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### Research Papers: Codes and Standards

J. Pressure Vessel Technol. 2017;139(4):041101-041101-6. doi:10.1115/1.4036531.

Shells and tubes usually fail in the form of buckling under external pressure. Charts are used in the design of shells and tubes in the standards of ASME VIII-1 and EN13445-3 and these simplify the calculation process, while the proportional law is a more effective and simple method. In this paper, the relationships between the proportional law and the charts used in the standards were researched; finite element method (FEM) was used to compare the accuracies of the proportional law and the charts. It was theoretically proved that the proportional law and the charts were using essentially the same method to calculate the critical buckling pressure; they were different forms of the same dimensionless tension stress–strain curves. The simulation results showed that the proportional law and the charts had effectively equal accuracies in calculated critical buckling pressures. Therefore, the proportional law can be a candidate method included in the standards for the design of shells and tubes under external pressure.

Commentary by Dr. Valentin Fuster

### Research Papers: Design and Analysis

J. Pressure Vessel Technol. 2017;139(4):041201-041201-12. doi:10.1115/1.4035935.

In this study, two failure modes, yield buckling of the compression ring section and strength failure in the roof-to-shell of the tank, have been proposed for a vertical vaulted tank. The failure criteria of the two failure modes in the roof-to-shell of vault tanks are established via finite element analysis of three tanks of 640 m3, 3200 m3, and 6800 m3 in volume. The finite element models are built with axisymmetric elements and spatial multi-elements. Based on the strength failure criterion, the failure pressure formula in the vaulted tank roof-to-shell is derived. The maximum relative error between the theoretical calculation and numerical simulation is 9.7%. Finally, we verify the strength failure criterion through a tank failure test; the maximum relative error between the test and theoretical calculation is 9.6%. The failure pressure of both failure modes has been compared and analyzed. The failure pressure of the yield buckling in the compression ring section is about 1.65 times that of the strength failure in the roof-to-shell of the tank.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041202-041202-7. doi:10.1115/1.4035980.

The stress function approach is revisited for the inverse determination of residual stresses and eigenstrains from limited pointwise data in spherically symmetric stress state. The robust least squares technique is utilized to minimize the deviation of the measurement data from the model predictions while a full range of continuum mechanics requirements are satisfied. The application of the newly proposed spherical stress function is effectively demonstrated for two cases of analytical and numerical solutions considering different material behavior models. Also, the eigenstrains are inversely determined satisfying the equation of strain compatibility and a closed form analytical solution is presented.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041203-041203-10. doi:10.1115/1.4036139.

The stress analysis method for fixed tubesheet (TS) heat exchangers (HEX) in pressure vessel codes such as ASME VIII-1, EN13445, and GB151 is based on the classical theory of thin plate on elastic foundation. In addition, these codes all assume a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the unit or one TS is needed to be considered. In this study, a refined general theory of stress analysis for TS is presented which also considers unequal thickness for two TSs, different edge conditions, pressure drop and deadweight on two TSs, the anisotropic behavior of the TS in thickness direction, and transverse shear deformation in TS. Analysis shows floating and U-tube heat exchangers are the two special cases of the refined theory. Theoretical comparison shows that ASME method can be obtained from the special case of the simplified mechanical model of the refined theory. Numerical comparison results indicate that predictions given by the refined theory agree well with finite element analysis (FEA) for both thin and thick TS heat exchangers, while ASME results are not accurate or not correct. Therefore, it is concluded that the presented refined general theory provides a single unified method, dealing with both thin and thick TSs for different type (U type, floating, and fixed) HEXs in equal detail, with confidence to predict design stresses.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041204-041204-9. doi:10.1115/1.4036143.

