Field of the invention

The present invention relates to a spread mooring arrangement for a vessel, which allows side-by-side connection of the vessel with a cargo ship so that a ship-to-ship transfer may take place. For instance, arrangement may be used for spread mooring a floating production vessel for hydrocarbons, such as liquefied nitrogen gas (LNG), and the cargo ship may be a floating storage unit (FSU). The invention is particularly suitable for use in shallow water, i.e. water having a depth of 35 meters or less, e.g. less than 25 meters. The invention further relates to use of such a spread mooring arrangement, and to a method of side-by-side mooring a cargo ship to a vessel that has been spread moored using the spread mooring arrangement.

Background art

From the prior art a catenary mooring system is known in which two mooring lines are each connected at one end to a vessel and at another end to the seafloor, and wherein a bar or a rigidly tensioned chain is connected between the two mooring lines to restrict separation of the mooring lines in a horizontal direction to a predetermined fixed distance. The bar or rigidly tensioned chain contributes to holding the mooring lines together, so that a cargo ship, e.g. an LNG carrier, can be moored in a side-by-side fashion to the vessel without the mooring lines interfering with the cargo ship.

A drawback of the known system is that, as the mooring lines are catenary, there may be some excursion of the moored vessel from an intended position of the vessel, for instance due to changing currents or changes in wind direction.

It is an object of the invention to provide a spread mooring arrangement, use thereof, and a method in which this drawback is at least partially overcome.

Summary of the invention

To this end, according to a first aspect, the present invention provides a spread mooring arrangement for mooring a vessel to the sea floor, the arrangement comprising: a first and a second forward catenary mooring line, each connected at one end to the sea floor and at another end to a bow portion of the vessel; a first and a second aft catenary mooring line, each connected at one end to the sea floor and at another end to a stern portion of the vessel; wherein the first and second forward catenary mooring lines are connected to each other below the water surface by a forward connecting line that is suspended from the first and second catenary mooring lines and hangs in a sagging configuration; and/or the first and second aft catenary mooring lines are connected to each other below the water surface by an aft connecting line that is suspended from the first and second aft catenary mooring lines and hangs in a sagging configuration. The weight of the connecting lines exerts a force on the corresponding catenary mooring lines which urges these catenary mooring lines together, at least at a portion of the catenary mooring lines in the vicinity of the connecting line. The connecting lines in this manner provide a restoring force on the catenary mooring lines for moving the vessel back to an intended position if the vessel has been displaced from its neutral position. The sagging configuration may for instance be a substantially U-shaped configuration, e.g. when the connecting line has a substantially uniform weight distribution along its length, or a substantially V-shaped configuration, e.g. when a clump weight or the like is attached at a center portion of the connecting line.

Both the bow and stern portions of the vessel are connected to catenary mooring lines, allowing the draft of the vessel to vary while the vessel is spread moored to the sea floor. When the vessel’s draft changes, e.g. increases during loading of the vessel or decreased during unloading, the sagging shape of the connecting line will change form as well, and the position of the centre of mass of the connecting line will shift correspondingly. The shape of the connecting line will generally become wider if the draft increases, and more narrow if the vessel’s depth decreases. Thanks to the flexibility of the connection line(s) and their sagging configuration, the points of connection of the connecting line to its catenary mooring lines can move away from each other, to some extent, both in a vertical direction and in a horizontal direction, while the absolute distance between the points of connection can vary. As the connecting line is deformable, its center of mass can shift relative to its points of connection with the catenary mooring lines.

Due to the use of catenary mooring lines the mooring arrangement also allows some excursion of the vessel from an intended position in the horizontal plane, and during such an excursion, two catenary lines that are connected by the connecting line are urged towards each other. The weight of each connecting line is shared between its associated catenary mooring lines, providing an enhanced catenary effect. By sharing the weight between two catenary lines, the total amount of weight used to restore the vessel to an intended position can be lower than if each of the catenary lines were separately provided with weights to provide a similar restoring force. Thus, at least some of the costs for weights to provide the restoring force, e.g. by providing heavy chains or clump weights, can be reduced when compared to prior art solutions in which separate mooring line are each separately provided with clump weights.

