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In order to study the response law of vortex-induced vibration (VIV) of marine risers under the combined action of roughness and interference effects, and to reveal the coupling mechanism of roughness and interference effects on the riser, a VIV experiment of rough risers in tandem arrangement was conducted in a wave–current combined flume. The experiment characterized the risers' roughness by arranging different specifications of attachments on the surface of the risers. Three rough risers with different roughness and smooth risers were arranged in tandem arrangement, with the rough risers arranged downstream. The experimental results indicate that the suppression of the attachments on the downstream risers' vibration are more significant both in the CF and IL directions as the reduced velocity increases. For the downstream riser, the amplitude response of rough riser is more significantly weakened compared with the smooth one at high reduced velocity. For the upstream risers, changes in the roughness and spacing ratio have an impact on their 'lock-in' region. When the roughness of downstream risers is relatively large (0.1300) and the spacing between risers is small (S/D=4.0), the reduced velocity range of 'lock-in' region in the CF direction of upstream risers is obviously expanded, and the displacement in the 'lock-in' region is severer. [ABSTRACT FROM AUTHOR]

This paper proposes a novel wake-induced vibration (WIV)-based energy harvesting system consisting of two bluff bodies. An inverted C-shaped bluff body is stationary installed at the upstream position to generate an interference wake street, and a cylinder bluff body equipped with a transducer is elastically suspended at the downstream position to harness WIV energy. The hydrodynamics and energy harvesting (EH) performance of the proposed system are investigated via experimental studies. The reduced velocity (U*) ranging from 2 to 14 (the corresponding Reynolds number ranging from 15100 to 106200) is considered in the present study. It is found that the wake generated by the inverted C-shaped bluff body significantly affects the EH performance. Enlarging the opening angle (α) of the C-shaped bluff body increases the vibration amplitude of the downstream cylinder, thereby increasing the harvested power. When the spacing (L) between two bluff bodies is two times the cylinder diameter (D), the wake-induced vibration (WIV) mode is observed, while the combined WIV and wake galloping (WG) mode occurs when α is 150°, and L equals 3D or 4D. The average drag coefficient becomes negative when L is 2D, 3D, or 4D. By carefully configuring a C-shaped bluff body, the wake generated by it can bring an augmenting effect on the vibration of the downstream EH cylinder. For example, the RMS power output of the proposed EH system reaches a maximum of 0.31 W at U* = 8 and L = 4D, which is 300% greater than that of its traditional counterpart. Furthermore, after a number of case studies, it is identified that the proposed EH system can achieve the best performance when α is 150° and L = 2D. [ABSTRACT FROM AUTHOR]

A vortex-induced vibration (VIV) experiment of rough risers with coupling interference effect under a side-by-side arrangement was carried out in a wave-current combined flume. The roughness of the riser was characterized by arranging different specifications of surface attachments on the surface of the riser. Rough risers with three different roughnesses were arranged side by side with smooth risers to explore the VIV response of the riser under the combined action of roughness and interference effect, and to reveal the coupling mechanism between roughness and interference effect. The experimental results show that, compared with that of a smooth riser, the VIV of a rough riser under the coupling interference effect has a wider "lock-in" region, and the displacement decreases more significantly at a high reduced velocity, which is more likely to excite higher-order modes and frequency responses. In addition, the displacement response and frequency response of the smooth riser are not significantly affected by wake interference from the rough riser, which is caused by the decrease of the wake region due to the delay of the boundary layer separation point of the rough riser. [ABSTRACT FROM AUTHOR]

A partition model of interference efficiency was constructed to study the coupling interference effect under combined internal and external flow. The concept of "internal flow efficiency", "velocity ratio" and "interference efficiency" were introduced to quantify the effect of internal flow and interference, and reveal the coupling mechanism among internal flow, external flow and interference effect. The results showed that the dynamic response of risers under variable angles was significantly different after considering the effect of internal flow. When the external flow velocity was smaller than 0.25 m/s, the vibration of risers was promoted by the internal flow. With the increase of external flow velocity, the effect of internal flow was weakened and the dynamic response of riser mainly depended on the external flow and interference effect. Under the effect of different internal flow, the interference efficiency had similar change trend. The interference effect amplified the complex secondary flow effect inside the riser, making the dynamic response of riser complex and random. In this paper, the overlap area and subdivision criterion of interference effect were constructed within the range of experimental velocity ratio, and the change curve of interference efficiency was obtained with an average meaning, which may have important practical meaning. [ABSTRACT FROM AUTHOR]

