STRAKE FOR A COLUMN OF A FLOATING PLATFORM

The present invention relates to strakes for a floating construction, particularly for a floating platform.

The main purpose of providing floating platforms with strakes is to prevent VIM motion (i.e. velocity induced motions) for extreme loop currents. Such currents occur for example in the Gulf of Mexico. When a floating platform is provided with strakes, they give a defined separation point of the laminar flow around the column shell, and thereby prevent extreme lateral motions which help prevent issues related to fatigue on mooring and riser arrangements. Platforms may at times be moved from one place to another in which the need for reducing velocity induced motions is greater than in the first place. It is therefore a need for strakes which can be mounted on existing structures. There is also a need for a strake which is simple in construction and easy to attach to a platform column, particularly platform columns which do not have a uniform outer diameter. It is therefore an objective of the present invention to provide one or more strakes which can be arranged on existing platforms which are already in use.

Another objective of the present invention is to provide a strake for a platform comprising columns with a varying outer diameter.

Furthermore, it is an objective of the present invention to provide strakes which are simple in construction, easy to attach on existing platform columns and cheap to manufacture.

These objectives are solved with a strake as defined in claim 1 , a floating structure as defined in claim 10 and a method for reducing velocity induced motions on a floating platform as defined in claim 18. Further embodiments of the strake are defined in dependent claims 2-9. Further embodiments of the floating structure are defined in dependent claims 11 -17, and further embodiments of the method for reducing velocity induced motions on a floating platform are defined in dependent claims 19-22.

There is provided a strake which is formed by at least one single plate, but preferably a plurality of plates, where the plates is adapted to be welded to the platform column and edge-to-edge to any neighbouring plate or plates of the strake such that a continuous strake is formed. Such plates are easy to cut out and to adapt to a column which has a varying diameter.

In order to absorb forces and moments acting on the strake, the strake is provided with at least one supporting bracket. The at least one supporting bracket is preferably attached to a strake plate such that the at least one supporting bracket is substantially horizontal when the strake is mounted on a platform column and the platform column is in a vertical position.

The at least one supporting bracket may be attached to the strake and the platform column in various ways, but is preferably welded to the strake and adapted to be welded to the platform column. The at least one supporting bracket may also be attached to the strake and the column with bolts or similar fastening means.

The size and number of supporting brackets that are needed will depend on several factors such as the size of the strake and the strength of water currents where the platform will be used.

Furthermore, the at least one support bracket is preferably formed by a single metal plate. If the strake is provided with two or more supporting brackets, the supporting brackets are preferably arranged on the same side of the strake.

The plate or plates that the strake is made up of may also be provided with at least one stiffener which will provide the plate or plates of the strake with additional strength. The at least one stiffer is preferably made of a single plate which is attached to the plate or plates of the strake by suitable fastening means, such as bolts, or preferably by welding. The at least one stiffener is preferably attached to the strake such that the at least one stiffener is substantially parallel to the surface of the platform column when the strake is attached to the platform column, i.e. the at least one stiffener extends substantially in the longitudinal direction of the strake. The size and number of stiffeners that are needed, will depend on several factors such as the size of the strake and the strength of water currents where the platform will be used. The at least one supporting bracket and the at least one stiffener are preferably provided on the same side of the strake. The plates of the at least one strake and/or the supporting brackets and/or the stiffeners are preferably made of a weldable metal like steel.

An embodiment of the present invention comprising a plurality of single plates which are welded to each other edge-to-edge and adapted to be welded to a platform column, supporting brackets which are formed by a single plate and welded to the strake and adapted to be welded to a platform column, and stiffeners which are welded to the strake, provides a strake which is sufficiently strong, is easy and cheap to manufacture, is easy to adapt to a platform column with a varying outer diameter and can easily be adapted to be attached to an existing platform.

There is also provided a floating structure comprising at least one column penetrating the water surface where the at least one column is provided with at least one strake. The at least one strake is formed by at least one single plate, but preferably a plurality of single plates, where the plates are welded to the platform column and edge-to-edge to any neighbouring plate or plates of the at least one strake such that a continuous strake is formed.

The at least one strake preferably extends downwards from the water line of the columns forming a helical shape.

The at least one strake is furthermore provided at least one supporting bracket. The at least one supporting bracket is attached to the at least one strake such that the at least one supporting bracket is substantially horizontal when the at least one strake is mounted on a platform column and the platform column is in a vertical position.

The at least one supporting bracket is preferably welded to the at least one strake and to the platform column at the same vertical level as the horizontal ring frames and decks inside the columns. The at least one supporting bracket may also be attached to the column and/or the strake with other means of fastening such as bolts. In a preferred embodiment of the floating structure all supporting brackets of the at least one strake are positioned on the same side of the at least one strake.

