FR2775993A1 - Self-lifting marine platform with submerged reservoir - Google Patents

Abstract

The platform has an upper floating chamber (10) which can be moved up or down its supporting legs (12) by a mechanical system (18), and a storage reservoir (14) designed to rest on the sea bed. The reservoir has a lower aperture (26) connecting its interior to the surrounding marine medium, and a bell chamber (30) containing an air bubble at the top. The reservoir is attached to the lower ends of the platform legs so that its position relative to the floating chamber can be varied by the mechanism (18).

Description

The present invention relates to a self-elevating platform of the type

  comprising a floating upper shell mounted movable along the length of the supporting legs, mechanical means for moving the legs relative to the shell, and a submerged storage tank, intended to rest on the bottom. It also relates to a method of setting up a submerged tank of 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 exploitation of oil fields. Before the extracted oil is transported, for example by boat, to 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 jack-up 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

  form. 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 is supported 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 controlled. However, for platforms that have to be moved several times on the oil field during their lifetime, the reservoir of

storage cannot be reused.

  Indeed, the tank being independent, its return to afloat is extreme-

delicate and risky.

  Thus, it is necessary for each new installation of the platform

  form 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

  cementing the upper shell creates an environmental nuisance. The object of the invention is to propose a self-elevating platform,

  as well as its installation and uninstallation process, allowing

  its successive implementations 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 of the

  aforementioned type, characterized in that the reservoir has a lower opening

  allowing the contact of the interior of the tank with the marine environment, which tank defines a bell-shaped cavity for confining an air bubble in its upper part, and in that the tank is linked to the end lower legs to ensure its movement relative

  to the hull from said mechanical means for moving the legs.

  According to particular embodiments, the platform includes one or more of the following characteristics: - it includes means for injecting gas under pressure into the tank; - the tank has a base whose surface is at least equal to

  the surface of the hull and at least one upper shield bordering partial-

  ment the base and extending along said shell when the latter is in the immediate vicinity of the base; - the or each shield is formed by a part of the tank;

  - the reservoir is of substantially polygonal shape, in particular trian-

  gular or rectangular, and shields extend along most of the length of at least two sides of the tank; and

  - the lower opening of the tank is fitted with a valve.

  The invention further relates to a method of setting up a

  submerged tank of a self-elevating platform as defined above

  above, characterized in that it comprises the following successive stages: a) - the tank is placed in communication with the marine environment by its lower opening, so that a pressure balance is established between the air bubble and water, the tank being initially solicited only by its own weight, b) - the legs are progressively lowered in relation to the hull kept constantly in flotation, allowing gradual entry

  of water in the tank through the lower opening, thus maintaining the pressure

  sion in the tank, substantially equal to the hydrostatic pressure at the depth of the tank, and

  c) - after applying the tank to the bottom, the hull is hoisted

  above the water level by displacement of the hull relative to the legs.

  According to particular modes of implementation, the method comprises

  has one or more of the following characteristics: - before step b) lowering the legs, a gas is injected into the tank; - the volume of the air bubble is adapted so that, during step b)

  lowering of the legs, during an initial phase, the means of

  placement urges the legs down and, during a subsequent phase, the displacement means retain the descent of the legs driven by the reservoir; and - during step b) lowering of the legs, pauses are observed

  allowing pressure balancing between the interior and exterior of the tank

  servoir (14) by entering water therein.

  The invention finally relates to a method of lifting a tank

  immersed in a self-elevating platform as defined above,

  characterized in that it comprises the following stages: a) - the tank is placed in communication with the marine environment through its lower opening; b) - an air bubble is trapped inside the tank, the pressure of the air bubble being equal to the hydrostatic pressure at the depth of the tank; c) - the legs are gradually raised with respect to the hull kept constantly in flotation, allowing the gradual exit of the water, thus maintaining the pressure in the tank, substantially equal to the

  hydrostatic pressure at the tank depth.

  According to particular modes of implementation, the method comprises one or more of the following characteristics: - before step c) of lifting the legs, a gas is injected into the tank; the volume of the air bubble is adapted so that, during step c) of

  lifting the legs, during an initial phase, the means of movement

  ment stress the legs upwards and, during a subsequent phase, the displacement means retain the ascent of the legs pushed by the reservoir; and - during step c) of lifting the legs, pauses are observed

  allowing pressure balancing between the interior and exterior of the tank

  serve by water outlet therein.

  The invention will be better understood on reading the description which will

  follow, given only as an example and made with reference to the

sins on which:

  - Figure 1 is a perspective view of three-quarters of a platform

  self-elevating form 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 elevation views of the platform

  form during successive phases of installation thereof; and - Figure 8 is a curve representing the force applied to the

  means of movement of the legs as a function of the depth of immersion

tank.

  The platform according to the invention, shown in the transport position in FIGS. 1 and 2, comprises an upper shell 10 mounted displaceable and adjustable in position on vertical legs 12. It also comprises a submerged storage tank 14 intended for rest on the bottom by

  via support plates 16. After installation, the reservoir 14 mu-

  nor of the 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. It is conventionally provided with operating equipment and living quarters, as well as a drilling tower arranged above a step.

  transverse sage 1 OA. 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 volume of the hull is

approximately 40,000 m3.

