GB1579698A - Concrete tower - Google Patents

Description

(54) CONCRETE TOWER (71) I, OLAV MO, Subject of the Kingdom of Norway, residing at Grnsundveien 94, 1360 Nesbru, Norway, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to structures for the development of sub-sea natural resources, particularly offshore platform structures of the articulated type used for drilling, production, storage, flaring, mooring/loading and other activities related to off-shore development of sub-sea resources; such as oil.

At present, suitable bottom-supported platforms exist for water depths up to approx. 200 m. When water depths exceed 200 m, bottom supported platforms are very expensive. On the other hand, other kinds of platforms all suffer from technical problems.

For example, floating platforms have problems with risers, and articulated platforms usually have problems with the link. Lack of sufficient pay-load capability and storage is also a common problem.

The preferred forms of the structures of the present invention, which will hereinafter be described in detail with reference to the drawings, have the following advantages: (1) Pay load and deck area are sufficient, even for large capacities; (2) storage is included; (3) the platform can be made of con crete, with all the well-known advan tages of concrete.

(4) the construction technique is very similar to the well-known technique of gravity platform construction; (5) some wells can be drilled before the platform is placed, improving the cash-flow; (6) drilling technique is very similar to conventional technique, allowing the wellheads to be placed on deck; (7) the hinge between the bottom temp late and the tower is very simple, and the key components can be placed without any shutdown of dril ling/production; (8) diving is almost not necessary; 9 the cost of the platform is moderate; 10) the concept covers a large depth range, may be from 100 to 100m.

(11) the platform is not volunerable to earthquakes; and (12) if required, the platform can be con structed and/or floated in a horizon tal position, which limits the neces sary water depths.

In accordance with the present invention there is provided a structure for the development of sub-sea natural resources, comprising two structural parts, the structural parts being a template fixed to the sea bed and a tower structure which is linked to the template, the link comprising a plurality of tension members placed in the central region of the link, the tension members being fastened to both structural parts and able to resist the vertical tension forces between them, and substantially vertical dowels separated from the tension members and placed away from the central region of the link, the dowels being able to move in the longitudinal direction relative to one of the structural parts such that lateral and twisting movements in the link are prevented but vertical and pivotal movements are not.

In the drawings which illustrate preferred embodiments of the invention: Figure 1 is a vertical section of a first embodiment of the platform; Figure 2 is a horizontal section of the first embodiment, taken along the line A-A of Figure 1; Figure 3 shows the first embodiment of the invention used as a mooring/loading platform; Figure 4 is a plan view of an embodiment of the template according to the present invention; Figure 5, 6 and 7 are various vertical sections of the bottom structure of Figure 4, taken along the lines B-B, C-C and D-D respectively; Figure 8 is a detail of one embodiment of the cable anchoring structure; Figure 9 is a vertical section of a second embodiment of the bottom structure according to the present invention; and Figure 10, appearing on the first sheet of drawings, is a horizontal section of a third embodiment of the platform.

The invention is best described by an example. The example is related to a water depth of 300 m.; and North Sea environmental conditions are assumed.

As best seen in Figure 1, the platform consists of three main parts: the template 1, the tower 2 and the deck structure 3. Between the template and the tower there is a link which permits the tower to oscillate. The template 1, as seen in Figures 4 to 7, consists of a core section 4, a slab 5 and a ring wall 6.

From Figure 2, it is seen that the tower consists of nine vertical cells, of which eight cells 7 are lying on the periphery and one cell 8 is in the center. Between the cells 7 and 8 is an open area 17. Further, the deck 3 is a concrete or steel structure of a conventional design and requires no further description insofar as the present invention is concerned.

Production and drilling equipment, living quarters, helideck etc., are placed on the deck in a conventional way.

Before any further dcscription of the platform, the construction technique will be described step by step. initially, the concer ete template 1 is fabricated ashore, is subsequently transported to the desired location by a bargc, and is lowered to the sea bed by a crane barge. A semisubmersible moves in and fastens the template by eight piles 9, illustrated in Figure 5, and also drills some wells. The wellheads (not shown) are placed on the slab 5 between the core 4 and the ring wall 6. The wells are not completed.

