Problem: A major production platform in West Africa receives up to 200 MMSCFD of dehydrated gas at 2500 psig from a new 16-inch pipeline, which is fed by a newly installed platform 16 miles away. The incoming pipeline gas is currently flowing directly to a departing pipeline but in the near future will be compressed and injected into the reservoir at the receiving platform. Installation of the future injection compressor and other planned platform upgrades will require repeated shut-ins and depressurizations of the incoming gas pipeline. However, due to the pipeline’s high operating pressure and large volume, and because of extreme limitations of the existing blowdown system, each depressurization of the pipeline will take approximately five days to accomplish. The unpalatable result is the shutting in of 150,000 barrels of oil at the new platform each day the pipeline is down.
EDG’s process engineers were tasked with developing a new blowdown system to be installed on the receiving platform that will significantly reduce the time required to depressurize the pipeline. There were several pressing technical challenges. First, the incoming gas is 55ºF, and if it is sent directly to the flare header, severe Joule-Thompson cooling will occur and prohibitively priced cryogenic piping materials would be required. Second, the pipeline gas is super-critical at 2500 psig, and large volumes of liquid will form at lower pressures. And third, there is little available real estate on the platform on which to install the new system.
Solution: EDG used its engineering acumen to develop a blowdown system that utilizes the platform’s existing process, contains specialized separation equipment and employs advanced automated process control and safety shutdown mechanisms. The new system uses flow control to depressurize the pipeline in three stages: first to the high-pressure production train at 1275 psig, second to the intermediate train at 560 psig, and then to the flare header. These staged pressure drops limit J-T cooling so that standard piping materials can be used. The system includes a compact, high-efficiency gas/liquid separator to remove liquids that form after the pressure reductions. The liquids from the separator are mixed with hot discharge gas from existing compressors before flowing to the flare header. The system takes up no more than 200 square feet of deck space. It is fully automatic, requiring minimal operator intervention. However, the system’s best feature is that it cuts the pipeline depressurization duration down to one day.
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