TECHNICAL FIELD

[1] The present disclosure relates to platforms for use with catenary mooring systems in particular, but not exclusively for use with offshore wind turbines.

BACKGROUND

[2] Small Catenary Anchor Leg Mooring (CALM) buoys and floating platforms are often employed as offshore loading facilities for transferring oil from an onshore or offshore location to an oil tanker, or from an oil tanker to a reception facility. These types of floating platforms are so named because they employ a plurality of catenary anchor chains to hold the floating platform generally in place.

[3] An advantage of CALM floating platforms is that they do not require construction of a structure that is required to be fixed to the seabed. However, since offshore loading facilities are often located in unprotected waters, CALM floating platforms must be designed to accommodate and withstand great environmental forces produced by large swells or waves, high winds and/or strong currents. These environmental forces can become particularly fierce when CALM floating platforms are placed in a relatively shallow location because waves tend to build up and become very steep before they break in the shallow water.

[4] Typically, CALM floating platforms are made with a circular or rectangular vertical cross-section which has a relatively high drag resistance, and are difficult to build in a simple shipyard if they are circular. In addition, CALM floating platforms are made so that they will attempt to climb over the waves as the waves pass by. As a result, very large forces are imposed both on the floating platform and the anchor chains holding the floating platform. In the past, this problem has been either avoided by moving the floating platform further offshore to deeper water in order to avoid steep breaking waves. Alternatively, the forces can be accommodated by increasing the diameter of the anchor chains. Generally, the placement of CALM floating platforms represents a compromise between moving the floating platform further offshore and increasing the diameter of the catenary anchor chains. However, both solutions increase the cost of the offshore loading facility considerably. [5] The present disclosure has been devised to mitigate or overcome at least some of the above-mentioned problems

SUMMARY

[6] According to an aspect of the disclosure, there is provided a platform for use in a catenary mooring system. The platform comprises a watertight bulkhead for rendering the platform floatable. The platform has a chamfered edge arranged in use to cause the platform to at least partially dive under waves which reach the platform. The chamfered edge reduces the drag coefficient of the platform, reducing stress on any catenary chains that may anchor the platform when the platform is in use. Waves that arrive at the platform will wash over the top of the platform due to the chamfered edge. A wave hitting one side of the platform causes that side of the platform to tilt downwards as the wave runs up and over the chamfered edge, thereby reducing the water plane on the forward section, and causing the platform to dive downwards at least partially into the wave. As the platform at least partially dives through oncoming waves rather than, for example, floating over the waves, this reduces tension on any catenary lines.

[7] An angle of the chamfered edge may be between approximately 10° and 70° from the waterline of the platform in use.

[8] The chamfered edge may be above or below the waterline in use. If the chamfered edge is above the waterline, the platform may comprise a second chamfered edge, wherein the second chamfered edge is below the waterline in use. An angle of the second chamfered edge is between approximately 10° and 70° from the waterline of the platform in use.

[9] The platform may comprise a collision bulkhead arranged around at least part of the watertight bulkhead. The watertight bulkhead may be encased by the collision bulkhead. The collision bulkhead is arranged to protect the integrity of the watertight bulkhead.

[10] The platform may comprise a set of side walls, wherein each side wall is perpendicular to the waterline in use, and each side wall in the set of side walls is substantially flat. For example each side wall may be a planar sheet of steel. The set of side walls may comprise four or more side walls. Preferably the set of side walls consists of six side walls. [11] The platform may comprise a plurality of attachment points for fixedly attaching catenary lines, such as chains, ropes, cables or wires. The platform may comprise means for fixedly attaching a wind turbine.

[12] There is also provided a catenary mooring system comprising the platform, and a plurality of catenary lines connected to the platform.

[13] There is also provided a floating wind turbine comprising the catenary mooring system.

BRIEF DESCRIPTION OF THE DRAWINGS

[14] Further details, aspects and embodiments of the disclosure will be described, by way of example only, with reference to the drawings in which:

Figures 1(a), 1(b) and 1(c) are schematic diagrams illustrating a catenary mooring system having a platform, wherein Figure 1(a) is a front view of the catenary mooring system, Figure 1(b) is a side view of the catenary mooring system, and Figure 1(c) is a top view of the catenary mooring system;

Figure 2 is a schematic diagram illustrating a side view of a catenary mooring system having a platform;

Figure 3 is a schematic diagram illustrating a side view of a catenary mooring system having a platform;

Figure 4 is a schematic diagram illustrating a top view of a catenary mooring system having a platform; and

Figure 5 is a schematic diagram illustrating a top view of a catenary mooring system having a platform.

