Gravity structure

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With time, a concrete gravity platform structure will induce considerable deformation within the supporting soil. The casing, particularly the marine conductor, will have, at least in part, to follow the displacements that take place in the soil.

The marine conductor and possibly the conductor casing therefore have to be designed such that the loads caused by soil settlement shear forces does not cause yield to occur. Casing capacity is not affected by this environment.

Modelling of these shear forces is generally performed by computer analysis.

In their analysis, such programs take into account:

  • self weight of marine conductor or conductor casing;
  • any loading from other strings and the wellhead/BOP;
  • temperature changes due to production;
  • platform settlements;
  • frictional forces from guides due to platform settlements;
  • bending moments due to platform rotation.

The loads are applied chronologically according to their appearance.

It should be noted that, as well as specifying the well geometry, the program user has to provide a production mode temperature profile. A soil settlement profile and soil/cement properties are also necessary.

The output typically consists of a plot of axial stress against depth, which can then be compared with the marine conductor or conductor casing yield strength.

While yield of the marine conductor may be tolerable, the conductor casing should be designed to withstand a combination of loads from the inner strings, wellhead, and soil settlement without yielding.

This can be achieved either by selection of an appropriate steel grade for the conductor casing, or by setting the marine conductor some 100 ft (30 m) below the depth of the deepest soil displacement, so that the conductor casing does not experience any soil settlement.

Transfer of settlement loads from the marine conductor to the conductor casing will, however, result from the existence of cement between the two pipes. Careful consideration should therefore be given to the cementation programme for the conductor casing, and this aspect should be included in computer simulations.

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