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Key takeaways
- A spool is a prefabricated section of piping sized to be transported and lifted into place.
- Shop fabrication gives better welding quality and productivity than field work, so projects aim to make as many welds as possible in the shop.
- Field welds and field fit welds connect spools on site and absorb dimensional differences.
The journey of a spool
- Isometric issued for construction (IFC) with spool breaks marked.
- Spool drawings: the fabricator splits each isometric into spool sheets with cut lengths and weld numbers.
- Material allocation: pipe, fittings and flanges are allocated with heat numbers for traceability.
- Cutting and bevelling: pipe is cut and the ends prepared, usually to a 37.5° bevel for butt welds.
- Fit-up and tack welding: components are aligned and held with tack welds. Fit-up is inspected before welding.
- Welding: to a qualified welding procedure (WPS) by a qualified welder, with the welder’s identification recorded for each weld.
- NDE: visual, then radiography or ultrasonic testing, and PT or MT as required by the piping class.
- PWHT where the material and thickness require it.
- Dimensional check against the spool drawing.
- Surface preparation and coating (blasting, painting or galvanizing) or pickling and passivation for stainless steel.
- Marking, end protection and shipping: each spool is tagged, ends are capped, and it is packed for transport.
Where it goes wrong
- Wrong material on the wrong spool, which is why traceability and PMI (positive material identification) exist for alloy piping.
- Dimensional errors from an outdated isometric revision.
- Missing branches or small-bore connections that get added in the field later.
- Damage in transport, especially to bevels, flange faces and coatings.
How big can a spool be?
It depends on transport: road limits, container sizes and site lifting. Many projects design spools that fit roughly 12 m long by about 3 m by 3 m, but always confirm with logistics.
Onshore
Onshore spools are trucked to site and erected on racks and around equipment. Field welds are common at tie-ins to equipment and between rack spools.
Offshore
Offshore, most spools are installed in modules in the fabrication yard. Hook-up spools between modules, or between the topsides and the hull, are measured after the modules are set and fabricated or trimmed to suit.
Code reference
ASME B31.3, Chapter V Fabrication, assembly and erection.
NORSOK L-004 Piping fabrication, installation, flushing and testing.
What to take back to your desk
Before issuing isometrics for fabrication, check every spool break against transport and lifting limits. A spool that doesn’t fit the truck gets cut, and that cut becomes an unplanned field weld.
Educational content. Always design to your project specification.
