The dynamics of long slender cylinders undergoing vortex-induced vibrations (VIV) is studied in this work. Long slender cylinders such as risers or tension legs are widely used in the field of ocean engineering. When ...The dynamics of long slender cylinders undergoing vortex-induced vibrations (VIV) is studied in this work. Long slender cylinders such as risers or tension legs are widely used in the field of ocean engineering. When the sea current flows past a cylinder, it will be excited due to vortex shedding. A three-dimensional time domain model is formulated to describe the response of the cylinder, in which the in-line (IL) and cross-flow (CF) deflections are coupled. The wake dynamics, including in-line and cross-flow vibrations, is represented using a pair of non-linear oscillators distributed along the cylinder. The wake oscillators are coupled to the dynamics of the long cylinder with the acceleration coupling term. A non-linear fluid force model is accounted for to reflect the relative motion of cylinder to current. The model is validated against the published data from a tank experiment with the free span riser. The comparisons show that some aspects due to VIV of long flexible cylinders can be reproduced by the proposed model, such as vibrating frequency, dominant mode number, occurrence and transition of the standing or traveling waves. In the case study, the simulations show that the IL curvature is not smaller than CF curvature, which indicates that both IL and CF vibrations are important for the structural fatigue damage.展开更多
An experimental study on a bare flexible cylinder as well as cylinders fitted with two types of cross-sectioned helical strakes was carried out in a towing tank. The main purpose of this paper is to investigate the ef...An experimental study on a bare flexible cylinder as well as cylinders fitted with two types of cross-sectioned helical strakes was carried out in a towing tank. The main purpose of this paper is to investigate the effects of strakes’ cross-section on the vortex-induced vibrations （VIV） suppression of a flexible cylinder. The square-sectioned and round-sectioned helical strakes were selected in the experimental tests. The uniform current was generated by towing the cylinder models along the tank using a towing carriage. The Reynolds number was in the range of 800–16000. The strain responses were measured by the strain gages in cross-flow （CF） and in-line （IL） directions. A modal analysis method was adopted to obtain the displacement responses using the strain signals in different measurement positions. The comparison of the experimental results among the bare cylinder, square-sectioned straked cylinder and round-sectioned straked cylinder was performed. The helical strakes can effectively reduce the strain amplitude, displacement amplitude, response frequencies and dominant modes of a flexible cylinder excited by VIV. And the mean drag coefficients of straked cylinders were approximately consistent with each other. In addition, the square-sectioned and round-sectioned strakes nearly share the similar VIV reduction behaviors. Sometimes, the strakes with round-section represent more excellent effects on the VIV suppression of response frequency than those with square-section.展开更多
This paper aimed at describing numerical simulations of vortex-induced vibrations（VIVs） of a long flexible riser with different length-to-diameter ratio（aspect ratio） in uniform and shear currents. Three aspect ra...This paper aimed at describing numerical simulations of vortex-induced vibrations（VIVs） of a long flexible riser with different length-to-diameter ratio（aspect ratio） in uniform and shear currents. Three aspect ratios were simulated： L/D= 500, 750 and 1 000. The simulation was carried out by the in-house computational fluid dynamics（CFD） solver viv-FOAM-SJTU developed by the authors, which was coupled with the strip method and developed on the OpenFOAM platform. Moreover, the radial basis function（RBF） dynamic grid technique is applied to the viv-FOAM-SJTU solver to simulate the VIV in both in-line（IL） and cross-flow（CF） directions of flexible riser with high aspect ratio. The validation of the benchmark case has been completed. With the same parameters, the aspect ratio shows a significant influence on VIV of a long flexible riser. The increase of aspect ratio exerted a strong effect on the IL equilibrium position of the riser while producing little effect on the curvature of riser. With the aspect ratio rose from 500 to 1 000, the maximum IL mean displacement increased from 3 times the diameter to 8 times the diameter. On the other hand, the vibration mode of the riser would increase with the increase of aspect ratio. When the aspect ratio was 500, the CF vibration was shown as a standing wave with a 3-（rd） order single mode. When the aspect ratio was 1 000, the modal weights of the 5-（th） and 6-（th） modes are high, serving as the dominant modes. The effect of the flow profile on the oscillating mode becomes more and more apparent when the aspect ratio is high, and the dominant mode of riser in shear flow is usually higher than that in uniform flow. When the aspect ratio was 750, the CF oscillations in both uniform flow and shear flow showed multi-mode vibration of the 4-（th） and 5-（th） mode. While, the dominant mode in uniform flow is the 4-（th） order, and the dominant mode in shear flow is the 5-（th） order.展开更多