Autofrettage is a metal working process of inducing compressive residual stresses in the vicinity of the inner surface of a thick-walled cylindrical or spherical pressure vessel for increasing its pressure capacity, fatigue life, and stress-corrosion resistance. The hydraulic autofrettage is a class of autofrettage processes, in which the vessel is pressurized using high hydraulic pressure to cause the partial plastic deformation followed by unloading. Despite its popularity, the requirement of high pressure makes this process costly. On the other hand, the thermal autofrettage is a simple method, in which the residual stresses are set up by first maintaining a temperature difference across the thickness of the vessel and then cooling it to uniform temperature. However, the increase in the pressure carrying capacity in thermal autofrettage process is lesser than that in the hydraulic autofrettage. In the present work, a combined hydraulic and thermal autofrettage process of a thick-walled cylinder is studied using finite element method package ABAQUS® for aluminum and SS304 steel. The strain-hardening and Bauschinger effects are considered and found to play significant roles. The results show that the combined autofrettage can achieve desired increase in the pressure capacity of thick-walled cylinders with relatively small autofrettage pressure. For example, in a SS304 cylinder of wall-thickness ratio of 3, an autofrettage pressure of 150 MPa enhances the pressure capacity by 41%, but the same pressure with a 36 °C higher inner surface temperature than outer surface temperature can enhance the pressure capacity by 60%.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041205-041205-10. doi:10.1115/1.4036427.

For the design of a transmission piping system, a stress intensification factor (SIF) is generally used for the stress calculations of piping components due to external forces, and the solutions for the single-walled piping components can be found in the existing design codes. However, it is quite difficult to obtain the reliable estimations for pipe-in-pipes (PIPs) from the existing solutions, because the PIPs show significantly different behaviors compared to the single-walled piping components due to the restraint effect induced by the outer pipe of the PIP. In this paper, the estimation schemes for the stress behaviors of the PIPs were proposed based on the detailed finite element (FE) analyses. In order to quantify the restraint effect, the FE analyses were conducted by considering various geometric variables of the PIPs under an internal pressure and a global bending moment. Based on the FE results, the tabular and closed-form solutions of the SIFs of PIPs were newly proposed. Finally, the proposed SIF estimations were validated against numerical results.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041206-041206-9. doi:10.1115/1.4035695.

In this paper, an inverse biharmonic axisymmetric elasticity problem is solved by invoking measured out-of-plane surface deformation values at discrete locations around a preloaded bolt head, in order to calculate the underhead contact stress and joint clamp load that would have caused that out-of-plane surface deformation. Solution of this type of inverse problem promises to improve the automation process of bolted joint system assembly, especially in critical and safety-related applications. For example, a real-time optically measured joint surface deformation can be utilized for automating process control of bolted joint assembly in a reliable fashion. This would be a significant reliability improvement as compared to the commonly used method in mass production using torque-only control method in which there is wide scatter in the torque–tension correlation due to the normal scatter in frictional variables. Finite element analysis (FEA) method is used to validate the inverse problem solution provided in this paper.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041207-041207-8. doi:10.1115/1.4036512.

The vibration-induced fatigue failure of small-bore piping is one of the common causes of failure trouble at nuclear power plants (NPPs). Therefore, the purpose of this study is to develop the measurement methods of vibration-induced stress for the screening to prevent from fatigue failure mechanism of small-bore piping. First, a measurement method using a single-mass model was introduced, and then, a measurement method using a two-mass model developed as an improved calculation model was proposed. These two kinds of models were validated by vibration tests using mock-up with small-bore branch piping. The results showed that the single-mass model could be used as the coarse screening. Additionally, the two-mass model was found to be suitable to the fine screening due to more accurate measurement of vibration-induced stress. Next, for small-bore piping with typical pattern configurations consisting of several masses and supports, the model considering the supports and the center of gravity being out of pipe centerline was developed and put into practical use. Finally, for the more complex small-bore piping with general piping configurations consisting of many bends, branches, or joints, the method based on the finite element analysis and using the measured values was developed. In the developed method, the differences between the natural frequency and the response acceleration obtained by the measurement and those values calculated using the analysis model are optimized to be enough small, and then, the vibration-induced stress is estimated by superposing the vibration modes of the small-bore piping with the static deformation representing the main piping vibration. In this study, the usability of the developed method was confirmed by the comparison with the numerical results without the measurement error, which were assumed to be the true values. The peak stress induced by vibration frequently occurs at the filet weld part between the small-bore piping and the main piping. The developed methods can be used for various weld geometries although the measurement method using strain gauges cannot be used for such weld parts. The failure possibility by vibration-induced fatigue can be evaluated by comparing the nominal stress measured by the methods in this study with the fatigue threshold stress divided by the stress concentration factor appropriate for the weld geometry.

