• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.4
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• pp.413-421
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• 2010

In this study, residual-stress distributions in welds with different strength used in natural gas pipelines are calculated by using finite-element analysis and simulating a realistic welding process. The temperature and residual-stress analysis results are compared with the real fusion profile and the application results of the Fitness-For-Service assessment code, API 579 in order to validate the finite-element analysis model and procedure. Parametric study is performed to assess the effect of welding and material variables such as mechanical strength mismatch, the strength of weld metal, reinforcement, and heat input on the residual stress distributions. Finally, on the basis of the parametric study results, the effects of these variables on residual stress distributions are investigated. In particular, the strength mismatch between base metals has an insignificant effect on residual-stress distributions.

• Journal of the Korean Society for Precision Engineering
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• v.31 no.6
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• pp.551-557
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• 2014

Ideally, it is preferable to obtain the fracture characteristics of a piping from the fracture toughness of real pipes. However, a fracture toughness test on real pipes not only incurs much expense, but is very difficult to perform. Therefore fracture toughness tests have been carried out with standard specimens instead of real pipes. But, the estimates of fracture toughness obtained from standard specimens are more conservative than those of real pipes owing to the difference in the constraint effect between real pipes and standard specimens. Therefore, we have been studied with equivalent stress gradient specimen (ESG) which is designed to behave equally compared to real pipe about stress gradient on crack tip. In this study, we will evaluate fracture characteristics of equivalent stress gradient specimen by using analytical methods and compare with those of real pipe. And finally investigate suitability of equivalent stress gradient specimen.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.1
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• pp.79-83
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• 2010

The conventional welding technique such as shield metal arc welding has been mostly applied to the piping system of the nuclear power plants. It is well known that this welding technique causes the overheating and welding defects due to the large groove angle of weld. On the other hand, the narrow gap welding(NGW) technique has many merits, for instance, the reduction of welding time, the shrinkage of weld and the small deformation of the weld due to the small groove angle and welding bead width comparing with the conventional welds. These characteristics of NGW affect the deformation behavior and the distribution of welding residual stress of NGW, thus it is believed that the residual stress results obtained from conventional welding procedure may not be applied to structural integrity evaluation of NGW. In this paper, the welding residual stress of NGW was predicted using the nonlinear finite element analysis to simulate the thermal and mechanical effects of the NGW. The present results can be used as the important information to perform the flaw evaluation and to improve the weld procedure of NGW.

• Structural Engineering and Mechanics
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• v.81 no.5
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• pp.565-574
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• 2022

The infrastructures such as offshore, bridges, power plant, oil and gas piping and aircraft operate in a harsh environment during their service life. Structural integrity of engineering components used in these industries is paramount for the reliability and economics of operation. Two regression models based on the concept of Gaussian process regression (GPR) and Minimax probability machine regression (MPMR) were developed to predict stress intensity factor range (𝚫K). Both GPR and MPMR are in the frame work of probability distribution. Models were developed by using the fatigue crack growth data in MATLAB by appropriately modifying the tools. Fatigue crack growth experiments were carried out on Eccentrically-loaded Single Edge notch Tension (ESE(T)) specimens made of API 5L X65 Grade steel in inert and corrosive environments (2.0% and 3.5% NaCl). The experiments were carried out under constant amplitude cyclic loading with a stress ratio of 0.1 and 5.0 Hz frequency (inert environment), 0.5 Hz frequency (corrosive environment). Crack growth rate (da/dN) and stress intensity factor range (𝚫K) values were evaluated at incremental values of loading cycle and crack length. About 70 to 75% of the data has been used for training and the remaining for validation of the models. It is observed that the predicted SIF range is in good agreement with the corresponding experimental observations. Further, the performance of the models was assessed with several statistical parameters, namely, Root Mean Square Error (RMSE), Mean Absolute Error (MAE), Coefficient of Efficiency (E), Root Mean Square Error to Observation's Standard Deviation Ratio (RSR), Normalized Mean Bias Error (NMBE), Performance Index (ρ) and Variance Account Factor (VAF).

• Nuclear Engineering and Technology
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• v.52 no.7
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• pp.1579-1586
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• 2020

Dissimilar metal joints (DMJs) are more common in the application of piping system of many industries. A 2- D and 3-D finite element analysis (FEA) is carried out on dissimilar metal Single Edged Notch Bending (DMSENB) specimens fabricated from ferritic steel, austenitic steel and Inconel - 182 alloy to study the behavior of DMJs with constraints by using linear elastic fracture mechanics (LEFM) principles. Studies on DMSENB specimens are conducted with respect to (i) dissimilar metal joint width (DMJW) (geometrical constraints) (5 mm, 10 mm, 20 mm, 30 mm and 50 mm) (ii) strength mismatch (material constraints) and (iii) crack lengths (16 mm, 20 mm and 24 mm) to study the DMJ behavior. From the FEA investigation, it is observed that (i) SIF increases with increase of crack length and DMJWs (ii) significant constraint effect (geometry, crack tip and strength mismatch) is observed for DMJWs of 5 mm and 10 mm (iii) stress distribution at the interfaces of DMSENB specimen exhibits clear indication of strength mismatch (iv) 3-D FEA yields realistic behavior (v) constraint effect is found to be significant if DMJW is less than 20 mm and the ratio of specimen length to the DMJW is greater than 7.4.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.3 s.258
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• pp.304-312
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• 2007