In a further embodiment of the plate or plates that the at least one strake is made up of, is provided with at least one stiffener. Preferably, the at least one supporting bracket and the at least one stiffener are attached to the at least one strake on the same side of the at least one strake.

Further embodiments of the supporting brackets and stiffeners of the floating structure is described above in connection with the strake.

There is also provided a method for reducing velocity induced motions on a floating platform where the platform comprises at least one column, where the method comprises the steps of welding at least one strake, formed by at least one single plate, but preferably a plurality of plates, directly on to the hull of the at least one column. Preferably, the at least one strake is provided with a helical shape extending downwards from the waterline of the at least one column.

The method further involves the step of welding at least one supporting bracket, comprising a single plate, directly to the strakes and to the hull of the at least one column. The method preferably also comprises the step of positioning the at least one supporting brackets of the at least one strake on the same side of said strake, and preferably also positioning the supporting brackets in same plane and at the same vertical level as the horizontal ring frames and decks inside the at least one column. It should be noted that a single plate in connection with the present invention means that that the plate is not a doubler plate.

A non-limiting embodiment of the invention will now be explained in more detail with reference to figures where Figure 1 shows strakes mounted on a platform column.

Figure 2 shows the same column as in figure 1 , but viewed from a different angle.

Figure 3 shows a strake.

Figure 4 shows the same strake as in figure 3, but viewed from a different angle.

Figures 1 and 2 discloses the same embodiment of the present invention, but seen from opposite sides, where strakes are arranged on a column 18 of platform.

Figures 3 and 4 show the strakes in more detail.

The embodiment of the floating structure shown in the figures comprises one or more columns 10 which penetrate the water surface. The column 10 may have a circular cross section along its entire longitudinal length (i.e. vertically), but the embodiment shown on the figures are divided into three sections, an upper section 12, which has a cylindrical shape with a circular cross section, a middle section 14 which is shown with a varying outer diameter, and a lower section 16 which has a cylindrical shape with a circular shape. The middle section 14 comprises an upper part 20 which has a conical shape, a middle part 22 which has a cylindrical shape with a circular cross section, and a lower part 24 which has a conical shape. The floating structure is designed such that the upper part 20 of the middle section 14 cuts through the water line of the sea (indicated with reference number 40 which indicates a belt or part of the column moving up and down through the water surface). The column 10 is provided with at least one, but preferably three strakes 18 as shown on the two figures. Any other number of strakes 18 may of course also be attached to a column if desired. The strakes 18 are made of one or more single plates which are welded to the columns 10. As already mentioned, a single plate means that that the plate is not a doubler plate. The strakes 18 are preferably made of two or more such single plates 19, as can be seen on figures 3, which are arranged edge-to-edge as indicated by lines 23, and welded together such that they form continuous strakes 18.

As can be seen on figures 1 -2, the strakes 18 are helically shaped, i.e. the strakes extends down the column along a helical path. Preferably the strakes start at the water line 40 of the column 10 and extend downwards to approximately the lower edge of the middle part 22 of the middle section 14. The strakes 18 may also extend further down if so desired. The height of the strakes (i.e. the radial height from the column to the outer edge of the strake) may be in the order of approximately 10% of the biggest diameter of the columns, which, for some embodiments of the invention may be about 2m.

As shown on the figures, the strakes 18 are supported by a plurality of supporting brackets 30. The supporting brackets 30 are also made of single plates without using doubler plates or any inside reinforcements. The supporting brackets 30 are welded to the plates 19 of the strakes 18 and to the column 10. The supporting brackets 30 are preferably positioned in the same plane and at the same vertical level as the horizontal ring frames and decks (not shown on the figures) inside the hull of the column 10. This will provide support for the supporting brackets 30 and help to absorb forces and moments acting on the strake 18. The strakes 18 are also shown with a plurality of stiffeners 21 which preferably extends in the longitudinal direction of the strake 18. The stiffeners 21 are preferably made of single plates which are welded to the plates 19 of the strake. As can be seen on figure 3 and 4, the stiffeners 21 are preferably attached to the same side of the strake 18 as the supporting brackets 30. The strakes 18 which have been described above, comprises single plates 19 to which there are attached supporting brackets 30 and stiffeners 21 , also made of single plates. Since the plates 19 preferably are welded edge-to-edge and the supporting brackets preferably are welded to the plates 19 and to the column 10 and the stiffeners 21 are welded to the plates 19, there is provided a strake 18 which is simple to manufacture on to mount on a platform column of both existing platforms and platforms which are under construction and which can easily be adapted to columns with a varying outer diameter if necessary which is particularly important when strakes are to be mounted on a column of an existing platform.