GB2090894A - Method of constructing an off shore tower and tower so constructed - Google Patents

Abstract

A method of constructing a free- standing tower (40) at an offshore site from a plurality of buoyant tubes 30-34 comprises the steps of submerging a first tube (30) below the water level utilizing cables (19) and anchors (17), floating a second tube (32) to a position directly over the first tube (30), allowing the buoyancy of the first tube (30) to lift the second tube (31) above the water level, making an interconnection between said first and second tubes (30, 32), submerging said tubes with said cables (19) and anchors (17), and repeating the above steps for the remaining tubes until the full tower (40) is constructed. <IMAGE>

Description

SPECIFICATION Method of constructing an offshore tower and tower so constructed It is known that a thermal energy conversion system can generate electric power utilizing the temperature difference which exists between the ocean's warm surface water and its cold bottom water. Such a system typically includes a floating platform that supports an electric power station which utilizes a working fluid, such as ammonia.

The ammonia is capable of evaporating and condensing over a small temperature range. Warm water from the surface of the sea is pumped into an evaporator where liquid ammonia is heated and vaporized. A turbo-generator converts the thermal energy of the ammonia vapor into mechanical energy and then into electric energy. A condenser receives cold water from a large floating tower than descends into the sea down to the level where relatively cold water is found. The condenser condenses the ammonia vapor, which leaves the turbine, into its liquid state. The liquid ammonia is then pressurized and returned to the evaporator, thereby completing the cycle.

This invention is concerned, in general, with the method of constructing a large offshore marine tower which can be used, for example, in the thermal energy conversion system.

The invention provides a method of constructing a free-standing tower at an offshore site from a plurality of buoyant tubes comprising the steps of: (a) submerging a relatively longer first tube below the water level utilizing cables and anchors; (b) floating a second shorter tube to a position directly over the first tube; (c) allowing the buoyancy of the first tube to lift the second tube above the water level; (d) making an interconnection between said first and second tubes; (e) submerging said tubes with said cables and anchors; and (f) repeating steps (b) through (e) for the remaining tubes.

It is preferred that the cables be controlled by winches which are mounted on a floating vessel positioned above the construction site, and that the anchors be suspended from the cables so that they are disposed on the sea bed in a predetermined pattern.

The method of the invention may be characterized in that each tube has buoyancy chambers, and in which at least some of the chambers are selectively floodable.

The method may be further characterized in that the first tube is provided with securing means for securing the outer ends of the cables, and the uppermost tube is provided with securing means for securing the inner ends of the cables, thereby anchoring the tower to the sea bed independently of the vessel and of the winches.

The invention includes a marine tower when constructed in accordance with the method of the invention.

The preferred method of the invention utilizes a platform which carries a plurality of winches. A barge which is adapted to become partially submerged consecutively transports the tubes, each having a net positive buoyancy provided by buoyancy chambers at least some of which can be flooded with water. On each winchcable is suspended an anchor which is deployed on the sea bed. On the bottom end of the lowermost tube are hooks to which the outer ends of the cables are connected. Utilizing the cables and the anchors, the lowermost tube is submerged first, then the second tube is positioned on top of the lowermost tube. The positive buoyancy of the lowermost tube is allowed to lift the second tube above the water surface. The buoyancy chambers in the lowermost and second tubes are fluidly interconnected.The interconnected lowermost and second tubes are then fully submerged, and a third tube is positioned on top of the second tube.

The positive buoyancy of the lowermost and second tubes is allowed to lift the third tube above the water level. The chambers in the second and third tubes are interconnected. The thusly constructed lowermost, second, and third tubes are fully submerged, and the process is repeated until the last or uppermost tube is secured to the next to the last tube. Utilizing the winchcables and by selectively flooding the buoyancy chambers with sea water, the fully constructed tower is submerged to a desired level in the water body and is anchored to the sea bed. The inner ends of the cables are detached from their winches. The uppermost tube carries a plurality of spaced hooks to which the inner ends of the winchcables are attached. The tower is then independent from the vessel and is self-stistaining with the aid of only the anchors which are suspended on the cables.

