Underground Column Buckling Behavior Of Theoretical And Experimental Study

Tubular strings used in petroleum engineering consist of petroleum tubes (pipes), joints and necessary tools, valves. As a special mechanical component, the strings, subjected to certain loads and stresses, may be longitudinally buckled like compressed rods. If the loads or stresses exceed allowable limits, some parts of the strings may be broken and operations will be failed. To avoid failures due to longitudinal buckling of tubular strings, this paper investigates the buckling behaviors of tubular strings in three dimensionally curved holes. After experimental and theoretical study, critical (sinusoidal, helical) buckling loads, distribution of contacting pressures as well as friction between (buckled) tubular strings and holes were obtained. Based on the experimental and theoretical study, equations and calculating (computing) methods, which could be used to guide the design or combination of tubular strings and selection of operating date, were derived.General differential equations, which could describe the buckling behavior of tubular strings, were derived by means of both traditional constitutional method and energy method. Parameters considered were: (1) loads subjected, (2) geometric and physical properties of tubing, (3) length, diameter, curvature of holes, (4) operation' date, (5) pressure and production of reservoirs. The general differential equations were further described with 5 dimensionless parameters: α,β,γ1, γ2, γ3 . From the 5 dime.isionless parameters, tubular buckling, influenced byparameters considered above could be revealed completely.With variation principles, the general differential equations of longitudinal buckled tubular strings were solved. According to the solutions, tubular strings in curved holes may be sinusoidally buckled, helically buckled and locked up. The 3 critical loads (threshold loads) of sinusoidal buckling, helical buckling and lockup were obtained during solving the general differential equations. By analyses of the 3 critical loads, it is found that the load released from hook could not be transmitted to parker or the bottom part of strings because of the friction between tubing and holes. The larger the released hook load, the larger the friction produced. With the increase of the friction, tubular strings may be locked up, and, parkers could not well set.On the bases of though investigations of buckling behavior, equations and methods for predicting loads, stresses and deformation of tubular strings were derived and presented. Due to steps between joints, parkers, well testing valves and tubing, reasonable continuous and boundary requirements should be taken into consideration.To invalidate the accuracy of theoretical analyses, experimental study was made inlaboratory and in oilfield. In laboratory experiments, longitudinal displacement, critical loads, friction between tubing and holes were measured. In 6 field experiments, loads, exerted to tubular strings during operations, were metered and recorded by specially made apparatus in 6 wells. With maximum relative error of 10%, the experiments agree to theoretical analyses well.