How to conduct integrity checks for offshore FPSOs?

At-a-Glance: A practical, campaign-based integrity verification program for FPSOs covering hull, turret/moorings, risers, and topsides to protect barriers, assure containment, and sustain uptime. Focus on risk-based inspection, non-intrusive techniques, and quantified acceptance criteria.

I. Objective Definition and Key KPIs

Maintain hydrocarbon containment and structural/functional integrity across hull, turret, moorings, risers/umbilicals, and topsides pressure systems without extending planned downtime.

• I.1 KPIs (targets):
  • Production uptime = 98.0%
  • Loss of primary containment (LOPC) frequency = 0.1 per 1 million work-hours
  • Overdue safety-critical inspections = 1% by count
  • Topside pressure boundary integrity: thickness below minimum = 0 events; PSVs in-service test compliance = 98%
  • Mooring line availability 100% (no failed lines); turret swivel leakage 0
  • Cathodic protection potentials within set range = 95% of test points
  • Unplanned deferment due to integrity = 1% of monthly potential
  • Flaring due to integrity trips = 1.0 MMSCFD (rolling average)
  • Emissions intensity within permit; fugitive leaks (LDAR) = 0.5% components leaking
• I.2 Scope boundaries: Hull and tanks; turret, swivel, bearings; moorings; risers/flexibles/umbilicals; offloading systems; topsides static and rotating pressure equipment; electrical and instrumented protective systems; cargo/ballast systems; corrosion management.
• I.3 Assumptions (estimated): Calendar-based major inspection window every 5 years; class and flag state survey regime baseline; dry-dock avoided via afloat/class-approved non-intrusive inspection where feasible.

II. Critical Parameters and Target Ranges

Key Formulas (for quick checks)

• UT-based corrosion rate: $CR=\dfrac{t_{0}-t_{n}}{\Delta t}$
• Remaining life: $RL=\dfrac{t_{n}-t_{\min}}{CR}$
• Hoop stress (thin wall): $\sigma_{h}=\dfrac{P\,D}{2\,t}$; Longitudinal: $\sigma_{L}=\dfrac{P\,D}{4\,t}$
• Approximate MAWP: $\mathrm{MAWP}\approx \dfrac{2\,S\,E\,(t-c)}{D}$
• Erosional velocity limit: $v_{e}=\dfrac{C}{\sqrt{\rho}}$ (C in consistent units)
• Miner’s cumulative fatigue damage: $D=\sum \dfrac{n_{i}}{N_{i}}\le 1$
• Paris law (crack growth): $\dfrac{da}{dN}=C\,(\Delta K)^{m}$
• Risk in RBI: $\text{Risk}=\text{PoF}\times \text{CoF}$

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

III.A Campaign Planning and Data Prep

• III.1 Define systems and corrosion/fatigue loops: Group by service (sour/sweet), temperature, solids, and materials; map Safety Critical Elements (SCEs).
• III.2 Review baselines: Design data sheets, P&IDs, GA drawings, mooring/riser design, last class survey, prior NDT, anomalies, deviations, open MoCs.
• III.3 RBI refresh: Update Probability of Failure and Consequence of Failure; rank with risk matrix; generate inspection scope and intervals.
• III.4 Permits and SIMOPS plan: Confined Space Entry, hot work, isolation plans, inerting/gas freeing, shuttle tanker offloading windows.
• III.5 Tools & access: Rope access teams, UAVs for flare/flare boom/vent stacks, ROV for hull/sea-chests, LRUT/PEC/PAUT sets, gas testing, CP meters, tension monitoring.

