Vortex-induced Motion Research Articles

Experimental Investigation of Towing of a Semi-submersible Floating Offshore Wind Turbine

Abstract The offshore floating wind energy sector is poised for exponential growth in the coming decades. A pivotal challenge within this context is the imperative to substantially decrease the levelized cost of energy (LCOE) to enhance competitiveness. A significant proportion of these LCOE costs are attributable to the towing operations entailed in both the installation of assembled floating wind turbines and the maintenance of major components. Semi-submersibles, characterized by multiple columns and pontoons, represent the prevailing conceptual approach in the industry due to their versatility across varying water depths and their cost-effectiveness in terms of mooring and transportation. Towing model tests for the semi-submersible INO WINDMOOR floating wind turbine have been conducted in the Ocean Basin at SINTEF Ocean. The primary objective of these tests is to investigate critical aspects such as towing resistance and dynamic responses, including vortex-induced motion (VIM) and galloping. This paper presents the experimental set-up and initial findings from the model tests. Through comparative data analysis of selected cases, an enhanced and insightful understanding of the intricate fuid-structure interactions is expected to be achieved. Additionally, it highlights the identification of knowledge gaps and research imperatives aimed at expanding the towing operational criteria, with the ultimate goal of reduction of installation and maintenance costs.

Polyester mooring system design and evaluation of a semi-platform in south China sea

Polyester rope offers numerous advantages over traditional steel catenary mooring systems and is considered an appealing option for deep-water mooring systems. In this paper, the design and evaluation procedure of the polyester mooring system for a semi-submersible platform located in the South China Sea is presented. A fully coupled numerical model of the semi-submersible platform, including all risers and mooring lines, has been established and calibrated through wave basin testing. To simulate the elongation behavior of polyester, a static-dynamic stiffness model is employed, and the corresponding procedure for mooring evaluation is established to simulate the mooring response under extreme environmental conditions. A comprehensive fatigue analysis is also conducted for the polyester mooring system using time domain dynamic theory. The effects of Vortex-Induced Motion (VIM) on mooring fatigue damage are also considered. The results indicate that the polyester mooring system could be safely operated at the target offshore field throughout its service life. Additionally, model test calibration is a crucial procedure during the entire mooring evaluation process, and the numerical model should be adjusted appropriately to accurately reflect the dynamic behavior of the coupled system. This study also illustrates that the stiffness of the rope plays a crucial role in polyester mooring design and global performance calculations. The proposed evaluation methodology can provide a foundation for the design of polyester mooring systems and for evaluating their safety and reliability in engineering practice.

Coupling Effects of a Top-Hinged Buoyancy Can on the Vortex-Induced Vibration of a Riser Model in Currents and Waves

In order to investigate the effects of the top-end dynamic boundary of risers caused by floater motions on their vortex-induced vibration (VIV) characteristics, a combined model comprising a buoyancy can with a relatively simple structural form and a riser is taken as the research object in the present study. The aspect ratios of the buoyancy can and the riser model are 5.37 and 250, respectively. A set of experimental devices is designed to support the VIV test of the riser with a dynamic boundary stimulating the vortex-induced motion (VIM) of the buoyancy can under different uniform flow and regular wave conditions. Several data processing methods are applied in the model test, i.e., mode superposition, Euler angle conversion, band pass filter, fast Fourier transform, and wavelet transform. Based on the testing results, the effect of low-frequency VIM on the high-frequency VIV of the riser is discussed in relation to a single current, a single wave, and a combined wave and current. It is found that the coupling effect of VIM on the riser VIV presents certain orthogonal features at low current velocities. The effect of the cross-flow VIM component on VIV is far more prominent than that of its counterpart, the in-line VIM, with increasing flow velocity. The VIM in the combined wave–current condition significantly enhances the modulation of vibration amplitude and frequency, resulting in larger fluctuation peaks of vibration response and further increasing the risk of VIV fatigue.

