Reliability Based Assessment of Minimum Reelable Wall Thickness for Reeling

The traditional calculation of minimum reelable wall thickness assumes that there is no variation in pipe properties, and that the curvature increases to a maximum on the reel. When a designer checks that a pipe will not buckle during reeling, the designer is usually checking that the curvature at which the peak plastic moment occurs is less than the vessel reel curvature. Design codes specify appropriate safety factors to ensure this requirement is met, where the curvature at which the peak plastic moment occurs is based upon the testing of pipes under pure moment loading. However, it is known that mismatches between adjacent pipe ends at a weld can cause high localised curvatures in excess of the reel curvature and in extreme cases buckling can occur. This paper examines the pipe behaviour during reeling when strength mismatches are present between adjacent pipes at a weld. It shows that the traditional, and currently used, method of calculating the minimum reelable wall thickness does not consider the maximum bending strain during the actual reeling process. The method does however consider a broadly equivalent process and it is shown that the simplified traditional approach is a credible criterion that provides a more than adequate margin to failure when combined with limited extra control of the specification of pipe purchased for reeling. The assessment procedure is outlined along with a description of the detailed Finite Element Analysis (FEA) modelling, full scale testing and field measurements performed to verify the method and results.