EP0942102B1 - Self-elevating platform with submerged reservoir and method for placing and raising the reservoir - Google Patents

Description

The present invention relates to a self-elevating platform of the type comprising a floating upper shell mounted displaceable according to length of load-bearing legs, mechanical means of movement legs relative to the hull, and a submerged storage tank, intended to rest on the bottom.

It further relates to a method of positioning and a method of lifting a reservoir. immersed in a self-elevating platform of the aforementioned type and a method of lifting of such a tank.

Such types of platforms are used for the operation of oil fields. Indeed, before the extracted oil is transported, for example by boat, even on land, the submerged tank is used to store oil.

For this purpose, it is known to provide a large storage tank capacity, on which the legs of the self-elevating platform rest.

In known platforms, the tank is generally made with concrete walls. It forms a base on which rests the rest of the platform.

When installing the platform, the storage tank is routed, separately, in flotation, to the place of installation of the platform. It is then immersed by ballasting it by adding water taken from the sea.

When the tank rests on the bottom, the hull of the platform carrying the legs is floated above the tank. The legs are then lowered, until their lower end take support on the upper surface of the tank. The hull is then hoisted above the water level.

Such a method of installing a platform is effective and well control. However, for platforms to be moved several times on the oilfield during their lifetime, the reservoir of storage cannot be reused.

Indeed, the tank being independent, its re-afloat is extremely delicate and risky.

Thus, it is necessary for each new implementation of the platform to initially set up a new tank. This increases considerably the cost of moving the platform.

In addition, the abandonment of the submerged storage tank after displacement of the upper shell creates an environmental nuisance.

Document FR-A-2 225 582 discloses a platform self-elevating type of tank submersible storage with controllable buoyancy.

The object of the invention is to propose a self-elevating platform, as well as its installation and uninstallation process, allowing updates successive operations of the platform in separate places, for a reduced cost with no consequences for the environment.

To this end, the invention relates to a self-elevating platform as defined in claim 1.

According to particular embodiments, the platform comprises one or more of the features defined in the dependent claims.

The invention further relates to a method for setting up a submerged tank of a jack-up platform according to claim 12.

The invention finally relates to a method of lifting a tank immersed in a jack-up platform according to claim 16.

• La figure 1 est une vue en perspective de trois-quarts d'une plate-forme auto-élévatrice selon l'invention en position de transport ;
• La figure 2 est une vue en élévation de la plate-forme de la figure 1 lors du transport ;
• Les figures 3, 4, 5, 6 et 7 sont des vues en élévation de la plate-forme lors de phases successives d'installation de celle-ci ; et
• La figure 8 est une courbe représentant l'effort appliqué sur les moyens de déplacement des jambes en fonction de la profondeur d'immersion du réservoir.

The invention will be better understood on reading the description which follows, given solely by way of example and made with reference to the drawings in which:

• Figure 1 is a three-quarter perspective view of a jack-up platform according to the invention in the transport position;
• Figure 2 is an elevational view of the platform of Figure 1 during transport;
• Figures 3, 4, 5, 6 and 7 are elevational views of the platform during successive phases of installation thereof; and
• FIG. 8 is a curve representing the force applied to the means for moving the legs as a function of the depth of immersion of the tank.

The platform according to the invention, shown in the transport position in Figures 1 and 2, includes an upper shell 10 movable mounted and adjustable in position on vertical legs 12. It further comprises a submerged storage tank 14 intended to rest on the bottom by via support plates 16. After installation, the reservoir 14 provided soles 16 form a base 17 for supporting the building formed of the shell 10 and legs 12.

The shell 10 comprises a hermetically closed box. She is conventionally provided with operating equipment and premises housing, as well as a drilling tower arranged above a passage transverse 10A. These are not shown in the figures.

The shell has for example the shape of an equilateral triangle on the side of length equal to 90 meters.

The height of the hull is equal to 10 meters. Its mass with the equipment it carries is around 25,000 t. The hull volume is around 40,000 m ³ .

