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The collapse failures of casings occur extremely easily under non-uniform load caused by plastic creep of mudstone, shale, salt rock and some soft rocks. In addition, only a limited number of specific research on the collapse mechanism of casings under non-uniform load have been undertaken, especially experimental studies, although non-uniform load has a great influence on the collapse properties of casings. Hence, full-scale crush tests for five types of N80 casings (200, 400, 600, 800 and 1000 mm) were conducted under non-uniform load using a self-developed testing system. Macro-analysis of the testing results provided the changes in radial displacement, morphology of casings before and after deformation as well as the correlation between these and the axial length of casings. Micro-analysis of testing results provided the strain-stress rules, yield load and instability load of the casings; specific focus on the influence of axial length on the collapse properties of casings under non-uniform load clarified the collapse failure mechanism of N80 casings under non-uniform load. Finally, the equations that can predict yield load and instability load of N80 casings with different axial lengths under non-uniform load were determined by fitting the experimental data.

Many studies focused on casing collapse strength under uniform load have been done, and the API 5C3 and ISO standards have been formed. However, only a limited number of researches on collapse failure mechanism of casing have been done under non-uniform load, especially experimental study, although the non-uniform load has a great impact on casing collapse strength. Hence, the collapse experiment is conducted for P110SS casing under opposed line load by using electro hydraulic servo pressure testing machine. The displacement variation rules of P110SS casing have been obtained under opposed line load. The strain of casing is measured in the process of collapse testing by the method of gluing strain-gauge on the outside surface of P110SS casing. The initial yield load of casing, instability load of casing and the yield load, stress-hardening rate, strain-hardening rate of casing after hardening are obtained. The stress-strain rules of casing after hardening are analyzed. The hardening characteristic and collapse failure mechanism of P110SS casing have been clarified under non-uniform load (under opposed line load). Experimental results can provide important references for the theoretical study on failure mechanism and collapse strength of casing under non-uniform load.

The corrosion of oil country tubular goods (OCTG) gets more and more serious especially in the acidic environment. So, it is very important to develop a perfect anticorrosion technology for exploring sour oil and gas fields economically and safely. Analysis indicates that the bimetal-pipe (BP) which consists of the base layer of low carbon steel and a corrosion resistant alloy (CRA) cladding layer is an economic and reliable anticorrosion technology and has broad application prospects in the transportation of acid medium. However, theoretical study of hydraulic expansion mechanism for BP is not enough. In this paper, the deformation compatibility condition of BP was obtained by studying the deformation rule of the (CRA) liner and the outer pipe of carbon steel in the forming process; the mechanical model which can compute the hydroforming pressure of BP has been established based on the nonlinear kinematic hardening characteristics of material; furthermore, based on the stress strain curve of inner pipe simultaneously, the calculation method of the plastic hardening stress has been proposed. Thus, the accurate method for computing the forming pressure was obtained. The experimental data show that results are consistent with results of the proposed model. It indicates that the model can be used to provide theoretical guidance for the design and production as well as use of BP.