Effectiveness of Hydrostatic Testing for High Strength Pipe Material

Pipeline systems are typically subjected to hydrostatic testing to help ensure pipeline integrity. It can be desirable to use the highest feasible test pressure to eliminate as many defects as possible. It is widely accepted that safe control of yielding can be achieved during hydrostatic testing and that the hydrostatic testing does not create a stress state that is less safe from the standpoint of pre-existing flaws. For a small percentage of cases, however, hydrostatic testing can produce flaws that were longer than the ones removed. In these few cases, the flaws can then fail at a lower test pressure than the original hydrostatic test. The low probability of these events, however, means that the effectiveness of the hydrostatic test is not significantly diminished in this case. Because crack growth from a pre-existing flaw is retarded in a plastically deformed material, it is also typically assumed that hydrostatic testing should not lead to accelerated crack growth. However, this does not take into account that the hydrostatic testing itself can cause some increment in crack growth and that for many higher strength pipe materials significantly large defects can survive hydrostatic testing. These longer defects can potentially grow after surviving a hydrostatic test. This paper discusses this difference in crack growth rates for cracks that have survived hydrostatic testing in different grade pipeline steels and the implications for hydrostatic testing.