A Self-Adaptive Planar Velocity Vector Sensor Based on Vortex-Induced Torsional Swing Motions

A Self-Adaptive Planar Velocity Vector Sensor Based on Vortex-Induced Torsional Swing Motions

Application of CFD on VIM of semi-submersible FOWT: A Case Study

This study examines the Vortex Induced Motions (VIM) of the INO WINDMOOR 12 MW semi-submersible Floating Offshore Wind Turbine (FOWT) platform using three-dimensional Computational Fluid Dynamics (CFD). The results from both model- and full-scale simulations are presented. The model scale results reveal varying VIM performances at different current headings, with detailed flow visualizations and analyses of hydrodynamic loads on FOWT components providing insights into the underlying wake interaction scenarios. One simulation shows bifurcated VIM, where regular motions are intermittently interrupted due to simultaneous suppression of vortex shedding on all FOWT columns. Preliminary full-scale CFD results suggest a strong scale effect. The paper concludes with a discussion on how CFD simulation can be more effectively used in VIM research.

The dynamic response of floating offshore wind turbine platform in wave–current condition

In this paper, the fluid–structure interaction of floating offshore wind turbine (FOWT) platforms under complex ocean conditions is investigated using OpenFOAM and in-house developed models. Two types of FOWT platform, i.e., a semi-submersible platform and a barge platform, are studied for their dynamic responses to either wave or current. The results reveal that a semi-submersible platform exhibits larger cross-flow motion and lock-in phenomenon, while a barge platform experiences smaller motion with no significant lock-in within the velocity range examined. The combined wave–current conditions are further studied for the semi-submersible platform, with different angles between wave and current, the current speeds, and wave parameters. Unlike other investigations focusing on colinear wave–current interaction, in which the waves usually mitigate vortex-induced motion (VIM); here, we find that waves might lead to an enhanced VIM with a large angle between current and wave. The evaluation on the interaction effect factor shows that the largest wave height in the lock-in region does not lead to the most dangerous scenario, herein, the largest platform motion. Instead, a smaller wave height with a large wave period can induce even larger motion.

CFD Simulation of Vortex Induced Motion of a Caisson during Installation Process

This paper presents the CFD investigation of the vortex induced motion (VIM) of a caisson model during the installation process. The simulations were performed by solving the unsteady 3D Navier-Stokes equation. The Dynamic Fluid-Body Interaction (DFBI) tool is utilized to simulate the motions. Typical draft conditions of a caisson during installation process are considered. The present methodology is verified through the free decay and the VIM of a cylinder simulations in two-dimension. Free decay simulations show that the natural frequency decrease with the increasing draft of the caisson. Maximum response amplitudes occur in the range of 10<Vr <13 for the draft Dp=0.1m. The resonance occurs when the crossflow response frequency f y is very close to the natural frequency f n. The inline/crossflow response frequency increases as the draft increases at the same reduced velocity V r. The response frequency in the inline direction is double of that in the crossflow direction. Eight-shaped trajectories are observed. The maximum displacements in the crossflow direction increase as the drafts increase.

Experimental Study on Vortex-Induced Vibration of Tension Leg and Riser for Full Depth Mooring Tension Leg Platform

According to the geometric parameters of the tension leg platform, the test model was made with a scale ratio of 1:61. The model was used to conduct the full-depth simulation test of uniform flow and wave current combination in the test pool. The model test results showed that when the reduced speed was between 5.5 and 8.5, and the lateral motion response of the platform was the most significant. In the interval of the reduced speed, the response frequency of transverse vortex-induced motion was close to the natural transverse frequency of the platform, and resonance occurred. The amplitude of surge motion increased with the increase of reduced speed. Due to the pull of the floating body, the tension of tension legs and risers increased with the flow rate but did not increase significantly in the floating body lock zone. The mooring tension had a certain limiting effect on the floating body sway. The displacement modes of tension legs and risers were greatly affected by the flow velocity. With the flow velocity increasing, the mode order increased. In addition, the increase in tension caused by the large displacement of the floating body had a certain impact on the displacement amplitude. The wave could reduce the sway of the floating body and strengthen the surge. Therefore, under the combined action of wave and current, the tension amplitude of the tension leg and riser was increased compared with that under the uniform flow. The conclusions obtained in this paper could be used for reference in the engineering design of tension legs and risers.