The connecting lines further keep their associated catenary lines together, so that the catenary lines, at least between the water surface and the bottom of the vessel’s hull, do not interfere with a cargo ship that is to be moored in a side-by-side configuration or has been moored side-by-side, during approach or departure of the cargo ship.

The anchor points at which the first and second forward catenary lines are connected to the sea floor are arranged in front of the bow of the vessel, and the anchor points at which the first and second aft catenary lines are connected to the sea floor are arranged rearward of the stern of the vessel. Preferably, the mooring arrangement and the vessel are located in waters having a depth of 35 meters or less.

The invention provides a mooring arrangement with a particularly small footprint, i.e. can be used when the area spanned by the anchors in the sea floor is relatively small.

In an embodiment the points of connection of the forward connecting line to the first and second forward catenary mooring line are at a distance which is at least 1 ,5 times smaller than the length of the forward connecting line; and/or the points of connection of the aft connecting line to the first and second aft catenary mooring line are at a distance which is at least 1 ,5 times smaller than the length of the aft connecting line. The connecting lines thus are of sufficient length to maintain their sagging configuration as the vessel’s draft varies.

In an embodiment the forward and/or aft connecting line hangs from its associated first and second forward catenary line substantially within a vertical plane. The weight of the connecting line keeps it hanging vertically, even during excursion of the vessel in the horizontal plane. The restoring force provided by the connecting line, and any weights connected thereto, thus is easily predicable. It will be understood that due to the sagging configuration, a lowest point of the connecting line is spaced apart from its points of connection with its associated forward and/or aft catenary lines.

In an embodiment a lowest point of the connecting line, i.e. of the forward and/or aft connecting line, is arranged for moving within said vertical plane relative to the points of connection of the connecting line with its catenary lines when the vessel moves relative to the sea floor. The position of the lowest point of the connecting line can thus shift substantially within the vertical plane relative to both its points of connection by different amounts.

In an embodiment, when seen in said vertical plane, the forward and/or said aft sagging connecting line spans a height defined as the vertical distance between a highest point of said connecting line and a lowest point of said connecting line, and spans a width defined as the maximum horizontal distance between any two points of said connecting line in the vertical plane, wherein the height divided by the width is between 0,5 and 2,0. When both a forward and an aft connecting line are used, then preferably these are of a substantially same length, though this is not required. The ratio of length of the connecting line to the distance between the points at which the connecting line is attached to its associated catenary mooring lines, provides a measure of the degree of sag of to the connecting line .

In an embodiment each of said catenary mooring lines has a point of contact with the vessel, wherein for each of the catenary mooring lines a straight line from said point of contact with the vessel to the point of connection with its corresponding connecting line is at a sharper angle to said vertical plane than a straight line from the point of connection to the connecting line to the point where the catenary mooring line is connected to the sea floor. Thus, for each catenary mooring line a portion from its point of connection with the vessel to its point of connection with its connecting line will slope more sharply towards the sea floor, than the remaining underwater portion of the catenary line below its point of connection with the connecting line. It should be noted that the straight lines described here are virtual line segments, rather than actual chains or ropes.

In an embodiment one or more weights are attached to the forward catenary line and/or to the aft connecting line for urging the first and second catenary line towards each other under the influence of gravity. Thus, instead of providing each of the two forward catenary mooring lines separately with their own weights, and/or providing each of the two aft catenary mooring lines separately which their own weights, the one or more weights are shared between the mooring lines via the connecting line. In this manner the total amount of weight needed to provide the restoring force may be reduced. In an embodiment each of the catenary mooring lines has a submerged weight per unit length that is smaller than a submerged weight of its associated connecting line and any weights attached to said connecting line, divided by the length of the connecting line. The combined weight of the connecting line and weights connected thereto, if any, divided by the length of the connecting line is thus significantly greater than the weight per unit length of the catenary mooring lines, e.g. by a factor 2 or more. When no weights are attached to the connecting line, e.g. the submerged weight per unit length of the connecting line may simply be greater than the submerged weight per unit length of each of its associated catenary mooring lines, e.g. by at least a factor 2.

