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answered Feb 5, 2015 by Normandy (21,040 points)

The following are typical design parameters  for an offshore skid:

Structural Steel

  • Structural steel skid design: AISC
  • Structural steel material: ASTM A36
  • Structural steel fabrication: AWS D1.1
  • Skid drain: 2”-150# flanged located in web of structural parameter beam at skid edge. Under skid piping will connect two drain sumps in skid. Drain sumps should be covered with fiberglass grating and flush with skid walking surface.
  • Deck surface: 3/8” thick smooth deck plate – seal welded to main structural members, then coated with non-skid paint system.
  • Containment border: 3” flat bar running along the outer perimeter of skid.
  • Lifting lugs: 4 pad eyes should be provided for single point lift capability.
  • Skid anchoring: mounting to platform deck and should be welded by the deck contractor. No anchor bolt hold is needed.
  • All skid deck penetrations (if any) shall receive curb sleeves that should be as tall as or greater than the height of the containment border at skid edge.

Piping and Tubing

  • All interconnecting equipment piping, within the skid boundaries, should be designed in accordance with ASME B31.3
  • Piping should be designed in a neat and direct manner on common planes as applicable. Low point pocketing should be minimized. Instrument an equipment items should be accessible for operation and removal.
  • Vents and drain connections should be properly sized in adequate number to provide necessary operations and testing.
  • Pipe supports should be fabricated in accordance with AWS D1.1 and should rest on one of the main structural steel skid members. Piping spool should be supported for operating loads in each of the (X and Y) directions. Each equipment item should be self-supported or supported by a fabricated structural frame that rest directly to one or more of the main skid structural members. Due to the unitized nature of construction, piping vertical (Z) loads and equipment nozzle loads should be minimized.
  • Piping should be attached to pipe supports by way of galvanized U-bolts with neoprene sheet gasket between U-bolts and dissimilar materials.
  • Piping / Tubing connections should terminate at skid edge and shall not extend past the vertical plane of the outer skid boundary.
  • Pipe flange connections should have stainless steel spiral wound gaskets.

Electrical / Instrumentation

  • Instruments and controls should be installed according to the manufacturers’ recommendations. Tubing lines should terminate in a bulkhead at skid edge. Tubing and Swagelok fittings should be 316 stainless steel. Controls should be functionally tested after completion of installation.
  • Electrical connections should be made by a CLX metal clad armored cable and gland system Fibergrate trays.
  • Equipment and materials should be Class I Div 2 Gr D classification type installation.
  • All on-skid electrical connections should be terminated in 316 stainless steel Class I Div 2 Gr D rated J-boxes located at skid edge.

Quality Control / Testing

  • Butt weld radiographic inspection of piping should be conducted in accordance with ANSI B31.3 code requirements (100% for ANSI 300# and below, 100% for ANSI 600# above).
  • All full penetration welds associated with the lifting eyes should be 100% UT/Magnetic Particle tested.
  • Piping system should be pressure tested after the pipe spools have been primed and installed on the skid. The hydrotest should be held for one hour and documented with pressure and temperature recorders. The hydrotest medium should be drained upon completion and shop air should be blow-out the system.
  • A standard functional test should include stroking the control valves, air test of the tubing, and a point to point continuity of the wiring.

Protective Coating

The skid should have the protective paint below as a minimum

  • Abrasive blast to near white metal blast with the following application
    • One coat inorganic zinc at 2.5 to 4.0 mils DFT
    • One coat high build epoxy at 3.0 to 5.0 mils DFT
    • One coat polyurethane at 1.5 to 2.0 mils DFT
    • Minimum total dry film thickness to be 7.0 mils
  • Stainless steel and galvanized surfaces should not be painted.
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