Field of the invention

The invention relates to an offshore system which includes a vessel hull provided with a moonpool in the form of vertical cylinder through the hull which is open at deck and keel level, a cylindrical shaped turret placed within the moonpool opening and rotatable hold within the moonpool via a bearing system consisting of at least one submerged bearing that is placed in the submerged part of the annulus formed between the moonpool and the turret, and mooring lines that are connected to the lower part of the turret and to the seabed, so that the vessel can weathervane around the turret when the vessel is moored offshore.

The invention also relates to a method to create a pressurized cavity within the space between the fixed and the rotating part of a turret mooring of an offshore system.

Background of the invention US4606727 discloses a columnar super structure tanker having a stern portion, which is anchored using a buoy which has an attached hollow columnar superstructure; the structure passes through a shaft which has an extension by way of an integrated expansion along the vertical axis. In this application, the anchoring system includes a pressure lock region disposed within the hollow columnar superstructure of the buoy disposed within said hollow columnar superstructure, the system including access from the tanker to an integrated space adjacent to said hollow shaft. According to this prior art, the columnar structure which is generally- flooded is blown out by means of compressed air and hence accessible to maintenance personnel via an air lock by means of gratings and ladders for inspection and repair work on the bearings or on the transfer lines and their connections. In this prior art the lower bearing is in the dry but only for the changing, maintenance of this bearing. The structure according to US4606727 is specially adapted for a small diameter turret provided with a segmented friction bearing, which are used because they act well in a permanent seawater environment. It is an object of the present invention to provide a turret moored structure provided with bearings and having the lower bearing structure in the dry even during operation. This solution enables to have a radial wheel design as a lower bearing and to keep the distance between the lower bearing and the chain table mooring connecting point as short as possible.

Summary of the Invention

According to a first aspect of the present invention, the invention relates to an offshore system which includes a vessel hull (2) provided with a moonpool (6) in the form of vertical cylinder through the hull (2) which is open at least at keel level, a cylindrical shaped turret (4) placed within the moonpool opening and rotatable hold within the moonpool (6) via a bearing system consisting of at least one submerged bearing that is placed in the submerged part of the annulus formed between the moonpool (6) and the turret (4), and mooring lines that are connected to the lower part of the turret (4) and to the seabed, so that the vessel (1) can weathervane around the turret (4) when the vessel (1) is moored offshore, characterized in that, within the annulus in the submerged moonpool (6) part both the turret (4) and moonpool (6) are provided with extensions that imbricate each other in which fluid or gas can be trapped so to create a dry area for the submerged bearing.

According to a preferred embodiment of the invention, a fluid flow path is created within the annulus. The fluid flow path is created within the annulus, such that when gas or fluid is injected via the gas or fluid injection means, which are located in the fixed part or in the rotating part of the turret structure within the annulus and create a pressurized chamber within the annulus the lower bearing is in the dry. The gas or fluid injected may be of any type such as nitrogen, air, oil, or other.

According to a preferred embodiment of the present invention, gas or fluid injection means are located in the turret and/or in the moonpool to inject fluid or gas within the submerged annulus and create a pressurized chamber within the annulus such that the submerged bearing is in a dry environment.

According to a preferred embodiment, the bearing system comprises radial wheels.

According to a preferred embodiment of at least one water barrage is located between the turret and the moonpool to dampen the effect of the vessel's movements on the water that is trapped into the annulus. According to a preferred embodiment a water barrage is located close to the keel of the vessels hull.

According to a preferred embodiment of the present invention, a water barrage is located close to the water level inside the annulus when the pressure chamber is created.

According to a preferred embodiment of the present invention, at least one inflatable seal is provided at one extremity of the annulus. According to a preferred embodiment of the present invention, bilge pumps are provided at the lower most location of the annulus within the turret or within the moonpool.

According to a preferred embodiment of the present invention, the sealing in between the upper part of the turret cylinder and the moonpool is frictionless and wear proof. According to a second aspect of the present invention, the invention relates to a turret for an offshore system placed within the moonpool opening and rotatable within the moonpool via a bearing system consisting of at least one submerged bearing that is placed in the submerged part of the annulus formed between the moonpool and the turret, and mooring lines that are connected to the lower part of the turret and to the seabed, so that the vessel can weathervane around the turret when the vessel is moored offshore, wherein the turret is provided with extensions that imbricates each other with extensions provided to the moonpool in which gas or fluid can be trapped so to create a dry area for the submerged bearing.

