Design Basis of EPC Projects in the Oil & Gas Industry
A practical, engineering-first reference for defining the basis of design (BoD) in oil & gas EPC projects—covering scope, codes & standards, process data, discipline design criteria, HSE, constructability, commissioning, and sample calculations.
Table of Contents
- Introduction
- Project Objectives
- Design Criteria
- Codes and Standards
- Process Requirements
- Safety Requirements
- Environmental Requirements
- Operability & Maintainability
- Project Interface Management
- Materials Requirements
- Equipment Requirements
- Civil & Structural Requirements
- Electrical & Instrumentation Requirements
- Construction Phasing
- Quality Requirements
- Documentation Requirements
- Schedule Requirements
- Conclusion
1) Introduction & Purpose
The Design Basis—often documented as the Basis of Design (BoD)—is the foundational reference that drives the lifecycle of an EPC (Engineering, Procurement, and Construction) project in the oil & gas sector. It translates business objectives and functional needs into engineering criteria, establishes design envelopes, and anchors decision-making across disciplines. A robust design basis prevents scope ambiguity, reduces change orders, and provides audit-ready traceability from FEED to handover.
Why it matters: The design basis aligns the owner’s requirements, code compliance, process performance, and execution constraints into a single authoritative source. It supports early risk reduction, bid comparability, and lifecycle cost control.
1.1 Scope & Depth — What belongs in a Design Basis?
The BoD should capture criteria and philosophies (what the plant must achieve and the rules used to design it), not every vendor-specific choice or detailed calculation. Think of it as the contractually controlled rulebook that informs FEED and detailed design. It states ranges, targets, methods, and references; the discipline design calculations and vendor selections are developed later and referenced back to the BoD.
| Belongs in BoD | Defer to Detailed Engineering / Vendor Docs |
|---|---|
| Design envelopes (pressures/temperatures/flows), MDMT/MDT rules, relief scenarios list, control & shutdown philosophies. | Valve-by-valve sizing worksheets, vendor relief device certified capacity reports. |
| Codes/standards hierarchy, materials selection philosophy, corrosion allowances, sour-service rules. | Final isometric bill of materials, heat numbers, weld maps (beyond traceability rules stated in BoD). |
| Power system topology, reliability class, short-circuit/arc-flash study methods and acceptance criteria. | Manufacturer-specific protection relay settings files and FAT procedures. |
| SIS lifecycle approach (HAZOP/LOPA method, SIL targets, proof-test intervals concept). | Per-loop SIF verification calculations and test scripts. |
| Firewater demand methodology, redundancy philosophy, impairment management. | Hydraulic model printouts and individual nozzle K-factor selections. |
2) Project Scope, Battery Limits & Objectives
2.1 Scope Definition
Define the facility type (onshore gas processing, offshore topsides, refinery unit, pipeline, LNG, tank farm), phase (greenfield/brownfield), and tie-ins. State design life (e.g., 25–30 years), availability (e.g., ≥ 98% for critical services), and reliability targets (e.g., MTBF, redundancy philosophy).
2.2 Battery Limits (B/L) & Interfaces
Plot limits: Describe the exact site boundary, right-of-way, and corridor width so everyone agrees where civil/utility ownership starts and ends.
Process battery limits: For each interface, specify pressure, temperature, flow, and composition so custody conditions are unambiguous.
Utility interfaces: Define connection points for power, instrument air, nitrogen, water, steam, flare, telecoms, and cybersecurity demarcations, including metering and isolation requirements.
Third‑party interfaces: Summarize dependencies with pipelines, grid operators, refineries, shipping terminals, or JV partners and how changes are controlled.
2.3 Objectives & KPIs
Meet rated capacity across normal and seasonal feed envelopes, including defined turndown limits.
Minimize OPEX via energy efficiency, waste heat recovery, and intelligent controls with measurable targets.
Design for maintainability and inspection without full shut-down where feasible, using access and lifting provisions.
Meet owner’s HSE targets (no LTI, emissions/noise limits, process safety KPIs) with auditable acceptance criteria.
