NR196801 Types of Pipe Defects

Abstract

Knowledge of the various types of defects and the examples used, have resulted from failure investigations sponsored in past years by individual pipeline companies and pipe manufacturers. One of the important observations from the examination of leaks or breaks is that, except in a very few cases of gross overload from secondary stresses, there has always been a defect of some type present at the origin of the failure. This report contains a summary of the various types of defects that have been observed to cause failure in line pipe. The description of defects has been divided into two categories: (a) those in the pipe prior to the test after construction and (b) those introduced into the pipe following the test after construction. The injurious defects in the pipe prior to the pre-service test can be eliminated by the hydrostatic test if the test pressure level is high enough-i.e; any of these defects that remain after a high-pressure test will not be serious enough to produce a leak or break later during service. The one exception to this is the occasional hard spot, which is present in the pipe wall prior to the pre-service test but is not removed by the test--such a defect has been known to later produce a leak or break because of the phenomenon of hydrogen-stress cracking.