III.B Topsides Static Pressure Equipment

• III.6 Vessels/columns/separators:
  • External visual (coating, CUI signs); thermography for wet insulation.
  • Thickness survey: UT spot and grids at TMLs; PAUT on nozzles, weld seams.
  • Internal (if required): scaffolding/RA, lighting, UT/MT/PT on welds; corrosion mapping; measure deposits; sample and analyze solids.
  • Compute $CR$ and $RL$; verify MAWP via thickness; reconcile PSV set pressure.
• III.7 Heat exchangers:
  • Eddy current/RFT/IRIS for tubes; tube pull if severe degradation suspected.
  • Channel/head UT; check for MIC; verify bundle support wear.
• III.8 Piping (including high-erosion zones):
  • LRUT on long runs; PEC for CUI; PAUT at deadlegs, elbows, reducers, tees.
  • Sand/erosion hotspots: use coupons/ER probes; verify $v_{\text{oper}}\le v_{e}$.
  • Clamps and temporary repairs: revalidate life and reinspection interval.
• III.9 Relief devices:
  • On-line verification: acoustic or lift-test where allowed; otherwise bench test during outage.
  • Orifice, set pressure, blowdown, seat leakage; recertify and seal.
  • Cause-and-effect validation for relief/flare header backpressure.

III.C Rotating, E&I, and Protective Systems

• III.10 Rotating equipment:
  • Vibration route (overall, spectra); lube oil analysis (PQ index, viscosity, TAN), alignment/soft foot checks.
  • Thermography; seal system leak checks; surge control tests on compressors.
• III.11 Instrumented protection:
  • Proof-test Safety Instrumented Functions per SIL; verify ESD valve close times and tight shutoff.
  • Loop checks for gas/ flame detectors, H2S, smoke; calibrate analyzers (O2, oil-in-water).
• III.12 Electrical and earthing:
  • Ex inspections (visual/detailed); IR scans on MCCs; UPS/battery capacity tests.

III.D Hull, Tanks, and Marine Systems

• III.13 Hull external:
  • ROV survey: shell plating, welds, sea-chests, bilge keels, fairleads; measure CP potentials/anode consumption.
  • UT thickness spot checks (from inside tanks) on high-stress bays and waterline strakes.
• III.14 Cargo/ballast tanks:
  • Gas-free or inert per task; CSE testing; sludge removal; staging.
  • Coating survey; UT grids; PAUT/TOFD on longitudinals, brackets, stringers; close-up on cracks and buckling.
  • Inspect heating coils, penetrations, sounding pipes for corrosion/holes.
• III.15 Marine systems:
  • Ballast/CPP/firewater/GS pumps condition; sea strainers; valve actuators; hull penetrations.
  • Helideck firefighting integrity; lifeboats and davits inspection and load tests.

III.E Turret, Moorings, Risers, and Offloading

• III.16 Turret/swivel:
  • Swivel seal leak checks; oil sampling for wear debris; bearing temperatures/vibration trending.
  • NDT on turret structure (PAUT/MT at hot spots); functional test of locking/turning gear.
• III.17 Moorings:
  • Live tension monitoring vs metocean; check fairlead and chain wear (caliper, 3D scan).
  • ROV inspect touchdown zones, connectors, clump weights; eddy-current for broken wires (if wire ropes).
  • Fatigue reassessment using measured loads; update Miner’s D.
• III.18 Risers/flexibles/umbilicals:
  • Annulus vent/pressure monitoring; sheath damage/strakes check; end-fitting inspections (NDT and penetrant).
  • CP potential on steel components; ROV visual of hang-off, I-tubes, bend stiffeners, VIV suppression.
• III.19 Offloading system:
  • Hose integrity (MFL/EDDY), pressure test; QCDC function; ESD-1/ESD-2 timings verified.
  • Hawser/MBC loads; chafe protection; PERC function test under control conditions.

III.F Documentation and Closeout

• III.20 Engineering assessments: Update MAWP, Fitness-For-Service (FFS) per recognized methodology; calculate $CR$, $RL$, and remaining fatigue life.
• III.21 Anomaly management: Grade defects; define mitigations (repair, de-rate, clamp, re-inspect interval); raise MoC where applicable.
• III.22 Class/flag reporting: Submit survey records and close findings; update corrosion management and RBI plans.
• III.23 Spares and CMMS: Load findings into CMMS with next due dates; adjust critical spares min/max and long-lead orders.

