Abstract During a gas pipeline rupture event, the crack propagation velocity can exceed 300 m/s and the crack can run for several hundreds of metres before arresting. The current model to predict arrest pressure is the Battelle Two Curve Method (BTCM) using the Charpy V-notch energy to characterize propagation toughness. It has been shown that this model can give non-conservative predictions for high-strength pipe steels. Hence, the Crack Tip Opening Angle (CTOA) has been introduced as a promising parameter to describe crack propagation. The objective of the current work was to study the crack propagation process in pipe by Finite Element Analysis (FEA) techniques to gain a better understanding of crack driving force and factors influencing CTOA. Implicit FEM simulations of dynamic crack propagation in pipes with diameters ranging from 355 mm to 1219 mm with a wall thickness of about 19 mm were performed using material properties representative of either X65 or X80 pipeline steel. The specification of a critical CTOA and the nodal release algorithm in the software WARP3D were employed to propagate the crack up to about two metres in the simulations. For a given critical CTOA, pipe diameter, and pipe thickness a set of simulations was performed where the initial applied gas pressure varied from as low as 4 MPa up to 60 MPa (which corresponds to about 80% of the yield strength of the material). The CTOA values used in the simulations ranged from 5° to 20° and corresponded to CTOA measurements obtained in concurrent work from Drop Weight Tear Tests performed on pipe steels. To accurately predict crack velocity, it was important to apply a flap loading profile near the crack front representative of the gas pressure response during pipe rupture. Comparison of the crack propagation response was carried out between a constant pressure profile just behind the crack front and a pressure profile that varied with circumference a round the pipe. The influence of soil pressure on the flap loading response was also considered in the models. The predicted pressure versus crack velocity profiles and the arrest pressure can then be subsequently used to predict the arrest length for a given CTOA.

Abstract This paper describes the development of a data base and associated interpolation tool used to perform the validation of FEA thermo-mechanical models designed to verify the structural integrity of a self-pressurized modular Reactor Pressure Vessel (RPV) and its nozzles under service transient thermal loads. The main goal is to assess the element’s size and time steps that provide a confidence level on the obtained solutions. The validation process implies the definition of the geometry under study, its material’s properties, thermal load conditions and type of mesh element. With all this information, the program gives the user a set of curves for maximum time steps vs. temperature change rates for each typical thickness section in the modeled geometry for a chosen element size. Any point located below those curves assures a solution underneath a user specified allowable error (e.g.: 5%). All calculations are processed using dimensionless variables in order to create a universal data base enabling the analysis of many different situations of geometries, materials and loads. To improve performance, an Artificial Neural Network algorithm was developed. The resulting application significantly reduces the convergence study time and efforts.

Abstract Bolted flange joints (BFJs) are very common in the process industries, and they play an important role in equipment connection. A lot of BFJs are working at high temperature. The temperature and internal pressure will lead to gasket stress redistribution, and then affect time-dependent leakage rate of BFJs. In this paper, the effect of internal pressure on gasket stress and leakage rate of BFJ during the long term service at high temperature was discussed. The time-dependent gasket stresses of BFJ during the long term service at high temperature under two tightness levels and three internal pressures were calculated by using the finite element method. And then, based on the leakage rate prediction model of BFJ during the long term service at high temperature proposed, the corresponding time-dependent leakage rates of BFJ during the long term service at high temperature were acquired. The results showed that the internal pressure had a significant impact on the time-dependent gasket stresses and time-dependent leakage rates of BFJ during the long term service at high temperature. This research will help the engineers and technicians to understand the role of the internal pressure on the time-dependent gasket stresses and time-dependent leakage rates of BFJ during the long term service at high temperature.

Abstract This contribution deals with the efficient numerical modeling of tapped thread joints. Commercial FE packages provide different strategies to tackle the problem of modeling threaded joints, which is a recurrent one for the design engineer. Different modeling techniques are characterised by how the screw is modeled: either three-dimensional elements (thetra, hexa or wedge) or mono-dimensional elements (beam) can be used. In the case of three-dimensional approaches, the thread helix is seldom modeled: the actual geometry is often replaced by a plain cylinder and a suitable choice of contact settings between the screw and the “threaded” hole. In the case of road vehicles, due to the high number of threaded connections to be modeled, it is paramount to reach a trade-off between modeling accuracy and computational effort. This paper aims at comparing two modeling approaches, namely a three dimensional approach (baseline) and a mono-dimensional one (simplified model). Based on several criteria, such as equivalent stress on the screw shank, pressure distribution at the interface of the plates and in the underhead region, optimal contact settings for the simplified model are suggested. These settings allow replicating the results provided by the three-dimensional approach for given load case. The comparison is carried out on single lap, single screw joints, by the ANSYS R17 software. The methodology can be easily extended to other softwares or joint configurations.