3) Applicable Codes, Standards & Regulations
Identify legal and corporate frameworks. Typical references include (non-exhaustive; confirm local jurisdictional requirements):
| Discipline | Primary References (examples) |
|---|---|
| Process & Safety | API 520/521 (relief systems), API 2000 (tanks), API 14C/14E (offshore), CCPS Guidelines |
| Mechanical | ASME BPVC Sec VIII (vessels), ASME B31.3/B31.4/B31.8 (piping), API 610 (pumps), API 617/618 (compressors) |
| Piping Materials | ASTM (material grades), NACE/AMPP MR0175/ISO 15156 (sour service), MSS-SP |
| Electrical | IEC 60034/60364, IEC 60079 (hazardous area), IEEE/IEC coordination; NFPA 70 where applicable |
| Instrumentation | IEC 61511 (SIS), IEC 61508 (functional safety), ISA‑5.1 (P&IDs), ISA‑84 |
| Civil/Structural | ACI/Eurocode, ASCE 7 (loads), API RP 2A (offshore), AISC Steel Construction Manual |
| Fire & Life Safety | NFPA 30/58/59A/70/72/2001, EN 13565, FM Global Data Sheets |
| Environment | Local permits, IFC EHS Guidelines, MARPOL (offshore), flare/emissions limits per regulator |
Note: The design basis should list the exact editions and any project-specific interpretations, plus the hierarchy when standards conflict (e.g., Local Law > Owner Spec > International Code > Industry Practice).
4) Process Design Basis
4.1 Feed & Product Definition
Feed envelope: Provide composition ranges (mol%), contaminants (H2S, CO2, mercaptans), density/viscosity vs. temperature, water cut, and solids so hydraulics and materials can be selected correctly.
Design cases: Define normal, turndown, start-up, shutdown, emergency, and seasonal extremes to size control elements and reliefs consistently.
Products/specs: State final product/dew-point specs (e.g., water/hydrocarbon dew point, sulfur, RVP/RON/MON) and export conditions (P/T) used to verify compliance.
4.2 Process Flow & Balances
Provide PFDs, heat & material balances (H&MB), and stream lists. Define utility balances (cooling water, seawater, air, nitrogen, steam, power).
4.3 Design Pressures & Temperatures
Design Pressure (DP): For vessels, typically ≥ governing case MAWP with margin; for piping per ASME B31.3 using the most severe coincident P/T (e.g., relief, blocked-in thermal expansion).
Design Temperature (DT): State max/min metal temperatures considering ambient, solar, process, depressurization, and auto‑refrigeration.
Example: For a hydrocarbon line with normal 25 barg @ 60°C
- Upset relief scenario: 34 barg @ 85°C
- Design Pressure: 1.1 × 34 = 37.4 barg (rounded per spec to 38 barg)
- Minimum Design Temperature (MDMT): -29°C (ambient + auto-refrigeration check)
- Maximum Design Temperature (MDT): 90°C (solar + process transient)
4.4 Process Utilities
Define all required utilities including cooling water, air, nitrogen, steam, electricity, and instrument air. Specify supply capacity, quality, pressure, temperature, redundancy, and control philosophy. Consider variations due to seasonal or operational changes and plan for peak demand conditions.
Utility segregation: Identify essential vs. non-essential services, backup provisions, and automatic/manual isolation points for maintenance or emergency scenarios.
4.5 Relief System Basis
Identify all pressure relief requirements for vessels, piping, and equipment. Include relief device type (PSV, rupture disc), sizing criteria, set pressures, allowable overpressure, and discharge location. Ensure compliance with API 520/521, API 2000, and any local regulations.
Design scenarios: Cover start-up, shutdown, upset, blocked-in thermal expansion, fire, and emergency depressurization. Document assumptions, safety factors, and relief network interconnections.
5) Mechanical Design Basis
5.1 Pressure Equipment
Vessels & Columns: Follow ASME Sec VIII; define corrosion allowances (e.g., 1.5–3.0 mm), MDMT methodology, PWHT rules, and nozzle load checks per WRC so vendors design to consistent limits.
Heat Exchangers: Apply TEMA; set shell & tube sizing rules (velocity limits, LMTD/approach), and specify when plate exchangers are acceptable, keeping final sizing consistent with vendor standards.
Air Coolers: State ambient extremes, fan power estimation method, and noise limits to manage plot space and community impact.
5.2 Rotating Equipment
Pumps (API 610): Require minimum NPSH margin (e.g., ≥ 1 m or ≥ 20% above NPSHr), minimum flow protection (recycle/VFD), mechanical seals per API 682 with allowable emissions for hazardous fluids, and vibration/acceptance criteria per API.
Compressors: Set minimum surge margin (e.g., ≥ 10–15%), dry gas seals where suitable, API-compliant lube systems with redundancy, and continuous vibration monitoring with shutdown criteria.
Turbomachinery: Define performance test standards, driver selection philosophy (electric vs gas turbine) based on availability and overall energy balance, and minimum efficiency guarantees at site conditions.
5.3 Piping Design
Line sizing: Use velocity and pressure-drop limits appropriate to service, with explicit two-phase criteria to mitigate slugging/erosion. Provide target velocity ranges by fluid type, allowable ΔP per 100 m, and noise thresholds near sensitive equipment.
