Page 313 - Hydrocarbon Exploration and Production Second Edition
P. 313

300                                                               Facilities






                                                              living quarters

                                                              utilities
                                                              water treatment

                                                              separation
                                                              flare
                                                              derrick

                                                              drilling
                                                              wellhead

                                                              water injection

          Figure 11.33  A steel jacket platform.


          Topside modules are either installed offshore by lift barges, or can be positioned before
          the platform is floated out.
             TLPs are used mainly in deepwater where rigid platforms would be both
          vulnerable to bending stresses and very expensive to construct. A TLP is rather like a
          semi-submersible rig, tethered to the seabed by jointed legs kept in tension. Tension
          is maintained by pulling the floating platform down into the sea below its normal
          displacement level. The ‘legs’ are secured to a template or anchor points installed on
          the seabed.
             Floating production systems are becoming much more common as a means of
          developing smaller fields which cannot support the cost of a permanent platform
          and for deepwater development. Ships and semi-submersible rigs have been
          converted or custom built to support production facilities which can be moved from
          field to field as reserves are depleted. Production facilities were initially more limited
          compared to the fixed platforms, though the later generation of floating production
          systems have the capacity to deal with much more variable production streams and
          additionally provide for storage and offloading of crude, and hence are referred to as
          FPSOs (floating production, storage and offloading). The newer vessels can provide all
          services which are available on integrated platforms, in particular three-phase
          separation, gas lift, water treatment and injection.
             Ship-shaped FPSOs must be designed to ‘weather vane’, that is must have the
          ability to rotate in the direction of wind or current. This requires complex mooring
          systems and the connections with the wellheads must be able to accommodate the
          movement. The mooring systems can be via a single buoy or, in newer vessels
          designed for the harsh environments of the North Sea, via an internal or external
          turret (Figure 11.34).
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