GB2319010A - Mooring device for use between two marine structures - Google Patents

Abstract

Mooring device for use between two structures 1, 2, at least one of which is floating, includes a) elastic connection means 13 1 , 13 2 ; 14 1 , 14 2 for urging the two structures towards each other and b) elastic buffer means 15 1 , 16 1 ; 15 2 , 16 2 placed between the two structures and activated by said structures coming into contact to attenuate forces transmitted between them. As shown the buffer means comprises two sets of horizontally separated members, each set comprising a respective bearing plate 15 1 , 15 2 articulated to structure 1 and bearing on a respective pneumatic fender 16 1 , 16 2 mounted on the side of structure 2. The plates 15 1 , 15 2 are mounted at the free end of a respective torsion rod 22 1 , 22 2 via arms 23 1 , 23 2 , 23 3 , 23 4 . The elastic connection means comprise two sets of lines 13 1 , 13 2 , 14 1 , 14 2 connected at one end to respective spring units 17 1 , 17 2 , 19 1 , 19 2 and at the other ends to repective points on the structures 1, 2. Applications include maintaining a tension leg platform 1 and a support barge 2 in close proximity.

Description

Mooring device for use between two marine structures.

The present invention concerns a mooring device for use between two marine structures, at least one of which is floating, and is more particularly concerned with a device of this kind for use between a compliant or moored structure and a moored or free floating structure.

A system comprising a support vessel moored to a drilling and/or production platform and accommodating services and/or equipment necessary for working on the platform (living quarters, tools and equipment, fluid and power supplies, etc) has recently been proposed for offshore oil exploration and production. Because of frequent transfers of equipment and personnel between the platform and the support vessel, they must remain in close proximity, possibly for long periods of time.

If one of the structures is fixed, for example a platform on a "jacket" type support on the seabed, a support barge or "tender" is conventionally moored to it and the position of the barge relative to the platform is controlled by means of its own mooring lines which hold the barge near the jacket. If environmental conditions (winds, currents and waves) become severe to the point that unacceptable shocks between the two structures are possible, work is stopped and the barge is moved away from the platform.

These provisions are not easily applied when the platform is compliant, because its excursions due to the action of the severe environment are then large and it becomes difficult to control the relative distance between it and the barge.

United States Patent n" 5,439,324 describes a device for mooring a support vessel to a compliant platform, this system including fixed fenders on the platform and on the support ship and a biasing system for pushing the support vessel and the platform one against the other, fender to fender. The system described assures effective decoupling of the two structures with regard to vertical forces. However, forces applied by one of these structures to the other in a horizontal plane are then transmitted through the fenders without any attenuation. These forces can be high given the very different hydrodynamic characteristics of a compliant platform and a surface vessel. The hull of the vessel or the floating part of the platform must then be strongly reinforced so that they are able to withstand the forces transmitted and the mooring and anchor lines of these structures must be strongly reinforced so that they can withstand the high forces to which they may be subjected. All these measures are costly and greatly increase the construction cost of the platform and of the support vessel.

An aim of the present invention is to provide a mooring device for use between such structures that does not have the disadvantages mentioned hereinabove and which therefore maintains the structures in close proximity through compliant control of their relative movements in a horizontal plane, thus attenuating the forces transmitted between them to avoid the need for the costly reinforcement of the structures referred to hereinabove.

Another aim of the present invention is to provide a device of the above kind allowing dynamic movements of the two structures, such as roll, pitch and heave which do not significantly affect the required proximity of the structures.

These aims of the invention and others that will emerge on reading the following description are achieved with a mooring device for use between two marine structures, at least one of which is floating, including a) elastic connection means for urging the two structures towards each other and b) elastic buffer means placed between the two structures and activated by said structures coming into contact to attenuate forces transmitted between them.

As will emerge later, the combination of the above means assure compliant mooring of the structures in a horizontal plane when the structures tend to move away from each other and when they move towards each other. In the latter case the forces transmitted between the structures are attenuated, avoiding the need for the reinforcement of the structures referred to above. The device of the invention maintains the two structures in close proximity at all times, in accordance with the essential aim of the invention.

