Helical Strakes Research Articles

Vortex-induced vibration experiments of flexible cylinders with floaters: effects of diameters, length, relative distance, total buoyancy and helical strakes

Vortex-induced vibration experiments of flexible cylinders with floaters: effects of diameters, length, relative distance, total buoyancy and helical strakes

Suppression of wake-induced galloping of tandem cylinders by helical strakes

Helical strakes as a classic passive control method for vortex-induced vibration (VIV), have been extensively studied, but wake-induced galloping (WIG) control of three tandem cylinders has received no previous attention. The current wind tunnel experimental study tries to fill this gap in the context of mechanical science. WIG of three tandem cylinders considering different structures of helical strakes is modeled and investigated. Helical strakes with varying heights G, pitches P, and cross-sectional shapes (square “S”, involute “I”, and D-shaped “D”) are first designed. Wind tunnel tests are conducted to experimentally study the dynamics responses of tandem cylinders (m*ξ = 0.09) at different spacings, and three vibration modes are obtained. This study delves into the characterization and quantification of the impact of various parameters of helical strakes on WIG of three tandem cylinders. These aspects, scarcely addressed in existing research, are thoroughly explored through statistical analysis, focusing on vibration amplitude, vibration frequency, and wavelet analysis. It is found that helical strakes can effectively suppress the multi-frequency vibration instability of bare cylinders. For wind speeds below 6 m/s, the "S" helical strakes achieves a vibration suppression efficiency of up to 665 %; Three-dimensional computational fluid dynamics (3D-CFD) demonstrate the coupling effect in the flow field, revealing that the "S" helical strakes has a greater impact on boundary layer formation than the "D" helical strakes; Reducing the pitch of the helical strakes can enhance the disturbance ability of the spanwise position, thereby increasing its suppression effect. This work improves vibration reduction and sheds light on how the helical strakes' cross-section controls vibrations in the tandem cylinders.

A comparison of immersed boundary and body-fitted methods: Transient vortex shedding in elaborate bio-mimetic cylinders

In this study, two contrasting computational fluid dynamics (CFD) methods are used to simulate flow around bio-mimetic cylinders. Cylindrical shapes are proposed to represent a cross-sectional view of complex geometries, such as cylinders with biofouling or cylinders with helical strakes. The non-conformal Cartesian immersed boundary method (IBM) and conformal curvilinear body-fitted method (BFM) are applied to solve the governing equations. A critical assessment of the accuracy and efficiency of IBM is presented by employing an open-source solver with similar discretization, algorithms, and properties. It has been found that IBM can reduce computation time by up to 49% for protrusions with different numbers of sides. Results show that IBM can reduce required computation time up to an outstanding 71% for protrusions with different heights. Results also indicate that transient duration doesn't significantly change the reduction trend in required computation time. For this reason, the onset of vortex shedding and eventually fully-developed flow takes place substantially sooner in IBM simulation than in BFM. It is also found that the height and shape of protrusions affect BFM performance but have no effect on IBM performance. This study proves that non-conformal IBM is a robust numerical approach when simulating vortex shedding in complex geometries.

Laboratory measurements on the suppression of vortex-induced vibrations and rotation in long-span riserless drill strings

Riserless drilling is a leading offshore technology, but the absence of a riser causes significant vibration during outflow and rotation. However, effective methods for addressing vortex-induced vibration coupling rotation are inadequate. Experiments were conducted on riserless drill strings, and the dynamic responses were analyzed by modal decomposition theory. Results indicate that structures with higher helix numbers and screw heights exit or delay entry to the lock-in region. Triple-helix designs with screw heights of 0.2 Diameter (D) and 0.25D achieve up to 93% suppression efficiency. Increasing helix number and screw height reduces rotating drill string frequencies and delays the occurrence of the dominant frequency jump phenomenon, resulting in enhanced suppression effects. Amplitude suppression primarily occurs in medium and weak regions, while helical strakes significantly reduce amplitudes in lock-in regions. For controlling vibration and minimizing fatigue damage in rotating drill strings, it is recommended to use triple-helix structures with lower screw heights.

