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Vortex-induced vibrations (VIVs) of a flexible cylinder subjected to multi-directional flows have been studied based on a wake oscillator model. The multi-directional flow comprises two slabs of flows in different directions, with each slab having a uniform uni-directional profile. The dynamics of the flexible cylinder is described based on the linear Euler–Bernoulli beam theory, and a wake oscillator model is uniformly distributed along the cylinder to model the hydrodynamic force acting on it. The dynamics of the coupled system has been solved numerically using the finite element method, and simulations have been conducted with the cylinder subjected to multi-directional flows with different angles between the two slabs. A large number of different initial conditions have been applied, and more than one steady-state response has been captured. The steady-state responses exhibit two different patterns: one is characterized by two waves traveling in opposite directions, while the other is dominated by a single traveling wave. The cross-flow VIV primarily occurs in the local cross-flow direction, and a transition of its vibrating direction happens at the interface of the two flows. Such transition is not observed in the inline VIV, and significant vibrations at the double frequency appear in both local cross-flow and inline directions. Energy analysis shows that this transition is boosted by a specific energy transfer pattern between the structure and the flow, which excites the vibration of the cylinder in some directions while damps it in others. [ABSTRACT FROM AUTHOR]

A frequency capture phenomenon in which the dominant response frequency of the downstream cylinder is the same as that of the upstream cylinder despite the differences in their physical characteristics was recently experimentally identified. The mechanism of this phenomenon is investigated by flow around two cylinders with unequal diameters undergoing flow-induced vibrations (FIV) using the open-source code OpenFOAM. Two FIV systems, a large stationary/vibrating upstream cylinder and vibrating downstream cylinder, are used for the simulation. The cylinders are free to vibrate in both the in-line and cross-flow directions. The forgetting factor least squares algorithm is applied for the time-varying excitation mechanism analysis. Simulation results show that the response of the downstream cylinder has a larger amplitude and contains multi-frequency components than the vortex-induced vibration of an isolated cylinder, making its trajectory more complicated. Simulations confirm that the frequency capture phenomenon is induced by the action of the upstream shedding vortex on the downstream cylinder. The energy transfers from vorticity to the structure when the frequency capture phenomenon occurs. The response of the downstream cylinder comprises a significant component of the vortex shedding frequency of the upstream cylinder. [ABSTRACT FROM AUTHOR]

In the present paper, the effects of external disturbances on cross-flow vortex-induced vibrations (VIVs) of a top tensioned riser are investigated based on a wake oscillator model. Two different flow conditions, namely uniform and linearly sheared profiles, and different combinations of disturbance frequency and amplitude were considered. Simulation results showed that synchronization between VIV and disturbance can occur when the disturbance frequency approaches the Strouhal frequency or when the disturbance amplitude is significant. This synchronization leads to a significant increase in riser response and the commonly used industry practice of simulating VIV and disturbance independently and summing both could underestimate the actual fatigue rate in such condition. However, in certain cases where VIV did not lock onto the disturbance, it was found that the disturbance could suppress the VIV and the industry practice tends to overestimate the fatigue rate. [ABSTRACT FROM AUTHOR]

• The hydrodynamics under a new energy competition force model in an oscillatory flow is studied. • The drag and excitation coefficients are inverse proportion to the KC number. • The added mass coefficients are independent of the KC number. • The empirical models for the drag and excitation coefficients in an oscillatory flow are developed. • A recommended value and modified curve for added mass coefficient in an oscillatory flow are established. The objective of this study is to reveal the characteristics of hydrodynamic force and coefficients undergoing vortex-induced vibration (VIV) of a flexible pipe in an oscillatory flow. The flexible pipe is forced to oscillate with different amplitudes and periods in the still water to simulate the equivalent oscillatory flow. Different from the traditional model, the hydrodynamic forces for the flexible pipe under the new energy competition force model in both in-line (IL) and cross-flow (CF) directions are divided into drag, excitation and added mass force. The identification methods of the hydrodynamic coefficients for the energy competition force model are then established. The detailed features of the hydrodynamic coefficients under the new force model in an oscillatory are illustrated. The distribution comparison of the two forces, i.e., drag force plays a role in energy dissipation and excitation force acts as energy input, explains the differences of the VIV response in magnitudes under different oscillatory flows. By summarizing the hydrodynamic coefficients, the drag and excitation coefficients are found to be inverse proportion to the Keulegan-Carpenter (KC) number. The independence of the added mass coefficients and KC number is revealed. On the basis of the observation, the empirical models for hydrodynamic coefficients are preliminarily proposed. The effects of the oscillatory flow on the hydrodynamics are established to be determined by the KC numbers under the same reduced velocity. The present investigations will provide a better guideline for developing flow-induced vibration prediction methods in future work. [Display omitted] [ABSTRACT FROM AUTHOR]

