Fracture Control Offshore Pipelines: Advantages of Using Direct Calculations in Fracture Assessments of Pipelines

2005 ◽  
Author(s):  
Christian Thaulow ◽  
Bjo̸rn Skallerud ◽  
K. R. Jayadevan ◽  
Espen Berg
Keyword(s):  
Internal Pressure ◽  
Biaxial Loading ◽  
Crack Depth ◽  
Deformation Capacity ◽  
Surface Cracks ◽  
Simple Method ◽  
Offshore Pipelines ◽  
Ductile Tearing ◽  
Fracture Control ◽  
Direct Calculations

Surface cracks pose major challenges for the structural integrity of pipelines. In fracture assessment programs the use of constraint parameters, such as the T-stress, along with K, J or CTOD are important to account for the limitations of single-parameter fracture mechanics. However, the three-dimensional nature of surface cracks precludes detailed 3-D finite element modeling for routine calculations. Here line-spring/shell-element models are demonstrated to be an efficient and reasonably accurate tool for constraint estimation even under large deformation levels when general yielding prevails in the pipe. Envisaging the potential use of this procedure in fracture analysis of pipelines, a new software, LINKpipe, has been developed. The program has been developed as a part of the Joint Industry project Fracture Control Offshore Pipelines. The objective of this project is to study the behaviour of defected girth welds in pipelines subject to construction and operational loads ever experienced before. The calculations have been performed in close cooperation with the project participants; see presentations of project-colleagues at OMAE 2005: Bruschi et al (2005), O̸stby (2005), Nyhus et al (2005) and Sandvik et al (2005). In this paper the line-spring calculations are compared with 3-D FE calculations and computations according to BS 7910. A pipe geometry, with OD = 400mm, was selected for the comparisons. The line-spring calculations were close to the 3-D calculations, while BS7910 was very conservative for long cracks and unconservative for short cracks. In highly ductile materials, such as pipeline steels, considerably amount of stable crack growth can be tolerated prior to the final failure of the structure. A simple method for simulating ductile tearing in surface cracked pipes with the line-spring model has been developed. A detailed parametric study has been performed to examine the effect of ductile tearing for pipes loaded in tensile, bending and with internal pressure. A significant reduction in deformation capacity from the stationary case is noticed. As the crack depth increases, the effect of ductile tearing becomes more important. And under biaxial loading a significant reduction of the deformation capacity is found as the internal pressure is increased. The development of the line-spring methodology paves the way for a transition from to-days rule-based design to direct calculations.

Fracture Control — Offshore Pipelines: Experimental Studies on the Effect of Crack Depth and Asymmetric Geometries on the Ductile Tearing Resistance

2005 ◽  
Author(s):  
Ba˚rd Nyhus ◽  
Erling O̸stby ◽  
Hans Olav Knagenhjelm ◽  
Scott Black ◽  
Per Arne Ro̸stadsand
Keyword(s):  
Fracture Toughness ◽  
Crack Depth ◽  
Experimental Studies ◽  
Single Edge ◽  
Offshore Pipelines ◽  
Ductile Tearing ◽  
Single Edge Notch ◽  
Edge Notch ◽  
Fracture Control ◽  
Tearing Resistance

Engineering critical assessment of offshore pipelines is usually very conservative if standardized single edge notch bend (SENB) specimens are used for the fracture mechanics testing. It is commonly accepted that the fracture toughness is dependent on the geometry constraint at the crack tip. The standardized SENB specimens have a high geometry constraint, and give lower bound fracture toughness for all geometries. For circumferential flaws in pipes the single edge notch tension (SENT) specimens is taken more into use, to establish more correct fracture toughness for the pipe in question. In this paper the effect of crack depth, misalignment and different wall thicknesses in SENT specimens have been studied. In addition the effect of crack depth and internal pressure in pipes have been studied with FE simulations.

Constraint Effects of Surface Crack Depth on Toughness: Experimental and Numerical Assessments

2019 ◽  
Author(s):  
G. Wilkowski ◽  
S. Kalyanam ◽  
Y. Hioe ◽  
F. W. Brust ◽  
S. Pothana ◽  
...  
Keyword(s):  
Surface Crack ◽  
Crack Depth ◽  
Limit Load ◽  
Initiation Toughness ◽  
Surface Cracks ◽  
Flat Plates ◽  
Depth Effect ◽  
Ductile Tearing ◽  
Wide Range ◽  
Axial Surface

