Study On The Characteristics Of Vortex-induced Vibration Of A Riser On Uneven Surface

The marine riser is an important channel connecting the seabed and the drilling platform in the development of marine oil and gas drilling.During the current flows through the riser,such as ocean currents,waves and ocean dark currents,the periodic vortex discharge will occur on the riser surface when the vortex discharge frequency is close.Or equal to the natural frequency of the riser itself,the riser will produce strong vibration,and the vibration of the riser will induce a more intense vortex shedding.The mutual coupling between the current and the riser is the Vortex-induced vibration of the riser.The continuous occurrence of vortex-induced vibration of the riser will affect its service life,which will bring serious safety hazards to marine oil and gas development.Therefore,studying the vortex-induced vibration characteristics of the riser and the vortex-induced vibration suppression method of the riser are urgent problems to be solved in the development of marine oil and gas.Based on the idea of installing a suppression tool on the surface of the riser,the protrusion of a certain size is arranged on the surface of the riser to suppress the vortex-induced vibration of the riser.By analyzing the vortex-induced vibration characteristics of the riser in different directions,numbers,heights and diameters of the protrusions,the results of the relevant dimensions of the protrusions when the vortex-induced vibration suppression effect of the protrusions is optimal are obtained.The research work carried out in this paper is as follows:(1)Firstly,this paper simplifies the riser flow model into a two-dimensional cylindrical flow model by numerical analysis related theory,and then embeds the UDF module that can realize the cylindrical double-degree-of-freedom vibration response equation by embedding in the fluent solver,and finally establishes the numerical calculation model.Based on the experimental data of Williamson,the flow pattern is identical to the experimental data.By comparing the numerical simulation results with Williamson’s experimental data,the correctness of the established two-dimensional flow model is verified.It is found that the smooth riser has obvious initial branch,upper branch and lower end when the vortex-induced vibration occurs.The branch proves that the vortex-induced vibration causes the vertical tube to undergo intense CF-direction vibration and IL-direction vibration.(2)Through the calculation and analysis of the two-dimensional numerical model of the number of protrusions with the diameter D=38.1mm,the number of protrusions is 1,3,5,7,9,and 11,respectively,and it is found that the riser of the riser is arranged on the downstream side of the riser will play a significant inhibitory role.And the number of protrusions is not increased,and the suppression effect is better.When the number of protrusions is 1,3,and 5,when the flow velocity is low(Ur=4-7),the suppression effect is not obvious;When the speed is large(Ur=8-12),the suppression effect is over 90%.Based on the quantitative results,the effects of different heights on the vibration of the riser were analyzed.It was found that the inhibition was good at the height of the analysis.Especially at the height of 1.00h,the results were good at all speeds;under the premise of determining the number and height of protrusions,the influence of the change of protrusion diameter on the vibration of the cylinder is analyzed.It is found that when the diameter of the protrusion is below 1.15d,the vibration is obviously suppressed,but when the diameter of the protrusion reaches 1.30d,On the contrary,it will have a significant enhancement to the vibration of the cylinder.(3)After analyzing the vortex-induced vibration characteristics of the riser after the protrusions on the backflow surface of the riser are arranged in the same direction as the downstream surface,it is found that the results are similar to those of the downstream surface.Arranging the protrusions can effectively suppress the vortex-induced vibration of the riser,and it is found that not the more the number of protrusions is,the better the suppression effect is.When the number of protrusions is 1,11,the inhibition effect is not obvious at low flow rate(Ur=4-6);When Ur=7-12,the inhibition effect is better,reaching over 90%;When the number of protrusions is 3,5,7,and 9,the suppression effect is excellent at each speed as compared with 1 and 11.Especially when the number of protrusions is 3,5,and 7,the suppression effect is 95%,and when the number of protrusions is 7,the suppression effect is 99%or more.By analyzing the height of the protrusion,it is found that the vibration of the riser is suppressed to different extents at the selected height,and the suppression effect is best when the height is 1.00h.Through the comparison of the data under different diameters,it was found that the vibration of the riser under each diameter was suppressed,especially when the diameter was 1.00d.(4)By comparing the vibration suppression effect of the cylinder after the protrusions are arranged on the downstream surface and the backflow surface,it is found that the result of the backflow surface is better than that of the downstream surface in the arrangement with the same number of protrusions and the same size parameters.Especially in the optimal arrangement,the suppression effect is still several times different.