Topics: Stress , Pipes , Vibration
Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041208-041208-9. doi:10.1115/1.4036656.

In this paper, applicability of net-section collapse load approach to circumferential multiple-cracked pipe assessment is investigated using finite element (FE) damage analysis. The FE damage analysis based on the stress-modified fracture strain model is validated against limited fracture test data of two circumferential surface-cracked pipes. Then, the systematic parametric study is performed using the FE damage analysis for symmetrical and asymmetrical surface-cracked pipes. It is found that predictions using the net-section collapse load approach tend to be more accurate with increasing the distance between two symmetrical cracks. For asymmetrical cracks, it is found that the deeper crack plays a more important role and that the existing net-section collapse load expression can be potentially nonconservative. Idealization to symmetrical cracks based on the deeper crack is proposed.

Commentary by Dr. Valentin Fuster

### Research Papers: Fluid-Structure Interaction

J. Pressure Vessel Technol. 2017;139(4):041301-041301-9. doi:10.1115/1.4035464.

A new cellular automaton technique was developed based on the finite difference scheme to analyze structures such as beams and plates as well as the acoustic wave equation. The technique uses rules for a cell, and the rules are applied to all the cells repeatedly. The technique is very easy to write a computer code and computationally efficient. Like the standard cellular automaton, many different boundary conditions can be applied easily to the new technique. The technique was applied to both structural and fluid–structure interaction problems. The fluid domain was modeled as either the acoustic medium without flow using the newly developed cellular automaton rules or the fluid flow medium using the lattice Boltzmann technique. Multiple example problems were presented to demonstrate the new technique. Those included dynamic analyses of beams and plates, acoustic wave problems, and coupled fluid–structure interaction problems.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041302-041302-9. doi:10.1115/1.4036429.

Natural gas is relatively clean, and its demand is currently increasing. In most cases, gas fields are located at the bottom of the sea. Therefore, floating production, storage, and offloading (FPSO) systems are now attracting considerable attention. This paper is related to the dynamical design of a FPSO system; in particular, it focuses on the free surface elevation induced by the waves in a horizontal cylindrical and axisymmetric liquid vessel with end caps. In this study, the theory of the wave height and resonant frequency in a horizontal cylinder subjected to pitching via external excitation is developed. Then, a theory taking into account the effect of perforated plates is introduced. A special discussion is made with regard to the number and location of the perforated plates and the effect of a partial opening in a perforated plate on the damping. Finally, the experimental data of resonant wave heights up to the third mode are shown in comparison to the theoretically derived results.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041303-041303-11. doi:10.1115/1.4036657.

Motivated by the fact that a leaking pipe can lose or gain energy from the leaking flow, this study attempts to explore the nonconservative leaking flow effect on the dynamic stability of a simply supported pipe with a constant velocity leakage. It employs a two-dimensional nonlinear longitudinal and lateral coupling model, and the leakage effect is accounted for by virtual work due to virtual momentum transport at the leaking point. The equations of motion are solved by Galerkin-based multimode approach and the Houbolt's finite difference time integration. It demonstrates that when there is a leaking flow, a stable pipe can be refined or destabilized via a static pitchfork bifurcation, and a buckling pipe can be stabilized or deteriorated into a worse divergence condition. The critical leaking flow velocities and the excited buckling modes depend on the leaking fluid mass and the leak's position. This study may provide some insights to assist the leak detection system (LDS) of a pipe transporting high-pressure oil or gas in modern engineering.