Visualization and Parametric Study on Vortex Shedding Suppression of Cylindrical Structures in Offshore Engineering Using Large Eddy Simulation

Cylindrical structures are widely used in offshore and marine engineering, but they may suffer from vortex-induced vibration under the influence of ocean or wave currents, which can lead to severe fatigue damage. In this study, we applied the open-source software Open-Source Field Operation and Manipulation (OpenFOAM) to investigate the characteristics of fluid flow around offshore cylindrical structures, taking into account the effect of helical strake parameters, such as pitch and strake number. The aim of this study is to explore the possibility of suppressing vortex shedding with different helical strake parameters. Numerical simulation results demonstrated that attaching a helical strake to the bare cylinder destroyed vortex shedding in offshore cylindrical structures. The vortex visualization showed that the helical strake destroyed the three-dimensional vortex structures. Moreover, the lift coefficient data showed that the vibration frequency of the cylinder decreased after attaching the helical strake, indicating that the vortex-induced vibrations on the wake flow tended to fade. The results suggest that the helical strake is a promising option for suppressing the wake vortex shedding of cylindrical structures in offshore engineering.

Scaled model testing for the helical strakes in reducing the vortex-induced vibrations

The accuracy of the prediction of the suppression efficiency of the triplex helical strakes for vortex-induced vibrations (VIV) is important in the offshore industry for the risers. Towing tank tests of the 1:1 section model have been used in the study of one degree of freedom; however, this method requires a long time for both the preparation and the testing. In this study, a scaled model testing method based on the flume and the oscillating frame is proposed, which measured the oscillating amplitude and the forces for both the bare pipe and the triplex helical strakes. The scaled tests were carried out in the TU Delft, with the range of Reynolds number between 6000 and 20000. The results were compared with the real model test applied in the towing tank to evaluate the accuracy. The measured strake efficiency of the scale model test agrees with the full-scale model test, providing support for the scaled model test in reducing the vortex-induced vibrations.

Wind-Induced Vibration and Vibration Suppression of High-Mast Light Poles with Spiral Helical Strakes

In this study, three-dimensional finite element models of high-mast light poles without and with spiral helical strakes were built using ANSYS software to investigate their vibration characteristics in a wind environment. Based on a two-way, fluid–structure interaction simulation method, the dynamic responses of the high-mast light poles under different windspeeds were analyzed. The results indicate that the high-mast light pole structure without spiral helical strakes may suffer from evident vortex-induced vibration, which is dominated by the third vibration mode in the windspeed range of 5~8 m/s, whereas the light pole with spiral helical strakes had no obvious vortex-induced vibration. The external helical strakes can amplify the along-wind response of the light pole to a certain extent, while significantly decreasing its crosswind vortex-induced response. The vibration suppression effect is better when the value of pitch P is small. Practically, if P = 7.5 D (D is the diameter of the dominant vibration mode), the vibration suppression effect is best. On the other hand, if the value of pitch P remains constant, the vibration suppression effect increases with the height H of the outer helical strakes. However, excessively high outer helical strakes may also increase the along-wind response of the structure. In general, when spiral helical strakes are used in design, the recommended values of P and H are P = 7.5 D and H = 0.20 D.

Benchmarking of VIV numerical analysis with prototype response for fatigue assessment of inverse catenary coldwater pipelines

High density polyethylene pipe has been used to draw coldwater for low temperature thermal desalination plants since the year 2006 at Lakshadweep Islands, India. The pipeline endured in-line oscillations due to shear current in one of the desalination plants, and this gave an opportunity to observe the response parameters of the prototype pipeline. The recorded oscillation parameters are utilized as a benchmark to verify the results of the vortex induced vibration (VIV) analysis in frequency domain and time domain, and the results show good concurrence. The in-line fatigue damage assessment is carried out for a range of current profiles showing the need for mitigation of pipeline VIV. The helical strakes on a partial length of the pipeline is modeled numerically showing significant suppression of VIV response as well as substantial increase in fatigue life of the pipeline. The benchmarking of the VIV response analysis with prototype measurements enhances the reliability of using numerical VIV fatigue analysis and mitigation measures.

An experimental study on vortex-induced vibration suppression for submarine multispan pipelines

To ensure structural safety, it is necessary to reduce the vortex-induced vibration (VIV) of free-span pipelines in offshore engineering applications. In this study, an experimental investigation on VIV suppression for submarine multispan pipelines was conducted. The VIV response was suppressed by attaching helical strakes to the pipeline. By comparing the vibration response of a smooth multispan pipeline with that of a pipeline with an installed suppression device, the suppression effect of the helical strakes was explored. The coverage of the helical strakes was changed, and differences in the VIV suppression effects were observed in the experiment. The helical strakes effectively suppressed VIV of the multispan pipeline, and the maximum suppression efficiency exceeded 90%. The inhibition effect increased with increasing strake coverage. Furthermore, the burial depth of the pipeline was found to influence the VIV suppression effect.