The existence of a bidirectionally sheared flow field caused by internal solitary waves has recently been confirmed according to a field survey in the South China Sea. A model test of a tensioned flexible pipe in bidirectionally sheared flow was performed in an ocean basin. The model was 28.41 mm in diameter and 7.64 m in length. The purpose of the model test was to understand the response performance and obtain benchmark data of vortex-induced vibration (VIV) in bidirectionally sheared flow. The test was performed on a rotating test rig to simulate bidirectionally sheared flow conditions. Fiber Bragg Grating (FBG) strain sensors were arranged along the test pipe to measure bending strains, and the modal analysis approach was used to determine the VIV response. Reduced velocities based on the tested first natural frequency in water reached 30.79. The cross-flow and in-line VIV amplitudes, response frequencies in statistics and time-domain analysis, traveling wave characteristics, phase synchronization and trajectories are presented in this article. The maximum root mean square (RMS) VIV amplitudes in the test reached 0.51 diameters and 0.18 diameters in the cross-flow and in-line directions, respectively. A relative low Strouhal number of 0.10 was found in the CF direction. • An experiment of a flexible pipe that was 28.41 mm in diameter and 269 diameters in length under bidirectionally sheared flow was conducted. The bending strain of VIV was measured in the CF and IL directions at 9 and 14 locations, respectively. A total of 58 test conditions with maximum flow velocities varying from 0.30 to 1.39 m/s were used in this experiment. • The RMS VIV response in the CF and IL directions reached maximum levels of 0.51 D and 0.18 D , respectively. The '2n/n' and '2n/2n-1' modal groups, modal groups '3/1' and '4/1' existed under low flow velocity due to the special flow profile. • The Strouhal numbers under the bidirectional sheared flow were approximately 0.10 and 0.24 in the CF and IL directions, respectively, which were smaller than the St numbers under uniform flow and linearly sheared flow. • A strong traveling wave and 'multi-frequency' response phenomenon were found, particularly in the IL directions. [ABSTRACT FROM AUTHOR]

Many unequal-diameter risers have been widely applied to satisfy different operational requirements in ocean oil and gas drilling engineering, and adjacent tandem risers can undergo complex vibration responses and even collisions under ocean flow conditions. To investigate the wake interference responses for multiple risers in tandem configuration, an experiment of double and triple unequal-diameter flexible cylinders was conducted under uniform flow with Reynolds numbers Re ranging from 2832 to 11,328 for the larger cylinder and from 1805 to 7220 for the smaller cylinder. The two cylinders were 8 m in length with aspect ratios equal to 282.49 and 443.21, respectively, and the corresponding diameter ratio was 0.64. Two different tandem configurations were arranged with a large cylinder upstream and downstream, and the wall spacing between adjacent cylinders was fixed at 4 D , where D is the diameter of small cylinders. Fiber Bragg grating strain sensors were used to measure both in-line (IL) and cross-flow (CF) strain responses. The modal superposition method was applied to reconstruct displacement responses based on measured strains, and drag forces were further inversely identified from the mean IL displacements. Equivalent wake velocities were calculated from drag force reductions. The results show that the downstream large cylinder in a triple-cylinder system endures a stronger wake interference effect than that in a double-cylinder system. Wake interference can decrease the dominant mode of downstream cylinders and increase the vibration amplitudes due to the reduction in the local wake velocity. In addition, a downstream cylinder undergoing wake-induced vibrations has the same dominant frequency in the CF and IL directions, and its CF frequency is identical to that of the upstream cylinder. • An experiment of double and triple unequal-diameter flexible cylinders in tandem was conducted, with entire models immersed vertically in water. In addition, the large and small cylinders were 8 m in length with aspect ratios equal to 282.49 and 443.21, respectively, and the corresponding diameter ratio was 0.64. • Compared to a double-cylinder system, the downstream large cylinder in a triple-cylinder system endures a stronger wake interference effect from the two upstream small cylinders. • Wake interference can decrease the downstream cylinders' dominant mode orders and increase their vibration amplitudes compared to a single cylinder, which results from the reduction of local wake velocity. • Upstream cylinders undergo classic VIV responses with a frequency ratio f IL /f CF of approximately 2.0, while the downstream cylinders may experience WIV responses with f IL /f CF equal to 1.0. In addition, the strong vortex shedding from an upstream cylinder can cause downstream SCs to vibrate at the same CF frequency as the upstream cylinder. [ABSTRACT FROM AUTHOR]

