Discussion on Plastic Collapse Criterion at High Temperatures in FFS Assessment Rules

Author(s):  
Kenji Oyamada ◽  
Shinji Konosu ◽  
Hikaru Miyata ◽  
Takashi Ohno

There are several Fitness-For-Service (FFS) standards with evaluation rules in terms of plastic collapse for a pressure vessel or piping component possessing a local metal loss area simultaneously subjected to internal pressure and bending moment. The authors have already reported the results of a comparative study of FFS rules, including the remaining strength factor (RSF) approach in Part 5 of API 579-1/ASME FFS-1 and the p-M diagram method, which pointed out that there could be significant differences in allowable flaw sizes. This paper describes an additional comparative study on the difference of allowable flaw size for local metal loss assessment between the RSF approach in Part 5 of API 579-1/ASME FFS-1 and the p-M diagram method, focusing on the effect of decreasing yield strength of the material at high temperatures, such as 350 degrees C. The allowable flaw depth at high temperatures derived from API579-1/ASME FFS-1 is larger than that derived by means of the p-M diagram method. However, it is verified by the finite element analysis that the allowable flaw size of the p-M diagram method is set on the stress state of general yielding near a local metal loss area if safety factor is not considered and it is possible to evade ratcheting due to cyclic bending moment in service, such as that caused by earthquake, etc.

Author(s):  
Gregory W. Brown ◽  
Lucie Parietti ◽  
Brian Rose ◽  
Ted L. Anderson

API 579-1/ASME FFS-1[1] Part 5 (2007 edition) contains procedures for assessing local metal loss based on failure by plastic collapse. Equation (5.10) defines “acceptable” tip radii for grooves to ensure a plastic collapse failure mode. Grooves failing the radius check must be treated as a crack-like flaws. The validity of Equation (5.10) is questionable, and it may be excessively conservative. This paper presents new rules for groove assessment based on brittle and ductile failure models. Computation of the Weibull stress using finite element analysis (FEA) was employed to determine the minimum groove radius required to eliminate the possibility of cleavage fracture. The Bao-Wierzbicki ductile failure model was used with FEA to evaluate burst pressure and to determine a new groove radius criterion defining the plastic collapse regime, allowing categorization as metal loss. Groove-like flaws categorized as neither sharp cracks nor as metal loss are evaluated using an effective toughness concept. This concept quantifies the difference in fracture response between a sharp crack and a notch with a finite tip radius. The upcoming 2016 API 579-1/ASME FFS-1 rules remove the excessive conservatism found in Part 5 of the 2007 edition and avoid the abrupt transition between crack and metal loss assessment types based on groove radius.


Author(s):  
Shinji Konosu ◽  
Kenta Ogasawara ◽  
Kenji Oyamada

This paper develops a procedure for plastic collapse assessment of vessel (run pipe) - nozzle (branch pipe) intersections with an arbitrarily positioned local thin area (LTA) under different loading conditions, namely internal pressure, external moment on a nozzle applied along various directions with respect to the vessel main axis, and pure bending moment on a vessel. Although simplified procedures for plastic collapse assessment based on the p-M (internal pressure ratio and external bending moment ratio) diagram method have been previously proposed for straight cylindrical vessels and pipe bends with an LTA, very few studies have dealt with the determination of plastic collapse load for an LTA in the critical region of intersecting vessels subjected to internal pressure and external moment loading. This is likely due to the complexity of the stresses caused by the applied loads in the critical region, which arises from geometric discontinuities. In this paper, simple and empirical formulae for predicting conservative plastic collapse loads for an LTA in the critical region of the intersecting vessels are proposed based on the analytical results of stresses at defect-free vessel-nozzle intersections by using linear finite element analysis (FEA). Localized elastic stress retardation factors are taken into account in the evaluation by the results of a non-linear FEA. Consequently, a p-M diagram method is developed for application to vessel-nozzle intersections with an LTA.


