The invention relates to a suppression element for vortex vibrations, wherein:

- the suppression element has a longitudinal direction, as well as a circumferential direction around a reference axis which is parallel to the longitudinal direction, and wherein the suppression element has an inner side and an opposite outer side, wherein the inner side has a concave shape in the circumferential direction and the outer side has a convex shape in the circumferential direction, and wherein the suppression element on both ends in the longitudinal direction has a first end edge and an opposite second end edge, and wherein the suppression element on both ends in the circumferential direction has a first longitudinal edge and an opposite second longitudinal edge;

- the suppression element is configured for partly enveloping, in the circumferential direction, a tubular element, in such manner that the suppression element with the inner side is facing the tubular element for forming, in operation, a tube segment, which is extending co-axially round said reference axis, and which can co-axially envelope the tubular element all round as a result of a pre-determined number of at least two specimens of the suppression element being mutually interconnected in the

circumferential direction, wherein the tube segment in the longitudinal direction has an overall length which is equal to the overall length of the suppression element in the longitudinal direction, and wherein said tube segment is configured for forming, in operation, a tube around the tubular element as a result of multiple specimens of said tube segment being mutually interconnected in the longitudinal direction; and

- the suppression element comprises a fin structure, which on said outer side is protruding at least in radial direction relative to said reference axis, and which is configured for reducing, in operation, vorticity shedding at the downstream side of the tubular element. Such suppression elements for vortex vibrations are for example known in the offshore industry as so-called‘Vortex Induced Vibration strakes’ or‘VTV strakes’. Such suppression elements are used on, for example, offshore drilling platforms to reduce the forces exerted by the water on a pipeline running from such a platform to an oil well or a so-called ‘off-loading vessel’.

A suppression element of the type as initially indicated above is for example known from W02004020777A1. In Figs. 1-5 of W02004020777A1 it is seen that the inner side and the outer side of the suppression element 1, including the inner side and the outer side of the fin structure 3, have corresponding shapes such that the suppression elements 1 are compactly stackable in a manner as shown in Fig. 5 of W02004020777A1. This compact stackability of such suppression elements is very important, since, especially with transport in offshore applications, the volume is an

important factor in the transportation costs of the suppression elements.

Another suppression element of the type as initially indicated above is for example known from US9140385B2. This other suppression element has another kind of fin structure than the fin structure known from W02004020777A1. Nowadays, this other kind of fin structure is often used because of its favourable effects on reducing vortex induced vibrations. In Figs. 1-2 of US9140385B2 it is seen that this other kind of fin structure consists of a series of multiple fins 7, of which the inner sides and the outer sides do not have corresponding shapes. Differently from the hollow, nestable fin structure 3 known from W02004020777A1, US9140385B2 discloses solid, non-nestable fins 7. Due to this, the suppression elements known from US9140385B2 are not so compactly stackable as the

suppression elements known from W02004020777A1.

The elements known from W02004020777A1 and the elements known from US9140385B2 have in common that in tubes, which have been formed with such elements, suppression elements of such a tube which are mutually adjacent in the longitudinal direction, are mutually positioned in a staggered manner in the circumferential direction. This is best seen in Fig.

4 of W02004020777A1. Thanks to such a staggered positioning of

suppression elements, the tubes being formed by them have a strong and reliable construction.

Furthermore, the elements known from W02004020777A1 and the elements known from US9140385B2 have in common that, for tubes which have been formed with such elements, the helically extending fin structures of the suppression elements each time are lying helically in-line relative to one another. Also this is best seen in Fig. 4 of W02004020777A1. Such fin structures, which are lying helically in-line relative to one another, are effective for reducing vorticity shedding.

It is an object of the invention to provide a solution according to which also suppression elements having a fin structure of which the inner sides and the outer sides do not have corresponding shapes (which occurs for example in case of sohd, non-nestable fins) are compactly stackable, while preserving the possibility to form with such elements a tube, which is strong and reliable, and which effectively reduces vortex-induced vibrations.

For that purpose the invention provides a suppression element according to the appended independent claim 1. Specific embodiments of the invention are provided by the appended dependent claims 2-8.

