There is a commonly held belief that internal operational pressure fluctuations of a gas pipeline is less likely to result in fatigue cracking than a liquid pipeline. A defensible engineering analysis process, demonstrating that this observation remains valid for all gas pipeline systems regardless of operation (i.e. transmission, bi-directional flow at storage facilities), existing features (i.e. dents or preexisting cracks), material properties (i.e. pipe vintage, grade) and pipeline geometry, needs to be developed. The work reported in this paper describes the work sponsored by the Interstate Natural Gas Association of America (INGAA) to support gas pipeline operators in completing Integrity Verification Programs (IVP) by defining which pipeline segments can reasonably be considered to be at risk of fatigue damage accumulation and which are not.
This paper presents an overview of the work completed to develop a defensible approach to demonstrate the conditions under which pipeline pressure fluctuation induced fatigue is not a concern. The development of this approach, presented in this paper, considers the results of a survey of the range of cyclic severities experienced by gas pipeline operators, typical feature types and sizes that could exist in a pipeline system (i.e. mechanical damage, cracking) and the role of material properties on these assessments. The paper includes the development of directions as to the collection and analysis of SCADA pressure data to produce a pressure spectrum severity indicator (SSI) characterizing the fatigue damage potential of an operational history. This SSI is used along with fatigue susceptibility tables indicating the conditions for which fatigue would not be a concern for a pipeline segment. The susceptibility criterion relates a pipelines operation, in terms of SSI to the maximum feature size that can exist in the system before fatigue is considered a threat to pipeline integrity.
The information presented in this paper would be of interest to pipeline operators, maintainers, consultants and regulators. The technical work presented in this paper was completed to support compliance with regulatory requirements and may also be used to influence the application of integrity management regulatory requirements on gas pipeline systems by providing information and processes to eliminate the need to consider fatigue as a damage accumulation process or integrity threat.