Remnant Life Estimation of Pipelines With Internal Corrosion

Situations can arise where the condition of a pipeline system is poorly known. This may be due to a variety of operational or commercial reasons. Failures will eventually occur if time dependent degradation mechanisms are active. While an appropriate response may be to inspect or hydrotest, this is generally not feasible within a short time frame and integrity assessments or replacements must therefore be prioritized. This paper looks at an ageing upstream pipeline system subject to internal corrosion. A case study is presented in which a system approaching its original design life is required to carry fluids from reservoirs now forecast to be productive for another 50 years. Fluids include sweet or sour gas, crude oil and injection water. Design data are available but inspection information is sparse with less than 10% of lines inspected by ILI; coupon data and well production forecasts are available. The challenge was to prioritize line replacements according to the remnant life of each pipeline, based on the limited available data. Current condition was measured for lines where ILI data were available. A corrosion risk assessment was conducted to identify credible degradation mechanisms. The pipelines were then grouped according to the fluids being transported. This enabled an estimate of current condition for all pipelines based upon the limited inspection and coupon data. In order to predict the remnant life it was necessary to estimate the future corrosion rates, again for all lines. A number of approaches could be used for estimating future corrosion rates. These include basing the rates on historical inspection data or using corrosion modeling techniques. The paper describes a hybrid method that synthesises these two approaches to allow a corrosion rate distribution to be postulated for calculating remnant life. In addition, the options for future corrosion rate estimation are described and the advantages and disadvantages of each one discussed.