L51398 Causes of Failures of Defects that have Survived a Prior Hydrostatic Test

The observance in service or during hydrostatic testing of an occasional pressure reversal has raised a question as to what causes these reversals and what is their magnitude. A pressure reversal occurs when a defect survives a given pressure level only to fail at a lower pressure level upon subsequent pressurization from 0 pressure.

The objective of this study is to investigate the causes of pressure reversals and their magnitude. In addition, the probability of the occurrence of a pressure reversal was investigated along with the identification of the factors in hydrostatic testing that tend to eliminate or reduce the occurrence of pressure reversals.The causes and magnitudes of pressure reversals were studied in this investigation using a number of full-scale experiments on line pipe. The experiments involved artificially induced defects that were purposely subjected to several cycles of near failure load and various hold times. Also, the pressure testing experience on numerous pipelines and the documented associated pressure reversals have been factored into this investigation. The major factors defined as contributing to pressure reversals in line pipe of normal toughness are the subcritical (stable) crack growth during prior loading, the proximity of the defect to failure, and compressive yielding at the flaw tip during unloading. The maximumpressure reversal observed in normal toughness line pipe in the experiments was a 25.2 percent reduction from the previous pressure level as a leak. The maximum recorded service pressure reversal of 62 percent was for a rupture that occurred during testing of old low toughness line pipe.