Flexibility analysis: Use CAESAR II/ROHR2 to verify thermal displacement, allowable nozzle loads, sustained/occasional load cases, and the need for springs or expansion joints. Define analysis load cases and allowable stress criteria in the BoD.
Stress categories: Follow ASME B31.3, explicitly defining occasional loads (seismic, wind, slug forces) and the limited use of expansion joints (approval, prohibited zones, inspection needs).
5.4 Sample Line Sizing Rule-of-Thumb
- Single-phase liquids: 1–3 m/s for suction, 2–4 m/s for discharge
- Hydrocarbon gas: 10–20 m/s (lower near compressors to manage noise)
- Steam: 20–35 m/s (main headers on the low end)
- Pressure drop targets: liquids 0.3–1.0 bar/100 m; gas 0.05–0.2 bar/100 m (context dependent)
6) Piping Materials & Corrosion Philosophy
6.1 Fluid Service Categories
Define categories (e.g., sour, lethal, flammable, erosive, cryogenic). Assign each service to a piping class that dictates materials, corrosion allowance, flange ratings, valve trims, gasket types, and NDT levels.
6.2 Materials Selection
- Base materials: Provide selection rules by service—carbon steel (A106/A53) for benign duties, LTCS (A333) for low temperatures, Cr‑Mo (P11/P22) for high-temperature creep resistance, austenitic stainless (304/316/316L) for general corrosion, duplex (UNS S31803/S32205) for chloride SCC risk, and Ni‑alloys for severe environments. Detailed MTO can be finalized during execution.
- Sour service: Comply with NACE/AMPP MR0175/ISO 15156; define hardness control limits, material restrictions, and PWHT requirements to ensure WPS/PQRs align with SSC resistance.
- Cladding/linings: Specify when internal linings or CRA cladding (e.g., 316L, Inconel overlay) are preferred over solid alloy based on lifecycle cost, repairability, and inspection access; finalize thicknesses during detailed design.
6.3 Corrosion & Erosion Controls
- Set corrosion allowances by service (e.g., 1.5 mm for utilities, 3.0 mm for produced fluids) with rationale tied to expected rates and inspection intervals.
- Define mitigation strategies: chemical inhibition, cathodic protection for buried/submerged systems, internal coatings/linings, velocity limits to reduce erosion-corrosion, and sand production management.
- Require brittle-fracture checks using MDMT and impact-tested grades where applicable; define acceptance criteria while procurement verifies MTRs.
6.4 Positive Material Identification (PMI) & Traceability
Specify PMI requirements for alloy lines (e.g., ≥ 5% sample or 100% for critical services), EN 10204 3.1/3.2 MTCs, and full heat-number traceability linking back to isometrics and weld maps.
7) Electrical Power System Design Basis
7.1 System Topology & Ratings
Define grid interconnection limits, on-site generation (GTGs/DGs), distribution voltages (e.g., 33/11/6.6 kV), ring vs radial philosophy, and target reliability class. Makes/protection settings are defined at later stages.
List studies: short-circuit, arc-flash, load flow/voltage profile, motor starting, and harmonics with THD limits at PCC. Specify study methods and acceptance limits rather than final trip settings.
7.2 Equipment & Area Classification
For hazardous areas, require selection per IEC 60079 with protection concepts (Ex d/e/i/n), correct gas group/temperature class, and appropriate IP ratings. Datasheets are detailed later.
State switchgear/transformer philosophies (e.g., ONAN/ONAF), UPS/inverter architectures, and minimum battery autonomy (e.g., 30–60 min); exact ratings are finalized in detailed engineering.
7.3 Earthing, Bonding & Lightning
Target overall grid resistance (e.g., ≤ 1–5 Ω depending on soil), perform mesh/step voltage checks, bond all metallic structures/piping, and specify lightning protection per IEC 62305 with coordinated surge protection.
7.4 Cables & Routing
Size cables by ampacity and voltage drop (e.g., < 3% for motor feeders), applying derating for ambient, grouping, and soil conditions. The BoD sets limits; the cable schedule is produced later.
Define routing/separation rules between power and instrumentation, respect fire/blast barriers and zoning, and maintain tray fill limits. Final route drawings are outputs of detailed design.
8) Instrumentation, Controls & Safety Systems
8.1 Control System Philosophy
Specify DCS/PLC topology with hot-standby controllers and redundant networks, historian requirements, and alarm rationalization per EEMUA 191 (e.g., max alarm rates/priorities). Cabinet layouts and I/O lists are detailed later.
