liner fitted directly onto the double hull. One of the main characteristics of this type of system is that it transfers to the double hull the loads induced by liquid motions inside the tanks. This hydrodynamic phenomenon, also known as sloshing, can lead to high magnitude impacts on the walls with potential consequences on the containment system response. This is why sloshing is extensively studied in the LNG shipping industry and more especially in GTT, where dedicated high-tech numerical tools and testing facilities have been developed for many years. At the same time, we have recently observed a rapid growth in the number of membrane LNG carriers in service, their cargo capacity and the variety of operating procedures (spot market, offshore regasification, etc). In parallel, a few unexpected incidents related to sloshing impacts have recently been recorded. No such incident had been observed since those isolated ones observed on former designs in the late seventies. Research and development effort into the sloshing phenomenon has never been so intense, particularly within GTT, and the overall knowledge on this subject has reached an unprecedented level. Particularly, the feed-back and lessons learned from these incidents are of inestimable interest for the scientific community, and help increase the phenomenon's knowledge.