Jeroen Van Wittenberghe

Parametric finite element model for large scale tension tests on flawed pipeline girth welds

The structural response of a pipe with a flawed girth weld, subjected to global plastic deformation is influenced by a large number of geometrical and material properties. Finite element models that aim to simulate this need to include all relevant influence factors, which causes high challenges in the creation of such models. In search for a high degree of flexibility, automation and ease of use, the authors have developed a parametric script that creates geometries for two common pipeline girth weld tension tests: the curved wide plate test and the full scale pressurized pipe tension test. The developed model allows to modify pipe geometry, test specimen geometry, flaw position (weld metal centre or heat-affected zone), flaw size, weld misalignment, pipe thickness variations, weld fusion line profile, and weld cap profile. The desired geometry is obtained by a coordinate transformation scheme that starts from a flat plate with a simplified weld geometry. A deliberate partitioning strategy is applied to obtain flexibility in the flaw location and full independence between a fine flaw mesh and a coarse body mesh. This article describes the approach, structure and governing equations of the model. An example geometry is discussed to illustrate the various possibilities. The proposed model provides inspiration for all who seek to develop parametric finite element models with a similar flexibility and ease of use.

Nonlinear Contact Analysis of Different API Line Pipe Coupling Modifications

Threaded pipe couplings are used to join pipelines when they have to be uncoupled frequently or as an easy to assemble alternative to welding. A large variety of patented coupling modifications are available, but little is known about their influence on the connection s behavior. In this study, the finite element model of an API line pipe threaded pipe connection is presented and its nonlinearities in material properties and contact behavior are discussed. Test results obtained from a four-point bending fatigue experiment are in good agreement with the results of the numerical simulations. A series of modifications of the standard connection are simulated to gain a better understanding in the influence of geometrical and material parameters on the connections performance. It was found that not all existing coupling modifications are improving the connections performance. It can be concluded that critical evaluation of the performance of existing coupling modifications is necessary and finite element analyses are proven to be a useful tool for this.

Resonant Bending Fatigue Test Setup for Pipes With Optical Displacement Measuring System

Pipes and tubular members are used in offshore applications as structural elements, such as columns or in transport pipelines, risers, etc. When subjected to dynamic loads, weld defects or geometrical stress raisers can initiate fatigue cracks, causing the columns or pipelines to fail prematurely. In order to investigate the fatigue behavior of pipe joints, a resonant bending fatigue setup was designed, suitable for testing pipes within a diameter range from 6 in. to 20 in. In this setup, the pipe, filled with water, is subjected to a dynamic excitation force with a frequency close to the natural frequency of the filled pipe. The force is applied using a unique drive unit with excentric masses. The pipe is supported in the nodes of its natural wave-form, so that no dynamic forces are transmitted to the setup. The deformation of the pipe is measured at discrete locations using an optical 3D dynamic measuring system. Through-thickness fatigue cracks can be detected by pressurizing the water in the pipe and applying a pressure gauge. In this paper, some unique aspects of the design of the resonant bending fatigue setup are discussed by presenting the results of a semianalytical model used for calculating the deformation and bending stress in the excitated pipe and by comparing these results to the deformation measurements made by the dynamic measuring system. The working principles of the setup are illustrated by showing the preliminary test results for a 12 in. diameter X65 steel pipe with a wall thickness of 12.7 mm. It is demonstrated that the model predicts the behavior of the pipe in the setup very accurately.

Fatigue Life Improvement of Threaded Pipe Couplings

In this study it is demonstrated experimentally that the fatigue life of threaded pipe couplings can be improved by providing a more uniform load distribution over the threads of the coupling. The load distribution was changed by applying both global and local stiffness reductions to the coupling. The obtained results were evaluated using finite element simulations of 2D axisymmetric models. To study the validity of these models a critical comparison was made with a full 3D model that accounts for non-linearities in material and contact properties.Copyright

Limit load and reference stress for curved wide plates

Curved wide plates are a valuable tool in the assessment of defective pipeline girth welds under tension. Throughout the years, Laboratory Soete collected an extensive database of curved wide plate test results. In an effort to investigate these results through FAD analysis, the authors recently developed a reference stress equation for curved plates. The approach followed is similar to the development of the Goodall and Webster equation for flat plates. This paper elaborates finite element analyses of the equation’s capability to predict plastic collapse. It is found that, although overestimated, the influence of plate curvature is correctly predicted in a qualitative way. For all simulations, the curved plate reference stress equation produced conservative estimations. This indicates that the proposed equation is suited to safely predict the plastic collapse of defective pipeline girth welds. An experimental validation is underway.Copyright

Non-Linear Contact Analysis of an API Line Pipe Coupling

In this study, the finite element model of an API Line Pipe threaded pipe connection is presented. The non-linearities in material properties and contact behaviour are discussed. A series of modifications of the standard connection are simulated to gain a better understanding in the influence of geometrical and material parameters on the connection’s performance. Finally, test results obtained from a four-point bending fatigue experiment are presented and compared with numerical simulations.

The Influence of Connection Geometry on the Fatigue Life of National Pipe Thread Threaded Pipe Couplings

In this paper, four-point bending experiments are performed to determine the fatigue life of several threaded pipe couplings. It is shown that fatigue life of these couplings can be improved by designing them with more uniform load distribution in the threads, achieved through reducing the global and local stiffness of the coupling. Finite element simulations of a 2D axisymmetric model show consistence between experimental observation and analytical modeling. A 3D finite element simulation, which includes nonlinear material behavior, elaborate contact properties and represents the coupling geometry more accurately, is also performed to evaluate the 2D axisymmetric finite element model.

Fatigue crack growth behavior of threaded pipe couplings

Threaded couplings are used in various applications to connect steel pipes. To maintain a secure connection, such couplings are preloaded and during service additional dynamic loads can act on the connections. The coupling’s threads act as stress raisers, initiating fatigue cracks, which can cause the connection to fail in time. Accurate knowledge of the fatigue behavior, taking into account crack initiation and propagation is necessary to understand the fatigue mechanisms involved. In this study, the fatigue behavior of tapered couplings with NPT threads is studied. This is done by analyzing the results of an experimental four-point bending test. The fatigue crack propagation is monitored using an optical dynamic 3D displacement measurement device and LVDTs to measure the crack opening. At certain times during the test, the load ratio is changed to apply a number of beach marking cycles. This way a fine line is marked in the fracture surface. These marked crack shapes are used as input for a finite element model. The measured deflection and crack opening are compared to the results of the numerical simulations. Using this methodology a distinction is made between fatigue crack initiation and propagation. By analyzing the fracture surface it was observed that once the crack is initiated, it propagates over a wide segment of the pipe’s circumference and subsequently rapidly penetrates the wall of the pipe. The observed crack growth rates are confirmed by a fracture mechanics analysis. Since the appearing long shallow crack is difficult to detect at an early stage the importance is demonstrated of accurate knowledge of the fatigue behavior of threaded connections in order to define acceptable flaw sizes and inspection intervals.