According to data on reportable incidents for the 20 years ranging from 1995 to 2014, excavation damage accounted for 16.4% of the incidents along 301,732 miles of gas transmission pipelines and 15. % of the incidents along 199,210 miles of hazardous liquid pipelines. In general terms, excavation damage is a major cause of incidents, ranking third 6following incidents caused by material/weld/equipment failure and corrosion.
For the purposes of this study, mechanical damage is separated into two categories: immediate failures and delayed failures. An immediate failure is one which occurs at the instant the damage is done to the pipeline. A puncture, for example, is an immediate failure. Delayed failures involve damage that is not sufficient to cause a leak
or a rupture at the time it is inflicted. On average, 14.6% of the mechanical damage incidents in gas transmission pipelines and 13.3% of the mechanical damage incidents in hazardous liquid pipelines can be classified as delayed failures.
Immediate failures are generally minimised through preventative measures and design efforts. For instance, this paper demonstrates that puncture probability can be calculated by comparing the likelihood of any given external load being imposed with the inherent pipe resistance.
While preventative measures serve to reduce the occurrence of delayed failures as well as that of immediate failures, delayed failures are largely mitigated for by in-line inspection and timely remedial action. The fact that the assessment methods for mechanical damage are generally not as robust as those for cracks and corrosion tends to limit the reliability of deterministic response-time calculations.
Therefore, in the study described herein, risk-based approaches to minimising delayed failures were developed. The study pursued three different approaches to deciding which dents need to be excavated after an ILI. One approach involves the use of reportable-incident rates based on the PHMSA statistics in conjunction with the number of ILI dent indications per mile to calculate a probability of failure. The second consists of a decision-making process based on the ILI reported-dent depths and the dent fatigue life probability-of-exceedance function. The third relates to a decision-making process based on successive excavations of dents located by ILI, in which the Bayesian method is applied to compare predicted versus actual severity and thereby determine the probability of failure associated with stopping after a specific number of excavations.
In this paper, the mitigation of mechanical damage is mainly approached from a risk standpoint. This is in conjunction with the fact that the assessment methods for mechanical damage are generally not as robust as those for cracks and corrosion. Three different angles have been pursued for case studies, including (1) a significant-incident probability based on the most up-to-date PHMSA reportable-incident rate; (2) a decision-making process based on full knowledge of POE function; (3) a decision-making process based on excavation, with no knowledge of POE function.
These investigations shed light on the remedying in-line inspection anomalies and the findings can be easily tailored to other called-out features, such as corrosion anomalies
or crack-like anomalies.
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