Taking the three-riser group arranged in tandem as the research subject, an experimental study was carried out on the risers arranged in tandem. The purpose is to explore the sensitivity of the dynamic response of each riser to spacing ratio and reveal the physical mechanism of riser groups under the interference effect. The spacing ratios of the adjacent risers are 4.0, 5.0, 6.0, and 8.0. At the spacing between the risers of 4.0D, the strong feedback effect increases the cross-flow (CF) displacement amplitude of the upstream riser. The shielding effect is the key factor affecting the interference effect on the midstream and downstream risers. At low reduced velocities, the shielding area initially appears, the displacement amplitude of the midstream and downstream risers varies greatly, the vibration of the two risers is still dominated by the first-order mode, and the transition between adjacent vibration modes is restrained. The multi-frequency superposition phenomenon is very significant at high reduced velocities. The most sensitive interference spacing under the test conditions is obtained. Due to the separation of the incoming flow and the double shielding effect of the upstream and midstream risers, the regular vortex-induced vibration in the wake area of the downstream riser is broken, and the vibration in the two directions is weakened. In general, the interference effect is more significant for the CF vibration of the three-riser groups than the in-line (IL) vibration. [ABSTRACT FROM AUTHOR]

In offshore engineering, the phenomenon of free span often occurs, and the pipeline may have multiple free spans adjacent to each other, forming a multi-span pipeline. The interaction of different spans makes the structural vibration characteristics more complex, which may change the fatigue characteristics of the pipeline and affect the safety of the structure. In this paper, model tests were designed to explore the vortex-induced vibration (VIV) characteristics of multi-span pipelines and investigate the multi-span interaction mechanism. The experimental study mainly focused on the dynamic response of double-span pipelines, and further extended to triple-span pipelines, hoping that the results can be applied to more complex environment. The effects of span-length ratio, buried depth and axial force on VIV of the pipeline were investigated and discussed. The dynamic response of the pipeline varied with the span length. There was obvious interaction between different spans of multi-span pipelines, and the pipe-sediment interaction obviously affected the vibration characteristics of pipeline. The differences of pipeline burial depth and axial force changed the structural stiffness. With the increase of buried depth, the response amplitude presented a downward trend. The spanwise evolutions were asymmetric caused by the pipe-sediment interaction and multi-span interaction. The results can help to identify multi-span pipelines in engineering, and realize the prevention and control of free spans. [ABSTRACT FROM AUTHOR]

Experiments were conducted on risers with different mass ratios to study the effect of mode conversion and spanwise correlation. The slenderness ratio of the riser model was set as 169, and the Reynolds numbers are 1600–14400. The dynamic responses of riser models versus reduced velocity were analyzed, and the spanwise displacement, frequency, and trajectory of the mode conversion from the lower to the higher mode were explored. The results revealed that the riser model with a higher mass ratio excites a higher number of modes. The conversion region of multi-mode competition exists and narrows with the increasing mass ratio. Mode conversion is continuous and manifests as the transmission of peaks and troughs in mode shape: the peaks and troughs of mode shape move up in the mode stable development region and move down in the mode conversion region. The single-mode dominating vibration exhibits a standing wave feature, and the traveling wave feature is significant in the mode conversion region. Furthermore, the frequency jump is always transmitted from the trough to the peak of the mode shape, and finally, all the axial positions vibrate at the same frequency. The trajectory in the mode conversion region deviates from the 8-shape and recovers the standard 8-shape at the middle and late stages of the mode stable development region. [ABSTRACT FROM AUTHOR]

The cross-flow (CF) vortex-induced vibration (VIV) of a deepwater steep wave riser (SWR) subjected to uniform or shear flow loads is investigated numerically. The model is based on a three-dimensional (3D) nonlinear elastic rod theory coupled with a wake oscillator model. In this numerical simulation, the nonlinear motion equations of the riser with large deformation features are established in a global coordinate system to avoid the transformation between global and local coordinate systems, and are discretized with the time-domain finite element method (FEM). A wake-oscillator model is employed to study the vortex shedding, and the lift force generated by the wake flow is described in a van der Pol equation. A Newmark-β iterative scheme is used to solve their coupling equation for the VIV response of the SWR. The developed model is validated against the existing experimental results for the VIV response of the top-tension riser (TTR). Then, the numerical simulations are executed to determine VIV characteristics of the SWR. The effects of both flow velocity and the spanwise length of the flow field on the drag coefficient in the inline (IL) direction and the lift coefficient in the CF direction are investigated systematically. The results indicate that compared with TTR, the low frequency and multi-modal vibration are the main components of the SWR due to the large deformation and flexible characteristics. For shear flow, the multi-frequency resonance dominates the VIV response of the SWR, especially at the hang-off segment. [ABSTRACT FROM AUTHOR]

