Gas associated with the crude produced from Occidental's Piper field is conserved by drying it and condensing out the heavier components. This renders the gas with water and hydrocarbon dew points acceptable for transfer to St. Fergus via Total's Frigg field pipeline. A process which includes a turbo expander/compressor is used to extract the condensate which is spiked into the crude pipeline for eventual recovery as liquid product and fuel gas at Flotta. The turbo expander can extract 30 percent more condensate than a simple Joule-Thompson expansion. Gas transferred to St. Fergus is 80 percent methane with a nett calorific value of 1000 BTU/SCF and a water dew point of −20°F at 1700 psig.

OVERALL PIPER GAS SYSTEM (See Figure 1)

At a crude production rate of 200,000 B/D the gas evolved at initial separation totals 75 MMSCF/D. The gas oil separation takes place at 150 psig. For entry to the Frigg system the gas must be processed and compressed to 1700 psig. In future 2100 psig will be required when the Frigg pipeline pressure is increased.

The first stage of compression is provided by centrifugal machines which discharge the gas at 650 psig. Cooling of the gas in sea water exchangers provides some condensate at this point. The major part of the gas processing then takes place between the first and second stages of reciprocating compressors. After compression to 1600 psig in the first stage the gas is dehydrated and the heavier components are condensed out by auto refrigeration. Recompression to export pipeline pressure of 1700 psig takes place in the second stage of the reciprocating machines.

You can access this article if you purchase or spend a download.