Commentary by Dr. Valentin Fuster

### Research Papers: Materials and Fabrication

J. Pressure Vessel Technol. 2017;139(4):041401-041401-8. doi:10.1115/1.4035884.

This research investigated the effects of global (in other words, furnace-based) and local post weld heat treatment (PWHT) on residual stress (RS) relaxation in API 5L X65 pipe girth welds. All pipe spools were fabricated using identical pipeline production procedures for manufacturing multipass narrow gap welds. Nondestructive neutron diffraction (ND) strain scanning was carried out on girth welded pipe spools and strain-free comb samples for the determination of the lattice spacing. All residual stress measurements were carried out at the KOWARI strain scanning instrument at the Australian Nuclear Science and Technology Organization (ANSTO). Residual stresses were measured on two pipe spools in as-welded condition and two pipe spools after local and furnace PWHT. Measurements were conducted through the thickness in the weld material and adjacent parent metal starting from the weld toes. Besides, three line-scans along pipe length were made 3 mm below outer surface, at pipe wall midthickness, and 3 mm above the inner surface. PWHT was carried out for stress relief; one pipe was conventionally heat treated entirely in an enclosed furnace, and the other was locally heated by a flexible ceramic heating pad. Residual stresses measured after PWHT were at exactly the same locations as those in as-welded condition. Residual stress states of the pipe spools in as-welded condition and after PWHT were compared, and the results were presented in full stress maps. Additionally, through-thickness residual stress profiles and the results of one line scan (3 mm below outer surface) were compared with the respective residual stress profiles advised in British Standard BS 7910 “Guide to methods for assessing the acceptability of flaws in metallic structures” and the UK nuclear industry's R6 procedure. The residual stress profiles in as-welded condition were similar. With the given parameters, local PWHT has effectively reduced residual stresses in the pipe spool to such a level that it prompted the thought that local PWHT can be considered a substitute for global PWHT.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041402-041402-8. doi:10.1115/1.4035976.

In this study, fatigue performances of the vehicle toroidal liquefied petroleum gas (LPG) fuel tanks were examined to estimate the fatigue life and its failure locations using both experimental and finite element analysis (FEA) methods. The experimental investigations performed as accelerated fatigue tests were carried out using a hydraulics test unit in which the tanks were internally pressurized by hydraulic oil. The LPG tanks were subjected to repeated cyclic pressure load varying from zero to service pressure (SP) of the tank. The computerized FEA modeling of these tanks were developed in three-dimensional (3D) form using nonuniform geometrical parameters and nonlinear material properties. These models were also subjected to zero-based high cycle fatigue pressure load considering the stress life approach. The FEA modeling process was also simulated in nonhomogeneous material conditions. Therefore, the fatigue life performance and failure location of the toroidal LPG fuel tanks were predicted using the computer-aided simulations and compared with the experimental results.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041403-041403-10. doi:10.1115/1.4036142.

The classic Kachanov–Rabotnov (KR) creep damage model is a popular model for the design against failure due to creep deformation. However, the KR model is a local approach that can exhibit numerically unstable damage with mesh refinement. These problems have led to modified critical damage parameters and alternative constitutive models. In this study, an alternative sine hyperbolic (Sinh) creep damage model is shown to (i) predict unity damage irrespective of stress and temperature conditions such that life prediction and creep cracking are easy to perform; (ii) develop a continuous and well-distributed damage field in the presence of stress concentrations; and (iii) is less stress-sensitive, is less mesh-dependent, and exhibits better convergence than the KR model. The limitations of the KR model are discussed in detail. The KR and Sinh models are calibrated to three isotherms of 304 stainless steel creep test data. Mathematical exercises, smooth specimen simulations, and crack growth simulations are performed to produce a quantitative comparison of the numerical performance of the models.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041404-041404-6. doi:10.1115/1.4036426.

The paper addresses the Bauschinger effect under complex stress state in materials with deformation-induced anisotropy whose strain hardening is described by the isotropic–kinematic (translational) type hardening hypothesis. The Bauschinger effect is analyzed using the model based on the yield surface conception and graphical–analytical method of construction of constitutive equations under complex loading. As an example, cylindrical pressure vessels with closed and open ends subjected to autofrettage are considered. The tension–compression Bauschinger effect in the axial and hoop directions as well as the Bauschinger effect under reversed torsion with respect to the longitudinal axis is determined. The role of such factors as the level of prestraining under autofrettage, relation between isotropic and kinematic components of the strain hardening, and chemical composition of the material is analyzed. The results obtained are presented in the form of plots.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041405-041405-7. doi:10.1115/1.4036141.