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

• IV.1 Confined space/atmosphere: Gas test continuous; ventilation; rescue plan; intrinsically safe equipment; pyrophoric risk wash-down and monitoring.
• IV.2 Hot work and ignition sources: Hot work permits; firewatch; blinds/spades verified; hydrocarbon-free certificates; portable gas monitoring.
• IV.3 Dropped objects/working at height: DROPs controls; secondary retention; exclusion zones; rope access supervision.
• IV.4 SIMOPS with shuttle tankers: Clear ESD matrix; communication checks; tug support per conditions; DP/heading control limits; mooring hawser integrity.
• IV.5 Electrical and ATEX/Ex: Equipment certification inspection; temporary power controls; earthing and bonding verified.
• IV.6 ROV/diving: Weather window and current limits; redundancy; emergency recovery plans; clear subsea exclusion zones.
• IV.7 Reliability/maintainability: A/B redundancy proof; bypass management for SCEs; spares kitting; post-maintenance testing before handover.
• IV.8 Environmental: OIW compliance checks; chemical handling; spill kits and drills; flare minimization during testing.

V. Optimization Levers (Data, Maintenance, Debottlenecking)

• V.1 Risk-Based Inspection (RBI) and corrosion loops: Focus UT/PAUT where risk is highest; extend low-risk intervals; integrate coupon/ER probe data.
• V.2 Non-intrusive inspection (NII): Replace shutdown internals with external NII where credible; justify via FFS and historical data.
• V.3 Robotics and remote access: UAVs for flare/topsides visuals; magnetic crawlers inside tanks; ROV-class cameras for hull and risers.
• V.4 Condition-based maintenance (CBM): Stream vibration, oil, thermography, CP, and annulus pressures; apply anomaly detection and trend limits.
• V.5 Digital twin/structural health monitoring: Calibrate mooring/riser models with measured tensions and motions to refine fatigue life and inspection scope.
• V.6 Chemical management: Optimize inhibitor/biocide/oxygen scavenger dosing with field residuals; link to corrosion KPIs; reduce over-dosing OPEX.
• V.7 Spares and logistics: ABC-classify integrity-critical spares; set min/max to cover weather downtime; pre-stage inspection kits.
• V.8 Campaign bundling: Combine rope access NDT, coating touch-ups, and PSV swaps per area to reduce mobilizations and PTW load.

VI. Verification & Monitoring Plan

VI.A Routine Monitoring (Online)

• VI.1 Daily–weekly:
  • Riser annulus pressures and rates-of-rise; turret bearing temps; swivel leak detectors.
  • Mooring tensions vs metocean; green-sea/slamming event logs.
  • Production solids and sand rates; corrosion inhibitor residuals; O2 in SWI.
  • Vibration overall values and alarms; oil condition quick checks.
• VI.2 Monthly–quarterly:
  • CP potential surveys (select points monthly, full quarterly); coupon retrievals and ER probe downloads.
  • LDAR surveys; offloading system hose integrity checks; ESD function tests (per cause & effect).
  • PSV inline checks where permitted; flare tip UAV visual (quarterly if safe).
• VI.3 Annual–multi-year:
  • ROV hull, moorings, risers visuals; UT campaigns on topsides TMLs; PEC/CUI program refresh before rainy season.
  • Bench-test PSVs during planned outage; internal of selected vessels per RBI; turret NDT hot spots.
  • Full offloading hose pressure test; hawser NDT and break strength certification.

VI.B Acceptance and Triggers

• VI.4 Thickness/CR triggers: If $RL\le 2$ years, implement mitigation: de-rate, clamp, repair, or shorten interval.
• VI.5 CP triggers: If potential > -0.75 V or < -1.15 V vs Ag/AgCl/seawater, adjust ICCP/anodes and inspect coatings.
• VI.6 Riser annulus triggers: Any sustained pressure rise trend above baseline; perform gas chromatograph of annulus vent; plan NDT of end fittings.
• VI.7 Mooring triggers: Tension exceedance or fatigue hot-spot growth; schedule ROV detailed NDT; update station-keeping analysis.
• VI.8 Protectives: Failed SIF proof-test ? immediate corrective; restore safety function before returning to service.

VI.C Reporting and Governance

• VI.9 Weekly integrity report: KPI dashboard, anomalies, barriers status, and actions.
• VI.10 Quarterly technical review: RBI updates, corrosion loop performance, fatigue life recalibration.
• VI.11 Class/flag compliance: Maintain traceable records, drawings redlines, and survey closeouts in the document control system.