Define OT cybersecurity: zone-and-conduit segmentation, remote-access rules, patch cadence, and time sync (NTP/PTP). Vendor-specific hardening is attached at execution.
8.2 Instrumentation Basis
Flow & level measurement technologies: Selection is based on service conditions and required performance, with the BoD stating criteria (not brand) for each technology:
- DP/orifice (flow): Suitable for clean, single-phase services where moderate accuracy (±1–2% of rate) and low cost are priorities; requires straight-run lengths and incurs permanent pressure loss.
- Coriolis (flow): Preferred for high-accuracy mass flow (±0.1–0.2% of rate), varying density/viscosity, or custody/chemical dosing; sensitive to two-phase slugs and line vibration.
- Ultrasonic (flow): Inline or clamp-on options for large diameters and bidirectional measurement with low pressure loss; performance depends on acoustic coupling and requires defined straight runs.
- Radar (level): Time-of-flight/FMCW radar handles wide ranges, vapor, and turbulence better than DP/bubblers; still requires consideration of nozzle geometry, internals, and dielectric constant.
For each service, specify minimum accuracy (% of reading), turndown ratio, straight-run needs, viscosity/solids/gas entrainment limits, and environmental constraints (temperature/pressure/hazardous area).
Control valves: Require trims suited to severity (anti-cavitation/multi-stage where flashing is credible), aerodynamic/hydrodynamic noise prediction with allowable SPL at 1 m, fail-safe action (FC/FO/FIP), minimum installed rangeability, leakage class per IEC/FCI, and digital positioners with diagnostics on critical loops.
8.3 Safety Instrumented System (SIS)
Follow IEC 61511: HAZID/HAZOP ➜ LOPA ➜ SIF definition ➜ SIL targets ➜ proof-test intervals ➜ bypass management, independent from the BPCS.
Define ESD/F&G interlocks, partial-stroke testing, common-cause mitigation, and proof-test coverage assumptions used in PFDavg calculations.
8.4 Fire & Gas Detection
Describe where point, open-path, and flame (IR/UV) detectors are used; require gas mapping for coverage; define redundancy and voting logic (e.g., 2ooN), alarm setpoints/time delays, and detector survivability for credible fire scenarios.
Define integration between F&G, ESD, PA/GA, and deluge/foam systems, including impairment procedures and override governance. Exact locations and I/O are produced in detailed design.
9) Civil, Structural & Architectural Basis
Geotechnical Investigations
Summarize SPT/CPT results, soil shear strength, settlement, groundwater, seismicity, flood/wind loads, and soil corrosivity so that foundations and corrosion protection can be designed based on real site data.
Foundations
Define selection criteria for piles, raft, or spread footings, include dynamic checks for rotating equipment foundations, and apply vibration isolation to avoid resonance with machinery.
Steel Structures
Provide load combinations (dead/live/wind/seismic/thermal), fireproofing thickness basis, and blast design where applicable.
Buildings
For CCRs, substations, and analyzer shelters, define HVAC classification, pressurization, gas-tight construction, and life-safety egress requirements.
Drainage
Set oily/water separation strategy (API separators), containment curbs, and emergency drainage routes to protect the environment and assets.
10) HSE, Risk, Fire & Environmental Basis
10.1 Risk Methodology
Maintain a project risk register; conduct qualitative and quantitative risk assessments (QRA), demonstrate ALARP, and use bow-tie analyses where they add clarity. Produce safeguarding diagrams; perform occupied building risk assessment (OBRA), explosion and fire studies (EFS), and document assumptions and mitigations.
10.2 Fire Protection
Demand & Application: Define the methodology to calculate firewater and foam demands for hydrants, monitors, hoses, deluge/foam systems, and storage, including duration and simultaneous scenarios used.
Hydraulics & Reliability: Require looped/sectionalized fire mains with hydraulic verification, impairment management, and redundancy (e.g., two diesel fire pumps + electric jockey) with auto-start logic and test regimes.
10.3 Environmental
Set air emission limits (NOx/SOx/VOCs), flare minimization strategy, LDAR program requirements, and site noise/light limits with measurement methods. Define effluent quality targets (oily/produced water), waste management segregation (hazardous/non-hazardous), and spill containment/response rules.
10.4 Human Factors & Life Safety
Plan escape, evacuation & rescue (EER), toxic gas muster strategies, egress routes, signage, and PA/GA coverage that match credible scenarios and occupancy. Address ergonomics: control room/HMI design, manual handling limits, access platforms, and clearances for routine tasks.
11) Constructability, Maintainability & Operability
Modularization
Define transport envelopes, lift studies, interface management plan, and the hook-up/commissioning split between yard and site.
