Papers by Keyword: Free Span

Abstract: The upper and lower courses of sea oil and gas exploitationare connected by submarine pipeline which is called life line project. Free span often occurs because of the unevenness and scour of seabed, and fatigue is one of the main failure modes.In this paper, with the finite element numerical simulation method, based on the harmonic response analysis, the research on the structural response of free span under the vibration induced by vortex was investigated, and the effect of the factors such as flow velocity, length of free span. According to the analysis results,the fatigue life of free span was evaluated.

Abstract: Improving the knowledge about the influence factors of the allowable free-span length which required in pipeline design, this study provides information about an evaluation of fatigue life due to vortex induced vibration (VIV). Standard finite element techniques are used to the eigen value analyses of a oil-conveying free-span on seafloor. Combined with VIV response model, the fatigue life under three different end boundary conditions (fixed, pinned and elastic seabed) is evaluated. The influence parameter study has shown that the allowable span length is very sensitive for structural natural vibration characteristic, controlled by axial force, end conditions and internal flow. The natural frequencies of free span increase while both the internal flow speed and compression decrease, and internal flow velocity was found less important for VIV response than other functional load factors, because the speed in reality is not high enough.

Abstract: The dynamic nature of marine environment is the major cause of fatigue failures in subsea gas pipeline structures. Since the severest loaded part of piping system is the free span, freespan inspection is performed periodically to ensure that no free span exceeds its critical length. The objective of this paper is to optimize free-span inspection interval by means of probabilistic fracture mechanics analysis. Simulation data is taken from previous work of Tronskar [1]. Stress intensity factors at the crack tip are calculated by crack closure technique. Fatigue crack growth is simulated by cycle-by-cycle integration technique. The fracture mechanics analysis is then expanded to probabilistic analysis to include stochastic input parameters. Probability of failure is computed by modified direct simulation method. Based on the result of direct simulation, the studied pipelines are recommended to be inspected every 3 years to make sure that no free span exceeds 30 m.

Abstract: Operating experience of steam generators has shown that cracks of various morphologies frequently occur in the steam generator tubes. These cracked tubes can stay in service if it is proved that the tubes have sufficient safety margin to preclude the risk of burst and leak. Therefore, integrity assessment using exact limit load solutions is very important for safe operation of the steam generators. This paper provides global and local limit load solutions for surface cracks in the steam generator tubes. Such solutions are developed based on three-dimensional (3-D) finite element analyses assuming elastic-perfectly plastic material behavior. For the crack location, both axial and circumferential surface cracks, and for each case, both external and internal cracks are considered. The resulting global and local limit load solutions are given in polynomial forms, and thus can be simply used in practical integrity assessment of the steam generator tubes, because the comparison between experimental data and FE solutions shows good agreement.