Hence, the invention provides a suppression element for vortex vibrations, wherein:

- the suppression element has a longitudinal direction, as well as a circumferential direction around a reference axis which is parallel to the longitudinal direction, and wherein the suppression element has an inner side and an opposite outer side, wherein the inner side has a concave shape in the circumferential direction and the outer side has a convex shape in the circumferential direction, and wherein the suppression element on both ends in the longitudinal direction has a first end edge and an opposite second end edge, and wherein the suppression element on both ends in the circumferential direction has a first longitudinal edge and an opposite second longitudinal edge;

- the suppression element is configured for partly enveloping, in the circumferential direction, a tubular element, in such manner that the suppression element with the inner side is facing the tubular element for forming, in operation, a tube segment, which is extending co-axially round said reference axis, and which can co-axially envelope the tubular element all round as a result of a pre-determined number of at least two specimens of the suppression element being mutually interconnected in the

circumferential direction, wherein the tube segment in the longitudinal direction has an overall length which is equal to the overall length of the suppression element in the longitudinal direction, and wherein said tube segment is configured for forming, in operation, a tube around the tubular element as a result of multiple specimens of said tube segment being mutually interconnected in the longitudinal direction; and

- the suppression element comprises a fin structure, which on said outer side is protruding at least in radial direction relative to said reference axis, and which is configured for reducing, in operation, vorticity shedding at the downstream side of the tubular element;

characterized in that

- the first longitudinal edge comprises at least one first fin longitudinal edge portion, wherein the at least one first fin longitudinal edge portion is extending helically around said reference axis;

- the fin structure comprises a first fin structure, which is extending along the at least one first fin longitudinal edge portion, and which on said outer side at the at least one first fin longitudinal edge portion is protruding in said radial direction for said reducing of said vorticity shedding; - the second longitudinal edge comprises at least one second fin longitudinal edge portion, wherein the at least one second fin longitudinal edge portion is extending helically around said reference axis;

- the fin structure comprises a second fin structure, which is extending along the at least one second fin longitudinal edge portion, and which on said outer side at the at least one second fin longitudinal edge portion is protruding in said radial direction for said reducing of said vorticity shedding; and

- the at least one first fin longitudinal edge portion and the at least one second fin longitudinal edge portion of the suppression element are configured such that, for each pair of, in the circumferential direction, mutually adjacent suppression elements of said tube segment, the at least one first fin longitudinal edge portion of one suppression element of said pair and the at least one second fin longitudinal edge portion of the other suppression element of said pair are lying helically in line relative to one another.

Hence, according to the invention the first fin structure and the second fin structure are extending along the first longitudinal edge and the second longitudinal edge, respectively. Furthermore, according to the invention the inner side and the outer side of the suppression element are shaped concavely and convexly, respectively, as seen in the circumferential direction. Thanks to the combination of the fin structures which thus are lying along the longitudinal edges, on the one hand, and said concave and convex forms of the inner side and the outer side of the suppression element, on the other hand, the suppression elements according to the invention are compactly stackable, and this also holds for suppression elements having a fin structure, of which the inner sides and the outer sides do not have corresponding shapes, which is for example the case for solid, non-nestable fins, such as the solid, non-nestable fins 7 in Figs. 1-2 of US9140385B2. In a preferable embodiment of a suppression element according to the invention:

- the first longitudinal edge comprises at least one first non-fin longitudinal edge portion, along which said first fin structure is not extending;

- the second longitudinal edge comprises at least one second non-fin longitudinal edge portion, along which said second fin structure is not extending;

- the at least one first non-fin longitudinal edge portion and the at least one second non-fin longitudinal edge portion of the suppression element are configured such that, for each pair of, in the circumferential direction, mutually adjacent suppression elements of said tube segment, the at least one first non-fin longitudinal edge portion of one suppression element of said pair and the at least one second non-fin longitudinal edge portion of the other suppression element of said pair are mutually adjacent.

In another preferable embodiment of a suppression element according to the invention, the suppression element comprises a positioning structure, which is configured for positioning of suppression elements of said tube, said suppression elements being mutually adjacent in the longitudinal direction, in fixed mutually staggered positions in the circumferential direction, in such manner that, for each pair of, in the longitudinal direction, mutually adjacent tube segments of said tube, each time the at least one first fin longitudinal edge portions and the at least one second fin

longitudinal edge portions of the suppression elements of one tube segment of said pair mutually are lying helically in line relative to first and second fin longitudinal edge portions of the other tube segment of said pair.

Thanks to such a positioning structure it is possible to precisely and reliable form tubes by suppression elements according to the invention, wherein the helically extending fin structures of the suppression elements each time are lying helically in-line relative to one another, which is effective for reducing vorticity shedding.

In the following, the invention is further elucidated with reference to a non-limiting embodiment and with reference to the schematic figures in the attached drawing, in which the following is shown.

Fig. 1 shows a perspective view onto the inner side of an example of an embodiment of a suppression element according to the invention.

Fig. 2 shows a perspective view onto the outer side of the suppression element of Fig. 1.