Four anchors for the later mooring of the platform are laid and tested. Approximatcly simultaneously with the fabrication of the template, a gravity dock is made. In this dock, the bottom raft of the tower is constructed. The dock is opened and the bottom raft is towed to a deep-watcr site and moored. The walls of the tower are slip formed, keeping the top of the structure at a constant freeboard by water ballasting. The deck is commenced approximately simultaneously with the fabrication of the template and the making of the graving dock.

The tower is submerged to a freeboard of six metres; the deck is towed over and fastened. The tower is raised to a tow-out draft and towed to the field in a vertical position.

The tower is placed in the pre-laid moorings.

lowered down over the template and fastened to the template by the link.

The pre-drilled walls referred to above are completed. This could be done either by keeping the wellheads on the template (subsea completion), or by placing the wellhead on the deck 3 and just lengthening casings and tubing. The remaining wells are drilled from the deck 3. Sub-sea pipelines are pulled in on the sea bed and connected to preinstalled risers.

As best seen in Figures 4 to 7, the core structure 4 of the template is made of concrete. There are eight through-going holes 10 for the piles 9. The holes may be slightly conical in order to increase the shear capacity between the pile and the core. The piles consist of steel tubes 11 which are drilled down to the desired depth, for instance 100 m.

Steel reinforcement is lowered down into the steel pipe and extends up to the top of the template. Finally, concrete is filled into the steel tube and the holes 10.

The basic object of the core structure is to form the base for the link. The main part of the link structure is 14 cables - 12 with a total capacity of approx. 10,000 t. The cables extend through holes 13 and are fastened in a chamber 14. The upper ends of the cables go through holes 1 5 and are fastened on the upper side of a slab 16.

The tower has an open area 17 inside the peripheral cells 7. The lower part of this area has one downward truncated cone 18 and one upward truncated cone 19. These cones are fastened in a cylinder 20 (of diam.

approx. 24 m) which is tangential to the peripheral cells and which is monolithically cast together with them. The slab 16 is situated on a cylinder 21 which, in turn, is constructed on the upper cone 19. The inner tower cylinder 8 goes all the way through these structures from the top to the bottom of the tower.

The described structures, that is, the piles 9, the core 4, the cables 12 and the concrete parts 16, 21. 19 and 20 of the tower, together form the part of the connecting link which is able to take tension. It is easy to see that the link is a very simple and tough structure which can easily be designed for very large capacities.

It is assumed that the tower will always have a positive buoyancy of a magnitude which is always greater than any downward force. for instance. caused by wave action.

Therefore, the cables will always be under tension and the tower can freely oscillate.

However, accidents could in some cases cause the tower to acquire a negative buoyancy. In such a case, the tower would "fall down" on the core, and come to rest on the central cell 8. The lower part of this cell is strnngthened to a ring foundation 22, which can transfer the forces to the lower cone 18.

An elastomeric bearing 23 is placed on the core just below the ring foundation 22. This bearing will dampen the fall of the tower and allow some movements of the tower without over-stressing the concrete.

The tower must also be prevented from moving sideways in the link and from twisting. These kinds of movements are prevented by two groups of dowels, primary dowels 24 and secondary dowels 25. The primary dowels 24, as seen in Figure 5, are fastened in the "star cell" between the peripheral cells 7 and the cylinder 20. The secondary dowels 25, as seen in Figure 6, are fastened in the slab 26 and the upper cones 19. Both kinds of dowels are placed in open holes, (27 for the primary dowels), so that they can be retracted and replaced, but they are prevented from falling down. The primary dowels extend down into openings 28 in the ring wall 6 and the secondary dowels in holes 29 in the core.

The dowels will then act as if they were fixed to the tower structure, while permitting limited angular and vertical movements in the template structure. As a result, the tower is prevented from any kind of horizontal movement at this elevation, but limited angular movements are permitted. The annulus between the dowel and the wall of the hole 27 should be filled with an elastomeric bearing to provide some flexibility.