Ways of carrying out the invention are described with reference to the accompanying drawings which illustrate specific preferred embodiments and in which: Figure 1 is a schematic representation of a suitable semi-submersible vessel which can be used during the construction of the marine tower; Figure 2 is a front view, partly in section, of the vessel shown in Figure 1; Figure 3 is a view of the vessel on line 3-3 in Figure 2; Figures 4 through 1 3 show successive stages in the deployment of a plurality of anchors on the sea bed; Figures 14 through 1 7 show the positioning of the first or lowermost tube relative to the vessel, and the attachment of the winchcables to the hooks on the bottom end of the first tube;; Figures 18 through 21 show the manner of submerging the lowermost tube, positioning the second tube on top of the first tube, and using the buoyancy of the lowermost tube to lift the second tube out of the water; Figures 22 through 23 show the successive steps in erecting the multi-tube tower, and sinking the tower to a predetermined height above the sea bed; Figure 24 shows the manner of anchoring the floating tower to the sea bed; Figure 25 shows a superstructure connected to the floating tower; Figure 26 is a horizontal sectional view of the lowermost tube showing its floodable compartments; Figure 27 is a vertical sectional view on line 27-27 of the tube shown in Figure 26; Figure 28 is a partial vertical sectional view of the upper end of the uppermost tube; and Figure 29 illustrates-the manner of mechanically interconnecting the tubes and fluidly interconnecting theirfloodable compartments.

The preferred method is illustrated with reference to the drawings, wherein is shown a conventional vessel, such as a semi-submersible vessel 10 (Figure 1), for example, of the type used in offshore drilling. Such a vessel 10 typically includes horizontal pontoons 1 5 (Figure 2) interconnected by vertical columns 14. On the periphery of a rectangular slot 12 in vessel 10 are mounted several winches 13. A conventional barge 1 6 (Figure 4) is used to successively transport large weights or anchors 17, each having a sheave 1 8 (Figure 4A).

The construction process starts when barge 1 6 is moved into slot 12 2 (Figure 5) so that the first transported weight 17 lies under a corresponding winch 13 (Figures 6-7). Then the outer free end of winchcable 1 9 is looped through sheave 1 8 (Figures 8-9) and is removably secured to a hook 20 on platform 10. The end of barge 1 6 which carries anchor 17 is then completely submerged, while winch 13 is rotated until its cable 1 9 fully supports anchor 1 7 (Figure 10).

Barge 16 is then removed from platform 10 to transport a second anchor 1 7 (Figure 11), while the first anchor 17 is lowered on cable 1 9 to the sea bed 21. In the manner above described, barge 1 6 will transport consecutively a number of anchors 1 7 which are deployed preferably in a circle on the sea bed 21 (Figures 12-1 3). The construction of tower 40 can begin utilizing many tubes 30-34.

Each tube can be made of any suitable material, such as concrete, steel, etc. The preferred material is lightweight, reinforced concrete. Each tube has an inherent positive buoyancy, that is, it has the ability to move from a fully to a partially submerged position. In addition, each tube has compartments or chambers 29 which can be flooded with water The buoyancy of a single tube or of several tubes can be adjusted by selectively flooding with sea water or emptying particular chambers 29.

With reference now to Figures 14-1 6, the first and lowermost, partially-submerged tube 30 is floated upright towards vessel 10 and is positioned within slot 12. Tube 30 can be towed by a tug (not shown). The bottom end of tube 30 has hooks 31. After tube 30 is centered within slot 12, the outer ends of the cables 1 9 are attached to respective hooks 31 (Figure 17). By rotating winches 13 to apply tension to their cables 1 9 (Figures 18-19), it is possible to sink tube 30 to any desired level.

At the same time, a second, shorter tube 32 (Figures 1 9, 1 9A) is positioned within slot 12 so as to be directly over the lowermost tube 30 (Figure 20). Then, cables 1 9 are loosened (Figure 21) to allow the inherent buoyancy of tube 30 to lift tube 32 above the water level 9. A mechanical connection 62 (Figure 29) is then made above the water level between tubes 30 and 32 (Figure 20), and a pipe connection 63 is made between their buoyancy chambers 29.

These construction steps are repeated for another short tube 32 (Figures 22-23) and for several longer tubes 33, as well as for the top or uppermost tube 34. In this manner the entire tower 40 becomes erected and fully submerged but floating above the sea bed 21.