The "riser group—fluid between risers" is taken as the carrier, and the experiment on vortex-induced vibration of tandem riser groups coupling interference effect under sensitive spacing is performed. The least-square method is used to linearly fit the reduced velocity and main frequency, and the rule of Strouhal numbers is analyzed. Each mode is separated based on the mode decomposition theory, and the mode conversion mechanism is also explored. The concept of "interference efficiency" is introduced to study the dynamic characteristics and response evolutions of different riser groups. The results show that the wake shielding effect widely exists in tandem riser groups, and the interference effect of midstream and downstream risers on their upstream risers is significantly lower than that of upstream risers on midstream and downstream risers. The trajectories of midstream and downstream risers lag behind their upstream risers due to multiple shadowing effects, the vibration frequency range of downstream riser is widened and the dominant frequency is extremely unstable. Compared with the isolated riser, wake interference suppresses the vibration displacement of the midstream and downstream risers in the in-line direction, and enhances the displacement of upstream and midstream risers in the cross-flow direction. The interference effect of the fluid between risers at low velocities is stronger than that at higher velocities, and the cross-flow displacements of upstream risers are always in the interference enhancement region. It is urgent to pay attention to the cross-flow displacement of upstream and midstream risers in tandem riser groups considering the safety design. [ABSTRACT FROM AUTHOR]

In this review, we primarily address the present state of the arts and latest progresses in a few frontier issues mostly relevant to free surface/interface in ocean engineering. They include TC (tropical cyclone) induced extreme surface wave, sloshing of LNG (liquefied natural gas), cavitation/bubble dynamics and VIM (vortex-induced motion) and VIV (vortex-induced vibration). In addition to general description, we mainly focus on the recent advances and challenging aspects of above-mentioned topics. Inspired by the achievements in the previous 70 years, mankind starts a new round of ocean exploration activities. Then, we can find obvious trends: the realm of ocean engineering is expanding from sea surface to deep sea, from low and middle latitude to polar region and from fossil to renewable energy in near future. [ABSTRACT FROM AUTHOR]

Vortex-induced vibration (VIV) for flexible cylinders under combined uniform and oscillatory flow is a challenging and practical issue in ocean engineering. In this paper, a time domain numerical model is adopted to investigate the characteristics of cross-flow VIV response and fatigue damage under different combined flow cases. Firstly, the adopted VIV model and fatigue analysis procedure are validated well against the published experimental results of a 4-m cylinder model under pure oscillatory flows. Then, forty-five combined flow cases of the same cylinder model are designed to reveal the VIV response characteristics with different non-dimensional oscillation period T* and combined ratio r. The combined flow cases are classified into three categories to investigate the effect of r on cylinder's dynamic response, and the effect of T* is described under long and short period cases. Finally, fatigue analysis is carried out to investigate how the structural fatigue damage varies with the variations of r and T*. The captured characteristics of structural response and fatigue damage are explained through the VIV mechanism analysis. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of internal friction of umbilical cable on its vortex-induced vibration (VIV) responses, the experimental study on VIV of bond umbilical cable (BUC) and un-bond umbilical cable (UBUC) was carried out in an experimental tank. A current generator in the laboratory simulated the uniform current, and the current velocities were observed in real time by using a Doppler Velocimeter. In addition, different sizes of top tension were applied to the umbilical cable model. The VIV responses of the umbilical cable model were measured by using Fiber Bragg grating (FBG) strain sensors. The displacement responses of umbilical cable model were reconstructed based on the experimental strain data processed by modal superposition method. In this paper, the traveling wave characteristics, the spatial-temporal distribution characteristics of frequency and fatigue damage of the BUC and UBUC under VIV are studied. The experimental results show that there are obvious differences between BUC and UBUC in the response characteristics of VIV. The UBUC appears the traveling wave sooner than BUC, but its standing wave characteristics are more obvious than those of BUC at high velocities. Compared with BUC, the spatial-temporal distribution of UBUC frequencies appears wide-band distribution sooner, but has narrower bandwidth in the "lock-in" state. The level of fatigue damage of BUC was approximately the same as that of UBUC. [ABSTRACT FROM AUTHOR]

