The Improved Measurements and Stress Analysis of Cylinder / Pipe With Out-of-Roundness

Abstract

The API 579 defines an out-of-roundness as the geometrical imperfection of a shell which is assumed to be constant in the longitudinal direction. In general, there are three types of out-of-roundness imperfections — a) the offset weld misalignment (misalignment of radius in the point of the circumference); b) angular misalignment (misalignment of the first derivative of the radius at some point); c) regular imperfection (radius is the continuous function along the circumference). The document API 579 [1] contains the procedure (or requirements) of the imperfections measurement as well as their analytical treatment in case of inner pressure loading. Regretfully, at least two principal drawbacks of the document should be pointed out: the formula for regular imperfection description based on the discrete measurements of the surface point is inaccurate. Besides, nothing is said about how to extract the values of the misalignments from them. It should be kept in mind that misalignments cannot exist alone without the continuous deforming of the remaining part of the circumference; notions of the local as well as the global peaking (angular lumps) is confusing. It is practically impossible to discern them physically. Furthermore, the two equal angular misalignments should lead to the same additional stresses irrespective of how we name this misalignment — be it local or global one. In both cases the formulas are incorrect. The proposed paper gives the solutions for the abovementioned problems. The results of the real measurements of the cross section form of different pipes are presented. The general approach for treating the out-of-roundness is given and an example of measurement of real pipe cross-section is presented. The stress calculation part of the procedure is based on the results presented in work [2].