Abstract Work published for the first time at the ASME PVP 2017 conference showed that when on the upper-shelf, the toughness measured directly from surface-cracked pipe tests decreased as the flaw depth increased. A similar trend existed in SENT tests. Initially it was found that this flaw depth sensitivity of the toughness occurred for a very tough material like TP304 stainless steel. The significance of that result was that even for a material where limit-load was thought to exist, as the flaw depth increased the toughness dropped appreciably, and the failure analysis mode changed from limit-load to elastic-plastic fracture. Experimentally, this made sense because it explained the observed phenomena of load-controlled leak-versus-break behavior for circumferential surface-cracked pipes (as will be shown for several pipe tests), but that LBB behavior is not predictable from circumferential flaw limit-load analysis. Furthermore, the flaw depth effect on toughness also exists for axial surface cracks and even in flat plates with surface cracks. For axial surface cracks the implication was that the long-used empirical surface-crack bulging factor from Maxey/Kiefner (incorporated in many international codes and standards) actually incorporated both the bulging factor and the toughness changes with flaw depth. Because of the change in toughness with flaw depth, when using detailed finite-element fracture analyses for the crack-driving force it is possible to have more error in the failure stress predictions if a constant toughness is assumed for all surface-flaw depths. In fact, in another paper in the ASME 2019 PVP conference it will be shown that the toughness in a wrought TP304 elbow at crack initiation of a circumferential surface crack that was 68% of the thickness was about 1/3rd of the toughness from a standard 1T CT specimen made from the same material. Those results will also be reviewed. Similar results of toughness decreasing with flaw depth in surface-cracked pipes and SENT specimens for various materials over a large range of strain-hardening behavior will show the toughness decrease trend with flaw depth is consistent. To understand these trends more theoretically, 3D FE analyses were also conducted for one initial set of TP304 SENT specimens with a wide range of a/w values (0.3 < a/w < 0.9). The initiation toughness decreased by a factor of 5 to 6 as the crack depth increased; however, the Q value coinciding to the load at the start of ductile tearing was constant for the wide range of a/W values. Q at the start of ductile tearing in the SENT (Qi) was more consistent at normalized distances from the crack tip, rσo/J that were in the range from 0.25 to 1.5 rather than just the popularly considered rσo/J = 2. Hence, by having one SENT test result with a single a/W value, the Ji value for any other a/W can then be calculated. This is consistent with the experimental trends to date, but unfortunately Ji was found to be not proportional to the Q values as is conventionally assumed by many researchers at this time.

Fracture Control — Offshore Pipelines: New Strain-Based Fracture Mechanics Equations Including the Effects of Biaxial Loading, Mismatch, and Misalignment

2005 ◽  
Author(s):  
Erling O̸stby
Keyword(s):  
Fracture Mechanics ◽  
Wall Thickness ◽  
Driving Force ◽  
Biaxial Loading ◽  
Crack Depth ◽  
Offshore Pipelines ◽  
Crack Driving Force ◽  
Fe Simulations ◽  
Definition Of ◽  
2D And 3D

In this paper a framework for a strain-based fracture mechanics crack driving force methodology for pipes with surface cracks, are presented. The model addresses the effects of crack depth, crack length, pipe diameter, wall thickness and yield to tensile ratio. Based on FE simulations, an equation to calculate the applied crack driving force, either through CTOD or J, has been derived. The equation is intended for use in cases where global plastic deformation occurs. A general approach to introduce the effects of biaxial loading, yield stress mismatch, and misalignment on the driving force, through definition of an effective wall thickness and an effective crack ligament height, is outlined. Models to quantify the effects of the different parameters are also derived. Finally, results are presented from comparison between 2D and 3D FE simulations and the predictions made by the proposed driving force equations.

J and CTOD Estimation Procedure for Circumferentially Cracked Pipes Under Combined Bending and Internal Pressure

2011 ◽  
Author(s):  
Lui´s F. S. Parise ◽  
Claudio Ruggieri
Keyword(s):  
Internal Pressure ◽  
Structural Integrity ◽  
Biaxial Loading ◽  
Numerical Solutions ◽  
Driving Forces ◽  
Crack Depth ◽  
Bending Moment ◽  
Estimation Procedure ◽  
Wide Range ◽  
Bending Load

This work provides an estimation procedure to determine the J-integral and CTOD for pipes with circumferential surface cracks subjected to combined bending load and internal pressure for a wide range of crack geometries and material (hardening) based upon fully-plastic solutions. The present investigation broadens the applicability of current evaluation procedures for J and CTOD which enter directly into structural integrity analyses and flaw tolerance criteria. Extensive 3-D nonlinear analyses of circumferentially cracked pipes with surface flaws having different crack depth (a) over pipe wall thickness (t) ratios and varying crack length for different strain hardening properties provide the dimensionless parameters relating the elastic-plastic crack-tip driving forces with the applied (remote) bending moment and internal pressure. The investigation provides a fairly comprehensive body of numerical solutions for J and CTOD in circumferentially cracked pipes subjected to biaxial loading.