Abstract During tightening bolts on the flange joint, a load is applied on each bolt. The deformation and stress on joint parts vary with magnitude and location of applying bolt forces. Therefore, sequence of tightening bolts has effects on the deformation and accumulated stress on joint parts. Finite element method is used to model the flange joint and to simulate effects of bolt tightening sequence on the deformation and stress on joint parts. The model includes bolts, nuts, raised face flanges, and gasket. Contact conditions are set on contact surfaces between contact parts. 3D elements are used to build the model. In order to prevent leaks, the bolt load and gasket pressure should be uniform. The accumulated stress of bolts and gasket are calculated. The deformation of gasket and flange are also simulated. The model is used to simulate the stress and deformation of flange joint generated by various bolt tightening sequences. The optimal tightening sequence is selected by that the variation of accumulated stress on gasket is the least.

Abstract The contact gasket stress reduces when the bolted gasketed pipe flange connections are subjected to internal pressure. In designing the bolted connections, it is needed to predict the reduced contact gasket stress, so, it is necessary to know the load factor. However, it is difficult to estimate the value of the load factor of the connections under internal pressure. In the previous paper (2018PVP), a more simpler calculation method was proposed. However, a more accuracy for obtaining the values of the load factor is desirable using the spring constants K tg and K cg . In the present paper, some calculation models for the spring constants are improved. Then, the values of the load factor for JIS 10K flange connections and ASME B16 flange connections with spiral wound gaskets are shown. The values of the load factor for the above connections are in a fairy good agreement with the FEM results. Using the obtained load factor, the residual contact gasket stress and an amount of gas leakage are predicted. The obtained calculated results of the load factor and the amount of the leakage are in a fairly good agreement with FEM results, and the measured results for 24” connection. As a result, the value of the load factor for the connections with larger nominal diameter is found to be negative and it decreases as the nominal flange diameter increases. In addition, a method how to determine the bolt preload for satisfying a give allowable leak rate is demonstrated.

Abstract It is known that the expected failure mode of piping systems under excessive seismic motion would be the fatigue failure at elbows or tees. Although there are a lot of experimental and analytical investigations on the strength and the failure behavior of elbows under seismic loading, there are a few studies on tee pipe joints particularly in the inelastic range. To clarify the characteristics of the strength of tee pipes under elastic-plastic region, investigations by FEM analysis were conducted. In this analytical investigation, steel butt-welding type tee pipe joints were considered. Two kinds of parameters were considered; 1) the loading direction (in-plane bending / out-of-plane bending), 2) the branch size (equal tee / unequal tee). From the analytical examination, the load-displacement relationship and the distribution of the accumulated plastic strain concentration position were obtained. The difference of the strength in the loading direction and the expected failure locations depending on the branch pipe size and the loading direction are discussed.

Abstract At ageing power plants, local thinning of pipework or vessel is unavoidable due to erosion/corrosion or other reasons such as flow accelerated corrosion (FAC) — one of the common degradation mechanisms in pipework of nuclear power plant. Local thinning reduces the structure strength, resulting in crack initiation from the corrosion pit or welding defect when subject to cyclic loading. General practice is to use the minimum thickness of the thinned area to calculate both limit load and stress intensity factor (SIF) in performing Engineering Critical Assessment (ECA) using Failure Assessment Diagram (FAD). Using the minimum thickness is normally overly conservative as it assumes that thinning occurs grossly instead of locally, leading to unnecessary early repair/replacement and cost. Performing cracked body finite element analysis (FEA) can provide accurate values of limit load and SIF, but it is time consuming and impractical for daily maintenance and emergent support. To minimise the conservatisms and provide a guidance for the assessment of locally thinned pipework or vessel using existing handbook solutions, a study was carried out by the authors on the effect of local thinning on limit loads. The study demonstrates that local thinning has significant effect on limit load if the thinning ratio of thinning depth to original thickness is larger than 25%. It concluded that the limit load solutions given in handbooks (such as R6 or the net section method) are overly conservative if using the minimum local thickness and non-conservative if using the nominal thickness. This paper discusses the effect of local thinning on SIFs of internal/external defects using cracked body finite element method (FEM). The results are compared with R6 weight function SIF solutions for a cylinder. A modified R6 SIF solution is proposed to count for the effect of local thinning profile. Along with the previous published paper on limit load it provides comprehensive understanding and guidance for fracture assessment of the local thinned pipework and vessel.

