Field Experience With a Model for Determining Hydrostatic Re-Test Intervals

At IPC 2006, a model was described that provided a scientific basis for determining hydrostatic re-test intervals for SCC in gas pipelines.[1,2] The model involves determining the maximum possible crack growth rate based upon previous hydrostatic-test intervals and pressures. It resulted in intervals that initially are short and subsequently get longer and longer. Compared to uniform intervals, this sequence is predicted to result in an equivalent level of safety with fewer re-tests. Several pipeline companies have adopted the model, and, in general, the model has been successful. The 2006 paper pointed out that the model was applicable to ruptures but not leaks. In addition, the model did not consider two possible, but unlikely, conditions. One is the possibility that a coating defect could develop after the first hydrostatic test and a severe chemical environment might develop under the defective coating. This possibility has never been observed. The second is the possibility that two or more nearly co-linear sub-critical cracks could coalesce to form a critical size flaw. That would cause a discontinuous step in the growth curve, which is not consistent with the model. The one and only exception to the model that has been observed to date was of this nature. Since this latter condition can occur for cracks at the toe of a double-submerged arc weld under tented tape coating, a special re-test schedule has been devised for this condition. The original assumption of the model that the failure pressure of a growing crack varies linearly with time was verified from a fracture surface that had markings corresponding to the position of the crack front at various known times during the history of the pipeline.