What are the procedures for decommissioning offshore platforms?

At-a-Glance: Offshore platform decommissioning follows a phased, risk-led sequence: plan and permit, plug and abandon (P&A) wells, clean and make-safe, disconnect pipelines/umbilicals, remove topsides and substructure, recover subsea assets, then verify seabed clearance and close out. Success hinges on barrier integrity, safe SIMOPS, metocean windows, waste/recycling performance, and cost/schedule control.

I. Objective Definition and Key KPIs

• I.1 Objective: Execute full or partial removal of offshore facilities to regulatory standards while eliminating hydrocarbon risk, minimizing HSE exposure, and optimizing cost, schedule, and environmental footprint.
• I.2 Scope (typical): P&A all wells, hydrocarbon removal and decontamination, electrical/mechanical isolation, pipeline/umbilical decommissioning, topsides and substructure removal, subsea recovery, seabed clearance, onshore dismantling and recycling, environmental monitoring.
• I.3 Key KPIs:
  • Safety: TRIR = 0.5; zero Major Accidents; PTW compliance = 99%.
  • Integrity: Barrier verification 100%; cutting success first-pass = 95%.
  • Throughput/Schedule: P&A days/well; heavy-lift critical path adherence = 95%; weather downtime = planned P50.
  • Cost/OPEX: Cost variance = ±10%; vessel utilization = 85%.
  • Environment: Recycling rate = 95% by weight; spills = 0; CO2e tracking within plan; discharge OIW within permit.
  • Uptime: DP incidents = 0; SIMOPS conflicts resolved within 24 hours.

II. Critical Parameters and Target Ranges

Assumptions marked “estimated” where jurisdiction-specific regulations apply.

II.A Key Engineering Formulas

• Hydrostatic head (well kill): \(P_h = \rho_m g \cdot TVD\), \(\Delta P = P_h - P_p\)
• Cement volume (annulus): \(V = \pi (r_o^2 - r_i^2) L\)
• Sling leg tension (static): \(T_{\text{leg}} = \dfrac{W \cdot \gamma}{n \cdot \cos\theta}\), include dynamic amplification: \(W\_\text{dyn} = W \cdot DAF\)
• Required crane capacity: \(C \ge W\_\text{dyn} \times SF\)
• Tow drag (order-of-magnitude): \(R \approx \tfrac{1}{2} \rho C_d A V^2\); required bollard pull \(BP \ge \dfrac{R}{\eta} \times 1.2\)
• Net buoyancy (refloat options): \(B\_\text{net} = \rho\_{w} g V\_{disp} - W\); stability criterion “GM” > project minimum.
• Emissions: \(CO2e = \sum(Fuel_i \times EF_i) + Flaring \times EF\_{flare}\)

III. Step-by-Step Procedure / Workflow / Checklist

III.1 Phase 0 — Front-End Planning and Permitting

• 3.1.1 Define end state: Full removal vs partial removal vs reefing (where allowed). “Estimated” decisions subject to regulator.
• 3.1.2 Engineering surveys: Structural integrity, topsides weight reconciliation, as-builts, subsea mapping (MBES/SSS), metocean, UXO screening.
• 3.1.3 Decommissioning program and permits: Submit P&A and removal plan, waste plan, environmental impact, navigational notices.
• 3.1.4 Contracting & logistics: P&A rigs, LWIV/DSV, heavy-lift vessel (HLV), transport barges, onshore yard selection, customs/ports.
• 3.1.5 SIMOPS plan: Interface matrix among P&A, cleaning, subsea, and lifting teams; 500 m safety zone management.

III.2 Phase 1 — Well Plug & Abandonment (P&A)

• 3.2.1 Well diagnostics: Pressure, integrity, cement bond logs, barrier verification.
• 3.2.2 Kill and cleanout: Establish overbalanced mud; circulate to clean; set mechanical plugs as needed.
• 3.2.3 Permanent barriers: Place qualified cement across reservoirs and cap rocks (100–150 m “estimated”); verify by logs/tag.
• 3.2.4 Casing/conductor severance: Cut at or below mudline; retrieve wellheads/trees.
• 3.2.5 Documentation: Barrier schematics, pressure tests, abandonment reports submitted to regulator.

