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Research Papers: Codes and Standards

J. Pressure Vessel Technol. 2018;140(4):041101-041101-9. doi:10.1115/1.4040096.

Pipelines used to transport oil and gas products are often subjected to external forces during its construction or operation, which can result in the formation of dents in the pipe. Various pipeline codes have stipulations on how a dent's severity can be ascertained in order to prioritize repairs. The most prominent being the depth-based criterion, which determines the severity of a dent by its depth. The depth-based criterion fails to consider the fact that the geometry of the dent can result in high strain concentration and eventually lead to integrity issues at the dented region. Alternatively, the strains associated with the dent can be an indicator of the dent's severity. Nonmandatory codified equations are available for evaluating the strains at the dented region of the pipe. The current implementation of these equations might fail to capture the strains that are not aligned with the most severe deformation profile of the dent and as such a global view of the strain distribution of the dented profile would be more informative as per the localized strain distribution. The study presented herein is the implementation of ASME B31.8 formulations together with the suggested modifications to evaluate the three-dimensional (3D) strain state of the dented pipe. The strain distributions obtained are compared against the strains predicted by finite element analysis (FEA) model. The correlation in the predicted strains indicates the possibility of the rapid and concise strain based characterization of dented pipes with the proposed technique.

Commentary by Dr. Valentin Fuster

Research Papers: Design and Analysis

J. Pressure Vessel Technol. 2018;140(4):041201-041201-14. doi:10.1115/1.4039844.

The paper is a review work devoted to dished heads of various meridian shapes. Geometry of the shells of revolution, the membrane state, and the edge effect occurring in the shells are described. Exemplary analytical and numerical finite element method (FEM) studies of torispherical, ellipsoidal, Cassini-ovaloidal, and untypical special dished heads are presented. The results of the above-mentioned two methods are compared. Moreover, numerical research of elastic buckling of the above-mentioned selected heads under external pressure is carried out. Literature related to each of the considered head types is quoted and discussed, with special attention paid to the works developed in the 21st century. In concluding remarks, the stress concentration and buckling of these structures are commented, with consideration of the head meridian shapes.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(4):041202-041202-10. doi:10.1115/1.4040421.

Bolted flange joints are widely used in the nuclear power plants and other industrial complexes. During their assembly, it is extremely difficult to achieve the target bolt preload and tightening uniformity due to elastic interaction and criss-cross talks. In addition to the severe service loadings, the initial bolt load scatter increases the risk of leakage failure. The objective of this paper is to present an analytical model to predict the bolt tension change due to elastic interaction during the sequence of initial tightening. The proposed analytical model is based on the theory of circular beams on linear elastic foundation. The elastic compliances of the flanges, the bolts, and the gasket due to bending, twisting, and axial compression are involved in the elastic interaction and bolt load changes during tightening. The developed model can be used to optimize the initial bolt tightening to obtain a uniform final preload under minimum tightening passes. The approach is validated using finite element analysis (FEA) and experimental tests conducted on a NPS 4 class 900 weld neck bolted flange joint.

Commentary by Dr. Valentin Fuster

Research Papers: Fluid-Structure Interaction

J. Pressure Vessel Technol. 2018;140(4):041301-041301-9. doi:10.1115/1.4040417.

Large eddy simulations (LES) are performed at low Reynolds number (2000–6000) to investigate the dynamic fluid-elastic instability in square normal cylinder array for a single-phase fluid cross flow. The fluid-elastic instability is dominant in the flow normal direction, at least for all water-flow experiments (Price, S., and Paidoussis, M., 1989, “The Flow-Induced Response of a Single Flexible Cylinder in an in-Line Array of Rigid Cylinders,” J. Fluids Struct., 3(1), pp. 61–82). The instability appears even in the case of single moving cylinder in an otherwise fixed-cylinder arrangement resulting in the same critical velocity (Khalifa, A., Weaver, D., and Ziada, S., 2012, “A Single Flexible Tube in a Rigid Array as a Model for Fluidelastic Instability in Tube Bundles,” J. Fluids Struct., 34, pp. 14–32); Khalifa et al. (2013, “Modeling of the Phase Lag Causing Fluidelastic Instability in a Parallel Triangular Tube Array,” J. Fluids Struct., 43, pp. 371–384). Therefore, in the present work, only a central cylinder out of 20 cylinders is allowed to vibrate in the flow normal direction. The square normal (90 deg) array has 5 rows and 3 columns of cylinders with 2 additional side columns of half wall-mounted cylinders. The numerical configuration is a replica of an experimental setup except for the length of cylinders, which is of 4 diameters in numerical setup against about 8 diameters in the experiment facility. The single-phase fluid is water. The standard Smagorinsky turbulence model is used for the subgrid scale eddy viscosity modeling. The numerical results are analyzed and compared to the experimental results for a range of flow velocities in the vicinity of the instability. Moreover, instantaneous pressure and fluid-force profiles on the cylinder surface are extracted from the LES calculations in order to better understand the dynamic fluid-elastic instability.

