Offshore LNG Transfer, A Practical System Based on Proven Oil Transfer Principles
Authors: Humphreys Vaughan , Jones Nicholas 
Published: October 2008

This paper focuses on the challenge of safely and efficiently transferring LNG from a LNG FPSO to a LNG carrier in exposed weather conditions. It analyses and proposes a practical LNG transfer system for two cases of tandem loading to project dedicated LNG carriers and to non-dedicated LNG carriers loading spot cargoes. This system is based on a new robust flexible LNG hose, and on the principles of oil transfer from oil FPSOs proven over the last 30 years.

Background.

BHP Billiton Petroleum (BHPB) was one of the pioneers of oil and gas FPSOs in the 1980's, and this understanding & belief in floating based offshore production facilities has continued through to today. In the early 1990's interest moved towards new methods of developing stranded gas resources offshore in remote locations with no local market. Even in those days the potential benefits of floating based LNG export facilities for this application were recognised, but the technology available at that time had not progressed to the point where this could be considered a practical reality. BHPB saw there were three main technical areas to be overcome before a LNG FPSO could be considered technically viable.

The first area was the selection of the LNG storage tank system suitable for a LNG FPSO. Proven land based LNG tanks could not practically be transferred to an efficient floating facility, and proven LNG carrier tank designs would need to be larger in volume and accommodate the potentially large dynamic motions at any fill level - i.e. the issue of LNG sloshing. The trend in LNG carriers was and still is to increase the storage capacity, and work was being considered to understand and overcome the sloshing problem. At that time BHPB believed that the industry would solve this technical issue in time for LNG FPSOs to become a reality, and indeed believe this is now the case.

The second area was the selection of a suitable LNG liquefaction process. Those processes with a good track record on land over many years could not easily be re-configured for use offshore in a dynamic marine environment on a confined vessel. BHPB and others believed development work was required in this area. Based directly on onshore gas plant experience with a cryogenic nitrogen removal plant using a simple nitrogen refrigeration cycle, BHPB developed and patented an optimised nitrogen cycle process for the liquefaction of LNG called compact LNG or cLNG? (ref. 1). This process was further developed and optimised in conjunction with Linde AG and although not quite as efficient as the larger land based plants using hydrocarbon refrigerants, this process offers critical advantages for offshore, including very low hydrocarbon inventory for safety, compact plot plan and tolerance of motions to suit a floating deck and ease of modularisation for construction. During this time, others in the industry have developed new hydrocarbon-refrigerant based processes, which are also more suitable for use offshore. BHPB therefore believe this second technical challenge is also now solved.