Vortex-induced Motion Research Articles

Nonlinear motion regimes and phase dynamics of a free standing hybrid riser system subjected to ocean current and vessel motion

A novel structural dynamic model for free standing hybrid riser (FSHR) is established based on the absolute nodal coordinate formulation (ANCF) in the present study. The position and slope coordinates are used in global framework to avoid coordinate transformation, and an accurate geometric relationship is introduced to describe geometric nonlinearity. The current loads on the risers and buoyancy can are simulated by Morrison equation and wake oscillator, respectively. After case validations, the vortex-induced motion (VIM) regimes of the buoyancy can and the phase dynamics of the riser and jumper in a FSHR system, subjected to the ocean current and vessel motion, is numerically investigated. It is found that the VIM of the buoyancy can experiences multiple motion switches among the periodic, quasiperiodic, multiple periodic, transition I and transition II regimes under different vessel motions. The y-motion of the jumper and both x- and y-motions of the riser present the phase trapping phenomenon along the structures. The phase trapping and locking, phase drifting and slipping, and other states are observed for x-motion of the jumper, which form a shape of “fish head” with opening mouth towards lower amplitude or higher period of vessel motion in regime map.

Numerical modeling to develop strake design of Spar platform for Vortex-Induced motions suppression

In this paper, a numerical study was performed aiming to reduce the Spar hull surge and sway response due to marine currents for various arrangements of strakes. Spar is a compliant platform consisting of a large-diameter single-cylinder, supporting a deck, moored in place vertically. Spar Vortex-Induced Motions (VIM) suppression is one of the most challenging offshore industries for assessing mooring, risers, and structure fatigue. This phenomenon is mostly occurring due to the interaction of marine currents with Spar platforms. Therefore, Spar VIM response has been the subject and focus of some research in the past few decades. In this research, the interaction of the marine current with a Spar model was numerically simulated. The results were compared with the VIM response of the existing Spars with conventional strake design to validate the model. Different strake patterns and a conventional bare hull Spar armed with strakes (Base model) were used to model the VIM response of the platform. The results demonstrated the possibility of Spar VIM response reduction due to the improving design of Spar strakes. Eventually, the best model was proposed with 58% suppression in sway motion and 9% in surge motion of VIM response and the results were discussed.

A new method to couple FEM mooring program with CFD to simulate Six-DoF responses of a moored body

The objective of this study is to develop an accurate and efficient coupled program, which incorporates computational fluid dynamics, a six-DoF motion solver, and nonlinear finite-element cable analysis program MOORING3D. The new coupled method aims at studying dynamic responses of moored structures in environments which cannot be modeled by conventional approach. The novelty of the coupled method also lies in that we develop the methodology for an overset grid system of embedding, overlapping, and moving structured grids. Large eddy simulation was used as the turbulence model. The level-set method was applied to capture free surface effects. Two buoys were studied. Wave-induced motions of the first buoy were simulated by both coupled CFD-mooring method and commercial software. The discrepancy was investigated to illustrate advantages of the coupled method in addressing viscous and nonlinear hydrodynamic effects. The wave-current-body interaction of another buoy was simulated with RAOs being compared against experiments and vortex-induced motion being investigated. The conclusions indicated that the coupled CFD-FEM method provides feasible solutions for floating systems exposed to complicated environmental conditions.

Experimental on the vortex induced motion suppression of floating spar-type wind turbine

To investigate the suppression performance of the spar-type wind turbine vortex induced motion(VIM) with the addition of splitter plate, VIM model tests were carried out in a circulating water channel. Two different splitter plates width conditions were tested. In all the conditions, the cylinder was free to oscillate with one DOF. Results regarding response amplitudes and characteristic frequencies were presented and discussed. There was no obvious lock-in phenomenon on the vortex excitation motion of the cylinders with splitter plate, only some synchronization was observed. W/D=0.2 has better suppression performance than W/D=0.4 in vortex induced motion.

