| As an important but weak component of the seabed resource development system,the marine riser is prone to VIV(VIV)and cause fatigue damage under the action of ocean currents,causing serious economic losses and environmental pollution,and has now attracted widespread attention.In mining projects,apart from the complex marine environment,the riser is extremely susceptible to the influence of internal flow fluids,exacerbating fatigue damage,and the coupling effect of the riser along the in-line and the cross-flow under the combined effect of internal and external flows will cause the riser dynamics to be more complicated Therefore,it is very important for the safe operation of marine engineering to study the two-degree-of-freedom VIV characteristics of the internal flow riser and comprehensively predict its fatigue life.Based on consulting a large number of documents,this paper analyzes the VIV mechanism and commonly used fatigue life calculation methods.Combining with the existing research conclusions,the two-degree-of-freedom VIV of the marine riser with internal flow are mainly studied in three aspects:(1)Based on the Euler-Bernoulli beam and the improved Van Der Pol wake oscillator,the VIV model of the riser is studied.The VIV predication model of marine risers is established considering the effects of internal fluid flow and cross-flow and coupled in-line,a differential equation is derived by using Hermit interpolation function,and Newmark-β method is used to solve the displacement of risers in time domain through iterative solution.Then a VIV experiment is conducted to verify the prediction mode.The results show that the model built in this paper can effectively predict the VIV characteristics and response of marine risers.(2)The research on the single-degree-of-freedom and two-degree-of-freedom VIV characteristics of the riser under internal flow.Applying the model to the research of the riser vibration characteristics under the influence of different internal currents,ocean currents,internal and external current interactions and top tension.The results show that the natural frequency decreases with the increase of internal flow velocity;the vibration amplitude obtained by considering the bidirectional degrees of freedom coupling effect of the riser is significantly higher than that without considering the coupling effect.Under the effect of internal flow,the cross-flow vibration amplitude is about 0.33 times of the in-line vibration amplitude,but the vibration frequency of the in-line is roughly twice that of the cross-flow.The average increase rate in the in-line is obviously greater than the average increase rate in the cross-flow.When the top tension changes,more attention should be paid to the VIV in the in-line.(3)Comprehensive prediction of riser fatigue life under the combined action of multiple influencing factors.Combining the riser VIV model and the vibration characteristics response law to establish a fatigue life prediction model,comprehensive riser bidirectional fatigue life,and put forward suggestions to improve the riser fatigue life.The results show that the shortest fatigue life is at the bottom of the riser.The vibration displacement of the riser increases with the increase of the flow velocity,and its fatigue life is reduced at the same time.By increasing the top tension,the influence of internal flow can be effectively eliminated.The fatigue life of the bidirectional degree of freedom VIV of the riser is roughly equal,especially when the sea level is higher,the damage of the riser in the in-line is large and cannot be ignored.Due to the time relationship,in this paper,the analysis of the riser VIV problem does not consider the offshore platform motion,wave combined load,etc.The influencing factors of the mirine riser are complex and diverse,and establishing a VIV prediction model that covering the combined effects of many influencing factors,It is also the focus of future research. |
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