Winding and unwinding a pipe with lders bands on a reel part I: Analysis of base case

Experiments and analyses have demonstrated that under bending tubes with Luders bands develop localized diamond -shaped patterns of inclined bands of higher strain on both the tensioned and compressed sides. The banding is progressive, accompanied by a sharp change in curvature and ovalization that can lead to collapse. When the pipe is wound onto a large diameter reel, in order for the line to conform to its curvature the bands form clusters separated by elastic zones. The present study uses finite element analysis to study the evolution of these events under repeated reeling/unreeling of pipelines. The analysis is coupled to a custom combined isotropic -kinematic hardening constitutive model; localization associated with Luders banding is reproduced by partial softening that covers the extent of the instability, and the Bauschinger rounding of unloading/reverse loading by nonlinear kinematic hardening. Part I studies in detail the repeated winding/unwinding of a 12 -inch pipeline with diameter -to -thickness ratio of 18 onto a 300 in. radius reel under a low back tension. The analysis demonstrates that the band clustering initiates close to the point of first contact with the reel and leads to localized curvature, wrinkling of the top and bottom surfaces of the pipe, and ovalization. Close to first contact with the reel, the section moment reaches a local maximum and drops to a lower level as it moves further down the reel. This unloading is linked to an uplift of a section of line from the reel adjacent to the point of first contact. During unwinding, the line straightens without sharp changes in curvature, the strain in the clusters is reduced, but new localization patterns nucleate in the elastic zones between the initial clusters. The straightened line ends with residual ovality, axial surface undulations, and residual stresses. During subsequent cycles, the line continues to bend in a smooth manner, but the clusters of bands reappear during winding and the secondary patterns during unwinding. The ovalization accumulates with every cycle, the undulations due to wrinkling remain, and so do the residual stress and strain fields.