Pipeline corrosion risk analysis – an assessment of deterministic and probabilistic methods

Abstract

PurposeThis paper compares and contrasts two approaches to the treatment of pipeline corrosion “risk” – the probabilistic approach and the more traditional, deterministic approach. The paper aims to discuss the merits and potential pitfalls of each approach.Design/methodology/approachProvides an outline of each approach. The probabilistic approach to the assessment of pipeline corrosion risks deals with many of the uncertainties that are common to the data employed and those with regard to the predictive models that are used also. Rather than considering each input parameter as an average value the approach considers the inputs as a series of probability density functions, the collective use during the assessment of risk yields a risk profile that is quantified on the basis of uncertain data. This approach differs from the traditional deterministic assessment in that the output yields a curve that shows how the “risk” of failure increases with time. The pipeline operator simply chooses the level of risk that is acceptable and then devises a strategy to deal with those risks. The traditional (deterministic) approach merely segments the output risks as either “high”, “medium” or “low”; a strategy for managing is devised based on the selection of an appropriate time interval to allow a reasonable prospect of detecting deterioration before the pipeline corrosion allowance is exceeded, or no longer complies with code. Applies both approaches to the case of a 16.1 km long, 14 in. main export line in the North Sea.FindingsThe deterministic assessment yielded a worst‐case failure probability of “medium” with a corresponding consequence of “high”; classifications that are clearly subjective. The probabilistic assessments quantified pipeline failure probabilities, although it is important to note that more effort was required when performing such an assessment. Using target probabilities for “high” and “normal” consequence pipeline segments, indications were that between 8.5 and 13 years was the time period for which the target (predicted) failure probabilities would be reached, again depending on how effective corrosion mitigation activities are in practice. Basing pipeline inspections in particular on the outputs from the deterministic assessment would therefore be conservative in this instance; but this may not necessarily always be so. That the probabilistic assessment indicates that inspections justifiably may be extended beyond that suggested by the deterministic assessment is a clear benefit, in that it affords the opportunity to defer expenditure on pipeline inspections to a later date, but it may be the case that the converse may be required. It may be argued therefore, that probabilistic assessment provides a superior basis for driving pipeline corrosion management activities given that the approach deals with the uncertainties in the basic input data.Originality/valueA probabilistic assessment approach that effectively mirrors pipeline operations, provides a superior basis upon which to manage risk and would therefore likely maximize both safety and business performance.