  In addition, the shell 10 is provided, for each leg 12, with a mechanism

  lifting mechanism 18. These mechanisms are adapted to ensure the movement

  cementing of the legs 12 relative to the shell 10, and in particular the

  cente of the legs and then the raising of the hull above sea level after the legs are supported on the seabed. Similarly, 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 triangular section. It consists of three vertical members, linked together 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. Thus, the longest side of the tank is 120 m long, while the two sides ending in the truncated top

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 re-

  upper edges 22 forming shields. These flanges 22 delimit on the base

  a triangular area 20A for supporting the shell 10. This area of support

  port 20A has a surface slightly larger than that of shell 10. It

  is open on a side opposite the truncated top of the tank.

  The base 20 has a thickness of 7 m. It is crossed by a step-

  vertical sage 28 for oil pipes.

  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

  above the support area 20A. Thus, the reservoir 14 has, lateral-

  lying on two sides, sides with a total height of 18 m. Volume

  total of the reservoir is approximately 60,000 m3, for a mass of approximately 7,200 t.

  The bottom wall of the reservoir 14 includes a valve 26 making it possible to selectively put the interior of the reservoir 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 se-

  it is substantially equal to 2 m. They are reported on the underside

  of the reservoir 14 in the corners thereof. Advantageously, the over-

  underside of the reservoir 14 covered by the soles 16 is less than half of the total interior surface of the reservoir. 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 integral ears, for one, of the reservoir and, for the other, of the sole.

to fix.

  For the construction of such an oil platform, the casing constituting the reservoir 14 is first manufactured.

then be floated.

  The shell 10 and the legs 12 are then built directly on the central support area 20A of the tank, while the latter floats. Thus, the manufacture of such a platform requires the use of a wedge only

  for the initial construction phase.

  After the shell 10 and the legs 12 have been completed, the soles 16 are conveyed to the lower surface of the tank 14. For this purpose,

  the soles 16 are prefabricated, then submerged and maintained on top

  face via buoys of suitable volume. They are slid under the corners of the tank 14 and are subjected 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 now

  bare above sea level. The platform is thus routed

than at its place of installation.

  During its transport, the lateral flanges 22 forming a shield protect

  swell, hull equipped with operating instruments.

  Shields 22 constitute protective freeboards having a height of about 11 meters above sea level, the tank

  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 tank 14 is open. Thus, 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 reservoir 14. In

  in particular, the air is compressed inside the flanges 22 constituting

seen in bell.

  When the pressure in the air bubbles 30 is equal to the pressure of

  water in the tank, the platform is in a balanced position

  bre, as shown in figure 3. In this position, the height of the

  air bubbles 30 is denoted B3. This height corresponds to the distance between

  rant the upper wall of the rims 22 of the liquid level in the reservoir

  see 14. For such a height B3 of the air bubbles, the volume of air trapped

  born in the reservoir 14 is about 33,000 m3.

  In this position of equilibrium, the platform having sunk into

  water, the shell 10 is partially submerged. It thus participates in the float-

  sound of the whole platform. In particular, in the example considered

  re, the hull 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 pushing

  sée d'Archimède applying on 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 m3.

  From this position, the valve 26 being kept open and we

  exerts, thanks to the lifting mechanism 18, a force on the legs 12 ten-

  to cause the tank 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 immersed, as shown in FIG. 4. In this position, the height B4 of the air bubbles 30 is reduced accordingly. Likewise, the immersion depth C4 of the shell 10 is reduced. The latter is then for example 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 balancing of the pressures between the exterior and the interior of the reservoir 14, following the entry of water into the interior of the reservoir 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 for the descent of the reservoir 14 continues as shown in the figure.

  gure 5. Thus, as the reservoir 14 is pushed in, the height

  B5 of the air bubbles 30 trapped in the reservoir 14 decreases. In

  at 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 cor-

  corresponds to the section of curve denoted A5. As shown in this figure, as the reservoir 14 is pushed in, the force applied by the lifting mechanisms decreases in absolute value. In fact, due to the increase in the hydrostatic pressure of water with depth, the

  volume of air contained in the air bubbles 30 decreases, thereby reducing the flow

tank stability 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

  to measure 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 shown 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

  pick. In addition, the legs 12 exerting traction on the shell 10, the pro-

  immersion founder, denoted C6, of this increases. It is then for example-

  full 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 reservoir 14 to avoid too rapid descent of this

  him. In particular, the lifting mechanisms 18 are released slowly enough to allow pressure balancing between the interior and

outside the tank 14.

  Finally, after the reservoir 14 is placed on the bottom by means of

  diary of the soles 16, the hull 10 is raised above sea level. 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-

  lying on the hull, avoiding prolonged stresses on the mechanisms

lifting 18.

  After setting up the platform, as indicated above

  ment, 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 constant

  equal, which avoids the application of high stresses on the walls

  of the tank. Thus, these can be carried out with sheets of relati-

thinly.

  In order to move the oil platform, it

  has just returned it 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.

