United States Department of Transportation
i
Validation for Flaw Acceptance of Mechanical Damage to Low Stress Natural Gas Pipelines
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2011-12-01
[PDF-6.05 MB]
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Abstract:The objective of the research project is to provide pipeline operators with a decision‐making tool regarding the repair options to pipeline’s mechanical damage. The research project addresses the gas pipelines operating at stress levels below 40% of their Specified Minimum Yield Strength (SMYS). The testing program evaluated a wide range of pipes and damage characteristics using a large‐scale dent‐gouge machine. The tests evaluated the effects of various sizes and shapes of dents and gouges, pipe stiffness (pipe diameters/wall thickness), and pipe grades. Third‐party damage was also simulated by applying a backhoe tooth on the pressurized pipes. A review was performed on the numerical models used to characterize failures due to mechanical damage. A comparative review of the various assumptions and analysis of these models was performed and the results of the tests were implemented in the European Pipeline Research Group (EPRG) Simplified Model. The simple EPRG model was conservative and the test results showed failures at higher stresses than the ones predicted by the model. Based on the test results, the EPRG simplified model can be conservatively used to evaluate mechanical damage in low MAOP pipelines. A web‐based computer program was developed to provide a simplified procedure for pipeline operatoSMYS. The program implemented the EPRG simple model for mechanical damages which do not cause leak or rupture of the pipeline. The program also evaluated the repair criteria for damages due to external corrosion based on the ASME B31G manual for determining the remaining strength of corroded pipelines. An investigation of the effect of wrinkle bends on pipe stresses was performed in Task 5 of the research project. This work provides a state‐of‐the‐art evaluation of the effects of wrinkles on buckles and bends in pressurized gas lines. The results of the investigation of the wrinkle bends incident records suggest that the vast majority of wrinkle bends do not pose a threat to pipeline safety under normal circumstances. The challenge is to use the information available to the operator to identify the small proportion of wrinkle bend installations that may pose a threat. The investigation showed that wrinkle bends with depths up to 2.5 percent of the diameter and aspect ratios (height of wrinkle over the wave length of the wrinkle) less than 0.13 are acceptable provided the following threats are not present: Aggressive longitudinal stress cycling of the line, Ground movement, i.e. mine subsidence or landslides, Corrosion, and stress corrosion cracking.rs to determine the criteria for repair needs of damaged pipelines operating below 40%.
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