Most capital projects in oil and gas, petrochemicals and energy follow a stage-gated path. Each phase adds definition, reduces uncertainty and ends with a decision. Knowing what each phase should deliver helps owners avoid moving forward on an incomplete design.
- Pre-FEED (basic engineering) fixes the design basis and concept.
- FEED defines the plant well enough for an EPC bid and investment decision.
- Detail engineering produces the IFC drawings used to buy, build and commission.
Pre-FEED: fixing the concept
Pre-FEED confirms the selected concept and freezes the process design basis. Typical deliverables are PFDs, heat and material balances, preliminary P&IDs, major equipment data sheets and a plot plan. The estimate is usually AACE Class 4.
FEED: ready for FID and EPC tender
FEED brings every discipline in — process, piping, mechanical, civil and structural, electrical, and instrumentation. HAZOP and SIL studies are completed, a 3D model supports layout and quantities, and the result is an EPC invitation-to-bid package with an AACE Class 3 estimate.
Detail engineering: design for construction
Detail engineering turns the FEED into issued-for-construction drawings, isometrics, specifications and vendor-integrated documents. Changes are expensive at this stage, which is why a thorough FEED pays for itself.
| Phase | Main purpose | Typical estimate class |
|---|---|---|
| Screening / feasibility | Choose the best concept | Class 5 / 4 |
| Pre-FEED | Freeze the design basis | Class 4 |
| FEED | Support FID and EPC bidding | Class 3 |
| Detail engineering | Issue for construction | Class 2 / 1 |
Skipping or compressing front-end phases is one of the most common causes of cost overruns. Investing a small share of total project cost in solid FEL work reduces risk across the whole project.