Offshore Gas Liquefaction Terminal - Development Of Astable Platform For North Sea Application
Authors: Metcalf P. , Stubbs S.B. 
Published: October 2008

Associated gas recovery from North Sea oilfields by means of liquefaction and storage offshore has been assessed to be an economical and technically feasible concept. Further development work is now proceeding and this paper reviews the results obtained to date. Acceptable process plant and storage tank configurations have been developed for use on specially constructed steel or concrete semi-submersibles or on purpose-designed barges for use as floating platforms.

It is considered that applications for this development will be found initially in the North Sea for associated gas recovery from producing oilfields which, due to their nature and geographic disposition, are unsuitable for gas gathering schemes based upon the use of pipelines. The paper analyses the quantitative study of platform motions and acclerations. From this information it has been possible to lay down detail specifications of environmental requirements for terminal plant and the LNG transfer system. Computer simulation techniques are being used to extend the data base of motions, accelerations and loads. Mooring system design and terminal main structure are now being developed using loads predicted by model testing and computer simulation.

Involvement of the Certifying Authorities is necessary to establish limits of safe operation and safety criteria which must be met. These criteria have a fundamental effect on the layout and operability of the terminal and, in particular, on availability of the LNG transfer process between terminal and shuttle carrier.

Environmental requirements for the process plant have been established as have basic safety criteria. The implications of these aspects on terminal design are highlighted and integrated into the overall design.

INTRODUCTION

Conservation is an integral part of energy policies of the world's nations. As energy requirements increase known reserves of hydrocarbon fuels diminish at an ever increasing rate and it is only by exploration in ever more remote and hostile areas of the world that new reserves can be found. All hydrocarbon fractions have a place in today's technological world and must, therefore, be recovered with the minimum of wastage.

Gas associated with oil production has often been considered as an unwanted by-product and it is only in recent years that much attention has been paid to the recovery of this valuable source of energy. In areas which are remote from potential markets and particularly where gas yields are low, the economics of gas recovery are subject to considerable debate and it is only as energy reserves decrease that these areas receive serious consideration.

With offshore oilfields the problem of associated gas recovery is compounded. Many ideas have been put forward for economic recovery of gas and a number of these have been seriously considered as possible solutions for the North Sea.

Pipelines are the simplest solution with minimum gas treatment being undertaken offshore and with, potentially, the highest availability. However, the cost of pipelining in deep hostile conditions of the North Sea is very high and any scheme requires a high throughput over a reasonable timescale to be considered economic.