The Offshore Production, Transfer, And Ship Transport Of The Gas Liquids Fraction From Crude Oil Or Condensate Fields
Authors: Cummings Donald R. , Leighton John , Braathen Colin W. 
Published: December 2007

Offshore loading of Natural Gas Liquids (NGL) enables low RVP crude to be loaded offshore while eliminating retrograde condensation problems in associated gas pipelines without resorting to flaring or re-injecting the NGL fraction. This is desirable from a conservation viewpoint, and is in many cases economically attractive. This paper describes a simple process for producing, storing, transferring to a carrier, and transporting NGL. The NGL is produced at ambient temperature under pressure, transferred to either subsea storage or pressure tanks mounted on a tethered tanker for storage, and subsequently transferred, via a single point mooring to an LPG carrier. All these actions are carried out at ambient temperature and modest pressure, 20 to 25 bar, i.e. well within the pressures commonly used for crude oil transfer. The ambient temperature NGL is transmuted into a low-temperature liquid at atmospheric pressure by the carrier's modified refrigeration system.

INTRODUCTION

The time is ripe for the offshore production industry to reappraise its methods of operation and we believe that offshore loading including NGL will prove to be a contending technology in the development of many fields.

The offshore oil industry has entered a new era; an era of maximum conservation of hydrocarbons by utilisation of the most efficient processing techniques. This is especially so in the North Sea where until now. the main question for management had been 'Can a satisfactory platform be designed to withstand the rigors of North Sea gales?'. We think that this question has been positively answered, and future developments will achieve improved processing, transportation and recovery factors. This paper shows that increased liquids recovery and better overall economics are possible using offshore loading and NGL recovery.

The continued improvement in the design, and experience with the use, of offshore loading systems has led us to believe that offshore loading of gas liquids is a technically feasible and economically attractive proposition. The scheme we propose is not cryogenic in nature, requiring new designs for the swivels and hoses, but is a method to transfer NGL at ambient temperatures and at pressures in the region of 14 to 20 bar through a conventional single point mooring to a suitable carrier. Once on board the carrier the gas liquids can be chilled to between â?'40 and â?'50 °C for conventional transport, or alternatively transported under pressure. We would normally expect that the NGL carried would have a molecular weight of between 40 and 50 and a bubble point temperature of 40 °C at a pressure of between 12 and 18 bar; such a material could be condensed from an NGL-rich gas stream using air or water as the coolant. A suitable process is described later in the paper.

The use of offshore loading in the North Sea is steadily increasing, even though some fields are using it merely as a temporary device for early production while pipelines and on-shore stabilisation and handling facilities are being built. Installing these facilities is proving more difficult than previously imagined, due, at least in part, to the desire to minimise any detrimental effect upon the local landscape, the desire of local communities to maximise the benefits likely to accrue to themselves, the remoteness of some of the sites and the construction problems encountered on large sites. These problems have caused delays to some on-shore sites, and have significantly diminished the degree of overall control which may be exercised over the total project. Associated with this loss of control is the possibility of significant reduction in the rate of return for such projects.