[15] Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. Like reference numerals have been included in the respective drawings to ease understanding.

DETAILED DESCRIPTION OF THE DRAWINGS

[16] With reference to Figures 1(a), 1(b) and 1(c), a platform 11 is arranged to float on the surface of a body of water. The platform 11 comprises a watertight bulkhead arranged to cause the platform 11 to float on water. The platform 11 includes a plurality of attachment points 20 for connecting to a first end of a respective catenary line 14 such as a wire, a cable, a rope, or a chain. A second end of each catenary line 14 may be connected to a seabed anchor or anchor pile 16. The platform 11 is anchored to the seabed 13 via the catenary lines 14. The platform 11 is intended to be located at a predetermined geographic location and when the platform 11 deviates from its intended location, for example due the effects of the wind, waves or water currents, the slack in one or more catenary line 14 is lifted. This generates a restoring force which tends to return the platform 11 to its equilibrium position, thus limiting the deviation of the platform 11 from its intended location.

[17] The platform 11 may comprise a collision bulkhead around at least part of the watertight bulkhead to protect the watertight bulkhead from impact e.g. with other floating objects or vessels.

[18] The platform 11 includes a section 58 that is arranged to support a wind turbine 50 for generating electricity. In other examples, the platform 11 may comprise tanks e.g. for storing oil products. The section 58 may include more structural reinforcement than other sections of the platform 11 in order to support the wind turbine 50. The wind turbine 50 is connected to a flexible power and data cable 26 for transmitting electricity and data to and from the wind turbine 50. The power and data cable 26 comprises a plurality of buoyancy modules 27 to support slack in the power and data cable 26 which enables continued operation in a range of weather conditions.

[19] The platform 11 comprises a chamfered edge 52 around its perimeter. The chamfered edge 52 may span 10% to 50% of the width of the platform 11 (where 50% span means that the chamfered edge reaches the centre of the platform). In the example of Figures 1 (a), 1 (b) and 1 (c), the chamfered edge 52 is an upper edge of the platform 11 which is normally above the surface of the body of water, i.e. the chamfered edge 52 is above the waterline of the platform 11. The chamfered edge 52 may have an angle 54 with the waterline (i.e. parallel to the still water level) of between 10° and 70°. In the example of Figures 1(a), 1(b) and 1(c), angle 54 is approximately 10°. The top and the bottom of the platform 11 is substantially flat.

[20] Waves that arrive at the platform 11 will wash over the top of the platform 11 due to the chamfered edge 52. Denoting the side of the platform 11 where a wave first hits as the forward side, the wave causes the front of the platform to tilt downwards as the wave runs up and over the chamfered edge 52, thereby reducing the water plane on the forward section, and causing the platform 11 to dive downwards at least partially into the wave.

[21] As the platform 11 dives at least partially through large oncoming waves rather than, for example, floating over the waves, this reduces tension on the catenary lines 14. Additionally, diving into the wave enables the platform 11 to be exposed to deeper regions of the body of water where water particle motion is lower. In turn, this reduces stress and wear on the catenary lines 14, the plurality of attachment points 20, and any anchors or anchor piles.

[22] The platform 11 has six substantially flat side walls that are substantially perpendicular to the waterline when the platform 11 is in use. In other words, the platform 11 has a generally hexagonal cross section when viewed from above (see Figure 1(c)). Advantageously, the hexagonal cross section of the platform 11 has a lower drag coefficient than a rectangular cross section used in typical prior art platforms for catenary mooring systems, whilst balancing fabrication costs. For example, a platform with a circular cross section would have a smaller drag coefficient than a hexagonal platform, however, a circular cross section would require double curvature plates to achieve the chamfered edge of the platform. Such curved plates are more time consuming and complex to manufacture whereas the platform 11 with hexagonal cross section can be assembled using conventional tools from flat plates. It is to be understood that similar advantages could be achieved with any number of substantially flat side walls, e.g. in a platform having, when view from above, a pentagonal, heptagonal, octagonal, nonagonal, decagonal etc. cross section. With a greater number of sides, the drag coefficient of the platform reduces. The cross section of the platform is not required to have sides of the same or equal lengths.

