Field bends in large diameter pipe are routinely used in the construction of oil and gas pipelines. Mild ripples are often unavoidable where such bends have a high D/t or high yield strength. Present regulations and industry standards differ in their treatment of mild ripples, ranging from silence to prohibition, depending upon interpretation. Consequently, the application of acceptance standards for such features is inconsistent, leading to variable standards of inspection and probable scrapping of otherwise sound bends. Finite element analysis (FEA) was used to estimate the effect of ripple magnitude and spacing on stresses due to pressure and bending. Stress concentration factors derived from the models were used with a suitable fatigue damage rule to estimate the effect of ripple parameters on service life.
Source: Rosenfeld, M. J., Hart, J. D. & Zulfiqar. N. (2008). Acceptance Criteria for Mild Ripples in Pipeline Field Bends.
Results were benchmarked against the available test data. The results indicate that (1) mild or shallow ripples up to two percent of the pipe diameter in crest-to-trough dimension would not be expected to be harmful in gas transmission pipelines operating under conditions normally encountered in the gas transportation industry; (2) mild or shallow ripples up to one percent of the pipe diameter in crest-to-trough dimension would not be expected to be harmful in hazardous liquid transportation pipelines operating under conditions normally encountered in the liquid transportation industry; and (3) the presence of ripples could eventually be harmful to long-term integrity in a severe cyclic loading (pressure, thermal expansion, flow-induced vibration) environment, or where soil movement could take place. Recommendations were made for ASME Code criteria specifically permitting mild ripples within stated limits.