On the study of vortex-induced vibration of a straked pipe in bidirectionally sheared flow

Helical strakes are widely used to suppress vortex-induced vibration (VIV) in ocean engineering. However, the VIV response and suppression efficiency of the straked pipe are unclear under the recently discovered bidirectionally sheared flow field. Experiments on vortex-induced vibration for a flexible pipe with three kinds of helical strakes were carried out in an ocean basin. The bare pipe model was 28.41mm in diameter and 7.64m in length. Three kinds of helical strakes with different pitch/height (17.5D/0.15D, 17.5D/0.25D,and 17.5D/0.35D) were applied. 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. The reduced velocities based on the tested first natural frequency of the bare pipe in water reached 30.79. The cross-flow and in-line VIV amplitudes, VIV suppression efficiency, and fatigue damage are presented in this article. The results showed that the helical strakes can suppress bidirectionally sheared flow-induced VIV significantly at a high reduced velocity regime with fatigue damage suppression efficiency over 99% in the CF and IL directions, but cannot suppress the in-line initial displacement, which was also found in previous studies as a drawback of helical strakes.

Experimental investigation on the effectiveness of serrated helical strakes in suppressing vortex-induced vibrations for a flexible riser

The effectiveness of an unconventional configuration of serrated helical strakes (SSs) in alleviating the vortex-induced vibrations (VIVs) of flexible risers was evaluated experimentally in the Reynolds number range of 2600–10,400. Five SSs with varying serration densities and lengths were designed and compared with conventional strakes (CSs) to highlight the unparalleled features of this new geometry. The VIV parameters of the bare and straked risers, including the displacement, frequency, fatigue damage, and suppression efficiency, were systematically investigated. The comprehensive evaluation of the helical strakes was also made considering the factors of amplitude suppression, frequency inhibition, drag force, and cost control. The experimental results confirmed that all the helical strakes were capable of suppressing the VIV responses of flexible riser models. With identical strake areas, the SSs were more effective in inhibiting the VIV amplitude and fatigue damage than the CSs; whereas, the serrated geometry had few effects in improving the reduction of VIV frequency. Moreover, serration density is a critical factor in determining the suppression performance of SSs under the same strake heights and areas. An increase in the strake area enhanced VIV suppression performance; however, excess strakes deteriorated the comprehensive evaluation index due to the increased cost.

An Experimental Investigation of the Effects of Helical Strakes on the Characteristics of the Wake around the Circular Cylinder

An Experimental Investigation of the Effects of Helical Strakes on the Characteristics of the Wake around the Circular Cylinder

Experimental Research on Vortex-Induced Force Characteristics of Flexible Riser with Buoyancy Module and Strakes

In this paper, the vortex-induced force characteristics of a flexible riser with buoyancy modules and strakes were experimentally investigated. The strain information caused by vortex-induced vibration (VIV) at different cross-sections on the flexible cylinder was measured in the model test. Combined with the modal superposition method and inverse analysis method for vortex-induced force, the vortex-induced forces on the flexible cylinder were inverted, including the excitation force in phase with velocity and added mass force in phase with acceleration. Then, the excitation coefficients and added mass coefficients at each time step were identified via a least-squares method. The results show that the added mass coefficient and excitation coefficient of the flexible riser covering buoyancy modules present a “sawtooth” distribution along the length of the pipe, while the total mass (sum of structural mass and added mass) is consistent along the pipe. The strake has little effect on the added mass coefficient of the buoyancy module and the bare pipe section, but when it covers the bare pipe section, the excitation coefficient of the bare pipe section decreases to a negative value, showing a damping effect. In addition, when the bare pipe section of the flexible riser is completely covered with helical strakes, only the vortex-shedding frequency component of the buoyancy modules remains in the VIV response of the pipe. The reason for this phenomenon is that helical strakes completely suppress the vortex shedding of the bare pipe section, resulting in the reduction in the VIV excitation energy of the original bare pipe section to 0.

Vortex-induced vibration fatigue damage prediction method for flexible cylinders based on RBF neural network

Vortex-induced vibration (VIV) is an important factor resulting in fatigue failure of cylindrical structures in offshore engineering applications (such as risers and cables). The traditional empirical model for VIV prediction often yields unsatisfactory results due to its dependence on the selection of empirical coefficients. Thus, establishing a general approach to VIV damage prediction based on artificial neural networks and engineering experience is of great significance to engineering guidance. In this paper, a VIV damage prediction model of a flexible cylinder was established using a radial basis function neural network (RBFNN) and model test data. Compared with back propagation neural network (BPNN), genetic algorithm-back propagation neural network (GA-BPNN), and generalized regression neural network (GRNN), the RBFNN exhibited excellent performance with high accuracy and the shortest training time in predicting the VIV damage of flexible cylinders. In addition, the VIV fatigue of flexible cylinders fitted with passive control devices (control rods and helical strakes) was predicted. Using three and four control rods, the VIV reduction effect was observed to be better when the attack angle θ was in the range of 30°–45° and 15°–30°, respectively. Further, regarding helical strakes, the damage values of the cylinder were significantly reduced; however, the VIV suppression effect was significantly affected by the yaw angles. The VIV damage prediction model of this paper is expected to serve as important engineering guidance for the preliminary design, installation arrangement, operation, and maintenance of flexible cylindrical structures in offshore engineering applications.

