+2 votes
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2 Answers

+2 votes
answered Jun 6, 2015 by russelbraley (435 points)

Typical requirements for an Offshore Loading System should have the followings to have a safe and technically sound design:

  • The Offshore Loading System should be able to transfer, at the design rate, hydrocarbon fluids from the seabed riser base to a dynamically positioned (DP) off-loading shuttle tanker.
  • The Offshore Loading System can be used in water depths ranging from 200 ft to 2000 ft (61 m to 610 m).
  • A DP reference system.
  • A secure flexible pipe connection between a sealed flowline and a shuttle tanker manifold connection.
  • A coupling for connection to a tanker manifold.  Once it is brought to the shuttle tanker manifold, connection of the coupling head to the manifold should be automatic.
  • A section tensioned by a subsurface buoy and a catenary section that connects to a shuttle tanker manifold.
  • Sufficient valves and hydro-acoustic controls to allow safe operation, installation, maintenance and replacement.
  • 360 degree shuttle tanker weathervaning.
  • A safe operating radius from its base.  The offset limits should be compatible with DP capability.  When not loading, the catenary section should rest on the sea floor.
+1 vote
answered Sep 22, 2015 by DeanBidot (360 points)

When analyzing or selecting an offshore loading system, you need to evaluate the following:

Functional Loads

All appropriate functional loads, such as in situ pre-stressing loads, temperature stress, internal and external pressure and shuttle tanker offset, should be included.

The shuttle tanker offsets should be computed with due consideration to the effects of wind, wave and current, in conjunction with the DP system operation, to counterbalance the offset and low frequency motions.

Accidental Loads

  • The following accidental loads should be evaluated:
    • Loss of buoyancy in the subsurface buoy
    • Shuttle tanker DP failure and emergency disconnect procedure
    • Pressure surge
    • Malfunction of valves
  • These loads should be taken in combination with the appropriate operating condition.
  • The governing design requirements should be taken from DNV Rules.
  • Load transfer through the entire system should be evaluated for all possible operating and survival configurations.

Vessel Motion

  • The vessel motions should be analyzed according to API RP 2SK.  It clearly describes the forces to be considered for the shuttle tanker motions and offsets.  It also outlines a procedure for combining the response statistics of the motion and offsets.
  • The riser top motions are governed by the motions of the shuttle tanker during off-loading operations.
  • Guidelines are likewise provided for the design of the DP system for station keeping.
  • The environmental forces acting on the vessel should be estimated according to OCIMF.
  • The mean forces from wind, current, and wave should be computed to obtain total steady forces.
  • The dynamic effects of waves should be computed by the use of a radiation-diffraction program for ship motions.
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