Winding and unwinding a pipe with Lders bands on a reel. Part II: Effect of problem parameters

The main features of the mechanical behavior of a pipeline with Luders bands wound/unwound repeatedly on a large diameter reel have been studied in detail in Part I for a particular pipe geometry, material properties, reel radius and back tension. Part II investigates how the complex behavior revealed and its degrading effects are affected by the parameters of the process. The extent of the Luders strain, the back tension, the reel radius, the material yield stress, and the pipe diameter-to-thickness ratio are varied within practical ranges, and their effects on the mechanical behavior of the pipeline are established. Typically, for a pipe under bending, Luders banding is in the form of higher strain inclined bands organized in diamond patterns. By contrast, on the reel the bands form clusters separated by elastic zones. This inhomogeneous deformation leads to localized curvature, higher ovality, wrinkling, and the danger of local buckling, all of which are aggravated by higher Luders strain. The spacing of the clusters on the reel depends on the difference between the Luders strain and the bending strain induced by the reel, with the banding becoming continuous when the Luders strain is smaller than the reeling strain. During unwinding, the line straightens free of sharp changes in curvature, the strain in the clusters is reduced, but new localization patterns nucleate in the elastic zones between the initial clusters. In subsequent cycles, the line continues to bend smoothly but the clusters reappear during winding and the secondary patterns during unwinding. The ovality accumulates with every cycle and the wrinkles are permanent. Higher tension reduces the possibility of collapse, but increases the ovality and the net elongation of the line. The reel-induced strain goes as the ratio of the pipe to the reel diameter and decides the cluster spacing and wrinkle amplitude. Ovality increases as the reel radius decreases, while the wrinkle amplitude increases as the reel strains gets closer to the minimum stress on the softening branch. The "strength" of the instability is governed by the ratio of the drop in stress in the softening branch of the stress-strain response and the Luders stress. Higher value of this ratio increases the degrading effect of the Luders strain. Increasing the pipe D/t increases the ovality induced by each cycle and makes the pipeline more prone to local collapse.