In an embodiment, the submerged weight of the connecting line and any weights attached thereto, on average is at least 3.500 kg per meter along the length of the connecting line.

In an embodiment the submerged weight of the forward connecting line and any weights attached thereto is greater than the combined submerged weight of the forward catenary mooring lines below their points of connecting with the forward connecting line, and/or the submerged weight of the aft connecting line and any weights attached thereto is greaterthan the combined submerged weight of the aft catenary mooring lines below their points of connection with the aft connecting line.

In an embodiment the forward and/or aft connecting line extends partially below a bottom side of the hull of the vessel. At least part of the connecting line will lie deep enough in the water not to interfere with other ships which approach the vessel. Preferably, at least half of the length of the connecting line extends below the bottom side of the hull, while the remaining portion extends above the bottom side of the hull.

In an embodiment the vessel has a vertical center plane having a first side and an opposite second side, wherein the first forward catenary mooring line and/or the first aft catenary mooring line is completely arranged on said first side, and wherein the second forward catenary mooring line and/or the second aft catenary mooring line is completely arranged on said second side. The horizontal component of a restoring force exerted by the connecting lines and any weights attached thereto will thus be towards the vessel’s vertical center plane. The mooring lines are preferably arranged symmetrically on either side of the vertical center plane.

In an embodiment, when seen in top view, the first and second forward catenary mooring line and/or the first and second aft catenary mooring line each extend from the vessel away from the longitudinal axis of the vessel to their points of connection with the sea floor, and away from the longitudinal axis of the vessel in a direction perpendicular to the longitudinal axis. The mass of the respective connecting line(s) will thus exert a restoring force on the vessel if the vessel has moved from its intended position, for restoring the vessel to the intended position.

In an embodiment, for each of said catenary mooring lines a distance along said line from its point of connection with the sea floor to its point of connection with the connecting line is at least 2,5 times greater than a distance along said catenary mooring line from its point of connection to with the connecting line to its point of connection with the vessel. The point of connection of each mooring line with the vessel will typically be at a corresponding winch of the vessel, though the lines could be connected to the vessel by other means instead. In an embodiment, each of the catenary mooring lines has an upper portion extending from the vessel to the point of connection with the connecting line, and a lower portion from the point of connection with the connecting line to the point of connection with the sea floor, wherein the upper portion hangs substantially more vertically from the ship than the lower portion. A horizontal distance spanned by upper portion of each mooring line will typically be at least a factor 3 smaller than a horizontal distance spanned by the lower portion thereof. A vertical distance spanned by the upper portion of each mooring line will typically be at least 0,8 times a vertical distance spanned by the lower portion thereof.

In an embodiment the forward catenary mooring lines, at portions between their points for connection to the vessel and their points of connection to the connecting line, are provided with buoyancy devices, for at least partially carrying the weight of the connecting line. Each buoyancy device is arranged for floating on the water surface. Preferably, the buoyancy devices for the first and second forward catenary lines are each adapted for providing an upward buoyancy force on said catenary line such that a tension force on the catenary line above the buoyancy device remains within 250.000 N.

In this embodiment, the vessel is preferably provided with winches for hauling and paying out the catenary mooring lines, and the buoyancy devices help to reduce the tension force that exerted by each catenary line on its respective winch. A separate winch may be provided for each of the catenary mooring lines, which, in combination with the sagging configuration of the connecting line suspended therefrom, allows for instance the first forward mooring line to be hauled in, using its corresponding winch, without a significant portion of the second forward mooring line being lifted by the same winch.

