A Study of Cases of Hydrostatic Tests Where Multiple Test Failures Have Occurred

The retesting of pipelines for integrity management purposes often involves testing of pipelines where multiple test failures can be expected. Multiple failures are most likely to occur when an existing pipeline is tested to a hoop stress level in excess of those used in prior tests of the pipeline. A major cause of such failures is seam manufacturing defects, but other types of defects such as mechanical damage or stress corrosion cracking may cause numerous failures as well. The occurrence of multiple failures can be costly in terms of the time the pipeline must remain out of service. Multiple failures sometimes involve pressure reversals that may affect confidence in the level of integrity sought by the pipeline operator. The study described in this paper involved a review of five actual cases of hydrostatic tests where multiple test failures occurred. On the basis of these cases a method was developed for predicting the ultimate number of failures required to reach a desired test level from the pressure levels of the first few failures. In addition, an improved method for estimating the probability of a pressure reversal of a given size was developed. Pipeline operators could use these techniques to decide when to terminate a hydrostatic test and to assess the effectiveness of the test in terms of a level of confidence that an integrity-threatening pressure reversal will not occur.