GB2034265A

GB2034265A - Devices for Mooring a Floating Installation to an Offshore Installation

Info

Publication number: GB2034265A
Application number: GB7939426A
Authority: GB
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 1978-11-14
Filing date: 1979-11-14
Publication date: 1980-06-04
Also published as: MX6867E; FR2442759A1; BR7907384A; GB2034265B; NO149305B; IT7927215A0; JPS5572491A; NO793639L; FR2442759B1; NO149305C; ES485959A1; CA1129260A; NL7908267A; IT1125669B

Abstract

A device for mooring a ship (1) to a buoy or column (2) anchored by lines (3) comprises a connecting arm (6) provided with roller means including wheels (11, 11a, 11b, 11c) which bear on a cylindrical portion of the outer wall (16) of the buoy (2). Belts (such as 23) encircling the buoy (2) and the roller means are tensioned to press the wheels (11 to 11c) against the cylindrical surface (16) and permit rotation of the arm (6) relative to the buoy (2). <IMAGE>

Description

SPECIFICATION Devices for Mooring a Floating Installation to an Offshore Installation The present invention relates to devices for mooring a floating installation to an offshore installation connected to a stationary point on the water bottom.

The floating installation may be a ship and the offshore installation may be a mooring buoy at some distance from the shore.

The offshore installation may instead be a buoy for loading and/or unloading the cargo of a ship or installation from which are performed or controlled such operations as drilling the sea bottom, recovering oil products; or alternatively a buoy, a floating caisson or a tank.

Systems are already known for mooring a ship to a structure such as buoyant caisson, or to an oscillating riser pipe, by means of pivoting arms.

Such systems are, for example, described in French Patent Specification 1,403,493 and in US Patent Specifications 3,354,479 and 3,908,212.

In such prior systems, the mooring structure is surmounted by a rigid arm secured to the ship and extending from its stem, the arm being articulated in the ship so as to accommodate pitching movements. To permit rotation of the arm and of the ship about the vertical axis of the mooring structure, the arm is connected to this structure by means of a pivot provided with ball bearings or roller bearings, this pivot forming a rotary table or orientation rings.

Such prior devices suffer from many drawbacks with regard to resistance, reliability and constructional yield.

Firstly, such rotary tables, which are very sensitive to marine corrosion are, like all roller bearings, designed for fast rotation and for successive and similar operation of all their constituent parts. Moreover, they are poorly suited to solving the problem of slow and occasional rotation where alternating forces developed by swell are for most of the time exerted without substantial rotation.of the table.

Since the prevailing winds statistically maintain a preferential heading of the ship and consequently of the connecting arm, the same area of the rotary table and thus the same balls of the bearing are permanently under load.

This mode of operation requires such an oversizing of these parts as to cause problems at the present time in the construction of large installations.

Moreover, in such a device, loads of generally very high values are concentrated by the rotary table at the level of the central part of the upper platform of the mooring structure. This requires significant reinforcements so as to distribute all these loads over the whole structure.

According to the present invention there is provided a device for mooring a floating installation to an offshore installation connected to a stationary point on the water bottom and provided over at least a portion of its length with a substantially cylindrical outer surface having a vertical axis, the device comprising a rigid connecting arm provided with roller means and coupling means maintaining said roller means in contact with a rolling path provided on the offshore installation, so as to permit relative rotation of the connecting arm and the offshore installation about said vertical axis.

Embodiments of the present invention described hereinbelow permit permanent mooring of a ship to an installation connected to a stationary point on the water bottom, these embodiments being more reliable, more resistant and offering a higher constructional yield than devices used before, and being moreover utilizable irrespective of the size of the installation.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, wherein: Figure 1 is a front view of a device constituting a first embodiment of the invention; Figure 2 is a top view of the embodiment of Figure 1; Figure 3 is a cross-sectional view along line ill of Figure 2; Figure 4 shows a second embodiment of a rotary connecting arm of the device; Figure 4A is a cross-sectional view along line IV of Figure 4; Figure 5 illustrates a third embodiment of the rotary connecting arm; Figure 5A is a cross-sectional view along line V of Figure 5; Figures 6A and 6B illustrate another embodiment of the invention; and Figure 7 shows an alternative embodiment of the invention.