According to Appendix L of the Boiler and Pressure Vessel Code Section XI, flaw tolerance assessment is performed using the stress intensity factor (SIF) even for low-cycle fatigue. On the other hand, in Section III, the fatigue damage is assessed using the design fatigue curve (DFC), which has been determined from strain-based fatigue tests. Namely, the stress is used for the flaw tolerance assessment. In order to resolve this inconsistency, in the present study, the strain intensity factor was used for crack growth prediction. First, it was shown that the strain range was the key parameter for predicting the fatigue life and crack growth. The crack growth rates correlated well with the strain intensity factor even for the low-cycle fatigue. Then, the strain intensity factor was applied to predict the crack growth under uniform and thermal cyclic loading conditions. The estimated fatigue life for the uniform cyclic loading condition agreed well with that obtained by the low-cycle fatigue tests, while the fatigue life estimated for the cyclic thermal loading condition was longer. It was shown that the inspection result of “no crack” can be reflected to determining the future inspection time by applying the flaw tolerance analysis. It was concluded that the flaw tolerance concept is applicable not only to the plant maintenance but also to plant design. The fatigue damage assessment using the design fatigue curve can be replaced with the crack growth prediction.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041406-041406-8. doi:10.1115/1.4035977.

This paper investigates the effect of initial residual stress and prestrain on residual stresses due to laser shock peening for Alloy 600 using numerical simulation. For simulation, the strain rate dependent Johnson–Cook hardening model with a Mie–Grüneisen equation of state is used. Simulation results are compared with published experimental data, showing good agreement. It is found that the laser shock peening (LSP) process is more effective for higher initial tensile residual stress and for larger initial prestrain in terms of compressive stress at the near surface. However, the effective depth decreases with increasing initial tensile residual stress and initial prestrain.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041407-041407-8. doi:10.1115/1.4036138.

To assure adequate fracture resistance of cryogenic pressure vessels designed to operate at a minimum design metal temperature (MDMT) colder than 77 K (−196 °C or −320 °F), current American Society of Mechanical Engineers (ASME) Code, Section VIII, Division 1, UHA-51 Impact Test rule requires that the weld metal (WM) meets or exceeds 0.53 mm (21 mils) lateral expansion at 77 K, i.e., LE77K ≥ 0.53 mm (21 mils), as determined using Charpy V-notch (CVN) impact testing. To the credit of this rule, cryogenic pressure vessels fabricated to date meeting the above requirement had continued to serve well—without any adverse incident—in numerous applications across the world, at cryogenic temperatures colder than 77 K. However, a critical examination of the underlying research which relied on a regression equation relating ratio of fracture toughness to yield strength obtained at 4 K, i.e., $[KIc/YS]$4K with LE77K, revealed that the technical basis for establishing the above requirement is metallurgically unsustainable. To successfully overcome this, the present research employed dimensional analysis and balancing of the previously published regression equations and proposed $[KIc/YS]$277K as a valid fracture resistance parameter applicable for MDMT 77 K and warmer, as well as MDMT colder than 77 K. Related efforts offered equivalent fracture resistance as an insightful concept, wherein the minimum fracture resistance parameter for a MDMT colder than 77 K is equated as a simple multiple of the minimum fracture resistance parameter at 77 K MDMT. Concluding efforts applied numerical analysis to the equivalent fracture resistance equation to reaffirm the current minimum 0.53 mm (21 mils) CVN LE77K requirement for WM when MDMT is colder than 77 K and to identify minimum required $[KIc/YS]$277K values for cryogenic service at MDMT 77 K and warmer, and MDMT colder than 77 K. Inherently, the use of $[KIc/YS]$277K as a fracture resistance parameter offers a tremendous benefit to cryogenic equipment manufacturers, particularly in schedule and cost savings, as LE, KIc, and YS measured at 77 K can be used to successfully assess the fracture resistance at MDMT 77 K and warmer, as well as MDMT colder than 77 K.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041408-041408-9. doi:10.1115/1.4036140.

