(VIV) IN STRUCTURES

Object of the Invention

The present invention relates to a device for passive suppression of Vortex-induced vibrations (known as VIV) in flexible structures or in elastically supported rigid structures.

Vortex-induced vibrations are a type of fluid-structure interactions that are consequence of the unsteady periodic forces generated in the wake of bluff bodies when affected by currents. The forces induced on the structures as a result of the large region of separated flow in the wake of the structure, result in vibrations. Vibrations can be fatal in engineering applications. Structures with low mass and damping are the ones in which VIV can be more important. In marine applications, most of the structures are characterised by combined low mass-damping. Although VIV is crucial in marine applications, the invention described here could also be used to prevent VIV in other non-marine scenarios such as structures affected by winds.

State of the Art

One of the most significant problems to overcome in offshore deep-water drilling is the effect of vortex-induced vibrations, VIV, particularly on pipes such as deep-sea risers, or any cylindrical structure that is exposed to external fluid flow. The study of vortex-induced vibrations (VIV) of bluff bodies is a classical problem in engineering.

Marine structures and ocean systems exposed to currents are prone to suffer vibrations induced by the hydrodynamic forces generated by the separated flow in their wake. Deep water drilling and production risers and other slender structures experience fatigue during their life span because of VIV. Vortex-induced motions are also typical of structures such as spar buoys in oil rigs or those found more recently in marine energy harvesting devices and floating wind turbines. In general, when systems are characterised by low mass (if compared to the fluid displaced mass) and low damping, VIV becomes a persisting problem and the structures tend to exhibit large amplitude vibrations in a wide range of operating conditions.

There has been an increased interest in devices to attenuate VIV in the last decades as a result of the industrial demand of such systems. One of the major industrial demands in relation to the VIV problem is to reduce the response of the structures, while keeping hydrodynamic forces contained. It must be done for all types of flow and omni-directionally. Different types of device have been developed to reduce the effect of VIV in low damping structures, such us the use of helical strakes around the riser, axial rods, perforated shrouds. In respect to helical strakes, they are very extended and have a good VIV reduction performance, but they considerably increase drag. They are also very expensive to manufacture and to install. Systems that streamline the structures are also well-known, for example fairings and splitter plates, which are known not only to reduce VIV but also drag. The main problem of those is that they are directional and if not, they can be extremely dependent on maintenance and on their ability to align with the flow. Other known suppression systems have been proposed in the past, see for example the use of tripping wires, control rods or suppression bumps.

W09526294 A1 describes a spar platform (1 ) for VIV suppression, which is configured in a cylindrical form shroud (4) arranged around a essentially vertical cylindrical buoyant vessel (7) with diameter D, wherein the shroud (4) comprises two essentially perpendicular intersecting sets of elements (5, 6). This shroud (4) discloses a radius (r) of: 0.53D<r<0.62D and a size of the clearance of the mesh (s) of: 0.05D<s<0.35D). However, this spar platform (1 ) do not produce optimal results of high VIV suppression capabilities, as the ones obtained with the present invention.

Consequently, there is a need for an improved device for eliminating (and not only reducing) such vibration effects, but at the same time reducing the drag. Purpose of the Invention The purpose of the invention is the passive reduction and attenuation of the Vortex- Induced Vibration dynamic response of engineering structures affected by currents or fluid flows.

The invention is of especial interest in engineering applications that involve low mass and low damping structures, as happens in ocean and offshore systems. The applicability is of especial interest in engineering systems such as ocean pipes, riser pipes, spar buoys, tendons, catenaries, mooring systems, etc. Those systems are typical in applications related to deep water offshore engineering, floating marine systems for wind turbines, oil rigs, and other systems for ocean exploration. Structures affected by winds are also prone to suffer VIV.

In the rest of the document we will sometimes use the term "marine structure" for referring to any of the previously described engineering systems, i.e. marine structures but also non-marine structures such as the ones affected by winds.

Description of the Invention According to the present invention, which is defined in claim 1 , a device for total or partial passive suppression of vortex-induced vibrations (VIV) in low mass-damping structures is provided, which is adapted for being arranged around an inner structure at a specific distance, and wherein the inner structure have an essentially cylindrical body with a diameter (D).

The device for passive suppression of VIV of the invention comprises a rigid mesh formed by rigid wires arranged in cylindrical form with radius (r), made of a series of intersecting wires with diameter of the wires (di) that form a grid. The space (s) between elements is defined here as the distance between two longitudinal or transverse consecutive elements in the mesh, therefore the parameter "s" indicates the size of the clearance of the mesh. The mesh forms a cylindrical structure of radius (r) that is to be placed around a structure of characteristic diameter (D), in order to mitigate VIV motions. Said rigid mesh is wrapped around the marine structure forming a rigid cylindrical configuration, wherein the radius (r) and the side or size (s) of the mesh are expressed as a function of the diameter (D) of the inner marine structure. These two parameters (r and s) determine the degree of VIV suppression achieved in the marine structure.