Thus, by tensioning cables 1 9 on which anchors 1 7 are suspended, each partiallyconstructed tower structure can be submerged in order to position over its top an additional tube. If necessary, ballasting of selected chambers 29 can also be employed in conjunction with the pull exerted by cables 1 9. Cables 19 are slackened each time to allow the inherent buoyancy of the structure to lift the last added tube above the water level 9.

Each of the chambers 29 is water tight. The water level in selected chambers can be controlled by valve means (not shown) while other chambers may be sealed at atmospheric pressure and may be designed to withstand the local sea pressure.

Such ballasting is well known in the art and no further description thereof is necessary.

The uppermost tube 34 has on its wall circumferentially-spaced hooks 35 (Figure 23).

When tower 40 is fully submerged and floating, the inner ends of cables 1 9 are detached from their winches 13 and are secured to corresponding hooks 35 (Figure 24). Tower 40 is self-sustaining when it is anchored to the sea bed 21 by anchors 17 suspended on cables 19.

Thereafter, vessel 10 can be moved away in the direction of the arrow 54.

A superstructure 50 is now moved over and is coupled to tower 40 preferabiy by a pivotable joint 51. In this manner, the movements of superstructure 50 are decoupled from tower 40.

Superstructure 50 can support an electric power generating station which draws cold water from tower 40, as above described.

The lowermost tube 30 is positioned at a sufficient distance above sea bed 21 to prevent silt from being drawn into the tower. A screen 60 (Figures 26-27) is provided to act as a filter.

In a preferred embodiment of tower 40, all the tubes 30-34 may have an internal diameter of approximately 50 meters. The length of the lowermost tube 30 may be on the order of 30 meters. The net buoyancy of tube 30 may be on the order of 50%. Tube 32 may have a length of about 10 meters and also a 50% net buoyancy.

Therefore, tube 30 should be submerged to about 7 meters below the water level 9, and it should have sufficient positive buoyancy to lift the first tube 32 out of the water. The sub-structure consisting of tubes 30 and 32 will lift the third tube 32 out of the water to a level of about 3 meters.

Tubes 33 can have a length, for example, of about 1 5 meters and a 25% net buoyancy. The sub-structure consisting of tube 30, the first tube 32 and second tube 32 is submerged to about 1 5 meters below the water level 9 so as to receive the first tube 33. The construction steps are then repeated for as many long tubes 33 and for the uppermost tube 34.

An important aspect of the construction process of this invention is the use of the anchors 17 on the sea bed 21 in cooperation with the winchcables 1 9 for submerging the sub-structures and finally for self-sustaining the fully constructed tower 40 at about 1 50 to 300 meters above the sea bed 21.

The lower portion of tower 40 should have a negative buoyancy, while its upper portion should have a positive buoyancy. In this manner tower 40 will have greater stability when fully submerged and floating.

Claims (6)

1. A method of constructing a free-standing tower at an offshore construction site from a plurality of buoyant tubes comprising the steps of: (a) submerging a first tube below the water level utilizing cables and anchors; (b) floating a second tube to a position directly over the first tube; (c) allowing the buoyancy of the first tube to lift the second tube above the water level; (d) making an interconnection between said first and second tubes; (e) submerging said tubes with said cables and anchors; and (f) repeating steps (b) through (e) for the remaining tubes until the full tower is constructed.

2. A method as claimed in Claim 1, in which said cables are controlled by winches which are mounted on a floating vessel positioned above said construction site, and said anchors are suspended from said cables.

3. A method as claimed in Claim 2, in which each tube has buoyancy chambers, and in which at least some of the chambers are selectively floodable.

4. A method as claimed in Claim 3, in which the first tube is provided with securing means for securing the outer ends of said cables thereto, and the uppermost tube of said tower is provided with securing means for securing the inner ends of said cables thereto, thereby anchoring said tower to the sea bed independently of said vessel and of said winches.

5. A method of constructing a free-standing tower at an offshore construction site substantially as hereinbefore described with reference to the accompanying drawings.

6. An offshore tower constructed in accordance with the method of any one of the preceding claims.