Richard E. Bravo

Strain History Effects on Fracture Mechanics Parameters: Application to Reeling

It is now well accepted that welded structures may contain flaws, and that these do not necessarily affect structural integrity or service performance. This is implicitly recognized by most welding fabrication codes that specify weld flaw tolerance, or acceptance, levels based on experience and workmanship practice. However, these levels are somewhat arbitrary and do not provide a quantitative measure of structural integrity, i.e. how “close” a particular structure containing a flaw is to the failure condition. This concept is of special interest in cases in which the pipe is subjected to loads that produce important deformations. In particular the reeling process, used to install offshore lines, produce large cyclic plastic deformation on the pipes. In this work the method to perform a structural reliability analysis (SRA) for a tube subject to reeling is considered in detail. A fracture mechanics based methodology is reviewed and the points that need to be resolved before extending the methods to include reeling are clearly identified. The effect of the strain history on the applied and material fracture mechanics parameters were studied. A theoretical model was developed to describe the crack driving force evolution through strain cycles. A criterion was proposed and corroborated to represent material fracture resistance behavior. An experimental program was carried out. The material analyzed was a X65 - tube 355.4 × 22.2 mm. Monotonic and cyclic fracture mechanic tests were performed on single edge notch in tension (SENT) specimens. The material fracture resistance curve was determined based on the monotonic tests. The cyclic tests were used to determine experimentally the applied fracture mechanic parameters evolution. A very good agreement between predicted and measured CTOD values was obtained for the cases analyzed. A methodology to perform a SRA for tube subjected to reeling is proposed.© 2005 ASME

Correlation of Fracture Behavior in Circumferentially Cracked Pipes Under Combined Load Conditions Using SENT Specimens: Effects on J-R Resistance Curves

Single edge cracked under tension (SENT) specimens appear as an alternative to conventional fracture specimens to characterize fracture toughness of circumferentially cracked pipes. The similarities of stress and strains fields between SENT specimens and cracked pipes are now well known. However, these similarities are not so well established for the case of circumferentially cracked pipes under combined loading conditions (i.e. internal pressure plus bending). This work presents a numerical analysis of crack-tip constraint of circumferentially surface cracked pipes and SENT specimens using full 3D nonlinear computations. The objective is to examine combined loading effects on the correlation of fracture behavior for the analyzed crack configurations. The constraint study using the J-Q methodology and the h parameter gives information about the fracture specimen that best represents the crack-tip conditions on circumferentially flawed pipes under combined loads. Additionally, simulations of ductile tearing in a surface cracked plate under biaxial loading using the computational cell methodology demonstrate the negligible effect of biaxial loadings on resistance curves.Copyright

Probabilistic Fracture Mechanics Methodology Applied to Pipes Subjected to Multiple Reeling Cycles

Reeling process is one of the more used methods for installations of linepipes in recent years. Pipes are welded onshore and subsequently reeled onto a drum. During installation, the line is unreeled, straightened, and then laid into the sea. The pipe is subjected to severe cyclic plastic deformation. Due to the characteristics of the process, it is necessary to guarantee the integrity of the components during and after the process. For this reason, structural reliability analyses are essential requirements. In a previous work [1], a fracture mechanics based methodology was developed to obtain a method to assess the structural reliability of reeled pipes. The problem of several reeling cycles was considered. In addition to a fracture mechanics methodology, a formulation considering fatigue crack growth (FCG) controlled by ΔJ parameter was developed. This formulation accounts for the crack growth produced during subsequent reeling cycles. In another work [2], a probabilistic fracture mechanics assessment approach to perform the structural reliability analysis of tubes subjected to a reeling process was developed. This procedure takes into account the statistical distributions of the material properties and pipe geometry, using a fracture mechanics approach and the Monte Carlo method. In this work, the probabilistic fracture mechanics approach was applied for the case of multiple reeling cycles that includes ΔJ-based fatigue crack growth and reliability analysis. A particular case of interest was studied and tolerable defect sizes were determined for different number of reeling cycles taking into account the parameters variability.Copyright

