Page 293 - Hydrocarbon Exploration and Production Second Edition
P. 293

280                                                     Oil and Gas Processing


          equipment needs, resulting in much smaller project footprints. Furthermore, the
          ability of multiphase pumps to handle gas in a closed system instead of venting and
          flaring reduces emissions to the environment. The ability to pressure boost well
          flows to remote centralised processing units and to handle very low inlet pressures
          also make the multiphase pump an ideal tool to develop marginal fields.
             Typically, multiphase pumps are installed in remote locations. They can be found
          on wellhead platforms offshore, on onshore fields far away from the production
          facilities or subsea. Such installations are often unmanned (obviously in the case of
          subsea) and therefore require rugged and reliable equipment.


          11.1.2.8. Subsea separation
          The development of reliable subsea separation technologies that allow companies
          to process offshore production without the need for above sea level production
          facilities have been progressing for some years, though as yet there are few systems
          operating other than on a field trial basis. In 2007, Statoil installed what was claimed
          to be the first commercial subsea processing station of its type in the Tordis Field
          (in the North Sea) which is a subsea separation, pressure boosting and water
          injection system. The system, which has been designed to allow the Tordis Field to
          continue production at high water cuts, includes a separator that removes water
          from the well stream, a multiphase pump for boosting the production rate and a
          water injection pump for the discharge of separated water in a disposal well.



          11.1.3. Upstream gas processing

          In this section, gas processing will be described in the context of ‘site’ needs and
          evacuation, that is how gas may be processed for disposal or prior to transportation
          by pipeline to a downstream gas plant. Gas fractionation and liquefaction will be
          described in Section 11.1.4.
             To prepare gas for evacuation, it is necessary to separate the gas and liquid phases
          and extract or inhibit any components in the gas which are likely to cause pipeline
          corrosion or blockage. Components which can cause difficulties are water vapour
          (corrosion, hydrates), heavy hydrocarbons (two-phase flow or wax deposition in
          pipelines), and contaminants such as carbon dioxide (corrosion) and hydrogen
          sulphide (corrosion, toxicity). In the case of associated gas, if there is no gas market,
          gas may have to be flared or re-injected. If significant volumes of associated gas are
          available, it may be worthwhile to extract natural gas liquids (NGLs) before flaring or
          re-injection. Gas may also have to be treated for gas lifting or for use as a fuel.
             Gas processing facilities generally work best at between 10 and 100 bar. At low
          pressure, vessels have to be large to operate effectively, whereas at higher pressures
          facilities can be smaller but vessel walls and piping systems must be thicker.
          Optimum recovery of heavy hydrocarbons is achieved between 20 and 40 bar.
          Long-distance pipeline pressures may reach 150 bar and re-injection pressure can be
          as high as 700 bar. The gas process train will reflect gas quality and pressure as well as
          delivery specifications.
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