Fig. 3 shows a perspective view onto the first end edge of the suppression element of Fig. 1.

Fig. 4 shows a perspective view onto an assembly of a tubular element and a tube around the tubular element, wherein the tube is formed by mutually identical tube segments which are interconnected with one another in the longitudinal direction of the tubular element, and wherein each tube segment is formed by three suppression elements, each of which being identical to the suppression element of Fig. 1, and which in the circumferential direction of the tubular element are interconnected relative to one another.

Fig. 5 separately shows suppression elements of the tube of Fig. 4, being mutually interconnected in the longitudinal direction, and in the same orientation and in the same perspective view as in Fig. 4.

Fig. 6 separately shows suppression elements of the tube of Fig. 4, being mutually interconnected in the circumferential direction, and in the same orientation and in the same perspective view as in Fig. 4.

Fig. 7 shows four mutually stacked suppression elements, each of which being identical to the suppression element of Fig. 1, in the same orientation and in the same perspective view as in Fig. 3. The reference signs used in the above-mentioned figures are referring to the above-mentioned parts and aspects of the invention, as well as to related parts and aspects, in the following manner

1 ^. tube

7 . tensioning strap

10 ^. tubular element

11, 12 ^. tube segment

100 ^. suppression element

101 ^. inner side

102 . outer side

107 ^. tensioning strap groove

111 ^. first end edge

111A, 11 IB ^. positioning structure

112 ^. second end edge

112A, 112B ^. positioning structure

121 ^. first fin longitudinal edge portion

131, 131A, 131B . first non-fin longitudinal edge portion

122 ^. second fin longitudinal edge portion

132, 132A, 132B . second non-fin longitudinal edge portion

141 ^. first fin structure

142 ^. second fin structure

C ^. circumferential direction

L ^. longitudinal direction

R ^. reference axis

Furthermore in Figs. 4-7 the reference numerals 200, 300, 400, 500, 600 are referring to suppression elements which are identical to the suppression element 100. For these suppression elements 200, 300, 400, 500, 600 the parts which correspond with parts of the suppression element 100 are indicated by the same reference numerals, however, increased by the numbers 100, 200, 300, 400, 500, respectively. For example, in Fig. 4 the first fin structure 541 of the suppression element 500 corresponds to the first fin structure 141 of the suppression element 100.

Based on the above introductory description, including the brief description of the drawing figures, and based on the above explanation of the reference signs used in the drawing, the shown examples of Figs. 1-7 are for the greatest part readily self-explanatory.

Figs. 1-3 show three different perspective views of the separate suppression element 100.

Fig. 4 shows a perspective view onto an assembly of the tubular element 10 and the tube 1 around the tubular element 10. The tube 1 is formed by mutually identical tube segments 11, 12 which in the longitudinal direction L are mutually interconnected. The tube segment 11 is formed by interconnection in the circumferential direction C of the three mutually identical suppression elements 100, 200, 300. The tube segment 12 is formed by interconnection in the circumferential direction C of the three mutually identical suppression elements 400, 500, 600.

The suppression elements 100, 200, 300, 400, 500, 600 of the tube 1 are held together by a number of tensioning straps 7. These tensioning straps 7 are mounted in tensioning strap grooves of the suppression elements. Fig. 2 shows the tensioning strap grooves 107 in the outer side of the suppression element 100.

Fig. 5 separately shows the suppression elements 100, 500 of the tube 1 of Fig. 4 which are mutually interconnected in the longitudinal direction L. Therein the suppression elements 100, 500, which are mutually adjacent in the longitudinal direction L, are mutually positioned in a staggered manner in the circumferential direction C. This is realized in that the suppression element 100 has the positioning structure 111A, 11 IB,

112A, 112B, also see Figs. 1-2, and in that the suppression element 500 has the similar positioning structure 511A, 51 IB, 512A, 512B. In Fig. 2 it is seen that the positioning structure of the suppression element 100 comprises, nearby the first end edge 111, a first slide-in portion 111A and an insert portion 11 IB, wherein the first slide-in portion l l lA has a smaller thickness than the insert portion 11 IB. In Figs. 1-2 it is further seen that the positioning structure of the suppression element 100, nearby the second end edge 112, comprises a recess 112A and a second slide-in portion 112B.

In the situation of Fig. 5 the insert portion 51 IB of the suppression element 500 has been inserted into the recess 112A of the suppression element 500, and the first slide-in portion 511A of the suppression element 500 and the second slide-in portion 112B of the suppression element 100 have been slided over one another.

