WO2017207746A1

WO2017207746A1 - Vortex induced suppression device

Info

Publication number: WO2017207746A1
Application number: PCT/EP2017/063426
Authority: WO
Inventors: Johannes Lambertus Fransiscus VAN KESSEL; Sanne Eva GROOT; Karst Meijer; Stephen John ROBINSON
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2016-06-02
Filing date: 2017-06-02
Publication date: 2017-12-07
Also published as: AU2017272811B2; BR112018074980A2; AU2017272811A1

Abstract

The invention relates to a vortex induced vibration suppression device (10) comprising: a riser pipe (12); a strake body (14) encircling the riser pipe (12); a vane (16) extending outwardly from the strake body (14), the vane (16) having a height (18) and a width (20); and a buffer (26) extending outwardly from the strake body (14), the buffer (26) having a height (32); wherein the height (32) of the buffer (26) is smaller than the height (18) of the vane (16) The buffer is provided to prevent damage of the vane from contact loads.

Description

VORTEX INDUCED SUPPRESSION DEVICE

FIELD OF THE INVENTION

The present invention relates to vortex induced vibration (VIV) suppression and more particularly to an improved device for suppressing VIV in riser pipes.

BACKGROUND

Conventionally, VIV suppression devices, such as described in WO2010085302, are installed on riser pipes using an underwater diver or divers. However, such installation can be costly and dangerous.

More recently, flexible vanes for the VIV devices have been proposed as an alternative. The flexible material is asserted to allow for the VIV devices to be lowered along with the riser pipe using a pipeline lay barge. For example, as illustrated in Figure 1, U.S. 6,695,540 utilizes a pipeline lay barge with a stinger and rollers to lower the riser to the seabed.

An alternative suppression element is described in US7458752, which describes a suppression element for vortex elements to envelop a tubular elements.

Some pipe laying vessels, however, do not include a stinger and rollers. For example, as illustrated in Figure 2, U.S. 8,366,352 illustrates a J-lay pipe vessel using a moon pool. In such vessels, the moon pool provides limited space and without rollers along a stinger, a pre- installed VIV device would encounter much larger shear forces as compared to those experienced with a stinger and roller vessel.

In Effectiveness of Polyehylene Helical Strakes in Suppression VIV Responses After Sustaining High Roller Load Deformation During S-lay Installation by Rafik Boubenider et. al. (OTC19289, Offshore Technology Conference 5 - 9 May 2008), the effectiveness of polyethylene strakes was tested having sustained permanent deformations due to a similated high contact load on the roller of a singer during an S-lay installation.

SUMMARY OF THE INVENTION

It is an object to provide an improved vortex induced vibration suppression device that is better able to withstand contact loads that occur during handling of the risers. Therefore there is provided a vortex induced vibration suppression device (10) comprising: a riser pipe (12); a strake body (14) encircling the riser pipe (12); a vane (16) extending outwardly from the strake body (14), the vane (16) having a height (18) and a width (20); and a buffer (26) extending outwardly from the strake body (14), the buffer (26) having a height (32); wherein the height (32) of the buffer (26) is smaller than the height (18) of the vane (16).

The height of the buffer may be in the range of 0.05 to 0.25 times the height (18) of the vane (16).

The buffer prevents the edge of the moon pool or other similar objects from

approaching the base of the vane too closely, thereby excessive bending forces at the base 24 of the vane are limited and damage to the vane is prevented or at least reduced.

According to an embodiment the buffer (26) has a width (27) along the circumference of the strake body (14) that is greater than the height (32) of the buffer (26). This makes the buffer better able to withstand shear forces.

According to an embodiment the height (32) of the buffer (26) is greater to or equal than

0.8 times the width (20) of the vane (16).

According to an embodiment a distance (30) between the vane (16) and the buffer (26) is greater than or equal to 0.8 times the height (18) of the vane (16).

According to an embodiment the height (32) of the buffer (26) and a distance (30) between the vane (16) and the buffer (26) create a gap (28) defining a cross-sectional area that is equal to or larger than a cross-sectional area defined by the height (18) and the width (20) of the vane (16).

The cross-sectional area is measured in a plane perpendicular to a longitudinal body axis of the riser pipe.

According to an embodiment, the gap (28) defines a distance between a top of the buffer

(26) and a base (24) of the vane (16), said distance being less than the height (18) of the vane (16).

