The Prediction Of Low Cycle Fatigue Life And Study Of Buckling Analysis Method For Coiled Tubing

Compared with the conventional threaded tubing, coiled tubing had the advantagesof no buckling, no shackling in the roundtrip. It was widely used in drilling, workover,fracturing, acidizing operations. However, it had been a key technical problem aboutcoiled tubing drilling operation that its low working life, high cost and difficulty axialcompression. This paper which adopted theoretical analysis, numerical simulation andexperimental method aimed at the life prediction of coiled tubing roundtrip operation,within the well casing axial compression buckling behavior to carry out the research.According to the failure forms and reasons of the coiled tubing, the geometryparameter of describing the damage of coiled tubing was defined: wall thicknessthinning amount Δt, ovality Φ, depth of pip H, local surface mechanical damagelength x, width w, depth h. Adopt existing electronic universal tensile testing machineCMT5105, develop the special pressure equipment, stretch bending and straighteningequipment, etc. Choose the23’’8and11’’4coiled tubing which commonly used inengineering as the test specimens, carry out no damage and damage including ovality,wall thickness, depth of pip, etc coiled tubing fatigue life bent and straight cumulativedamage experiment. A lot of experimental data were obtained. Through data analysisand the regression process, the coiled tubing ovality, wall thickness, and calculationformula of damage parameters changing with cycles were obtained.According to the working state of the coiled tubing, coiled tubing was analyzedthrough the injection head, guide frame, roller and mechanical behavior in the wellcasing. The coiled tubing in the ground and well casing non-linear finite elementanalysis method were established. The site, guide frame, cylinder head and coiledtubing within the well casing dangerous section of generalized stress (axial stress,circular stress) and radial stress and strain calculation formula were obtained. Accordingto the damage form of coiled tubing, using the numerical simulation and mathematicalregression method, the pits and sag calculation formula of the stress concentration factorin coiled tubing were obtained. Using elastic mechanics theory, with the hoop stress of coiled tubing ovality damage calculation formula was deduced. A nondestructive anddamage calculation method of the coiled tubing fatigue life prediction was established.Through indoor experiment and field test, its theory to predict the fatigue life ofgeometric parameters and damage error were less than20%.Select the coiled tubing in the vertical wells, slant wells and curved wells as theresearch object, coiled tubing within the well casing based on energy method in bottomhole pressure under the action of the sinusoidal and spiral buckling instability criticalload formula were deduced. According to sinusoidal or helical buckling compressionstring geometry, considering well casing trajectory (curvature and hole deviation),coiled tubing sinusoidal buckling, spiral deformation after contacting with well casingstatus, bring in the coiled tubing gravity and the friction resistance along the axial workitem to Lagrange multiplier method. The vertical wells and slant wells, bendingcompression coiled tubing in the well with well casing contact force, friction anddeformation calculation formula were obtained. Aimed at coiled tubing in well casingpost-buckling occurred sinusoidal or helical deformation problems, special roof boltspost-buckling experiment device was developed. Draw roof bolt’s change law underdifferent load in the sinusoidal and helical buckling instability critical load and friction.Its experimental results were basically identical with the theoretical calculation results,provided a theoretical basis for coiled tubing drilling and repairing work.