PRCI Report 206

L51684e

Battelle Memorial Institute

Need: Corrosion affects tees and elbows as well as straight pipe. The effect of metal-loss corrosion on the remaining strength of straight pipe can be predicted using ASME B31G, "ANSI/ASME B31G Manual for Determining the Remaining Strength of Corroded Pipelines." Modifications to the B31G methodology to account for the defect profile have changed its accuracy for straight pipe. These techniques have been successfully used by pipeline operators for several years to assess whether corrosion damage to a pipeline warrants repair or replacement or whether it may be safely left in service.The B31G method does not address corroded tees and elbows, and an ongoing program, which is developing a new procedure for corroded pipelines under axial and pressure loads, does not cover curved pipe. Hence, pipeline operators have no proven method of evaluating corroded fittings. Currently, fittings are occasionally removed from service even when the corrosion damage is considered relatively minor. This can be extremely costly, for example, in offshore installations.

Benefit: This report summarizes the development of a method for assessing the strength of corroded elbows. The method is actually an extension of the NG-18 surface-flaw equation for straight pipe. Modifications were made to the NG-18 equation to account for differences in the yield and ultimate strengths, different stress rates around an elbow vs. a straight pipe, and the effects of curvature on local bulging. Only the modification for different yield and ultimate strengths actually brought predicted results closer to actual test data. Predictions made by the modified NG-18 equation were conservative compared to the test data and reasonably accurate.

Results: The results of this study indicate that corrosion defects in elbows can be conservatively evaluated using an equation with the flow stress substituted for the ultimate strength. This method uses the local wall thickness at the defect area and accounts for the stress distribution in an elbow by incorporating a Lorenz-factor effects. Using the ultimate strength rather than the flow stress gives a more accurate prediction. The more accurate predictions can sometimes be slightly nonconservative.

Product Details

Published:

05/01/1993

Number of Pages:

42

File Size:

1 file , 310 KB