In addition, the shell 10 is provided, for each leg 12, with a mechanism lifting 18. These mechanisms are adapted to ensure movement legs 12 relative to the shell 10, and in particular the descent legs then the elevation of the hull above the level of the sea after the legs are supported on the seabed. Likewise, these lifting mechanisms 18 are adapted to allow a rise of the legs 12 and tank 14.

Each vertical leg 12 has, in this case, a section triangular. It consists of three vertical members, connected between them by a lattice of metal tubes. The lower end of each leg is fixed by welding to the upper surface of the reservoir 14.

The total mass of the legs is around 5000 t.

The reservoir 14 has a general shape of an equilateral triangle. One of the vertices is truncated. So the longest side of the tank has a length of 120 m, while the two sides ending in the truncated summit have a length of 95 m.

The reservoir 14 is formed by a metal box. It has a base 20 with a surface greater than that of the shell 10. This base is bordered laterally along two sides adjacent to the top truncated by two edges upper 22 forming shields. These edges 22 delimit on the base 20 a triangular area 20A for supporting the shell 10. This support area 20A has a surface slightly larger than that of the shell 10. It is open along a side opposite the truncated top of the tank.

The base 20 has a thickness of 7 m. It is crossed by a passage vertical 28 for oil exploitation conduits.

The flanges 22 forming shields are delimited by the metal box forming the reservoir. Thus, they delimit in their thickness a part of the reservoir 14. The flanges 22 extend over a height of 11 m above the support zone 20A. Thus, the reservoir 14 has, laterally along two sides, sides with a total height of 18 m. The total volume of the reservoir is approximately 60,000 m ³ , for a mass of approximately 7,200 t.

The bottom wall of the reservoir 14 includes a valve 26 allowing to selectively put inside the tank 14 and the surrounding marine environment.

In addition, the interior of the reservoir 14 is connected, by a pipe 27A passing along a leg 12, to an air compressor 27 installed on the shell 10.

The soles 16 are formed by heavy massive blocks. The total mass of the soles is around 6000 t. The height of each sole is substantially equal to 2 m. They are reported under the underside of the reservoir 14 in the corners thereof. Advantageously, the surface lower of the tank 14 covered by the soles 16 is lower to half of the total interior surface of the tank. The soles have, for example, the shape of equilateral triangles 30 m side.

The soles are fixed under the tank 14 by means of retaining means, releasable when the base 17 is placed on the bottom. These means consist for example of a mechanical lock or any other suitable means, for example a key engaged through two ears secured, for one, to the tank and, for the other, to the sole to fix.

For the construction of such an oil platform, we proceed first to the manufacture of the box constituting the reservoir 14. This can then be floated.

The shell 10 and the legs 12 are then built directly on the central support zone 20A of the reservoir, while the latter floats. So the manufacturing such a platform requires the use of a wedge only for the initial construction phase.

After the completion of the shell 10, and of the legs 12, the soles 16 are routed to the lower surface of the reservoir 14. For this purpose, the soles 16 are prefabricated then immersed and maintained on the surface through buoys of suitable volume. They are slipped under the corners of the tank 14 and are subject thereto by means of the means of restraint.

In order to install the platform according to the invention, we performs the successive steps illustrated in Figures 2 to 7.

In the configuration shown in Figures 1 and 2, while the valve 26 is closed and the reservoir 14 is empty, the shell 10 is maintained above sea level. The platform is thus routed up to its place of installation.

During transport, the side edges 22 forming a shield protect swell, the hull fitted with operating instruments.

The shields 22 constitute protective freeboards having a height of about 11 meters above sea level, the reservoir being sunk into the water to a depth of about 7 m.

Once arrived at the place of operation of the platform, the valve 26 provided in the lower part of the reservoir 14 is open. So under the action of the total weight of the platform, it gradually sinks in water as the reservoir 14 fills.

The reservoir 14 having no opening in its upper part, the air initially contained therein is compressed in one or more air bubbles 30 confined in the upper parts of the tank 14. In particular, the air is compressed inside the flanges 22 constituting cavities in bell.

