The Study Of Suppression Effects Of Auxiliary Pipes On Offshore Riser

Marine oil and gas transport riser are impacted by the current in the complex marine environment,that will lead to vortex-induced vibration,and vortex-induced vibration is one of the important reasons to induce failure of offshore riser.Vortex-induced vibration suppression device are installed around the riser for increasing its service cycle and reduce the probability of failure in offshore petroleum industry.In recent years,although the study of vortex vibration and its suppression has been developed in China,there are still some lags in the research work of flexible riser with large diameter and related suppressions.With the Chinese government attaches importance to marine resources,and the rapid development of marine oil and gas engineering,flexible riser vortex vibration and suppression research work to be carried out.In this paper,based on the idea of fully utilizing the auxiliary pipes around the offshore riser,the experimental study and numerical simulation experiment on the vortex-induced vibration of the flexible riser and the effect of the auxiliary cylinders on the vortex-induced vibration are studied.A series of experiments on the suppression effects of different cylinders at different flow rates on different numbers and different mounting positions were carried out.First of all,flow rate test of experimental tank is carried,the natural frequency is measured according to the selected diameter and length and then convert the reduced speed ranges.Experimental tank is transformed due to the low flow rate.Non-invasive measurement works through the pixel contrast method to determine whether the measured object is moved,black stripes is marked by waterproof marker in the circumferential direction of flexible riser for enhancing identification of cameras.By placing the bare flexible riser and the flexible riser with the auxiliary cylinders in a uniform flow at different flow rates,the first-order vibration mode is observed only by the lower branch of the first-order vibration due to the limitation of the experimental sink.As a result of the first-order vibration state,the amplitude reaches maximum on the midpoint of riser,the vibration frequency increases as linear with the increase of the flow rate,the riser trajectory are "8" shape within the flow rate ranges,the majority of vertical vibration shapes are symmetry.The auxiliary cylinders can effectively suppress the vortex-induced vibration response of the flexible riser,and the suppression effect is up to 90%.The variation law of the vibration frequency of the flexible tube after installation of the auxiliary cylinders is basically the same,and the frequency is lower than that of the bare one.The auxiliary cylinders will lead the vibration trajectory into messy.The attachment cylinders is located at the amplitude of the amplitude to be superior to the mounting end,and the asymmetric mounting of the attachment cylinder causes the flexible riser to be shifted from the mounting side to the mounting side.In order to make reasonable use of the computational resources,two-dimensional numerical simulation of the real size offshore riser was carried out.The experimental is valid by using physical experimental parameters,and the simulation results are similar to the short straight rigid cylindrical vibration.When the number of attached cylinders is 8,the adaptability is the best,and the difference is less than 30%compared with the other two.Considering the limitation of the experimental conditions,the vortex shedding pattern and the real-time vibration shape cannot be observed.The CFX and Transient Structural are used to study three-dimensional solid-fluid coupling simulation.Compared with the two-dimensional simulation,the three-dimensional simulation results are closer to the physical experimental results,and the reduced velocity of the maximum amplitude of the first-order vibration is smaller than that of the two-dimensional difference.In the first-order resonance region,different wake vortices can be observed at different axial distances,with 2S at both ends and P+S and 2P in the middle of the span.The IL and CF amplitudes of flexible riser with rods are significantly reduced.Vortex shedding has also been disturbed,making the fluid force more stable.