A CFD Study of Vortex-Induced Motions of a Semi-Submersible Floating Offshore Wind Turbine

Vortex-induced motion (VIM) is a critical issue for floating structures made of one or more columns, due to its significant impacts on their operational stability. Supported by column-type floating platforms, floating offshore wind turbines (FOWTs) may also experience large-amplitude VIM responses in current flow. Existing research on FOWTs has mostly focused on their wind/wave induced responses, yet less attention has been paid to their responses in current flow. In this paper, the VIM of the OC4 semi-submersible FOWT platform is studied numerically over a wide range of flow velocity. Three incidence angles, i.e., 0°, 90°, and 180°, are considered and the effect of current incidence on platform VIM is analysed. Results show that the so-called lock-in phenomenon is present and that a large transverse response amplitude of more than 0.3D persists until Vr = 30, with its maximum reaching over 0.8D at Vr = 8. Meanwhile, the transverse response amplitude for cases with the incidence angle of 180° is generally smaller, with a narrower lock-in regime, than those under the other two incidence scenarios. Flow field visualisation reveals that upstream vortices continuously interact with the downstream side column when the incidence angle turns to 180°, impacting the vortex shedding process and consequently fluid forces of the downstream column.

Numerical study of vortex-induced autorotation of an elliptic blade in lid-driven cavity flow

Large-scale eddies in a lid-driven cavity are potential sources of angular momentum which can induce rotational effect in a free-to-rotate inertial body due to fluid–structure interaction. The novel objective of the present study is to investigate vortex-induced autorotation of an elliptic blade hinged at the centre of a lid-driven cavity. The governing equations are numerically solved using iterative direct forcing immersed boundary method. The impact of Reynolds number and blade length on dynamics characteristics of the blade are analysed. Considering left to right motion of horizontal top lid, four different vortex-induced modes are identified as the steady blade response, including stationary position, small-amplitude fluttering, clockwise autorotation and counter-clockwise autorotation. Long blades are mostly potential for steady clockwise autorotation, particularly in higher Reynolds numbers, due to dominance of principal near-wall cavity vortex compared to other vortices. In contrast, effective role of central counter-clockwise vortex in a short blade and weak interaction of such blade with the near-wall cavity vortex leads to a steady counter-clockwise rotation, particularly in high Reynolds numbers. In the case of low Reynolds numbers or blade with moderate length, vortex-induced blade motions in clockwise and counter-clockwise directions are fairly balanced, leading to stationary position or small-amplitude fluttering modes.

Experimental studies on the effects of draft condition, current heading and mooring stiffness on vortex-induced motions of a Tension-Leg Platform

The paper presents experimental studies on the effects of draft conditions, current headings and mooring stiffnesses on the Vortex-Induced Motion (VIM) responses of a Tension-Leg Platform (TLP) with four circular columns. The nominal transverse and yaw amplitudes, and the corresponding spectra obtained by fast Fourier Transform (FFT) and continuous wavelet transform (CWT) are analyzed to examine the characteristics of the motion responses. Four current headings in the range of 0° to 45° are adopted in the model test. The results show that the maximum response amplitude in both transverse and yaw directions are achieved at 0° current incidence. For the transverse response, the maximum nominal amplitude decreases and the lock-in range narrows as the current heading increases. The spectral analyses for the transverse and yaw motions at 0° current heading show that the dominant peak frequencies increase as the reduced velocity increases. It is observed that the low-frequency spectral peak of the in-line motion amplitude spectra increases as reduced velocity increases. A color band or a ribbon concentrates stably around the frequency of f<0.05 in the wavelet analysis. It is primarily caused by the initial acceleration of the model in the towing tank. The absolute values of correlations between the spring tensions and the transverse motion of the platform are much larger than those between the spring tensions and the in-line or yaw motions. Moreover, it is found that the VIM trajectories of the TLP are primarily always along the direction perpendicular to the current heading.

