On this page
Key takeaways
- The hydraulic rules are the same: short, pocket-free suction, enough NPSH, and low nozzle loads.
- Onshore, the pump sits at grade under the rack with plenty of room for straight runs and maintenance.
- Offshore, the pump is skid-mounted on a moving deck, so support, motion and removal routes drive the design.
The duty
A centrifugal pump takes liquid from the bottom of a separator and sends it to the next stage. Two identical pumps run as duty and standby. The same duty appears on a refinery unit and on a floating production unit.
Onshore: the refinery version
- The separator is on a concrete plinth, elevated to give the pumps enough NPSH.
- The pumps sit at grade in a row under the pipe rack, each on its own foundation.
- Suction lines drop vertically from the vessel outlet, turn horizontally and run straight into the pumps, with an eccentric reducer (flat on top) and a temporary strainer.
- Discharge lines rise to the rack with a check valve and isolation valve at operating height.
- There is space for a mobile crane to lift out a motor, and for the operators to reach every valve from grade.
The main design checks are NPSH, nozzle loads from the hot discharge line, and the thermal difference between the running and standby pumps.
Offshore: the FPSO version
- Deck area is precious, so the pumps sit on a skid close to the separator, often on a lower deck.
- Suction lines are short and compact, but there is less room for straight runs, so the arrangement is agreed early with the pump vendor.
- The skid, the separator and the deck all move with the hull. Piping between the separator and the skid has to absorb hull deflection and motion, which may call for extra flexibility or a shared support frame.
- Accelerations from vessel motion add loads on the piping and the pump nozzles.
- Pump and motor removal needs lifting beams or monorails and a clear route to a laydown area.
- Liquid level control must handle sloshing in the separator, which affects suction conditions.
Comparison
| Item | Refinery | FPSO |
|---|---|---|
| Pump mounting | Individual foundations at grade | Skid on deck |
| Suction length | Can be generous | Very short and compact |
| Extra load cases | Wind, seismic | Motion, hull deflection |
| Maintenance access | Mobile crane, vehicles | Lifting beams, monorails |
| Main layout driver | Access, NPSH | Space, weight, motion |
Code reference
API 610 Centrifugal pumps, including nozzle loads.
ANSI/HI 9.6.6 Rotodynamic pumps for pump piping.
DNV-RP-D101 Structural analysis of piping systems.
What to take back to your desk
When a layout from an onshore project is reused offshore, check the three things that don’t transfer: motion loads, relative displacement between structures, and maintenance access.
Educational content. Always design to your project specification.
