Fatigue Damage Analysis Of Steel Catenary Riser

With the exploitation of oil/gas resources in deep sea, the ocean riser has been taken into practice from shallow water to deep sea water and even in extra deep water. Riser is the main device to connect the floater with the subsea pipeline or oil well. There are many types of risers such as Steel Catenary Riser (SCR), Top Tension Riser (TTR) and flexible riser. Compared to top tension riser, SCR doesn't need top tension and can overcome large drift and heave motions of floating platforms. Compared to flexible riser, SCR can undertake worse environmental loads, especially in high temperatures and pressures. So SCR is a preferred selection for deepwater oil/gas exploitation. However, it's a complex problem to evaluate the fatigue damage of SCR reliably. That's the reason that the fatigue damage of SCR not only induced by the hydrodynamic force but also by the motions of floating platforms. There, the fatigue damage analysis of SCR is of great theoretical significance and engineering value.According to the flexible characteristics of SCR, a numerical analysis has been performed to evaluate the fatigue damage of a steel catenary riser attached to a Spar platform in present thesis. It contains modal analysis, structural response analysis of vortex induced vibration, parameters analysis of vortex induced vibration fatigue damage, suppression device study for vortex induced vibration, evaluation of wave induced Spar motions, analysis of fatigue damage induced by wave induced Spar motions, evaluation of vortex induced Spar motions, analysis of fatigue damage induced by vortex induced Spar motions.The modal characteristics of steel catenary riser have been studied. The theoretical modal analysis is carried on. Natural frequencies, modal shapes and modal curvatures of the SCR are deduced based on catenary equation. For validating the reliability of the theoretical analysis, the numerical analysis of the SCR has been developed using finite element software. A typical deep-water riser is tested, and its numerical results agree with the theoretical results very well.The procedures for calculating the response of vortex induced vibration (VIV) and evaluating the fatigue damage of SCR induced by VIV are both deduced in detail. The response of vortex induced vibration is calculated using mode superposition method, and the fatigue damage induced by VIV is evaluated based on Rayleigh distribution with Gaussian narrow band. The vortex induced vibration responses of a typical ocean riser are analyzed. The parameters analyses of vortex induced vibration fatigue damage on SCRs are carried out for different current velocities, inner fluid volume densities, outer diameters and riser thicknesses. Some useful conclusions have been obtained for ocean enginnering from these analyses. The devices to suppress the SCR vortex induced vibration are studied. Several common methods for suppressing vortex induced vibration of ocean risers are introduced. Then the fatigue damages with different suppression device distributions are analyzed. The results show that:the helical strake can be used to suppress the riser's vortex induced vibration very well.The wave induced fatigue damages for the SCR are analyzed. The motion response amplitude operators (RAOs) of Spar platform are evaluated using boundary element method, and wave induced fatigue damages are evaluated using spectral analysis method. There are two linear systems containing the platform system and the riser system during the analysis process. Then the wave induced fatigue damage of a typical SCR attached to a Spar platform is analyzed. The results show that the wave induced fatigue damages of SCR have close relation with significant wave heights, the fatigue damage increases exponentially with the increasing the significant wave height.The riser's fatigue damages induced by vortex induced Spar motions (VIM) are also analyzed in detail. The amplitudes and periods of the motions for the Spar platform are calculated with the wake oscillator model, and then the fatigue damages induced by VIM are evaluated using finite element method by considering the boundary condition of the upper floater. The SCR's fatigue damages are calculated for several sea states. It can be observed that the amplitudes and periods of the vortex induced Spar motions are much bigger than the usual vortex induced vibrations of small-size structures.