J. Pressure Vessel Technol. 2017;140(1):010201-010201-2. doi:10.1115/1.4038633.

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Commentary by Dr. Valentin Fuster

Research Papers: Design and Analysis

J. Pressure Vessel Technol. 2017;140(1):011201-011201-10. doi:10.1115/1.4038226.

Application of thin-walled high strength steel has become a trend in the oil and gas transportation system over long distance. Failure assessment is an important issue in the construction and maintenance of the pipelines. This work provides an engineering estimation procedure to determine the J-integral for the thin-walled pipes with small constant-depth circumferential surface cracks subject to the tensile loading based upon the General Electric/Electric Power Research (GE/EPRI) method. The values of elastic influence functions for stress intensity factor and plastic influence functions for fully plastic J-integral are derived in tabulated forms through a series of three-dimensional (3D) finite element (FE) calculations for a wide range of crack geometries and material properties. Furthermore, the fit equations for elastic and plastic influence functions are developed, where the effects of crack geometries are explicitly revealed. The new influence functions lead to an efficient J estimation and can be well applied for structural integrity assessment of thin-walled pipes with small constant-depth circumferential surface cracks under tension.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):011202-011202-8. doi:10.1115/1.4038310.

This work investigates the applicability of the flaw shape idealization methods to carry out the structural integrity assessment of steam generator (SG) tubes under internal pressure with complicated axial inner and outer surface flaws that were typically found during the in-service-inspection (ISI). In terms of flaw shape, three different shapes of flaws which can be detected during an actual ISI are considered, i.e., long symmetric flaw, asymmetric inclined flaw and narrow, symmetric deep flaw. As for flaw shape idealization methods for the predictions of burst pressures of these flaws, four different flaw shape idealization models, i.e., semi-elliptical, rectangular, maximum length with effective flaw depth and weakest subcrack model proposed by the Electric Power Research Institute (EPRI) are employed in this work. In order to validate the applicability of these idealization methods, the burst pressures of SG tubes with these flaws are investigated by using the finite element (FE) analyses. By comparing the predictions of the burst pressures based on the four different flaw shape idealization methods with those based on actual flaw shapes, it is found that the weakest subcrack model proposed by the EPRI and maximum length with effective flaw depth model provide the better agreement with actual complex flaw.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):011203-011203-11. doi:10.1115/1.4038516.

Based on a brief review of existing tubesheet (TS) design standards and the pertinent technical literature, a unified analytical method of stress analysis for fixed TS heat exchangers (HEXs), floating head and U-tube HEXs is proposed by removing the midplane symmetry (MPS) assumption, which assumes a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the HEX or one TS needs be considered. The unified method can be successfully extended to the situations for different TS materials, unequal TS thicknesses, different TS edge conditions, different TS temperatures, pressures drop and dead weights on two TSs. The effects of pressure in TS perforations and temperature gradient in TS thickness direction are also considered by the unified method. Theoretical comparison shows that ASME method can be obtained from the special case of the simplified mechanical model of the unified method. Numerical comparison indicates that predictions given by the unified method agree well with finite element analysis (FEA), while ASME results are not accurate or not correct.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):011204-011204-14. doi:10.1115/1.4038723.

Aboveground vertical steel storage tanks use stiffener rings to prevent their shell wall from buckling under wind loading. The existing stiffener rings design rules from API 650 standard is known to be overly conservative. This study investigates the possibility of modifying the design rules by reducing the required size of the top stiffener ring to the same size as the intermediate stiffener ring. In this study, we used finite element analysis (FEA) to perform linear bifurcation analysis (LBA) and geometrically nonlinear analysis including imperfections (GNIA) to obtain failure load of modeled tanks. The buckling pressure load was obtained to ensure it is larger than the design pressure. Moreover, the effects of higher strength materials, different buckling modes, and various wind profiles were also studied to ensure the design suggested by this study is practical and universal to different situations. The results show that for cylindrical storage tanks, which only needs one intermediate stiffener ring, the size of the top stiffener ring can be set to the same size as the intermediate stiffener ring.