Austenitic stainless steel of type X6CrNiNb18-10 exhibits advantageous mechanical and chemical properties and is a common material for numerous applications in the nuclear power plant and chemical industries. Besides the mechanical strain induced by high pressure, the fatigue life in welded pipelines is affected by additional thermomechanical strains due to thermal loading. The welding process mainly determines the geometry and metallurgical constitution of the welded joint. Therefore, the butt welds additionally influence the strain gradient along the component and reduce its lifetime. While the base and weld material are similar, they show different softening and hardening behavior, especially at ambient temperature. Cyclic hardening occurs in the base material, whereas cyclic softening can be observed in the weld material. The hardness distribution along the welded joint reveals no clear differentiation of the base material, the heat affected zone, and the weld material. The attributes of the individual materials cannot be transferred to the welded joint automatically. Thus, the analysis of the interaction between the materials along the welded joint is a main topic of this research. To this end, digital image correlation (DIC) is used for different kinds of specimens and load conditions. The position along the testing area at which fatigue failure occurs depends on the specimen type and the load condition but not on the temperature. Further, isothermal and anisothermal fatigue tests on welded cruciform specimens are presented. The common practice of the effective strain is discussed for the analyzed conditions.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041409-041409-14. doi:10.1115/1.4036430.

Shop–welded, flat-bottom tanks for storage of production liquids are designed and fabricated in specific dimensions and capacities for internal pressures close to atmospheric pressure in accordance with the API 12F specification. This study addresses the failure pressure on the eleven (11) current API 12F shop-welded steel tanks as well as two proposed sizes through finite element and stress analysis of more than 350 different tank models. An elastic analysis was carried out to determine the yielding pressure of the shell-to-bottom and roof-to-shell joints. An elastic buckling analysis and a post-buckling analysis including imperfections was performed to determine the buckling modes of the equipment. The redistribution of stresses due to inelastic deformations and plastic collapse were evaluated through a plastic stress analysis considering the stress–strain hardening of the ASTM A36 mild steel material. Moreover, the design pressure increase to failure pressure or 24 oz/in2 (10.3 kPa) was investigated regarding the stress levels and bottom uplift of the 13 flat-bottom tanks. The presented research provides meaningful insights and engineering calculations to evaluate the current design of the API 12F shop-welded, flat-bottom tanks as well as to establish new design internal pressures guaranteeing a safe performance of the equipment.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041410-041410-6. doi:10.1115/1.4036532.

This contribution deals with the determination of the reference temperature of A533 Cl.1 steel using miniaturized specimens. The dimensions of the miniaturized specimens used were 3 × 4 × 27 mm (thickness × width × length). This specimen type allows utilizing a limited amount of test material or the broken halves of precracked Charpy or larger specimens. The test material comes from the broken halves of 0.5 T SE(B) specimens previously tested for the determination of the reference temperature at Ciemat, Madrid. The fracture toughness tests were performed in the transition region of the steel according to the recommendations of the standard ASTM E1921 and according to Wallin's recommended temperature range of miniaturized specimens. The reference temperature of the Master Curve was very similar to the ones obtained from three-point-bend specimens of sizes 0.2 T, 0.4 T, and 0.5 T. The results obtained confirm a necessity to conduct tests at low temperatures and to test a sufficient number of specimens in order to generate enough valid data for the determination of the reference temperature.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041411-041411-9. doi:10.1115/1.4036534.

This paper presents the analysis of repair of pipelines using nanofiller dispersed composites. Steel pipe with part wall loss as per ISO 24817 is repaired using glass/epoxy composites with and without nanoclay reinforcement and burst test is performed in order to assess the performance and effectiveness of nanocomposites-based repair system. A simple methodology is developed to find out the failure pressure of pipelines and is compared with the experimental and ISO 24817 repair code results. The thickness of the composite wrap is predicted analytically for 1–5% nanofiller dispersion in the epoxy for 30–80% of pipe wall loss and live pressure of pipe. The results are analyzed to find effectiveness of clay dispersion and effect of live pressure for 30–80% of pipe wall loss. It is observed that the dispersion of nanofiller improves the bursting resistance of composite wrapping over outer surface of pipe.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041412-041412-10. doi:10.1115/1.4036533.