According to a third aspect of the present invention, the space between the turret and the moonpool of a weathervaning offshore system that comprises a submerged bearing, the method comprising the following steps: - apply at one gas or fluid injection location a pressure PI to start flushing the water out of the annulus by the exit close to the vessel keel;

increase PI up to the value of P2 such that part of the water keeps being flushed out by the exit close to the vessels keel whereas the water trapped inside the siphon tends to move up inside the annulus to the direction of the exit close to the sea line;

maintain the pressure P2 such that water levels remain static inside the annulus creating a pressurized cavity where the lower bearing can be located.

Brief description of the drawings

The invention will be further described below in connection with exemplary embodiments with reference to the accompanying drawings, wherein

FIG. 1 shows a cross section of the turret assembly of a vessel according to the prior art; FIG.2A and FIG.2B show schematics illustrating the principle applied to a turret where by means of a hydrostatic pressurized compartment, the radial wheels are maintained in a dry compartment;

FIG.3A shows a cross section of the turret assembly of a vessel according to the present invention;

FIG.3B shows a detailed part of FIG.3A with the pressurized cavity in which the lower bearing is maintained the dry.

FIG. 1 shows a vessel-turret assembly with an opening 3 in the hull 2 of the vessel 1 wherein a turret 4 is rotatably received. This turret 4 is rotatably arranged in the shaft through bearings 8 and 10. The turret is connected through anchor lines 5 with the sea bottom and line 21 extends to the sea bottom to an underwater oil well or the like. The upper bearing part 8 is consisting of axial bearings, between the hull structure 2 and the turret cylinder, or moonpool 6, that have a number of circumferentially spaced bogies running on raceways.

In FIG.2A this turret 4 has been illustrated schematically as a tube. It should be understood that such a turret is known per se and therefore needs no further explanation, but in reality it will have a far more complicated structure with a number of components which are not shown here.

Usually, upper bearing 8 is in relatively dry condition, lower bearing 10 is subjected to the influence of sea water because as to make journaling of the turret 4 as rigid as possible bearings 8 and 10 are spaced apart as much as possible which implies that frequently the lower bearing 10 is located below water level. In the present invention the aim is to minimize the distance between the lower bearing 10 and the chain table. Further, consequently to diameter limitation of single piece roller bearing, large internal turret are now designed using weathervaning system made of axial bogie bearing and radial wheel, thus, allowing to virtually remove the diameter limitation of the bearing system. Actually, the radial wheel design does not allow being under water for a long period of time. Additionally, for maintenance reasons, but also for safety reason during inspection, it is preferable to have the radial wheel located in dry as shown in the embodiment of FIG.l. This has as direct consequence to have the distance between the lower and upper bearings 8 limited to the vessel depth combined with the maximum draft experienced on site.

This distance between the lower and upper bearings 8 is mainly linked to the horizontal mooring force applied at the chain table elevation and the maximum quantity of radial wheel that can be physically fitted around the turret diameter, where the wheel race, but also to the actual working load limits of the radial wheel.

To overcome this distance limitation DL, a solution is proposed to place the radial wheel in a dry compartment but at an optimum elevation located below the sea water level. This allowing removing the distance limitation of the previous system in the maximum extent of the vessel depth as shown in FIG.3A. The final aim being to keep the distance between the lower bearing 10 and the chain table mooring connecting point as short as possible. FIG. 2A and FIG. 2B show schematics illustrating the principle applied to a turret 4 where by means of an hydrostatic pressurized compartment, the radial wheel are maintained in a dry compartment or chamber 46 (see FIG.3B). According to the invention, the need of a dynamic seal between the turret fixed and rotating part is hence not essential.

A possible advantage according to the present invention is that it is possible, when pressurized, to flush the radial wheel with treated water to avoid corrosion.

In FIG. 2A and 2B the fixed part has an extension 30 and the rotating part has an extension 31 such that the fixed part is imbricated within the rotational part hence, the space between the fixed and the rotating part forming an annulus having an S configuration (as clearly shown in FIG.2B). Even if it is not shown, it should be noted that the inverse configuration is also possible where the rotational part is imbricated within the fixed part at the annulus location to form an S configuration.

