George E. Varelis

Buckling of High-Strength Steel Cylinders Under Cyclic Bending in the Inelastic Range

The present paper examines the structural behavior of elongated steel hollow cylinders, referred to as tubes or pipes, subjected to large cyclic bending, through a rigorous finite element simulation. The bent cylinders exhibit cross-sectional distortion, in the form of ovalization, combined with excessive plastic deformations. Those deformations grow under repeated loading and may lead to structural instability in the form of local buckling (wrinkling) and, eventually, failure of the loaded member. The study focuses on relatively thick-walled seamless cylindrical members made of high-strength steel, which exhibit local buckling in the plastic range of the steel material. The analysis is conducted using advanced nonlinear finite element models capable of describing both geometrical and material nonlinearities. A cyclic plasticity model that adopts the “bounding surface” concept is employed. The material model is calibrated through special-purpose material testing, and implemented within ABAQUS, using a user-subroutine. The finite element model is validated by comparison with two experiments on high-strength steel tubular members. Special emphasis is given on the increase of ovalization and the gradual development of small-amplitude initial wrinkles with repeated loading cycles. A parametric numerical study is conducted, aimed at determining the effects of initial wrinkles on plastic buckling performance.

Experimental and Numerical Investigation of Pressurized Pipe Elbows Under Strong Cyclic Loading

The present work examines the behavior of pipe elbows subjected to strong cyclic in-plane bending loading in the presence of internal pressure. In the first part of this work the experimental procedure is presented in detail. The tests are conducted in a constant amplitude displacement-controlled mode resulting to failures in the low-cycle fatigue range. The overall behavior of each tested specimen, as well as the evolution and concentration of local strains are monitored throughout the testing procedure. Different internal pressure levels are used in order to examine their effect on the fatigue life of the specimens.The above experimental investigation is supported by rigorous finite element analysis. Using detailed dimensional measurements and material testing obtained prior to specimen testing, detailed numerical models are developed to simulate the conducted experiments. An advanced cyclic plasticity material model is employed for the simulation of the tests. Emphasis is given on the local strain development at the critical part of the elbow where cracking occurs. Finally, the results of the present investigation are compared with available design provisions in terms of both ultimate capacity and low-cycle fatigue.Copyright

Finite Element Analysis of Cyclically-Loaded Steel Pipes During Deep Water Reeling Installation

Thick-walled steel pipes during their installation in deep-water are subjected to combined loading of external pressure and bending, which may trigger structural instability due to excessive pipe ovalization with catastrophic effects. The loading path followed during the reeling installation process is characterized by strong cyclic loading of the pipe material and results in residual stresses and deformations of the pipe cross-section, undermining the structural capacity of the pipe. Using advanced material tools, the present study examines the effect of reeling on the structural response and resistance of offshore pipes during the installation process.Copyright

Numerical Simulation of UOE Pipe Process and its Effect on Pipe Mechanical Behavior in Deep-Water Applications

Large-diameter thick-walled steel pipes during their installation in deep-water are subjected to a combination of loading in terms of external pressure, bending and axial tension, which may trigger structural instability due to excessive pipe ovalization with catastrophic effects. In the present study, the UOE pipe manufacturing process, commonly adopted for producing large-diameter pipes of significant thickness, is considered. The study examines the effect of UOE line pipe manufacturing process on the structural response and resistance of offshore pipes during the installation process using nonlinear finite element simulation tools.Copyright

Effects of UOE Manufacturing Process on Pressurized Bending Response of Offshore Pipes

Thick-walled steel pipes manufactured through the UOE process are used in deep-water pipeline applications for the safe and cost-effective transmission of hydrocarbon energy resources. Such pipes are subjected to bending loads in the presence of high external pressure during their installation stage. The combination of bending and external pressure often triggers the development of structural instability due to excessive ovalization of the pipe with catastrophic effects. In the present study, the effect of UOE line pipe manufacturing process on the bending response of externally-pressurized thick-walled pipes is examined, using finite element simulation tools.Copyright

Structural Performance of Steel Pipe Tee-Junctions

The behavior of steel pipe junctions (Tees) subjected to strong loading in the presence of internal pressure is examined in the present study. The analysis is based on a set of monotonic and cyclic out-of plane bending tests under constant and increasing amplitude displacement-controlled loading schemes leading to low-cycle fatigue failure.Rigorous finite element models are developed to support the experiments, accounting for detailed dimensional measurements and material testing results obtained prior to testing. A parametric analysis is also conducted focusing on the effect of the geometrical characteristics on the overall junction behavior. The performance of the Tee-junctions with varying geometries under out-of plane bending, in-plane bending and axial loading is also examined numerically accounting for the presence of internal pressure.Copyright

Experimental and Numerical Investigation of Pipe T-Junctions Under Strong Cyclic Loading

Tee pipe junctions are piping components widely used in industrial and pipeline applications. Their performance under severe loading conditions may be critical for the structural integrity of an industrial facility. When these components are subjected to repeated loading associated with cyclic plasticity, failure is possible. The present work is a combined experimental and numerical effort which examines the behavior of piping branch T-junctions subjected to strong cyclic out-of-plane bending. The first part of this paper describes the experimental investigation of the junction performance. Tests are conducted in a constant and varying amplitude displacement-controlled mode resulting to failure in the low-cycle fatigue range. The overall behavior of each specimen in terms of fatigue life, as well as the evolution and concentration of local strains are monitored throughout the testing procedure.The experimental investigation is supported by finite element modeling, developed to simulate the experiments. Advanced cyclic plasticity material models are employed and emphasis is given on the local strains developed at the critical part of the T-junctions where first cracking occurs.Copyright

Pipe Elbows Under Strong Cyclic Loading

Motivated by the response of industrial piping under seismic loading conditions, the present study examines the behavior of steel process piping elbows, subjected to strong cyclic loading conditions. A set of experiments is conducted on elbow specimens subjected to constant-amplitude in-plane cyclic bending, resulting into failure in the low-cycle-fatigue range. The experimental results are used to develop a low-cycle-fatigue curve within the strain-based fatigue design framework. The experimental work is supported by finite element analyses, which account for geometrical and material nonlinearities. Using advanced plasticity models to describe the behavior of elbow material, the analysis focuses on localized deformations at the critical positions where cracking occurs. Finally, the relevant provisions of design codes (ASME B31.3 and EN 13480) for elbow design are discussed and assessed, with respect to the experimental and numerical findings.Copyright