This paper discusses testing and analysis work done to establish the effects of pipeline dents (without gouges) under cyclic internal pressure loading. Operators were surveyed regarding the expected dent shapes and failures from dents that had been experienced. A full scale test program utilizing 12-3/4" pipe with controlled dents and cyclic pressure was carried out. Elastic and plastic finite element analyses was used to determine stresses so that a fatigue analysis could be done.
The results indicate that plain smooth dents less than 5% of the diameter for pipe with a diameter to thickness of less than 30 are not a problem for normal pipeline service. Plain dents are probably not a concern for normal gas line service. An analytical procedure was developed to predict the fatigue life of a pipeline subjected to a combination of plain dents and cyclic pressure.
Extensive research has provided an understanding in the behavior of pipelines subjected to mechanical damage [2–111, The presence of a gouge or the combination of a gouge and a dent has been proven detrimental to the burst strength of pipelines [2–91. The research has also revealed that plain smooth dents, in the absence of a defect, do not significantly reduce the burst strength of pipelines [5,7,8,91. On the other hand, plain dents locally increase the pipe stress due to pressure, Thus, fatigue failures from dents can occur from normal operating pressure fluctuations in the lines over a period of time [7].
The purpose of this research was to develop an understanding of the effects of plain dents subjected to cyclic pressure such that guidelines can be provided which tell operators:
Which dents should be repaired immediately?
Which dents are of no concern?
Which dents could be tolerated for a definable period of time with perhaps a lowered operating pressure?
The main goals of this project were:
Review of past failures through historical data [10,12, 13] and published works [2-9,11] to determine the general shapes of the most common dents, and the general operating pressure variations which gas and oil lines experience. From this a "specification" for a typical dent was developed.
Conduct experiments to measure fatigue lives of dented pipelines.
Predict the stress level and therefore the fatigue life as a function of all relevant variables using the finite element method.
One of the objectives of this task was to find specific data regarding failure situations such as dent size and shape, pipeline size, thickness and grade, the operating pressure history of the line, a description of the failure and any special circumstances of the failure. Other data that is necessary for fatigue analysis purposes is the tropical cyclic pressure variations (amplitude and frequency) of both oil and gas pipelines, The sources reviewed were.
