Disconnectable mooring assembly

The invention relates to a disconnectable mooring assembly for a vessel, comprising a mooring buoy and a swivel positioned above the mooring buoy, wherein the mooring buoy is provided with a central member for being anchored to the seabed and comprising a number of passages each adapted for receiving a riser, the mooring buoy further comprising an outer member surrounding the central member and capable of a rotation relative thereto, which outer member is adapted to be housed in and locked to a corresponding receiving opening of the vessel.

Such a disconnectable mooring assembly allows a vessel to weathervane around the mooring buoy for minimising loads resulting from external factors, such as wind, current and waves. Under severe conditions (for example at the arri- val of a hurricane) the mooring buoy can be disconnected from the vessel and the vessel can navigate to a safe location. The swivel of such a mooring assembly provides a rotating connection between the risers of the mooring buoy and corresponding lines on board of the vessel, which have a variable position relative to each other as a result of the vessel weathervaning around the mooring buoy {more specifically the central member of the mooring buoy) which basically is kept geostatic by anchor lines connecting the mooring buoy to the seabed. In a known disconnectable mooring assembly of the above type the swivel is connected directly on top of the mooring buoy (more specifically the central member thereof) and supported thereby in the operational position of the mooring assembly. Such a state of the art configuration, however, has a number of drawbacks. For connecting and disconnecting (e.g. a controlled lowering through wire/hoisting means} the outer member of the mooring buoy to and from, respectively, the vessel the respective region of the vessel (which generally

is located near the keel of the vessel) has to be accessible, such as for (de) bolting, cleaning and inspection. This means that the swivel has to be moved to a position away from the mooring buoy. This calls for complicated moving mechanisms and when the swivel is moved upward or sideward such a movement requires a respective receiving space to be defined in the vessel as well as flexible lines that need to be disconnected or that need to be able to cope with an upward and/or sideward moving swivel (typically with less reliable jumper hoses); in addition the moving mechanisms require a high position flexibility and position accuracy for re-connection of the (typically heavy) swivel on top of the mooring buoy.

Similar disadvantages are valid for the connections between the mooring member and the swivel. In the operational position of the mooring assembly (swivel connected to and supported by the mooring buoy) the presence of the swivel makes the establishment or disconnection of such connections difficult, whereas an inspection and maintenance of such connections is very troublesome. In addition, space between the mooring buoy and the swivel is known to be small and is troublesome for inspection, maintenance and manifold features, such as control racks and pig handling (the need for pig launchers and/or receivers between mooring buoy and swivel) . A further disadvantage is, that during operation the weight of the swivel is supported by the mooring buoy, which adds to the loads on the bearing between the central member and the outer member of the mooring buoy, and indirectly to the loads acting on the vessel at said region.

With respect to the above it is an object of the present invention to provide an improved disconnectable mooring assembly of the type referred to above.

Thus, in accordance with the present invention a disconnectable mooring assembly is characterised in that the swivel is located above the mooring buoy in such a manner that an interspace is defined between the mooring buoy and the swivel.

Depending on the specific embodiments of the inventive disconnectable mooring assembly according to the present invention, among others the following advantages are offered, alone or in combination: - connecting the mooring buoy to a vessel or disconnecting it there from can be carried out easier and with less complicated means, such that a shorter response time is achieved on altering conditions (e.g. an arriving hurricane) as well as a reduced production downtime; - there is no need for complicated means and/or structures for moving the swivel to a position away (e.g. sideward) from the mooring buoy; the swivel may remain in its original position during connect and disconnect procedures and relative flexible connections may take care of the align- ment between the swivel and mooring buoy;

- the connections (e.g. large and small bore piping, optical, control and power cables, bolting and clamping means) between the mooring buoy and swivel are readily accessible, making inspection, maintenance, and the establishment and disconnection thereof easier and thus more reliable and, thus, safer;

- the loads on the mooring buoy and on the respective region of the vessel are minimised due to the removal of swivel loads, allowing a larger swivel (independent from the mooring buoy) and larger mooring loads within an existing mooring assembly;

- the interspace between the mooring buoy and the swivel allows the introduction of hoisting means required for lifting or lowering the mooring buoy during connect or dis- connect operations as well as the provision of (advanced) manifolding between the swivel and the mooring buoy;

- under circumstances the number of risers passing through the mooring buoy may be increased.

A number of illustrative embodiments of the discon- nectable mooring assembly in accordance with the present invention are shortly discussed below.

When the swivel has a stationary position relative to and is supported substantially entirely by the vessel, the overall constructive measures can be minimised.

When the swivel is supported in the vessel by means of a bearing assembly, a direct transmission of the weight of the swivel towards the vessel is obtained.

It is noted, that a swivel basically comprises components stationary with respect to the vessel and components rotating with respect thereto. The latter components are sup- ported by means of the said bearing assembly.

It is possible that the swivel at least partly extends below the level of the deck of the vessel. In such a case the swivel is received in a recess in the deck of the vessel. Said recess may or may not be enclosed and/or made inert.