In an embodiment the forward and aft connecting lines are spaced above the sea floor by at least 2 m. Thus, the weight of the connecting lines pull on the catenary mooring lines, rather than the connecting lines resting on the sea floor. The connecting line and any weights connected thereto are preferably connected only to the vessel and the sea floor via the corresponding mooring lines.

According to a second aspect, the present invention provides a method of side-by-side mooring a cargo ship to a vessel, wherein the vessel has been spread moored using the mooring arrangement according to the first aspect of the invention, the method comprising the steps of: letting the cargo ship approach the vessel until the cargo ship is arranged along a longitudinal side of the vessel; and side-by-side mooring the cargo ship to the vessel. Preferably, during a last part of said approach immediately prior to said side-by-side mooring, the direction of approach of the cargo ship is substantially parallel to a longitudinal direction of the vessel. Once the cargo ship has been moored to the vessel, ship-to-ship transfer of material, e.g. LNG, from the vessel to the cargo ship may be effected.

In an embodiment the vessel is moored to the sea floor by means of the mooring arrangement in waters having a depth of 35 meters or less. The system is particularly useful in shallow waters. In an embodiment the vessel is moored to the sea floor by the means of the mooring arrangement in waters wherein a ratio of the depth of the vessel below the water to the water depth is less than 1 .

In an embodiment the cargo ship is moored to the vessel by means of a flexible mooring system which allows vertical movement between the vessel and the cargo ship. Thus it is substantially prevented that ship-to-ship transfer of material between the cargo ship and the vessel which causes a change in draft depth of either of both, will cause the vessel and/or the cargo ship to roll.

According to a third aspect, the present invention provides a system comprising a vessel that is moored to the sea floor by means of a mooring arrangement according to the first aspect of the invention.

According to a fourth aspect, the present invention provides a use of a mooring line arrangement according the first aspect of the invention for mooring a vessel to the sea floor, wherein the first and second forward catenary mooring line are each connected at one end to the sea floor and at another end to the bow portion of the vessel, and wherein the first and second aft catenary mooring line are each connected at one end to the sea floor and at another end to the stern portion of the vessel; wherein:

- the first and second forward catenary mooring lines are connected to each other below the water surface by the forward connecting line that is suspended from the first and second forward catenary mooring lines and wherein the forward connecting line hangs in a sagging configuration; and/or

- the first and second aft catenary mooring lines are connected to each other below the water surface by the aft connecting line that is suspended from the first and second aft catenary mooring lines and wherein the aft connecting line hangs in a sagging configuration.

Brief description of the drawings

The present invention will be discussed in more detail below, with reference to the attached drawings, in which Fig. 1 shows a top view of the mooring arrangement according to the present invention;

Figs. 2A and 2B show a front view of the mooring arrangement of Fig. 1 , respectively with a vessel at two different draft depths;

Fig. 3A and 3B respectively show a side view of the mooring arrangement of Figs 2A and

2B; and Fig. 4A and 4B respectively show a top view and a side view of a mooring arrangement of the invention, wherein the catenary mooring lines are further provided with buoyancy devices for supporting part of the weight of the catenary lines and the connecting line.

Detailed description of the drawings In the figures, like reference numerals refer to like structures. Fig. 1 shows a top view of a mooring arrangement 1 according to the invention, for mooring a vessel 4 to the sea floor 3. In the present example, the vessel is floating production storage and offloading (FPSO) vessel, though in other examples the vessel could be any kind of vessel which collects materials, such as oil, gas, and even sand or rocks, from the sea floor. The mooring arrangement 1 comprises a forward part 10 with two forward catenary mooring lines 11 , 12, each connected at one end to the bow 5 of the vessel 4, and connected at another end to the sea floor 3. The forward catenary mooring lines 11 ,12 extend in a catenary manner respectively from point 15 from the bow to anchor point 31 , and from point 16 from the bow to anchor point 32 on the sea floor. The mooring arrangement 1 further comprises a rearward part 20 with two aft catenary mooring lines 21 , 22, each connected at one end to the stern 6 of the vessel 4, and connected at another end to the sea floor 3. The aft catenary mooring lines 21 ,22 extend in a catenary manner respectively from point 25 from the stern of the vessel to anchor point 33, and from point 26 from the stern to anchor point 34 on the sea floor 3. In the top view of Fig. 1 , each of the mooring lines extends from the vessel 4 to its anchor point, at an angle of less than 45 degrees to a vertical center plane P of the vessel 4.

