TECHNICAL INSPECTIONS
ADVOCATING FOR RISK-BASED INSPECTIONS IN THE US Ingrid Pederson looks at the drawbacks of time-based inspections and the improvements gained by switching to risk-based inspections
POLITICAL OBSTINACY frequently cancels new pipeline projects while the energy demand continues to grow. This dichotomy is requiring existing infrastructure to be maintained longer and pushed harder. At the same time oil price crashes and a global pandemic have squeezed maintenance budgets. The conservatism built into American Petroleum Institute (API) Standard 653: Tank Inspection, Repair, Alteration and Reconstruction, can result in some aboveground storage tanks (AST) taken out of service more frequently than necessary while following a time-based inspection schedule. The Pipeline and Hazardous Materials Safety Administration (PHMSA) has incorporated by reference the 2008 edition of API 653 excluding section 6.4.3, thus disallowing risk-based inspections (RBIs) for ASTs for any owner/operator that is federally regulated in the US. TANK INTEGRITY MANAGEMENT PROGRAMMES The purpose of a tank integrity management programme is to maintain the condition of the tank so that it remains operable. Ensuring safe operation also
minimises unplanned outages and maximises the value gained from the asset. Tank bottom evaluation cannot typically be assessed while the tank is in service, thus this tends to be the driving factor in internal inspections, also known as out-of-service (OOS) inspections. One method of improving the efficiency of an integrity management plan is to incorporate risk in the evaluation of a tank’s inspection interval. An RBI approach can be used to compare the risk trend of a tank against a risk target to determine whether an internal inspection is required earlier or later than the timebased approach currently allowed by the Code of Federal Regulations (CFR) for all Department of Transportation (DOT) regulated ASTs. Risk can be leveraged directly as part of a risk-based inspection programme with discrete risk targets or limits. Alternatively, it can be utilised as one of multiple criteria in a risk-informed inspection program which incorporates the risk while allowing for other inputs. An example of what RBIs for ASTs might look like can be taken from API RP 581, Risk-based Inspection Methodology. This RBI modelling provides semi-quantitative calculation methods to determine inspection plans. API RP 581 combines probability of failure (PoF) model and the
consequence of failure (CoF) combined to provide a risk ranking. Inputs into the PoF model include construction data, basic operating data and inspection results. PoF is typically flat for a leak failure mode until minimum allowable remaining thickness (MRT) is reached, at which point it becomes exponential. Both MRT and the repair threshold are inputs into the 581 model. The repair threshold is dependent on what the owner/operator sets their reinspection interval at within the maximum allowed by API 653. REASONS FOR NON-INCLUSION PHMSA declined to incorporate the latest edition of API 653 in their 2015 Federal Register ruling1. Rather this ruling retains API S653–2001 (3rd edition, December 2001), with the exception of section 6.4.3, ‘Alternative Internal Inspection Interval.’ PHMSA stated they have excluded section 6.4.3 because of concerns that the RBI procedures described in section 6.4.3 of the standard do not require adequate or consistent assessment factors for establishing an alternate internal inspection interval. Operators utilising an RBI program for their ASTs only had two years to comply with the 2015 ruling. This change caught
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