Fracture Control — Offshore Pipelines: Probabilistic Fracture Assessment of Surface Cracked Ductile Pipelines Using Analytical Equations

2005 ◽  
Author(s):  
Andreas Sandvik ◽  
Erling O̸stby ◽  
Arvid Naess ◽  
Gudfinnur Sigurdsson ◽  
Christian Thaulow
Keyword(s):  
Fracture Behaviour ◽  
Crack Growth Resistance ◽  
Bending Moments ◽  
Line Pipe ◽  
Offshore Pipelines ◽  
Crack Driving Force ◽  
Ductile Tearing ◽  
Fracture Assessment ◽  
Fracture Control ◽  
Probability Of Fracture

Since modern pipelines usually display ductile fracture behaviour, fracture assessments accounting for ductile tearing should be used. In this work we use a simplified strain-based fracture mechanics equation in the probabilistic fracture assessments. Furthermore, we use the traditional tangency criterion between the crack driving force and the crack growth resistance, in calculation of the onset of critical ductile tearing. Additionally, two types of external load on the line-pipe are considered, namely strains due to external bending moments and internal pressure. We establish the probability of fracture for line-pipes with relevant diameter to thickness ratios, and thicknesses, for J-laid or S-laid offshore pipelines. The distinction between system effects, in which all defects are likely to be subject to the same loading, and cases where only a small part of the pipeline will experience high loading, is also discussed.

Stress-Intensity Factors for Internal and External Surface Cracks in Cylindrical Vessels

1982 ◽  
Vol 104 (4) ◽  
pp. 293-298 ◽  
Author(s):  
I. S. Raju ◽  
J. C. Newman
Keyword(s):  
Stress Intensity Factor ◽  
Finite Element ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Internal Pressure ◽  
Stress Intensity Factors ◽  
Crack Depth ◽  
Surface Cracks ◽  
Stress Distributions ◽  
Intensity Factors

The purpose of this paper is to present stress-intensity factor influence coefficients for a wide range of semi-elliptical surface cracks on the inside or outside of a cylinder. The crack surfaces were subjected to four stress distributions: uniform, linear, quadratic, and cubic. These four solutions can be superimposed to obtain stress-intensity factor solutions for other stress distributions, such as those caused by internal pressure and by thermal shock. The results for internal pressure are given herein. The ratio of crack depth to crack length from 0.2 to 1; the ratio of crack depth to wall thickness ranged from 0.2 to 0.8; and the ratio of wall thickness to vessel radius was 0.1 or 0.25. The stress-intensity factors were calculated by a three-dimensional finite-element method. The finite-element models employ singularity elements along the crack front and linear-strain elements elsewhere. The models had about 6500 degrees of freedom. The stress-intensity factors were evaluated from a nodal-force method. The present results were also compared to other analyses of surface cracks in cylinders. The results from a boundary-integral equation method agreed well (±2 percent), and those from other finite-element methods agreed fairly well (±10 percent) with the present results.

Fatigue Behaviour of Dented Pipes Under Internal Pressure: A Numerical-Experimental Approach

2018 ◽  
Author(s):  
Mario A. Polanco-Loria ◽  
Håvar Ilstad
Keyword(s):  
Fatigue Life ◽  
Internal Pressure ◽  
Fatigue Behavior ◽  
Crack Depth ◽  
Fatigue Behaviour ◽  
Experimental Methodology ◽  
Experimental Program ◽  
Metal Loss ◽  
Life Predictions ◽  
Dent Depth

This work presents a numerical-experimental methodology to study the fatigue behavior of dented pipes under internal pressure. A full-scale experimental program on dented pipes containing gouges were achieved. Two types of defects were studied: metal loss (plain dent) and sharp notch. Both defects acting independently reduce the fatigue life performance but their combination is highly detrimental and must be avoided. We did not find a severity threshold (e.g. dent depth or crack depth) where these defects could coexist. In addition, based on numerical analyses we proposed a new expression for stress concentration factor (SCF) in line with transversal indentation. This information was successfully integrated into a simple fatigue model where the fatigue life predictions were practically inside the window of experimental results.

J-Integral Analysis of Surface Cracks in Round Bars under Bending Moments

2011 ◽  
Vol 52-54 ◽  
pp. 43-48 ◽  
Author(s):  
Al Emran Ismail ◽  
Ahmad Kamal Ariffin ◽  
Shahrum Abdullah ◽  
Mariyam Jameelah Ghazali ◽  
Ruslizam Daud
Keyword(s):  
Finite Element ◽  
Crack Depth ◽  
Crack Tip ◽  
Bending Moment ◽  
Limit Load ◽  
Fe Model ◽  
Surface Cracks ◽  
Element Analysis ◽  
Reference Stress ◽  
Proposed Model

This paper presents a non-linear numerical investigation of surface cracks in round bars under bending moment by using ANSYS finite element analysis (FEA). Due to the symmetrical analysis, only quarter finite element (FE) model was constructed and special attention was given at the crack tip of the cracks. The surface cracks were characterized by the dimensionless crack aspect ratio, a/b = 0.6, 0.8, 1.0 and 1.2, while the dimensionless relative crack depth, a/D = 0.1, 0.2 and 0.3. The square-root singularity of stresses and strains was modeled by shifting the mid-point nodes to the quarter-point locations close to the crack tip. The proposed model was validated with the existing model before any further analysis. The elastic-plastic analysis under remotely applied bending moment was assumed to follow the Ramberg-Osgood relation with n = 5 and 10. J values were determined for all positions along the crack front and then, the limit load was predicted using the J values obtained from FEA through the reference stress method.

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