An Experimental Study of Buckling and Post-Buckling of Laterally Constrained Rods

Abstract

The effects of friction and curvature on buckling, post-buckling, and unbuckling behavior of rods laterally constrained within an enclosure are studied experimentally. The experimental apparatus, measurement procedures, and uncertainty analysis are described. Results indicate that friction significantly delays the onset of buckling, and causes noticeable hysteresis in the post-buckling behavior. As a result, the unbuckling loads are always less than the corresponding buckling loads. The drag-related loss, which eventually leads to lock-up, is also measured and reported in this work. Friction is also a cause of post-buckling snapping and reversals in the direction of wrap of the helix. As expected, the effects of friction become less significant as the inclination decreases. It is shown that predictions of current theory agree with experimentally measured unbuckling rather than buckling loads. When friction is significant, current theory underestimates the compressive loads at which buckling occurs in straight or curved wellbores. Ignoring friction or curvature limits the estimated weight on bit well below the safe load that can be used in many drilling and completion operations in extended reach or horizontal wells. Moreover, the hysteresis effect of friction means that once buckling has occurred, the compressive loads must be reduced to values much below the buckling initiation loads to fully straighten the buckled pipe.

How Drillstring Rotation Affects Critical Buckling Load?

“…For a non-rotating pipe, friction causes a hysteresis effect since buckling loads (increasing compressive load) are greater than unbuckling loads (decreasing compressive load). This hysteresis effect has been observed experimentally by Salies 18 and Suryanarayana 19 , and has not yet been reproduced numerically to the authors knowledge.…”

“…Many experimental tests have shown some helix reversal effects 18,19,21 , that is, a change in the winding direction of the helical shape (from right-hand to left-hand helical shape, and vice versa). This reversal phenomenon is explained by friction 19 and may probably exist in the field.…”

Buckling of Tubulars in Actual Field Conditions

“…Due to friction, the buckling loads measured in the laboratory were 40-70 % higher than the corresponding unbuckling loads, showing that available theoretical expressions underestimate the critical sinusoidal and helical buckling loads in straight wellbores. Moreover, many experimental tests have shown some helix reversal effects 19,21,22 , that is a change in the winding direction of helical shape (from right-hand to left-hand helical shape, and vice versa). This reversal phenomenom is explained by friction 19 and may probably exist in the field.…”

“…Moreover, many experimental tests have shown some helix reversal effects 19,21,22 , that is a change in the winding direction of helical shape (from right-hand to left-hand helical shape, and vice versa). This reversal phenomenom is explained by friction 19 and may probably exist in the field. Zdvizhkov 22 showed in the laboratory that torsion can be induced in helically buckled pipe without externally applied torque, and that the sign of the induced torsion depends on the winding direction of helical shape.…”

“…Many experimental studies have been carried out in laboratory with reduced size pipes in order to better understand the mechanisms involved in the buckling phenomenom 16,18,19,20,21 . An extensive experimental study was conducted by Salies 21 to investigate the helical buckling of tubulars in inclined wellbores, from vertical to horizontal.…”