A cargo ship 50 has been moored to the vessel 4 in a side-by-side manner using a standard berthing hawser arrangement 8a, 8b and flexible fluid transfer pipet 9 for Ship-to-Ship transfer, here shown schematically. T. In the example shown, the cargo ship is a liquid natural gas floating storage unit (LNG FSU), though in other examples it could be any kind of cargo ship, e.g. a floating storage regasification unit (FSRU), or a transport ship, e.g. for transport of sludge, sand or rocks.

A cryogenic fluid transfer hose 9 extends between the vessel 4 and the cargo ship 50, for transferring LNG from the vessel 4 to the cargo ship 50. Though not visible in the top view of in Fig. 1 , a flexible conduit connects to the sea floor 3 at one end and to the vessel 4 at another end, for transferring LNG from the sea floor 3 to the vessel 4. The flexible conduit, which is indicated in Figs. 3A and 3B by reference numeral 40, can accommodate limited movement of the vessel 4 with respect to the sea floor in the XY plane as well as along the Z-axis.

The forward catenary mooring lines 11 ,12 are arranged mirror symmetrically with respect to each other in the vertical center plane P of the vessel, as are the aft catenary mooring lines 21 ,22. The catenary lines 11 ,12,21 ,22 allow some excursion of the vessel 4 in the horizontal plane XY, and allow some variation in draft height of the vessel 4 along axis Z, for instance when part of the contents of the vessel 4 is transferred to the cargo ship 50.

Figs. 2A and 2B respectively show bow-side views of the vessel 4 of Fig. 1 at two different draft depths of the vessel, i.e. with two different distances dr1 > dr2 from the bottom 7 of the hull of the vessel to the water surface 2. The vessel 4 floats in shallow water which has a depth of less than 35m, i.e. a distance between the water surface 2 and the sea floor 3 is less than 35m. In Fig. 2A the vessel 4 is loaded with more weight than in Fig. 2B. For instance, when the vessel is an FPSO, in Fig. 2A its storage tanks may be filled with LNG, whereas in Fig. 2B the vessel is loaded with less weight due to some of the LNG having been transferred to the cargo ship 50.

The two forward catenary mooring lines 11 ,12 are connected to each other below the water surface 2 by a connecting line 13 to which a clump weight is 14 attached, in such a manner that the connecting line 13 is suspended from the two catenary mooring lines 11 ,12 and hangs in a sagging configuration therefrom, such that the clump weight is shared by the two mooring lines. Though not shown in Figs. 2A and 2B, the two aft catenary mooring lines are also connected to each other below the water surface by a connecting line 23 to which one or more clump weights 24 are attached, the connecting line being suspended from the aft catenary mooring lines 21 , 22and also hanging in a sagging configuration therebetween. It will be understood that instead of a single clump weight 14, 24 a number of clump weights could be attached to the connecting lines 13,23.

The connecting line 13 hangs in its sagging configuration from points A,B where it is connected to its catenary mooring lines 11 ,12, in such a manner that points A,B can move relative to each other along orthogonal axes X and Z, without the connecting line being in rigid tension. For instance, in Fig. 2A, in which the vessel’s draft depth is dr1 , the points A,B are at a horizontal distance dw1 from each other. In Fig. 2B, due to the draft depth having decreased to dr2 < dr1 , the points A,B have moved horizontally closer to each other such that the horizontal distance there between now is dw2, with dw2 < dw1. Likewise, in Fig. 2A, the vertical distance dh1 of the points A,B to the lowest point of the connection line 13 is smaller than the vertical distance dti2 of the points A,B, to the lowest point of connection line in Fig. 2B. In general, points A and B will be at a said vertical level regardless of the draft depth of the vessel, though this is not required. It is even possible to attach the points A and B at different distances from points 15,16 of contact with the vessel at which the mooring lines 11 ,12 exert a force on the vessel 4.