Abstract A nonstandard flanged and dished head is frequently used in the horizontal storage tank for quick and full access to the internal in oil and gas industry. The head is forged into an elliptical shape with a flat edge at its peripheral. The flat edge serves as a flange while the other mating half coming from the tank shell. The flange pair is installed within a C-shaped clamp and secured by compression bolts at its head side. A gasket is sandwiched between the mating surfaces of the flange pair to provide proper seal. While the head can be removed easily by unscrewing the compression bolts, this disintegrated structure does increase the complexity in component design and unique requirement for installation. The bolt compression load not only affects the pressure capacity of the storage tank, but also governs the stresses in flange pair and C-clamp. In this study, the flanged and dished head assembly has been modeled and analyzed by finite element method for stresses and gasket seal performance subject to installation and operation loads. Both elastic and elastic-plastic analysis has been performed. The tightening force of the bolts is examined against component stresses and gasket seal performance. Optimized bolt load is recommended based on acceptance criteria for stresses and leakage prevention.

Abstract Cylindrical shell structure is widely used in pressure vessels. In this paper, the orthotropic cylindrical shell structure is analyzed based on the theory of elastoplastic mechanics and the Hill48 yield criterion, the elastoplastic limit load expression of the orthotropic cylindrical shell and the corresponding three-dimensional stress formulas at different stages are obtained. The effect of the radius ratio and the yield strength ratio on the elastic limit load and plastic limit load of the cylindrical shell are also discussed. Finally, the orthotropic cylindrical shell structure is simulated by finite element method, the numerical results verify the correctness of the analytical solutions.

Abstract Pipelines subjected to displacement-controlled loading such as ground movement may experience significant longitudinal strain. This can potentially impact pipeline structural capacity and their leak-tight integrity. Reliable calibration of the tensile strain capacity (TSC) of pipelines plays a critical role in strain-based design (SBD) methods. Recent studies were focused mostly on high toughness modern pipelines, while limited research was performed on lower-grade vintage pipelines. However, a significant percentage of energy resources in North America is still being transported in vintage pipelines. Eight full-scale pressurized four-point bending tests were previously conducted on X42, NPS 22 vintage pipes with 12.7 mm wall thickness to investigate the effect of internal pressure and flaw size on TSC. The pipes were subjected to 80% and 30% specified minimum yield strength (SMYS) internal pressures with different girth weld flaw sizes machined at the girth weld center line. This paper evaluates the TSC of X42 vintage pipeline by utilizing ductile fracture mechanics models using damage plasticity models in ABAQUS extended finite element method (XFEM). The damage parameters required for simulating crack initiation and propagation in X42 vintage pipeline are calibrated numerically by comparing the numerical models with the full-scale test results. With the appropriate damage parameters, the numerical model can reasonably reproduce the full-scale experimental test results and can be used to carry out parametric analysis to characterize the effect of internal pressure and flaw size on TSC of X42 vintage pipes.

Abstract A periodic inspection of a reactor pressure vessel in refinery was scheduled. Prior to that inspection, criteria need to be established to determine what flaw indication would be tolerable so that the vessel can safely be put back in to service in a timely manner, or in the worst case, identify what flaw indication would create a very strong case for repair or replacement criteria for the vessel. A flaw tolerance criterion that can be applied to the refinery inspection process was developed for numerous potential flaw locations in this vessel. The finite element alternating method was used to determine the appropriate fracture parameters to assist in this flaw assessment procedure. These computational efforts involved examining the fracture response of the system in preparation for planned inspections. Stress intensity factors were evaluated for a total of ten (10) cracks inserted into the refinery pressure vessel at several locations and crack orientations. Most of the cracks had depth to thickness ratios of 0.25 and a half width 3 times this depth. The crack sizes are chosen based on the assumed maximum initial flaw sizes expected to be found from NDI. The stress intensity factor for residual stress loading was conservatively estimated by placing a unit tensile pressure on the crack face for all 10 cracks. The approximation of crack face pressure loading to simulate residual stress is also shown to be accurate. Therefore, one can estimate the contribution to stress intensity factor by multiplying the residual stress value of K by the estimated residual stress ratio. The final estimate of crack driving force for a crack, K I , is obtained by adding the contributions of the pressure loading with the residual stress contribution. Internal pressure loading of this vessel is the only significant source of loading in this vessel.

