Flow-induced Vibration Analysis Of Offshore Riser Under Different Current Flow Profiles

With the rapid development of China's economy,the demand for petroleum energy is increasing day by day.In order to meet the increasing oil demand,the focus of China's oil resource extraction has gradually shifted from land to sea,and from shallow sea to deep sea.Although China has made remarkable research achievements in the field of shallow sea oil exploitation,it has built and operated multiple offshore platforms and obtained rich experience in shallow sea oil production.However,compared with the shallow sea,the deep sea marine environment is more complicated and the operating conditions are more severe.China's deep sea oil exploration still faces many challenges.Top Tension Riser(TTR)is an important component structure of offshore platforms such as Tension Leg Platform(TLP)and Single Column Platform(Spar).Widely used in deep sea oil and gas exploitation.Due to the complexity of the deep sea current,when the current flows through the TTR riser,it will cause the riser to cause Flow-induced vibration(FIV)or Vortexinduced vibration(VIV)lock-in.It reduces the safety of the riser structure and even leads to the occurrence of fatigue damage.Therefore,it is very necessary to study the dynamic response and fatigue damage of the deep sea TTR riser under the action of ocean currents.At present,the load model in the analysis of marine riser dynamic response mainly focuses on simplifying the ocean current into uniform flow,inclined flow or gradient flow,etc.,which cannot reflect the true profile behavior of ocean current load.In order to understand the environmental information of Liuhua in the South China Sea in more detail,the research team cooperated with CNOOC Research Institute to establish a marine environment prototype monitoring system on “Nan Hai Tiao Zhan” FPS,and collected more detailed monitoring data.Based on many years of current monitoring information and using big data analysis methods,28 typical current profiles with different shapes are clustered.This article first briefly describes the“Nan Hai Tiao Zhan” monitoring system,which can be subdivided into an independent power supply system,a remote transmission system,and a data acquisition system.Furthermore,in view of the optimization of the layout of monitoring sensors for large aspect ratio structures in underwater structure monitoring,the wet mode effect of the underwater structure is considered,and combined with the Effective independent method(EI),a suitable for marine TTR riser structure is developed.Research on sensor placement optimization method.Then based on the long-term current profile monitoring information,the distribution of 28 typical current profiles based on big data clustering is given.Based on the fluid-solid coupling function of the Ansys-Fluent platform,the large eddy simulation(LES)method was used to carry out a three-dimensional numerical simulation of the flow-induced vibration response of the TTR riser of a tension leg platform in the South China Sea under different types of current profiles,and the analysis of different current profiles The multi-modal characteristics of the riser vibration under the action.The numerical simulation results show that with the increase of the average velocity of the current profile,the modal order of riser vibration gradually increases,and the main frequency of vibration gradually increases.At the same time,it was found that the shape of the current profile will affect the vibration behavior of the riser.When the average velocity of the current profile is the same,the riser will show higher-order modal vibration as the slope of the current profile increases.The standing wave and standing wave analysis of the standpipe vibration found that the low-order mode vibration is mainly standing wave,and the high-order mode vibration is mostly standing wave-traveling wave mixed wave,of which traveling wave is dominant.Finally,the method of combining rain flow counting method,S-N curve and Miner linear cumulative damage criterion was used to carry out fatigue damage analysis under the action of dynamic behavior of the riser,and the overall service life of the riser was evaluated.