Owing to the complex environmental conditions, suspension could induce complicated forces on submarine pipelines and even cause vortex-induced vibration, resulting in fatigue damage of pipelines. Through aiming at the 28-inch submarine pipeline in the East China Sea, the pipeline was segmented according to the similarity, considering the factors of pipe assembly, typhoon, current, wave and seabed topography. The effects of span length on natural frequency in each section of submarine pipeline were analyzed by finite element model. The maximum safe span length allowed by each pipeline section was verified by fatigue cumulative damage theory, and the fatigue life of each pipeline section were predicted. The results showed that each order natural frequency of the pipeline decreased with the increase of span length. The calculated results of empirical formulas were much smaller than those of the FEM analysis. The increase of the gap between the suspended pipeline and the seabed was beneficial to enhance the fatigue life of the suspended pipeline. [ABSTRACT FROM AUTHOR]

A numerical study based on a wake oscillator model was conducted to determine the response performance of vortex-induced vibration (VIV) on a long flexible cylinder with pinned-pinned boundary conditions subjected to linear and exponential shear flows. The coupling equations of a structural vibration model and wake oscillator model were solved using a standard central finite difference method of the second order. The VIV response characteristics including the structural displacement, structural frequency, structural wavenumber, standing wave behavior, travelling wave behavior, structural velocity, lift force coefficient and transferred energy from the fluid to the structure with different flow profiles were compared. The numerical results show that the VIV displacement is a combination of standing waves and travelling waves. For linear shear flow, standing waves and travelling waves dominate the VIV response within the low-velocity and high-velocity zones, respectively. The negative values of the transferred energy only occur within the low-velocity zone. However, for exponential shear flow, travelling waves dominate the VIV response and the negative energy occurs along the entire length of the cylinder. [ABSTRACT FROM AUTHOR]

This study proposed a method to obtain hydrodynamic forces and coefficients for a flexible riser undergoing the vortex-induced vibration (VIV), based on the measured strains collected from the scale-model testing with the Reynolds numbers ranging from 1.34E5 to 2.35E5. The riser is approximated as a tensioned spatial beam, and an inverse method based on the FEM of spatial beam is adopted for the calculation of hydrodynamic forces in the cross flow (CF) and inline (IL) directions. The drag coefficients and vortex-induced force coefficients are obtained through the Fourier Series Theory. Finally, the hydrodynamic characteristics of a flexible riser model undergoing the VIV in a uniform flow are carefully investigated. The results indicate that the VIV amplifies the drag coefficient, and the drag coefficient does not change with time when the CF VIV is stable. Only when the VIVs in the CF and IL directions are all steady vibrations, the vortex-induced force coefficients keep as a constant with time, and under “lock-in” condition, whether the added-mass coefficient changes with time or not, the oscillation frequency of the VIV keeps unchanged. It further shows that the CF excitation coefficients at high frequency are much smaller than those at the dominant frequency, while, the IL excitation coefficients are in the same range. The axial distributions of the excitation and damping region at the dominant frequency and high frequency are approximately consistent in the CF direction, while, in the IL direction, there exists a great difference. [ABSTRACT FROM AUTHOR]

A series of fully three-dimensional (3D) numerical simulations of flow past a free-to-oscillate curved flexible riser in shear flow were conducted at Reynolds number of 185-1015. The numerical results obtained by the two-way fluid-structure interaction (FSI) simulations are in good agreement with the experimental results reported in the earlier study. It is further found that the frequency transition is out of phase not only in the inline (IL) and crossflow (CF) directions but also along the span direction. The mode competition leads to the non-zero nodes of the rootmean- square (RMS) amplitude and the relatively chaotic trajectories. The fluid-structure interaction is to some extent reflected by the transverse velocity of the ambient fluid, which reaches the maximum value when the riser reaches the equilibrium position. Moreover, the local maximum transverse velocities occur at the peak CF amplitudes, and the values are relatively large when the vibration is in the resonance regions. The 3D vortex columns are shed nearly parallel to the axis of the curved flexible riser. As the local Reynolds number increases from 0 at the bottom of the riser to the maximum value at the top, the wake undergoes a transition from a two-dimensional structure to a 3D one. More irregular small-scale vortices appeared at the wake region of the riser, undergoing large amplitude responses. [ABSTRACT FROM AUTHOR]