Abstract From linear elastic fracture mechanics (LEFM), it is well accepted that only the singular stress near the crack tip contributes to the fracture event through the crack tip stress intensity factor K. In the biaxial loading, the stress component that adds to the T-stress at the crack tip, affects only the second term in the Williams’ series solution around the crack tip. Therefore, it is generally believed that biaxial load does not change the apparent fracture toughness or the critical stress intensity factor (K c ). This paper revisited several specimen geometries under biaxial loading with finite element method. The sources of discrepancy between the theory and the test data were identified. It was found that the ideal biaxial loading would not be achieved for typical fracture specimens with finite geometry. Comparison to available test data shows that, while the biaxial load could affect the apparent fracture toughness, the contribution is relatively small.

Abstract In the numerical simulation of welding residual stress (WRS), the thermal fields distribution of the weld beads during welding process is the basic data to calculate the residual stress due to thermal expansion and extraction. Different welding heat input control method had been proposed and used in the WRS finite element method (FEM) analysis, such as input based on theory, input controlled by temperature monitoring point and so on. Some disadvantages like less calculating efficiency, unexpected temperature variation have been found in these methods. Based on the study of the FEM result of temperature distribution through the weld butter field by using constant unit heat input, the heat transfer activities of each weld bead were researched. And a novel heat input fitted equation was proposed to describe the suitable heat input data for each weld bead of the welding process. More reliable and uniform melting temperature of weld beads with different location could be achieved. And the welding residual stresses from different welding thermal field of theory method, temperature monitoring point method, and novel heat input equation method are compared with the measured WRS results. The results show that the WRS achieved from the novel heat input equations is close to the measured result and has more computational efficiency. The uncertain WRS data used for probability safety assessment were also proposed based on the FEM results with the novel heat input equation method.

Abstract This study presents some measurements of the effective hydrogen diffusivity in a cold-rolled, Type-304 stainless steel. Steel plates rolled under various cold working ( CW ) ratios were prepared. Disk specimens, referred to as LT and SL specimens, were sampled from the plates to determine the diffusivity. The rolling direction is perpendicular to the thickness direction for LT specimens and parallel for the SL specimens. Fraction and distribution of α′ phase islands resulting from strain-induced martensite were characterized by electromagnetic induction (EMI) method and electron backscatter diffraction (EBSD) analysis, respectively. The diffusivity of the LT and SL specimens exposed to high-pressure hydrogen gas was determined experimentally through desorption methods. Hydrogen permeation tests for LT and SL specimens were simulated using the finite element method (FEM) by considering a model material containing an inhomogeneous distribution of α′ phase islands. The EMI measurements established that the fraction of the α′ phase increases with the CW ratio. The phase maps from the EBSD analysis revealed an important difference in α′ phase distribution on planes perpendicular and parallel to the rolling direction (LT and SL planes). For CW = 60%, the diffusivity of the SL specimen was five times larger as compared to the LT specimen, although the fraction of the α′ phase is equal. The simulation of the permeation tests also showed a strong difference in the diffusivity between both specimens, and therefore supports the experimental results. Both experiments and simulations suggested that the anisotropic nature of the effective hydrogen diffusivity (in LT and SL specimens) could be attributed to the inhomogeneous distribution of the α′ phase islands in the cold-rolled material.

Abstract Fusion welding of steel joints is common through history of industrial applications. Among those, Shielded Metal Arc Welding (SMAW) and Gas Tungsten Arc Welding (GTAW) are most common. Fusion welding process comprises of rapid heating and cooling cycles. Each cycle produces a non-uniform and transient temperature distribution and causes rapid thermal expansion followed by thermal contraction. Thus plastic deformation and thermal residual stresses can be induced in a welded joint when it cools down gradually to room temperature. In this study, temperature profiles of a hand-weld mild steel butt weld are analyzed by means of the finite element method (FEM) through ANSYS Mechanical APDL The moving heat source is simulated using the Gaussian distribution heat source model. A parametric study was then performed to evaluate the importance of certain key process parameters that affect the quality of a weldment. The effects of temperature profile on hardness numbers inside and away from the heat affected zone (HAZ) are discussed. It was found that the residual stress results obtained from the simulation agree with the distribution of hardness numbers tested on the weldment sample.