Figures 1, 2 and 3 show a first mooring device embodying the invention for mooring a floating installation 1, such as a ship, to an offshore installation 2, such as a floating buoy of cylindrical shape connected to a stationary point on the water bottom by means of wire line moorings 3, 3a, 3b, 3.... and mooring checks 4, 4a, 4b, 4c., whereby the buoy 2 has a substantially stationary position with respect to the water bottom.

The water level is diagrammaticaily indicated by a line 5.

The device comprises a rigid connecting arm 6 which is, for example, of cross-braced tubular structure. The arm 6 extends from the stem 7 of the ship 1 and is articulated by means of bearings 9, where it is connected to the stem 7, which is reinforced by iron fittings 8, about a horizontal axis 12 perpendicular to the longitudinal plane of symmetry of the ship.

At its end opposite to the bearings 9, the arm 6 carries at a first level two idle pulleys 10 and 1 Oa and four wheels 11, 1 1 a, 1 1 b, 1 1 c fitted with tyres.

Similarly, at a second level, the arm 6 carries two idle pulleys 1 Ob and 1 Oc and four wheels 1 1 d, lie, 1 1 f, 1 lgflttedwith tyres. The axes of rotation, such as 13 and 14 (Figure 3) of the pulleys and wheels are held parallel to the vertical axis 1 5 of the cylindrical buoy 2 when the treads of the tyres of the wheels contact the outer wall 16 of the buoy.

Each shaft (such as shafts 1 7 and 1 8) carrying roller bearings such as 19, 1 9a of the wheels and 20, 20a of the pulleys, connects two junctions, for example 21 and 22, of the tubes of the crossbraced structure forming the connecting arm 6.

The eight wheels 11 to 1 1g are held in contact with the wall 16 of a buoy 2 by means of two straps or flexible belts 23, 24 forming closed loops which encircle, each at its respective level, both the idle pulleys and the outer wall of the buoy. The upper belt 23 is stretched or tensioned by the pulleys 10 and 1 Oa and the lower belt 24 is stretched or tensioned by the pulleys 1 Ob and lOc.

The tubes which form the cross-braced structure of the arm 6, as well as the wheels and pulleys carried by the arm, are advantageously constituted as closed watertight chambers.

Preferably, the assembly formed by the arm 6, the wheels 11 to llgand the pulleys 10 to 1 Oc has a positive buoyancy so that it floats without pressing substantially on the contact area of the wheels with the outer wall 1 6 of the buoy 2.

Advantageously, the contact surface of the belt, of the buoy and of the pulleys on one side of the contact surfaces of the wheels and of the buoy on the other side, will have complementary shapes whereby an accurate relative positioning of these elements can be maintained. For example, as shown at 25 in Figure 3, these surfaces may form horizontal corrugations, but such corrugations may instead be vertical or inclined to the vertical.

The operation of the above-described device will now be explained.

The tyres of the wheels 11 to 1 g being deflated, the arm 6 is moved towards the buoy 2 until the wheels contact the outer surface of the wall 1 6 of the buoy. The belts 23 and 24 are then positioned. The tyres of the wheels 11 to 11 g are inflated, thereby stretching the belts 23 and 24 through the intermediary of the pulleys 10 to 1 Oc.

The connecting arm 6 will thus follow pitching and rolling movements of the buoy 2. On the other hand, the arm 6 can pivot about the vertical axis 1 5 of the buoy 2. This pivoting movement is permitted both by the wheels 11 to 1 Ig rolling along the lateral wall 1 6 of the buoy 2 and by the belts 23 and 24 which are moved apart as the arm 6 is rotated, thus permitting the Idle pulleys 10 and 1 Oe to roll along the internal surfaces of the belts during their rotation.