According to an embodiment the device is further comprising:

a second buffer (126) extending outwardly from the strake body (14) on an opposite side of the vane (16) as the first buffer (26), the second buffer (126) having a height (132); wherein the height (132) of the second buffer (126) is smaller than the height (18) of the vane (16).

The second buffer may have the same dimensions as the (first) buffer and may be positioned on the same distance from the vane as the (first) buffer.

According to an embodiment the vane (16) forms a path (34) along a length of the strake body (14) and the buffer (26) forms a path along a length of the strake body (14) parallel to the path (34) of the vane (16). In case of a second buffer, the second buffer may also form a path along a length of the strake body parallel to the path (34) of the vane (16).

According to an embodiment the vane (16) forms a helical path (34) along a length of the strake body (14).

According to an embodiment the device further comprises a second vane (116) extending outwardly from the strake body (14), the second vane (116) having a height (118) and a width (120), wherein a distance between the second vane (116) and the buffer (26) is greater than or equal to 0.8 times the height (118) of the second vane (116).

The second vane may have similar or even identical dimensions as the (first) vane. Both vanes may form parallel paths along a length of the strake body (14). The buffer(s) associated with the second vane may have the same relative positioning with respect to the second vane as the buffers associated with the (first) vane.

According to an embodiment the device further comprises a standoff element (36) at an end of the riser pipe (12), wherein, proximate a base (24) of the vane (16), the standoff element (36) has a thickness (38) that is greater than a thickness (40) of the strake body (14).

According to an embodiment proximate the buffer (26), the standoff element (36) has a thickness (42) equal to or greater than the thickness (40) of the strake body (14) and the thickness (42) is smaller than or equal to the thickness (40) of the strake body (14) plus the height (32) of the buffer (26).

According to an embodiment the strake body (14), the vane (16), and the buffer (26) are integrally formed.

According to an embodiment the strake body (14), the vane (16), and the buffer (26) are formed of polyurethane and/or polyethylene.

According to an embodiment a leading edge (46) of the vane (16) is chamfered. According to an embodiment the buffer (26) has a cross- sectional profile configured to reduce or increase drag.

According to an aspect, there is provided a riser comprising a vortex induced vibration suppression device as defined above.

The vortex induced vibration suppression device may extend along the entire length of the riser or over a portion of the riser.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a drawing showing a side view of a prior art pipeline lay barge with a stinger and rollers.

Figure 2 is a drawing showing a perspective view of a prior art pipe vessel having a moon pool.

Figure 3 is a drawing of a side view of a vortex induced vibration (VIV) suppression device.

Figure 4 is a drawing of a cross sectional view of the VIV suppression device of Figure

3.

Figures 5A and 5B are drawings of a cross sectional view of a VIV suppression device without a buffer, as a point load is encountered.

Figures 5C and 5D are drawings of a cross sectional view of a VIV suppression device with buffer, as a point load is encountered.

Figure 6 is a drawing of a cross sectional view of the VIV suppression device of Figure 3, showing an alternative buffer configuration to that shown in Figure 4.

Figure 7 is a drawing of a longitudinal cross section view of the VIV suppression device of Figure 3, illustrating a standoff element, among other things. Notably, for simplicity, Figure 7 does not illustrate an optional helical configuration of certain elements.

DETAILED DESCRIPTION

Referring now to Figures 3 and 4, an improved design for a vortex induced vibration (VIV) suppression device may mitigate the undesirable shearing that occurs when, for instance, an edge of the moon pool contacts the vanes. Such a VIV suppression device 10 may include a riser pipe 12, a strake body 14 encircling the riser pipe 12, a vane 16 extending outwardly from the strake body 14. When no force is applied to the vane 16, the vane 16 extends in radial direction from the strake body 14. The vane 16 has a height 18 and a width 20. The height 18 is greater than the width 20. Typically the height is more than a factor 5 or 10 greater than the width 20. Heights are measured in radial direction of the riser pipe 12. All widths are measured along the circumference of the strake body 14. All indicated widths and heights of vanes and buffers relate to their undeformed state.

The strake body 14 may be formed modularly from a number of modular suppression elements, which when assembled encircle the riser pipe 12 over a certain length.

The vane 16 may extend along the entire length of the riser pipe 12 or along a predetermined portion of the length of the riser pipe 12.

