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Separation of gas and liquids is a key processing function for any production operation. Several approaches exist to accomplishing this separation subsea, as described on this page. Which is most appropriate to use depends on the fluids and conditions specific to the particular location.
Gas/Liquid Separation and Liquid Pumping
By separating the gas and liquid phases and pumping the liquid stream, this simplest of systems will capture most of the benefits of subsea processing. It will reduce backpressure to the wells and eliminate problems associated with multiphase flow. Although the liquid (oil and water) stream must still be processed at the host platform, two-phase separation may be the best compromise for cost, function, operability, and maintainability.
Two keys to success for subsea systems are:
Gas/liquid separation can be achieved in a relatively small vessel (see the page on separators). Typically, vapor/liquid equilibrium can be achieved with liquid residence time of 1 minute or less, in contrast to oil and water separation, which requires 5 minutes or more. In addition, compact designs based on cyclonic principles are available for gas/liquid separation, which allows those units to be even smaller. Because the subsea vessels for deep water must be designed for external collapse pressure (because of seawater hydrostatics), compact vessels can significantly reduce system weight and cost. In addition, the lighter and more compact separator will allow whole system retrieval without heavy-lift vessels, making installation, retrieval, and maintenance costs lower. Reliability will improve because the facilities can be tested as an integrated system on the surface prior to installation.
Water Separation and Disposal
With some reservoirs, water breakthrough can significantly reduce pipeline capacity and increase surface treating costs. For existing facilities, such as Troll “C,” where limited platform space makes expansion of the water separation and treatment system difficult and costly, subsea separation and water injection may be an attractive solution. In the Troll Pilot (see previous discussion), produced water is removed in a three-phase separator and injected in subsea disposal wells. Supplemental injection pressure is provided by electric-motor-driven subsea pumps. Oil and gas are commingled and transported to the host platform in a mixed-phase pipeline. Whether or not to transport separated oil and gas in a single-phase pipeline is an economic decision. The value of the production uplift, flow-assurance benefits, and operability improvement must outweigh the extra cost of a two-pipeline system. Because Troll is in relatively shallow water, produced-water removal at the seabed has the greatest impact on flow-induced friction, while the reduction in hydrostatic gradient is less significant. Troll chose to use a single mixed-phase pipeline. For deepwater fields or longer tieback distance, the economics may be different.
The ultimate goal of subsea processing is to achieve efficient gas/oil/water separation, gas compression, oil pumping, and water disposal. This would transfer the bulk of the production facilities to the seabed and enable lower separator pressure than otherwise can be achieved. But before embarking on such an ambitious goal, one should consider the following:
- The larger and heavier three-phase separator that would be required, and the implication that it has on modularization, installation, and maintenance options.
- Heat input or chemical demulsifier injection that may be required to effect good oil/water separation.
- Water quality and monitoring required to maintain adequate disposal-well injectivity.
- Technology required to supply the large amount of power for gas compression.
- Operation and maintenance of relatively complex compression equipment.
Given the developing state of subsea processing, it is better to start with simple systems that yield the largest impacts and progress slowly into the more complex systems. Until the industry has gained confidence and greater know-how on equipment marinization, deepwater installation, mechanical equipment operations, and maintenance, the risks associated with subsea three-phase processes are quite high.
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