In one preferred embodiment of the invention, the elastic buffer means include at least one set of two members, one of the members comprising a pneumatic fender mounted on one of the structures and the other member taking the form of a bearing plate articulated to the other structure and reacting elastically against the pneumatic fender. The buffer means preferably comprise two sets of two members, the two sets being horizontally separated and located between the two structures. Each bearing plate is mounted on a torsion rod defining a rotation axis for the plate and generating an elastic force on the plate when the latter is moved away from a neutral position. The elastic connection means for urging the two structures towards each other comprise at least first and second pairs of elastically tensioned lines, the lines of each pair being arranged between the two structures and at a distance from each other, the lines of the first and second pairs being oriented so as to resist primarily sway and surge, respectively, of a floating structure.

Other features and advantages of the present invention will become apparent from a reading of the following description and an examination of the accompanying drawings, in which: - figure 1 is a perspective view of a floating platform such as tension leg platform and a support barge connected together by the mooring device of the invention, - figure 2 is a perspective view of the upper part of the platform only, showing more clearly the various units constituting the device of the invention, - figure 3 is a schematic plan view of the device of the invention used to explain how the device works, and figure 4 is a schematic representation of the structure of one of a plurality of spring units forming part of the device of the invention.

Figure 1 of the accompanying drawings shows the mooring device of the invention applied to connecting a tension leg platform 1 and a floating support barge 2, this non-limiting application being described by way of illustrative example only.

The platform conventionally comprises a deck 3 mounted on a floating hull with columns 41 through 44, these columns being connected to the seabed (not shown) by tethers 51, 52, etc tensioned by the positive buoyancy of the platform. The platform carries on the deck typical equipment such as a flare stack 6, a crane 7 and a derrick 8 as used in exploration and/or production through oil wells drilled in the seabed and connected to the platform by conductors that are not shown.

A platform of this kind is "compliant" and can therefore be moved away from its vertical equilibrium position by forces applied to it by winds, currents and waves, for example. The installation of platforms of this type is currently envisaged for undersea oil wells at great water depths, for example 1000 metres. This solution can be rendered particularly economic by installing minimum equipment on the deck of the platform, such as wellheads for example. Other usual equipment such as living quarters, equipment storage areas, fluid storage tanks, helideck 9, crane 10, etc.. can be deployed on a support barge 2 which can be reused without modification at another site as the support barge for another platform. The barge is conventionally moored by lines lii, 112, etc forming part of a mooring system.

This solution implies the installation of reliable means of communication between the platform and the barge, for example a walkway 12 for personnel and equipment, as well as cables and pipes for power and fluid supply (not shown). The presence of such means between the platform and the barge requires them to remain in close proximity and the connection established between them for this purpose must be able to withstand the most severe local environment without risk of damage to these means of communication or to the structures that they connect.

To this end, in accordance with the present invention, a mooring device is installed to interface between the barge 2 and the platform 1, this device including elastic connection means 131, 132; 141, 142 adapted to urge the two structures towards each other and elastic buffer means 151, 161; 152, 162 placed between the platform and the barge and activated by the latter coming into contact to attenuate forces transmitted between them.

The arrangement of the above means can be seen more clearly in figure 2, which shows the face of the platform that is in contact with the barge 2, the barge being omitted from the figure for the sake of simplicity. Figure 2 shows that the elastic connection means comprise two breast lines 131, 132 and two surge lines 141, 142. The lines 131, 132 are each connected at one end to a respective spring unit 171, 17 fixed to the platform 1 and at the other end to a respective hard point 181, 182 on the barge (see figure 1). Similarly, the lines 141, 142 are each connected at one end to a respective spring unit 191, 192 fixed to the platform and at the other end to a respective hard point 201, 202 on the barge. Figures 1 and 2 show that these lines pass through fair-leaders 211, 212 before they are attached to the spring members 171, 172, 191, 192 mounted on the platform.

The elastic buffer means of the device in accordance with the invention comprise at least one set of horizontally separated members, and preferably two such sets, each set comprising a respective bearing plate 151, 152 articulated to the platform 1 and bearing on a respective pneumatic fender 161, 162 mounted on the side of the barge 2. The plates 151, 1 52 can be made of steel and they are mounted at the free end of a respective torsion rod 221, 222 the other end of which is fixed rigidly to the platform. The torsion rods 221, 222 thus each define a rotation axis for the plates 151, 152 that they support via respective arms 231, 232; 233, 234. Rotation of a torsion rod due to pressure applied to a bearing plate generates a return force tending to return that plate to its initial position, in the conventional manner of a torsion rod.