A modified wake oscillator model for modelling vortex-induced vibrations (VIVs) of a free-hanging riser subjected to vessel motion is presented. Different from the conventional wake oscillators which have typically utilized in the prediction of steady flow VIVs , the new model can be applied to simulate VIVs introduced by the vessel motion where the flows around the riser are not known in advance. This has been achieved by introducing an equivalent flow based on the relative flow velocity and coupling the wake oscillator with the acceleration of the riser in the lift direction. The modified wake oscillator model is tuned against the experiments of riser VIVs subjected to steady flows and a set of empirical parameters is obtained. The model is then applied to simulate the VIVs of a large-scale free-hanging water intake riser (WIR) subjected to different vessel motions and the simulated results are compared to those of experiments. The comparison shows that the proposed model is able to predict response frequency in good agreement with the experiment, and generates satisfactory prediction with respect to the response amplitude for three test cases out of five that have been studied in the present paper. • A modified wake oscillator model is proposed for vessel-motion induced VIV. • An equivalent flow and acceleration coupling in the lift direction are introduced. • The proposed model is able to predict response in good agreement with the experiment. [ABSTRACT FROM AUTHOR]

With the development of ocean engineering, freely hanging water intake riser (WIR) has aroused widespread interests. The WIR with one free end would be easier to deflect remarkably, along with more complex vortex-induced vibration (VIV) responses and hydrodynamic forces. To elucidate the VIV characteristics of WIR under uniform flow, a series of VIV experiments for a scaled WIR model are carried out. Then a hydrodynamic identification method for VIV along with large offset is developed. The response and hydrodynamic characteristics of the WIR in cross-flow (CF) and in-line (IL) directions are further analyzed. Results indicate that VIV displacements with higher exciting modes are unstable when uniform flow velocity gets large. The IL-to-CF dominant frequency ratio of VIV is found to be 1.0, not 2.0 in conventional VIV knowledge and the IL harmonic at nearly twice the dominant CF frequency is observed more complex for flow velocity greater than 0.3 m/s. The vortex-induced force coefficients are found to be time-varying. Moreover, it is found that within the spectrum of Reynolds number from 2 × 103 to 1.1 × 104, the drag coefficient distribution changes along the WIR model, and the average drag coefficient is in range of 1.0–1.5, which is slightly smaller than the results predicted by the other published empirical formulae. • A series of VIV experiments for a scaled water intake riser model were carried out. • A hydrodynamic identification method for VIV of water intake riser was developed. • The vortex-induced force coefficients of WIR under uniform flow were time-varying. • The drag coefficient is slightly smaller than the results of empirical formulae. [ABSTRACT FROM AUTHOR]

Different from the previous studies of the vortex-induced vibration (VIV) dominated by first mode of flexible pipe in an oscillatory flow, the features of a higher mode dominated are experimentally investigated in the ocean basin. The flexible pipe is forced to harmonically oscillate with different combinations of a period and amplitude. The design dominant mode consists of first and second modes under the maximum reduced velocity (VR) of approximately 5.5 with a KC number ranging from 22 to 165. The VIV responses between only the excited first mode and the excited higher mode are compared and studied using displacement reconstruction and wavelet transform methods. The discrepancies of spatial and temporal response between smaller and larger KC numbers (KC = 56 and 121) are first observed. The strong alternate mode dominance and lock-in phenomena occur in the case of larger KC numbers, while they cannot be observed in the case of smaller KC numbers under higher modes. The VIV dominant frequency in the in-line (IL) direction is found to be always triple the oscillatory flow frequency and not twice that in the cross flow (CF) direction. The dominant frequency in the CF direction can be predicted by the Strouhal law, and the Strouhal number is approximately 0.18 under VR = 5.5, which is not affected by the excited mode. Moreover, differences of response motion trajectory are also revealed in this paper. The present work improves the basic understanding of vessel motion induced VIV and provides helpful references for developing prediction methods of VIV in an oscillatory flow. [ABSTRACT FROM AUTHOR]

The Norwegian Public Road Administration is planning to upgrade Coastal Highway E39 by replacing ferry connections with floating bridges or submerged floating tunnels (SFTs). This study considers a potential pontoon-supported curved SFT designed for crossing Sognefjorden at a submergence of 12 m. It consists of two identical tubes with a diameter of 12.6 m each in a tandem configuration and with a length of approximately 4 km. The natural frequencies of the low-order modes are well within the energy content in the spectra of the second-order difference-frequency wave excitation forces and the vortex shedding-induced forces. In this paper, numerical simulation of wave-induced hydroelastic response and flow-induced vibrations of the twin-tube SFT is performed. Long- and short-crested waves, the first and second order wave loads, are considered. A time-domain approach to simulate crossflow vortex-induced vibration (VIV) and VIV-amplified inline drag forces, partly based on the coefficients obtained experimentally, is established and applied. The focus is on extreme conditions – relating to ultimate strength limit states. The second-order wave load substantially affects the lateral motion and lateral bending moment, as expected. The short-crested waves influence the response in both the lateral and vertical directions by exciting asymmetric eigenmodes. In strong flow conditions, once VIV is excited, the standard deviation of the vertical motion (of about 30% of the diameter) and the bending moment about the horizontal axis is more that an order of magnitude larger than that induced by the wave loads. The simulation of the wave- and flow-induced load effects provides a good reference for the design of SFTs. • VIV of a twin-tube submerged floating tunnel is analyzed by a time-domain method using self-oscillation experimental data. • The contributions from different components of the wave excitation forces on the hydroelastic response are investigated. • A new understanding on the comparison between the wave-induced hydroelasticity and flow-induced vibration is achieved. [ABSTRACT FROM AUTHOR]

