| As the dependence of mankind on crude oil is increasing,the mining progress of onshore oil gradually can not meet the needs of industrial development.The oil and gas exploration is shifting from the land to the ocean and from shallow water to deep water.In the offshore oil and gas exploration,the riser is the important equipment that connects the offshore platform to the submarine oil and gas wells.Due to the continuing effects of waves and currents,riser may be broken by vortex-induced-vibration(VIV),which may causes serious loss of economic,ecological and life.Therefore,it is of great engineering significance to study the hydrodynamics and VIV of the riser and multi-cylinder.Using the drilling riser as the engineering background,this paper has studied deeply the the flow interference and the VIV of multi-cylinder in the ocean structure.The main research works are as follows:(1)By embedding the module for solving the motion of the multi-cylinder and modifying the module for solving the dynamic mesh,this paper has completed the secondary development of the OpenFOAM and obtains a solver for simulating the flow interference and the VIV of the multi-cylinder.Based on the new OpenFOAM module,a lot of simulations on classical issues(flow past and VIV of single-cylinder and double-cylinder)are carried out.The numerical models and methods are validated by comparing with the experimental results or numerical results of other researchers.It is found that the models and numerical motheds adopted in this paper can accurately simulate the interference effect and VIV characteristics of multi-cylinder.(2)The flow past and the VIV of a real drilling riser,which is used for development of the oil and gas in the South China Sea,are simulated.The effect of incidence angle on the fluid force and vortex shedding process of the riser system is analyzed.The hydrodynamic characteristics of the multi-cylinder,multi-size and irregularly arranged cylinder system are preliminarily understood,which can provide the basis for further simplification of the model.It has been found that the auxiliary lines can effectively suppress the vortex shedding from the main line and reduce the lift and drag coefficients on the main line.As multiple auxiliary lines are located downstream of the main line,the main line may not shed vortex due to the clamping of the downstream auxiliary lines,where the lift coefficient on the main line may be negligible.The VIV model of the riser system with auxiliary line is established using the cylinder system moving synchronously.The influence of the auxiliary pipe on the vortex-induced vibration characteristics of the riser was analyzed.Due to the effect of the auxiliary lines,the upper branch does not appear in the VIV response of the real drilling riser.The VIV amplitude of the riser is much smaller than that of the single cylinder.As multiple auxiliary lines are located downstream of the main pipe,the VIV of the riser system is so weak that the riser almost remain stationary.(3)By placing a smaller control rod around the main cylinder,the cylinder system with just one control rod is modeled.The interferences between the control rod and the main cylinder at different position parameters are discussed by analyzing the hydrodynamic characteristics of the main cylinder.It is found that the influence of the single control rod on the hydrodynamic characteristics of the main cylinder can be divided into four regions.The drag and lift coefficients on the main cylinder are significantly reduced when the gap ratio and the position angle are small(such as G/D<0.7 and?<50~o).As the gap ratio is large enough(G/D>0.2)and the position angle is near 110~o,the effect of the position parameters on the drag and lift coefficients of the main cylinder is not obvious,and the drag and lift coefficients of the main cylinder are almost equal to those of the single cylinder;When the position angle is close to 180~o,the control rod can reduce the lift and drag coefficients on the main cylinder to a certain extent,and the effect vary little with different position parameters;As the gap ratio is small(G/D<0.2)and the position angle close to 110~o,the control rod can significantly increase the drag and lift coefficients of the main cylinder,and the effect of the control rod increases with decreasing gap ratio.The VIV of the cylinder system with a tandem(upstream or downstream)control rod is simulated using the cylinder system moving synchronously.The effect of the single control rod on the VIV characteristics of the cylinder system is discussed.The upstream tandem control rod does not affect the trend of the vibration amplitude of the cylinder system with the reduced volecity.At any gap ratio,the upstream tandem control rod can reduce the maximum VIV amplitude of the cylinder system,and the effect of the upstream tandem control rod is related to the gap ratio.The downstream tandem control rod can cause the sub-harmonic resonance of the VIV of the cylinder system.There are multiple discontinuous lock-in regimes in the VIV of the cylinder system.The larger the gap ratio is,the smaller the range of the second lock-in regime is.