Oil prices have been driven to very high levels by ever increasing global energy demands. Gas-to-liquids (GTL) technology, which converts natural gas into liquid products, is set to play a major role in meeting these demands. GTL technology has already been utilized at several onshore locations and it won't be long before this important technology is applied offshore. It is particularly applicable for realizing the potential of stranded gas reserves, where physical or economic factors have hindered development.

The lack of infrastructure normally prevents development of stranded gas offshore. Offshore GTL conversion offers a solution, as it enables produced hydrocarbons to be transported using existing infrastructures, such as trading tankers and tanker terminals worldwide.

As most stranded fields are located in deep water, a floating GTL production platform with storage capacity is required. From this specification, a concept evolved that merged GTL technology with a Floating Production, Storage and Offloading Vessel (FPSO) capable of oil and condensate production. This coupling of conventional and GTL technology provides a solution to monetize a gas source that might have been flared or reinjected otherwise. Since most utility systems are common to oil producing FPSOs and offshore GTL plants, the shared infrastructure and additional revenues from oil production could greatly improve potential project economics. A feasibility study conducted by Bluewater Energy Services B.V. and Syntroleumi, supported by Amec Oil & Gas, has determined the viability of combining these two technologies.

The synergy between FPSO and GTL utilities has been given much attention and resulted in a steam-dominated plant with maximized heating and cooling integration. Virtually all GTL by-products - heat, steam, low-BTU gas and process water - are used by the FPSO systems, requiring only minor additional fuel gas consumption.

Figure 1 - GTL FPSO (Available in full paper)

1 Introduction

GTL technology development has reached a stage where its marinisation may be considered for FPSO application. The Syntroleum GTL technology is the most suitable for an FPSO. This is because the majority of GTL processes use pure oxygen for the production of syngas, requiring a costly oxygen plant. However, Syntroleum proprietary GTL technology uses air, eliminating this need.

Feed gas for the GTL process will be produced from oil and gas wells connected to the FPSO, rather than imported. FPSOs typically receive only oil well fluids, so associated gas is either reinjected or exported. Therefore, to maximize production levels, the GTL FPSO will be equipped with an oil separation train and a gas/condensate separation train. Integrating a GTL plant with both oil and gas production facilities is a further design challenge.

Bluewater Energy Services BV, based in Hoofddorp, in The Netherlands, together with Syntroleum Corporation, based in Tulsa, Oklahoma, USA and supported by Amec Oil & Gas, in London, UK, have conducted an economic and technical feasibility study of an FPSO equipped with a conventional oilgas- water separation plant and a gas-to-liquid (GTL) plant.

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