Daniel Vasilikis

Bending Deformation Capacity of Large-Diameter Spiral-Welded Tubes

Numerical simulations are conducted to define the bending deformation capacity of large-diameter spiral-welded tubes, towards efficient strain-based design. Under bending loading, the principal failure mode of those tubes is local buckling (wrinkling) of the tube wall. Bending moment, curvature and ovalization are monitored through the numerical analysis, and comparison is conducted with available test data on two 42-inch-diameter tubes, with D/t ratio of 67 and 119, described in detail elsewhere.The analysis accounts for the actual material properties. Initial geometric imperfections (profile, thickness, ovalization) are obtained from the tested specimens. Furthermore, residual stresses are also considered in the analysis, as computed by a numerical simulation of the cold bending process. A parametric analysis is also conducted on the influence of material properties, geometric initial imperfections and residual stresses on local buckling of spiral-welded tubes. Finally, a comparison with design equations for tube bending deformation capacity is conducted.Copyright

Buckling of Clad Pipes Under Bending and External Pressure

The present paper concerns the structural behaviour of clad pipes. This is a double wall pipe, composed by two pipes that are in contact through an appropriate manufacturing procedure; a thick-walled carbon steel “outer pipe”, and a thin-walled corrosion-resistant inner pipe, referred to as “liner” pipe. To predict the bending response and the buckling curvature of the thin-walled liner, it is necessary to account for its contact with the confining thick-walled outer pipe. Because of this confinement, existing numerical solutions or analytical predictions for the bending buckling resistance of unconfined thin-walled tubes are inadequate to predict the buckling resistance of the bent liner. In the present work, the problem is solved numerically, using nonlinear finite elements to simulate the clad pipe, accounting for the interaction between the liner and the outer pipe. First, the manufacturing process of the clad pipe is simulated to determine the liner hoop prestressing. Subsequently, bending curvature is applied (with and without the presence of external pressure). Stresses and strains are monitored throughout the deformation stage with emphasis on possible detachment of the liner from the outer pipe and the formation of local buckling on the liner wall.© 2011 ASME

Buckling Design of Confined Steel Cylinders Under External Pressure

Thin-walled steel cylinders surrounded by an elastic medium, when subjected to uniform external pressure may buckle. In the present paper, using a two-dimensional model with nonlinear finite elements, which accounts for both geometric and material nonlinearities, the structural response of those cylinders is investigated, toward developing relevant design guidelines. Special emphasis is given on the response of the confined cylinders in terms of initial imperfections; those are considered in the form of initial out-of-roundness of the cylinder and as an initial gap between the cylinder and the medium. Furthermore, the effects of the deformability of the surrounding medium are examined. The results indicate significant imperfection sensitivity and a strong dependency on the medium stiffness. The numerical results are employed to develop a simple and efficient design methodology, which is compatible with the recent general provisions of European design recommendations for shell buckling and could be used for design purposes.

Wrinkling of Lined Steel Pipes Under Bending

Lined pipes are used in energy pipeline applications (oil, gas, etc.); a corrosion-resistant thin-walled liner is fitted inside a carbon-steel outer pipe. The paper focuses on wrinkling of lined pipes (sometimes also referred to as “mechanical clad” pipes), which are candidates for offshore pipeline applications. The lateral confinement of the liner pipe due to the deformable outer pipe and its interaction with the outer pipe has a decisive influence on the wrinkling behaviour of the thin-walled liner. The problem is solved numerically, using nonlinear finite elements to simulate the lined pipe and its interaction with the outer pipe. Nonlinear geometry with large strains is taken into account, and the material of both pipes is elastic-plastic. Stresses and strains are monitored throughout the deformation stage with emphasis on possible detachment of the liner from the outer pipe and the formation of wrinkles. It is shown that the behaviour is characterized by a first bifurcation in a uniform wrinkling pattern, followed by a secondary bifurcation and finally a localization of the buckled pattern. The values of curvature at which liner wrinkling occurs are determined. The numerical results are compared with available experimental results.Copyright

BUCKLING OF UNCONFINED AND CONFINED THIN-WALLED STEEL CYLINDERS UNDER EXTERNAL PRESSURE

The present paper investigates the structural stability of thin-walled steel cylinders subjected to uniform external pressure. A brief discussion of unconfined steel cylinder buckling is presented first. Subsequently, motivated by the design of buried pipelines, buckling of confined steel cylinders, surrounded by an elastic medium, is examined. A two dimensional model is developed, assuming no variation of load and deformation along the cylinder axis. The cylinder and the surrounding medium are simulated with nonlinear finite elements that account for both geometric and material nonlinearities. The external pressure response of confined thin-walled steel cylinders is examined, in terms of initial the out-of-roundness of the cylinder, the initial gap between the cylinder and the medium, and the stiffness of the surrounding medium. Numerical results show a rapid drop of pressure after reaching the maximum pressure level. Finally, the numerical results show good comparison with a simplified closedform expression, proposed elsewhere, which could be used for buried pipeline design purposes.