Based on a tee joint, the simplified creep design methods of ASME code, elastic-perfectly plastic (EPP) analysis of Code Case N-861 (CC N-861) and the inelastic analysis by isochronous stress strain (ISS) curve, were evaluated and compared to nonlinear finite element creep analysis (FECA). Results indicate that both EPP and FECA lead to a greater inelastic strain than ISS curve-based results. By contrast, the ISS-based analysis induces a smaller usage fraction (damage) than FECA due to the underestimated inelastic strain. In addition, the smallest usage fraction (damage) is obtained by using EPP analysis of CC N-861 in which case the remarkable bending strain is involved.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041413-041413-8. doi:10.1115/1.4036658.

The U.S. nuclear power industry is seeking U.S. Nuclear Regulatory Commission (USNRC) approval to use high-density polyethylene (HDPE) in safety-related applications. The USNRC had granted approval for the use of HDPE for safety-related service water applications, with limitations, to Catawba (Duke Energy Corp., Catawba, SC) and Callaway (Union Electric Co., Callaway, MO) based on separate relief requests submitted by the licensees. The nuclear industry continues to show increasing interest in utilizing HDPE in safety-related piping systems. In order to evaluate and maintain the structural integrity of HDPE pipes, the material properties and the fracture resistance behavior must be fully characterized. Although there has been extensive work on material property development of HDPE, most of the investigations have been focused on the parent (base) material. Hence, the material property and fracture resistance behavior of the butt-fusion region have not been comprehensively investigated. In this paper, tensile, dynamic mechanical analysis (DMA), and slow crack growth (SCG) tests were performed for unimodal PE 4710 HDPE material. Specimens were machined from both parent piping material and butt-fusion regions. The test results indicate that the tensile and DMA properties show no significant differences between parent and butt-fusion joint materials. However, in terms of SCG resistance, the time to failure for butt-fusion joint material was an order of magnitude lower than that of the parent material.

Commentary by Dr. Valentin Fuster

### Research Papers: Operations, Applications and Components

J. Pressure Vessel Technol. 2017;139(4):041601-041601-9. doi:10.1115/1.4035979.

A three-way water hydraulic pressure reducing valve (PRV) was developed in this paper for a test equipment in laboratory for adapting complex conditions. The designed PRV has a damping chamber with an orifice located at the spring chamber. Two types of throttles and orifice diameter were investigated through dynamic simulation and optimization, and their dimensions were determined and applied to the manufactured valve prototype. The static and dynamic performances of the valve were tested by experiments. At the preset pressure of 5.0 MPa, the outlet pressure variations for the pressure-reducing port and the relief port, are 0.73 MPa and 1.44 MPa, respectively, while the flow variation is up to 18.0 l/min. The experimental rising times and settling times of the PRV under the inlet pressure step for preset pressures of 5.0 MPa are 33.7 ms and 120.2 ms, respectively, and the overshoot is 3.76%. The test results at each preset pressure agree well with the simulation which verifies that the simulation model can be used to predict the dynamic performance of the PRV. The experimental results for the valve under flow step input conclude that it can work stably at small flow state. The research indicates that making the spring chamber a damping chamber by using an orifice is a feasible way to increase the pressure stability and the dynamic performance of the PRV. However, the damping effect of this structure is insufficient at high working pressure.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):041602-041602-8. doi:10.1115/1.4036428.