Author(s):  
Kenji Oyamada ◽  
Shinji Konosu ◽  
Takashi Ohno

Remaining Strength Factor (RSF) approach in Part 5 of API 579-1/ASME FFS-1 is an assessment method for a cylindrical component with a local metal loss based on surface correction factors. Also, reference stress solutions that are applied in the Failure Assessment Diagram (FAD) method for a cylindrical component with a crack-like flaw are provided in Annex D using surface correction factors. In the p-M diagram method that has been recently developed, reference stress solution for local metal loss evaluation in a cylindrical component is derived using bulging factors, which are similar but not identical to the surface correction factors used in API 579-1/ASME FFS-1. This paper describes the results of a comparative study among the RSF approach, reference stress solutions for the FAD method, and the p-M diagram method, in terms of plastic collapse evaluation of a cylindrical component. These results were compared with the FEA and experimental results to confirm how those estimated stresses could be validated. The results of the study also contain proposals for prospective modifications of API 579-1/ASME FFS-1.


Author(s):  
Nobuyuki Yoshida ◽  
Atsushi Yamaguchi

Fitness-For-Service (FFS) assessment using Finite Element Analysis (FEA) has been a problem in deciding yes-no which vary from evaluator to evaluator. The difference in decision making is caused by the degree of freedom in modeling a FEA model. In this study, burst pressures of pipes with local metal loss were calculated by using FEA in order to investigate the influence of thickness measurement intervals on FFS assessment. The analyzed pressures by FEA were verified by burst tests. A pipe specimen, which was thinned by corrosion under insulation in the actual plant, was used for the burst tests. Shape of the pipe specimen was measured by laser displacement meter and extracted at several types of interval. It is concluded that the analyzed pressures in various measurement intervals showed almost no difference, but were higher than the actual burst pressure of the specimen.


Author(s):  
Kenji Oyamada ◽  
Shinji Konosu ◽  
Takashi Ohno

Pipe bends are common elements in piping system such as power or process piping, and local thinning are typically occurred on pipe bends due to erosion or corrosion. Therefore, it is important to establish the plastic collapse condition for pipe bends having a local thin area (LTA) under combined internal pressure and external bending moment. In this paper, a simplified plastic collapse assessment procedure in p-M (internal pressure ratio and external bending moment ratio) diagram method for pipe bends with a local thin area simultaneously subjected to internal pressure, p, and external out-of-plane bending moment, M, due to earthquake, etc., is proposed, which is derived from the reference stress. In this paper, only cases of that an LTA is located in the crown of pipe bends are considered. The plastic collapse loads derived from the p-M diagram method are compared with the results of both experiments and FEA for pipe bends of the same size with various configurations of an LTA.


Author(s):  
Şefika Elvin Eren ◽  
Tyler London ◽  
Yang Yang ◽  
Isabel Hadley

The British Standard, BS 7910 Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures is currently under revision [1]. Major changes have been undertaken, especially in the fracture assessment routes, and this paper specifically addresses the assessment of proximity to plastic collapse, usually expressed as the parameter Lr via either a reference stress or limit load approach. In the new edition of BS 7910, the reference stress approach has been retained for the assessment of many geometries, mainly for reasons of continuity. However, new limit load solutions (originating in the R6 procedure) are given for use in the assessments of strength mismatched structures or clad plates. In general, a reference stress solution and a limit load solution for the same geometry should deliver the same value of Lr. However, recent comparative studies have shown differences in the assessment of plastic collapse depending on whether the reference stress solutions in BS 7910:2013 or the limit load solutions in R6 are used for the calculation of Lr. In this paper, the extent of the difference in the assessment results with respect to the choice of solutions and boundary conditions are discussed. The results of the assessments in accordance with BS 7910 and R6 are compared with the results of numerical assessments obtained via Finite Element Analysis (FEA). The collapse loads observed in various wide plate tests conducted in the last 20 years are also compared with the collapse loads predicted by BS 910:2013, R6 and FEA. Finally, observations regarding the accuracy of different Codes and FEA are discussed.


Author(s):  
Szabolcs Sza´vai ◽  
Gyo¨ngyve´r B. Lenkey

The most important question for the user is if pipelines having metal loss defect could be operated safely, if any pipe sections should be repaired or replaced, and how much is the reserved safety against a possible failure. There are several engineering methods for determining the safety margin of operation but those are usually quite conservative. For this reason Lenkey has proposed safety diagrams based on finite element analysis of external corrosion defects in underground pipelines [4]. These safety diagrams could be used to determine safety factors in a less conservative way for critical situations during the pipeline operation. The FEM calculations have been verified by burst tests carried out on several pipe sections. In the present paper the results of some further analyses are presented about the difference between the measured, numerically and analytically determined failure pressure values.


2010 ◽  
Vol 132 (2) ◽  
Author(s):  
Shinji Konosu ◽  
Masato Kano ◽  
Norihiko Mukaimachi ◽  
Shinichiro Kanamaru

General components such as pressure vessels, piping, storage tanks, and so on are designed in accordance with the construction codes based on the assumption that there are no flaws in such components. There are, however, numerous instances in which in-service single or multiple volumetric flaws such as local thin areas are found in the equipment concerned. Therefore, it is necessary to establish a fitness for service rule, which is capable of evaluating these flaws. The procedure for a single flaw or multiple flaws has recently been proposed for assessing the flaws in the p-M (pressure-moment) diagram, which is an easy way to visualize the status of the component with flaws simultaneously subjected to internal pressure p and external bending moment M due to earthquake, etc. If the assessment point (Mr,pr) lies inside the p-M line, the component with flaws is judged to be safe. In this paper, numerous experiments and finite element analysis for a cylinder with external multiple volumetric flaws were conducted under (1) pure internal pressure, (2) pure external bending moment, and (3) subjected simultaneously to both internal pressure and external bending moment, in order to determine the plastic collapse load at volumetric flaws by applying the twice-elastic slope (TES) as recommended by ASME. It has been clarified that the collapse (TES) loads are much the same as those calculated under the proposed p-M line based on the measured yield stress.


Author(s):  
Atsushi Ohno ◽  
Yoshiaki Uno ◽  
Takayasu Tahara

Recently, Codes and Standards for FFS assessment has been developed and applied in United States and other countries such as API RP579 as a series of maintenance procedures for pressure equipment. Activities developing FFS assessment procedures in conjunction with new safe inspection standards are also progressing in Japan. In order to prove applicability of the FFS procedure for assessment of damaged pressure equipment, it is also important to validate how much of inservice safe margin is derived from the FFS assessment procedures in compared with design margin of pressure equipment. Local metal loss assessment procedure specified by API RP579 is studied using finite element analysis and discussed how much of in-service safe margin will be sufficient as standardized FFS assessment procedure.


Author(s):  
J. Chattopadhyay ◽  
W. V. Venkatramana ◽  
B. K. Dutta ◽  
H. S. Kushwaha

A throughwall axial crack may develop in an elbow or pipe bend due to service related degradation mechanism. It is very important to know the plastic collapse moment (PCM) of an elbow in the presence of a throughwall axial crack. The existing PCM equations of throughwall axially cracked (TAC) elbows are either too conservative or inadequate to correctly quantify the weakening effect due to the presence of the crack. Further, they do not differentiate between closing and opening modes of bending although deformation characteristics under these two modes are completely different. Therefore, the present study has been undertaken to investigate through 3-D elastic-plastic finite element analysis. A total of 84 elbows with various sizes of axial cracks (a/Dm = 0–1), different wall thickness (R/t = 5 — 20), different elbow bend radii (Rb/R = 2,3) and two different bending modes, namely closing and opening have been considered in the analysis. Elastic-perfectly plastic stress-strain response of material has been assumed. Both geometric and material non-linearity are considered in the analysis. Crack closing is observed in most of the cases. To capture the crack closure effect, contact analysis has been performed. Plastic collapse moments have been evaluated from moment — end rotation curves by twice-elastic slope method. From these results, closed-form equations are proposed to evaluate plastic collapse moments of elbows under closing and opening mode of bending moment. The predictions of these proposed equations are compared with the test data available in the literature. Matching between predictions and experimental results is found to be satisfactory.


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