Experimental investigation of the vortex-induced motion of tension leg platform system with suppression system under current

The vortex-induced motion (VIM) of a tension leg platform (TLP) and vortex-induced vibration (VIV) of tension legs and risers occur only under the action of water flow. The VIM results in a high amplitude motion in the cross-flow (CF) direction, and the VIV leads to fatigue failure of the tension legs and risers. A three-dimensional (3D) physical model investigation was conducted on the overall dynamic response of the platform and acceleration at different positions of the tension legs in the in-line (IL) and CF directions. Different lengths of the splitter plate behind the TLP columns were observed, and the suppression effect of the VIM at six flow velocities was investigated. The experimental results indicated the obvious VIM phenomenon of the platform without the splitter plate, and multi-frequency time-varying coupling characteristics were observed when the heading angle was 45°. Furthermore, the results revealed that the VIM phenomenon of the platform was evidently suppressed, in amplitude and frequency, and the force coefficients are reduced significantly, after the splitter plates were arranged behind each column of the platform. However, the splitter plate slightly affected the VIV of the tension legs.

Comparative Test of Characteristics of Vortex-Induced Motion and Galloping of Classic Spar Platform

Comparative Test of Characteristics of Vortex-Induced Motion and Galloping of Classic Spar Platform

NUMERICAL SIMULATION ON A DEEP-DRAFT SEMI-SUBMERSIBLE

A numerical study is conducted to determine the Vortex Induced Motion (VIM) effects on Deep-Draft Semi-Submersibles (DDSS). The VIM phenomena is a crucial problem that can cause severe impact on the fatigue life of mooring risers in DDSS. Therefore, a comprehensive numerical simulation is conducted using the Acusolve computational fluid dynamics (CFD) software. Five models of immersed columns with different aspect ratios (ie. 0.6, 0.8, 1.0, 1.2 and 1.4) are numerically investigated under two different incidence angles, which are 0° and 45°. The transverse and in-line vibration amplitude, amplitude of lift force coefficient and vortex shedding are analyzed. The numerical measurements are obtained to see the response of horizontal plane motions, which are transverse, in line and yaw motions. This study with detailed numerical results from parametric data will contribute future studies and the comparisons are made to demonstrate the capability of the present CFD approach.

A Few Frontier Issues in Ocean Engineering Mechanics

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.

Experimental study on vortex-induced motions of a semi-submersible platform with four square columns, part III: Effects of the collinear irregular and regular wave incidence and current

Aiming to complete the results presented before in Gonçalves et al. (2012. Ocean Eng. 54, 150–169) and Gonçalves et al. (2013. Ocean Eng. 62, 10–24), this third part of the research work studied the behavior of large-volume SS - semi-submersible platforms due to the VIM - vortex-induced motions under the combined action of current and waves. Small-scale towing tests were carried out to understand the effects of the collinear irregular and regular waves on the VIM of a SS. The results showed that waves influenced the VIM phenomenon and decreased its response amplitudes in specific cases, although the source of this mitigation process does not depend on the wave nature (regular or irregular). Moreover, by using plots of velocity ratio α as function of the KC - Keulegan-Carpenter number, regions of predominance were identified for inertia and viscous forces. The VIM in waves was mitigated for cases with inertia-force predominance, yet it had appeared even in low amplitudes for cases of large KC number at the viscous-force region.

Spoiler plate effects on the suppression of vortex-induced motions of a single circular cylinder

Several methodologies have been proposed to mitigate vortex-induced motions (VIM) on offshore structures, such as the use of strakes, splitter plates, and spoiler plates. Among the mitigation devices, spoiler plates remain insufficiently explored. Given the situation, a parametric experimental investigation was undertaken to investigate the suppression of vortex-induced motions of a circular cylinder with a low aspect ratio using spoiler plates. The experiments were carried out in a towing tank. The circular cylinder tested had an aspect ratio (L/D) of 1.5, which is typically employed in monocolumns and columns of semi-submersible (SS) platforms. Several configurations were investigated by changing the spoiler plate aspect ratio (AR,S), the number of rings with spoiler plates along the vertical axis of the cylinder (nL), and the number of spoiler plates per ring (nD). The reduced velocities investigated varied from 1 to 12, and the Reynolds number from 12,500 to 95,000. The results demonstrated the occurrence of a desynchronization of the vortex shedding, which led to a significant reduction of the in-line and transverse amplitudes. Moreover, a decrease in drag forces of approximately 50% occurred for the most improved case, compared with a bare cylinder, demonstrating a potential solution for this mitigation device.

