WO2006058704A1

WO2006058704A1 - Semisubmarine provided with improved mooring units

Info

Publication number: WO2006058704A1
Application number: PCT/EP2005/012741
Authority: WO
Inventors: Sandro Foce
Original assignee: Sandro Foce
Priority date: 2004-12-01
Filing date: 2005-11-29
Publication date: 2006-06-08
Also published as: ATE410359T1; EP1866200B1; EP1866200A1; DE602005010309D1; ITMI20042314A1

Abstract

An off-shore semisubmarine comprising a main deck supported by columns (13) forming hulls of the semisubmarine and a plurality of mooring units each comprising a mooring cable (14), a traction winch for the double-drum cable (20) and a storage winch (21) for the cable pulled by the double-drum winch and characterized in that the double-drum winch (20) and the storage winch (21) are arranged basically at the base of a column of the semisubmarine with the mooring cable between the double-drum winch and the storage winch being returned by an intermediate pulley (19) arranged at a level higher than that of the winches by at least one third the height of the column (13).

Description

"SEMISUBMARINE PROVIDED WITH IMPROVED MOORING UNITS"

This invention relate to an offshore semisubmarine structure designed for example for applications in the petroleum field and provided with an improved anchorage system.

The technical problem relative to anchorage in the open sea of semisubmarine platforms designed for example for petroleum well drilling is known. In recent years the problem has been felt in even greater measure since it is required ever more frequently to build offshore platforms suited for operating even at more than 500 m depth in zones where the force and depth of the sea are such as to impose the use of cables of considerable length and large diameter whereas the cable length is usually chosen equal to 5 times the platform operating depth.

In general, to realize the anchorage, from 4 to 12 steel mooring cables departing from the corners of the platform are used to reach the anchors.

In fact, to pull the mooring cables in the past, single- drum winches were used both for pulling the cable and for storing it. This solution however proved poorly suited to satisfying the need for reaching ever greater depths and, indeed, when winding considerable lengths of cable on the only drum of the winch, the coils overlapped several times on themselves causing an increase in the effective winding diameter of the cable and drastically reducing the maximum possible pull. Therefore the use of double-drum traction winches in combination with a storage drum for cable pulled by the double drum was proposed. This configuration allowed obtaining a constant maximum pull independent of the quantity of cable collected and allowing mooring of the platform either at slight or great depth.

In general, the double-drum winch is installed at the height of the main deck of the platform near one of the semisubmarine columns at the vertices of the platform while the storage winch is arranged at a lower height inside the lower hull of the column.

The double-drum winch with its considerable masses is thus found at a relatively high level, which has a negative influence on the stability of the semisubmarine platform in the sea. It is noted that the stability problem in this type of off-shore application is very real both because of the conditions of the sea which can occur and because of the wind load acting transversely on the platform tower (even several tens of meters high). Beyond this, it is necessary to prepare an adequate circuitry along the entire height of the platform column for transmission of the power and control signals necessary for coordinating the operation of the storage winch and the double-drum winch. In addition, although the height of the submersible column (and consequently also the distance between the double drum and the storage winch) could exceed 15 to 20 meters, the fleeting angle for the storage winch (which measures the maximum slope of the cable with respect to the plane normal to the winch axis) is relatively big, so much so that mechanical cable-winding systems become necessary at the inlet of the storage winch to ensure its correct winding on the drum; nevertheless, these systems, because of the strong angle component, do not always succeed in winding the cable correctly to avoid excessive wear thereof. Still, the arrangement of the double-drum winch on the main deck engages a considerably ample space which could be used differently. Lastly, the double-drum winch remains exposed on the main deck to the sea environment and to the mercy of the corrosive action of humidity and marine salts. As an alternative to this type of anchorage system, there have been proposed dynamic positioning systems provided with a set of propellers that are operated so as to always maintain correct positioning of the semisubmarine platform. These systems are used mainly for off-shore applications with very deep operating level but have very high maintenance costs and burdensome power consumption because of the frequent maintenance of the system.

The general purpose of this invention is to remedy the above mentioned shortcomings by making available a semisubmarine structure provided with anchorage means allowing realization of mooring even at great depth of the sea while having a simple and economical structure.