Investigation on vortex-induced motions of four-square columns in a square configuration considering galloping under different current velocity and incident angle

The study of vortex-induced motions (VIM) is practical significance for offshore oil and gas development. The VIM of multi-column structures, which has not been completely understood, is more complex compared to that of an isolated column structures. The VIM of four rigidly coupled square columns in a square configuration was investigated numerically using Reynolds-averaged Navier–Stokes (RANS) equations with the SST k–ω turbulence model for incident angles of α = 0°, 15°, 30°, and 45°. The time histories of the displacement, the motion amplitudes, the frequency characteristics, the hydrodynamic forces, the motion trajectories, and the vortex flow patterns were analyzed and discussed. The results indicate that the motion behavior of the four-square columns structure is more likely to be a combined motion of VIM and galloping. More specifically, the critical reduced velocities for occurrence of sway and surge galloping are both approximately 9.23, while yaw galloping may occur at a lower reduced velocity. It is found that the complex motion frequency components cause irregular motion trajectories and vortex flow patterns in the pre-lock-in and post-lock-in cases; but unitary motion frequency components lead to regular motions in the lock-in cases. Additionally, there is a strong coupling relationship between the sway and yaw motion, which is independent of the incident angle.

Hybrid turbulence models for flows around a stationary smooth circular cylinder

The aim of this paper is to evaluate the accuracy, stability and efficiency of the different hybrid turbulence models via the benchmark computations of flows around a stationary smooth circular cylinder. The hybrid turbulence models have been validated and applied a lot in turbine flow, combustion, aerodynamic and aeroacoustic problems, while not much in hydrodynamic problems. In order to verify the simulation performance of the newly proposed hybrid strategies, this paper uses the SST-PANS, SST-SAS and SST-IDDES to compare the numerical simulation of the large separation flow problem. The model performance is assessed by a detailed comparison of predictions regarding the turbulence characteristics, the hydrodynamic characteristics, and the distribution of eddy-viscosity. The self-developed CFD solver, vim-FOAM-SJTU, is used in the present simulations. From the assessment, it is confirmed that the hybrid models have the abilities to calculate the small-scale motions, and all models can be employed to predict the unsteady characteristics of wake vortices advantageously. Both the SST-PANS and SST-IDDES models can capture complex flow structures and physics mechanism. The SAS model using the Lvk scale with features of local-adaptive and grid-independent can predict the eddy-viscosity reasonable. When the constant Cs = 0.11 contained in the Lvk limiter is modified to be Cs = 0.08, SST-SAS is also able to achieve better performance in the same mesh. The hybrid model can calculate and simulate the 3D vortex structure well, which is closer to the real physical phenomenon. And SST-IDDES has the best simulation results. The numerical results from SST-IDDES show its comparable capabilities for simulation of massively separated hydrodynamic flows and its potential application in the prediction of industrial turbulent flows for vortex-induced motions (VIM).

Two identical tandem square prisms with rounded edges and hard marine fouling at incidence in cross-flow: Effect of spacing and Reynolds number on unsteady fluid dynamics

Because of their long submerged columns, deep-draft semi-submersible floating structures are particularly susceptible to vortex-induced motions. This paper focuses on the steady and unsteady forces that act on such a column pair, as well as on the frequency and strength of the shed vortices behind the downstream column. Wind tunnel experiments were performed on two rounded and lightly rough square-section prisms in cross-flow for Reynolds numbers from 100,000 up to 7 million. They are arranged inline at centre-to-centre distances of S/D = 2.8, 4, and 5.6 and at 0° or 45° incidence angle. The trend of the drag curve for the upstream prism deviates at S/D = 2.8 and 4 sharply from that for a single prism. The drag force on the downstream prism strongly depends on both S/D and α. At S/D = 2.8, a thrust force occurs either in certain (α = 0°) or in all (α = 45°) flow states. Larger spacing values lead to an overall higher drag and a flattening of the Cd2 (ReD) curve. The highest fluctuating lift forces on the downstream prism are obtained at α = 45°. Regarding the vortex shedding frequency, differences between both incidence angles occur at S/D = 5.6 only.