III.3 Phase 2 — Hydrocarbon Removal and Decontamination

• 3.3.1 Inventory isolation: ESD/ESV closure; line blinds/spades; verify zero energy.
• 3.3.2 Drain/vent: Drain liquids to slops; inert and vent gas systems; ensure LEL < 1%.
• 3.3.3 Chemical cleaning/pigging: Circulate detergents/solvents; mechanical pigging for pipelines; sample OIW until within permit.
• 3.3.4 Waste handling: Segregate slops, NORM, mercury, PCB paint “if present”; manifest and ship to licensed facilities.
• 3.3.5 Gas-free certification: Authorized testing for hot work/entry.

III.4 Phase 3 — Electrical/Mechanical Isolation

• 3.4.1 De-energize and verify: LOTO on power, hydraulics, pneumatics; test-for-dead and ground.
• 3.4.2 Remove hazardous materials: Batteries, refrigerants, chemicals, fire suppression mediums.
• 3.4.3 Structural prep: Install padeyes, grillage, sea-fastenings; cut module interfaces per lift plan.

III.5 Phase 4 — Pipelines, Umbilicals, Cables

• 3.5.1 Disconnection: Depressurize; blind flanges; install pipeline isolation plugs where required.
• 3.5.2 Decommission-in-place vs removal: “Estimated” per permit; trenching/rock-dump if left in situ; recover spools, jumpers, PLEMs.
• 3.5.3 Verification: Pressure at zero; double-block and bleed confirmed; end caps fitted.

III.6 Phase 5 — Topsides Removal

• 3.6.1 Weight/CoG confirmation: As-built survey; reconcile to within ±5% for lift plan.
• 3.6.2 Lifting method: Single-lift (HLV) or modular reverse-installation. Validate sling geometry and \(T_{\text{leg}}\).
• 3.6.3 Execution window: Metocean within limits; DP footprint established; exclusion zone enforced.
• 3.6.4 Lift, transfer, sea fastening: Execute lift; place on barge; weld/grout grillage; voyage-ready inspection.

III.7 Phase 6 — Substructure (Jacket/GBS) Removal

• 3.7.1 Pre-cut prep: Flood/ballast plans; marine growth survey; anode and appurtenance removal as required.
• 3.7.2 Pile cuts: Internal abrasive water jet cutting preferred; explosives only per permit and risk ALARP. Cut = 3 m below seabed.
• 3.7.3 Jacket lift or refloat: Verify lift points; confirm \(C \ge W\_\text{dyn} \times SF\). For refloat, ensure \(B\_\text{net} > 0\) and GM margin.
• 3.7.4 Transport: Barge stability and seafastening calculations; weather route planning; bollard pull check.

III.8 Phase 7 — Subsea Infrastructure Recovery

• 3.8.1 ROV campaigns: Recover mattresses, grout bags, clump weights, anchors, subsea structures.
• 3.8.2 Cable/umbilical handling: Cut, cap, and lift; reel recovery where feasible.

III.9 Phase 8 — Seabed Clearance and Verification

• 3.9.1 Debris trawl/ROV inspection: Clear debris; confirm no protrusions above acceptance criteria.
• 3.9.2 Hydrographic survey: MBES + side-scan sonar; produce digital terrain model; third-party verification if required.
• 3.9.3 AtoN updates: Notify maritime authority; remove marks/aid as instructed.

III.10 Phase 9 — Onshore Dismantling and Waste Management

• 3.10.1 Yard intake: Radiation/NORM screening; environmental controls; stormwater protection.
• 3.10.2 Segregation & recycling: Ferrous/non-ferrous, plastics, concrete; hazardous streams isolated.
• 3.10.3 Material tracking: Weighbridge logs; chain-of-custody; recycling certificates.

III.11 Phase 10 — Close-Out

• 3.11.1 Regulatory close-out: Submit abandonment and clearance reports; lessons learned.
• 3.11.2 Environmental monitoring plan: Post-decommission surveys per permit interval.

