RESIDUAL STRENGTH ASSESSMENT OF CORRODED PIPELINES

Pipelines are one of the most efficient means for transporting hydrocarbons from one point to the other point, which may be routed within onshore or offshore locations. There is a great risk while operating these pipelines due to defects occurring during the service life. Corrosion is one of the most common defects observed in many instants. At the point of corrosion, the wall of the pipe section becomes thinner and starts to lose its mechanical resistance. Therefore, appropriate defect assessment method is necessary in order to decide whether to keep them into continual operation or to make a shutdown for necessary maintenance or replacement of sections of the pipeline.
Methods for assessing metal loss defects have been available for many decades, as for instance the NG-18 equation and ANSI/ASME B31G code. Throughout the years, many modifications to the original equations have been made and newer methods like Modified B31G and RSTRENG were adopted. Moreover, these days, there are several in-house methods and commercial codes. A quantitative study on the prediction by five most applicable current assessment methods showed big bias and large scatters against burst test database. For example, the burst capacity prediction made by B31G criteria showed an average bias of about 31% under estimation with up to 72% lower predictions. Hence, these methods enforce either unnecessary maintenance or premature replacement of pipelines. But pipeline operators need a reliable defect assessment methodology not only to assure safe operation but also to implement optimum operation cost.
This research was conducted to develop a new method for the residual strength assessment of corroded pipeline based on burst test and a series of nonlinear finite element (FE) analyses. The burst test samples were taken from API X52 pipeline retired from service due to corrosion. Burst tests were conducted in order to study the failure mode and to validate the FE approach for the assessment of corroded pipelines.
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The burst test showed that the failure of the corroded pipeline is due to plastic collapse. The FE simulations corresponding to the test samples well matched with burst test results within less than 5% error. Thus, the FE simulation was used as a complement to the burst test database in order to develop a new corrosion assessment method. Stress-based criterion based on plastic instability analysis was used to predict the failure pressure.
This research contributed to the development of an alternative corrosion defect assessment method. The New Method can predict the burst pressure of corroded pipelines with better accuracy than the currently used corrosion assessment codes and norms. The New Method agreed with the burst test database with predictions evenly distributed within about ±7% along the actual value with an average error of only about 0.30%. For the same burst test database, the Modified B31G gave conservative predictions with a mean bias of about 24% with as low as 52% predictions than the actual value. Therefore, pipeline operators and engineers will benefit from this research.