The primary dowels will be those acting under normal circumstances. The reason for introducing the secondary dowels is to have additional safety if the primary dowels fail for some reason. The total capacity of the dowels is very dependent on the conditions in each separate case. It could be as low as 100 tons, and as high as 5,000 tons horizontal load. When installing the tower (construction step 12), the primary dowels are the lowest part. They have to enter the holes 28; and to ease this operation, cones 30 are fastened to the ring wall. The capacity of one dowel should be lower than the point resistance capacity of the template. If so, only the dowel will be hurt if a collision occurs between a dowel and the template. The preinstalled wellheads are lower than the ring wall 6 and are therefore protected.

When installing the tower, the structure is lowered by water ballasting. The primary dowels enter the holes 28 and the lowering continues until the tower is resting on the bearing 23 with a small weight. The cables 12, which are pre-installed in the holes 15, are lowered down through the holes 13 and fastened. Therafter, water is pumped out until the desired positive buoyancy is obtained, and the tower is "hanging" in the cables. Now, the platform can cope with any weather conditions. Max. horizontal movement at sea level is approx. 15 m, giving a maximum deviation from vertical of 1:20.

A very important point is access to the structural parts forming the link, both during installation, maintenance, and removing of the platform. Access can be obtained by three methods. Divers can go down in the open area 17, through holes (not shown) in the cones 19 and 18, and down on the slab 5).

From here, there is a horizontal tunnel 31, as seen in Figures 4 and 7, and further an opening 32 through which the diver can enter the chamber 14. Diver access could also be inside the center cell 8, through openings (not shown) in the cone 19 and the slab 26 down on the core. Further access to the chamber 14 would be through the holes 34.

The tunnels 31 could be closed by water-tight hatches 35.

The second method for access is made possible by introducing the slab 36, as seen in Figure 1, in the centre cell. Compressed air is pumped in the centre cell below this slab and presses out the water in the centre cell below the slab 36 down to the tip of the ring foundation 22. Through a sluice in the slab 36 men can go down and work in the lower part of the centre cell in air.

The third method for access is only possible during good weather conditions. The tower is ballasted so that it rests on the bearing 23. When the hatches 35 are closed, it is possible to pump out all the water in the center cell and in all the openings in the template core. People have then access through the centre cell to the cables and to the secondary dowels under atmospheric conditions.

The centre cell is filled with a noncorrosive liquid to prevent corrosion on the parts contained in this cell. A membrane 37 is fastened between the ring foundation 22 and the template core to prevent the noncorrosive liquid escaping. For the same reason, the hatches 35 are normally closed.

A reduced amount of such liquid is necessary if all openings in the slab 36 are closed and the non-corrosive liquid is filled below this slab only.

The slab 36 has access openings and openings to permit cables 12 to be lowered directly from the deck into position. As seen in Figure 8, the cables 12 have steel plates 38 permanently fixed at both ends thereof. The holes 15 and 13 have diameters slightly larger than the plates 38 and the cables can therefore be lowered through the holes.

They are fastened by steel plates 39 which are split and are therefore installed from the sides. Varying thicknesses of steel plates 39 can be used to compensate for small differences in cable lengths.

Pipes with curved walls 71 are placed in the annular spaces between the cable and the hole's wall so as to prevent sharp bends on the cables when the tower oscillates. The cables must have a certain length, decided in each separate case, in order to achieve the desired flexibility. This is obtained by employing the cylinder 21, the length of which can be varied for each separate platform.

The peripheral cylinders of the tower are those which give the buoyancy. Each has a bottom dome 67 and two intcrnal domes, a lower dome 39 and an upper dome 40. Seven of the cells are storage cells 41, for the storage of, for instance, oil. The eighth cell is a utility cell 42 for storing pipes, machinery, etc. In each storage cell is a lower compartment 43 which is used for storage. The lower compartment 43 is always filled with liquid, either sea water or oil. A middle compartment 44 is air-filled, but is alsc, used for ballasting. Ballasting is necessary for lifting or lowering the tower, as well as to compensate for different weights.This may be caused by changing the payload or by the different specific weight of oil and water, which will change the weight of the tower according to the amount of oil in storage. Diffcrcntial ballasting may also be necessary, for example, to compensate for horizontal forces caused by wind or current, giving the tower a heel. or to give the tower a desired stability (GM).

