Sloshing in membrane LNG carriers and its consequences from a designer's perspective

The worldwide utilisation of natural gas has led pr oducing countries to look for transportation solutions. Marine transport ation becomes economically feasible for long distances or when pi pe laying becomes unpractical. In order to ship natural gas in suffic ient quantities to make a complete energy supply project viable, it is liqu efied 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 t ransfers 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 sloshi ng is extensively studied in the LNG shipping industry and more especially in GTT, where dedicated high-tech numerical tools and testi ng 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, o ffshore regasification, etc). In parallel, a few unexpected incidents related to sloshing impacts have recently been recorded. No su ch incident had been observed since those isolated ones observed on former designs in the late seventies. Research and development effort into the sloshing p henomenon 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 fro m these incidents are of inestimable interest for the scientific communit y, and help increase the phenomenon’s knowledge. However, some particularities of these incidents and thus of the sloshing phenomenon itsel f, still have not been as yet explained to our entire satisfaction. Given the strong expectations from the industry to improve understanding and thus better tackle the sloshing p henomenon and its consequences, this paper will introduce most of the research studies that have been performed recently or which are curr ently in progress as well as a major evolution of the methodology for sl oshing assessment. Each of these items will be then described and disc ussed in detail during dedicated ISOPE presentations.