The vane 16 may, at least partially, be constructed of a flexible material such that a tip 22 of the vane 16 can move relative to a base 24 of the vane 16. Such flexibility may allow for mitigation of failure from shearing caused by the edge of the moon pool MP or other focused load points. Referring briefly to Figure 5A, rather than shearing off, i.e. breaking or being permanently damaged, when contacting a point load, the tip 22 of the vane 16 may take the path of least resistance and simply rotate or fold relative to the base 24 of the vane 16.

In some circumstances, however, flexibility alone is insufficient to prevent shearing failure. For instance, referring briefly to Figure 5B, if the shear force is applied near the base 24 of the vane 16, shear failure may still occur even in flexible materials.

Thus, the VIV suppression device 10 may include a buffer 26 extending outwardly in radial direction from the strake body 14 , for example as illustrated in Figure 4. In instances where the tip 22 of the vane 16 rotates or bends about the base 24 of the vane 16 by up to approximately 90 degrees, the vane 16 may at least partially fit within a gap 28 between the vane 16 and the buffer 26, thus minimizing the likelihood that shear failure will occur.

Buffer in the context of the present application is directed to a device or piece of material for reducing shock or damage to the vanes due to contact, for instance in radial direction. The buffer herein serves as a protective barrier, protecting the strake body 14. The buffers of the disclosure provide a protective stand-off between their radially outermost contact surface and the body 14 of the VIV strake. The buffers of the disclosure absorb energy upon radial impact. Thus protecting the vanes and resulting in reduced contact loads on the VIV strake (for instance during installation). The buffers herein may indicate monolithic bodies. Monolithic herein means that each buffer is basically a block of material. The buffers may be made of a material which is elastic to allow the buffers to absorb shocks upon impact. The material may also be relatively dense and robust, suitable for application offshore.

The buffer 26 prevents the edge of the moon pool MP (or other similar object) from approaching the base of the vane 16 too closely thereby limiting excessive bending forces at the base 24 of the vane. Depending on the relative dimensions and positioning of the vane 16 and the buffer 26, the vane 16 may completely fit in the gap 28 created by the buffer 26 (an example of which is shown in Fig. 5C) or the vane 16 may partially fit in the gap 28 created by the buffer 26, for instance the vane 16 may bend towards the buffer 26 and may land with its side on top of the buffer 26, thereby being prevented from further bending (an example of which is shown in Fig. 5D).

Such a gap 28 might be defined as a distance 30 between the vane 16 and the buffer 26 as measured along the outer surface of the strake body 14 before the vane 16 has deformed to fill the gap 28. Distance 30 is preferably measured from a side 27 of buffer 26 directed to the vane 16 and a side 17 of the vane directed away from the buffer 26. For the buffer 26 to function properly, certain dimensions may be considered. The distance 30 between the vane 16 and the buffer 26 may be greater than 0.5 times the height 18 of the vane 16, greater than 0.8 times the height of the vane, greater than 0.9 times the height 18 of the vane 16 or greater than or equal to the height 18 of the vane 16. The distance 30 is preferably less than 1.25 times the height of the vane 16.

The buffer 26 also has a height 32. The height 32 of the buffer 26 may be greater than 0.5 times the width 20 of the vane 16, greater than 0.8 times the width 20 of the vane 16, greater than 0.9 times the width of the vane 16 or may be equal to or greater than the width 20 of the vane 16. When applying a height that is greater than the width 20 of the vane 16 it is possible to use slightly less flexible materials without creating excessive bending forces at the base 24 of the vane 16. The height of the buffer 26 is preferably less than 1.25 times the width of the vane 16.

As described above, the height 32 of the buffer 26 and the distance 30 between the vane 16 and the buffer 26 may be directly correlated with the height 18 and the width 20 of the vane 16 and possibly with the flexibility of the vane 16. According to an embodiment, the distance 30 between the vane 16 and the buffer 26 is equal to or greater than the height 18 of the vane 16 and the height 32 of the buffer 26 is equal to or more than the width of the vane 16 to create a gap 28 large enough to receive the vane 16.

The volume or cross-sectional area may be used as a substitute for the specific dimensions mentioned here.