Pressure vessels are subject to deterioration processes, such as corrosion and fatigue, which can lead to failure. Inspections and repairs are performed to mitigate this risk. Large industrial facilities (e.g., oil and gas refineries) often have regularly scheduled shutdown periods during which many components, including the pressure vessels, are disassembled, inspected, and repaired if necessary. This paper presents a decision analysis framework for the risk-based maintenance (RBM) planning of corroding pressure vessels. After a vessel has been inspected, this framework determines the optimal maintenance time of each defect, where the optimal time is the one that minimizes the total expected cost over the lifecycle of the vessel. The framework allows for multiple defects and two failure modes (leak and burst), and accounts for the dependent failure events. A stochastic gamma process is used to model the future deterioration growth to determine the probability of vessel failure. The novel growth model presents a simple method to predict both the depth and length of each corrosion defect to enable burst analysis. The decision analysis framework can aid decision makers in deciding when a repair or replacement should be performed. This method can be used to immediately inform the decision maker of the optimal decision postinspection. A numerical example of a corroding pressure vessel illustrates the method.

Commentary by Dr. Valentin Fuster

### Research Papers: Pipeline Systems

J. Pressure Vessel Technol. 2017;139(4):041701-041701-8. doi:10.1115/1.4036217.

When offshore pipelines approach the end of their design life, their condition could threaten oil flow continuity as well as become a potential safety or environmental hazard. Hence, there is a need to assess the remaining life of pipelines to ensure that they can cope with current and future operational demand and integrity challenges. This paper presents a methodology for assessing the condition of aging pipelines and determining the remaining life that can support extended operation without compromising safety and reliability. Applying this methodology would facilitate a well-informed decision that enables decision makers to determine the best strategy for maintaining the integrity of aging pipelines.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Pressure Vessel Technol. 2017;139(4):044501-044501-3. doi:10.1115/1.4035975.

This note is concerned with two circular-hole cracks of the same size in an infinite plate in tension by means of the generation of Bueckner's principle and the hybrid displacement discontinuity method. Many numerical results which can reveal the interactions of two circular-hole cracks are given.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):044502-044502-4. doi:10.1115/1.4035978.

Casing and tubing is widely used as protective conduits during all the phases of operations and productions for the oil and gas industry. Recently, casing and tubing burst failure accidents often take place in high pressure and high temperature (HPHT) oil and gas wells during production. Therefore, it is very important to accurately predict casing and tubing bust strength in the casing and tubing design and operation process. PD CEN ISO-TR 10400 presents the ductile rupture model for capped-end conditions, but capped-end casing and tubing applied in oil fields is few. For this case, this document establishes the ductile rupture model for capped-open conditions under combined loads on the base of PD CEN ISO-TR 10400. Numerical and experimental comparisons show that the ductile rupture model for capped-open conditions under combined loads prediction values essentially coincides with burst data provided by PD CEN ISO-TR 10400.

Topics: Stress , Rupture , Pressure , Tubing , Design
Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(4):044503-044503-7. doi:10.1115/1.4036144.

Optimization of piping supports is a well-known problem. The paper considers the optimization of piping supports with respect to cost and the loads transmitted to the supporting structural elements, when the orientation of the supporting structure is to be determined. This is the case, when new structural elements need to be added to the existing building structure to support components and piping systems that come as a new addition to a nuclear plant. The analytical target cascading (ATC) method is used for the optimization, combining the support loads from different piping analyses in a hierarchical framework. It is shown that the ATC method can be used for an optimized location of structural elements simultaneously supporting complex piping systems and implemented in a structural analysis software.

Commentary by Dr. Valentin Fuster

### Discussion

J. Pressure Vessel Technol. 2017;139(4):045501-045501-1. doi:10.1115/1.4034878.

In the paper by Moustafa et al. (2012, “Leak Localization in Pipelines Via Computational Pipeline Monitoring,” ASME J. Pressure Vessel Technol., 134(4), p. 041701), they have described a scanning procedure to locate multiple leaks in a pipeline by software, which is equivalent to an optimization procedure looking for the nodes in the spacial discretization model of the fluid. The purpose of this correspondence is to make comments and remarks about it.

Commentary by Dr. Valentin Fuster

### Errata

J. Pressure Vessel Technol. 2017;139(4):047001-047001-2. doi:10.1115/1.4036659.
FREE TO VIEW

This erratum corrects errors in the originally published paper.

Commentary by Dr. Valentin Fuster