Commentary by Dr. Valentin Fuster

Research Papers: Materials and Fabrication

J. Pressure Vessel Technol. 2017;140(1):011401-011401-11. doi:10.1115/1.4038435.

This paper describes the results of static loading tests using a half-scale thick rubber bearing to investigate ultimate properties application for a sodium-cooled-fast-reactor (SFR). Thick rubber bearings which have a rubber layer that is roughly two times thicker in comparison with existing rubber bearings have been developed by the authors to ensure seismic safety margins for components installed in the reactor building, and to reduce the seismic response in the vertical direction as well as the horizontal direction. The thick rubber bearings, 1600 mm in diameter at the full scale, have been designed to provide a rated load of about 10,000 kN, at the compressive stress of 5.0 MPa, with a horizontal natural period of 3.4 s and a vertical natural period of about 0.133 s. The restoring-force characteristics, including variations, and breaking points, for the thick rubber bearings have not been cleared yet. These validations are essential from the point of view of probabilistic risk assessment (PRA) for a base-isolated nuclear plant as well as a verification of the structural integrity of the thick rubber bearings. The purpose of this paper is to indicate the variation of the stiffness and damping ratios for restoring force characteristics, and the breaking strain or stress, as ultimate properties through static loading tests using half-scale thick rubber bearings. In addition, an analytical model for the thick rubber bearings which is able to express the nonlinear restoring force, including the breaking points, is presented.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):011402-011402-9. doi:10.1115/1.4038227.

The paper presents a method for modeling and measuring the residual stress (RS) field in axisymmetric autofrettaged elements. The method is based on the assumption that an Initial Strain Distribution (ISD), originated by the plastic strain previously induced during the autofrettage process, is the source of RSs. The ISD is the quantity to be evaluated and, after being determined, it can be used, by means of a dedicated finite element (FE) model, to evaluate the RS field in the component or in any part extracted from it. The ISD is obtained by elaborating the relaxed strains produced by cutting the autofrettaged component in incremental steps. The elaboration is based on solving a set of Fredholm's integral equations in which the unknown function is the ISD and the kernel is an Influence Function (IF) correlating the measured relaxed strain to the ISD. After a general discussion of the RS induced by the autofrettage and the effect of the plastic properties of the material under process, the methods for obtaining the relaxed strains by a rational experimental setup and the procedures for obtaining the IFs are presented and discussed. The whole methodology is applied to evaluate the RS field in a hollow cylinder for which the autofrettage was modeled by a FE simulation. The consistency of the method is verified and useful indications for the experimental activities were obtained.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):011403-011403-10. doi:10.1115/1.4038525.

In this paper, a cyclic-plasticity-based fully mechanistic fatigue modeling approach is presented. This is based on time-dependent stress–strain evolution of the material over the entire fatigue life rather than just based on the end of live information typically used for empirical S∼N curve-based fatigue evaluation approaches. Previously, we presented constant amplitude fatigue test based related material models for 316 stainless steel (SS) base, 508 low alloy steel base, and 316 SS-316 SS weld which are used in nuclear reactor components such as pressure vessels, nozzles, and surge line pipes. However, we found that constant amplitude fatigue data-based models have limitation in capturing the stress–strain evolution under arbitrary fatigue loading. To address the aforementioned limitation, in this paper, we present a more advanced approach that can be used for modeling the cyclic stress–strain evolution and fatigue life not only under constant amplitude but also under any arbitrary (random/variable) fatigue loading. The related material model and analytical model results are presented for 316 SS base metal. Two methodologies (either based on time/cycle or based on accumulated plastic strain energy (APSE)) to track the material parameters at a given time/cycle are discussed and associated analytical model results are presented. From the material model and analytical cyclic plasticity model results, it is found that the proposed cyclic plasticity model can predict all the important stages of material behavior during the entire fatigue life of the specimens with more than 90% accuracy.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):011404-011404-6. doi:10.1115/1.4038526.

Polyethylene (PE) pipes have the advantages of low weight, corrosion resistance, high impact resistance, and superior flexibility, and have been widely used for various urban gas engineering. With the increase of service life, the aging of PE pipes has become a safety issue that needs to be solved. So far, the aging performance of PE pipes are researched at home and abroad, but there are few reports on the aging performance of PE pipes under different pressures which are similar to actual urban gas working condition. Therefore, an accelerated aging test of gas PE pipes under different pressures was carried out by a thermal oxygen aging experimental setups. After that, mechanical properties of the aged PE pipes were tested by a tensile test. Then, based on the tensile test's results, empirical equations of pressured urban gas PE pipes were got by Arrhenius fit of the data, and finally, a life prediction model of pressured urban gas PE pipes was proposed. The results show that tensile strength (TS) of the aged gas PE pipes reduces with the increasing internal pressure. The lives of the PE gas pipes with internal pressure of 0.1 MPa, 0.2 MPa, 0.3 MPa, and 0.4 MPa are 10%, 22.4%, 34.7%, and 44% shorter than those without internal pressure, respectively. This life prediction method is not only suitable for pressured urban gas PE pipes, but also for other plastic pipes in similar environments.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):011405-011405-13. doi:10.1115/1.4038311.