However, in some instances, depending on the materials, the shape of the gap 28 may be wider, or taller or otherwise configured. Generally, the volume of the vane 16 fits within the gap 28. Stated another way, the height 32 of the buffer 26 and the distance 30 between the vane 16 and the buffer 26 create the gap 28 defining a cross-sectional area along the surface of the strake body 14 that is larger than the cross- sectional area defined by the height 18 and the width 20 of the vane 16.

According to an alternative embodiment, the distance 30 between the vane 16 and the buffer 26 and the height 32 of the buffer 26 may be selected such that the vane 16, when bended, lands on top of the buffer 26, thereby preventing excessive bending. For instance, the height 32 of the buffer 26 may be in the range of 0.4 - 0.9 times the height 18 of the vane 16, e.g. 0.5 times the height 18 of the vane 16, and the distance 30 may be selected to be in the range of 0.4 - 0.9 times the height 18 of the vane 16.

Preferably, the buffer 26 has a width 27 that is greater than the height 32 of the buffer 26. More preferably, the width of the buffer is greater than 2 times the height 32 of the buffer 26. These dimensions make the buffer 26 better capable of withstanding shearing forces.

Referring still to Figures 3 and 4, the vane 16 may have two associated buffers 26, 126, allowing the vane 16 to flex in either direction when a load is applied. In such instances, the second buffer 126 may be identical to the (first) buffer 26 with a similar height 132 and distance 130 between the vane 16 and the second buffer as the first buffer 26. The dimensions of the second buffer 126 and its relative position with respect to the vane 16 may be as indicated above for the first buffer 26. Alternatively, the dimensions and placement of the second buffer 126 may be altered in any of the ways described above with respect to the buffer 26, whether the buffer 26 is so altered or not.

Referring to Figure 3, the vane 16 may form a helical path 34 along a length of the riser pipe 12. In order to maintain appropriate spacing, the buffers 26, 126 may form a similar helical path 35. The helical path 34 may be advantageous as helical strakes can deal with any water current direction efficiently. Reference is made to C. Scruton and D.E.J. Walshe, A Means for Avoiding Wind-excited Oscillations of Structures with Circular Or Nearly Circular Cross-section, National Physics Laboratory (Great Britain), 1957 and Allen, D.W., An

Experimental Evaluation of Vortex Suppression Devices, Technical Progress Report BRC 22- 89, Shell Development Co., Bellaire Research Center, Houston, 1989.

Referring back to Figure 4, any number of vanes and associated buffers may be used. The examples of one vane 16 and one buffer 26 and of one vane 16 and two buffers 26, 126 are described above. With reference to the Figure 4, a further example includes three vanes 16, 116, and 216, with six associated buffers 26, 126, 226, 326, 426, 526, each vane 16, 116, 216 having two associated buffers, one on each side of the vane. The details, dimensions and relative positioning of these vanes and buffers would be similar to that described above and is therefore not repeated here. However, it is notable that the number of buffers may be greater or equal than the number of vanes, depending on the specific design. Some specific designs envisioned include 2 vanes offset from one another by 180 degrees, 3 vanes offset from one another by 120 degrees, and 4 vanes offset from one another by 90 degrees.

Referring now to Figure 6, a single buffer 26 may be sufficient for one, two or more vanes 16, 116. As illustrated in Fig. 6, a single buffer 26 may be used to create the gap 28 associated with the first vane 16 and can be used to create a similar gap associated with second vane 116. In this example, the second vane 116 extends outwardly in radial direction from the strake body 14. Like the first vane 16, the second vane 116 has a height 118 and a width 120. Similarly, as described above, height 32 of the buffer 26 may be selected in relation to the width 120 of the second vane 116 and a distance 230 may be selected in relation to the height 118 of the second vane 116, according to the criteria set out above.

Similarly, the volume or cross-sectional area may be used as a substitute for the specific dimensions mentioned here.

Referring now to Figure 7, in addition to any or all of the various elements described above, the VIV suppression device 10 may further include a ramp up section or standoff element 36 at an end of the riser pipe 12. The standoff element 36 may have an inclined outer surface 37 (as shown) or may have a stepped outer surface 37 (not shown) in axial direction.