On the other hand, the rigid mesh does not need to occupy all the length (L) of the marine structure; therefore, the rigid mesh may have a span of equal, greater or smaller length than that of the marine structure.

In one possible embodiment of the invention, said grid pattern of the mesh is formed by a uniform perpendicular set of intersecting elements that form a polygonal arrangement, preferably in squares. Nevertheless, alternatively, other polygonal patterns may be used, with elements forming rhombus, rectangles or other polygonal shapes.

Furthermore, in one preferred embodiment of the invention, the rigid mesh is made of rigid wires. Nevertheless, other equivalent materials may be used. In one preferred embodiment of the invention, the mesh has a radius (r) in the range that goes from 0,5 and 3 times the diameter (D) of the structure. The size (s) of the side of the unitary cell is comprised between 0, 1 and 0,7 times the diameter of the marine structure (D). In other preferred embodiment of the invention the mesh has a radius (r) comprised between 0,7 ^* D (i.e. 70% of the structure diameter (D)) and 3 ^* D (i.e. 300% of the structure diameter (D)); and the size (s) of the side of the unitary cell is comprised between 0, 1 and 0,7 times the diameter of the marine structure (D). In one preferred embodiment of the invention, the diameter (di) of the elements forming the mesh is comprised between 0.01 and 0.1 times the diameter of the structure (D) (i.e. from 1 % to 10% of the structure diameter).

In the preferred embodiment, which has been proved experimentally as showing the best results, the rigid mesh has a radius (r) comprised between 0,5 and 1 ,3 times the diameter (D) of the marine structure and a size (s) of the side of the square comprised between 0, 1 and 0,36 times the diameter (D) of the marine structure.

The preferred value for the radius (r) of the mesh expressed as a function of the marine structure is: 1 ,3 ^* D, with a square mesh size (s) of 0,1 ^* D.

The device of the invention can advantageously reduce by more than 95% vortex induced vibration (VIV) in structures which are exposed to currents, reducing simultaneously the drag forces (approximately a 20% of drag reduction inside the lock-in region).

The device of the invention can be manufactured very easily and economically.

The device of the invention operates in an omni-directional manner, because it is wrapped around the whole perimeter of the marine structure, so its effectiveness is independent of the current direction.

Additionally, the device has the advantage that it operates without any other standoff mechanism to be fitted or attached to the marine structure. It simply needs to be rigidly supported to the main structure. It does not require power as it is completely passive.

Other details and characteristics will be shown over the course of the description below, which refers to the drawings that accompany this report, in which there is a graphic representation of the invention and the results of laboratory experiments carried out, for the purpose of illustration, not limitation.

Description of the Figures The invention will now be described in more detail and by way of example with reference to the accompanying figures, in which:

Figure 1 is a schematic three-dimensional view of a cylindrical model structure used to the represent the marine structure (2) fitted with the suppression wire mesh (1 ) described as the invention. Figure 2 is a schematic view of the cylinder model that represents the marine structure (2), fitted with the suppression mesh (1 ), which is a preferable embodiment of the present invention.

Figure 3 shows a comparison of the amplitude A ^* (in plot a)) and frequency f ^* (in plot b) ) response of the cylinder model that represents the marine structure in the experiments, with and without suppression meshes. All the plots are for meshes with square grid size s = 0.1 D. The mean drag coefficient (Cd) appears in plot c). Symbols indicate the different sizes (r) of the mesh. The abscissa is for the reduced velocity U ^* and the Reynolds number Re.

Figure 4 shows a comparison of the amplitude (A ^* ) (in plot a)) and frequency (f ^* ) (in plot b)) response of the cylinder model that represents the marine structure in the experiments, with and without the suppression meshes. All the plots are for meshes with square grid size with s = 0.36D. The mean drag coefficient (Cd) appears in plot c) . Symbols indicate the different sizes (r) of the mesh. The abscissa is for the reduced velocity U ^* and the Reynolds number Re. Figure 5 shows the phase averaged dimensionless vorticity maps for the marine structure model without suppression meshes, at a reduced velocity U ^* ~ 6 (where the largest VIV oscillations take place), at eight cylinder positions of the oscillation cycle. Clockwise vorticity appears in red. Flow is left to right and the phases are 0, ττ/4, ττ/2, 3π/4, π, 5ττ/4, 3π/2, 7ττ/4, from the upper left corner to the right.