When, under the combined action of the wind, water current and waves, the ship 9 Is subjected to a horizontal drag, the drag produces a traction force which is transmitted, through the arm 6, to the bearings of the pulleys 10 to 1 Oc and thus stretches the two belts 23 and 24 held by the anchored buoy 2, on the lateral wall 16 of which they are pressed over a large area.

When under the orbital movements of the swell, the ship 1 is subjected to a pounding movement, any displacement of the ship toward the buoy 2 is stopped by the wheels 1 1 to 1 g bearing against the lateral wall 1 6 of the buoy, thus resulting in a compression of the tyres of the wheels.

When the direction of the wind, water current or waves varies, the ship 1 pivots like a weathercock about the axis 1 5 of the buoy 2, as described above, and takes up a new position.

When, under the action of a swell directed along the longitudinal axis of the ship 1, the ship and the buoy 2 are subjected to pounding movements and pitch which create large differences of level between them, the assembly of the arm 6 and of the buoy 2 oscillates about the axis 12 of the bearings 9.Depending on its orientation, the torque developed by the arm 6 connected to the buoy 2 puts the upper belt 23 under tension, producing a compression of the tyres of the lower wheels 1 1 d, lie, 1 If and lig against the lower part of the wall 1 6 of the buoy, or stretches the lower belt 24 producing a simultaneous compression of the tyres of the upper wheels 11, 1 la, lib, 1 1 c against the upper part of the wall 16 of the buoy.In this case the shear force at the level of the connection between the arm 6 and the buoy 2 is transmitted by adherence of the tyres of the wheels or through mutuai shearing of the side walls of the corrugations 25 of the outer surface of the tyres.

When the ship 1 moves to put its head into the wind, the transverse swell produces rolling of the ship. A torque is induced in the arm 6 through the bearings 9 and is transmitted to the buoy 2 by adherence of the tyres of the wheels 11 to 1 g or through mutual shearing of the side walls of the colrugations 25 of the outer surfaces of the tyres.

The torque and the shear stress resulting from pitching movements can be more efficiently transmitted by employing one of the embodiments illustrated in Figures 4 and 4A and Figures 5 and SA.

In the embodiment illustrated in Figures 4 and 4A use is made of profiled wheels 26a, 2gob, 26c and 26d having the shape of tip truck wheels which eo-operate with rails 27a and 27b fast with the buoy 2.

The pulleys 10 and iCe, around which the belt 23 passes, are journalled on shafts 28 of the wheels 26a, 2gob, 26c and 26d. Auxiliary pulleys 29 and 30 carried by the arm 6 provide for a sufficient contact length between the belt 23 and the pulleys 10 and 1 Ca. A hydraulic or mechanical jack 31 secured to the arm 6 permits adjustment of the tension of the belt 23 by displacing a pulley 32 secured to the end of a movable stem 33 of the jack.

Figures 5 and Sa show another embodiment of the connecting arm 6 whereby the ship 1 can be moored to a floating column or riser pipe 2 connected to the water bottom by means of anchoring lines 36, 36a, etc. and through the intermediary of mooring chocks 37 and anchoring points 38 secured to the water bottom 39.

In this embodiment the arm 6 is formed from two separate elements 6a and 6b hingedly connected to each other on a shaft 40 parallel to the articulation axis 12. The first element 6a has the shape of a frame supporting the pulleys 10, 1 Oa for tensioning the belts 23 and 24, and supporting the wheels 11,11....

The second element 6b is a beam hinged on a shaft of the ship 1 arranged along the axis 12. The wheels 10, 10a, 106 ... and the belts 23 and 24 bear on the lateral wall of cylindrical flanges 34a and 34b provided on the column 2 so as to provide ring-shaped flat rolling paths. Wheels 41, 41s, 41b, whose axes are radial with respect to the column 2, bear on the ring-shaped flat side walls of the cylindrical flanges 34a and 34b provided on the periphery of the column 2.