The standoff element 36 may be present along the entire circumference of the riser 12 or may only be present proximate vanes and buffers. A standoff element may be present for each vane and associated buffers. In the latter case, the standoff element 36 may also have an inclination or stepped portion in the circumferential direction, to create a smooth transition between portions of the circumference having a standoff element and portions without standoff elements. In some applications, it may be deemed useful to include a standoff element 36, 136 at each end of the riser pipe 12. The standoff element 36 serves to further reduce shear stress created by point loads. To that purpose, proximate the base 24 of the vane 16, the standoff element may have a thickness 38 that is greater than the thickness 40 of the strake body 14. Similarly, proximate the buffer 126, the standoff element 36 may have a thickness 42 that is substantially equal to the thickness 40 of the strake body 14 plus the height of the buffer 26.

As illustrated in Figure 7, the vane 16 and/or buffers may extend substantially along the entire length of the riser pipe 12 as a continuous strake. However, it is also possible for the vane 16 and/or buffers to be a series of smaller fins aligned to act as a discontinuous strake. Similarly, the other vanes 116 and 216 may have either type of construction.

Various portions of the above-described devices may be integrally formed. For example, the strake body 14, the vane 16 and the buffer 26 may be integrally formed. Similarly, with additional vanes and/or additional buffers, integral formation may be possible.

Materials suitable for the elements described herein may include materials typically used in such applications. For example, riser pipes are well-known and are commonly constructed of steel. Similarly, the other portions relating to the vane(s), buffer(s), strake body and optionally the standoff element may all be constructed of materials known for VTV suppression. For example, polyurethane and/or polyethylene may be used. For the standoff element or ramp up section steel may be used. Also, (aluminum) anodes may be used to serve as ramp up section or standoff element 36. The ramp up section may be formed to function as an anode and may at least partially be formed of an anode suitable material, such as aluminum or zinc.

The standoff element may be unitarily formed with other elements or may be a separate element that is attached to the remaining elements.

In some applications, a chamfer 44 in the leading edge 46 of the vane 16 may provide further resistance to shear of the vane 16. In case the vane 16 is formed as a series of smaller fins aligned to act as a discontinuous strake the chamfer 44 may extend over two or more smaller fins. The chamfer 44 is present towards the end of the riser 12.

The primary function of the buffers is to keep the edge of the moon pool MP or the stinger/rollers at some distance from the strake body 14 thereby reducing bending and shearing of the vanes. Furthermore, the buffers provide gaps in which the vanes can lie in circumstances where the vanes might otherwise shear.

The cross-sectional profile (in a direction perpendicular to the body axis of the riser 12) of the buffers may be chosen to influence the drag of the riser. The buffers may have a relatively smooth profile configured to reduce drag. Alternatively, in instances where increased drag may be preferred, the profile of the buffers might include more rough edges

(e.g., square or rectangular cross section). Likewise, the width of the buffers might be selected to optimize drag. In addition to protecting the vanes, the buffers may also protect clips or other elements used to attach the vanes to the riser pipe.

The riser pipe 12 may serve as the base of the VIV suppression device 10 with the other elements attached thereto by straps or other securing mechanisms.

Notably, while the vanes/buffers have been described as having a width and a height, it should be noted that both dimensions may vary in the radial direction along the vane/buffer or along the axial direction of the riser 12. For example, it is possible that the width at the base of the vane may be bigger than the width at the tip. Thus, when length or width are being used, one having ordinary skill in the art may need to make appropriate adjustments to

accommodate for the actual dimensions.

It is believed that the design described above has advantages over previous designs that require installation over the edge of a moon pool or over rollers that significantly lower contact loads thereby reducing or preventing damage to the associated VIV strakes or vanes. In some applications where (axial) shear loads are high, the vanes of traditional designs may shear off and the resulting system is thus ineffective for VIV suppression, particularly when the VIV strakes are installed over a fixed surface, e.g., moon pool edge, instead of over rollers. In the inventive design, however, the shear loads may be displaced, resulting in reduced contact loads during installation. Without cause catastrophic failure of the vanes, more effective VIV suppression can be provided without the need for a spar and rollers during dispatch. Generally VIV strakes are being installed over rollers, which results in significantly lower contact loads and no or limited damage to the VIV strake. When (axial) shear loads are high the vanes of a VIV stake may shear of and system is not effective to suppress VIV motions anymore, this is particaluarly the case when the VIV strakes are being installed over fixed surfaces instead of rollers. Some vendors use a chamfer on the first vanes to reduce the axial shear load on the vane when it enters the rollers on a stinger of a pipelay vessel during installation. However, none of the conventional technology solves the problem when VIV strakes are to be installed over a fixed surface. The device according to the present disclosure including buffers obviates damage to or shearing of vanes even when VIV strakes are to be installed over a fixed surface.