The ductile crack propagation behavior of pressure equipment has always been the focus of structural integrity assessment. It is very important to find an effective three-dimensional (3D) damage model, which overcomes the geometric discontinuity and crack tip singularity caused by cracking. The cohesive force model (CZM), which is combined with the extended finite element method (XFEM), can solve element self-reconfiguration near the crack tip and track the crack direction. Based on the theory of void nucleation, growth and coalescence, the Gurson-Tvergaard-Needleman (GTN) damage model is used to study the fracture behavior of metallic materials, and agrees well with the experimental results. Two 3D crack propagation models are used to compare crack propagation behavior of pipe steel from the crack tip shape, fracture critical value of CTOA and CTOD, constraint effect, calculation accuracy, efficiency and mesh dependence etc. The results show that the GTN model has excellent applicability in the analysis of crack tip CTOD/CTOA, constraint effect, tunneling crack and so on, and its accuracy is high. However, the mesh of crack growth region needs to be extremely refined, and the element size is required to be 0.1–0.3mm and the calculation amount is large. The CZM model combined with XFEM has the advantages of high computational efficiency and free crack growth path, and the advantages are obvious in simulating the shear crack, combination crack and fatigue crack propagation. But, the crack tip shape and thickness effect of ductile tearing specimen can not be simulated, and the CTOA value of local crack tip is not accurate.

Flexural vibration behavior of a cracked straight pipeline, divided into two complete pipelines which were jointed together with a massless torsion spring simulating the outer surface circumferential crack, was analyzed with the Laplasse transform method and the transfer matrix theory, based on a straight pipeline FSI transverse vibration 4-equation model. At the same time, the equivalent spring method was also applied to simulate the elastic supports at both ends of the cracked straight pipeline, assuming that the pipeline was symmetrically supported and there were a line spring and a torsion spring at each end. The transverse vibration matrix equations model without or with frequency domain excitation force were finally obtained and then the natural frequencies were solved with Matlab. The natural frequency calculated results were compared with those of the finite element method to verify the correctness of the analysis process, and at last the influences of elastic support coefficient, crack angle, depth and location on the dynamic behaviors of the cracked straight pipeline were calculated and discussed.

Creep crack growth behavior of the Inconel625/BNi-2 brazed joint considering the diffusion zone at 650 °C was investigated by a continuum damage mechanics approach based on the finite element method. The results show that creep crack nucleate and develop at the region of the brazing filler metal. The crack initiates at about 0.2 mm ahead of the crack tip. When the load is 1000 N, the crack initiation time of the CT specimen is 1664 hour. While when the load is 1135 N, the crack initiation time is only about 891 hour. The simulated results correspond well with the experimental data, presenting that the used finite element method can accurately simulate the creep damage behavior of the brazed joint. When the mechanical properties of the diffusion zone are not considered, the crack initiation time and fracture time decrease significantly compared to the result with properties of the diffusion zone included, indicating that the result from the conventional simulating method without considering the diffusion zone is quite conservative compared to the experimental life of the component.

When multiple flaws are detected in pressure retaining components during inspection, the first step of evaluation consists of determining whether the flaws shall be combined into a single flaw or evaluated separately. This combination process is carried out in compliance with proximity rules given in the Fitness-for-Service (FFS) Codes. However, the specific criteria for the rules on combining multiple flaws into a single flaw are different among the FFS Codes. In this context, revised and improved criteria have been developed, to more accurately characterize the interaction between multiple subsurface flaws in operating pressure vessels. This improved approach removes some of the conservatism in the existing ASME Code approach, which was developed in the 1970s based on two flaws interacting with each other. This paper explains in detail the methodology used to derive improved flaw proximity rules through three-dimensional FEM and XFEM analyses. After the presentation of the calculations results and the improved criteria, the paper also highlights the multiple conservatisms of the methodology using several sensitivity analyses.