Hydrostatic testing of pipelines that are subject to pressure cycling and fatigue damage can alter the intrinsic characteristics of flaws that survive the test. The effect is generally favorable, as test pressures well above the maximum operating pressure (MOP) can significantly reduce the subsequent rate of fatigue crack growth. The phenomenon is known as fatigue retardation, which is caused by crack closure due to compressive residual stresses created by plastic deformation during the hydrotest.

Fatigue retardation following an overload event is a well-known phenomenon in metallic structures, but there has been little or no effort to take advantage of this beneficial effect in pipelines. This paper presents a modeling procedure aimed at quantifying fatigue retardation following a hydrostatic test. A series of 3D elastic-plastic finite element simulations have been performed to model fatigue crack growth following a pressure test. The effect of test pressure and MOP on plasticity-induced crack closure was studied. The relative effect of fatigue retardation on remaining life was demonstrated with several examples. In some cases, the results were counter intuitive.

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