API 581-Based RBI Assessment for Pressure Vessels at a Canadian LNG Facility

A Canadian LNG production and storage facility operated a group of pressure vessels — including vessels in flammable vapour, liquid, and multiphase service — that had not been inspected during a previous scheduled shutdown. Access constraints related to insulation, coating, and fireproofing condition, combined with operational limitations, had prevented inspection from proceeding as planned.

The operator needed to understand whether the vessels could remain safely in operation through to the next available inspection window, whether projected risk would remain within the facility's tolerable risk criteria, and what future inspection or mitigation actions would be required. The objective was a technically defensible integrity basis — not a conservative assumption that would force unnecessary intrusive inspection or an unplanned outage.

The central challenge was establishing a credible, defensible integrity basis for eight pressure vessels without the benefit of recent internal inspection data. The absence of inspection data introduces uncertainty into probability of failure assessments — and that uncertainty must be managed through conservative assumptions, sensitivity analysis, and targeted confirmatory actions rather than ignored.

Several vessels were insulated or had coating and fireproofing systems whose condition could not be directly verified without removal, introducing additional uncertainty around external degradation mechanisms including Corrosion Under Insulation and atmospheric corrosion. For stainless steel insulated vessels, chloride stress corrosion cracking under insulation was a credible mechanism requiring specific assessment consideration.

The assessment needed to cover a pilot scope of eight vessels — each with distinct service conditions, materials of construction, inspection histories, and degradation profiles — while maintaining methodological consistency and producing actionable, vessel-specific integrity management plans.

LNG pressure vessels can carry high consequence potential due to the flammable and hazardous nature of process fluids. High consequence combined with data uncertainty creates a challenging assessment environment — conservative assumptions must be applied, but excessive conservatism produces misleading risk rankings and drives unnecessary inspection activity.

The damage mechanism landscape for LNG pressure vessels is broad. Internal thinning, external atmospheric corrosion, Corrosion Under Insulation, CUI chloride stress corrosion cracking for applicable stainless steel vessels, and brittle fracture considerations for low-temperature service all required systematic screening and credibility assessment — not a generic software template exercise.

Coating and fireproofing condition on several vessels could not be directly confirmed, introducing conservatism into external corrosion and CUI assessments. Managing this uncertainty required sensitivity analysis to identify which assumptions were driving risk results and to define targeted confirmatory inspection actions that could validate or improve those assumptions.

Non-intrusive inspection methods needed to be identified and prioritised where feasible — but the suitability of each technique depends on vessel geometry, insulation type, access configuration, and the specific damage mechanism being targeted. Matching inspection method to mechanism required engineering judgement beyond standard software outputs.

TES Canada converted an inspection access challenge into a structured, defensible integrity risk assessment. A pilot scope of eight pressure vessels was assessed under API 581 methodology, with vessel-specific datasets, damage mechanism reviews, and risk calculations produced for each asset.

All assessed vessels were demonstrated to be within the client's tolerable risk range at the time of assessment. Conservative assumptions were applied where data uncertainty existed, and sensitivity analysis identified the specific assumptions driving risk results — providing the operator with a clear picture of where confirmatory inspection would have the greatest impact on risk confidence.

Vessel-specific future integrity management plans were produced, prioritising non-intrusive inspection techniques and defining action windows for future confirmatory inspections. The assessment reduced unnecessary intrusive inspection by providing a technical basis for deferring internal access where risk remained within acceptable limits, while ensuring that targeted confirmatory actions were defined to validate key assumptions. The pilot framework established a reusable methodology for extending RBI to the wider vessel population at the facility.