Investigation of mooring damping effects on vortex-induced motion of a deep draft semi-submersible by coupled CFD-FEM analysis

The vortex-induced motion (VIM) of semi-submersible is gaining increasing attention from both industry and academia with the recent development of deep draft semi-submersible platforms. Field measurements of semi-submersibles reveal that the VIM responses are typically found to be smaller than those observed in model tests. The possible factors causing the VIM response reduction in the field measurements include Reynolds number, wave effects, mass ratios, and external damping from mooring lines. The objective of this study is to investigate the mooring-induced damping effects on VIM. The Finite Analytic Navier-Stokes (FANS) code is integrated with an in-house six degree-of-freedom (DoF) motion solver and an in-house nonlinear finite element (FEM) code MOORING3D, for the coupled CFD-FEM analysis of the VIM of a moored semi-submersible. The large eddy simulation (LES) turbulence model is used to provide accurate estimation of hydrodynamic loading. Simulations are performed for the 1:70 model of the platform under varying reduced velocities. Full-scale mooring systems with different water depths and mooring line numbers are designed to represent different levels of mooring damping effects. The simulated VIM response is compared with the experiments. The study reveals that mooring-induced damping is a critical factor contributing to the VIM response reduction in the field.

Effects of column shape and configuration on the vortex-induced motions of semi-submersibles

The vortex-induced motions (VIM) of offshore floaters occur when the frequency of the vortex shedding is close to the natural frequency, and this phenomenon can severely affect the fatigue life of risers and moorings. During the design and operation stage of the semi-submersible (SS), VIM behavior has emerged as an important issue in offshore engineering. The shape of the columns, circular or square, and the wake interference, characterize the VIM of the SS. Numerical studies using the validated Detached Eddy Simulation (DES) method are carried out for parametric analysis of the VIM performance of various SS models consisting of different column shapes: with four rounded square columns, four circular columns and the mixed square-circular columns. The results show that the most significant transverse responses at 0∘ current incidence are observed for the SS with four circular columns and the one with two circular upstream columns. At a diamond configuration (i.e., 45∘ and 135∘), the SS models with a square upstream column all present the most pronounced VIM. The analysis shows that the vortex shedding characteristics of each column at different current incidence govern the VIM of the SS models. The square and circular columns both do large positive work on the transverse motions, and thus improve the VIM response at a certain current incidence. Therefore, the design of SS with square, circular and multi-shape columns can not mitigate the VIM behaviors efficiently. Designing a SS model with irregular polygon-shaped columns or twisted columns to avoid the strong vortex shedding may work.

Experimental Characterization of High-Amplitude Fluid–Structure Interaction of a Flexible Hydrofoil at High Reynolds Number

Abstract A fluid–structure interaction (FSI) experiment was performed to study low-frequency (∼10 Hz), high-amplitude (±3.5% of the span) fin motion. This was achieved by placing an Inconel swept-fin at −9.6 deg angle-of-attack within the wake of a roughened cylinder. Speeds between 2.5 and 3.6 m/s produced cylinder diameter-based Reynolds numbers between 190,000 and 280,000, respectively. Detailed descriptions of the geometry, material/structural behavior, fluid properties, and initial conditions are provided to facilitate computational model development. Given the initial conditions, the resulting forced fin behavior was characterized with measurements of the mean and fluctuating velocity upstream of the fin (i.e., within the cylinder wake), fin tip/surface motion, and fin constraint forces/moments. This work provides a detailed experimental dataset of conditions mimicking a crashback event that is also a challenging FSI benchmark problem involving turbulent, vortex-induced structure motion. It has been used as a validation condition for FSI simulations, and it can be used to validate other FSI models as well as identifying strengths and weaknesses of various modeling approaches.