Passive flow control of vortex-induced vibrations of a low mass ratio circular cylinder oscillating in two degrees-of-freedom

Offshore wind turbine towers are often placed vertically before installation at ports. At certain wind speeds, these towers are subject to vortex-induced vibrations and can lose up to a third of their fatigue live within a few days. So far, helical strakes have primarily been used in the upper part of the towers. These have so far proven to be not effective enough to suppress vortex-induced vibrations. The aim of this study was to develop a passive flow control method for offshore wind turbine towers and to experimentally investigate its efficiency.Experiments were performed in a circulating water tank to investigate using fin plates to suppress vortex-induced vibration of fully submerged flexible cylinders in flows at Reynolds numbers, referred to the cylinder diameter, between 1.4 × 104 and 8.0 × 104. The corresponding non-dimensional reduced velocities (normalized via the free stream velocity, the excitation frequency, and the diameter of the cylinder) were between 1.1 and 6.1. The cylinders’ of aspect ratio 18.8 had a mass ratio of 0.535. This mass ratio, referred to the mass of an oscillating cylinder normalized by the mass of displaced fluid, was below the critical mass ratio of 0.545. Each cylinder consisted of a polyvinyl chloride pipe mounted as a cantilever beam on a six degrees-of-freedom piezoelectric transducer. The top end of the cylinders was free to oscillate in the streamwise and lateral directions. An accelerometer was placed near their tops to measure accelerations and, via twofold integration, to retrieve top-end cylinder motions. A Laser Doppler Velocimetry system was employed to measure free stream velocities at the inlet of the water channel and wake velocities behind the cylinder. The wake velocities were used to calculate shedding vortex frequencies. Dynamic forces and displacements were also measured. Results demonstrated that fin plates attached to the cylinders mitigated their vibrations considerably. Optimized fin plate arrangements reduced amplitudes of streamwise and lateral vibration by 81 and 75%, respectively, and amplitudes of lateral forces acting on the cylinders by between 40 and 75%. Low uncertainty estimates of measured values substantiated the reliability of the results.

Novel geometries of serrated helical strakes to suppress vortex-induced vibrations and reduce drag

We present an experimental investigation on the effectiveness of unconventional geometries of serrated helical strakes to suppress the cross-flow vortex-induced vibrations (VIV) of a circular cylinder with low mass and structural damping. The VIV responses of five models of strakes are compared with that of a bare cylinder at moderate Reynolds numbers: one continuous helical strake, two serrated strakes and two inverted strakes. While the conventional strake suppressed 88% of the peak amplitude of response with a 48% drag reduction, the most efficient serrated strake reduced the peak amplitude of vibration by 95% producing 54% less drag than a bare cylinder. When the models were not allowed to respond to VIV they all increased drag in relation to that of bare cylinder. We verified that simply inverting the local angle of attack of the individual blades in relation to the helical pitch of the strake did not produce a favourable result in terms of VIV suppression, but reduced drag and fluctuating lift on a fixed body. Visualization of the flow around the blades helped to clarify the hydrodynamic mechanisms governing flow separation in the near wake, disrupting the formation of coherent vortices and reducing VIV.

The effectiveness of helical strakes in suppressing vortex-induced vibration of tandem circular cylinders

Abstract An experiment study of vortex-induced vibration (VIV) of two rigid cylinders equipped with and without helical strakes (pitch of 10.0D and height of 0.15D) in a tandem configuration was investigated at a critical spacing distance of 3.5D. Here, the cylinders were attached to a structural test rig, which would allow them to oscillate in the cross-flow direction only. The oscillation amplitude and frequency data were collected. Accordingly, the results revealed that in the case of the two bare cylinders, the amplitude response of the upstream cylinder was higher compared to the case of single cylinder usage. Furthermore, outcomes obtained confirmed that the helical strakes successfully reduced the oscillation amplitude for the upstream and downstream cylinders both, whereas they were ineffective if merely applied at the downstream cylinder. Hence, it can be concluded that the arrangement of the strakes influences the VIV suppression in the context of two-cylinder cases.