Maintainability
Require removable spools, davits/monorails, laydowns, and online maintenance provisions with clear access envelopes to ensure safe and efficient routine work.
3D Reviews
At 60% and 90% model completion, set clash resolution targets, egress/access checks, and maintainability walkdown actions that must be closed before IFC release.
Spares & Predictive Maintenance (PdM)
Define critical spares strategy, including two-year running spares, lubrication/oil management, and predictive maintenance sensors/data requirements to maximize equipment uptime.
12) Procurement, QA/QC & Inspection Basis
Vendor Qualification
Use an Approved Vendor List (AVL), conduct audits, and review past performance; include cybersecurity expectations for smart/connected devices.
Technical Requisitions
Standardize datasheets, design conditions, Inspection & Test Plans (ITPs), and hold/witness points so vendor bids are comparable and aligned with project requirements.
QA/QC
Apply ISO 9001 practices, welding WPS/PQR rules, and nondestructive testing (RT/UT/PAUT/MPI/DPI), along with painting and lining specifications to ensure quality compliance.
FAT/SAT
Define functional and performance tests, reliability runs, and receipt inspections with clear acceptance criteria for equipment and systems before handover.
13) Pre-commissioning, Commissioning & Start-Up
Systemization & Handover
Define systemization and handover of sub-systems, discipline check sheets, punch categorization (A/B/C), and turnover dossier structure.
Pre-commissioning Activities
State requirements for cleaning, flushing, blowing, leak tests, instrument calibration, loop checks, motor solo runs, and cause & effect testing.
Operational Readiness
Set operational readiness criteria including procedures, training, spares, CMMS master data, and performance test acceptance methods.
Commissioning Basis Example
Hydrostatic tests per ASME with test pressure = 1.5 × design pressure for vessels; piping tests per class; pneumatic tests only by exception with enhanced controls.
14) Document Control, Data Models & BIM/3D
Document Control
Define numbering, revisions, status codes (IFR/IFA/IFC/As-Built), transmittals, and distribution matrices.
Data-Centric Engineering
Maintain a single source of truth for tag registry, line/equipment lists, cable schedules, loop index, and keep it referenceable from vendor data.
3D/BIM
Set model maturity gates, intelligent P&IDs, use of laser scans for brownfield tie-ins, and define the scope for digital twins (if any).
Information Handover
Standardize structured data formats (e.g., ISO 15926 concepts), vendor data books, and O&M manuals.
15) Change Management & Deviation Control
Define Management of Change (MOC) workflows: triggers, approvals, impact analysis (safety/cost/schedule/quality), and as-built capture. Clarify deviation/waiver process for departures from codes/specifications, with risk sign-off and mitigation actions.
Warning: Uncontrolled changes are a leading cause of cost/schedule overrun. Enforce discipline by routing all technical and scope changes through MOC.
16) Design Basis Deliverables & Checklists
16.1 Core Deliverables
- BoD report with signed approvals and revision history.
- Process: PFDs, H&MB, control narratives, safeguarding diagrams, relief load summary, utility balances.
- Mechanical: equipment datasheets, line sizing basis, stress criteria, rotating equipment philosophies.
- Piping: piping classes, line list, valve specs, insulation/painting specs, PMI/traceability plan.
- Electrical: SLDs, load list, earthing, lighting, heat tracing, hazardous area classification drawings.
- Instrumentation: I/O list, control system topology, F&G detection philosophy, SIL targets, alarm strategy.
- Civil/Structural: design criteria, geotechnical summary, load tables, foundation types, drainage philosophy.
- HSE/Environment: risk basis, firewater/deluge basis, emissions/effluent limits, waste plan.
16.2 Review & Approval Gates
30% Concept Freeze → 60% Discipline Alignment → 90% IFC‑Ready → 100% Approved for Use.
Hold/witness reviews with Owner, PMC, Certifying Authorities (as required), and Insurers for high‑risk systems.
16.3 Checklists (Extract)
- Have all design cases been identified (normal, upset, start‑up/shutdown, emergency)?
- Are relief scenarios quantified and traced to the flare/vent system hydraulic model?
- Are materials suitable for MDMT, corrosion, and sour service? PMI/traceability defined?
- Are hazardous areas classified and reflected in electrical/instrument layouts?
- Is the firewater demand calculated, with redundancy and impairment plans?
- Are operability and maintainability reviews actioned with 3D model updates?
17) Conclusion
A clear, criteria‑driven design basis creates alignment across stakeholders, reduces change orders, and shortens commissioning by removing ambiguity early. Use this page as a template and tailor the acceptance criteria, code editions, and environmental constraints to your project and jurisdiction.