Arnold M. Gresnigt

Failure of Locally Buckled Pipelines

Mechanical damage in steel pipelines in the form of local buckles due to excessive bending deformation may severely threaten their structural integrity. The present paper describes experimental and numerical research conducted to assess the structural condition of buckled pipes, subjected to both bending and internal pressure. Fatigue failure under repeated loading is mainly investigated, whereas pipe burst due to internal pressure is also examined. Three full-scale buckled pipe specimens are tested under pressure and bending loads to determine their structural capacity. In addition, using nonlinear finite element tools, an extensive parametric study is conducted to determine the critical locations at the buckled area at which maximum strain variation occurs, as well as to investigate the influence of several geometrical and mechanical parameters. Using the maximum strain range from the finite element computations and a simple S-N approach, reasonable predictions are obtained for the number of cycles to failure observed in the tests. The results of the present study demonstrate that, under repeated loading, fatigue failure occurs in the buckled area at the location of maximum strain range. It is also found that the burst pressure may not be affected by the presence of buckles.

Nonlinear Response and Failure of Steel Elbows Under In-Plane Bending and Pressure

The paper investigates the response of elbows under in-plane bending and pressure, through nonlinear finite element tools, supported by experimental results from real-scale tests. The finite element analysis is mainly based on a nonlinear three-node tube element, capable of describing elbow deformation in a rigorous manner, considering geometric and material nonlinearities. Furthermore, a nonlinear shell element from a general-purpose finite element program is employed in some special cases. Numerical results are compared with experimental data from steel elbow specimens. The comparison allows the investigation of important issues regarding deformation and ultimate capacity of elbows, with emphasis on relatively thin-walled elbows. The results demonstrate the effects of pressure and the influence of straight pipe segments. Finally, using the numerical tools, failure of elbows under bending moments is examined (cross-sectional flattening or local buckling), and reference to experimental observations is made.

Lateral Loading of Internally Pressurized Steel Pipes

This paper examines the denting response of pipes subjected to lateral (transverse) quasistatic wedge loading, in the presence of internal pressure. Pipes are modeled with nonlinear shell finite elements and a simplified analytical model. The analysis focuses on the significant influence of internal pressure on the denting resistance. Furthermore, the effects of wedge denting device orientation on the denting resistance are briefly discussed. Motivated by the experimental and numerical results, a two-dimensional heuristic model is proposed, which yields closed-form expressions for the denting force in terms of the corresponding displacement. The finite element results and the model equations are in good agreement with the experimental results and illustrate pipe denting response in an elegant manner.

Ultimate Bending Capacity and Buckling of Pressurized 90 deg Steel Elbows

The paper examines the nonlinear elastic-plastic response of internally pressurized 90 deg pipe elbows under in-plane and out-of-plane bending. Nonlinear shell elements from a general-purpose finite element program are employed to model the inelastic response of steel elbows and the adjacent straight parts. The numerical results are successfully compared with real-scale experimental measurements. The paper also presents a parametric study, aimed at investigating the effects of diameter-to-thickness ratio and moderate pressure levels on the ultimate bending capacity of 90 deg elbows, focusing on the failure mode (local buckling or cross-sectional flattening) and the maximum bending moment. Special attention is given to the response of 90 deg elbows under out-of-plane bending moments.

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

MAIN ISSUES ON THE SEISMIC DESIGN OF INDUSTRIAL PIPING SYSTEMS AND COMPONENTS

A significant number of damages in piping systems and components during recent seismic events have been reported in literature which calls for a proper seismic design of these structures. Nevertheless, there exists an inadequacy of proper seismic analysis and design rules for a piping system and its components. Current seismic design Codes are found to be over conservative and some components, e.g., bolted flange joints, do not have guidelines for their seismic design. Along this line, this paper discusses about the main issues on the seismic analysis and design of industrial piping systems and components. Initially, seismic analysis and component design of refinery piping systems are described. A review of current design approaches suggested by European (EN13480:3) and American (ASME B31.3) Codes is performed through a Case Study on a piping system. Some limits of available Codes are identified and a number of critical aspects of the problem e. g., dynamic interaction between pipes and rack, correct definition of the response factor and strain versus stress approach, are illustrated. Finally, seismic performance of bolted flange joints based on the results of experimental investigations carried out by the University of Trento, Italy, will be discussed.

Ultimate Capacity of Pressurized 90 Deg Elbows Under Bending

In this paper the response of internally pressurized pipe elbows under bending loads (in-plane and out-of-plane) is examined. The investigation is conducted through advanced finite element analysis tools, supported by real-scale test data. The numerical results are successfully compared with the experimental measurements. In addition, a parametric study is conducted, which is aimed at investigating the effects of the diameter-to-thickness ratio on the ultimate capacity of 90 deg elbows, focusing on the failure mode (buckling or flattening) and the maximum bending strength. The effects of moderate pressure levels on the ultimate moment and the failure mode are examined extensively.Copyright

Stability of spiral welded tubes in Quay Walls

A European research project (RFCS) has started to provide economic and safe guidance for the design of spirally welded tubes in combined walls. The main motivation for this project called COMBITUBE is that the current Eurocode 3 regulations for tubes in quay walls lead to uneconomic designs, because of poor local buckling design rules for these tubes. Because the most important load in quay walls is bending due to earth load, economic design implies a high diameter to wall thickness ratio. For relatively thin walled shells, the Eurocode 3 rules provide good results for the local buckling stress. For thicker walled shells where local buckling occurs when a part of the cross section has yielded, rather poor and uneconomic estimates of the local buckling bending moment are obtained. Also, no information on the deformation capacity is given. A solution for this problem is the application of strain based design methods, where the strain at local buckling is determined, from which the curvature (deformation capacity) and via the stress strain relationship, the bending moment can be obtained. The advantages of strain based design are explained. In comparison with longitudinally welded tubes and seamless tubes not much bending test results are available for spirally welded tubes. Therefore in the project full scale four point bending tests on spirally welded tubes with diameters up to 1060 mm are performed. Test results obtained so far are presented. The test results are used for the validation of FEA models for parameter studies where the effect of geometrical conditions (spiral welds and girth welds, geometrical imperfections), material properties and loading (mainly combinations of bending moment, normal force and earth loads) on the local buckling curvature and bending moment capacity are determined. Therefore, in the test program accurate measurements are performed of these conditions. Moment - curvature and curvature - ovalisation relationships are presented and compared with theoretical predictions

Discussion of “Mechanics of Confined Thin-Walled Cylinders Subjected to External Pressure,” (Vasilikis, D., and Karamanos, S., 2014, Appl. Mech. Rev., 66(1), p. 010801)

The collapse pressure of confined cylinders depends on many factors. In addition to the thorough investigations of Vasilikis and Karamanos, more factors can be candidates for further investigation, such as the effect of variations in the material mechanical properties of the liner pipe in compression and the effect of residual stresses. The mechanical response of the materials in compression depends on the type of steel and the stress-strain history, which depends on the fabrication method of the cylinder. This is illustrated with theoretical and experimental results on pipes under external pressure, as used in offshore applications. There is a need for more experimental test results for validation. More applications of confined cylinders are mentioned that are worth investigation.

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