Vortex Induced Motion Response Research Articles

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.

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.

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 new passive control technique for the suppression of vortex-induced motion in deep-draft semisubmersibles

The control of vortex-induced motion (VIM) of deep-draft semisubmersibles has recently emerged as a serious concern in offshore semisubmersible platforms subjected to high ocean currents. With the increased draft of semisubmersibles, the coherent wake vortex fields behind the multi-column platforms become quite significant, which increase the impact of the coupled fluid-structure dynamics on the platform motion. In particular, the effective mitigation of the synchronization of wake vortices in a deep-draft semisubmersible is important for the reliable station-keeping and to extend the fatigue life of risers and mooring systems. The present study aims at the development of a passive control technique for the vortex synchronization of deep-draft semisubmersibles (DDS) via continuous cross-sectional twisting of the rounded square column along the spanwise direction. The computations are performed using a hybrid URAN-LES turbulence model based on the finite volume method. A validation study of the numerical results is performed with the available experimental data for the stationary and vibrating configurations of deep-draft semisubmersible. To begin, we first examine the VIM performance of a single column of the semisubmersible with a twisted angle of 45 °. We next examine the design of twisted column to understand its effect on the vorticity dynamics and the VIM response characteristics. In comparison to the untwisted square-column configuration, the twisted column produces the reduction in the transverse (sway) VIM amplitude up to 90% at the peak lock-in condition. The streamwise (surge) amplitude for the twisted-column semisubmersible is also found to be reduced by approximately 1/3 of the amplitude of the square-column counterpart. The twisted surface on the column results in a continuous spanwise (column-wise) variation of the shear-layer separation points. The three-dimensional (3D) variation of separation lines and the pressure distribution along the twisted and the square columns are analyzed to understand the underlying mechanism of the VIM suppression. We find that the modification of vortex shedding due to the variation in the separation line along the twisted column significantly influences the mean and fluctuating hydrodynamic forces. We extend the twisted column concept to the deep-draft semisubmersibles and demonstrate its effectiveness to produce a remarkable degree of VIM suppression. We explore the effects of the orientation of twisted columns and the incidence angle of the oncoming flow and present the 3D wake vortex structures and the motion histories of the DDS configuration.

Vortex-Induced Motion Response of Semi-Submersible Platform in Deep Water: II. Investigation on Hull Optimization

Vortex-Induced Motion Response of Semi-Submersible Platform in Deep Water: II. Investigation on Hull Optimization

Vortex-Induced Motion Response of Semi-Submersible Platform in Deep Water: I. Investigation on Key Characteristics

Vortex-Induced Motion Response of Semi-Submersible Platform in Deep Water: I. Investigation on Key Characteristics

Numerical studies on vortex-induced motions of a multi-column deep-draft oil and gas exploration platform

This paper presents numerical studies on vortex-induced motions (VIM) of a multi-column floating oil and gas exploration platform. Numerical computations are performed using an improved delayed detached eddy simulation (IDDES) together with a moving grid approach. The transverse (sway) and yaw motion responses, motion trajectories, motion frequencies and power spectral density of motions are computed and analyzed systematically. After extensive comparisons with experiments, it is confirmed that the present numerical solutions using IDDES agree well with the experimental results and are better than those via delayed detached eddy simulation (DDES). The differences of transverse motion responses between computational results and experiments are less than 10% in the lock-in region. The numerical simulations reveal that the transverse VIM responses occur in a range of reduced velocities from 7.0 to 14.0 at H/D = 1.44 (H and D are the column height and width, respectively). The largest nominal transverse amplitude, around 35% of the column width, occurs for 22.5∘ current incidence. It is found that the VIM responses mainly perform along the platform diagonals for 15∘, 22.5∘ and 45∘ current incidences. The transverse and yaw motion frequencies for 15∘, 22.5∘ and 45∘ current incidences are higher than those for 0∘ current incidence. The energy levels of the yaw motion responses for 15∘, 22.5∘ and 45∘ current incidences are about 10% of that for 0∘ current incidence. Moreover, parametric studies have been performed to examine the effect of submerged column height on VIM. It demonstrates that VIM in transverse direction grows significantly when submerged column height H/D is greater than 1.0. To be more specific, compared to the case with H/D = 1.44, VIM in transverse direction at H/D = 3.0 increase by around 120%.