Probabilistic Fracture Mechanics Structural Reliability Analysis of Reeled Pipes

Structural integrity analyses are used to guarantee the reliability of critical engineering components under certain conditions of interest. In general, the involved parameters have statistical distributions. Choosing a single set of values for the parameters of interest does not show the real statistical distribution of the output parameters. In particular, offshore pipes installation by reeling is a matter of concern due to the severe conditions of the process. Since it is necessary to guarantee the integrity of the pipes, a probabilistic fracture mechanics reliability analysis seems to be the most adequate approach. In this work, a probabilistic fracture mechanics assessment approach to perform the structural reliability analysis of tubes subjected to a reeling process was developed. This procedure takes into account the statistical distributions of the material properties and pipe geometry, using a fracture mechanics approach and the Monte Carlo method. Two-parameter Weibull distributions were used to model the variability of the input parameters. The assessment procedure was implemented as a self-contained executable program. The program outputs are: the statistical distribution of critical crack size, amount of crack extension, final crack size and the cumulative probability of failure for a given crack size. A particular case of interest was studied; a seamless tube - OD 323.9 × wt 14.3 mm, was analyzed. Tolerable defect size limits (defect depth vs. defect length curves) for different probability of failure levels were obtained. A sensitivity analysis was performed; the effect of material fracture toughness and misalignment was studied.Copyright

Effect of the Yield to Tensile Ratio, Y/T, on Structural Reliability of Linepipes Subject to Bend Loading

A model based on elastic-plastic fracture mechanics (EPFM) and plasticity theory, was developed to study the effect of the Yield (Y) to Tensile (T) ratio, Y/T, on the structural reliability of linepipes with part through the thickness (PTT) circumferential defects subject to bend loading. The analysis allows for load or deformation control situations. The results are shown in terms of curves of critical defect size vs. the controlling variable, i.e. load or deformation. For each one of the materials studied, different cases with different Y/T values were considered. Even for the lower limits of experimental data, i.e. larger Y/T, the materials have adequate defect tolerance. A Leak Before Break Analysis was also conducted.Copyright

Material Behavior of Two High Grade Pipe Steels Under Different Triaxility Levels

Particular geometries and loading conditions may have important effects on the stress fields of a given component promoting complex triaxial stress states and modifying the hydrostatic stress level. The yield condition of a ductile material is represented by the von Mises stress. However, the triaxial stress states have important effects on material toughness and ductility. This work presents a study of the effects of stress concentrators (different triaxial stress states) on material rupture. The aim is to determine the effects of hydrostatic stresses on the strain at failure in two low alloy high strength mill steels employed in field well and linepipe applications. Cylindrical specimens with different notch radius were tested to obtain different hydrostatic to von Mises stress ratios during tensile tests (h = σh /σVM ). The considered notch radii were 2.0, 0.8, 0.4 and 0.25 mm. The notched specimens were loaded in tension and applied load vs. reduction of transversal area data were recorded during the tests. Numerical simulations of the tensile tests allow reproducing the test in the numerical model and calculating the stress and strains fields during each stage of the applied loading. Finally, tables of strain at failure vs. stress triaxiality are obtained for both steels that allow determining the most appropriate material for critical applications.Copyright

Offshore Installation of Welded Pipe: Local Buckling Evaluation

Up to the present, most of the pipes used in offshore applications installed with methods introducing plastic deformation have been seamless pipes; however, welded pipes can also be used. Welded pipes offers benefits over seamless pipe in terms of improved lead times, lower project costs, tighter dimensional tolerance and good control of mechanical properties and chemistry resulting in excellent weldability. During installation of welded pipes, failure by fracture, plastic collapse and local buckling may occur. In this work, the occurrence of the local buckling phenomenon, produced during the installation method, was evaluated. Numerical models were developed to study the effect of materials and geometrical parameters on the local bucking of pipes subjected to bending. Specifically, the loads and strains at which the plastic instability occurs were determined for each particular condition. In addition, the influence of longitudinal and girth welds on the local bucking occurrence was assessed.Copyright

