Abstract
To investigate the effects of breathing cracks on the dynamic characteristics, i.e., natural frequencies, vibration displacement, slope angle, and bending moment, of a drilling riser, a time-domain finite element method based on the fracture mechanics theory is developed. The correctness of the proposed method is verified, and the performance of the different dynamic characteristics for crack identification is evaluated. First, the influence of the breathing and open cracks on natural frequencies is explored. The maximum frequency shifts vs crack location and water depth are studied. The results of the small difference of natural frequencies between intact and cracked risers illustrate that the traditional frequency-based crack-detection method is not applicable for the risers. Second, the time-domain motions, orbit plots, Poincaré maps, and fast Fourier transform diagrams are extracted for discussing the effects of the breathing and open cracks and crack depth and location. Finally, the second and fourth derivatives of the root-of-mean-square (RMS) of the dynamic response characteristics are defined for crack identification. It is found that the moment RMS has a good performance in single- and multiple-crack identifications and that not only locations but also degrees of the breathing cracks can be well detected by the proposed indices. In short, several conclusions drawn are a benefit for the safety of a serviced drilling riser system.
Highlights
Due to complex sea loads, especially the extreme wind and harsh waves and current, corrosion, high operation loads, low temperatures, wearing, fatigue, and other factors, cracks are unavoidably generated and propagated and significantly affect the safety of a drilling riser system
It is found that the moment RMS has a good performance in single- and multiple-crack identifications and that locations and degrees of the breathing cracks can be well detected by the proposed indices
Zhou et al.5 studied the dynamic characteristics of a platform with a single-side open crack and concluded that crack sizes play an effective role in dynamic behaviors, where the maximum crack depth reaches half of the thickness of the beam
Summary
Due to complex sea loads, especially the extreme wind and harsh waves and current, corrosion, high operation loads, low temperatures, wearing, fatigue, and other factors, cracks are unavoidably generated and propagated and significantly affect the safety of a drilling riser system. The vibration-based damage-detection method, recently, has become a new trend to structural health monitoring and crack identification for marine structures, e.g., platforms, drilling risers, and pipelines. Zhou et al. studied the effect of surface-open cracks on natural frequencies and mode shapes of a hang-off riser during deployment and retrieval. As evidenced in the literature, the studies on damage detection of marine risers are mainly based on the simple stiffness-reduction assumption and the open-crack assumption. To investigate the influence of small cracks on the dynamic characteristics of a marine riser, a time-domain finite element method (FEM) based on the modeling of a breathing crack is developed in this study.
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