Background of the invention

Floating systems are frequently used for activities such as exploration, production, drilling and storage in the oil industry, both offshore and inshore. These floating installations may comprise cylindrical platforms that are kept in position by means of mooring lines which are anchored to the seabed.

US 3,138,135 describes a mooring device including mooring lines anchored to the seabed. Buoyancy bodies are attached to the mooring lines such that a central part of the mooring lines is oriented horizontally. Radially outwards of the horizontally oriented part, the mooring lines hang rather loose such that they may touch the seabed under extreme conditions.

The floating systems known from the state of the art are, in terms of station-keeping performance and motion behavior, not optimal for shallow waters. For example, floating platforms may be required to operate at water depths in the order of 15 to 20 meters. Such very shallow waters impose severe restrictions on allowable platform drafts in order to avoid seabed impact. A floating platform operating at these waters depths may typically have a ballast draft of about 11 meters and a loaded draft of about 12 meters, i.e. a draft variation of only 1 meter. Maintaining the mooring lines at predetermined depths, avoiding surfacing and seabed contact, is also a challenge in such very shallow waters. It is therefore a need for optimizing the existing floating systems such that they are in particular suitable for very shallow waters.

Summary of the invention

It is an objective of the present invention to provide a mooring system especially suitable for very shallow water. This objective is achieved with a mooring system according to the independent claim. Advantageous further developments are subject of the dependent claims.

It is thus provided a mooring system, comprising at least one mooring line, one end of which is attached to a floating body configured for floating in water, and the other end of which is adapted to be anchored to a seabed; wherein at least two buoyancy elements are attached to the mooring line for generating a buoyant force acting on the mooring line, and wherein the buoyancy elements are spaced apart along the mooring line. The mooring system is characterized in that - said at least two buoyancy elements have different sizes and buoyancies;

- the larger buoyancy element is located farther away from the floating body than the smaller buoyancy element;

- said at least two buoyancy elements are elongated bodies of different lengths, which are attached to the mooring lines at one of their ends; and

each buoyancy element is configured such that a smaller elongated part of the buoyancy element is above the water surface and a larger elongated part of the buoancy element is below the water surface, whereby each buoyancy element exhibit variable buoyancy, depending on the weight with which the mooring line (9) pulls on the buoyancy element.

This has the advantage that the buoyancy elements exhibit variable buoyancy. Namely, the more the weight of the mooring line pulls the buoyancy elements downwards, the larger the elongated part becomes which is below the water surface and the larger the buoyancy. By choosing the elongated shape, the variation range of the buoyancy force is larger than with for example a rounded buoyancy element. In this manner, the mooring lines may be kept from touching the seabed and (if buoyant) from floating up to the water surface.

According to another embodiment of the invention, in a normal operating condition, the elongated length of the buoyancy elements is longer than the distance between an attachment point at which a respective buoyancy element is attached to the mooring line and the water surface vertically above this attachment point. With buoyancy elements of such a large elongated dimension, the range of the above mentioned effect of the variable buoyancy is enlarged.

The elongated shape of the buoyancy elements will prevent the mooring line from floating to the surface, thus avoiding the possibility of it being damaged by ships and other surface vessels.

According to another embodiment of the invention, the buoyancy elements are adapted such that in use, a smaller portion of the elongated length of the buoyancy elements is above the water surface and a larger portion is below the water surface.

According to another embodiment of the invention, the floating body has an upper portion, a lower portion and a middle portion connecting the upper and lower portion. The comparatively large lower portion is advantageous for reducing heave motion of the floating body.

According to another embodiment of the invention, at least a segment of the mooring line is formed by a fibre rope, prefereably a synthetic fibre rope. In one embodiment, the segment comprises a polyester line. This could be one line, or more polyester lines connected in a row. Polyester line(s) provides the main stiffness element of the mooring line, in particular the polyester line provides the mooring line with axial stiffness. In particular, a segment of the mooring line which is arranged in between the buoyancy elements is a fibre rope line.

According to another embodiment of the invention, the mooring line comprises a lower chain segment which is adapted to be anchored to the seabed. In particular, the chain segment may have a length which is longer than the depth of the water vertically above the anchor. This has the advantage that after installation of the anchor and bottom chain segment, the top end of the chain segment can be retrieved by a vessel to the water surface, in order to connect further segments of the mooring line such as a first fibre rope segment or buoyancy elements to it.

According to another embodiment of the invention, the largest buoyancy element of the at least two buoyancy elements is attached to the end of the lower chain segment opposite to the end adapted to be anchored to the seabed.

According to another embodiment of the invention, the mooring line comprises an upper chain segment which is adapted to be attached to the floating body. This upper chain segment has the advantage that a conventional fairlead and chain stopper arrangement can be used at the floating body.

