| Accelerating the development of offshore oil and gas resources is of strategic significance for reducing China’s dependence on foreign oil and gas,alleviating the energy crisis and ensuring the healthy development of national economy.Marine Riser,connecting subsea wellhead and offshore floating production platform,is the key equipment and fragile component in the development of deep-sea oil and gas resources.Vortex-induced vibration(VIV)is an important factor causing fatigue damage of riser,which seriously threatens structural safety.It is necessary to investigate the vortex-induced vibration characteristics of riser and suppress the vibration.At present,most domestic and foreign researches on VIV of riser assume that the incoming flow direction is perpendicular to the axial direction of riser and only subjected to constant pretension.In practical engineering,risers face complex environmental conditions.The direction of incoming flow is not always perpendicular to the axial direction of the riser,and the axial tension at the top usually changes periodically.The VIV characteristics and suppression methods of risers under inclined flow and time-varying axial force remain to be further studied.With the support of The National Natural Science Foundation of China,this paper conducted a systematic study on the characteristics and suppression of vortexinduced vibration of riser under the condition of inclined flow and time-varying axial force.The main research contents,innovations and conclusions are as follows:(1)Model tests on VIV of riser under the condition of inclined flow was conducted to reveal the effect of the inclined angle on VIV dynanmic and hydrodynamic characteristics.The application scope of the independence principle(IP)is given for desin and analysis.When the inclined angle exceeds 30°,the higher order mode VIV is more easily excited,and the higher the inclined angle,the lower the equivalent reduction velocity corresponding to the higher order mode excitation.The vibration displacement does not decrease with the increase of inclined angle.The movement trajectory of the riser is generally "8" shape and crescent shape,and elliptical trajectory appears in specific conditions.When the inclined angle is less than15°,the VIV fluid force is less affected.When the inclined angle exceeds 30°,the hydrodynamic characteristics change significantly.The empirical formula of average resistance coefficient is fitted to provide guidance for the design of riser.Considering the influence of inclined angle on VIV response and fluid force,the IP is valid when inclined angle is less than 30°.When the inclined angle exceeds 30°,the dominant mode,vibration displacement and hydrodynamic force coefficient change,and the IP is not applicable.(2)The dynamic characteristics of the riser fitted with helical strakes and control rods under inclined flow were experimentally studied to reveal the influence of inclined angle on the VIV suppression effect of helical strakes and control rods,providing basis for VIV suppression of marine risers.Under the inclined flow,the helical strakes can reduce the in-line(IL)response frequency,but it has limited effect on the cross-flow(CF)response frequency.The response frequencies in CF and IL directions are close,and the motion trajectory is usually elliptic.With the increase of inclined angle,the suppression effect of helical strakes decreases significantly.The suppression efficiency in the CF and IL direction can reduce to 50% and 20%,respectively.Regardless of vertical flow or inclined flow,the control rod can effectively reduce the vibration displacement.With the increase of equivalent reduction velocity,the suppression effect is more significant.When the mounting angle of the control rods is 45°,the suppression efficiency of vibration displacement is the highest,and the suppression efficiency does not decrease with the increase of inclined angle.The suppression efficiency in the CF and IL direction can reach 80%and 90%,respectively.(3)Model tests on VIV of the riser under the combined excitation of timevarying axial force and vortex was carried out.The effects of amplitude ratio and frequency ratio on the VIV were systematically investigated,and the mechanism of the parametric excitation and vortex excitation was revealed.When the reduced velocity is low,VIV is not excited,but the axial excitation excites the vibration of the riser,and the vibration amplitude increases with the increase of the amplitude ratio.The higher mode vibration can be excited earlier,and the participation rate of each mode was greatly affected by the frequency ratio.The multi-mode vibration features are significant.The vibration of riser has amplitude modulation,which is more significant with the increase of frequency ratio.When parametric resonance is not excited,the dominant frequency is the vortex excitation frequency,and the harmonic components are mainly the sum frequency and difference frequency.When parametric resonance is excited,the energies of parametric resonance and the vortex excitation are concentrated at a same frequency,the harmonic components of the sum frequency and difference frequency disappear.The peak value of the vibration displacement increases 65%。(4)The dynamic characteristics of riser fitted with helical strakes under timevarying axial force were experimentally studied to reveal the effects of amplitude ratio and frequency ratio on the suppression effect of the helical strakes.With the increase of amplitude ratio,the suppression effect of helical strakes decreases.When the reduced velocity is low,the vibration of the riser with helical strakes can excite higher order modes due to parametric resonance.With the increase of reduced velocity,the axial excitation frequency component and the vortex frequency component are more significant.Due to the coupling of parametric excitation and vortex excitation,sum and difference harmonic components appear.A suppression method based on energy flow was proposed,and a theoretical analysis model of vibration suppression was established to verify the method.By wrapping damping material in the energy dissipation area,the increase of damping makes the energy attenuation ratio reach the critical value of 2.3,and the energy transmitted to the energy dissipation area is dissipated,which can effectively suppress the vibration of the riser. |
PDF Full Text Request