Hugo A. Ernst

Experimental results on fatigue crack closure for two aluminum alloys

Abstract Fatigue crack closure measurements were performed on two aluminum commercial alloys with different yield strength levels. The opening loads were determined on the basis of the non-linear reversible correlation between the applied load and the crack opening displacement. An experimental procedure which amplifies such a non-linear behavior was used. The experimental data indicate that a strong correlation between the opening stress intensity factor, K op , and the maximum value of the stress intensity factor during the fatigue cycle, K MAX , exists. Such a correlation would not depend on the load ratio, R , and it suggests to consider a “master” curve which represents the effective stress intensity range, ΔK EFF , as a function of K MAX for a given material. This curve allows to rationalize several basic features of the fatigue crack growth behavior.

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

Modeling the Effect of Material Behavior and Mild Thermal Treatments on Collapse Resistance of UOE Pipes

Oil exploration and production of offshore sources is continuously shifting towards increasing depths and more severe environmental conditions. Ultra deep waters are an objective in, e.g., the pre-salt layer off the Brazilian coast and in the Gulf of Mexico. Under these conditions, resistance to collapse of pipelines is a main concern. Increasing the collapse pressure pc is thus a primary objective, which would lead to a reduction of material and installation costs.To increase pc, it is fundamental to understand which variables affect it, and how to control these variables. For instance, it is well known that ovality, residual stresses, and material constitutive behavior have a direct effect on pc. Current efforts for improving pc of large diameter UOE pipes include an increase in flow stress by the application of a thermal cycle, similar to those typical of coating processes. These thermal treatments recover at least part of the early yielding due to the Bauschinger effect that develops during the collapse test, after the expansion stage.Predictive modeling of pc, based on an appropriate set of input variables, allows for an adequate design of deep- and ultra-deep water projects. In the present work, an assessment by finite element analysis of the requirements on material characterization tests for a reliable prediction of pc has been performed. The most appropriate testing direction is the transverse compression. Moreover, since for large diameter pipes the plastic strain levels attained at collapse are often below 0.2%, the sample should allow for an accurate determination of compression behavior in this very low deformation range. This is particularly relevant for cold-formed pipes, as with the UOE process. Based on these guidelines, a testing sample geometry and compression data processing methodology has been designed.The methodology has been applied to a series of UOE processed pipes that had been thermally treated. On one hand, compression samples were extracted and used for the FE calculation of pc. On the other hand, collapse tests were performed on the same pipes. Both the absolute values of pc, and the enhancement of pc due to thermal cycling, were accurately predicted.In addition, both the flow stress after thermal cycling, and the measured pc values, clearly show that the fabrication factor αfab used in the standard DNV OS-F101 should be set to αfab≥1 for an adequate rating of the pipes.Copyright

Modeling of the Collapse and Propagation Behavior of UOE SAW Pipes Under External Pressure: Influence of Thermal Treatments for Typical Coating Applications

Large diameter UOE pipes are being increasingly used for the construction of offshore pipelines. Since oil discoveries are moving towards ultra deep water areas, collapse resistance is a key factor in the design of the pipelines. It has been demonstrated in previous works that the application of typical coating thermal treatments increases the collapse resistance of the pipes recovering the original strength of the plate. To improve the understanding of these effects, the Tenaris has embarked on a program of both, experimental testing and finite element modeling.Previous phases of this work formulated the basis for model development and described the 2D approach taken to model the various stages of manufacture, from the plate to the final pipe and the collapse test. More recent developments included some modeling enhancements, sensitivity analyses, and comparison of predictions to the results of full scale collapse testing.In the present work, 3D finite element analyses of collapse were performed and compared with the latest collapse and propagation tests performed by Tenaris, where the effect of typical coating thermal treatments was studied and significant increments in the collapse pressure of pipes were obtained. The numerical results show a good agreement with the experimental ones and could predict the increment produced in the collapse pressure by the effect of the thermal treatments. Comparison of the results with the predictions from API RP 1111 and DNV OS-F101 equations was also performed.The outcomes of this study will be employed to further optimize the collapse resistance of subsea linepipe in order to reduce material and offshore installation costs through the increment of the fabrication factor as stated in the DNV OSF101 standard.Copyright

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

The Influence of the UOE-SAWL Forming Process on the Collapse Resistance of Deepwater Linepipe

The UOE-SAWL pipe manufacturing process introduces considerable plastic deformations and residual stresses to feedstock plate material. Previous experimental and analytical studies have demonstrated that the effects of this process, predominantly in its final expansion stage, significantly reduce the collapse resistance of deepwater linepipe. Finite element analyses, sensitivity analyses and full-scale tests were conducted by Tenaris and C-FER Technologies (C-FER) over the last several years to better comprehend the impact of cold forming on collapse resistance. This paper presents the findings of the latest segment of this ongoing study, the objective of which was to optimize the collapse resistance of UOE-SAWL linepipe by varying three key thermal ageing parameters: time, temperature and number of thermal cycles. Six X70M and four X80M UOE pipe samples were manufactured and thermally treated with varied parameters. Full-scale collapse and buckle propagation tests were then carried out in an experimental chamber that simulates deepwater conditions. These experimental results were evaluated with respect to collapse predictions from API RP 1111 and DNV OS-F101. Material and ring splitting tests were also performed on samples obtained from these pipes to better assess the extent of the UOE pipe collapse resistance recovery. The outcomes of this study will be employed to further optimize the collapse resistance of subsea linepipe in order to reduce material and offshore installation costs.Copyright

Influence of Weld Mismatch on the Structural Integrity of Pipes for Reeling

Structural integrity analysis of tough materials based on Elastic-Plastic Fracture Mechanics (EPFM) has been successfully employed in the assessment of components. EPFM has originally been developed for homogeneous materials and its applicability to inhomogeneous materials has some peculiarities. In particular, Fitness for Service design of welded pipes requires to know the weld fracture toughness and to estimate accurately the J-integral applied on the actual structural member. In this work, finite element analyses of simulated welds have been carried out in order to qualify and quantify the lack of accuracy of experimental methodologies for measuring fracture toughness of welds and the influence of welds on the applied J-integral in a pipe under bending. Different weld widths and cracks positions are characterized for single edge notch specimens in tension (SE(T)) and pipes. It has been found that inhomogeneity affects elastic-plastic fracture parameters for cracks centered in welds of certain widths. Moreover, the applied J-integral on pipes with circumferential cracks depends significantly on the weld width and crack position.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