The objective of this study is to develop a local failure criterion for a wall thinning defect of piping components. For this purpose, a series of tensile tests was performed using several types of simulated specimens with different stress states, including smooth round bar, notched round bar (five different notch radii), and grooved plate (three different groove radii). In addition, finite element (FE) simulations were performed on the simulated specimen tests and the results were compared with the test results. From the comparisons, the equivalent stress and strain corresponding to maximum load and final failure of notched specimens were proposed as failure criteria under tensile load. The criteria were verified by employing them to the estimation of failure of grooved plate specimens that simulate the wall thinning defect. It showed that the proposed criteria accurately estimate the maximum load and final failure of grooved plate specimen tests.

• Journal of the Society of Naval Architects of Korea
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• v.48 no.4
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• pp.325-329
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• 2011

Floating, production, storage and offloading (FPSO) is a production facility that refines and saves the drilled crude oil from a drilling facility in the ocean. The flare system in the FPSO is a major part of the pressure relieving system for hydrocarbon processing plants. The flare system consists of a number of pipes and complicated connection systems. Decision of pipe support types is important since the load on the support and the stress in the pipe are influenced by the pipe support type. In this study, we optimally determined the pipe support types that minimized the support cost while satisfying the design constraints on maximum support load, maximum nozzle load and maximum pipe stress ratio. Performance indices included in the design constraints for a specified design were evaluated by pipe structural analysis using CAESAR II. Since pipe support types were all discrete design variables, an evolutionary algorithm (EA) was used as an optimizer. We successfully obtained the optimal solution that reduced the support cost by 27.2% compared to the initial support cost while all the design requirements were satisfied.

• Journal of the Korean Society of Industry Convergence
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• v.23 no.4_2
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• pp.653-659
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• 2020

In this study, seismic verification of the bellows used in the plant field was conducted. The pressure used in the analysis was analyzed by applying the design pressure of 15.7bar. For the seismic analysis, the natural frequency of the bellows system was obtained and the stability of the system was evaluated by static seismic analysis comparing the lowest order natural frequency with the dominant frequency of 33 Hz. The material of the bellows system is STS304, and the safety factor is obtained in comparison with the allowable stress. For the seismic analysis, the design response spectrum was prepared and the maximum acceleration was applied to the static seismic analysis and the stability of the entire system was confirmed. Compared to the structural analysis results, the maximum stress of the bellows system increased by about 16.4% and the maximum strain increased by about 3 times when seismic analysis was performed.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.40 no.3
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• pp.245-252
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• 2016

In this study, an alternative bend design is proposed to overcome the aging problem in piping bends. In this design, the foam pad is not included. Finite element analysis was performed based on the total pipe diameter. From this analysis, the shape of the Shear Control Ring (SCR) was determined. Temperature, stress, and other data of the proposed reinforced pipe were acquired and analyzed after the test was performed. The value of the thermal stress for the reinforced steel pipe satisfied the required standard without the foam pad based on the manufacturing of the reinforced fitting and construction site of the test. The reinforcement provided a shear strength level for the foam pad that resulted in maximum shear stress less than stress based on the original foam pad applied at the pipe bend. Additionally, an increasing factor of safety effect for the reinforced fitting application was discovered.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.5
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• pp.742-749
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• 2003

Failure of a pipeline due to local wall thinning is getting more attention in the nuclear power plant industry. Although guidelines such as ANSI/ASME B31G and ASME Code Case N597 are still useful fer assessing the integrity of a wall thinned pipeline, there are some limitations in these guidelines. For instance, these guidelines consider only pressure loading and thus neglect bending loading. However, most Pipelines in nuclear power plants are subjected to internal pressure and bending moment due to dead-weight loads and seismic loads. Therefore, an assessment procedure for locally wall thinned pipeline subjected to combined loading is needed. In this paper, three-dimensional finite element(FE) analyses were performed to simulate full-scale pipe tests conducted for various shapes of wall thinned area under internal pressure and bending moment. Maximum moments based on true ultimate stress(${\alpha}$$\sub$u,t/) were obtained from FE results to predict the failure of the pipe. These results were compared with test results, which showed good agreement. Additional finite element analyses were performed to investigate the effect of key parameters, such as wall thinned depth, wall thinned angle and wall thinned length, on maximum moment. Also, the effect of internal pressure on maximum moment was investigated. Change of internal pressure did not show significant effect on the maximum moment.