An Experimental Investigation of Forces on a Textured Cylinder at Subcritical Reynolds Numbers

Spanwise deviations from a perfect circular cross-section, affect the aerodynamics and hydrodynamics of cylindrical structures. In this study the ability of a novel wavy cylinder, the textured cylinder, to reduce applied drag and lift forces is investigated experimentally. Three different configurations of the proposed textured geometry, as well as a bare cylinder and a cylinder with helical strakes are 3D printed and tested in an open circuit wind tunnel. Tests are conducted at Reynolds numbers from 3.5×104 to 8.0×104. Results show that the time-averaged drag and fluctuating lift forces on the textured cylinder can be 20% and 15% lower than those of a bare cylinder, respectively. Amongst the tested cylinders, the straked cylinder exhibited the lowest fluctuating lift forces. However, the lowest time-averaged drag forces were measured on the textured cylinder, Tex30, through the range of Reynolds numbers tested. The Strouhal number of the Tex30 was found to be between those of the bare and straked cylinders.

The effect of time-varying axial tension on VIV suppression for a flexible cylinder attached with helical strakes

Flexible cylindrical structures, such as top-tensioned risers (TTRs) and tethers of tension leg platforms (TLPs), usually vibrate under the combined excitation of vortex shedding and time-varying axial tension. Helical strakes are widely used to suppress vortex-induced vibration (VIV). The VIV suppression effectiveness of helical strakes for a flexible cylinder under axial tension excitation is still unknown. Model tests were conducted to investigate the effect of time-varying axial tension on VIV suppression for a flexible cylinder fitted with helical strakes. Three tension amplitude ratios (Tv/Tc = 0.1, 0.2 and 0.3, where Tv is the amplitude of the varying tension and Tc is the constant axial tension) and four tension frequency ratios (fv/f1 = 0.5, 1.0, 2.0 and 4.0, where fv is the frequency of the varying tension and f1 is the fundamental natural structural frequency) were considered. The vibration displacements were reconstructed using the measured strains by the modal analysis method. The dominant mode, response frequency and displacement amplitude were determined to show the influence of axial excitation on the dynamic characteristics of the flexible cylinder attached with helical strakes. When the reduced velocity is low, the second-order mode vibrations of the straked cylinder with axial excitation are excited. Due to the axial excitation, the straked cylinder has large cross-flow (CF) and in-line (IL) displacements at low reduced velocities. The displacement contours in the case of fv/f1 = 1.0 are remarkably different from those in other cases. When the reduced velocity increases, the effect of vortex shedding is enhanced. Both fv induced by axial excitation and the fn induced by vortex shedding are prominent, meaning that both vortex shedding and axial excitation affect the vibration of the straked cylinder. Under axial excitation, the IL displacements of the straked cylinder are enlarged, especially in the case of fv/f1 = 1.0. Axial excitation can reduce the VIV suppression efficiency of helical strakes.

Performance of helical strakes in suppressing the FIV fatigue damage of two long flexible cylinders in a tandem configuration

Helical strakes are the most common vibration suppression devices for cylindrical structures undergoing vortex-shedding. Many problems with the performance of helical strakes on flow-induced vibration (FIV) fatigue damage suppression of two tandem cylinders have not been solved yet. The effectiveness of three-strand helical strakes with a pitch of 17.5D and a height of 0.25D (where D is the structural diameter) in suppressing fatigue damage of two tandem long flexible cylinders undergoing FIV was investigated in this paper. A constant spacing ratio (center-to-center distance to cylinder diameter) of 8.0 was selected. Four cases of the two cylinders, i.e., naked-naked, naked-straked, straked-naked and straked-straked, were executed under subcritical Reynolds number (Re ≤ 1.6 × 104). Based on the strain responses from the experimental tests, fatigue damage was estimated by using S–N curves and Miner's rule. To highlight the role of helical strakes in FIV fatigue damage suppression, an isolated naked cylinder was considered as a benchmark. It is shown that helical strakes exert substantially the same effect on FIV fatigue suppression of the upstream cylinder as they do on an isolated cylinder. The upstream cylinder significantly affects the performance of helical strakes on FIV fatigue suppression of the downstream cylinder. Both vortex breaking and prevention of interaction of two shear layers predominate the primary mechanism of the downstream straked cylinder when the upstream one is naked, which reinforces its cross-flow fatigue damage and hereby lowers the performance of helical strakes. In contrast, due to wake-induced vibration (WIV), the downstream straked cylinder laid behind a straked one might encounter much more serious fatigue damages in both the cross-flow and in-line directions than the case when the upstream cylinder is naked.