Such an arrangement provides a high constructional yield. The forces generated by the connecting arm 6 are applied to the buoy or column 2 and distributed over a large contact area, so that low bearing pressures are exerted on a structure of cylindrical shape which is specially designed to withstand such compression loads without excessive reinforcements. These loads, which are transmitted through the belts and the wheels at two levels substantially distant from each other, enable the transmission of the embedding moment of the connecting arm 6 to the column or buoy 2. These reaction forces at the location of the bearings are reduced as the length of the lever arm 6 increases.Moreover, elastic and plastic characteristics of the belts and of the wheels equipped with tyres make it possible to dampen the loads developed by certain sudden impacts produced by dynamic alternating actions of swell.

Moreover since the cylindrical columns 2 do not include any moving parts, they can be economically constructed such as by welding, without requiring precise tolerances, or machining, and the connections by belts and tyres are completely unaffected by marine corrosion, in contrast to mechanisms using ball bearings.

Whenever required, should extreme meteorological-oceanographic conditions occur of such an amplitude that the Installation, as a whole, cannot withstand them without damage, it is possible to rapidly disconnect the ship 1, so as to preserve the integrity of the buoy 2 and thus avoid deterioration of the installations and underwater connections. To achieve such a fast separation between the buoy 2 and the ship, it suffices to cut off the retaining belts 23, 24, for example, at the location where the band forming each of these belts, as illustrated in Figure 2, is folded on itself, by means of metal fittings 51 and 51 a, bolted to each other and which pinch the ends of this band, this connection being destroyed by remotely actuating exploding bolts 52.

Figures 6A and 6B illustrate an alternative embodiment of the invention. In this embodiment relative sliding of the belts 23, 24 and the buoy 2 in a direction parallel to the axis of the buoy is prevented by positioning the belts in grooves provided on the buoy 2. These grooves are, for example, delimited by the vertical wall of the buoy and by flanges 61 fast therewith. Moreover, shear stresses at the level of the connection between the arm 6 and the buoy 2 are transmitted through the belts.

In fact, as is apparent from Figure 6B, the outer surface 62 of each belt (e.g. 23) has a concave section opposite to its contact area with the cylindrical surface of the buoy 2. This outer surface 62 constitutes a rolling path for the roller means whose wheels are equipped with tyres having a convex outer surface substantially complementary to the outer surface 62.

To ensure appropriate rolling of the roller means on the belts, each wheel assembly is carried by a support member articulated about a shaft 63 which extends radially with respect to the buoy 2 in a plane parallel to the axis 12.

Changes to the above-described embodiments may be brought about without departing from the scope of the present invention. For example, although the drawings illustrate the anchoring of a ship to a surface buoy, it is possible to use other items of offshore equipment, such as a tower, provided such items of equipment have an outer cylindrical wall over at least a portion of their length.

The number of wheels and pulleys may be different from that illustrated. For example, as diagrammatically shown in Figure 7, for each belt 60 a single pulley 10 may be associated with two wheels 11, these three rotary elements being placed at the apices of a triangle.

Moreover, the number of belts can vary and will be determined by those skilled in the art in dependence on the loads to be supported by the device, provided the belts properly maintain the wheels and the rolling path thereof on the marine installation 2 in contact with each other.

It is also possible to replace the floating buoy 2 by a buoy submerged at a greater or lesser depth, the arm 6 being of corresponding shape and suitably ballasted.

The belts may be constructed in a known manner. They may be made, for example, of a plastics or resilient material reinforced with wire layers, strips, or strands or with layers or braided wires.

Such reinforcements for withstanding the mechanical loads applied to the belts may be made of metal, glass fibres, or plastics fibres, such as those sold under the registered trade mark Kevlar, etc.

The belts may also be formed of cables or chains.

All the above embodiments of the invention are only indicated by way of example.

