Abstract

During the last years, the interest in LNG FPSO's has been revived. One of the challenges in designing a LNG FPSO is the liquid sloshing in partial filled tanks. One of the unique features of the spherical tank is that it is certified for operation with part filling to any level. It is therefore well suited as containment system for floating LNG production units.

The spherical tank was developed as a containment system for LNG carriers more than 30 years ago. The tank is essentially a shell supported by a cylindrical skirt through a patented transition joint. The spherical shape results in membrane type operational stresses and bending moments are limited to the transition area where they are carefully controlled. The tanks are today constructed entirely of aluminum by specialized shipyards around the world. The spherical LNG tank is installed in close to 70 LNG carriers, some of which are nearly 30 years old and have recently been certified for another 20 years service. All vessels built are still in service today and no serious incidents directly attributed to the tank construction have ever been reported.

Through a number of studies for major oil companies we have acquired a solid database and a good overview of the technical and economical challenges which must be met and the safety requirements that have to be adressed. This paper will outline the unique features of a floating LNG production facility based on the spherical tank system. It will be shown how the overall safety is enhanced through the use of the spherical tank and how it can be adapted to suit requirements for harsh as well as mild weather conditions, different production capacities and feed compositions.

INTRODUCTION

Although having been the subject of numerous concept studies and proposals for the last years, the idea of LNG production on a floating structure is by no means new. In the mid-to late 1970's a LNG FPSO was offered to Iran for the Kangan field. A full basic engineering design including comprehensive safety assessments was carried out and Det Norske Veritas prepared a Rule Proposal. The unit consisted of 6 spherical LNG tanks and a 2-train APCI process in a steel hull. The hull was weathervaning on a single point mooring and the unit had a LNG production capacity of 2,8 MMTPA and a storage capacity of 165 000 m3.

Complete front-end engineering was carried out for this project including comprehensive risk assessment. The project was terminated for political reasons. The main features of the Kangan LNG FPSO are shown in Figure 1.

Figure 1 - General arrangement and main features of the Kangan LNG FPSO(Available in full paper)

Since then, a number of concept studies have been done for oil companies on the same subject. However, recently these activities have become more and more frequent due to thegeneral interest throughout the LNG industry.

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