Commentary by Dr. Valentin Fuster

Research Papers: Materials and Fabrication

J. Pressure Vessel Technol. 2018;140(4):041401-041401-11. doi:10.1115/1.4039843.

In the course of the service of long-distance oil/gas pipelines, due to corrosion, abrasion, and other reasons, the possibility of pipeline leakage is growing. In-service welding is an advanced technique employed in the repair of pipelines, and it has wide application in guaranteeing the safe transmission of petroleum or gas. The present studies on in-service welding, including experiments and numerical simulations, all assumed that the inner wall of the pipeline was in good condition without considering the influence of defects. This paper started from internal corrosive defects, through the finite element simulation method, investigated how the pressure of inner medium and defect size influence the burn-through of in-service welding. The results show that, compared with the intact pipe, pipeline with internal corrosive defect is more prone to burn-through. With the increase of medium pressure, the maximum radial deformation, the von Mises stress, and hoop stress at the defect area increase. The radial deformation has a certain time effect. The depth of defect has an evident impact on the radial deformation and the stresses. The radial deformation, the von Mises stress, and hoop stress increase with the deepening of the defect, while the impacts of the defect's length and width are less obvious.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(4):041405-041405-9. doi:10.1115/1.4040315.

Residual stress distributions as welded and after local postwelding heat treatment (PWHT) of butted weld joint of a huge cylinder with ultra-thick wall were investigated by finite element (FE) simulations and measurement. Sequential coupling thermal-mechanical analyses were conducted with a generalized plane strain two-dimensional (2D) model to simulate the welding procedure bead by bead, combining with three-dimensional (3D) double-ellipsoid moving heat source and mixed isotropic–kinematic hardening plastic model. The simulation was validated by X-ray diffraction (XRD) measurements. Simulation results showed that local PWHT with heated band width of 0.5Rt can significantly reduce the residual stress on the outer surface of weld joint, but bring about harmful high tensile stress on inner surface due to bending moment induced by local radial thermal distortion. For the purpose to find out the appropriate heated band width of local PWHT, relations between stress relief and size of heated band were studied. Results show that the stresses on the inner surface reach a maximum value when the heated band width is less than 1Rt. Based on the simulation results and from the view point of lowering the stress level on the inner surface, the optimum width of 3Rt for heated band was proposed.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(4):041406-041406-7. doi:10.1115/1.4040274.

Submerged arc welding of SA 516 grade 60 pressure vessel grade steel was conducted with different heat plate thicknesses and the influence of cooling rate on microstructure, Vickers hardness, and impact toughness of heat affected zone (HAZ) of weldment was systematically investigated. Weld cooling rates vary with change in heat input or variation in plate thickness of base metal. Results showed that thin plates accumulate the heat, which cause grain coarsening and loss of acicular ferrite (AF) microstructure, which is further responsible for lower impact strength of welded joint. It is deemed that faster cooling rates due to heat sink in thickness direction with thick plates cause high percentage of AF with finer grain and enhanced hardness values. Improved impact strength with thick plates with same heat input signifies that supplying heat more than required to thin plates may cause microstructural deterioration and responsible for impact strength loss of weldments. Test demonstrates that the cooling rate should be above 15 °C/s to keep impact strength loss within considerable limits.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(4):041407-041407-7. doi:10.1115/1.4040275.

Fracture behavior of a high-pressure vessel for food processing under monotonic and fatigue loadings was investigated by conducting both experiments and finite element analysis (FEA) based on abaqus and zencrack software. Finite element analysis results showed that cracks nucleated at the filets of pin-hole and propagated faster near the inner surface than near the outer surface of the pressure vessel, progressively deflected, and eventually coalesced with other cracks initiated from the counter pin hole under monotonic loading. Such crack growth behavior coincided with the experimental result of hydraulic pressurizing test. Based on fatigue crack growth test of the pressure vessel material, 17-4PH stainless steel, a new equation to express the da/dNΔK curves including threshold region, has been proposed and embedded into the zencrack software to simulate the fatigue behavior of the pressure vessel. The simulation results showed that fatigue lives could be accurately estimated including low pressure range. The present simulation methods would be the useful design tool for pressure vessel under monotonic and cyclic loadings.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(4):041408-041408-12. doi:10.1115/1.4040314.

The structural integrity of underground pipelines are subject to a major threat from permanent ground displacements when they cross active tectonic (e.g., strike-slip) faults, because of large strains potentially induced in pipes, leading to pipe buckling and possible rupture. In this paper, the buckling behavior of X80 pipe is studied numerically with an emphasis on the effects of steel stress–strain characteristics. A rigorous mechanics-based nonlinear finite element (FE) model of a buried X80 pipe crossing a strike-slip fault is developed using shell elements and nonlinear springs for the pipe and soil resistance, respectively. The pipe steel material in the FE model is characterized by a novel and versatile stress–strain relationship, which was established to successfully capture both the round-house (RH) type and the yield-plateau (YP) type stress–strain behaviors. This allows investigating the significant effects of the stress–strain characteristics, as observed in this paper, on the buckling behavior of pressurized and nonpressurized pipes.