According to another embodiment of the invention, the mooring line comprises a lower chain segment which is adapted to be anchored to the seabed, an upper chain segment which is adapted to be attached to the floating body, and at least one fibre rope line connecting the lower and upper chain segments. This combines the above mentioned advantages with respect to the lower and upper chain segments and the fibre rope line.

According to another embodiment of the invention, the mooring system includes three or more mooring line groups which are attached to the floating body at three different circumferential positions, each group comprising a plurality of the mooring lines. Three mooring line groups are beneficial in terms of cost and position stability.

According to another embodiment of the invention, each mooring line group includes one or more mooring lines.

According to another embodiment of the invention, each mooring line group is attached to the floating body at an attachment position, wherein the attachment positions for each group are spaced apart from each other in a circumferential direction of the floating body.

The floating body comprises internal ballast tanks and cargo tanks. In one embodiment, the cargo tanks are arranged concentrically around a central tank area and the ballast tanks are arranges concentrically around the cargo tanks.

According to another embodiment of the invention, the mooring line is inclined downwards from the end attached to the floating body towards the anchored end.

According to another embodiment of the invention, the lower chain segment has a length which is longer than the the depth of the water in which the body (1) is floating. These and other embodiments of the invention are described in the following in more detail with reference to the figures. Brief description of the drawings

Fig. 1 schematically illustrates a side view of a floating body according to an embodiment of the invention;

Fig. 2 shows a top view of the mooring system including three groups of mooring lines,

Fig. 3 schematically illustrates one of the mooring lines, its attachment to the floating body and its anchoring to the seabed, and

Fig. 4 is a perspective sectional view at the nominal water line 15 (cf. Fig. 1) illustrating a typical tank configuration.

Detailed description of a preferred embodiment

Fig. 1 schematically illustrates a front view of a floating platform 1 according to an embodiment of the invention, floating in a body of water above a seabed 10. The floating platform 1 has preferably a substantially circular cross-section in a horizontal plane, and is adapted for floating in water. These so-called cylinder platforms are usually used for floating production, drilling and storage of oil and gas. Also a square, rectangular or other cross-section is possible. The floating platform 1 comprises internal tanks for storage of oil, ballast or other fluids. A tank configuration is shown in Fig. 4, schematically illustrating central tanks 23 surrounded by concentric cargo tanks 24 and ballast peripheral tanks 22. The central tanks 23 may typically be used for slop, ballast and other fluids. It should be understood that the tank configuration shown in Fig. 4 is an example, and that the sizes (volumes) and lay-out may be arranged as suited for the given application.

The floating platform 1 together with its mooring system is in particular suitable for use in shallow waters.

The floating platform 1 can be imaginary divided into an upper portion 2, a middle portion 3 and a lower portion 4. The upper portion includes a process deck 5 and a main deck 6. The lower portion 4, often referred to as a "skirt", has the larger diameter, for example 80m, the middle portion has the smaller diameter, for example 62m, and the upper portion has a maximum diameter in between these two, for example 71m. The preferred total height of the floating platform in the illustrated embodiment is 26m. It should be noted that the invention shall not be limited to the aforementioned dimensions, as they merely indicate one possible embodiment.

As shown in Fig. 1, the diameter of the floating platform reduces stepwise from the skirt 4 to the middle portion 3. The lower part of the upper portion 2 increases its diameter continuously from the diameter of the middle portion 3 to the maximum diameter of the upper portion. The same relationship as described in connection with the diameters applies to the circumferences of the upper, middle and lower portion. In the illustrated embodiment, the lower portin 4 has a greater diameter than the middle portion 3.

Fig. 2 shows a top view of a mooring system 7, including three groups 8 of mooring lines 9 (only one of which is provided with a reference numeral). In the illustrated embodiment, a group 8 comprises three mooring lines 9, one end of each is attached to the floating platform 1 and the other end of each is adapted to be anchored to a seabed (not shown in Fig. 2) located below the floating platform. The mooring system 7 shown in Fig. 2 thus comprises nine mooring lines 9 and buoyancy elements 13, 14 (only two of which are given a reference sign in Fig. 2). The three groups 8, in particular the middle mooring lines of these, are in the illustrated embodiment spaced from each other by 120° about the vertical, as it is indicated by the angle a in Fig. 2. All mooring lines 9 within a group 8 are attached to the floating platform 1 at substantially the same position or close to each other, preferably with the same distance in between adjacent ones. The ends of the mooring lines 9 within a group 8, which ends are attached to anchors 12 on the seabed, are spaced apart from each other. Alternatively, also three groups 8 each having four mooring lines 9 can be used advantageously. Further, the invention may be beneficially realized with a different number of groups 8 and/or mooring lines 9.

