This paper describes how deformations, forces and stresses of buckled tubing and casing in inclined holes can be predicted. It is also shown that wall frictional forces on buckled pipe can be reduced greatly by increasing the axial force above the normal operating value and then slacking off.
Failure of tubing or casing in offshore wells can lead to loss of wells and platforms in some instances. It is well known that tubulars, when subjected to compressive axial loads due to landing, thermal or pressure effects, will buckle. Research results reported in this paper demonstrate that the post-buckling behavior of tubular members in inclined holes, while quite complex, can be predicted. Hysteresis effects occur such that there can be a large variation of axial force without change of helical pitch of the tubular, and the history of loading plays a dominant role in determining the downhole configuration.
The hysteresis effect during loading and unloading of helically buckled pipe is described and simple laboratory experimental confirmation is presented. A convenient way of plotting the force/pitch relationship is used to assist in the explanation of the phenomena occurring during changes of axial loading on a helically buckled pipe. Consideration is given to computing the stresses in buckled pipe constrained within a circular hole and wall contact forces are evaluated for both vertical and inclined holes. Based upon simple equilibrium considerations a relationship is developed for predicting the critical hole inclination angle. For angles less than this critical angle the pipe will buckle into a helix and for angles greater than the critical angle the pipe loses contact with the high side of the inclined hole. Finally, design considerations are discussed briefly.
Equations describing the helical post-buckling configuration of a weightless circular column confined within a cylinder as developed by Lubinski et al.1 are frequently applied to the analysis of oil field tubing and casing.2–5 Recently Cheatham and Pattillo have extended this analysis by showing that the Lubinski equations apply to situations in which the axial load is being increased (loading) and a different force/pitch relationship applies when the axial loading is being decreased (unloading). Additional research has been done to investigate the implications of this hysteresis effect during the loading and unloading of, helically buckled pipe.7–9
The geometry of a helically buckled pipe is illustrated by Figure 1. In the analysis reported in this paper the angle 8 is measured from the low side of an inclined hole, p refers to the pitch of the helix and r refers to the effective radius or the clearance between the pipe and the wall of the hole. The axial force vs. pitch relationship is given by the following equation (Available in full paper)