PASSIVE CONTROL OF VORTEX-INDUCED MOTIONS ON DEEP-DRAFT SEMISUBMERSIBLE PLATFORMS

The deep-draft semisubmersible (DDSS) platform is well known due to its favourable vertical motion routine. However, DDSS platforms face a serious challenge in vortex-induced motion (VIM) due to the changing forces of the column. A computational fluid dynamic (CFD) simulation using AcuSolve is conducted to investigate the effect of a twisted-square column with different twisted angles, 0°, 22.5°, 45.0°, 67.5° and 90.0°, on the VIM responses and performance at current incidence angles of 0° and 45°.Platform models at a scale of 1:70 are studied to determine that the alteration of vortex shedding due the separation line at the twisted column expressively influences the mean and fluctuation of hydrodynamic forces and motion. In particular interest is the drag and lift coefficients, surge, sway amplitudes, as well as the yaw motion of the DDSS platform. The simulation was conducted at a velocity of 0.16 m/s.

State-of-the-Art Review of Vortex-Induced Motions of Floating Offshore Wind Turbine Structures

The motivation for this study is the fast development of floating offshore wind energy and the immature methodology and engineering practice related to predictions of vortex-induced motions (VIM). Benefiting from the oil and gas industry, in the past several decades, extensive knowledge and experience on vortex-induced vibrations (VIV) on slender marine structures has been gained. As the learnings from these efforts should be transferred and adapted to the renewable energy industry, a state-of-the-art review on influential VIM research has been carried out in this paper, focusing on: (1) engineering practice, (2) model tests, (3) numerical calculation, and (4) field measurement. Engineering gaps and potential research topics are identified as future work.

On the submerged low-Cauchy-number canopy dynamics under unidirectional flows

The unsteady dynamics and induced turbulence associated with flow development of a fully-submerged canopy with relatively low flexibility were explored experimentally under small Cauchy numbers Ca≲9 using particle image velocimetry (PIV) and particle tracking velocimetry (PTV). Inspection of the flow around a rigid canopy supported the identification of significant modulation of the flow dynamics due to minor blade reconfiguration and oscillations within the first rows. The canopies consisted of relatively short arrays of rectangular blades of height h=37.5mm placed in a staggered layout and extending Δx/h=5 in the streamwise direction. The results show a downstream shift in the onset of the internal boundary layer. Canopy top friction velocity was lower than its rigid counterpart, even where blades undergo negligible deflection, evidencing the impact of the leading structure dynamics on flow adjustment. A transition from vortex-induced blade motions to dominance by natural blade frequency occurs with distance from the canopy edge. A frequency synchronization occurs several rows into the canopy, resulting in peak displacement intensity of the blades. The relationship between flow-induced and blade natural frequencies is described through a developed transfer function, enabling the determination of the characteristic vortex frequency. Natural and flow-induced frequencies dominate the flow-structure interaction near the leading edge region, resulting in increased small-scale turbulence.