Topics: Stress , Pipes , Buckling , Steel
Commentary by Dr. Valentin Fuster

Research Papers: NDE

J. Pressure Vessel Technol. 2018;140(4):041501-041501-10. doi:10.1115/1.4040360.

Neural network technology is applied to the detection of a pipe wall thinning (PWT) in a pipe using a microwave signal reflection as an input. The location, depth, length, and profile geometry of the PWT are predicted by the neural network from input parameters taken from the resonance frequency plots for training data generated through computer simulation. The network is optimized using an evolutionary optimization routine, using the 108 training data samples to minimize the errors produced by the neural network model. The optimizer specified not only the optimal weights for the network links but also the optimal topology for the network itself. The results demonstrate the potential of the approach in that when data files were input that were not part of the training data set, fairly accurate predictions were made by the network. The results from the initial network models can be utilized to improve the future performance of the network.

Commentary by Dr. Valentin Fuster

Research Papers: Pipeline Systems

J. Pressure Vessel Technol. 2018;140(4):041701-041701-13. doi:10.1115/1.4039698.

This paper investigates the rupture of thin-walled ductile cylinders with isolated corrosion defects, subject only to internal pressure. It aims to propose a new solution for predicting the maximum load limit that will rupture a corroded pipeline, regardless of its material, its geometric ratio, or the dimensions of the existing corrosion defect. This solution is the result of several numerical simulations by variation of the length and depth of the defect with the assumption that the width of the defect has a negligible marginal effect. In all our numerical simulation analyses, the rupture was controlled by the Tresca failure criterion which is expressed in terms of material hardening exponent and the ultimate material stress. The proposed solution was then compared with the currently used coded methods, first B31.G, its improved version 0.85dL, and then DNV-RP F101, using an experimental database compiled from the existing literature. As a result, our proposed solution has been validated and has resulted in rupture ratios ranging from approximately 0.7 to 1. Furthermore, it has a tight prediction range compared to the B31.G, 0.85dL, and the DNV-RP F101 methods.

Commentary by Dr. Valentin Fuster

Research Papers: Technology Review

J. Pressure Vessel Technol. 2018;140(4):044001-044001-6. doi:10.1115/1.4040139.

Pipeline valves are the largest, heaviest, and most important valves on an offshore platform with long delivery time. A pipeline valve is either a ball type or through conduit gate valve type with a top entry design. The top entry design provides advantages such as a lower risk of leakage, greater mechanical strength against pipeline loads, and ease of maintenance (online maintenance) compared to the side entry design. A 30 in pipeline ball valve in class 1500 and carbon steel body material was chosen for stress analysis in this paper. The valve was connected to the pipeline through pup pieces from both sides. The pup pieces were connected to the body of the valve through transition pieces. The large 30 in valve has an emergency shut down safety function and is equipped with a hydraulic actuator. The valve is designed based on the American Petroleum Institute (API) 6D Specification for Pipeline and Piping valves. The proposed formula of wall thickness calculation in this paper is based on the American Society of Mechanical Engineers (ASME) Section VIII, Division 2, Boiler and Pressure Vessel Code (BPVC) instead of the ASME B16.34 standard. The wall thickness values given in the ASME B16.34 standard of “Valves Flanged, Threaded and Welding End” are very conservative and thick, which makes pipeline valves heavier and more expensive. Noticeably, ASME B16.34 requires an even higher thickness due to assembly loads, actuation (opening and closing) loads, and shapers other than circular that are applicable for pipeline valves. These valves should withstand loads from pipeline systems such as axial, torsion, and bending moments. ASME B16.34 does not specify the body wall thickness of the pipeline valves under the pipeline loads and moments. This paper aims to create a model to prove that the 30 in Class1500 pipeline valve will withstand the loads and moments with the thickness of the valve calculated using ASME Section VIII, Division 2 methods. The criteria and the model used to prove the suitability of the valve against the loads and moments are based on ASME Section VIII, Division 2.

Commentary by Dr. Valentin Fuster

Technical Brief: Technical Briefs

J. Pressure Vessel Technol. 2018;140(4):044501-044501-3. doi:10.1115/1.4039882.

Documented thermowell failures designed to PTC 19.3TW and earlier, when evaluated with the drag crisis invoked, reveals the potential for enhanced reliability of the current standard in reducing the risk of failure. The code calculation remains largely intact apart from a conservative Strouhal number in conjunction with Reynolds number criteria marking the onset and terminus of the drag crisis.

Commentary by Dr. Valentin Fuster

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