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.

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.

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. In a standard cycle the welded pipes are reeled onto a drum, reeled off, aligned and straightened. High plastic deformation is introduced in the pipe. 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.

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.

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