FIG.2B shows the water level within the annulus along the step of the method according to the present invention to create a pressurized cavity 46 (see FIG.3B) within the space between the fixed and the rotating part of a turret 4.

At start, the annulus between the fixed and the rotating part is full of water up to the sea water level.

A pressure PI equal to Ro.g.Hl (with Ro being the water density and g the earth gravity) is applied in the upper cavity of the S configuration to start flushing the water out of the annulus by the exit close to the vessel keel. Then, PI is increased up to the value of:

P2 with P2 = Ro.g.H4.

The shape of the S configuration is such that part of the water keeps being flushed out by the exit close to the vessel keel whereas the water trapped inside the S configuration tends to move up inside the annulus to the direction of the exit close to the sea line. An equilibrium is then realised so that:

V2 = V3 and P2 = Ro.g.(H2 + H3).

By maintaining the pressure P2, the level H4 and H3 remains static inside the annulus, creating a pressurized cavity where the radial wheel can be located.

FIG.3A shows a cross section of the turret assembly of a vessel 1 according to the present invention and FIG.3B shows a detailed part of FIG.3A with the pressurized cavity 46 in the annulus between the turret 4 and the vessel 1. According to the present invention to minimise the water level variation inside the annulus, due to the vessel motion, a water barrage 40 is located close to the keel, the closest as possible. An additional water barrage 41 is located close to the water level i.e. ideally 1 meter, inside the annulus to minimise the water transit when the vessel 1 is rolling or pitching. However it should be noted that this ideal distance of 1 meter is also depending on the roll and pitch of the vessel 1.

Additionally, for safe inspection and maintenance purpose at least one inflatable seal 43 is located at the keel entry 44 of the annulus. When required, the turret 4 is locked, the seal 43 is inflated and the pressure released inside the annulus. Bilge pumps are used to purge the remaining water and allow for safe and dry access to the radial wheel.

In the preferred embodiment, the distance LI is made maximum by design to minimize the height of the volume of water pushed above the sea water level (H2 and V2).

The distance L2 is, by opposite, made minimum to "accelerate" the evacuation of the trapped water thanks to the equality of the volume V3 and V2. The system according to the invention is adapted for very large diameters turrets which generate very large forces. That means that these large forces cannot be taken up with a slide bearing only. In order to allow these forces, the invention proposes the combination of a lower bearing system comprising radial wheels which are placed at or near the chain table, where the horizontal mooring forces are acting.

The effect is that, contrary to the solution according to the prior art, the system according to the invention enables to have a radial wheel at a lowermost position which is always in the dry, even during operation. Some of the obtained advantages are: the creation of the pressurized cavity 46 renders the need of a dynamic seal between the turret fixed part and rotating part not essential, instead a volume of water V3 ensures the sealing function, hence the "seal" is completely frictionless.

it enables to have the distance between the radial bearing and the chain table mooring connecting point as short as possible, which renders the bearing more efficient.

- as the bearing is located nearer from where the effort is applied on the

turret, the lever arm effect between the mooring point and the radial bearing is smaller, thus the moment transferred on the boogie units is smaller;

hence the number of bogie units required is smaller; hence the installation is cheaper.

- when needed it is far less time consuming to access the bearing for

maintenance or change out.

the lower bearing is not lubricated by seawater.

The radial wheel is located so that, even under maximum roll or pitch, the wheel remains in dry condition.

In the text, it is stated that it is possible to inject fluid or gas within the submerged annulus and create a pressurized chamber 46 within the annulus such that the 25 submerged bearing is in a "dry" environment. It should be understood that the annulus could be filled with a fluid such as oil. Therefore the term "dry"

environment, as used in the text should be understood to be an environment in which the sea water has been totally eliminated. Or in other words: an environment completely filled with gas such as air or with fluid - other than sea water - such as oil.

The only way to have radial wheels work over time correctly is to have them placed in a permanent dry area or in a non-corrosive fluid. To create this environment a gas or fluid can be injected to create a pressurized chamber. Above, it is mentioned that to flush the radial wheel treated water is used to avoid corrosion; Hence, the wording "dry" is to include a permanent pressurized area containing the bearing (or radial wheels) in which seawater is replaced by a less or non-corrosive gas or fluid.

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.