A flow past a circular-section cylinder with a perforated conic shroud, in which the perforation is located at the peak of the conic disturbance as the shroud installed on the cylinder and uniformly distributed with several circular holes, is numerically simulated at a Reynolds number of 100. Two factors in the perforation are taken into account, i.e. the attack angle relative to the direction of incoming flow and diameter of holes. The effect of such perforation on the drag, lift and vortex-shedding frequency is mainly investigated. Results have shown that variation of the attack angle has a little effect, especially on the drag and vortex-shedding frequency, except in certain cases due to the varied vortex-shedding patterns in the near wake. The increasing hole diameter still exhibits a little effect on the drag and frequency of vortex shedding, but really reduces the lift, in particular at larger wavelength, such as the lift reduction reaching almost 66%-68% after introducing the perforation. [ABSTRACT FROM AUTHOR]

To analyze the stress wave propagation associated with the vortex-induced vibration (VIV) of a marine riser, this paper employed a multi-signal complex exponential method. This method is an extension of the classical Prony's method which decomposes a complicated signal into a number of complex exponential components. Because the proposed method processes multiple signals simultaneously, it can estimate the 'global' dominating frequencies (poles) shared by those signals. The complex amplitude (residues) corresponding to the estimated frequencies for those signals is also obtained in the process. As the signals were collected at different locations along the axial direction of a marine riser, the phenomena of the stress wave propagation could be analyzed through the obtained residues of those signals. The Norwegian Deepwater Program (NDP) high mode test data were utilized in the numerical studies, including data sets in both the in-line (IL) and cross-flow (CF) directions. It was found that the most dominant component in the IL direction has its stress wave propagation along the riser being dominated by a standing wave, while that in the CF direction dominated by a traveling wave. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental investigation on the variation in the tension and the distribution of drag force coefficients along flexible risers under vortex-induced vibration (VIV) in a uniform flow for Reynolds numbers ( Re) up to 2.2×10. The results show that the mean tension is proportional to the square of the incoming current speed, and the tension coefficient of a flexible riser undergoing VIV can be up to 12. The mean drag force is uniformly and symmetrically distributed along the axes of the risers undergoing VIV. The corresponding drag coefficient can vary between 1.6 and 2.4 but is not a constant value of 1.2, as it is for a fixed cylinder in the absence of VIV. These experimental results are used to develop a new empirical prediction model to estimate the drag force coefficient for flexible risers undergoing VIV for Reynolds number on the order of 10, which accounts for the effects of the incoming current speed, the VIV dominant modal number and the frequency. [ABSTRACT FROM AUTHOR]

The dynamic response of two flexible model risers in tandem arrangement immersed in a stepped current was analyzed. The risers, with an external diameter of 20 mm and a total length of 6200 mm, had an aspect ratio of 310. They were hinged to the support structure at the center-to-center distances away 3-12 times the external diameter. The top 1200 mm was exposed to a uniform current at a speed which was up to 0.9 m/s (the Reynolds number was 18000) and the rest in still water. The dynamic responses, which were obtained through the Fiber Bragg Grating strain gauges mounted on the surface, were analyzed by studying the cross-flow amplitudes and modal weights. The cross-flow vibration were observed up to the third mode, and the modal transformation from the second mode to the third mode was clearly observed. The experiment confirmed that the typical vortex-induced vibration (VIV) had occurred on the up-stream riser. But for the down-stream riser, the main excitation mechanism was wake-induced vibration (WIV). The modal transformation of WIV was more complex than that of VIV, which might be helpful for other researchers to study the interference effect. [ABSTRACT FROM AUTHOR]

Laboratory tests were conducted on a flexible riser with and without helical strakes. The aim of the present work is to further understand the response performance of the vortex induced vibration (VIV) for a riser with helical strakes. The experiment was accomplished in the towing tank and the relative current was simulated by towing a flexible riser in one direction. Based on the modal analysis method, the displacement responses can be obtained by the measured strain. The strakes with different heights are analyzed here, and the response parameters like strain response and displacement response are studied. The experimental results show that the in-line (IL) response is as important as the cross-flow (CF) response, however, many industrial analysis methods usually ignore the IL response due to VIV. The results also indicate that the response characteristics of a bare riser can be quite distinct from that of a riser with helical strakes, and the response performance depends on the geometry on the helical strakes closely. The fatigue damage is further discussed and the results show that the fatigue damage in the CF direction is of the same order as that in the IL direction for the bare riser. However, for the riser with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction. [ABSTRACT FROM AUTHOR]