The weight of the connecting line 13 and the clump weights 14 attached thereto exerts a restoring force on the catenary mooring lines 11 ,12 which urges the catenary mooring lines toward each other, and in turn urges the vessel 4 to move to a location in the horizontal plane XY at which the connecting line 13 can reach a lowest point. By sharing the weight of the connecting line and the clump weights attached thereto, less weight is required to provide such a restoring force than if weights were attached separately to the lines 11 and 12. Moreover, the force exerted by the weight can shift between the two mooring lines until an equilibrium is reached. Though in the exemplary figures the connecting lines 13, 23 are shown provided with clump weights 14, 24 it will be understood that instead heavy connecting lines may be used without additional weights attached thereto. For instance as connecting lines, metal chains may be used having links which each link weighing 250 kg or more. Generally, the submerged weight per unit length of each connecting line will be significantly higher, e.g. by a factor 2 or more, than the submerged weight per unit length of the catenary mooring lines it is connected to. In the example shown, the submerged weight of connecting line 13 and the weight 14 attached thereto is about 60.000 kg, which is greater than the combined submerged weight of the catenary mooring lines it is connected to. In the present example, the mooring lines 11 ,12 each have a submerged weight of about 20.000 kg. In Figs. 2A and 2B, the length of the connecting line along its sagging shape remains constant, though a ratio of said length to a distance from point A to point B varies. For example, in Fig. 2A the shortest distance between points A and B is about 1 ,5 times smaller than the length of the connecting line 13 along its sagging shape, and in Fig. 2Bthe shortest distance between points A and B is about 1 ,6 times smaller than said length. The points 15,16 at which the mooring lines 11 ,12 exert a force on the vessel 4 are arranged symmetrically on either side of the vessel’s vertical center plane P, as are the corresponding anchor points 31 ,32, and the plane P intersects connecting lines 13 and 23.

Figs. 3A and 3B show a longitudinal side-view respectively of Fig. 2A and 2B. As can be seen, each of the connecting lines 13, 23 hangs in a sagging configuration vertically from its catenary lines, so that each connecting line is arranged substantially within a vertical plane. The connecting lines 13,23 and the weights 14, 24 attached thereto, remain spaced apart from the sea floor 3, both when the vessel 4 has draft depth dr1 , as shown in Fig. 3A, and when the vessel has draft depth dr2, as shown in Fig. 3B.

The portions of the catenary lines from the vessel to their points of connection with the connecting lines 13,23 hang more steeply downward than the portions of the catenary lines below said points of connection with the connecting lines. In the example shown, if a straight line were drawn from point 16 to point B, then this line would be at a sharper smaller angle to vertical plane V1 than a straight line from point B to anchor point 32.

Figs. 4A and 4B respectively show a top view and a side view of a further embodiment of a mooring arrangement according to the invention, wherein each of the catenary mooring lines 11 , 12, is provided with a buoyancy device 17, 18, at positions on the mooring line above the connecting line 13 and spaced apart from the points of connection A, B between the mooring lines 11 ,12 and the connecting line 13. The buoyancy devices that are attached to the first and second catenary mooring lines 11 ,12 together provide sufficient buoyance force to support more than half of the weight of the connecting line and the weight 14 attached thereto. As in the embodiment of Fig. 1 , instead of attaching weights to the connecting line, the connecting lines may be heavy themselves, e.g. such that a submerged weight per unit length of the connecting line is at least twice a submerged weight per unit length of the catenary mooring lines.

Thanks to the buoyancy devices, which during use float on the water surface, less tension force is exerted on winches 41 placed at the bow of the vessel than would be the case is no buoyancy devices were provided.

The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.