When the pressure in the air bubbles 30 is equal to the pressure of the water in the tank, the platform is in an equilibrium position, as shown in FIG. 3. In this position, the height of the air bubbles 30 is denoted B3. This height corresponds to the distance separating the upper wall of the flanges 22 from the level of liquid in the reservoir 14. For such a height B3 of the air bubbles, the volume of air trapped in the reservoir 14 is approximately 33,000 m ³ .

In this position of equilibrium, the platform having sunk into water, the shell 10 is partially submerged. It thus participates in the flotation of the whole platform. In particular, in the example considered, the shell 10 is immersed over a depth denoted C3 of approximately 5 m.

In this position, the force applied to the lifting mechanisms corresponds to an upward push of the legs under the action of the push Archimedes applied to reservoir 14. This state corresponds to point noted A3 in Figure 8, where the effort is negative.

While the platform is in this position, we proceed to the injection of compressed air inside the reservoir 14.

Thus, the thrust exerted by the reservoir 14 via the legs on the lifting mechanisms 18 increases and corresponds to a state represented by point A3 'in Figure 8.

In this position, the height of the air bubble B3 is increased and the immersion depth C3 of the shell 10 is reduced accordingly.

The air volume in the tank is then for example 55,000 m ³ .

From this position, the valve 26 being kept open and we exerts, thanks to the lifting mechanism 18, a force on the legs 12 tending cause the reservoir 14 to sink.

During this depression, the upper part of the flanges 22 crosses the surface of the water, so that the entire reservoir 14 is submerged, as shown in Figure 4. In this position, the height B4 of the air bubbles 30 is reduced accordingly. Similarly, the depth immersion C4 of the shell 10 is reduced. The latter is then by example of about 1 m. For the considered depth of the reservoir 14, the force applied by the lifting means 18 corresponds to point A4 of the figure 8.

The descent of the reservoir 14 is carried out sufficiently slowly in order to guarantee a pressure balance between the outside and the inside of the tank 14, following the entry of water inside the tank 14 through the valve 26. In particular, breaks are operated at regular intervals during the descent.

The force applied by the lifting means 18 on the legs 12 in view of the descent of the reservoir 14 continues as shown in the figure 5. Thus, as the reservoir 14 is pushed in, the height B5 air bubbles 30 trapped in the reservoir 14 decreases. In the same time, the immersion height of the shell 10, denoted C5, increases.

In FIG. 8, this initial phase of descent of the reservoir 14 corresponds to the section of curve marked A5. As shown in this figure, as the reservoir 14 is pushed in, the force applied by the lifting mechanisms decrease in absolute value. Indeed, due to the increase in hydrostatic pressure of water with depth, the volume of air contained in air bubbles 30 decreases, thereby reducing buoyancy of the reservoir 14.

For a given immersion depth, denoted Pi, which in the example considered is substantially equal to 40 m, the force applied to the lifting mechanisms cancel.

For depths greater than this immersion depth Pi, the air bubbles 30 confined in the reservoir 14 are insufficient to ensure the buoyancy of it. Also, the reservoir 14 exerts traction on the legs 12. The latter affects the lifting mechanisms 18 which are then subjected to a positive force, as represented in FIG. 8 by the section of curve A6.

This subsequent phase of descent of the reservoir 14 corresponds to the phase shown in Figure 6, where the height B6 of the air bubbles is reduced. In addition, the legs 12 exerting traction on the shell 10, the depth immersion, denoted C6, of it increases. It is then for example of the order of 7 m, when the reservoir 14 is in the immediate vicinity of the ground.

During the entire phase corresponding to the section of curve A6, the shell 10 retains the reservoir 14 to avoid a too rapid descent thereof. In particular, the lifting mechanisms 18 are released sufficiently slowly to allow pressure balancing between the interior and outside the tank 14.

Finally, after the reservoir 14 is placed on the bottom via soles 16, the shell 10 is raised above the level of the sea. The air contained in the tank is extracted for example through the vertical pipe installed in the legs of the legs.

The force exerted on the lifting means then corresponds to point A7 of Figure 8. In this position, the legs 12 are locked directly on the hull, avoiding prolonged stresses on the mechanisms lifting 18.