Respective tube arrangements 241, 242 operate on the two support arms of the plates 151, 152 to guide rotation of these plates.

The pneumatic fenders 161, 162 can take the form of cylindrical permanent fenders of an elastic material with the exterior surface covered with a net of automobile vehicle tyres, as shown in figure 2. Cylindrical permanent fenders of this kind are known in the art as ce Yokohama type fenders.

The units forming part of the device in accordance with the invention can been seen more clearly in figure 3, which is a schematic plan view. The operation of this device will now be described with reference to this figure, which shows that the beast lines 131, 132 are placed in a substantially horizontal plane and oriented in a direction close to the normal to the vertical plane of contact P of the pneumatic fenders 161, 162 of the barge 2 with the bearing plates 151, 152 fixed to the platform. Thus the set of lines 131, 132 operates principally by applying the resultant of the forces to the structures that they connect; which is substantially normal to the plane P.

These lines are tensioned by spring units 171, 172 when the platform and the barge move away from each other, for example due to the effect of winds, currents or waves (position 2' of the barge 2). When the platform and the barge move towards each other (position 2" of the barge), these lines slacken but the fenders 161, 162 are then crushed between the barge and the bearing plates 151, 152 which then pivot about the respective torsion rods 211, 222 constituting their articulation axes, generating an elastic reaction force at the fenders that opposes the forces applied to the plates by the crushing of the fenders. These arrangements attenuate forces between the platform and the barge substantially in the direction of sway movements of the barge, indicated by the direction of the doubleheaded arrow F1 in figure 3.

Figure 3 also shows that the surge lines 141, 142 cross and are tensioned by the respective spring units 191, 192. These lines operate on the platform and on the barge in directions steeply inclined to the plane P and therefore they act primarily through the components of the forces that they transmit to the structures that they correct, which are parallel to this plane.

Accordingly the surge lines 141, 142 elastically resist surge movements of the barge along its main axis, which is paraliel to the double-headed arrow F2 in figure 3.

Given the elastic connection forces established by the lines 131,132 and by the forces generated by the units 151, 161, 152, 162, on the one hand, in the direction of the double-headed arrow F1, and the forces generated by the lines 141, 142, on the other hand, in the direction of the double-headed arrow F2, it is clear that the device of the invention effectively controls any movement of the barge relative to the platform in a horizontal plane, including of course yaw movements about the axis of the arrow F3 in figure 3, perpendicular to the plane of the figure and to said horizontal plane.

A floating structure has six degrees of freedom. Thus in addition to sway, surge and yaw as referred to hereinabove, the barge, for example, can be subject to pitch about the axis of the doubleheaded arrow F1, to roll about the axis of the double-headed arrow F2 and to heave along the axis of the arrow F3.

These three movements, which do not degrade significantly the close proximity to be maintained between the platform and the barge, are advantageously not restricted by the mooring device of the invention. Thus the relaxation of these movements in this way, prevents unnecessary stresses that could lead to accidents or to breakages of liaison means between the structures.

Figure 4 shows by way of illustrative and non-limiting example only one of the spring units 171, 172, 191, 192 used to balance the tension in the respective lines 131, 132, 141, 142. The line 131, for example, is connected to a rod 28 that enters a tube 25 containing an axial stack of spring members 261, 262,..., etc etc (A ranging from 1 through 11, for example). The rod 28 passes through these members and is fixed to a thrust block 27 mobile in the tube.

Each member 26j can comprise a plurality of coil springs with different diameters fitted concentrically inside each other. With no tension in the line 131, the thrust block 27 occupies the position 27' at the end of the tube. When tension is applied to the line 131 the thrust block leaves the position 27' and moves towards the other end of the tube, compressing the members 26j so as to balance the force exerted on the line 131.

The stiffness of the device must prevent dynamic amplification of movement. In particular, the natural period associated with movement of the platform and the barge away from each other or towards each other must be kept outside the range of wave excitation periods, i.e. typically above 20 to 25 seconds.

The stiffness of the device of the invention in compression and in tension is determined by finding a compromise between, on the one hand, the necessity for the dynamic effects to result primarily in relative movement and in minimised dynamic loads generated in the mooring device and, on the other hand, the need to minimise relative displacement under a static load.