Another purpose of this invention is to make available a semisubmarine provided with anchoring means allowing having a satisfactory distribution of the masses such as to obtain satisfactory stability of the semisubmarine in the water. Another purpose of this invention is to make available a semisubmarine structure provided with anchorage means not requiring costly additional reinforcing structures nor costly power transmission control circuits. Another purpose of this invention is to make available a semisubmarine structure having more usable space on the main deck compared with the prior art platforms. Another purpose of this invention is to make available a semisubmarine on which the mooring means do not remain exposed to the sea environment.

In view of this purpose it was sought to provide in accordance with this invention an off-shore semisubmarine comprising a main deck supported by columns forming hulls of the semisubmarine and a plurality of mooring units each comprising a mooring cable, a double-drum traction winch for the cable and a storage winch for the cable pulled by the double-drum winch and characterized in that the double- drum winch and the storage winch are arranged basically at the base of a column of the semisubmarine with the mooring cable between the double-drum winch and the storage winch being returned by an intermediate pulley arranged at a higher level than that of the winches by at least one third of the column height. To clarify the explanation of the innovative principles of this invention and its advantages compared with the prior art there is described below with the aid of the annexed drawings a possible embodiment thereof by way of non- limiting example applying said principles. In the drawings: 5 012741

FIG 1 shows a diagrammatic top view of a semisubitiarine platform in accordance with this invention, FIG 2 shows a diagrammatic side view of the platform of FIG 1, FIG 3 shows the arrangement of the platform of the mooring cable traction and storage winches inside a platform column, and

FIG 4 shows another view of the traction winches arranged inside of a platform column. With reference to the figures, FIG 1 shows diagrammatically an off-shore semisubmarine platform 11 designed for example for drilling of a petroleum well.

The platform 11 has a rectangular shape and floats supported by six partially submerged columns 13 that hold up a main deck 12. The lower parts of the column 13 form the lower hulls of the platform inside of which are in general arranged the ballasts that contribute to obtaining an adequate distribution of the masses of the platform and give them the necessary stability in the sea. From the main deck 12 rises a drilling tower 15 shown diagrammatically in the figures. On the platform there can be made various rooms 45 for the workers who live aboard the off-shore semisubmarine. From the vertices of the platform 11 there depart eight steel mooring cable 14 designed for anchoring the semisubmarine to the bottom of the sea. In the realization shown in the figure, there are arranged two cables for each vertex of the platform. It is noted however that the cables could also be more numerous depending on the type of platform and the environmental conditions in which it is designed to operate.

FIG 2 shows a side view of the platform 11 in which are also shown two mooring units of the semisubmarine. In particular, the figure shows diagrammatically the main deck 12, two columns 13 and the tower 15. Reference number 70 designates the sea level line under operating conditions with the mooring cables 14 tight. For each mooring cable 14 there is provided a double-drum traction winch 20 (with the two drums arranged vertically one over the other) and a cable storage winch 21. As described below, both the winches are arranged at the base of a column 13. In particular they are arranged inside a lower hull 16 of the platform 11 (also called 'pontoon'). Between the two winches the cable is returned by a pulley

19 placed over the column just below the main deck.

FIG 3 shows diagrammatically the configuration of a mooring unit inside a column 13 of the platform.

At the base of the column 13 are arranged side by side the cable storage winch 21 and the double-drum traction winch

20 designed to produce the pull on the cable. Between the storage winch 22 and the double-drum winch, the cable 14 is returned by a pulley 19 arranged almost vertically over the storage winch 21 at the height of the main deck 12. The pulley 19 is installed in a manner sliding in the direction of its axis.

Downstream from the traction winch 20 the cable 14 is started toward the pulley 18 which is arranged nearly vertically over the double drum 20 at the height of the main deck 12 and returns the cable 14 to the anchor 30. Advantageously, between the pulley 18 and the anchor 30 there is inserted another return point 31 realized with the so-called 'fairlead' fastened on the outside of the column 13 below the pulley 18. In FIG 3 the anchor 30 is shown in non-operative position_^ resting on an appropriate support 32.

The positioning of the traction winch 20 at the column bottom level allows carrying huge masses downward (on the order of hundreds of tons in case of large platforms) to contribute to obtaining a much _^improved stability of the semisubmarine in the water.

