Modification of LNG Carriers for Regasification

Abstract

Introduction This paper addresses the issues surrounding the modification of existing Liquefied Natural Gas (LNG) carriers to act as floating storage and provide regasification facilities. In the context of studies undertaken, 'existing' may refer to an LNG vessel of any age ranging from one in excess of 20 years age to one still in the Builder's shipyard. The key point is that the modification is of a vessel whose original design purpose was that of a conventional sea-going LNG carrier. The modified vessel is known generically as a Floating Storage and Regasification unit (FSRU). The need for such an FSRU may arise because a modified LNG carrier is seen as a possible solution where there is a high value given to achieving early gas delivery into a new market. The FSRU can be mobilised and in operation for the initial period whilst the conventional receiving terminal is being constructed. An alternative scenario is the case where acquisition of land for the receiving terminal, or obtaining the necessary permits is likely to be a difficult and drawn out process. The concept may also be applicable for the case where the supply contract is too short to economically justify the building of a land based terminal. This paper reflects the evolution of ideas over a number of project proposals studied. The Basics In terms of hardware and duty, what are we expecting of the FSRU? The FSRU is to be moored permanently or semipermanently in an appropriate location. The FSRU is to receive, store and vaporise LNG and to send out regasified LNG into a pipeline distribution system. Other issues relating to management of boil-off gas, power generation and manning arrangements are also addressed. Selection of Candidate Vessels The studies embraced a range of vessels for conversion from some more than 20 years old to one project which looked at modifying vessels whilst still in the construction yard. Clearly the 'flat deck' designs (membrane and IHI-SPB) offered some advantages with respect to lay out of new equipment, but studies of Moss spherical tank design vessels did not find any insuperable difficulties. Within the industry, there are questions about the susceptibility of membrane designs to damage caused by excessive liquid motions in the cargo tanks - 'sloshing' damage. Should the candidate be a membrane ship then specific studies for the site conditions would be conducted by well established procedures to demonstrate acceptability. Interruptible Supplies When considering any FSRU project proposal, one of the first issues to address is that of interruptibility of supply. Are interruptible supplies acceptable and on what terms? The attraction of accepting an interruptible supply basis is that it may reduce some of the technical requirements for the FSRU. In the evolution of projects, it often starts off that interruptible supplies are acceptable, but as serious commercial negotiations progress the penalties involved are such as to drive the project towards higher and higher levels of continuity of supply. In this paper, the assumption is that the basis is that of maintaining uninterrupted supply.