Experimental Study Of Curvature And Frictional Effects On Buckling
Authors: McCann R.C. , Suryanarayana P.V.R. 
Published: October 2008

Buckling and post-buckling lock-up place a limit on the reach of extended-reach and horizontal wells. Although buckling has received considerable theoretical attention in the past, no serious attempt has been made to study this process experimentally. This paper describes results from experiments on buckling, unbuckling and post-buckling behavior of rods laterally constrained in a cylindrical enclosure, with particular emphasis on the effects of friction. The experimental apparatus, procedures, and uncertainty analysis are described. Results indicate that friction significantly delays the onset of buckling (both sinusoidal and helical buckling), and causes noticeable hysterisis in the post-buckling behavior, As a result of this hysterisis, the unbuckling loads are always less than the corresponding buckling loads. Mitigation of friction reduces the hysterisis. Friction is also a cause of post-buckling snapping and reversals in the direction of the helix. As expected, the effects of friction become less significant as the inclination decreases. For inclinations (from vertical) less than 15 degrees, the effects of friction are negligible for the initiation of sinusoidal buckling, but are significant once the rod has buckled into a full helix. Curvature also delays the onset of buckling, and both curvature and friction have a stabilizing effect on the tubulars.

Current theory is re-visited and re-interpreted irr the light of these experimental observations, and its limitations are discussed with an emphasis toward operational practices and field applications. It is shown that currently used theory actually predicts unbuckling and not buckling. When friction is significant, current theory underestimates the compressive loads at which sinusoidal and helical buckling of tubulars occur in straight wellbore intervals. Ignoring friction limits the 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 hysterisis 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.

INTRODUCTION

Buckling of tubulars such as drill string, casing, or tubing places operational limits on extended reach wells. In particular, buckling and subsequent "lock-up" are limiting factors to the reach of horizontal wells. Buckling occurs when the compressive load on a tubular exceeds some critical value. This critical load is the load beyond which the equilibrium shape is no longer stable, and small increases in load lead to large lateral deflections. Typically, compressive load is provided by the weight on bit (WOB) or the slack-off friction. However, unlike Euler buckling of bars, the lateral deflection of buckled tubulars in wells is limited by the outer constraint of the wellbore. As a result, buckling progresses differently. When the compressive load reaches the critical value, the straight shape of the string is no longer stable. A slight increase in the load from this value causes the string to deflect (buckle) into a sinusoidal shape along the lower portion of the hole. This is referred to as sinusoids/buckling.