In addition, insertion of the return pulley 19 between the storage winch 21 and the traction winch 20 allows forming a path of various tens of meters for the cable 14 so as, allow easy winding of the cable on the storage winch drum 21. Indeed, it is noted that the storage drum 21 can even be wider than 2 m and that, with the winches 20 and 21 arranged so close together inside the hull 16 of the column, the fleeting angle of the cable 14 on the storage winch 21 would have been impossible without the aerial return of the pulley 19.

But thanks to insertion of the return of the pulley 19 it is possible to wind the cable 14 on the drum 21 without even using a mechanical cable-guide device since the fleeting angle (which, as mentioned above, measures the slope of the cable with the plane normal to the drum axis) is much reduced and the cable is guided directly by the coils already wound on the drum without particular efforts or additional tensions.

To perform its function correctly, the pulley 19 must be positioned at a higher level than the winches by at least one third of the height of the column 13 (by column height is meant here the deck height with respect to the bottom of the column) .

In a preferred embodiment of this invention as above- mentioned, the pulley 19 is positioned at the height of the main deck 12 just below it. This stratagem allowed making the length of the path of the cable 14 between the winch 20 and the winch 21 maximum while reducing the fleeting angle to a minimum. The pulley 19 is installed so as to slide axially in the direction of its axis according to known mechanical engineering techniques. As mentioned above, the pulley 18 also is advantageously located on the main deck 12 near the outer edge of the column 13.

As shown in the figures, the double-drum winch 20 is arranged in contact with the wall structure of the column turned towards the open sea and the storage winch 21 is arranged turned towards the inside of the semisubmarine. It was found particularly advantageous to arrange the two powered drums 37 and 38 of the -traction winch 20 vertically one over the other so as to reduce to a minimum the space occupied at the base of the column 13. This stratagem still allows reserving ample spaces at the base of the column for insertion of ballast 17. This new arrangement allows increasing the useful load of the platform since the shifting downward of the weights of the winches has allowed forming a permanent ballast instead of a liquid ballast (sea water) .

Advantageously as shown in the* figure, the cable 14 is wound on the two drums 37 and 38 of the traction winch 20 with figure-eight coils. This stratagem allows having greater angular amplitudes of the arcs of the cable 14 in contact with the traction drums allowing best use of the surface of the drums for producing the pull on the cable. In addition, the particular configuration allows avoiding crossings between the cable branch incoming to the winch and the outgoing one.

It is noted however that the cable could also be wound in round coils. The double-drum winch 20, which can reach a weight on the order of hundreds of tons, undergoes under operating conditions the pull of the cable 14 upward comparable with the intensity of its own weight. This situation allows considerably reducing the supporting structures 39 necessary for fastening the traction winch 20 to the frame of the column 13 since its own weight compensates for a large part of the stress of traction to which it is exposed when the cable 14 is pulled. The particular funicular geometry adopted thus allows saving many tons of steel which would be necessary to fasten the winches to the platform structure.

It is noted that the sliding pulley 19 added between the storage winch and the double-drum winch must not support a great load because the pull of the cable 14 is very high downstream of the traction winches 20 but relatively low upstream thereof since the storage winch 21 performs only the task of stretching to avoid excluding the cable. Therefore, insertion of the return of the pulley 19 does not require high costs in terms of reinforcement structures 33 for support of the pulley.

It is also noted that the reinforcement structure 34 that supports the pulley 18 near the outer edge of the platform deck is sized in a manner far less costly than in the prior art where the double-drum winch was arranged on the deck and the pulley for return of the cable to the anchor projected considerably overhanging the edge of the deck and thus required a massive use of steel for its support. FIG 4 shows a diagrammatic view of the column 13 cross sectioned along plane of cut IV-IV as shown in FIG 3. The figure shows the double-drum winch 20 with the two drums 37 and 38 and the cable 14 returned from the pulley 18 to the anchor (not shown).