Research on the effects of in-line oscillatory flow on the vortex-induced motions of a deep draft semi-submersible in currents

A Deep Draft Semi-submersible (DDS) under certain flow conditions could be subjected to Vortex-Induced Motions (VIM), which significantly influences the loads on and life fatigue of the moorings and the risers. To investigate the VIM of a DDS with four rectangular section columns in waves coupled with a uniform current, a numerical study using the computational fluid dynamics (CFD) method was conducted. The issues of the VIM of multi-column floaters can be conveniently converted to the issues of oscillating cylinders in fluid cross flows. This paper looks into the CFD numerical simulation of infinite cylinders having rectangular sections in a two-dimensional sinusoidal timedependent flow field coupled with a uniform current. The resulted hydrodynamic forces and motion responses in different oscillatory flows plus currents both aligned in the same direction for the incidence of 135° of the DDS relative to the flow are compared with the ones in current only cases. The results show that the VIM response of this geometric arrangement of a DDS with four rectangular columns in a current combined with oscillatory flows is more evident than that in the current only case. The oscillatory flows and waves have the significant influence on the VIM response, forces and trajectory, in-plane motions of the DDS.

Experimental and numerical studies of the pontoon effect on vortex-induced motions of deep-draft semi-submersibles

The vortex-induced motion (VIM) is a critical issue in mooring and riser system design for column-type deepwater platforms. As regards to deep-draft semi-submersibles (DDS), even though VIM is mainly excited by vortex shedding around columns, the large-volume pontoons beneath the columns are also responsible for the wake interference, implying a non-negligible influence on VIM behavior. An experimental study and three-dimensional numerical simulations were performed to analyze the pontoon effect on the VIM of two semi-submersibles and a four-column structure without pontoons. The numerical results using Detached Eddy Simulation (DES) are in good agreement with the experimental measurements obtained from the towing model tests. The present investigations indicate that the resonant phenomenon is observed for all configurations. However, the four-column structure without pontoons shows the most significant transverse responses and yaw motions at both 0°- and 45°-incidences owing to the largest fluctuating lift forces induced by the well-established wake. Additionally, the negative values of work done by the pontoons at all reduced velocities confirm their damping effect on the VIM response.

Parametric study on the vortex-induced motions of semi-submersibles: Effect of rounded ratios of the column and pontoon

The vortex-induced motions (VIMs) of semi-submersibles have emerged as an important issue in offshore engineering, as they pose a threat to safe and reliable operations and severely affect the fatigue lives of risers and mooring systems. The VIM response depends on the shape of the submerged structure and thus is significantly influenced by the design parameters related to the columns and pontoons. Numerical simulations by the detached Eddy simulation method are validated by experimental data and then used for parametric analysis of the VIM performance of various semi-submersibles with different column rounded ratios (Rc/L) and pontoon rounded ratios (Rp/Lp). The results show that the transverse amplitudes of a semi-submersible with circular columns at a 0° current heading are twice as large as those at a 45° current heading. However, the semi-submersible with rounded square columns shows more significant transverse motions at a 45° current heading than at a 0° current heading. Furthermore, at the 45° current heading, the transverse amplitudes of the semi-submersibles show a rapid increase as the column radius increases in the range of Rc/L<0.1. The peak values remain roughly the same for 0.1≤Rc/L≤0.2 and then decrease as the column radius increases (Rc/L≥0.3). In addition, the effect of the pontoon shape on the transverse response is negligible for semi-submersibles with sharp square columns, while for semi-submersibles with rounded square columns or circular columns, the sharp rectangular pontoons greatly mitigate the VIM response.