Application Progress of Computational Fluid Dynamic Techniques for Complex Viscous Flows in Ship and Ocean Engineering

Complex flow around floating structures is a highly nonlinear problem, and it is a typical feature in ship and ocean engineering. Traditional experimental methods and potential flow theory have limitations in predicting complex viscous flows. With the improvement of high-performance computing and the development of numerical techniques, computational fluid dynamics (CFD) has become increasingly powerful in predicting the complex viscous flow around floating structures. This paper reviews the recent progress in CFD techniques for numerical solutions of typical complex viscous flows in ship and ocean engineering. Applications to free-surface flows, breaking bow waves of high-speed ship, ship hull–propeller–rudder interaction, vortex-induced vibration of risers, vortex-induced motions of deep-draft platforms, and floating offshore wind turbines are discussed. Typical techniques, including volume of fluid for sharp interface, dynamic overset grid, detached eddy simulation, and fluid–structure coupling, are reviewed along with their applications. Some novel techniques, such as high-efficiency Cartesian grid method and GPU acceleration technique, are discussed in the last part as the future perspective for further enhancement of accuracy and efficiency for CFD simulations of complex flow in ship and ocean engineering.

An experimental study on vortex-induced motion responses of a moored semi-submersible with and without riser

This article presents an experimental study on the “vortex-induced motion” responses of a moored “semi-submersible without catenary riser” and “semi-submersible with catenary riser.” An eight-point catenary mooring system is adopted for the study in which eight rollers are installed on the pontoons for guiding the mooring lines and the semi-submersible design is of two rectangular pontoon and two circular columns on each of the pontoons. To replicate the real-world scene, the experiments are conducted in wave-cum-current flume with five directions of current (i.e. 0°, 30°, 45°, 60°, and 90°) to study the vortex-induced motion under the surge and sway degrees of freedom at high dimensionless reduced velocities (i.e. 6< Urs<50.86). Along with the surge and sway degrees of motion, the roll and pitch degrees of motion are investigated too with their paths for semi-submersible (i.e. amplitude traces). A “stainless steel flexible braided hose” with an aspect ratio of 157 till the touchdown point is used as a catenary riser. Results are presented with the definitions of degrees of freedom with respect to the axes defined at the center of gravity of semi-submersible and show that above the reduced velocity of 25 for the sway and surge degrees of freedom, the vortex-induced motion responses show a complex variation without any monotonicity. Furthermore, it is observed that (1) in traces there are no eight-shaped trajectories; (2) for both the semi-submersible without catenary riser and semi-submersible with catenary riser, the higher responses occur along the diagonals; (3) there are couplings of the pitch with surge, and roll with sway dofs; (4) the catenary riser reduces the vortex-induced motion responses for most of the current directions; and (5) galloping instabilities may be occurring and higher amplitudes may be related to them. Finally, it is shown that the vortex-induced motion response of semi-submersible reduces the vortex-induced vibration response of riser and that the motions in pitch and roll are likely to become critical and they demand a detailed analysis to be addressed in future to improve the operational range of semi-submersible and catenary riser in deep water depths under harsh marine environment of high current velocities.

A Study on Presentation of Wind Blowing with Air Vortex-induced Apparent Tactile Motion

A Study on Presentation of Wind Blowing with Air Vortex-induced Apparent Tactile Motion

Undamped transverse-only VIV of a diamond cylinder at low Reynolds numbers

There have been only a few investigations lately to analyze the flow-induced vibrations of a diamond cylinder, i.e. a square cylinder inclined at 45 ∘ to the free-stream at low and high Reynolds numbers, Re. However, for transverse vortex-induced motions of diamond cylinders at low Re, features of response branching, mean surface pressure, wake modes and fluid loading are less explored. We investigate numerically at Re=100, undamped hydroelastic vortex-induced vibrations of a diamond cylinder over a well-resolved U∗ range of 1 to 12. The order of magnitude of the maximum response as well as branching of response of the diamond oscillator are analogous to those of the circular cylinder and depart considerably from the ones of a square cylinder at zero incidence. Interestingly, the mean lift from this nominally symmetric cross-section deviates from zero at U∗ values of 3.5 and 4.2 corresponding to the quasi-periodic initial branch and periodic lower branch, respectively. At U∗=3.5 corresponding to low amplitude oscillations, three distinct asymmetric wake modes are identified: asymmetric 2S and two one-sided modes of different nature. The well known asymmetric P+S mode is identified at U∗=4.2 relating to high amplitude oscillations.