In one embodiment of this invention, on the bottom of the column 13 there can be arranged another double drum traction winch 40 beside the winch 20 and designed to produce the pull on another cable 14 returned at a pulley 41 to another anchor of the platform. Another storage winch will be combined with the winch 40 not shown in the figure and quite similar to the winch 21 already described. The figure also shows the operation system 42 of the two traction winches 20 and 40 which includes the power motors and reducers and is itself realized in accordance with the prior art. The system can also be realized with independent traction systems for the vertical double drums or with a single common power unit.

It is now clear that the preset purposes have been achieved. An anchoring system has been made available for a semisubmarine that allows increasing considerably the operating depths of the off-shore semisubmarine with no need for costly dynamic positioning systems. In addition, the semisubmarine in accordance with this invention has satisfactory stability in the sea since the huge masses of storage winches and double-drum winches have been taken to a relatively low level oh the bottom of the columns. Weight distribution is still better when the platform is laid out for transfer from one site to another with the cables wound on the storage drum. Again, on the main deck of the platform a considerable amount of space has been freed which, in prior art platforms, was typically occupied by the mooring cable traction winches, which are transferred in accordance with this invention to the height of the lower hulls of the semisubmarine. In addition, the particular funicular geometry and the arrangement of the storage and traction winches saves considerably on the weight of the reinforcing structural steel needed to support the mooring means. Again, thanks to insertion of the intermediate pulley 19 between the storage drum and the traction drums, the cable can wind onto the storage drum with very low winding angle with no need of using specific mechanical cable-winding systems and thus reducing considerably the wear to which the cable is subjected. In addition, thanks to the. particular arrangement of the traction winches vertically one over the other it was possible to reduce to a minimum the space occupied at the base of the column and thus reserve ample zones of the lower hull for insertion of ballast.

Naturally the above description of an embodiment applying the innovative principles of this invention is given by way of non-limiting example of said principles within the^' scope of the exclusive right claimed here. For example, the platform could be square and not rectangular as described above and be provided with only four semisubmerged columns instead of six.

Claims

1. Off-shore semisubmarine comprising a main deck supported by columns (13) forming hulls of. the semisubmarine and a plurality of mooring units each comprising a mooring cable (14), a traction winch for the double-drum cable (20) and a storage winch (21) for the cable pulled by the double-drum .winch and characterized in that the double-drum winch (20) and the storage winch (21) are arranged basically at the base of a column of the semisubmarine with the mooring cable between the double- drum winch and the storage winch being returned by an intermediate pulley (19) arranged at a level higher than that of the winches by at least one third the height of the column (13).

2. Semisubmarine in accordance with claim 1 characterized in that the intermediate pulley (19) is arranged at the height of the main deck (12) of the semisubmarine.

3. Semisubmarine in accordance with claim 1 characterized in that the intermediate pulley (19) is arranged nearly vertically above the storage winch.

4. Semisubmarine in accordance with claim 1 characterized in that the mooring cable (14) is returned by the double- drum winch (20) to an anchor (30) by a pulley (18) arranged nearly vertically above the double-drum winch (20).

5. Semisubmarine in accordance with claim 4 characterized in that the return pulley (18) for the cable to the anchor (30) is arranged at the height of the main deck (12) of the semisubmarine.

6. Semisubmarine in accordance with claim 1 characterized in that the storage winch (21) and the double-drum^" winch (20) are arranged one beside the other.

7. Semisubmarine in accordance with claim 6 characterized in that the double-drum winch (20) is arranged near a wall structure of ^' the column turned outward from the semisubmarine and the storage winch (21) is arranged toward the inside of the semisubmarine.

8. Semisubmarine in accordance .with claim 1 characterized in that the storage winch (21) does not comprise guide means for winding of the cable on its drum.

9. Semisubmarine in accordance with claim 1 characterized in that the two drums of the double-drum winch (20) are arranged vertically one above the other.

10. Semisubmarine in accordance with claim 1 characterized in that it comprises from four to twelve mooring cables (14).

11. Semisubmarine in accordance with claim 1 characterized in that it comprises from four to ten columns (13).

12. Semisubmarine in accordance with claim 1 characterized in that the intermediate pulley (19) is sliding in the direction of its axis.

13. Semisubmarine in accordance with claim 1 characterized in that said winches (20, 21) occupy an area contained on the bottom of the column.

14. Semisubmarine in accordance with claim 1 characterized in that it comprises a drilling tower (15) for petroleum wells.