A model test investigation on vortex-induced motions of a buoyancy can

This paper presents an experimental investigation on 6-DOF vortex-induced motion response of a tethered buoyancy can under conditions of different tether lengths. The buoyancy can was mounted vertically, with the diameter of 150 mm and the length of 700 mm. The responses of trajectory, amplitude, frequency and other parameters were analyzed in detail under conditions of different tether lengths and height above the water. In addition, during the experiment, obvious yaw motion is observed and the yaw characteristics were investigated. The results show that when the reduced velocity is small (less than about 3.5), the motion amplitude is small and irregular and when the reduced velocity is large (3.5<Ur < 15), the in-line vibration frequency is two times the transverse vibration frequency and the trajectory of the cylinder is in “figure 8” shape. A strong connection between the yaw motion frequency and the vibration frequency was also found. Moreover, the mean drag coefficient of the buoyancy can was measured and the amplification effect of VIV on drag coefficient was observed.

Experimental study on vortex-induced motions of a semi-submersible with square columns and pontoons at different draft conditions and current incidences

The Vortex-induced Motions (VIM) phenomenon of semi-submersibles is relevant for the fatigue life of moorings and risers. Model tests regarding the VIM behavior of a semi-submersible with four square columns were conducted in order to investigate the effects of the current incidence and the aspect ratio of the immerged column. The experimental results show that the largest transverse amplitudes are around 70% of the column width at 30° and 45° incidences in a range of reduced velocities from 5 to 8 when the aspect ratio of the immerged column is 1.90. The largest yaw motion occurs at 0° incidence with the peak value around 4.5°. Similar characteristics of the VIM response are observed for the semi-submersible with aspect ratios of 1.90 and 1.73. When the aspect ratio decreases 50% to 0.87, 30% decrease in the peak transverse amplitudes can be seen.

Investigation of the VIMs of a spar-type FOWT using a model test method

This article presents the research findings of an extensive model test investigation of the dynamic response to vortex-induced motions (VIMs) of the OC3 spar-type floating offshore wind turbine (FOWT). Particulars of this research are to investigate the unique and crucial effects of wind load on the spar-type FOWT, which differs considerably from other traditional spar-type floating systems utilized in the gas and oil industry, on the VIMs. The model test was performed with a sequence of current, wind, and irregular wave sea states to analyze the nature of the coupled dynamic response behavior of VIMs. Many unique characteristics were found and analyzed. The lock-in phenomenon of sway in the cross flow (CF) direction was found to occur first, followed by the lock-in phenomenon of surge in the in-line (IL) direction. For the current-only case, the remaining responses, including the other 4-degree-of-freedom motions, mooring tensions, and turbine bearing loads, were found to be coupled by sway/surge VIMs. Moreover, the oscillation amplitudes of all of the responses increased significantly with increasing current velocity, particularly after locking-in. Furthermore, the wind load had a clear suppression effect on the CF and IL VIM responses. The experimental measurements demonstrated that the wave load excites wave-induced oscillations for some CF responses and all IL responses, and it further restrains the VIM oscillation amplitudes on the foundation of the wind suppression effect. In particular, when a wind load is involved, the influence of current or wave is limited to the yaw and IL tower-top bending moment.

Experimental study on vortex-induced motions of a semi-submersible platform with four square columns, Part I: Effects of current incidence angle and hull appendages

An experimental study on Vortex-Induced Motion (VIM) of the semi-submersible platform concept with four square columns is presented. Model tests were carried out to check the influence of different headings and hull appendages (riser supports located at the pontoons; fairleads and the mooring stretches located vertically at the external column faces; and hard pipes located vertically at the internal column faces). The results comprise in-line, transverse and yaw motions, as well as combined motions in the XY plane, drag and lift forces and spectral analysis. The main results showed that VIM in the transverse direction occurred in a range of reduced velocity 4.0 up to 14.0 with amplitude peaks around reduced velocities around 7.0 and 8.0. The largest transverse amplitudes obtained were around 40% of the column width for 30° and 45° incidences. Another important result observed was a considerable yaw motion oscillation, in which a synchronization region could be identified as a resonance phenomenon. The largest yaw motions were verified for the 0° incidence and the maxima amplitudes around 4.5°. The hull appendages located at columns had the greatest influence on the VIM response of the semi-submersible.