An upper compartment 45 is also airfilled.

and is also used as a water level tank. This is necessary because the whole tower structure should be kept at a certain underpressure, to obtain compressive forces in the structure.

The storage compartment is in direct communication with the water level tank, and therefore the storage is kept at the same pressure. In this case, the underpressure in the storage will be approx. 40 m.

In the utility cell, the middle compartment 33 is used for the main machinery as, for instance. oil pumps, sea water pumps etc.

The lower compartment 46 contains an inner cylinder 47 which houses the ballast pumps and other machinery that should be placed close to the bottom of the tower. Also, the upper compartment 48 has an inner cylinder 49. The reason for this is to protect the machincry against flooding if a collision should cause flooding of the upper compartment 48.

A major reason for the compartmcntization by the domes 39 and 40 is to obtain acceptable damage conditions. Risers 72 are prc-installed in separate riser cells 50. These cells are slipformcd together with the main cells. At the bottom, the cell is terminated by a slab 51. As seen in Figure 7, the pipeline 52 to be connected to the riser has a bend 53 in the end. The bend is supported by the sledge 54 and is prevented from tilting by the sledge. The pipeline is pulled under the tower until the vertical part of the bend is exactly under the pre-installcd riser. The bend is then lifted and the vertical part enters a hole 55 in the slab 51. The annulus between the slab and the pipeline is scaled and the riser cell 50 is pumped dry. Now, the weld 56 between the riser and the pipeline can be done under atmospheric conditions.When this is done, the riser cell is filled, the seal is removed, the riser lowered and the pipeline sledge 54 will again rest on the sea bed, as shown in Figure 7.

The area 17 between the peripheral cells 7 and the centre cell 8 is always waterfilled, and in this area the conductors are situated.

Just one conductor 57 is shown in Figure 7.

The conductor extends through holes 58 and 59 in the cones, and through a hole 60 in the template slab. The holes 59 or 58 could be temporarily closed during construction and installation to increase buoyancy, but will be open when the tower is installed. Conductor guides (not shown) are installed at different levels in the area 17. When movements are sufficiently small, no link will be introduced in the conductors. The movements are taken by bending only.

Some figures for the embodiment shown shall be given. The water depth is 300 m. The outer diameter of the peripheral cells 7 is 15 m., giving a diameter of the open area 17 of approximately 24m. The diameter of centre cell 8 is 8 m. The total displacement is more than 400,000 m3, and the concrete volume approximately 100,000 m3. The deck load could be in excess of 30,000 t.

More figures could be found directly from these figures. Figures 1 and 2, which are approximately to a scale of 1:2000; while Figures 4 - 7 are approximately to a scale of 1:200.

A second embodiment of the invention is shown in Figure 9. In this embodiment, only the link design differs from that of the first embodiment. The second embodiment provides a structure where the link is not only for tension, but where the vertical forces could be both tension and compression.

The cables 12 with anchors are in principle the same as for the first embodiment. However. in the centre, the cables are omitted and a support 61 is placed there. The support consists of an upper steel slab 62, and a lower steel slab 63. The slabs are curved and are very solid, having a diameter of 1 - 2m. In this case. the upper slab 62 is dome shaped (convex), and the lower slab 63 is slightly concave. When the tower oscillates, the upper slab will roll on the lower slab and form a support for the tower.

The upper slab 62 is fixed on the lower cone 18 and the lower slab 63 is fixed on the template core 4. The central part of the core is heightened somewhat, so that the upper part is enclosed by the lower part of the central cylinder 8. When the central cylinder is filled with compressed air, the water level will be below the access openings 64. This means that the chamber 65 could also be air filled and that under-water work could be avoided when inspecting and removing the cables 12 and the support 61. The support 61 should preferably be made removable. This could be done by removing some of the cables, pulling the whole support 61 to the side and replacing it with another similar support. During this operation, the tower must be ballasted to positive buoyancy. The cables can be replaced when the tower has a negative buoyancy.Then, the support forces will be taken by the support 61. The support 61 could also be effected in other ways than described, for instance, by elastomeric bearings.

The dowels are, in principle, the same for the first and second embodiments. It might be necessary, however, to move the centre of rotation somewhat to get the same centre of rotation as the support 61. The primary dowels must also be longer for the second embodiment, due to the heightened core.