Figure 6 shows the phase averaged dimensionless vorticity maps for the marine structure with a suppression mesh with radius r = 1.3D and s = 0.1 D, at a reduced velocity IT ~ 6 (where the largest VIV oscillations take place when the cylinder is not equipped with the suppression device), at eight cylinder positions of the oscillation cycle. Clockwise vorticity appears in red. Flow is left to right and the phases are 0, TT/4, TT/2, 3TT/4, TT, 5TT/4, 3TT/2, 7TT/4, from the upper left corner to the right.

Description of the experiments The effectiveness of the wire mesh (1 ) in suppressing vortex-induced vibrations (VIV) has been investigated experimentally in detail.

A series of square wire meshes with different densities and sizes have been tested. The key parameters regarding the size of the mesh are its radius (r) and its mesh density (s), i.e. the distance between consecutive parallel wires in the mesh forming a square. Both parameters are expressed in terms of the characteristic size or diameter (D) of the marine structure in which the mesh is to be installed for suppressing VIV. The work focuses on the VIV response description as well as on the analysis of the hydrodynamic drag force associated with the device.

Different sizes (r) of the mesh have been studied in order to alter locations in the wake with high sensitivity to VIV, while keeping the system omnidirectional. The effect of the mesh density (s) is inferred by testing two different mesh porosities.

In particular, the square meshes that have been experimentally investigated in the hydrodynamics laboratory, have the following parameters:

- radius (r) of the mesh: 0,5 ^* D; 0,6 ^* D; 0,9 ^* D; 1 ,3 ^* D; and

- size (s) of the of side of the squares formed by the wires in the mesh:

0,1 ^* D; 0,36 ^* D.

diameter (di) of the wires that form all the meshes: 0.02 ^* D.

The results clearly show that all the meshes investigated were able to attenuate significantly the VIV response of the marine structure. At the same time a reduction of drag forces was achieved to different degrees.

The mesh that provided better VIV suppression results had a size (s) of 0,1 ^* D and a radius (r) of 1.3 ^* D. a) Facility and structural model:

Experiments were conducted in a laboratory closed loop recirculating free surface water channel. The water channel has a cross-section of 1 * 1.1 m ² downstream a 6 to 1 (cross-sectional area) three-dimensional contraction, and it is able to deliver over 0.7 m ³ /s. The flow is generated by two axial pumps controlled by two frequency sources, and the profile in the working section is characterized by a very low velocity variability, with a maximum deviation in velocity of 1.58%. This facility has been widely used in the past 7 years for producing state of the art research.

A circular cylinder model was used for the study, in order to model a generic marine structure, and consisted of a rigid transparent acrylic tube with and immersed length L = 0.5 m and with an external diameter D = 0.05 m (aspect ratio L/D = 10). The model had an end-plate attached to his bottom end in order to prevent three- dimensional effects. Two springs were connected to the model providing restoring forces to the system, that hung from a one-degree-of-freedom frictionless air bearing rig. The only motions allowed to the cylinder were those in a direction perpendicular to the flow direction or cross-flow. A load cell was installed linking the cylinder model to an air bearing carriage, allowing direct measurement of the drag force acting on the cylinder. The displacements of the cylinder in the y direction along the air bearing rig, were measured using a non-intrusive laser displacement sensor. The experiments were conducted by sampling at 2kHz the displacement and force signals during a period of time of 90s, ensuring more than 90 cycles of oscillation per run. Decay tests in air showed a very low damping in the structural model designed for the experiments, (0.54% of the critical damping). The total weight of the structural system, yield a mass ratio (structural mass divided by fluid added mass) of 2.38. The values achieved in these two dimensionless parameters are typical of marine structures.

The Reynolds number (ratio of inertial to viscous forces in the experiments) based on the cylinder model diameter ranged from 7000 to 23000. The ratio between the free stream velocity in the water channel and the average velocity of the cylinder when oscillating is defined as the reduced velocity (IP) and ranged from 4 to 14 approximately. These values of reduced velocity are also the typical ones at which marine structures show VIV response when exposed to marine currents in the ocean. b) Suppression meshes: A series of cylindrical wire meshes were installed around the cylinder in order to study their effects on the VIV response of the system. All meshes were rigid and made of wires of approximately 1 mm diameter (0.02D or 2% of the marine structure diameter) arranged in a square pattern. Two square sizes s = 5 mm (0.1 D or 10% of the marine structure diameter) and s = 20mm (0.36D or 36% of the marine structure diameter), were used in order to study the effect of the density (porosity) of the mesh, that is the size of the squares of the mesh. Meshes of different radius (r) were installed around the cylinder model covering the whole perimeter of the structure, with its radius ranging from 0.5D (50% of the marine structure diameter) to 1.3D (130% of the marine structure diameter). Schematics of typical systems used to model the marine structure in the laboratory, with a mesh installed, appear in figures 1 and 2.

