Experimental Strain Measurements on Large Diameter Mitered Pipe Joints

Historic pipeline construction utilized miter joints to enable small directional changes in pipeline routing, and this legacy construction remains in today’s pipelines. Current codes and regulations impose a limit on the maximum miter angle to less than three degrees of the total pipeline direction change, for pipeline operating with pressure over 30-percent SMYS (Specified Minimum Yield Stress). In anticipation of an operational pressure increase, an experimental and simulation effort was undertaken recently to determine the stress amplification due to miters in 30-inch diameter, 0.5-inch thick gas transmission pipelines. Experiments were conducted on six miter joints ranging in miter angle from 0° to 8° degrees of total pipeline direction change. Three of the miter joints were removed from the field (1950’s original installation), while the remaining three were specifically fabricated for the testing. All the miters considered were X42 pipeline steel. The miter pipe joint specimens were tested with pure pressurization, pure bending, and combined pressure and bending using a custom designed loading apparatus. Hoop and axial strains were measured using internally and externally mounted strain gauges. Pressure, as well as four point bending loads and deflections were recorded. One 3.8° field miter specimen was tested to burst. Experimental data, analytical solutions, and finite element results are compared at the miter joint section for the three loading cases. The study is limited to pipe radius to thickness ratio values of 30, and hence the results presented in this study are useful near this value. Results showed that miter joints increase stresses in the vicinity of the miter joint for pressure and/or bending loads. The peak stresses are on the exterior at the intrados. The pressure induced peak stress values increase proportional to the miter angle, and bending further increases the miter stress magnitudes. The ovalization effects significantly compromise the use of linear superposition of pressure and bending stresses even though material behavior remains elastic. Findings from this study demonstrate that in-situ miters on the pipeline in question do not compromise the integrity of the line, and stress additions for small angles over three degrees are comparable to stress risers occurring from normal pipeline features. The results of this work are important for performing structural integrity assessments and for making informed regulatory decisions for mitered pipeline operation.