Simulation Of The Vortex-induced Vibrations Of Riser By Les And Accumulated Fatigue Damage Analysis

With the development of offshore oil and gas production into deepwater,marine risers are getting longer and the structure is becoming more flexible.Vortex-induced vibration (VIV) is an important issue in the design of deepwaterriser systems, including drilling, production and export risers. The VIV canproduce a high level of fatigue damage in a relatively short period of time for risersexposed to severe current environments. And the damages of risers caused by VIVcan decrease the service life of the risers, even endanger the safety of platforms.This thesis presents a transient FSI coupling method for numerical simulatingthe Vortex-Induced Vibration of a flexible riser by using commercial software:computational fluid dynamics software CFX to simulate the flow field and finiteelement analysis software ANSYS to simulate the structural vibration. Thetransient coupling between the fluid and the structure is accomplished by thedynamic data exchanging between the dynamic fluid loads and the structuraldeformation based on the commercial software Workbench.In this thesis the turbulence flow is solved by Large Eddy Simulation (LES)which can successfully realize the three dimensional simulation ofvortex-induced vibration of flexible risers. The work of this thesis consists threemain parts. Firstly, the VIV of the riser with a small aspect ratio in the uniformcurrent profile are numerically simulated and analyzed. Results show that theriser with a small aspect ratio vibrates at low modes, and its vibrationdisplacement is very small under the act of the uniform current. In particularly,the axial amplitude of the riser can be ignored. The VIV of the deep-sea riserwith a high aspect ratio in the uniform current are numerically simulated andanalyzed. When compared, the simulated results are in good agreement with theempirical data. Results further reveal that the crossing flow vibration responsesof the deep-sea riser with a high aspect ratio remarkably influence the dynamiccharacteristics of vortex. The influence of stiffness degradation on riser’svibration response has been analyzed, and it will lay the foundation for the risers’fatigue effect study. Secondly, the VIV of the deep-sea riser with a high aspect ratio in the sheared current are numerically simulated and the results arecompared with that of the uniform current situation. It has been shown that theshear current may spatially increase the complexity of riser’s VIV behaviors,while the vibration intensity of the riser in the shear current tends to be smallerthan that in the uniform current. Finally, the prediction method based on the S-Ncurves for the VIV-induced fatigue damage in deep sea risers has beeninvestigated. Under the conditions of the ladder current, uniform current andshear current, fatigue damage of risers have been predicted respectively. Theresults show that the current velocity is the most important factor which affectsVIV-induced fatigue damage of risers.