Expanders Do Payout Offshore North Sea
Authors: Barnwell J. , Wong W. 
Published: December 2007

Introduction

A turbo expander has been installed on the North Sea Occidental Piper Platform, which was first brought onstream in 1976. The installation, as Platform, which was first brought onstream in 1976. The installation, as originally designed, may produce 200,000 B/D [31 797 std m3/d] of crude plus 75 × 10 3 scf/D [ 2.12 × 10 3 std m3/d] of gas. The crude is exported plus 75 × 10 3 scf/D [ 2.12 × 10 3 std m3/d] of gas. The crude is exported to the Flotta terminal and at an early stage the gas was directed into the Frigg/MCP-01/ St. Fergus pipeline. To match the pipeline specifications, the associated gas is dried and heavy hydrocarbons removed. Initially, for Phase 1, triethylene glycol (TEG) was employed for dehydration and a Phase 1, triethylene glycol (TEG) was employed for dehydration and a Joule-Thompson (J-T) expansion for recovering the condensate, which then is spiked into the crude (see Fig. 1). Besides the need to meet the gas specification, the spiked crude also must achieve the required bubblepoint pressure for pipeline transportation to the Flotta terminal. This imposes pressure for pipeline transportation to the Flotta terminal. This imposes a second design criterion by restricting the quantity of light hydrocarbons that may be present in the condensate that is mixed into the crude oil. While Phase 1 was in operation it became apparent that additional condensate could be recovered from the rich associated gas with the application of extra cooling. The gas exported still would meet the pipeline specifications shown in Table 1, and the extra condensate would pipeline specifications shown in Table 1, and the extra condensate would not cause the vapor pressure of the crude oil to rise above the acceptable level. The extra, potentially recoverable condensate was the economic driving force for implementing Phase 2, the gas conservation project. This addition to the original topsides involved the use of molecular sieves for dehydration and a turbo expander for chilling (Fig. 2). The selection of a turbo expander was based on favorable process and mechanical aspects. The change to molecular sieves for drying was a result of their capability of achieving the desired water content of 5 ppm in the dehydrated gas. If the dewpoint specification had been -40F [-40C] or higher at the dryer conditions, a glycol dryer could have been used. The gas conservation project was commissioned in July 1980 and has operated satisfactorily. On the basis of this proven application of turbo expanders offshore, the various process schemes for recovering condensate from rich gas offshore have been re-examined. The purpose is to illustrate that an expander system has merit for treatment of rich associated gas for North Sea platforms. The study results are identified and followed by a discussion of the main mechanical features of installing a turbo expander offshore.

Process Design Options Process Design Options Three basic process options to extract condensate from the rich associated gas were considered: J-T expansion, external refrigeration, and turbo expansion. These schemes were compared with one another by using the restraining criteria common for North Sea platforms:space conservation andprocess equipment weight minimization.

Also, such factors as the use of proven, reliable, easy-to-maintain equipment suitable for installation in a confined space was taken into account. The comparison was based on dry gas from the molecular sieve dehydrator with the conditions given in Table 2. The product specifications and utility values are typical of North Sea application. The design of the three compared process schemes was optimized to illustrate the differences that would be found in practice. Fig. 3 shows the extent of the equipment affected by the study as bounded by the dotted line.

J-T Expansion (Case 1). When the rich associated gas exits the molecular sieve driers it is divided and part of it is cooled with cooling water (see Fig. 4 and Table 3). The remainder of the inlet gas is cooled against the process column reboiler and overhead heater and then recombined and further process column reboiler and overhead heater and then recombined and further reduced in temperature before entering a J-T expansion valve. Operating conditions of 290 psig [2 MPa] and -51F [-46C] are controlled at an optimum at the exit of the J-T valve to meet the sales gas specification. The resultant mixture is separated and the lean gas heated against the total inlet fluid. Then it is compressed and cooled before entering the booster compressor suction. The separated liquid is directed to the demethanizer column, where light components are stripped to ensure that the condensate product, when spiked into the crude, does not cause the export crude to exceed the vapor pressure requirement.

External Refrigeration (Case 2). For this process design option, a closed- loop Freon (TM) refrigeration cycle at -22F [-30C] is provided to assist the J-T expansion cooling (see Fig. 5 and Table 4). The remainder of the process is similar to Case 1 except that the gas compressor now is not process is similar to Case 1 except that the gas compressor now is not needed with the separator operating at 965 psig [6.7 MPa] and -29F [-34C].

Expander Plant (Case 3). The turbo expander design process flowsheet is shown in Fig. 6 and Table 5. Again the scheme is similar to Case 1 except that two separators are needed. The final separation after the turbo expander is operated at 645 psig [4.5 MPa] and -24F [-31C].

JPT

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