Study On The Internal Flow Characteristics And Vibration Characteristics Of The Marine Riser System

The stability of offshore riser system is one of the most concer ned problems in offshore oil and gas exploration activities.External vortex excitation and internal two-phase flow make the riser system under complicated dynamic loadings.At present,the research on the vibration of marine riser is mainly focused on the external vortex induced vibration.The research on the influence of internal flow are not very much.In order to study the vibration characteristics of the marine riser system caused by the internal flow,and its influence on the vortex induced vibration,a series of studies have been carried out.By building an indoor experimental device to simulate the ocean riser system,a variety of flow patterns in the riser system are found,the vibration of riser system is also measured.The following conclusions are drawn from both time and frequency domain analysis of the pressure signal at the bottom and the top of the riser and the the vibration signal at the bottom of the riser.(1)The flow patterns can be distinguished by the time domain and frequency domain characteristics of the bottom pressure signal or the top pressure signal.(2)The vibration signal at the bottom of the riser has a good corresponding relationship with the bottom pressure signal,which can be used to evaluate the vibration of riser system.Two dimensional and three dimensional numerical model of the two-phase flow in riser system are established,and the calculation cost of the two models are compared.The simulation results are compared with the experimental results,which verifies the accuracy of the numerical model.On the basis of this,the impact of different flow patterns on the impact force of the elbow,and the influence of the geometry size on the severe slug flow parameters are obtained.The natural frequency of the riser system under different support modes are obtained by the modal analysis of the riser.It is found that the possibility of resonance caused by internal flow is very small.The dynamic response of the riser is obtained by loading the impact force of the internal flow to the elbow in workbench,and the dynamic response of the riser is obtained,which is consistent with the experimental results.Two dimensional vortex induced vibrations model of the riser considering two-way coupling effects was established,the influence of internal flow is seen as the mass ratio change of the riser,and it's been found that risers with different mass ratios will be motivated by the different forms of vibration.