FIM - flow-induced motions of four-column platforms

Experiments regarding the FIM – flow-induced motion on an array of four floating cylinders with low aspect ratio, H/L=1.5, and three different distances between column centers, or spacing ratio, S/L=2, 3 and 4, were carried out in a towing tank; where H is the column height below the water area, L is the length of the face dimension of the columns, and S is the distance between center columns. The array of four columns was elastically supported by a set of four linear springs to provide low structural damping to the system. Three different section geometries of the columns were tested, namely circular, square, and diamond. Two different current incident angles were tested, 0 and 45°. These configurations of the four-column arrays were selected to cover the range of the main SS – semi-submersible and TLP – tension leg platforms. The aims were to understand the FIM nature of the four-column systems and to provide knowledge to the designers of offshore systems. The range of Reynolds number covered 10,000 < Re < 110,000. Concerning the geometry of the columns sections, the amplitude results showed larger amplitudes for the four-column array than the single cylinder case for the circular and diamond cases, in which VIM – vortex-induced motions dominated response was presented. In turn, the amplitudes for the single square case were higher than for the array; in this case, the galloping-dominated response was observed. Concerning the distance between column centers, the amplitude results for the four-column arrays with S/L=3 and 4, were very similar. The four-cylinder array with S/L=2 acted as a single cylinder or changed the wake interference significantly because of the proximity of the columns. Finally, the presence of pontoons had a significant effect on the motions in the transverse direction, and this presence was responsible for decreasing the amplitudes.

Large Eddy Simulation of Free Motion of Marine Riser using OpenFOAM

In this study, the free motion of a riser due to vortex shedding was numerically simulated with Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) turbulence models. A numerical simulation program was developed by applying the Rhie-Chow interpolation method to the pressure correction of the OpenFOAM standard solver pimpleDyMFoam. To verify the developed program, the vortex shedding around the fixed riser at Re = 3900 was calculated, and the results were compared with the existing experimental and numerical data. Moreover, the vortex-induced vibration of a riser supported by a linear spring was numerically simulated while varying the spring constant. The results are compared with published direct numerical simulation (DNS) results. The present calculation results show that the numerical method is appropriate for simulating the vortex-induced motion of a riser, including lock-in phenomena.

Numerical simulation of vertex induced motion for Spar platform with helical strakes

Objectives Long-lasting Vortex Induced Motion(VIM)may cause the Spar platform mooring system fatigue and even the structure damage. Therefore, the adverse effects of VIM must be fully taken into account. Methods In this paper, the CFD solver naoe-FOAM-SJTU and dynamic grid method developed by ourselves are used to make numerical simulation of the characteristics of the vortex induced flow field around the Spar platform. The time Delayed Detached-Eddy Simulation(DDES) turbulence model based on shear-transport-stress equation is used to simulate the three-dimensional fine wake vortex structure of the Spar platform with helical strakes. The longitudinal motion, transverse motion and yawing of the Spar platform with helical strakes at different reduced velocities are studied. The platform mooring system is stimulated by using the linear spring system. The research also takes into account the effects that the platform coupling effect at different degrees has on VIM. Then the numerical simulation results are compared with the model test results, to analyze the time history, spectral characteristics and locking of longitudinal and transverse VIM of Spar platform with helical strakes and to reveal the intrinsic mechanism of VIM. Results The results show that the helical strakes can effectively reduce the response amplitude of Spar platform's VIM; the CFD solver naoe-FOAM-SJTU has good accuracy and reliability for the calculation and simulation of the offshore platforms' VIM. Conclusions The numerical simulation and analysis of the VIM of Spar platform with helical strakes is of great significance to the practical design of Spar platform.