Hydrostatic Retesting of Existing Pipelines

An analytical model is presented for predicting hydrostatic retest intervals in liquid pipelines which are subjected to frequent large pressure cycles. The model utilizes pressure cycle history, hydrostatic test history, and fatigue crack growth rate data for the pipe material to calculate time to failure for the largest possible defect which could have survived a previous hydrostatic test. An example problem is described which shows the value of maximizing the margin between test pressure and operating pressure in order to achieve long time intervals between tests.