The mooring system 7 is based on a passive mooring system with no use of thrusters, in order to maintain the position of the floating platform 1 with respect to the seabed. Please note that the arrangement and orientation of the mooring lines in Fig. 2 is only of exemplary nature. The actual orientation of the groups 8 and the mooring lines 9 will have to be determined based on the actual environmental conditions at the site.

Fig. 3 schematically illustrates one of the mooring lines 9, its attachment to the floating platform 1 and its anchoring to the seabed 10. The end of the mooring line 9 which is adapted to be anchored to the seabed 10 (referred to as "anchored end") is attached to an anchor 12. The anchor 12 may be a driven or drilled pile, a drag anchor, a suction anchor or other type of anchor, depending on the soil condition of the seabed 10. The mooring line 9 is inclined downwards from the end attached to the floating platform 1 towards the anchored end. A plurality of buoyancy elements 13, 14 is attached to the mooring line 9. The number of the buoyancy elements 13, 14 in the shown embodiment is two, but the invention could also be beneficially used with more than two buoyancy elements. The buoyancy elements 13, 14 are elongated bodies which, when immersed into water, lead to an upwards force exerted by the water that opposes the weight of an immersed object, such that the buoyancy elements 13, 14 float at the area of the water surface 15, in particular a smaller elongated part of the buoyancy elements 13, 14 being above and a larger elongated part of them being below the water surface 15. This has the advantage that the buoyancy elements 13, 14 exhibit variable buoyancy. Namely, the more the weight of the mooring line 9 pulls the buoyancy elements 13, 14 downwards, the larger the elongated part becomes which is below the water surface and the larger the buoyancy. By choosing the elongated shape, the variation range of the buoyancy force is larger than with for example a rounded buoyancy element. The buoyancy elements 13, 14 are attached with one elongated end to the mooring line 9, such that in use they are oriented vertically in the water (due to water movement this orientation is not fixed, of course). In more detail, the elongated end of the buoyancy elements 13, 14 is attached directly to the mooring line 9 or by means of a short chain segment and/or a connection element, such as an H-shackle. The buoyancy elements 13, 14 attached to the same mooring line 9 have different lengths (and different diameter), such that their length increases away from the floating platform 1. In other words, the buoyancy element 13 being closer to the floating platform is shorter in length than the buoyancy element 14 being closer to the anchor 12. This has the advantage that the mooring lines 9 are maintained within a predetermined depth range, and prevented from sinking to the seabed. The inclined orientation of the mooring line 9 is particularly favorable for shallow water. The buoyancy elements 13, 14 are useful in case that the mooring line 9, or parts of it (as will be described below) has itself a buoyant effect, or a buoyancy close to neutral, but they are primarily intended for mooring lines that do not float, such as polyester lines.

The segment of the mooring line 9 which segment is located in between the buoyancy element 14 (which is farthest away from the floating platform 1) and the anchor 12, is a lower chain segment 16. The chain segment 16 has a length which is longer than the depth of the water, in the exemplary embodiment approximately 40 m. This has the advantage that after installation of the anchor 12 and bottom chain segment 16, the top end of the chain segment 16 can be retrieved by a vessel to the water surface, in order to connect a first line segment 17, comprising a fibre rope, preferably a synthetic fibre rope . The first line segment 17 is located in between the buoyancy elements 13, 14. A second line segment 18, also comprising a fibre rope, preferably a synthetic fibre rope, is connected to the first line segment 17 at the attachment point of the buoyancy element 13.

The fibre rope line segments 17 and 18 are in one embodiment polyester lines. The polyester lines provide the main stiffness element of the mooring line 9 (utilizing the axial stiffness of the polyester line). The buoyancy element 13 is located between the two line segments 17 and 18. The end 19 of the second line segment 18, which is closer to the floating platform 1, is connected to an upper chain segment 20 which is connected to the floating platform 1. This upper chain segment 20 is introduced in order to utilize a conventional fairlead and chain stopper arrangement at the floating platform 1. The different segments of the mooring line 9 may be connected with each other by connection elements 11, such as H-shackles.

The buoyancy elements 13, 14 are used in order to ensure that the line segments (e.g. polyester line) are lifted off the seabed and kept below the water surface in all operating conditions, thus avoiding damage to the fibre line segments. The largest buoyancy element 14 is connected by a short segment to the H-shackle connecting the lower chain segment 16 and the first line segment 17. The smaller buoyancy element 13 is connected between the two line segments of the mooring line 9.

In operation in very shallow water with short distance between hull bottom and seabed, the floating body 1 ballast tank 22 volume and cargo tank 24 volume are kept more or less equal, in order to maintain the platform within predetermined draft tolerances. For the same purpose, active ballasting operations are employed during loading and offloading. The skirt 4 contributes to reducing platform heave movements.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive and it is not intended to limit the invention to the disclosed embodiments. The mere fact that certain dimensions are recited in mutually different dependent claims does not indicate that a combination of these dimensions cannot be used advantageously.