Alternatively, the swivel extends above the level of the deck of the vessel, which may improve the access to components of the assembly.

To ensure that the components of the swivel rotat- ing relative to the vessel do maintain a rotational position in unison with the central member of the buoy without applying a torque on lines connecting the mooring buoy to the swivel, it is possible that the central member of the mooring buoy and the swivel are interconnected by a torque member, for example a centrally positioned, vertically extending torque rod.

Hereinafter the invention will be elucidated while referring to the drawing in which embodiments thereof are illustrated. Fig.l shows, in a schematical side elevational and sectional view a first embodiment of the disconnectable mooring assembly according the present invention, and

Fig.2 shows, in a schematical side elevational and sectional view a second embodiment of the disconnectable mooring assembly according the present invention.

Firstly referring to figure 1 a disconnectable mooring assembly for a vessel 1 is illustrated. It comprises a mooring buoy 11 which is provided with a central member 2 for being anchored to the seabed by anchor lines 3 and fur- ther comprises a number of passages each adapted for receiving a riser 4. Said risers may be connected, for example, to a sub sea oil or gas well and can be connected at the lower end of the mooring buoy or can be (partly) pulled through the mooring buoy. The mooring buoy 11 further comprises an outer member 5 surrounding the central member 2 and capable of a rotation relative thereto (see, for example, bearing assemblies 6) ₌ The outer member 5 is adapted to be housed in and locked (by appropriate releasable locking means not illustrated here, but known per se) to a corresponding receiving opening 7 of the vessel 1 (generally, but not exclusively, located in the region of the keel 8 of the vessel 1) .

The disconnectable mooring assembly further is provided with a swivel 9 positioned at such a level above the mooring buoy 11 that an interspace 10 is defined between the mooring buoy and the swivel.

Basically the swivel may be of a known type and therefore a detailed description thereof will not be needed within the context of the present invention. It only is noted in general, that such a swivel 9 provides a rotating connection between the risers 4 of the mooring buoy 11 and corresponding lines 12 on board of the vessel 1, which have a variable position relative to each other as a result of the vessel weathervaning around the mooring buoy (more specifi- cally the central member 2 of the mooring buoy) which basically is kept geostatic by the anchor lines 3 connecting the mooring buoy 11 to the seabed.

The expression that the swivel 9 has a stationary position relative to the vessel 1 means that its general po- sition will remain the same, also during connecting and/or disconnecting the mooring buoy 11. Of course the swivel 9

will comprise components which will rotate relative to the vessel, but still in total the swivel is considered to be s- tationary.

The swivel 9 is supported in the vessel by means of a bearing assembly 13 positioned between the vessel 1 and a swivel support frame 14.

In the embodiment illustrated in figure 1 the swivel 9 at least partly extends below the level of the deck 15 of the vessel 1. Specifically, the swivel is received in a respective recess 16 in the deck 15 of the vessel.

Because the rotating components of the swivel 9 should rotate in unison with the central member 2 of the buoy member relative to the vessel 1 when latter weathervancs around the buoy member 11, the central member 2 of the moor- ing buoy and the swivel 9 are interconnected by a torque member 17 which, as illustrated here, is connected to the swivel support frame 14. As a result lines 18 connecting the risers 4 to corresponding lines (not shown in detail) within the swivel 9 will not be torqued. _. Preferably the torque member 17 is a centrally positioned, vertically extending torque rod, but also other embodiments are conceivable. Generally, the torque member will be designed such that swivel supporting loads on the mooring buoy are eliminated or at least minimised. Referring to figure 2 an alternative embodiment of the disconnectable mooring assembly is illustrated in a view similar to figure 1. Most of the components of this alternative embodiment are similar to components of the first embodiment, and thus are not explained again. The main differ- ence with the previous embodiment is the feature that the swivel 9 now extends above the level of the deck 15 of the vessel 1. The swivel support frame 14' now is prolonged in a vertical direction compared to swivel support frame 14 of the figure 1 embodiment. Whereas according to figure 1 the bear- ing assembly 13 is provided at the lower side of the swivel support frame 14 and cooperates with a bottom wall 19 of the

recess 16 in the deck 15 of the vessel, according to figure 2 the upper end of the swivel support frame 14' carries a bearing assembly 13' cooperating with the deck 15 of the vessel or mounts 15' attached thereto. The swivel support frame 14' again is housed partly in a recess 16' . The lower end of the swivel support frame 14' now does not contact the bottom of the recess 16' .

Although the above embodiments have been described with respect to a mooring assembly in which the mooring buoy is received in a corresponding receiving opening near to or at the keel of the vessel (that means within the confines of the hull of the vessel) , it is noted that the scope of the invention is not limited to such embodiments. It is conceivable too that the mooring buoy is received in a receiving opening defined elsewhere. Specifically it is possible to provide such a receiving opening in an outrigger connected to and projecting from the vessel, such that the mooring assembly then is positioned at least partially, and often fully outside the confines of the hull of the vessel. The invention is not limited to the embodiments described above which may be varied widely within the scope of the invention as defined by the appending claims.