Fig. 6 separately shows the suppression elements 100, 200 of the tube 1 of Fig. 4 which are mutually interconnected in the circumferential direction C. In Figs. 1-2 it is seen that the first longitudinal edge 121, 131,

131A, 13 IB of the suppression element 100 comprises the first fin

longitudinal edge portions 121, as well as the first non-fin longitudinal edge portions 131, 131A, 13 IB. The first non-fin longitudinal edge portion 131A is, as seen in perpendicular projection relative to the longitudinal direction L, situated between respective first fin longitudinal edge portions 121 of the suppression element 100. In Figs. 1-2 it is furthermore seen that the second longitudinal edge 122, 132, 132A, 132B of the suppression element 100 comprises the second fin longitudinal edge portions 122, as well as the second non-fin longitudinal edge portions 132, 132A, 132B. The second non- fin longitudinal edge portion 132A is, as seen in perpendicular projection relative to the longitudinal direction L, situated between respectieve second fin longitudinal edge portions 122 of the suppression element 100.

In the situation of Fig. 6 the second non-fin longitudinal edge portions 132A, 132B of the suppression element 100 are adjacent to the second non-fin longitudinal edge portions 23 IB, 231A of the suppression element 200, respectively. In Fig. 4 it is seen that the second fin structure 342 of the suppression element 300, the first fin structure 141 of the suppression element 100, the second fin structure 442 of the suppression element 400, and the first fin structure 541 of the suppression element 500, respectively, are lying helically in-hne relative to one another. In correspondence therewith also the second fin longitudinal edge portions of the suppression element 300, the first fin longitudinal edge portions of the suppression element 100, the second fin longitudinal edge portions of the suppression element 400, and the first fin longitudinal edge portions of the suppression element 500, respectively, are lying helically in-line relative to one another.

Likewise in Fig. 4 it is seen that the second fin structure 142 of the suppression element 100, the first fin structure 241 of the suppression element 200, the second fin structure 542 of the suppression element 500, and the first fin structure 641 of the suppression element 600, respectively, are lying helically in-line relative to one another. In correspondence therewith also the second fin longitudinal edge portions of the suppression element 100, the first fin longitudinal edge portions of the suppression element 200, the second fin longitudinal edge portions of the suppression element 500, and the first fin longitudinal edge portions of the suppression element 600, respectively, are lying helically in-line relative to one another.

Likewise in the situation of Fig. 4 the second fin structure 242 of the suppression element 200, the first fin structure (niet getoond) of the suppression element 300, the second fin structure (niet getoond) of the suppression element 600, and the first fin structure 441 of the suppression element 400, respectively, are lying helically in-line relative to one another. In correspondence therewith also the second fin longitudinal edge portions of the suppression element 200, the first fin longitudinal edge portions of the suppression element 300, the second fin longitudinal edge portions of the suppression element 600, and the first fin longitudinal edge portions of the suppression element 400, respectively, are lying helically in-line relative to one another.

Fig. 7 shows the four mutually identical suppression elements 100, 200, 300, 400 in a mutually stacked manner, and in the same orientation and in the same perspective view as in Fig. 3. It is seen that the suppression elements are compactly stackable. Thanks to the invention this compact stackability has been obtained despite the fact that the fins of the first and second fin structures 141 and 142 do not have corresponding shapes, since the fins are solid, non-nestable fins, just like for example the solid, non- nestable fins 7 in Figs. 1-2 of US9140385B2. It is furthermore thanks to the invention that the compact stackabihty of the suppression elements is obtained while preserving the strong and reliable construction of the tubes formed by the suppression elements, and while preserving the effectivity in reducing vortex induced vibrations.

It is remarked that the above-mentioned examples of embodiments do not limit the invention, and that various alternatives are possible within the scope of the appended claims. It is furthermore remarked that parenthesized reference signs used in the claims are not to be construed as limiting features of a claim concerned.

For example various variations are possible in the shapes, dimensions and materials of a suppression element according to the invention. If, for example, a suppression element according to the invention comprises said positioning structure, then, instead of the shown

combination of the first slide-in portion 111A, the insert portion 11 IB, the recess 112A and the second slide-in portion 112B, various other

embodiments of a positioning structure of a suppression element according to the invention are possible.

A suitable material for manufacturing a suppression element according to the invention is for example a foamed plastic, and more in particular a polyethene (PE). Because of this, the element not only is lightweight, but it can also be manufactured from recycled plastic, which is environment -friendly. Another suitable material is polypropylene (PP). Such a material has good shape-retaining properties, also at high temperatures, and can for example be applied to pipings through which a fluid is transported under increased temperature.

These and similar alternatives are deemed to fall within the scope of the invention as defined in the appended claims.