Pressure vessels designed in accordance with the ASME BPVC code are protected against local ductile failure. Recent work has shown that local ductile failure highly depends on the stress state characterized by both stress triaxiality (T) and the Lode parameter (L). In this paper, the effect of stress state on the ductility of a tubular steel is studied. Two ring specimen configurations were optimized to allow the determination of the ductile failure locus at both tensile and plane strain loadings. The geometry of both ring specimen configurations was optimized to achieve a plane strain (L=0) condition and a generalized tension (L=-1) condition. Notches with different radii were machined on both types to achieve a wide range of stress triaxiality. Specimens were manufactured from SA-106 carbon tubular steel and were tested to determine the ductile failure loci as a function of T and L. Failure locus of SA-106 steel was constructed based on the failure instants and was found to be independent of the Lode parameter. The ASME-BPVC local failure criterion showed close agreement with experimental results (EXP).

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):011406-011406-15. doi:10.1115/1.4038309.

Lamination is one of common defects in the manufacturing process of seamless metallic pipes. In this paper, the interaction between the circumferential Lamb waves and lamination in the midplane of an aluminum pipe is studied. The used circumferential Lamb waves are CL0 and CL1 modes generated with a finite element method code. Lamination along the circumferential direction is established by the demerging-node method. Numerical results of arrival time are compared with theoretical results in order to verify the accuracy of the excitation ways. The interaction between circumferential Lamb waves and lamination in a damaged full circular pipe is analyzed by composing the received waveforms of the corresponding receivers when CL0 and CL1 modes are excited at different excitation positions: the inner subpipe, the outer subpipe, and the main pipe. The composed waveforms fit well with the original waveforms. When CL0/CL1 mode reaches the entrance and exit of a lamination, it generates new mode and undergoes multiple reverberations, diffraction, and mode conversion between the two ends of the lamination. Based on the detailed analysis of the waveform in detail, some phenomena, which are different from those in a plate, are observed.

Topics: Waves , Pipes , Lamination
Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):011407-011407-12. doi:10.1115/1.4038594.

A comprehensive testing program to determine the fatigue crack growth rate (FCGR) of pipeline steels in pressurized hydrogen gas was completed. Four steels were selected, two X52 and two X70 alloys. Other variables included hydrogen gas pressures of 5.5 MPa and 34 MPa, a load ratio, R, of 0.5, and cyclic loading frequencies of 1 Hz, 0.1 Hz, and 0.01 Hz. Of particular interest was whether the X70 materials would exhibit higher FCGRs than the X52 materials. The American Petroleum Institute steel designations are based on specified minimum yield strength (SMYS), and monotonic tensile tests have historically shown that loss of ductility correlates with an increase in yield strength when tested in a hydrogen environment. The X70 materials performed within the experimental spread of the X52 materials in FCGR, except for the vintage X52 material at low (5.5 MPa) pressure in hydrogen gas. This program was developed in order to provide a modification to the ASME B31.12 code that is based upon fatigue, the primary failure mechanism in pipelines. The code modification is a three-part Paris law fit of the upper bound of measurements of FCGR of pipeline steels in pressurized hydrogen gas. Fatigue crack growth data up to 21 MPa (3000 psi) are used for the upper bound. This paper describes, in detail, the testing that formed the basis for the code modification.

Commentary by Dr. Valentin Fuster

Research Papers: NDE

J. Pressure Vessel Technol. 2017;140(1):011501-011501-10. doi:10.1115/1.4038517.

Research using microwaves (MWs) to detect pipe wall thinning (PWT) distinguishes the presence of wall thinning, but does not accurately locate the discontinuities. Ultrasonic testing (UT) is capable of accurately locating the PWT defect, but cannot do so without time-consuming linear scanning. This novel work combines the MW technique as a way to predict the location of a series of PWT specimens, and the UT technique as a way to characterize the PWT specimens in terms of location, depth, and profile shape. The UT probe is guided to the predicted location derived from the Phase One MW results, generating the Phase Two results to determine accurate location, depth measurement, and profile shape detection. The work uses the previously successful experimental setup for testing of an aluminum pipe with 154.051 mm inner diameter (ID) and 1 m length. A vector network analyzer (VNA) generates a MW sweeping frequency range of 1.4–2.3 GHz. This signal is propagated within reference pipes with both open end and short-circuit configurations for calibration of the system. The calibrated system is used to detect the presence and location of six PWT specimens, with two profile shapes, at three depths of thinning and three locations along the pipe. The predicted locations from Phase One are then used to guide a calibrated, manually guided straight beam UT probe to the predicted position. From that point, the UT probe is used in order to accurately localize and determine the depth and shape profile of the specimens.