[23] The platform 11 of Figures 1(a), 1(b) and 1(c) has a substantially flat underside 56 which is normally below the surface of the body of water. In other examples, the edge of the platform 11 which is below the waterline is also chamfered.

[24] With reference to Figure 2, a platform 111 is arranged to float on the surface of a body of water (not illustrated). The platform 111 is substantially the same as platform 11 but with the addition of a second chamfered edge 113 around the perimeter of the platform that is below the waterline. The second chamfered edge 113 may have an angle 115 with the waterline (i.e. parallel to the still water level) of between 10° and 70°. In the example of Figure 2, angle 115 is approximately 10°. The angle 54 of the chamfered edge and the angle 115 of the second chamfered edge are independent of each other and so may be different angles. The second chamfered edge 113 may span 10% to 50% of the width of the platform 111 (where 50% span means that the chamfered edge reaches the centre of the platform).

[25] With reference to Figure 3, a platform 211 is arranged to float on the surface of a body of water (not illustrated). The platform 211 is substantially the same as platform 11 but with the addition of a second chamfered edge 213 around the perimeter of the platform that is below the waterline. The platform 211 also differs from platform 11 in that the bottom of the platform 211 is not flat. The second chamfered edge 213 may have an angle 215 with the waterline (i.e. parallel to the still water level) of between 10° and 70°. The second chamfered edge 213 spans 50% of the width of the platform 211 , and so forms a point in the centre of the underside of the platform 211.

[26] The platform may comprise a plurality of interconnected modules, each module having a watertight bulkhead. The modules may be coupled directly to each other, e.g. via welding, or may be coupled together using rigid members and/or chains. When coupled together, the modules form a platform. Each module comprises a chamfered edge on at least one side and, when arranged into a platform, the chamfered edges are around the outside of the platform. For example, in a platform in which the modules are welded directly together, the platform may be substantially the same as platform 11, 111, 211. Advantageously, a platform comprising interconnected modules may be assembled closer to the installation location than the location where the modules or platforms are fabricated, thereby simplifying the manufacturing and transport processes. Figures 4 and 5 illustrate example platform in which the modules are coupled with rigid members.

[27] With reference to Figure 4, a platform 400 includes six substantially similar modules 402, 402a. In other examples, the platform may include a different number of modules, for example, three to sixteen modules, preferably six or eight modules. One of the modules 402a may include more structural reinforcement than other modules in order to support the wind turbine. The modules 402, 402a are coupled together with a plurality of support members 406 such as beams or chains. One edge of each module 402 comprises a chamfered edge 404 and the modules 402 in the platform 400 are arranged such that the chamfered edges 404 are around the perimeter of the platform 400. The chamfered edges 404 are substantially similar to any one of chamfered edges 52, 113, 213. Each module 402, 402a includes an attachment point 20 for connecting to a first end of a respective catenary line 14. In other examples, the modules 402, 402a are directly welded together without any support members 406.

[28] With reference to Figure 5, a platform 500 includes six substantially similar modules 502, 502a and a central module 502b. The modules 502, 502a are arranged around the central module 502b. In other examples, the platform may include a different number of modules, for example, three to sixteen modules, preferably six or eight modules. The number of edges of the central module 502b corresponds to the number of modules 502, 502a. In the example of platform 500, the central module 502b has a hexagonal cross-section as there are six substantially similar modules 502, 502a surrounding the central module 502b. One of the modules 502a may include more structural reinforcement than other modules in order to support the wind turbine. The modules 502, 502a and the central module 502b are coupled together with a plurality of support members 506 such as beams or chains. One edge of each module 502, 502a comprises a chamfered edge 504 and the modules 502, 502a in the platform 500 are arranged such that the chamfered edges 504 are around the perimeter of the platform 500. The chamfered edges 504 are substantially similar to any one of chamfered edges 52, 113, 213, 404. Each module 502, 502a includes an attachment point 20 for connecting to a first end of a respective catenary line 14. In other examples, the modules 502, 502a and the central module 502b are directly welded together without any support members 506.

[29] Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also, the inclusion of a feature in one category of claims does not imply a limitation to this category, but rather indicates that the feature is equally applicable to other claim categories, as appropriate.

[30] Furthermore, the order of features in the claims does not imply any specific order in which the features must be performed and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus, references to ‘a’, ‘an’, ‘first’, ‘second’, etc. do not preclude a plurality.

[31] Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognise that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term ‘comprising’ or “including” does not exclude the presence of other elements.