Evaluation of Crack Tip Constraint in Pipes Subjected to Combined Loading

Single edge cracked under tension (SENT) specimens appear as an alternative to conventional fracture specimens to characterize fracture toughness of circumferentially cracked pipes. The similarities of stress and strains fields between SENT specimens and cracked pipes are now well known. However, these similarities are not so well established for the case of circumferentially cracked pipes under combined loading conditions (i.e. internal pressure plus tension, internal pressure plus bending, etc.). This work presents a numerical analysis of crack-tip constraint of circumferentially surface cracked pipes and SENT specimens using full 3D nonlinear computations. The objective is to examine combined loading effects on the correlation of fracture behavior for the analyzed crack configurations. The constraint study using the J-Q methodology and the h parameter gives information about the fracture specimen that best represents the crack-tip conditions on circumferentially flawed pipes under combined loads.Copyright

Evaluation of Crack Tip Constraint Effects on the Assessment of Pipes Subjected to Combined Loading Conditions

Under certain conditions, pipelines may be submitted to biaxial loading situations. In these cases, questions arise about how biaxial loading influence the driving force (i.e.: CTOD, J-integral) of possible presented cracks and how affects the material fracture toughness. For further understanding of biaxial loading effects on fracture mechanics behavior of cracked pipelines, this work presents a numerical analysis of crack-tip constraint of circumferentially surface cracked pipes and SENT specimens using full 3D nonlinear computations. The objective is to examine combined loading effects on the correlation of fracture behavior for the analyzed cracked configurations. The constraint study using the J-Q methodology and the h parameter gives information about the fracture specimen that best represents the crack-tip conditions on circumferentially flawed pipes under combined loads. Additionally, simulations of ductile tearing in a surface cracked plate under biaxial loading using the computational cell methodology demonstrate the negligible effect of biaxial loadings on resistance curves.Copyright

Structural Reliability Analysis of Pipes Subjected to Multiple Strain Cycles: Application to Reeling Process

The reeling process is one of the most important methods for offshore installations of linepipes. Pipe segments are welded onshore and subsequently bent over a cylindrical rigid surface (reel) in a laying vessel. The pipe is significantly cyclically strained. Due to the high loading condition, the high costs of operations and the severe failure consequences, it is necessary to guarantee the integrity of the components during the process. Conventional defect assessment procedures are not explicitly developed for situations with large cyclic plastic strains. In previous work, a fracture mechanics based methodology was developed to obtain an appropriate specific method to assess the structural reliability of reeled pipes. A description of the material resistance toughness and the crack driving force evolution through strain cycles was proposed. This methodology was experimentally verified. In order to expand this model, in this work the case where several reeling cycles are applied is considered. In addition to the fracture mechanics methodology previously developed, a fatigue crack growth (FCG) formulation controlled by ΔJ parameter was developed. This formulation accounts for the crack growth produced during subsequent reeling cycles. Several fatigue laws and methods to calculate ΔJ were evaluated. An experimental program was carried out. Girth welded joints from two different seamless steel pipes were analyzed. Monotonic and cyclic fracture mechanics tests were performed using single edge notch tension (SENT) specimens. Cyclic tests were used to determine experimentally the cyclic crack growth. Experimental measurements were compared to predicted fatigue crack growths for different ΔJ calculation methods and fatigue laws. Comparison between theoretical and experimental results led to the selection of the most realistic fatigue law. A methodology to assess the reliability of pipes during multiple reeling cycles, based on fracture and elastic-plastic fatigue crack growth, was developed. A particular case of interest was presented, tolerable defect sizes were determined for different number of applied reeling cycles. The proposed methodology seems to be an accurate method to assess cases where multiple plastic cycles are taken into account.© 2006 ASME