A Nonlinear Fluid Damping （NFD） in the form of the square-velocity is applied in the response analysis of Vortex-Induced Vibrations （VIV）. Its nonlinear hydrodynamic effects on the coupled wake and structure oscil...A Nonlinear Fluid Damping （NFD） in the form of the square-velocity is applied in the response analysis of Vortex-Induced Vibrations （VIV）. Its nonlinear hydrodynamic effects on the coupled wake and structure oscillators are investigated. A comparison between the coupled systems with the linear and nonlinear fluid dampings and experiments shows that the NFD model can well describe response characteristics, such as the amplification of body displacement at lock-in and frequency lock-in, both at high and low mass ratios. Particularly, the predicted peak amplitude of the body in the Griffin plot is in good agreement with experimental data and empirical equation, indicating the significant effect of the NFD on the structure motion.展开更多
The fatigue life of top tensioned risers under vortex-induced vibrations (VIVs) with consideration of the effect of internal flowing fluid on the riser is analyzed in the time domain.The long-term stress histories of ...The fatigue life of top tensioned risers under vortex-induced vibrations (VIVs) with consideration of the effect of internal flowing fluid on the riser is analyzed in the time domain.The long-term stress histories of the riser under VIVs are calculated and the mean stresses,the number of stress cycles and amplitudes are determined by the rainflow counting method.The Palmgren-Miner rule for cumulative damage theory with a specified S-N curve is used to estimate the fatigue life of the riser.The corresponding numerical programs numerical simulation of vortex-induced vibrations (NSVIV) which can be used to calculate the VIV response and fatigue life of the riser are compiled.Finally the influences of the riser's parameters such as flexural rigidity,top tension and internal flow velocity on the fatigue life of the riser are analyzed in detail and some conclusions are drawn.展开更多
The vortex-induced vibrations(VIV)is an important topic of study in many different scientific and engineering fields.While VIV can be of benefit in some cases,oftentimes,it is an undesirable phenomenon that can be qui...The vortex-induced vibrations(VIV)is an important topic of study in many different scientific and engineering fields.While VIV can be of benefit in some cases,oftentimes,it is an undesirable phenomenon that can be quite dangerous.In particular for offshore pipelines,VIV can lead to fatiguing of the pipe structure and can cause disastrous consequences if left unchecked.A number of different methods have been applied to the measurement of VIV,especially for the elongated,thin cylindrical structures.The use of fiber optic fiber Bragg gratings(FBGs)in particular has gained popularity over the recent years due to their distinct properties.However,FBGs are also very fragile and are susceptible to failure when placed in harsh environments without protection.In this paper,56 FBGs encapsulated in stainless steel tubes were applied to the measurement of VIV in a28-m model pipeline under controlled and uncontrolled conditions.Tests show that the encapsulated sensors possessed good sensitivity as well as fatigue life(>80000 cycles).The measurements from FBGs were also high enough to allow frequency domain analysis of the pipeline VIV under the two conditions.The authors conclude that the encapsulated FBGs are a viable tool for the study of VIV in pipeline structures.展开更多
The Generalized Integral Transform Technique （GITT） was applied to predict dynamic response of Vortex-Induced Vibration （VIV） of a long flexible cylinder. A nonlinear wake oscillator model was used to represent th...The Generalized Integral Transform Technique （GITT） was applied to predict dynamic response of Vortex-Induced Vibration （VIV） of a long flexible cylinder. A nonlinear wake oscillator model was used to represent the cross-flow force acting on the cylinder, leading to a coupled system of second-order Partial Differential Equations （PDEs） in temporal variable. The GITT approach was used to transform the system of PDEs to a system of Ordinary Differential Equations （ODEs）, which was numerically solved by using the Adams-Moulton and Gear method （DIVPAG） developed by the International Mathematics and Statistics Library （IMSL）. Numerical results were presented for comparison to those given by the finite difference method and experimental results, allowing a critical evaluation of the technique performance. The influence of variation of mean axial tension induced by elongation of flexible cylinder was evaluated, which was shown to be not negligible in numerical simulation of VIV of a long flexible cylinder.展开更多
基金Supported by the National Natural Science Foundation of China (Grant No 10532070)the Knowledge Innovation Program of Chinese Academy of Sciences (Grant No KJCX2-YW-L07)the LNM Initial Funding for Young Investigators
文摘The dynamics of long slender cylinders undergoing vortex-induced vibrations (VIV) is studied in this work. Long slender cylinders such as risers or tension legs are widely used in the field of ocean engineering. When the sea current flows past a cylinder, it will be excited due to vortex shedding. A three-dimensional time domain model is formulated to describe the response of the cylinder, in which the in-line (IL) and cross-flow (CF) deflections are coupled. The wake dynamics, including in-line and cross-flow vibrations, is represented using a pair of non-linear oscillators distributed along the cylinder. The wake oscillators are coupled to the dynamics of the long cylinder with the acceleration coupling term. A non-linear fluid force model is accounted for to reflect the relative motion of cylinder to current. The model is validated against the published data from a tank experiment with the free span riser. The comparisons show that some aspects due to VIV of long flexible cylinders can be reproduced by the proposed model, such as vibrating frequency, dominant mode number, occurrence and transition of the standing or traveling waves. In the case study, the simulations show that the IL curvature is not smaller than CF curvature, which indicates that both IL and CF vibrations are important for the structural fatigue damage.
