What is the optimal inspection interval
for an aging pipeline?

Our clients was at risk of being forced to repair or renew the pipeline at short notice if a leak or pipeline burst were to occur or the corrosion allowance was exceeded.
Re-scheduling the next inspection would reduce the risk of forced repairs and downtime by c.$31,000/month
Replacement would significantly reduce risks and need for monitoring.
Introduction of flexible inspection schedules for critical cases, instead of fixed interval corrosion monitoring
Problem description

Our clients were managing an aging pipeline with integrity concerns: how often to inspect and what if renewal instead? The plant’s Chief Inspector was concerned that insufficient inspection was being carried out on the aging ethylene pipeline. The pipe was being monitored annually using ultrasonic wall thickness measurements, following a Risk-Based  Inspection (RBI) study. Nevertheless, he considered that the company was at risk of being forced to repair or renew the pipeline at short notice if an ethylene leak or pipeline burst were to occur or the corrosion allowance was found to have been exceeded.

We were called to support a multi-disciplined team study of the optimal inspection program using the SALVO Process. This provides a structured navigation and evaluation discipline to ensure all factors, stakeholder
interests, uncertainties, and scenarios were explored, quantified, and evaluated on a consistent and auditable basis. This case involved a small team comprising inspectors, operations, maintenance, finance and insurance,  risk management, and process safety personnel. It used a mixture of known historical data (previous measurements, costs, etc.), expert knowledge, and some prior RBI studies to help in estimating the ongoing degradation rates (if such information had not been available, the process facilitates the capture of optimistic and pessimistic extremes of expert opinion and explored these for decision sensitivity).

It needed just 2 hours to build the base case scenario, using the appropriate SALVO storyboard and DST Inspection Evaluator™ software to model the risks, costs, and operational options available. The team also developed a number of alternative scenarios, such as the effect of pipeline re-lining or replacement, to explore the impact of the optimum solution.

The study we performed

The study, including testing for sensitivity for all sources of data uncertainty, revealed the following:

  • There was an opportunity to introduce flexible inspection schedules for critical cases such as this, instead of fixed interval corrosion monitoring. This dynamic inspection strategy is useful for aging assets where inspections should become more frequent as deterioration progresses.
  • If inspections can only be carried out during shutdowns, the evaluation process includes a facility for identifying which ‘opportunity’ occasions are best exploited, as well as a potential ‘pit stop’ shutdown specifically for the purposes of the inspection.
  • The replacement of the pipeline would naturally result in longer inspection intervals and reduce planned inspection costs and levels of risk associated with the pipeline. This study included an evaluation of how much this benefit would be (see graph above). However, the significant capital investment required for the pipeline replacement, the optimal timing for this, and the future pipe life cycle costs would need to be evaluated in a corresponding SALVO Process, supported by the relevant cost/risk/ benefit analysis, using the DST Lifespan Evaluator™ software module.
  • The difference between RBI-derived strategies and the true optimal cost/risk decision was clear. Whereas RBI is an extremely powerful systematic method of developing risk-proportionate monitoring strategies, it does not adequately consider the costs (and potential downtime costs) of inspections, the probabilistic exposure to exceeding corrosion allowances, and other economic factors which may justify earlier or later intervention.

Scheduling the next inspection in 3 months’ time instead of waiting for the RBI-derived timing of 12 months, would reduce the risk of forced repairs and downtime by c.$31,000/month. This earlier intervention is mainly justified by the risk of forced repair and downtime if the corrosion allowance is exceeded (blue line in the graph) rather than the integrity risk (red line).  The study also considered the effect of replacing the pipe on a planned basis. This would significantly reduce the risks and need for monitoring: below, the comparison graph shows the cost/ risk reduction if such a renewal were to occur: these form inputs to the Capex valuation, which would use the corresponding SALVO evaluation the process to justify investment and optimal timing.

The Woodhouse Partnership would be delighted to show you how the SALVO process and Decision Support Tools™ can be implemented to benefit your business, contact us now for a free initial consultation. We look forward to hearing from you.

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John Woodhouse

With 30 years’ experience in utilities, oil & gas, transport and other sectors, John is one of the most widely known experts in integrated asset management projects for some of the largest companies in the world.

John is a Founder and Life Fellow of the IAM; he has written 4 books, chaired the development of BSI PAS55 and represents the UK on the ISO55000 committee.

He also led the international MACRO and SALVO collaboration projects in optimised asset management decision-making.

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