The flow field around the model was quantified using Time-Resolved Digital Particle Image Velocimetry (TR-DPIV). This is an optical measurement technique that allows to instantaneously quantify the flow velocity in planar regions around the structure. c) Results of the experimental investigation:

Figures 3 and 4 summarize the main results of the experiments carried out. Amplitude (A ^* =A/D) and frequency response (f ^* =f/f _n ) appear together with the drag coefficient (Cd) as a function of the reduced velocity (IT). Amplitude in the plots appears in dimensionless form (as a function of the diameter of the marine structure), and has been computed as the mean of the maximum excursion of the cylinder in each cycle (mean of the envelope of the displacement signal measured). The dominant frequency of the oscillation appears in dimensionless form, scaled with the natural frequency of the system. Frequencies were computed from the displacement signals by using Fourier analysis. Having the natural frequency of the system fixed, the reduced velocity was varied by varying the free stream velocity in the channel which in turn meant variations in the Reynolds number, indicated also in the abscissa of the plots.

The results obtained with the cylinder without meshes (plain cylinder) appear in both figures with solid black dots for reference. As it can be seen the marine structure model experiences very large vibrations when exposed to currents. The maximum amplitudes reached by the marine structure model when vibrating was over 0.8D (more than 80% of its diameter). Just for reference, typical spar platforms used in oil rigs with similar damping and mass ratio to the experimental model used here, can be 200m long with diameters near 50m, meaning that their VIV motions would be in the order of 80 m peak-to-peak.

Figure 4 is for the results obtained with the less dense mesh (s = 0.36D) having the larger squares in the grid and figure 3 for the ones with the denser mesh (s = 0.1 D), with the smaller squares in the mesh Amplitude plots provide a clear indication of how good each mesh is in attenuating VIV. In both figures, each symbol and color indicates a different size of mesh r (radius of the mesh).

It is evident from plot 4a) that in general all the meshes of density s = 0.36D studied, yield VIV reduction when compared to the plain cylinder response. The larger one, with r = 1 .3D, produced a reduction in the VIV amplitudes of nearly 40% at the reduced velocities in which amplitudes were maximum. The reduction of drag coefficient when using the mesh that leads to the larger VIV reduction (r = 1.3D) is near a 20% inside the range where amplitudes were the largest.

The results in figure 3 with the denser mesh (s = 0.1 D) are even more remarkable in the sense that even larger VIV suppression is achieved. There is a practically total suppression of VIV (95% of suppression) with the larger mesh (r = 1.3D) at all reduced velocities.

With the r = 1.3D mesh, the frequency response is at all reduced velocities smaller than the expected shedding frequency. The drag coefficient is only reduced in the range 5 < U ^* < 8 and it becomes larger, by a factor of almost 2, outside the main response region.

TR-DPIV measurements were obtained in some specific experiments, with the intention of elucidating the fluid flow mechanisms that were governing the response of the cylinder with the meshes, and their suppression capabilities.

The DPIV measurements were made in some of the experiments conducted with a reduced velocity near 6, where the largest oscillations were expected, inside the lock- in region. Dimensionless vorticity maps appear in figures 5 and 6. In all that figures, the vorticity appears phase averaged at eight cylinder positions inside its oscillation cycle.

Figure 5 shows a standard vortex street with a series of alternating vortices that are periodically shed from the marine structure without the wire mesh invention. Hydrodynamic forces cause by the vortices generate the large VIV oscillations observed in the structures.

In figure 6 it can be seen how when the wire mesh with radius r = 1.3D and density s = 0.1 D is installed in the marine structure model, the wake is totally suppressed, there are no vortices being shed from the cylinder. Therefore, vibrations disappear thanks to the invention presented here. d) Conclusions of the experiments:

The invention presented here demonstrates the VIV suppression capabilities of rigid cylindrical wire meshes that are installed around marine structures subject to uniform flows. It has been empirically shown how VIV can be totally suppressed at all the reduced velocities and Reynolds numbers investigated, therefore eliminating the classical VIV locked-in region of the response that generates large vibrations in marine structures. The suppression of VIV response in the lock-in region is advantageously accompanied by a reduction in the drag coefficient up to a 20%.

Having sufficiently described the present invention in relation to the attached figures, it is easy to understand that any modifications can be made in the details as deemed suitable, as long as it doesn't change the essence of the invention, which is summarized in the following claims.