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Subsea Structures Installation Tolerances

asked Jan 31, 2015 in Subsea by pasmith (3,320 points)

Hi, I am interested in learning about Subsea Structures Installation Tolerances (dimensions of the target box, tolerances regarding the hubs elevation and orientation) for the following equipment:

  • PipeLine End Termination (PLET)
  • Production Manifold
  • Production Tree
  • Riser Base Gas Lift subsea structures (if different from PLET installation tolerances)

2 Answers

+1 vote
answered Jan 31, 2015 by jakangaru (2,010 points) | edited Feb 1, 2015 by jakangaru

The subsea structures (PLETs, SSIVs, anode sleds, etc.) should be installed within the specified target established by each Operating Companies. Target boxes defined below are nominal and should be confirmed for each project on a case-by-case basis, in conjunction with the appropriate engineering analysis. The following parameters as said are for study only:

  • PLET or pipeline laydown flange target box should be no more than ± 6.6 feet (2 m).
  • Subsea structure (PLET) tolerances should be a maximum of ±1° from vertical at the hub absolute vertical axis, or as determined by connector manufacturer.
  • Riser or pipeline transition piece target box should be ± 20 feet (6 m).
  • Manifold target box should be ± 6.6 feet (2 m).
  • Anode sleds target box should be ± 6.6 feet (2 m).
  • Suction pile target box should be ± 3 feet (1 m) (± 1 degree vertical).
  • Installation Supplier should ensure by design that PLETs land in accordance within these tolerances and do not sink into the seabed by more than 4 inches (100 mm) after landing.
  • For trees/wellheads tolerances, the target should be 9.8 feet (3m) radius, ± 1 feet (.3m) height, 1 degree maximum vertical misalignment, and  ± 5 degree heading.
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answered Feb 1, 2015 by jakangaru (2,010 points)

For the Suction Piles Supporting Subsea Equipment

  • Note: Development of project-specific design tolerances are recommended for suction piles supporting subsea equipment because the requirements for different packages of subsea equipment vary.
  • The effects of design tolerances and pile deflections under design and operational loads should be explicitly considered in evaluation of (a) pile capacity, and (b) stresses on jumpers, connectors, flowlines, and structural components.
  • In the absence of project-specific guidance, the following design tolerances should be taken into account in determining loads and sizing suction piles supporting subsea equipment:
    • Inclination: ±3 degrees.
    • Orientation (heading): ±5 degrees.
    • Embedment: 5 feet less than targeted.
    • Erosion: 5 feet local erosion (piping, plowing, or scour) with full embedment.
    • Position: 5 feet from targeted position.

The initial elastic settlement of foundation pile should not exceed 3 inches due to elastic compression of pile and soil system under submerged weight of subsea equipment that is installed atop support pile.

In lieu of more stringent equipment requirements, long-term consolidation settlement of suction piles supporting subsea equipment should not exceed 3 inches.

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