Experimental Study Of Curvature And Frictional Effects On Buckling

Abstract

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.

A review of down-hole tubular string buckling in well engineering

“…Experimental results also have indicated that friction force on the tubular string can delay the onset of the sinusoidal buckling (McCann and Suryanarayana 1994). Mitchell (2007) pointed out that the initial buckling with friction is in the form of pipe rolling but not sliding.…”

“…Experiments (McCann and Suryanarayana 1994;Salies 1994) have shown that the build wellbore curvature has a stabilizing effect on tubular string buckling. The tubular string may have been in ''lock-up'' or yielded while the tubular string does not enter the buckling state in the build section (Kyllingstad 1995).…”

Equilibrium equations for nonlinear buckling analysis of drill-strings in 3D curved well-bores

“…Results are listed in Table 1. For comparison, experimental results for the case of T=0 [17] are also list in Table 1. From Table 1 it is seen that the numerical result agrees very well with the experimental result for the case of T=0.…”

“…To verify our model, a simple example having experimental results [17] The helical buckling loads are obtained by using the Rayleigh-Ritz method. Results are listed in Table 1.…”

Design of a mechanical clutch-based needle-insertion device

“…Drill strings are often in compression and have the tendency to buckle and ''lock'' (jam) inside the well if the compressive load exceeds a critical threshold (7). This critical locking point depends on the buckling point of the drill string and length and shape (curvature and number of curves) of the drill hole.…”