Gas plants are capital-intensive, multi-discipline beasts. Miss on scope or sequence and costs explode, schedules slip, and confidence fades. 

In large capital programs broadly, reputable studies show chronic budget and schedule slippage, vast majorities of megaprojects run over. It is exactly why the front end has outsized leverage on outcomes.

This guide clarifies when to use FEED (Front End Engineering Design) versus jumping straight to detailed design, and how that choice affects risk, cost accuracy, procurement timing, and delivery.

What FEED Actually Does

Think of FEED as the bridge from concept to buildable intent. It locks the process basis and key design criteria, producing PFDs/P&IDs, plot plans, preliminary equipment specs, and the safety backbone. HAZOP and SIS/SIL planning per IEC standards, for example.

The payoffs are tighter estimates and fewer surprises. In the AACE estimate-class framework, moving from conceptual (Class 5/4) toward Class 3 typically improves accuracy to roughly −10%/−20% to +10%/+30% depending on complexity. It is far better than the ±50% conceptual range often cited for early studies.

On cost, FEED commonly falls in the ~2–3% of TIC range (some programs cite ~3–5% depending on depth and complexity), but that spend underwrites sharper scope, procurement strategy, and construction planning.

Safety and operability analysis belong here. Use IEC 61882 to structure HAZOPs and IEC 61511 to frame SIS lifecycle/SIL determination.

What Detailed Design Delivers

Detailed design transforms FEED intent into construction-ready drawings, isometrics, data sheets, cable schedules, control narratives, and procurement packages across mechanical, electrical, I&E, civil/structural. At this point, estimate precision typically tightens again (toward Class 2/1 bands) with narrower ranges suitable for bids and Final Investment Decisions.

The distinction in plain language: FEED defines the right plant; detailed design defines every bolt of the right plant.

1) Cost Accuracy You Can Defend

FEED narrows uncertainty from conceptual swings (±50%) toward Class-3-like ranges (often in the ~±15–30% envelope for process industries), enabling credible budgets, contracting strategy, and financing.

Independent and vendor literature report that robust FEED/front-end loading correlates with lower total installed cost and shorter execution. Some benchmarks cite material cost reductions and schedule improvements when FEED is thorough.

2) Fewer Technical Surprises

FEED is where you validate process simulations, unit operations, and operability/maintainability. Run HAZOP; assign SIL to SIFs; and stress-test tie-ins.

Doing it here prevents costlier changes later and anchors mandatory safety/protection requirements for detailed design.

3) Schedule You Can Actually Hit

A complete FEED lets you start procurement in parallel with detailed design. Critical when long-lead packages (e.g., large compressors, major electrical gear) run 20–60 weeks or more, with some compression equipment out a year+ in tight markets. Early identification and pre-bid/vendor engagement protect the critical path.

For LNG and large gas processing, FEED itself can take 12–18 months, but that time produces a package that de-risks EPC and informs long-lead buys.

• Technical/Process: complex separations, acid gas removal, sulfur recovery, and high-consequence safety envelopes—best handled with standards-driven HAZOP/SIS governance.
• Commercial: inflation, supply-chain volatility, and scarce specialty resources. Sector-wide analyses still see 30–45% average budget/schedule variance on major programs without better controls/visibility.
• Regulatory/ESG: tightening emissions and permitting expectations add steps you want planned, not discovered.
• Operational: 20–30-year life cycles demand flexibility for feed changes and future debottlenecking.

Cost discipline early. Use FEED to standardize equipment, simplify process trains, and remove bespoke one-offs. Lock an AACE-aligned basis of estimate and contingency logic; socialize it with financiers and partners to avoid late-stage resets.

Safety first, on paper. Complete HAZOP and define SIS lifecycle/SIL targets before detailed design. Treat the outputs as design requirements, not advice.

Procurement strategy early. Identify long-lead items during FEED; pre-qualify vendors and launch RFPs on the first safe opportunity. Many MEP/electrical packages (switchgear, AHUs, large valves) now see 20–60-week windows; large compression skids may extend 12+ months.

Parallelize smartly. With process requirements frozen and key specs set, detailed design can progress while long-lead orders and early works start—shortening your critical path.

Green lights typically require:

• Technical maturity: simulations closed, FEED-level P&IDs/plot plan, HAZOP actions addressed, preliminary 3D/constructability passes done.
• Commercial readiness: budget approved, funding plan in place, contracting model set, long-lead procurement strategy defined.
• Permitting/ESG: material approvals on track to avoid EPC stalls.
• Risk posture: if you must accelerate, quantify what’s “at risk” (and cap it). Sector analyses warn that under-cooked front ends are a common root cause of overrun/overrun.

1. Own a living risk register from FEED onward, covering technical/commercial/schedule/regulatory line items with owners and triggers.
2. Favor proven tech unless the business case justifies pilot/prototype risk; if you must push tech, secure vendor guarantees and performance bonds.
3. Plan compliance in FEED—early agency engagement, environmental baseline work, and submissions sequenced to your long-lead timeline.
4. Build real contingency: technical alternates, schedule recovery options, and cost mitigation actions you can actually execute.

• Use thorough FEED for high-complexity, first-of-a-kind, brownfield tie-ins, constrained sites, or tight safety envelopes.
• Consider acceleration only when market timing justifies it and you can quantify the added risk (and carry the contingency).
• Don’t skip FEED to “save” 2–3%—front-end investment routinely saves multiples downstream via fewer changes, cleaner procurement, and faster commissioning.
• Match to capability: experienced owner’s teams may compress phases; others should buy rigor with experienced FEED partners.

• Scope clarity: Process basis frozen? Battery limits clear?
• Safety: HAZOP complete (key actions closed), SIS/SIL targets set per IEC 61511?
• Estimate maturity: AACE-aligned class with documented assumptions/contingency?
• Procurement: Long-lead list finalized; RFPs/tenders staged; vendor shortlist agreed?
• Schedule logic: FEED→detailed design overlap defined; early works identified; critical path driven by long-lead reality (not hope)?
• Permits/ESG: filings sequenced to avoid EPC stalls?
• Change control: frozen-line philosophy and governance in place?

1. Adopt a narrative template mapped to ISA-106 states plus ISA-18.2 alarm hooks; pilot it on one complex unit.
2. Publish an alarm philosophy one-pager (priorities, KPIs, standing-alarm rules) and socialize it at the console.
3. Stand up a role-based training index tied to your OQ program (API RP 1161/PHMSA FAQs) so every trainee knows the modules to complete before CSU.