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Evaluation of the resistance of cylindrical couplings to axial loading in repair of circumferential joints in pipelines

One of the efficient methods of restoration of the loadcarrying capacity of operating gas and oil pipelines with surface damage and loss of metal, is the installation of metallic or glass–plastic composite couplings. 2 The repair structure of the coupling–pipe for this type of defect is calculated for strength, taking into account permissible circumferential stresses. The defects in circular joints, so-called planar defects, detected in the service of pipelines, are oriented in the circumferential direction and reduce the resistance of pipelines to longitudinal loading. The presence of these defects indicates that it is essential to evaluate the axial strength of a repaired structure. In the present work, analysis is made of the stress state of a coupling–pipe type structure under the effect of longitudinal loading, comparable with the yield limit of the parent metal, with special reference to the tasks of repair of circumferential joints in transmission pipelines. The diagram of the coupling–pipe system is shown in Fig. 1. Zone 0 is the wall of the pipe below the coupling. Zone 1 is the layer of the adhesive, securing the coupling to the pipe. The thickness of the zone is considerably smaller than that of the remaining zone. Zone 2 is a cylindrical coupling, examined as a monolithic structure, having a characteristic circular depression above the convex part of the welded joint. This depression is filled with a sealing compound, whose strength is not taken into account. For the approximate evaluation, it is assumed that the stress state is uniaxial, the normal stresses are uniformly distributed in the thickness of the appropriate zones. In the equations of the static equilibrium of forces, the product σ 1 δ 1 is insignificantly small. Consequently, the equation of equilibrium of the forces of the pipe, situated below the coupling is: