ECAs, FE and Bi-Axial Loading: A Critique of DNV-OS-F101, Appendix A

2015 ◽  
Author(s):  
Andrew Cosham ◽  
Kenneth A. Macdonald
Keyword(s):  
Finite Element ◽  
Driving Force ◽  
Longitudinal Strain ◽  
Axial Loading ◽  
Limit Load ◽  
Fe Analysis ◽  
Plastic Limit ◽  
Crack Driving Force ◽  
Failure Assessment ◽  
Plastic Limit Load

Controlled lateral buckling in offshore pipelines typically gives rise to the combination of internal over-pressure and high longitudinal strains (possibly exceeding 0.4 percent). Engineering critical assessments (ECAs) are commonly conducted during design to determine tolerable sizes for girth weld flaws. ECAs are primarily conducted in accordance with BS 7910, often supplemented by guidance given in DNV-OS-F101 and DNV-FP-F108. DNV-OS-F101 requires that finite element (FE) analysis is conducted when, in the presence of internal over-pressure, the nominal longitudinal strain exceeds 0.4 percent. It recommends a crack driving force assessment, rather than one based on the failure assessment diagram. FE analysis is complicated, time consuming and costly. ECAs are, necessarily, conducted towards the end of the design process, at which point the design loads have been defined, the welding procedures qualified and the material properties quantified. In this context, ECAs and FE are not an ideal combination for the pipeline operator, the designer or the installation contractor. A pipeline subject to internal over-pressure is in a state of bi-axial loading. The combination of internal over-pressure and longitudinal strain appears to become more complicated as the longitudinal strain increases, because of the effect of bi-axial loading on the stress-strain response. An analysis of a relatively simple case, a fully-circumferential, external crack in a cylinder subject to internal over-pressure and longitudinal strain, is presented in order to illustrate the issues with the assessment. Finite element analysis, with and without internal over-pressure, are used to determine the plastic limit load, the crack driving force, and the Option 3 failure assessment curve. The results of the assessment are then compared with an assessment using the Option 2 curve. It is shown that an assessment based Option 2, which does not require FE analysis, can potentially give comparable results to the more detailed assessments, when more accurate stress intensity factor and reference stress (plastic limit load) solutions are used. Finally, the results of the illustrative analysis are used to present an outline of suggested revisions to the guidance in DNV-OS-F101, to reduce the need for FE analysis.

The Effects of Classification of Misalignment-Induced Stresses in Engineering Critical Assessments of Welded Joints With Some Misalignment

2010 ◽  
Author(s):  
Liwu Wei
Keyword(s):  
Welded Joints ◽  
Driving Force ◽  
Limit Load ◽  
Surface Cracks ◽  
Plastic Limit ◽  
Secondary Stress ◽  
Butt Weld ◽  
Crack Driving Force ◽  
Plastic Limit Load

In the ECA of a structure or component such as a pipeline girth weld, the bending stress component arising from misalignment across the weld is often classified as primary, partly because standards such as BS 7910 and API 579-1/ASME FFS-1 do not give definitive guidance on this subject. This approach may be over-conservative as the σmis is localised. In order to obtain a more realistic assessment of the structural integrity of structures containing misalignment, it is necessary to understand the conservatism or non-conservatism in an ECA associated with the classification of σmis. To address the above concerns, systematic investigations were carried out of surface cracks in a plate butt-weld including some misalignment, external circumferential surface cracks and external fully circumferential cracks in a misaligned pipe connection. FEA of these cracked welded joints with some misalignment (typically from 1mm to 2mm) was performed to calculate crack driving force and plastic limit load. The results from FEA were compared with the existing solutions of KI and σref in BS 7910 generated by assuming three options of treating the σmis. The three options were: (1) classification of σmis wholly as primary stress; (2) 15% of σmis as primary and 85% of σmis as secondary stress; and (3) classification of σmis wholly as secondary stress. Variations in parameters (eg misalignment, crack size, loading, weld overmatch and base material properties) were taken into account in order to determine the effects of these parameters on plastic limit load and crack driving force. The implication of different classifications of σmis in terms of ECAs of misaligned welded joints was revealed by conducting BS 7910 Level 2B assessments with the use of a FAD. It was found in this work that for the cases examined, use of the σmis as entirely primary bending in an ECA was over-conservative, and even treatment of σmis as entirely secondary bending was generally shown to be still conservative, when compared with the assessments based on FEA solutions. Furthermore, caution should be exercised in using the solutions of KI and σref given in the existing BS 7910 for crack-containing structures subjected to a bi-axial or tri-axial stress state. A non-conservative estimate may result from the use of these solutions which have been derived based on a uniaxial stress condition.

Plastic Limit Loads for Slanted Through-Wall Cracks in Cylinder and Plate Based on Finite Element Limit Analyses

2014 ◽  
Vol 136 (3) ◽  
Author(s):  
Doo-Ho Cho ◽  
Young-Hwan Choi ◽  
Nam-Su Huh ◽  
Do-Jun Shim ◽  
Jae-Boong Choi
Keyword(s):  
Finite Element ◽  
Crack Opening ◽  
Limit Load ◽  
Correction Factors ◽  
Plastic Limit ◽  
Opening Displacement ◽  
Limit Loads ◽  
Axial Tension ◽  
Reference Stress ◽  
Plastic Limit Load

The plastic limit load solutions for cylinder and plate with slanted through-wall cracks (TWCs) are developed based on the systematic three-dimensional (3D) finite element (FE) limit analyses. As for loading conditions, axial tension, global bending, and internal pressure are considered for a cylinder with slanted circumferential TWC, whereas, axial tension and internal pressure are considered for a plate and a cylinder with slanted axial TWC. Then, the verification of FE model and analysis procedure employed in the present numerical work was confirmed by employing the existing solutions for both cylinder and plate with idealized TWC. Also, the geometric variables of slanted TWC which can affect plastic limit loads were considered. Based on the systematic FE limit analysis results, the slant correction factors which represent the effect of slanted TWC on plastic limit load were provided as tabulated solutions. By adopting these slant correction factors, the plastic limit loads of slanted TWC can be directly estimated from existing solutions for idealized TWC. Furthermore, the modified engineering estimations of plastic limit loads for slanted TWC are proposed based on equilibrium equation and von Mises yield criterion. The present results can be applied either to diverse structural integrity assessments or for accurate estimation of fracture mechanics parameters such as J-integral, plastic crack opening displacement (COD) and C*-integral for slanted TWC based on the reference stress concept (Kim, et al., 2002, “Plastic Limit Pressure for Cracked Pipes Using Finite Element Limit Analyse,” Int. J. Pressure Vessels Piping, 79, pp. 321–330; Kim, et al., 2001, “Enhanced Reference Stress-Based J and Crack Opening Displacement Estimation Method for Leak-Before-Break Analysis and Comparison With GE/EPRI Method,” Fatigue Fract. Eng. Mater. Struct., 24, pp. 243–254; Kim, et al., 2002, “Non-Linear Fracture Mechanics Analyses of Part Circumferential Surface Cracked Pipes,” Int. J. Fract., 116, pp. 347–375.)

scholarly journals Plastic limit load of ellipsoidal pressure vessel head with nozzle under internal pressure loading

2013 ◽  
Vol 18 (4) ◽  
pp. 1263-1274 ◽  
Author(s):  
V.N. Skopinsky ◽  
N.A. Berkov ◽  
A.B. Smetankin
Keyword(s):  
Finite Element ◽  
Internal Pressure ◽  
Limit Load ◽  
Rate Of Change ◽  
Pressure Loading ◽  
Plastic Limit ◽  
Elastic Plastic ◽  
Limit Pressure ◽  
Plastic Limit Load ◽  
Plastic Limit Pressure

Abstract A new method and numerical procedure for determining the plastic limit load in an ellipsoid-cylinder intersection using the elastic-plastic finite element analysis are presented. The proposed method is based on the maximum criterion of the rate of change of the relative plastic work. For the elastic-plastic analysis of the nozzle connections the 2D finite element method and plasticity theory with strain hardening are used. The results of the comparison of the plastic limit pressure obtained on the basis of different known criteria and the proposed criterion are presented. A parametric study of ellipsoidal heads with a nozzle under internal pressure loading was performed. The effects of nondimensional geometric parameters of shell intersection on the plastic limit pressure are discussed.

Analysis and Experiments on the Plastic Limit Load of Elbows under Combined Pressure and In-Plane Closing Bending Moment

2013 ◽  
Vol 774-776 ◽  
pp. 1090-1097 ◽  
Author(s):  
Zhi Xiang Duan ◽  
Kun Shi
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Safety Assessment ◽  
Bending Moment ◽  
Limit Load ◽  
Experimental Results ◽  
Plastic Limit ◽  
Element Analysis ◽  
Limit Pressure ◽  
Plastic Limit Load

This paper discusses the plastic limit load of elbows without defects and with local thinned area (LTA) in the extrados under combined pressure and in-plane closing bending moment. Finite element analysis (FEA) and experiments have been used. The results of FEA show that, for the elbows without defects, when the ratio of pressure to the limit pressure (P/PL) is smaller than 0.469, the limit moment of elbows increases with the increasing pressure; when the ratio (P/PL) is bigger than 0.469, the limit moment of elbow decreases with the increasing pressure. For the elbows with LTA, the FEA results show that with different LTA the variation of the limit load of elbows to the pressure is different. Perhaps, the limit moment of elbows always decreases with the increasing pressure. It is also likely that the limit moment of elbows increases with the increasing pressure and then decreases with the increasing pressure. The results of FEA are consistent with the experimental results. By fitting the results of FEA, the safety assessment figure for elbows under combined pressure and in-plane closing bending moment is drawn.

The Effect of Bi-Axial Stress on Limit Loads of Structures Containing Surface-Breaking Flaws and its Influence on Structural Integrity Assessments

2012 ◽  
Author(s):  
Liwu Wei ◽  
Isabel Hadley
Keyword(s):  
Surface Crack ◽  
Structural Integrity ◽  
Axial Loading ◽  
Limit Load ◽  
Plastic Limit ◽  
Integrity Assessment ◽  
Reference Stress ◽  
Fracture Assessment ◽  
Plastic Limit Load ◽  
Assessment Procedures

Fracture assessment diagram (FAD) based fracture assessment procedures are universally adopted by standards/documents including BS7910, R6, API579-1/ASME FFS-1 and FITNET. In the use of a FAD for structural integrity assessment, one important consideration is to determine the load ratio (Lr) which is defined by two equivalent definitions: Lr is either defined as the ratio of reference stress (σref) to yield strength (σY) as in BS7910, or as the ratio of applied load to plastic limit load as in R6. The solutions of reference stress or limit load are given in the assessment procedures for commonly encountered flawed structures such as a plate containing a surface crack and a cylinder containing an external surface crack. Although the solutions given in the various standards are not all the same, they were invariably derived on the basis of analysis of the force and moment equilibrium with regard to a flawed section and none of them has taken into account the effects of bi-axial stressing on a flawed section, thus leading to the likelihood of an overly conservative assessment. In this work, finite element analysis (FEA) of various flawed geometries (plate and cylinder containing surface cracks) was performed to compute plastic limit load, with the focus on understanding the effects of bi-axial stressing on plastic limit load. The geometries assessed include a plate with a surface crack subjected to both uni-axial and bi-axial loading, and a cylinder with circumferentially internal and external surface cracks sustaining a combination of axial loading and internal pressure. The investigation of these cases has demonstrated a significant increase in plastic limit load arising from bi-axial stressing. Comparison of the results of plastic limit load obtained from FEA with those derived from BS 7910 reference stress solutions was carried out to assess the extent of conservatism when the standard solutions are used in the applications containing bi-axial stresses. The implication for structural integrity assessment due to bi-axial stressing was also addressed. A comparison between BS 7910 Level 2B (material-specific FAD) and Level 3C (based on a FAD generated with FEA) procedures was also made and it was shown that whether the Level 3C procedure can reduce the conservatism in an assessment is dependent on individual cases.

Limit Load Solutions for Pipes With Through-Wall Crack Under Single and Combined Loading Based on Finite Element Analyses

2006 ◽  
Vol 129 (3) ◽  
pp. 468-473 ◽  
Author(s):  
Nam-Su Huh ◽  
Yun-Jae Kim ◽  
Young-Jin Kim
Keyword(s):  
Finite Element ◽  
Internal Pressure ◽  
Limit Analysis ◽  
Limit Load ◽  
Combined Loading ◽  
Thickness Effect ◽  
Plastic Behavior ◽  
Plastic Limit ◽  
Integrity Assessment ◽  
Plastic Limit Load

The present paper provides plastic limit load solutions for axial and circumferential through-wall cracked pipes based on detailed three-dimensional (3D) finite element (FE) limit analysis using elastic-perfectly plastic behavior. As a loading condition, axial tension, global bending moment, internal pressure, combined tension and bending, and combined internal pressure and bending are considered for circumferential through-wall cracked pipes, while only internal pressure is considered for axial through-wall cracked pipes. In particular, more emphasis is given for through-wall cracked pipes subject to combined loading. Comparisons with existing solutions show a large discrepancy in short through-wall crack (both axial and circumferential) for internal pressure. In the case of combined loading, the FE limit analyses results show the thickness effect on limit load solutions. Furthermore, the plastic limit load solution for circumferential through-wall cracked pipes under bending is applied to derive plastic η and γ factor of testing circumferential through-wall cracked pipes to estimate fracture toughness. Being based on detailed 3D FE limit analysis, the present solutions are believed to be meaningful for structural integrity assessment of through-wall cracked pipes.

J-Integral and Plastic Limit Load of Hole-Edge Crack in Plate under Biaxial Loadings

2014 ◽  
Vol 881-883 ◽  
pp. 1412-1415
Author(s):  
Yan Qing Zhang ◽  
Zhong Xian Wang ◽  
Wei Xu
Keyword(s):  
Edge Crack ◽  
Load Ratio ◽  
Limit Load ◽  
Fe Analysis ◽  
J Integral ◽  
Plastic Limit ◽  
Elastic Plastic ◽  
Hole Edge ◽  
Biaxial Load ◽  
Plastic Limit Load

Elastic-plastic J-integral and plastic limit load were described by finite element (FE) analysis for single hole-edge crack in plate under far field biaxial load. In this paper, the effect of biaxial load ratio (from -1 to 1) on elastic-plastic J-integral and plastic limit load was studied, the values of coefficient h1 for J-integral engineering estimation formulas were given and the effect of material hardening on plastic limit load was discussed. The result obtained from this work provides reference for fracture evaluation, so it can be used in engineering accurately and expediently.

Plastic Analysis for Cylindrical Vessels Under In-Plane Moment on the Nozzle

2010 ◽  
Vol 132 (6) ◽  
Author(s):  
B. H. Wu ◽  
Z. F. Sang ◽  
G. E. O. Widera
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Three Dimensional ◽  
Limit Load ◽  
Cylindrical Vessel ◽  
Plastic Limit ◽  
Element Analysis ◽  
Test Models ◽  
Scale Test ◽  
Plastic Limit Load

The objective of this paper is to determine the plastic limit moment for cylindrical vessels with a nozzle under in-plane moment loading. Three full scale test models with different d/D ratios were fabricated for the experiment. A three-dimensional nonlinear finite element analysis was also performed. The plastic limit moment of the cylindrical vessel-nozzle connections was determined approximately by the twice-elastic-slope criterion. The results indicate that the plastic limit moments obtained by the experiment and finite element analysis are in good agreement. On the basis of the above results, a parametric analysis of the plastic limit moment for cylindrical vessels under in-plane moment on the nozzle was carried out, and an empirical formula is proposed. The results can serve as a supplement to the available data of plastic limit load for cylindrical vessel-nozzle connection structures under external load.

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