The various above-mentioned technological solutions may be combined with each other in different manners so as to solvent the overall problem arising from a particular application to a given case.

Rotary devices embodying the invention for mooring a ship to a cylindrical buoy are particularly useful in the construction of installations for temporary production of oil from an underwater oil well where the anchored buoy supports flexible pipes 63 connecting underwater well-heads to the water surface, and wherein the ship is a tanker equipped with production processing means.

For this purpose, as illustrated in Figures 1 and 2, the tubular structure of the arm 6 supports rigid pipes 53, 54... for conveying the fluid, these pipes being connected to flexible pipes 55, 56, 57... connected in turn to piping fitted to tlle ship 1 and buoy 2.

The tubular structure of the arm 6 may also be fitted with gangways 58 permitting passage of maintenance personnel between the ship 1 and buoy 2, and with a crane 59 for unloading equipment onto the buoy.

Claims

1. A device for mooring a floating installation to an offshore installation connected to a stationary point on the water bottom and provided over at least a portion of its length with a substantially cylindrical outer surface having a vertical axis, the device comprising a rigid connecting arm provided with roller means and coupling means maintaining said roller means in contact with a rolling path provided on the offshore installation, so as to permit relative rotation of the connecting arm and the offshore installation about said vertical axis.

2. A device according to claim 1 , wherein said connecting arm is articulated to the floating installation about a substantially horizontal articulation axis, and wherein said roller means comprises at least one assembly which includes at least two substantially coplanar wheels whose axes of rotation are substantially orthogonal to the articulation axis of the connection arm, said two wheels being located on opposite sides of the longitudinal axis of the connecting arm.

3. A device according to claim 2, comprising at least two assemblies of at least two coplanar wheels, said two assemblies being located at separate levels of the offshore installation and at a distance from each other.

4. A device according to claim 3, wherein the coupling means comprises at least one pulley carried by the connecting arm and having an axis of rotation which is substantially parallel to the axes of the wheels, at least one belt forming a closed loop passing around the pulley and the offshore installation, and means for stretching the belt.

5. A device according to claim 4, wherein the wheels are fitted with tyres which, upon inflation, stretch said belt.

6. A device according to claim 4 or claim 5, wherein the rolling path is formed by an outer cylindrical wall of the offshore installation, wherein the outer surfaces of the wheels and the cylindrical surface of the offshore installation have complementary shapes preventing sliding of said surfaces relative to each other in a direction parallel to the vertical axis of the offshore installation, and wherein the outer cylindrical wall of the offshore installation, the inner surface of the belt and the surface of the pulley have complementary shapes.

7. A device according to claim 4, wherein the rolling path is formed by a rail, secured to the offshore installation, the wheels and the rail have co-operating complementary shapes to permit relative rotation of the connecting arm and offshore installation, and the means for stretching the belt comprises a movable pulley carried by the connecting arm and in contact with the belt.

8. A device according to claim 4, wherein the connecting arm carries at least two guide wheels co-operating with flanges of the offshore installation to prevent relative displacement of the arm and the offshore installation in a direction parallel to the vertical axis of the offshore installation.

9. A device according to claim 4, comprising the same number of pulleys and belts and of assemblies of coplanar wheels, the external wall of each belt constituting a rolling path for an assembly of coplanar wheels, each belt being maintained in a groove provided in the cylindrical portion of the offshore installation, and the wheels being carried by a support member articulated about a horizontal axis extending radially of the offshore installation.

10. A device according to claim 9, wherein the external surface of each belt, which is opposite to its surface in contact with the cylindrical wall of the offshore installation, is concave and cooperates with the surfaces of the wheels to prevent relative sliding of the arm and of the offshore installation in a direction parallel to the vertical axis of the offshore installation.

11. A device for mooring a floating installation to an offshore installation, the device being substantially as herein described with reference to Figures 1 to 3, Figures 4 and 4A, Figures 5 and 5A, Figures 6A and 6B or Figure 7 of the accompanying drawings.