Abstract
To ensure the safe operation of a high-pressure gas pipeline a periodic assessment of the integrity is required. At present External Corrosion Direct Assessment (ECDA) is a well-established method (although still subject to discussion) to assess the threat of corrosion to the integrity of non-piggable pipelines. However, most of the data collected during this process is subject to uncertainty. This paper describes a structural reliability based model that can be used to quantify the effect of failure mitigating measures such as coating surveys and bell-hole excavations on the integrity of the pipeline. The model can also be used to optimize the scheduling of future inspections and maintenance activities.
The developed model consists of six steps. In the first step, the current reliability of the pipeline is established based on prior knowledge about material properties, pipeline geometry, corrosion defects and corrosion growth. The second step utilizes the results from a coating survey technique such as Direct Current Voltage Gradient (DCVG) or Pearson to update the number of coating defects on the pipeline through Bayesian statistics. In the third step, the results from a “corrosion detection” technique such as Close Interval Potential Survey (CIPS) or Intensive Measurements are utilized to update the number of corrosion defects. In the last three steps bell-hole excavations are used to update the performance indicators of the utilized survey techniques, the number of coating and corrosion defects, the defect size distributions and the corrosion rate.
A small diameter pipeline that has recently been inspected is used as a pilot study to illustrate the model. For this pipeline the probability of failure (PF) due to external corrosion threats decreases from an initial value of PF =2.7·10-2 based on the available prior knowledge, to a value of PF = 2.4·10-3 after DCVG and CIPS, and finally to a value of PF = 4.25·10-8 after the bell-hole excavations.
