Risk Based Asset Management Model for Managing External Corrosion on Natural Gas Transmission Pipelines

Abstract Gasunie operates a natural gas transmission system in the Netherlands. External corrosion is one of the threats. Gasunie applies coating, cathodic protection and in-line inspection (ILI) as the best-industry practices to manage external corrosion. Globally, there is a general trend to increase ILI. Gasunie started its comprehensive ILI-program in 2000. This has resulted in solid historical corrosion data. Furthermore, Gasunie has been and still is improving CP. Currently, (re)inspection intervals are calculated with pipeline parameters and assumptions on defect dimensions and corrosion rate. This is independent of risk. Gasunie applies risk based asset management to balance expenditures on and risks of the transmission system. Risk of unwanted events, such as a pipeline rupture, is monetized. Specifically for external corrosion, Gasunie aims to determine (re)inspection intervals based on risk, i.e. risk based (re)inspection intervals. This means shorter (re)inspection in case of a higher risk and vice versa. Risk of external corrosion is defined by failure frequency as function of time, failure mode and consequences of failure. Failure frequency is determined based upon historical corrosion data, especially defect dimensions and defect densities. Consequences are determined for the following categories financial damages, environment, transportation reliability and safety. This model is unique, because it determines the net present value of monetized risk as a function of time for external corrosion taking into account local differences in relevant parameters and local conditions. Moreover, (re)inspection intervals are determined based upon risk. This paper presents work in progress of this risk model.