Scoping an RBI Assessment With Sparse Piping Data
A terminal plans its first RBI assessment covering 63 pressure vessels and 180 piping circuits. The corporate standard requires all equipment in the unit to be assessed before the next turnaround, 10 months away. The team has 1.5 full-time engineers and a contractor for 6 weeks. Thickness data exist for 108 circuits; the remaining 72 have only commissioning records and one external visual inspection. A junior engineer proposes excluding the 72 circuits from the RBI because 'there is no data to assess.' The unit has had two small leaks in sour water piping in the last 5 years, both in circuits without UT data.
1. How should the planning team respond to the proposal to exclude the 72 circuits?
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Excluding them is not a defensible RBI scoping decision. RBI scope is set by equipment boundaries and risk, not by data convenience. The two recent leaks are evidence that the low-data circuits are not low-risk; excluding them would bias the assessment and leave known sour-water exposure unranked.
The practical response is to include the circuits with a documented data-quality flag. Use conservative assumptions (e.g., nominal thickness, credit only confirmed linings, assume the higher credible corrosion rate) and a screening-level assessment until UT data are collected. Prioritize thickness surveys on the sour-water circuits and any circuit whose failure could affect occupied areas. Record the assumptions and data gaps in the assessment so the risk ranking reflects uncertainty rather than hiding it.
2. Select an assessment approach and justify it given the resources and timeline.
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A single quantitative method across all 243 items is not realistic with 1.5 engineers in 10 months if data are missing. A tiered approach is more defensible:
- Screen all vessels and circuits qualitatively or semi-quantitatively to identify high-consequence and high-uncertainty items.
- Apply quantitative RBI to high-consequence items and to the sour-water circuits with leak history.
- Use the contractor for data collection and screening, while the owner's engineers review assumptions and risk drivers.
This allocates effort where the risk is concentrated and where data gaps matter. The plan should state that qualitative results are provisional and must be revisited when UT and process data are available. It should also identify the specific circuits whose ranking is sensitive to missing data.
3. What must the documented assessment plan contain to make this defensible?
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The plan should define:
- Scope and equipment boundaries, including the 72 low-data circuits and why they are included.
- Methodology and rules, such as consequence endpoints, probability approach, and how data gaps are treated.
- Data sources and quality, with known missing thickness and process data called out.
- Key assumptions and their basis, including conservative thickness and corrosion-rate assumptions.
- Roles and responsibilities, including contractor review and owner approval.
- Schedule and resource allocation, with data collection prioritized before final ranking.
- Acceptance criteria and triggers for reassessment when new UT, process, or leak data arrive.
Without these, the assessment becomes an undocumented opinion and cannot be updated reliably.