The stiffness of each of the components of the mooring device, and consequently the nature of these components, depends on the two structures concerned and on the relevant environmental conditions. The dimensions of these components must therefore be determined for each individual application.

In the application described by way of example in which a barge with a catenary mooring in connected to a drilling and production tension leg platform, for example, the total stiffness of the mooring connection can be about 400 kN/m in the direction perpendicular to the plane P in which the two structures bear on each other.

It is clear that the device of the invention has many advantages over and above its essential advantage of maintaining compliant close contact between two marine structures whilst enabling movements of these structures that do not significantly affect this contact. It is simple, robust and entirely passive. It can be used to moor the support barge to the platform for long periods or occasionally, depending on the work in hand. In a moderately severe environment, the mooring connection can withstand hundred-year storms. In a more severe environment, in the event of a hurricane, for example, the structures can be moved further apart.

Of course, the invention is not limited to the embodiment described and shown, which is for example only. Thus the spring units described hereinabove could be replaced with fluid springs or with elastic nylon, polyester or rubber lines, for example. Likewise, the torsion rods that support the bearing plates could be replaced with visco-elastic springs or coil springs operative between the hard points of the columns 42 and 44 supporting the deck 3 of the platform and the bearing plates, or by leaf springs arranged vertically along these columns to absorb impact between the plates and the columns. Also, the respective positions of the bearing plates and the pneumatic fenders on the two connected structures could be interchanged.

Nor is the invention limited to mooring a support barge to a compliant tension leg platform. It can be used between any two marine structures provided that at least one of them is floating. Thus one of the structures can be a semi-submersible platform, a compliant tower, a moored barge or ship, a fixed platform with a jacket support or gravity base, or even a quay. The other structure can be moored or free. Apart from a barge, it can be a ship such as a tanker or a bulk carrier, a semi-submersible structure or another tension leg platform.

Claims (13)

1. Mooring device for use between two marine structures, at least one of which is floating, including a) elastic connection means for urging the two structures towards each other and b) elastic buffer means placed between the two structures and activated by said structures coming into contact to attenuate forces transmitted between them.

2. Device according to claim 1 wherein said elastic buffer means include at least one set of two members, one of the members comprising a pneumatic fender mounted on one of the structures and the other member taking the form of a bearing plate articulated to the other structure and reacting elastically against the pneumatic fender.

3. Device according to claim 2 wherein said buffer means comprise two sets of two members1 the two sets being horizontally separated from each other and located between the two structures.

4. Device according to claim 2 or claim 3 wherein each bearing plate is mounted on a torsion rod defining a rotation axis for the plate and generating an elastic return force on the plate when the latter is moved away from a neutral position.

5. Device according to any one of claims 1 to 4 wherein the elastic connection means for urging the two structures towards each other comprise at least first and second pairs of elastically tensioned lines, the lines of each pair being mounted between the two structures at a distance from each other and the lines of the first and second pairs being oriented so as primarily to resist sway and surge, respectively, of a floating structure.

6. Device according to claim 5 wherein the lines of the second pair are crossed.

7. Device according to claim 5 or claim 6 wherein the tension in each of the lines forming part of the elastic connection means is balanced by a compression spring unit acting on a thrust block fixed to one end of the line and mobile axially within a tube attached to one of the structures.

8. Device according to any one of claims 1 to 7 wherein said elastic connection means are adapted to allow heave, roll and pitch of a floating structure substantially without reaction forces.

9. Device according to any one of claims 1 to 8 wherein one of the structcrres is selected from the group comprising a compliant or moored platform, a barge, a moored ship, a fixed platform and a quay and the other structure is moored or free and is selected from the group comprising a barge, a ship, a semi-submersible structure and another moored platform.

10. Device according to claim 3 wherein one of the structures is a tension leg platform and the other is a support barge, the platform carrying the bearing plates forming part of the elastic buffer means.

11. Set of two docked marine structures connected together by a mooring device according to any one of claims 1 to 10.

12. A device for mooring two marine structures substantially as hereinbefore described with reference to the accompanying drawings.

13. A set of two marine structures connected together substantially as hereinabove described with reference to the accompanying drawings.