IV. Risk & Mitigation (HSE, Reliability, Redundancy)

• IV.1 Hydrocarbon/pressure hazards: P&A barrier failure; Mitigation: double barriers, pressure tests, independent verification, emergency well control on standby.
• IV.2 SIMOPS conflicts: P&A vs lifting vs subsea; Mitigation: SIMOPS matrix, single point of control, phased exclusion zones, dedicated marine controller.
• IV.3 Lifting/dropped objects: Overload or sling failure; Mitigation: engineered lift plans, NDT on padeyes, certified rigging, real-time heave/DAF monitoring, secondary retention.
• IV.4 Cutting operations: Explosive risk, kickback; Mitigation: prefer internal abrasive cutting, blast-design and marine life mitigation if explosives, cut verification runs.
• IV.5 DP/vessel incidents: Loss of position; Mitigation: DP2/DP3 redundancy, critical spares, SIMOPS DP watch, weather abort criteria.
• IV.6 Environmental releases: Residual hydrocarbons, NORM; Mitigation: thorough cleaning, containment, closed-loop waste, continuous OIW and gas detection.
• IV.7 Weather downtime: Narrow windows; Mitigation: seasonal planning, probabilistic weather routing, standby windows, go/no-go gates.
• IV.8 UXO/unknown seabed hazards: Clearance survey, ALARP standoff, qualified EOD response if required.
• IV.9 Personnel exposure: Minimize manual intervention via ROVs, modularization, remote monitoring; strict PTW and confined-space protocols.

V. Optimization Levers (Cost, Schedule, Emissions)

• V.1 Campaign sequencing: Batch P&A by well type; batch cutting and recovery to reduce re-mobilizations.
• V.2 Lift strategy: Single-lift for large integrated topsides when metocean/HLV allow; modular reverse-installation where weight/CoG uncertainty is high.
• V.3 Digital/analytics: Weight reconciliation using LIDAR/photogrammetry; dynamic lift simulations with real sea states; predictive weather downtime models (P50/P90).
• V.4 Subsea efficiency: Pre-weakening cuts; smart pigging to minimize chemical usage; AUV pre-surveys to shorten ROV time.
• V.5 Contracting/logistics: Integrated vessel spreads; back-to-back charters to raise utilization; shore-base proximity to yard to cut transit fuel.
• V.6 Emissions reduction: Optimize DP power management; low-sulfur fuels; hybrid power for accommodation; minimize flaring by inerting and recovery.
• V.7 Waste value recovery: Early scrap market engagement; segregate high-value alloys; concrete reuse pathways.

VI. Verification & Monitoring Plan

• VI.1 What to measure:
  • Barriers: Pressure tests; cement log outcomes.
  • Process cleanliness: OIW mg/L; LEL readings; sludge %.
  • Lifts: Real-time hook load vs plan; DAF; crane utilization %.
  • Subsea clearance: MBES/SSS coverage = 100% with 20–30% overlap; ROV visual confirmation.
  • Waste/recycling: Tonnes by stream; recycling %; hazardous manifests closed out.
  • Emissions: Fuel by vessel/day; CO2e by activity; variance vs plan.
  • HSE/SIMOPS: PTW audits; TRIR; lifting audits; DP event logs.
• VI.2 Frequency:
  • Daily: HSE, fuel, DP, PTW, vessel utilization.
  • Per-lift: Pre-/post-lift checks, hook load trace, NDT as specified.
  • Per-well: Barrier verification record; abandonment report.
  • Per-system: OIW and gas-free certificates before disconnection or hot work.
  • Campaign end: Hydrographic survey and clearance certificate; waste/recycling reconciliation; emissions roll-up.
  • Post-closure: Environmental monitoring per permit (e.g., 1, 3, 5 years “estimated”).
• VI.3 Acceptance criteria: All barriers verified; all hazardous materials removed; seabed clearance within permit; recycling KPI met; no open actions with regulator.
• VI.4 Documentation: Master Decommissioning Dossier: permits, engineering calcs, lift plans, cutting logs, survey data (raw + processed), waste certificates, emissions ledger, lessons learned.