A third embodiment of the invention is shown in Figure 10. In this case it is the tower itself that differs from the first embodiment.

The section shown in Figure 10 corresponds to that shown in Figure 2. The third embodiment has some small buoyancy cells 66 in addition to those in the first embodiment.

The smaller cells 66 reach from the bottom of the tower to a level somewhat below sea level, for instance to an elevation of - 40 m.

The background for the cells 66 is the following. When a wave passes the structure, there will be pressure variations in the water.

The variations on the lower domes 67 will create a varying vertical force on the tower and consequently also in the link. By introducing the cells 66, this force will increase, but the pressure variations will also act on the upper domes of the cells 66, and in the opposite direction. In addition, the pressure variation is larger at el. - 40 than at el. - 290. This means that by correct designing the vertical variations can be considerably reduced.

An example will show the effect of a wave on the cells 66. The lower domes 67 of the periphery cells 7 have an area of 1400 m2.

The domes of the cells 66 have an area of 630 m2, if the cell diameter is 10 m. For a certain wave, the pressure variation may, at 290 m depth, be 3 t/m2, and at 40 m depth 8 t/m2.

In this case, the first embodiment of the invention will give a pressure variation of 1400 x 3 = 4200 t. The third embodiment will give 2030 x 3 = 6090 tin one direction and 630 x 8 = 5040 tin the other direction.

This means that the resultant force, as a result of the introduction of the cells 66, will be reduced from 4200 t to 1050 t.

Figure 3 shows a mooring/loading system connected to the platform. This is a conventional system and does not require a detailed explanation. When a ship 68 swings around the platform due to the weather, a mooring system 69 and a loading system 70 follow on rails going around the tower. By placing the mooflng/oading system directly on the tower, a special loading buoy is avoided, as is a great deal of subsea work.

It is possible also to place flare structures etc., on the deck. This means that all the facilities needed for an offshore development of a field could be incorporated in one platform. This is a great advantage because, in this case, no sub-sea pipelines are needed and the amount of diving will be greatly reduced. In fact, it is possible that diving could be completely avoided.

When constructing and towing the tower 2, it has been assumed that the tower should at all times be in a vertical position. This may be difficult in many cases, due to depth limitations. Thus, it should be emphasized that it is also possible to construct the tower in a horizontal position in a dock, tow it to the field or to a deep-water fitting-out site in a horizontal position, and turn it to a vertical position at the site, by a proper ballasting of the cell compartments.

It is also possible to construct the tower in a vertical position (which is a great advantage because slipforming can then be used), turn it to a horizontal position for towing over shallow areas, and then turn it back to a vertical position. A towing at a certain tower heel is also possible.

The template 1 should have protecting caps during installation. The central area of the core, inside the piles, should have a steel cap to cover the elastomeric bearings and the area inside during all construction steps until the tower is to be installed. The area outside should have a steel cap (with holes for the piles) during the piling period.

The structure described is well fitted to be used in earthquake areas. If the horizontal shear force in the link exceeds the capacity of the dowels, the dowels will yield somewhat.

This is no catastrophy, as it is possible to cut the dowels and to replace them with new dowels.

Vertical forces from earthquakes will usually be taken by the deflection of the cables.

However, it might be necessary to place the support 61 on elastomeric bearings to obtain sufficient flexibility in the support. The elastomeric bearing should be larger than the support, for instance, 5 x 5 m and should be stiffened by a steel plate. The steel plate and the bearings must then have holes for the cables.

The embodiments described are examples only, and the design could be varied within the scope of the present invention, e.g., by using steel rods instead of cables.

A structure is also envisaged where all the link components could be installed and renewed from above the sea surface. Looking at Figure 9 the slab forming the upper cable support is lifted to above water level and the length of the cables is increased accordingly. The cables can then be handled from surface level. The lower locking device must in this case be automatic. The dowels can be handled from the surface when they are hanging in cables or rods. The lower cone 18 must have a hole in its center formed so that the support 61 can be lowered through it, rotated 90 degrees and thereby be supported by the concrete. The support 61 should preferably be hung on a stiff rod during these operations.

