| The marine riser is the key equipment connecting the offshore platform and the underwater oil and gas development system.It causes complex fluid-solid coupling problems under the action of floating body movement,environmental load and internal flow.When the vortex shedding frequency is close to the natural frequency of the riser,the system will be excited to lock-in and vibrate within a certain flow velocity range,resulting in the instability of the riser structure,resulting in violent large vibrations and accelerated fatigue damage of the riser.In many cases,marine risers are widely used in actual deep-water projects in the form of "strings","rows" and "groups".When the current flows through the riser groups,the wakes and gap flow of adjacent risers interfere with each other.The linear superposition and the diversity of wakes change the vortex-induced vibration law of the riser.The wake vortex shedding pattern,structural stress and fluid-structure coupling mechanism are significantly different from the vortex-induced vibration of the isolated riser.As the depth of oil and gas development increases,the slenderness ratio and flexibility become greater and greater,and the effects of interference effect become more apparent,significantly increasing the service risk of the riser.Researching the vortex-induced vibration mechanism of the riser group coupling the interference effect is of great engineering significance to the safe service of the riser,and is also one of the key issues that need to be solved urgently in the current riser vortex-induced vibration.In this paper,the "riser group structure-inter-riser fluid" is used as a carrier,and the"three-riser group system" is selected as the research object,and the variable angle vortex-induced vibration incomparable experiment of the standing riser group coupling interference effect is carried out in the wave-current combined channel of the Ocean University of China.In the riser group test,the adjacent risers are arranged at equal intervals of 4 times the diameter of the riser.The external flow excitation changes within the flow velocity range of 0.1-0.6m/s.The angle between the axis of the riser group and the direction of the incoming flow is changed,and the angle is configured as 0°,30°,45°,60°,and 90°.The bare fiber grating technology(BFBG)is applied to the underwater vortex-induced vibration test,which is monitored by a high-definition camera,and the wake oscillator model is established for numerical simulation of the isolated riser vortex-induced vibration,and the simulation results are compared with the test data.Based on the strain signal obtained by BFBG and the displacement amplitude at any position of the riser group obtained by the modal decomposition theory,and based on the fast Fourier transform and wavelet analysis method,the structural vibration frequency is studied in the frequency domain and time domain,and the cumulative fatigue damage of the riser group is calculated using the S-N curve method and the Miner linear criterion.The concepts of "frequency interference ratio","displacement interference efficiency",and "fatigue damage interference efficiency" are introduced to divide the interference enhancement and suppression regions,and the riser group frequency,displacement,and the change laws of the frequency response,displacement and fatigue damage of the riser group coupling interference effect relative to the dynamic response of the isolated riser under different angles are analyzed.The results indicate that the changes in the dynamic response parameters of the riser measured by the model test are in good agreement with the numerical simulation results,which verified the feasibility of the model test process control and the accuracy of the test data.The wake shadowing effect constitutes the main constraint on the VIV of the midstream and downstream risers at angles 0° and 30°,which leads to the gradual reduction of the Strouhal numbers of the riser group and the sudden reduction in the dominant frequencies of the midstream and downstream risers at 0°.The boundary effect is mainly reflected in the 45°,60°,and 90° riser groups,which increases the vibration frequency as a whole.The interference effect of multiple fluids on the variable-angle riser group causes the intensity of the mode transition to be different,and modal transition causes the relative displacement of the 45° riser group to be greatest in the IL direction,and its vibration trajectory is deflected toward the incoming flow direction.The fatigue damage in CF direction is up to 1-2 orders of magnitude than that of the IL direction.The 45° and 60°-riser groups in the CF direction should be paid more special attention to the fatigue damage than other angles,and the dominant frequency with Strouhal number as the reference reaches the peak,and the vibration mode conversion marked by the traveling wave property is the strongest at 60°.For the variable-angle riser group,the CF and IL interference effects are significantly different,and the frequency interference ratio,displacement interference efficiency,and fatigue damage efficiency do not show consistency.At low reduced velocities,the multi-frequency input causes the riser fatigue loss to be greatly increased,and the interference effect of the fluid between the risers on the adjacent riser is higher.Therefore,the interference effect of wake feedback effect and gap flow at low reduced velocities can not be ignored. |
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