The features of hydrodynamic coefficients including drag and added mass coefficients on a flexible pipe fitted with helical strakes in an oscillatory flow are studied experimentally. The experiment of a flexible straked pipe was conducted in an oscillatory flow with the Keulegan-Carpenter (KC) number varying from 9 to 165 and the maximum reduced velocities ranging from 4 to 8. Strain response in both in-line and cross-flow directions are measured by fiber bragg grating sensors. Using displacement reconstruction and inverse analysis methods, displacement response and hydrodynamic force of the straked pipe are identified. Then, through the least squares method, corresponding drag and added mass coefficients are extracted. The asymmetric features at the acceleration and deceleration stages of hydrodynamic force on a flexible pipe in the in-line direction under oscillatory flow are first observed. Compared with a bare pipe, helical strakes can effectively reduce the higher frequency fluctuating force and enhance the wake effects. The hydrodynamic coefficients on the straked pipe are significantly magnified in the case of a relatively small KC number. The maximum mean drag coefficients can reach approximately 10. The presented work suggests that the risk of helical strakes application should be fully evaluated through flexible pipe experiments in both steady flow and unsteady flow. • The hydrodynamics of flexible pipe fitted helical strakes in an oscillatory flow are first revealed. • Asymmetric features of hydrodynamic force at the acceleration and deceleration stages are first observed. • Significantly amplification of drag coefficients for the straked pipes was found. • Helical strakes covered on riser may have an extremely adverse influence. [ABSTRACT FROM AUTHOR]

The cross-flow vortex-induced vibration features of a submerged floating tunnel element, which is composed of two rigidly connected cylinders in a tandem configuration, were investigated via a self-oscillation model test in a steady flow. The Reynolds number ranged from 2 × 104 to 9 × 104, and the ratio of the center-to-center distance between the two cylinders and cylinder diameter varied within a range of 2-4. The vortex induced vibration responses and lift forces on the up- and downstream cylinders were studied under different spacing ratios and compared with those on a single cylinder. The results show that the spacing ratio plays an important role in VIV until the ratio reaches 4. For a small spacing ratio, a significant difference between the lift forces on the up- and downstream cylinders appears and induces a large torsional moment. For the convenience of engineering application, a torsional coefficient was proposed. The maximum torsional coefficient can reach 2.9, 1.2 and 0.98 for spacing ratios of 2, 3 and 4 at the reduced velocity of 5, respectively. Considering the vortex induced vibration responses as well as the torsional moment, a spacing ratio of 3 was recommended for tandem floating tunnel design. • Self-oscillation test of a twin-tube submerged floating tunnel segment model. • Lift forces on stationary and self-oscillation twin-cylinder are compared. • A torsional moment is induced due to the difference of lift force on the cylinders. • Spacing ratio heavily affects the vortex induced response, lift and torsion. [ABSTRACT FROM AUTHOR]

Helical strakes are widely used to suppress vortex-induced vibration (VIV) in offshore engineering. However, the VIV response and suppression efficiency of the straked pipe remain unclear in an oscillatory flow. In this paper, an experimental study of VIV for a flexible pipe fitted with helical strakes was conducted in an oscillatory flow with the Keulegan-Carpenter (KC) number varying from 21 to 165 and maximum reduced velocities ranging from 4 to 12. The effects of the helical strakes on the VIV response, Strouhal number, suppression efficiency and fatigue damage in an oscillatory flow were investigated. The results show that the suppression efficiency and fatigue damage reduction ratio are not as ideal in oscillatory flow as those in steady flow. Moreover, the VIV dominant frequency is obviously magnified and a large Strouhal number is observed, reaching 0.439 under a lower reduced velocity of 4. As the reduced velocity further increases to 6, 8, 12, two branches of Strouhal numbers are clearly seen. The dominant frequency decreases at the first branch with a small KC number and increases at the second branch with a large KC number. •An experimental study of a flexible pipe fitted with helical strakes in oscillatory flow was first conducted. •Under a lower reduced velocity (V R = 4), the helical strakes can significantly increase the VIV dominant frequency and the Strouhal number can reach 0.439. •Under a higher reduced velocity, two distinct branches in the variation of the St number against the KC number are observed. • The suppression efficiency of helical strake in an oscillatory flow is not as high as that under steady flow. [ABSTRACT FROM AUTHOR]