基金supported by the National Natural Science Foundation of China(Grant Nos.51479135,51525803 and 51679167)the Science Fund for Creative Research Groups of the National Natural Science Foundation of China(Grant No.51621092)+1 种基金the Major State Basic Research Development Program of China(973 Program,Grant No.2014CB046801)the Natural Science Foundation of Tianjin(Grant No.15JCQNJC07700)
文摘An experimental study on a bare flexible cylinder as well as cylinders fitted with two types of cross-sectioned helical strakes was carried out in a towing tank. The main purpose of this paper is to investigate the effects of strakes’ cross-section on the vortex-induced vibrations （VIV） suppression of a flexible cylinder. The square-sectioned and round-sectioned helical strakes were selected in the experimental tests. The uniform current was generated by towing the cylinder models along the tank using a towing carriage. The Reynolds number was in the range of 800–16000. The strain responses were measured by the strain gages in cross-flow （CF） and in-line （IL） directions. A modal analysis method was adopted to obtain the displacement responses using the strain signals in different measurement positions. The comparison of the experimental results among the bare cylinder, square-sectioned straked cylinder and round-sectioned straked cylinder was performed. The helical strakes can effectively reduce the strain amplitude, displacement amplitude, response frequencies and dominant modes of a flexible cylinder excited by VIV. And the mean drag coefficients of straked cylinders were approximately consistent with each other. In addition, the square-sectioned and round-sectioned strakes nearly share the similar VIV reduction behaviors. Sometimes, the strakes with round-section represent more excellent effects on the VIV suppression of response frequency than those with square-section.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51490675,51379125,11432009 and 51579145)
文摘This paper aimed at describing numerical simulations of vortex-induced vibrations（VIVs） of a long flexible riser with different length-to-diameter ratio（aspect ratio） in uniform and shear currents. Three aspect ratios were simulated： L/D= 500, 750 and 1 000. The simulation was carried out by the in-house computational fluid dynamics（CFD） solver viv-FOAM-SJTU developed by the authors, which was coupled with the strip method and developed on the OpenFOAM platform. Moreover, the radial basis function（RBF） dynamic grid technique is applied to the viv-FOAM-SJTU solver to simulate the VIV in both in-line（IL） and cross-flow（CF） directions of flexible riser with high aspect ratio. The validation of the benchmark case has been completed. With the same parameters, the aspect ratio shows a significant influence on VIV of a long flexible riser. The increase of aspect ratio exerted a strong effect on the IL equilibrium position of the riser while producing little effect on the curvature of riser. With the aspect ratio rose from 500 to 1 000, the maximum IL mean displacement increased from 3 times the diameter to 8 times the diameter. On the other hand, the vibration mode of the riser would increase with the increase of aspect ratio. When the aspect ratio was 500, the CF vibration was shown as a standing wave with a 3-（rd） order single mode. When the aspect ratio was 1 000, the modal weights of the 5-（th） and 6-（th） modes are high, serving as the dominant modes. The effect of the flow profile on the oscillating mode becomes more and more apparent when the aspect ratio is high, and the dominant mode of riser in shear flow is usually higher than that in uniform flow. When the aspect ratio was 750, the CF oscillations in both uniform flow and shear flow showed multi-mode vibration of the 4-（th） and 5-（th） mode. While, the dominant mode in uniform flow is the 4-（th） order, and the dominant mode in shear flow is the 5-（th） order.