Those of skill in the art will appreciate that many modifications and variations are possible in terms of the disclosed embodiments, configurations, materials, and methods without departing from the scope of the appended claims. Accordingly, the scope of the claims and their functional equivalents should not be limited by the particular embodiments described and illustrated, as these are merely exemplary in nature. For instance, elements described with respect to separate embodiments may be combined.

Claims

1. A vortex induced vibration suppression device (10) comprising:

a riser pipe (12);

a strake body (14) encircling the riser pipe (12);

a vane (16) extending outwardly from the strake body (14), the vane (16) having a height (18) and a width (20); and

a buffer (26) for protecting the vane by absorbing energy upon radial impact, the buffer (26) extending outwardly from the strake body (14) and having a height (32);

wherein the buffer height (32) of the at least one buffer (26) is smaller than the height (18) of the at least one vane (16).

2. The device (10) according to claim 1, wherein the buffer (26) has a width (27) along the circumference of the strake body (14) that is greater than the height (32) of the buffer (26).

3. The device (10) according to any one of the preceding claims, wherein the height (32) of the buffer (26) is greater to or equal than 0.8 times the width (20) of the vane (16).

4. The device according to any one of the preceding claims, wherein a distance (30) between the vane (16) and the buffer (26) is greater than or equal to 0.8 times the height (18) of the vane (16).

5. The device according to any one of the preceding claims, wherein the height (32) of the buffer (26) and a distance (30) between the vane (16) and the buffer (26) create a gap (28) defining a cross-sectional area that is equal to or larger than a cross-sectional area defined by the height (18) and the width (20) of the vane (16).

6. The according to any one of the preceding claims, wherein the gap (28) defines a distance between a top of the buffer (26) and a base (24) of the vane (16), said distance being less than the height (18) of the vane (16).

7. The device (10) according to any one of the preceding claims, further comprising: a second buffer (126) extending outwardly from the strake body (14) on an opposite side of the vane (16) as the first buffer (26), the second buffer (126) having a height (132); wherein the height (132) of the second buffer (126) is smaller than the height (18) of the vane (16).

8. The device (10) according to any one of the preceding claims, wherein the vane (16) forms a path (34) along a length of the strake body (14) and the buffer (26) forms a path along a length of the strake body (14) parallel to the path (34) of the vane (16).

9. The device (10) according to any one of the preceding claims, wherein the vane (16) forms a helical path (34) along a length of the strake body (14).

10. The device (10) according to any one of the preceding claims, further comprising: a second vane (116) extending outwardly from the strake body (14), the second vane

(116) having a height (118) and a width (120), wherein a distance between the second vane (116) and the buffer (26) is greater than or equal to 0.8 times the height (118) of the second vane (116).

11. The device (10) according to any one of the preceding claims, further comprising: a standoff element (36) at an end of the riser pipe (12),

wherein, proximate a base (24) of the vane (16), the standoff element (36) has a thickness (38) that is greater than a thickness (40) of the strake body (14).

12. The device (10) of claim 11, wherein, proximate the buffer (26), the standoff element (36) has a thickness (42) equal to or greater than the thickness (40) of the strake body (14) and the thickness (42) is smaller than or equal to the thickness (40) of the strake body (14) plus the height (32) of the buffer (26).

13. The device (10) according to any one of the preceding claims, wherein the strake body (14), the vane (16), and the buffer (26) are integrally formed.

14. The device (10) according to any one of the preceding claims, wherein the strake body (14), the vane (16), and the buffer (26) are formed of polyurethane and/or polyethylene.

15. The device (10) any one of the preceding claims, wherein a leading edge (46) of the vane (16) is chamfered. 16. The device (10) any one of the preceding claims, wherein the buffer (26) has a cross- sectional profile configured to reduce or increase drag.

17. Riser comprising:

a riser pipe (12);

a strake body (14) encircling the riser pipe (12);

a buffer (26) extending outwardly from the strake body (14), the buffer (26) having a height (32);

wherein the height (32) of the buffer (26) is smaller than the height (18) of the vane (16).