It may be an advantage to omit the dowels 24 and just use the dowels 25. In this case all link members are placed within the cell 8 and there is easy access.

WHAT I CLAIM IS: 1. A structure for the development of sub-sea natural resources, comprising two structural parts, the structural parts being a template fixed to the sea bed and a tower structure which is linked to the template, the link comprising a plurality of tension members placed in the central region of the link, the tension members being fastened to both structural parts and able to resist the vertical tension forces between them, and substantially vertical dowels separated from the tension members and placed away from the centril region of the link, the dowels being able to move in the longitudinal direction relative to one of the structural parts, such that lateral and twisting movements in the link are prevented but vertical and pivotal movements are not.

2. A structure according to claim 1, wherein the tension members are cables.

3. A structure according to claim 1 or 2 wherein the link also comprises a bearing to take downward forces from the tower.

4. A structure according to any preceding claim, wherein the tower comprises a plurality of vertical cells, which have the same horizontal section above and below the water surface.

5. A structure according to any of the preceding claims, wherein the tower comprises a plurality of vertical cells, at least one of the cells extending unchanged from the bottom to above water level and at least one of the cells terminating below the water level.

6. A structure according to any of the preceding claims. further comprising a central cylinder extending from the tower bottom to above the water level. link members being placed within the central cylinder.

7. A structure according to claim 6, wherein the central cylinder can be lowered down to the template and be emptied of water.

8. A structure according to any of the preceding claims. further comprising a drilling/production plant. living quarters. a flare system and/or a mooring/loading system.

9. A structure according to any preceding claim wherein all the link members can be installed and renewed from above sea surface.

10. A structure for the development of sub-sea natural resources substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of installing pipelines in a structure according to any of the preceding claims, wherein the pipelines are pulled under the tower on a supporting sledge, lifted up into a hole in a bottom slab of the tower, welded to a riser and lowered down again.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. and the length of the cables is increased accordingly. The cables can then be handled from surface level. The lower locking device must in this case be automatic. The dowels can be handled from the surface when they are hanging in cables or rods. The lower cone 18 must have a hole in its center formed so that the support 61 can be lowered through it, rotated 90 degrees and thereby be supported by the concrete. The support 61 should preferably be hung on a stiff rod during these operations. It may be an advantage to omit the dowels 24 and just use the dowels 25. In this case all link members are placed within the cell 8 and there is easy access. WHAT I CLAIM IS:

1. A structure for the development of sub-sea natural resources, comprising two structural parts, the structural parts being a template fixed to the sea bed and a tower structure which is linked to the template, the link comprising a plurality of tension members placed in the central region of the link, the tension members being fastened to both structural parts and able to resist the vertical tension forces between them, and substantially vertical dowels separated from the tension members and placed away from the centril region of the link, the dowels being able to move in the longitudinal direction relative to one of the structural parts, such that lateral and twisting movements in the link are prevented but vertical and pivotal movements are not.

2. A structure according to claim 1, wherein the tension members are cables.

3. A structure according to claim 1 or 2 wherein the link also comprises a bearing to take downward forces from the tower.

4. A structure according to any preceding claim, wherein the tower comprises a plurality of vertical cells, which have the same horizontal section above and below the water surface.

5. A structure according to any of the preceding claims, wherein the tower comprises a plurality of vertical cells, at least one of the cells extending unchanged from the bottom to above water level and at least one of the cells terminating below the water level.

6. A structure according to any of the preceding claims. further comprising a central cylinder extending from the tower bottom to above the water level. link members being placed within the central cylinder.

7. A structure according to claim 6, wherein the central cylinder can be lowered down to the template and be emptied of water.

8. A structure according to any of the preceding claims. further comprising a drilling/production plant. living quarters. a flare system and/or a mooring/loading system.

9. A structure according to any preceding claim wherein all the link members can be installed and renewed from above sea surface.

10. A structure for the development of sub-sea natural resources substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of installing pipelines in a structure according to any of the preceding claims, wherein the pipelines are pulled under the tower on a supporting sledge, lifted up into a hole in a bottom slab of the tower, welded to a riser and lowered down again.