(4)By placing another control rod around the cylinder system with a single control rod,a new cylinder system model is established.By analyzing the difference of the hydrodynamic characteristics on the main cylinder,the combined effects of the two control rods in different configurations(Model 1:combination of upstream tandem control rod and staggered control rod;model 2:combination of downstream tandem control rod and staggered control rod;model 3:combination of symmetrical control rods)are discussed.Among these three arrangements,the control rods in model 2 can significantly reduce the lift and drag coefficients on the main cylinder,and the effect does not vary obviously with the changing position angle,which is more suitable to suppress the lift and drag coefficients on the main cylinder than the other models.The effect of the double control rods in model 1 on the hydrodynamic characteristics of the main cylinder can be considered as a linear superposition of the effect of the single control rod at any position angle and gap ratio,namely,the research results of the cylinder system with singe control rod are also applicable to model 1.In the other two models,the double control rods can cause additional effects,namely,the effect of the double control rods on the hydrodynamic characteristics of the main cylinder is significantly different from that of the single control rod.The VIV of the cylinder system with double control rods is simulated using the cylinder system moving synchronously.The effect of double control rods on the VIV characteristics of the cylinder system is discussed.The sub-harmonic resonance can occur in the VIV of the cylinder system with double control rods,and the smaller the gap ratio is,the more obvious the sub-harmonic resonance response is.In all these three models,the control rods can amplify the maximum amplitude of the cylinder system in the range of G/D<0.2&90~o<?<135~o,but the effects of the control rods in model 2(amplified to 285.9%of the maximum amplitude of the single cylinder)and model 3(amplified to 310.3%of the maximum amplitude of the single cylinder)is significantly better than that of the control rods in model 1(amplified to 141.4%of the maximal amplitude of the single cylinder).Both the control rods in model 1 and model 3 can effectively reduce the maximal amplitude of the cylinder system in the range of G/D<0.4&15~o<?<55~o,but the effect of the control rods in model 3(reduced to 1.4%of the maximal amplitude of the single cylinder)is better than that of the control rods in model 1(reduced to 39.9%of the maximal amplitude of the single cylinder).In model 2,the control rods in the range of50~o<?<90~o&0.6<G/D<1.5 and?>165~o&0.45<G/D<1.6 can effectively reduce the maximal amplitude of the cylinder system(reduced to 43.8%of the maximal amplitude of the single cylinder),but the effect is not as good as that of the control rods in model 1 and model 3.(5)Comparing the effects of the control rods in different configuration(single control rod,model 1,model 2 and model 3)on the hydrodynamic characteristics of the main cylinder and the VIV responses of the cylinder system,the law on the effect of the control rods can be summarized.First,both the single control rod and the double control rods can cause the sub-harmonic resonance in the VIV the cylinder system.Second,the effect of the double control rods is more pronounced than the single control rod at any position parameter(G/D and?).Third,the effect of the control rods in model 3 on the hydrodynamic characteristics of the cylinder system is significantly different from that of the single control rod,which can more significantly amplify or reduce the maximum amplitude of the VIV response of the cylinder system.Forth,the boundary layer control mothed can effectively increase the range of the lock-in regime and the vibration amplitude in the VIV of the cylinder system.As the gap ratio is small(such as G/D=0.3),the shear layer control method can cause the VIV of the cylinder system unstable,and effectively reduce the vibration amplitude of the cylinder system.As the gap ratio is large(such as G/D=0.7),the shear layer control method and the boundary layer control method have the similar influence on the VIV of the cylinder system,but the effect of the former is not as obvious as that of the latter.The coming flow control method can reduce the VIV amplitude of the cylinder system,but the effect of the reduction is not as obvious as the other control methods and is not sensitive on the varing position parameters.The wake stability control method may cause the VIV of the cylinderl system to appear multiple discontinuous lock-in regimes.(6)Based on multiple riser systems on TLP,a four-array cylinder system is used to model the risers on adjacent wells by samplifing the single riser as a single cylinder.By discussing the relationship between the hydrodynamic characteristcs of different cylinders,the interference among the flow around the stationary cylinders is discussed.The drag and lift coefficients of each cylinder are periodical in the range of G/D>1,and the periods on all cylinders are the same.The hydrodynamic characteristics of the side-by-side cylinders are always symmetrical about the center line.The phase relationship of the drag and lift coefficients on the tandem cylinders is related to the gap ratio,which maybe in-phase,out-of-phase and anti-phase respectively.The hydrodynamic characteristics on the upstream cylinders vary little with the gap ratio.The mean drag coefficient on the downstream cylinder increases with increasing gap ratio,while the lift coefficient amplitude decreases gradually.The VIV of four cylinders moving independently are simulates to study the interference of the adjacent risers on TLP.The VIV responses of the upstream cylinders are almost the same at different gap ratios.As the reduced velocity is small(U_r<14),the VIV responses of the downstream cylinders are not sensitive to the gap ratio.As the reduced velocity is large(U_r>14),the mean in-flow displacements of the downstream cylinders at G/D=3 are larger than those at G/D=5 and 7,while the cross-flow amplitudes of the downstream cylinders at G/D=3 and 5 are significantly larger than those at G/D=7.The VIV responses of the upstream and downstream cylinders are not essentially different at the gap ratios studied in this paper.Although the downstream cylinders have a larger lock-in regimes and VIV amplitudes than the upstream cylinders as the reduced velocity is large,but the maximum amplitudes of the upstream cylinders and the downstream cylinders are comparative. |
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