After setting up the platform, as indicated above, the enclosure formed by the reservoir 14 is used for the storage of the oil extracted by the platform.

It is understood that during the entire descent of the reservoir 14, the pressures inside and outside of it are substantially constantly equal, which avoids the application of high stresses on the walls of the tank. Thus, these can be carried out with sheets of relatively thin.

In order to move the oil rig, it is advisable to bring it back to its transport position shown on the figure 2.

To this end, the steps implemented for the descent of the tank 14 are reproduced in reverse.

In particular, an air bubble of height B7 is first injected at inside the tank.

One then exerts a traction on the legs 12 via lifting mechanisms 18. This traction is exerted until the reservoir 14 reaches the inversion depth Pi.

Then, the reservoir 14 tends to rise by itself to the surface. To limit its ascent rate, the lifting mechanisms 18 push the legs 12 so that they stress the tank down. In this configuration, the shell 10 weighs on the tank.

Thus, the ascent rate can be reduced, so that at any time the pressure difference between the inside and the outside of the tank either substantially zero.

Compliance with decompression stops allows satisfactory circulation water through valve 26 so that the pressure in the tank 14 is equal to the hydrostatic pressure at the immersion depth of the tank.

Thus, in all circumstances, both during installation and operation or dismantling, the tank is under pressure.

In addition, with a platform as described here, the force applied to the lifting mechanisms is relatively reduced for the installation of a tank high capacity.

In addition, during the descent and ascent of the tank, it performs a stabilization function for the hull floating on water.

During manufacture, the particular shape of the tank allows it to play the role of a floating dock, which makes it possible to manufacture the hull and legs directly on the floating tank.

Under certain particularly difficult conditions, especially when the seabed is loose, there is a risk of sticking of the lower part of the base 14 on the bottom.

In this case, before proceeding to raise the reservoir 14, we frees the retaining means securing the reservoir 14 to at least some soles 16. This release can be done for example by a diver. Thus, some soles 16 remain on the bottom allowing the ascent of the reservoir 14.

Indeed, due to the thickness of the soles 16, when the base 17 is placed on the bottom, the central region of the reservoir 14 is spaced from the bottom by an interval I (figure 7) of a height of about 2 m. So the surface lower part of the reservoir 14 is not firmly held against the bottom by collage and it is then possible to reassemble it.

Such soles can be used for any type of tank or submerged element intended to come to rest on the seabed and then to be reassembled surface.

Claims (19)

1. A self-elevating platform comprising bearing legs, a floating upper hull (10) mounted so that it can move along the length of the bearing legs (12), mechanical means (18) for moving the legs (12) in relation to the hull (10), and a submersible storage tank (14), intended to rest on the seabed, wherein the tank (14) comprises a lower opening (26) allowing the inside of the tank (14) to be brought into contact with the marine environment such that the pressures inside and outside the tank are substantially constantly the same, which tank defines a bell-shaped cavity for confinement of an air bubble (30) in its upper part, and the tank (14) is connected to the lower end of the legs (12) so that it can be moved in relation to the hull (10) from said mechanical means (18) for moving the legs, the mechanical means (18) being adapted to apply to the legs, on the one hand, a force for making the tank (14) sink, and, on the other, an opposite force for making the tank (14) rise.
2. The platform according to Claim 1, characterised in that it comprises means (27, 27A) for injecting gas under pressure into the tank (14).
3. The platform according to Claim 1 or 2, characterised in that the tank (14) comprises a base part (20A), the surface area of which is at least equal to the surface area of the hull (10) and at least one upper shield (22) partially bordering the base part (20A) and extending along said hull (10) when it is in immediate proximity to the base (20A).
4. The platform according to Claim 3, characterised in that the or each shield (22) is formed by a part of the tank (14).
5. The platform according to Claim 3 or 4, characterised in that the tank (14) is substantially polygonal shaped, notably triangular or rectangular shaped, and in that shields (22) extend along most of the length of at least two sides of the tank (14).
6. The platform according to any one of the preceding claims, characterised in that the lower opening of the tank is equipped with a valve (26).
7. The platform according to any one of the preceding claims, characterised in that it comprises at least one mat (16) for support on the seabed, the or each mat (16) being made integral with the underside of the tank (14) by retaining means, which can be released, when the base (17) is supported on the seabed, so as to allow the tank (14) to rise without at least one mat (16).
8. The platform according to Claim 7, characterised in that the lower surface area of the bearing structure (14) covered by the or each mat (16) is less than half of the total lower surface area of the bearing structure (14).
9. The platform according to Claim 7 or 8, characterised in that the or each mat (16) is disposed in proximity to the outer contour of the tank (14), leaving the central area of the tank (14) free.
10. The platform according to any one of Claims 7 to 9, characterised in that the height of the or each mat (16) is adapted to keep a free space (I) between the seabed and the area of the underside of the tank (14) which has no mat.
11. The platform according to any one of Claims 7 to 10, characterised in that the releasable retaining means comprise a mechanical lock.
12. A method of installing a submerged tank (14) of a self-elevating platform according to any one of Claims 1 to 11, the tank (14) being initially immediately below the hull (10), whereby said hull (10) floats at the water surface,
    characterised in that it comprises the following successive steps:

    a) the tank (14) is brought into communication with the marine environment via its lower opening (26), such that a pressure balance is established between the air bubble (30) and the water, the tank (14) being initially acted upon solely by its own weight,

    b) the legs (12) are gradually lowered in relation to the hull (10) which is kept constantly afloat, by allowing water to gradually enter into the tank (14) via the lower opening (26), thus keeping the pressure in the tank (14) substantially equal to the hydrostatic pressure at the depth at which the tank is situated, the legs (12) being gradually lowered first by applying to the legs from the mechanical means (18) a force for making the tank (14) sink, and then by applying to the legs from the mechanical means (18) a force for making the tank (14) rise, and

    c) after applying the tank (14) to the seabed, the hull is hoisted above water level by moving the hull in relation to the legs.
13. The method according to Claim 12, characterised in that, prior to step b) for lowering the legs, a gas is injected into the tank (14).
14. The method according to Claim 7 or 8, characterised in that the volume of the air bubble (30) is adapted such that, during step b) for lowering the legs, at an initial stage, the moving means (18) urge the legs (12) downwards and, at a subsequent stage, the moving means (18) act as a restraint against the descent of the legs (12) driven by the tank (14).
15. The method according to any one of Claims 12 to 14, characterised in that, during step b) for lowering the legs (12), pauses are observed to enable the pressures inside and outside the tank (14) to be balanced by water entering therein.
16. A method of raising a submerged tank (14) of a self-elevating platform according to any one of Claims 1 to 11, the tank (14) initially being placed on the seabed, the hull (10) floating at the water surface, characterised in that it comprises the following steps:

    a) the tank (14) is brought into communication with the marine environment via its lower opening (26);

    b) an air bubble (30) is trapped inside the tank (14), the pressure of the air bubble (30) being equal to the hydrostatic pressure at the depth at which the tank (14) is situated;

    c) the legs (12) are gradually raised in relation to the hull (10) which is kept constantly afloat, by allowing the water to gradually exit, thus keeping the pressure in the tank (14) substantially equal to the hydrostatic pressure at the depth at which the tank (14) is situated, the legs (12) being gradually lifted first by applying to the legs from the mechanical means (18) a force for making the tank (14) rise, and then by applying to the legs from the mechanical means (18) a force for making the tank (14) sink.
17. The method according to Claim 16, characterised in that, prior to step c) for raising the legs, a gas is injected into the tank (14).
18. The method according to Claim 16 or 17, characterised in that the volume of the air bubble (30) is adapted such that, during step c) for raising the legs, at an initial stage, the moving means (18) urge the legs (12) upwards and, at a subsequent stage, the moving means (18) act as a restraint against the rising of the legs (12) pushed by the tank (14).
19. The method according to any one of Claims 16 to 18, characterised in that, during step c) for raising the legs (12), pauses are observed which enable the pressures inside and outside the tank (14) to be balanced by the exit of water therein.

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