基金Project supported by the National High Techology Research and Development Program of China (863 Program, Grant No2006AA09Z350)the Chinese Academy of Sciences (Grant No KJCX2-YW-L02)
文摘A Nonlinear Fluid Damping （NFD） in the form of the square-velocity is applied in the response analysis of Vortex-Induced Vibrations （VIV）. Its nonlinear hydrodynamic effects on the coupled wake and structure oscillators are investigated. A comparison between the coupled systems with the linear and nonlinear fluid dampings and experiments shows that the NFD model can well describe response characteristics, such as the amplification of body displacement at lock-in and frequency lock-in, both at high and low mass ratios. Particularly, the predicted peak amplitude of the body in the Griffin plot is in good agreement with experimental data and empirical equation, indicating the significant effect of the NFD on the structure motion.
基金supported by the High Technology Research and Development Program of China (863 Program,Grant Nos.2006AA09Z356 and 2007AA09Z313)
文摘The fatigue life of top tensioned risers under vortex-induced vibrations (VIVs) with consideration of the effect of internal flowing fluid on the riser is analyzed in the time domain.The long-term stress histories of the riser under VIVs are calculated and the mean stresses,the number of stress cycles and amplitudes are determined by the rainflow counting method.The Palmgren-Miner rule for cumulative damage theory with a specified S-N curve is used to estimate the fatigue life of the riser.The corresponding numerical programs numerical simulation of vortex-induced vibrations (NSVIV) which can be used to calculate the VIV response and fatigue life of the riser are compiled.Finally the influences of the riser's parameters such as flexural rigidity,top tension and internal flow velocity on the fatigue life of the riser are analyzed in detail and some conclusions are drawn.
基金supported by the Science Fund for Creative Research Groups of the National Natural Science Foundation of China(Grand No.51121005)the National Natural Science Foundation of China(Grand No.51108059)+2 种基金the Special Fund for Basic Research on Scientific Instruments of the National Natural Science Foundation of China(Grand No.51327003)the Special Project of China Earthquake Administration(Grand No.2015419014)China Scholarship Council(Grand No.201206060081)
文摘The vortex-induced vibrations(VIV)is an important topic of study in many different scientific and engineering fields.While VIV can be of benefit in some cases,oftentimes,it is an undesirable phenomenon that can be quite dangerous.In particular for offshore pipelines,VIV can lead to fatiguing of the pipe structure and can cause disastrous consequences if left unchecked.A number of different methods have been applied to the measurement of VIV,especially for the elongated,thin cylindrical structures.The use of fiber optic fiber Bragg gratings(FBGs)in particular has gained popularity over the recent years due to their distinct properties.However,FBGs are also very fragile and are susceptible to failure when placed in harsh environments without protection.In this paper,56 FBGs encapsulated in stainless steel tubes were applied to the measurement of VIV in a28-m model pipeline under controlled and uncontrolled conditions.Tests show that the encapsulated sensors possessed good sensitivity as well as fatigue life(>80000 cycles).The measurements from FBGs were also high enough to allow frequency domain analysis of the pipeline VIV under the two conditions.The authors conclude that the encapsulated FBGs are a viable tool for the study of VIV in pipeline structures.
基金financial support provided by CNPq,CAPES and FAPERJ ofBrazil for their research workfinancial support provided by China Scholarship Council
文摘The Generalized Integral Transform Technique （GITT） was applied to predict dynamic response of Vortex-Induced Vibration （VIV） of a long flexible cylinder. A nonlinear wake oscillator model was used to represent the cross-flow force acting on the cylinder, leading to a coupled system of second-order Partial Differential Equations （PDEs） in temporal variable. The GITT approach was used to transform the system of PDEs to a system of Ordinary Differential Equations （ODEs）, which was numerically solved by using the Adams-Moulton and Gear method （DIVPAG） developed by the International Mathematics and Statistics Library （IMSL）. Numerical results were presented for comparison to those given by the finite difference method and experimental results, allowing a critical evaluation of the technique performance. The influence of variation of mean axial tension induced by elongation of flexible cylinder was evaluated, which was shown to be not negligible in numerical simulation of VIV of a long flexible cylinder.