  A traction is then exerted on the legs 12 by means of the 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 sur-

  face. 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

  as the pressure difference between the inside and the outside of the tank is

substantially zero.

  Compliance with decompression stops allows for satisfactory circulation.

  making water through the 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 re-

large capacity.

  In addition, when lowering and raising the tank, it

  performs a stabilization function for the hull floating on water.

  During manufacture, the particular shape of the tank allows it to

  ci 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, particularly

  as the seabed is soft, there is a risk of sticking of the lower part

lower of the base 14 on the bottom.

  In this case, before proceeding to raise the reservoir 14, we

  releases the retaining means securing the reservoir 14 to at least certai-

  nes soles 16. This release can be done for example by a plun-

  geur. Thus, some soles 16 remain on the bottom allowing the ascent

of the reservoir 14.

  Indeed, because of 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 (FIG. 7) of a height of approximately 2 m. Thus, the lower surface of the reservoir 14 is not securely 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 be

surface mounted.

Claims (14)

  1.- Self-elevating platform of the type comprising a top shell   floating upper (10) mounted movable along the length of the supporting legs   teuses (12), mechanical means (18) for moving the legs (12) relative to the shell (10), and a submerged storage tank (14), intended to rest on the bottom, characterized in that the tank (14) has a lower opening (26) allowing the interior of the   reservoir (14) with the marine environment, which reservoir defines a closed cavity   confinement of an air bubble (30) in its upper part, and in this   that the reservoir (14) is linked to the lower end of the legs (12) for   secure its displacement relative to the shell (10) from said means   mechanical (18) movement of the legs.   2.- Platform according to claim 1, characterized in that it comprises means (27, 27A) for injecting pressurized gas into the tank (14).   3.- Platform according to claim 1 or 2, characterized in that the tank (14) comprises a base (20A) whose surface is at least equal to the surface of the shell (10) and at least one upper shield (22) partially bordering the base (20A) and extending along said shell (10)   when the latter is in the immediate vicinity of the base (20A).   4.- Platform according to claim 3, characterized in that the or   each shield (22) is formed by a part of the reservoir (14).   5.- Platform according to claim 3 or 4, characterized in that   the reservoir (14) is of substantially polygonal shape, in particular triangular-   square or rectangular, and in that shields (22) extend along   most of the length of at least two sides of the tank (14).   6.- Platform according to any one of the preceding claims-   tes, characterized in that the lower opening of the tank is provided with a valve (26).   7.- Method of setting up a submerged tank (14) of a platform   self-elevating form according to any one of claims 1 to 6, the   tank (14) being initially immediately below the hull (10), which hull (10) floats on the surface of the water, characterized in that it comprises the following successive stages: a) - the tank (14 ) is placed in communication with the marine environment through its lower opening (26), so that a pressure equilibrium is established between the air bubble (30) and the water, the reservoir (14) being initially stressed only by its own weight,   b) - the legs (12) are gradually lowered relative to the co-   that (10) kept constantly in buoyancy, allowing the entry pro   water in the tank (14) through the lower opening (26), now   thus reducing the pressure in the reservoir (14), substantially equal to the pressure   hydrostatic sion at the depth of the tank, and c) - after application of the tank (14) on the bottom, the hull is hoisted above the water level by displacement of the hull relative to the legs. 8. - Method according to claim 7, characterized in that, before step b) lowering the legs, a gas is injected into the reservoir (14). 9. Method according to claim 7 or 8, characterized in that the volume of the air bubble (30) is adapted so that, during step b)   lowering of the legs, during an initial phase, the means of   placement (18) stress the legs (12) downwards and, during a phase   later, the displacement means (18) retain the descent of the jams   bes (12) driven by the reservoir (14).   10.- Method according to any one of claims 7 to 9, ca-   characterized in that, during step b) lowering the legs (12), pauses are observed allowing pressure balancing between the interior   and the outside of the reservoir (14) by entering water therein.   11.- Method of lifting a submerged tank (14) from a platform   self-elevating form according to any one of claims 1 to 6, the   tank (14) being initially placed on the bottom, the hull (10) floating on the surface of the water, characterized in that it comprises the following stages: a) - the tank (14) is placed in communication with the marine environment through 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 of the tank (14); c) - the legs (12) are gradually raised relative to the hull (10) constantly maintained in flotation, allowing the progress output-   water, thus maintaining the pressure in the tank (14), sensitive-   equal to the hydrostatic pressure at the depth of the tank (14).   12.- Method according to claim 11, characterized in that, before   step c) of lifting the legs, a gas is injected into the reservoir (14).   13.- Method according to claim 1 1 or 12, characterized in that the   volume of the air bubble (30) is adapted so that, during step c) of raising   turn of the legs, during an initial phase, the displacement means (18) urge the legs (12) upward and, during a subsequent phase, the   displacement means (18) retain the rise of the legs (12) pushing seized by the reservoir (14).   14.- Method according to any one of claims 11 to 13, ca-   characterized in that, during step c) of raising the legs (12),   serve breaks to balance pressures between the interior and   outside the tank (14) by water outlet therein.