Commentary by Dr. Valentin Fuster

Research Papers: Pipeline Systems

J. Pressure Vessel Technol. 2017;140(1):011701-011701-11. doi:10.1115/1.4038224.

An integrative numerical simulation approach for pipeline integrity analysis is presented in this work, combining a corrosion model, which is the main focus of this paper, with a complementary structural nonlinear stress analysis, using the finite element method (FEM). Potential distributions in the trapped water existing beneath pipeline coating disbondments are modeled in conjunction with reaction kinetics on the corroding exposed steel surface using a moving boundary mesh. Temperature dependencies (25 °C and 50 °C) of reaction kinetics do not greatly affect final corrosion defect geometries after 3-yr simulation periods. Conversely, cathodic protection (CP) levels and pH dependencies within the near-neutral pH range (6.7–8.5) strongly govern depth profiles caused by corrosion, reaching a maximum of ∼3 mm into the pipeline wall. A 0.25 V amplification of CP potential combined with a 0.5 mm widening in disbondment opening size reduces defect penetration by almost 30%. Resulting corrosion defect geometries are used for stress examinations and burst pressure evaluations. Furthermore, nonlinear elastic–plastic stress analysis is carried out using shell elements in order to predict the burst pressure of corroded pipes. Corrosion is modeled by reducing the stiffness of a damaged element that has the dimensions of the defect. The predicted burst pressures are in good agreement with those obtained using an experimental-based formula.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):011702-011702-9. doi:10.1115/1.4038720.

Burst pressure models are used for the fitness-for-purpose assessment of energy pipelines. Existing burst pressure models for corroded pipelines are unable to predict the pipe capacity correctly. In this paper, an improved burst pressure model is developed for corroded pipelines considering the burst pressure of flawless pipes and a reduction factor due to corrosion separately. The equation for the burst pressure of flawless pipe is revised based on the theory of the thick wall cylinder. A new model for the Folias factor is proposed for calculating the reduction factor. The new model for the Folias factor incorporates the depth of corrosion defect, whereas the existing models do not account for the effect of the defect depth. The authors' earlier work revealed that the Folias factor depends on the depth of defect. The proposed burst model reasonably predicts the burst pressures obtained from finite element (FE) analysis conducted in this study and the burst test results available in the published literature.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Pressure Vessel Technol. 2017;140(1):014501-014501-4. doi:10.1115/1.4038484.

In this paper, the ultimate hoop strength of an industrial (±55 deg)9 filament-wound glass-reinforced epoxy (GRE) pipe as a short-term test is determined according to the ASTM D-1599 standard by performing the internal hydrostatic pressure test. After the test, the failure surface of the pipe is photographed by a high magnification camera, and in addition, the explanations are presented about the type of failure. The main purpose of this study is to compare the results obtained for the ultimate hoop strength and failure mechanisms of the pipe by using the internal hydrostatic pressure test with that by the split disk test method according to the ASTM D-2290 standard in the previous work.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;140(1):014502-014502-4. doi:10.1115/1.4038225.

As an economical alternative to solid corrosion resistant alloy (CRA) and clad pipes, mechanically lined or sleeved CRA pipes are proven to be effective in the transport of corrosive fluids in oil and gas industry. A major issue with these pipes is that pressure drop or fluctuations may cause buckling of the liner, resulting in irreparable and costly damage. This issue should be resolved in order to fully implement this type of pipes in oil and gas industry. In this study, post-buckling analysis of liner pipe encased in carbon steel outer pipe is carried out following the hydraulic expansion manufacturing process. Commercially available abaqus finite element software is employed. The proposed model is partly verified with an analytical solution and other numerical results under the condition of no residual contact pressure. Results of the parametric study reveal that increasing the residual contact pressure and decreasing the magnitude of geometric imperfection can both contribute to enhancing the buckling resistance.

Commentary by Dr. Valentin Fuster

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