An experimental assessment of the effect of current on wave buoy measurements

Wave measurement buoys provide characterisation of wave climates that forms the basis for the design of offshore systems. These buoys are commonly subjected to currents which affect the resulting wave measurements, and if not accounted for will result in errors in the estimated sea state parameters. The present work provides results and observations from experiments aimed at assessing the impact that currents have on wave buoy measurements, thereby informing processing techniques to more accurately include this effect. Through scaled testing (circa 1:15) in a combined wave–current test tank, buoy motions (diameter, D = 0.24 m) are recorded in current only, waves only, and combined wave–current including oblique conditions. From these, the wave-induced motions are extracted and compared against three prediction methods based on established transfer function approaches as well as a frequency-domain hydrodynamic coefficient (HC) model based on potential flow. The scaled buoy was observed to have large, complex, irregular oscillatory vortex-induced motions (VIM) exceeding the buoy diameter. Both the magnitude and frequency of these oscillations was found to be significantly altered by the mooring stiffness and configuration whilst the addition of collinear waves was found not to affect the magnitude of VIM. Furthermore, due to the lack of VIM heave response and a large difference between the frequencies of the vortex-induced and wave induced horizontal motions, it was found that the VIM did not significantly alter the interpretation of the wave climate for the tested conditions. The HC model was found to accurately capture the observed modified hydrodynamics for opposing wave–current conditions, where larger horizontal motions than (typically) predicted are observed for all frequencies. This behaviour is concluded to result from increased excitation forces owing to the higher wavenumbers. The experiments highlight the potential effects of VIM on wave measurement performance of wave buoys, along with the complex and mooring-dependent nature of the response. Altered dynamics in the presence of currents are described which must be accounted for to avoid errors and the presented prediction methods provide a mechanism to account for these effects in wave processing methodologies which can subsequently reduce uncertainty in our understanding of the offshore environment.

Effect of initial roll or pitch angles on the vortex-induced motions (VIM) of floating circular cylinders with a low aspect ratio

The vortex-induced motions (VIM) is a phenomenon that can promote large oscillatory amplitudes on circular platforms in the horizontal plane, such as spar or monocolumn production, storage, and offloading system (MPSO). Floating offshore wind turbines (FOWT) can use a spar-type platform to support the turbine, and due to the wind incidence, roll or pitch angles can occur. The main goal of this work was to experimentally investigate the effect of the initial roll or pitch angles on the VIM amplitudes of a floating circular cylinder with a low aspect ratio. Four different initial angle conditions were carried out, namely vertical, roll angle, positive pitch angle, and negative pitch angle. The Reynolds number range performed was 12,000<R<100,000. The results of VIM amplitudes showed that the yaw motion was affected by the initial pitch angle. The initial pitch angles proved to be responsible for creating a geometric asymmetry of the body and promoted more significant yaw motions than the case without initial angle conditions. The yaw motions started to be substantial after the mean pitch angle of the cylinder reached 3°. The yaw motion increased as the reduced velocity was increased and showed to be essential to be correctly modeled and analyzed in the case of VIM on FOWT.

An experimental investigation on vortex-induced motion (VIM) of a tension leg platform in irregular waves combined with a uniform flow

Vortex-induced motion (VIM) of a tension leg platform (TLP) is critical to avoid structural and fatigue damages to the tendons and risers. Many previous studies have conducted model tests for TLPs in towing tanks. However, the models were usually established using linear springs to replicate the underwater moorings. In this study, a TLP model with full-water-depth tendons and top tension risers (TTRs) was developed. A series of model tests were conducted in a large-scale wave tank at three incidence angles (θ=0°, 22.5°, and 45°) for the reduced velocity ranging from 4 to 11.5 and two extra cases of flow combined with irregular waves. The maximum transverse amplitude of 0.32D was observed at a reduced velocity of 7.0 when θ=0°. On the other hand, the in-line and yaw motion experienced increasing amplitudes with reduced velocities and no lock-in region was identified. The most significant responses of transverse and yaw motion appeared at 0° and 22.5° headings, respectively, while no critical heading was recognized for surge motion. The competing mechanics between VIM and the wave-induced motion were revealed in the cases of combined flows and irregular waves. The tensions in tendons and TTRs that result from VIM were found as significant as those induced by wave loads, which highlights the necessity of considering VIM in the design of TLP. Furthermore, the result showed that the VIM amplitudes were considerably suppressed under the nonlinear stiffness and larger damping provided by the tendons and TTRs.