Jack-up platform having a submerged tank and methods for installing and raising the tank

A jack-up platform of the type having a buoyant upper barge (10) mounted so that it can be moved along the length of bearing legs (12), mechanical mechanism (18) for moving the legs (12) relative to the barge (10), and submerged storage tank (14) intended to rest on the bottom. The tank (14) has a lower opening (26) allowing the inside of the tank (14) to be placed in contact with the marine environment. The tank defines a bell-shaped/inverted U-shaped cavity confining an air bubble (30) in its upper part. The tank (14) is connected to the lower end of the legs (12) so that it can be moved relative to the barge (10) from the mechanical mechanism (18) for moving the legs.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a jack-up platform of the type comprising
 a buoyant upper barge mounted so that it can be moved along the length of
 bearing legs, mechanical means for moving the legs relative to the barge,
 and a submerged storage tank intended to rest on the bottom.
 It furthermore relates to a method of installing a submerged tank of a
 jack-up platform of the aforementioned type and to a method of raising
 such a tank.
 2. Description of the Related Art
 Such types of platform are used for operating oil fields. What happens is
 that before the extracted oil is transported, for example by boat, as far
 as land, the submerged tank is used for its temporary storage.
 It is known practice for this purpose to provide a large-capacity storage
 tank on which the legs of the jack-up platform rest.
 In known platforms, the tank is generally made with concrete walls. It
 forms a footing on which the rest of the platform rests.
 When installing the platform, the storage tank is floated out separately to
 the site where the platform is to be installed. It is then submerged by
 weighting it down by filling it with water taken from the sea.
 When the tank is resting on the bottom, the barge of the platform bearing
 the legs is floated out to a location over the tank. The legs are then
 lowered until their lower end rest on the upper surface of the tank. The
 barge is then jacked up above the water level.
 Such a method of installing a platform is effective and well mastered.
 However, in the case of platforms which have to be moved around the oil
 field a number of times during their life, the storage tank cannot be
 reused.
 This is because as the tank is independent, it is extremely tricky and
 risky to re-float it.
 Thus, each time the platform is moved to a new site, it is necessary
 initially to install a new tank. This considerably increases the cost of
 re-siting the platform.
 Furthermore, abandoning the submerged storage tank once the upper barge has
 been re-sited is harmful to the environment.
 SUMMARY OF THE INVENTION
 The object of the invention is to propose a jack-up platform and a method
 of installing and uninstalling it, that allow the platform to be employed
 a number of times in turn at distinct locations, for a lower cost and with
 no impact on the environment.
 To this end, the subject of the invention is a jack-up platform.
 Another subject of the invention is a method of installing a submerged tank
 of a jack-up platform.

DETAILED DESCRIPTION OF THE INVENTION
 The platform according to the invention, depicted in the transport position
 in FIGS. 1 and 2, comprises an upper barge 10 which is mounted so that it
 can be moved and so that its position can be adjusted on vertical legs 12.
 It further comprises a submerged storage tank 14 intended to rest on the
 bottom via bearing soles 16. After installation, the tank 14 with the
 soles 16 form a base 17 that supports the structure formed of the barge 10
 and of the legs 12.
 The barge 10 comprises a hermetically sealed caisson. It is fitted in the
 conventional way with production equipment and living quarters, and with a
 boring tower arranged above a transverse passage 10A. These are not
 depicted in the figures.
 The barge has, for example, the shape of an equilateral triangle with sides
 90 meters long.
 The height of the barge is 10 meters. Its mass with the equipment it
 carries is about 25,000 t. The volume of the barge is about 40,000
 m.sup.3.
 Furthermore, the barge 10 is equipped, for each leg 12, with a lifting
 mechanism 18. These mechanisms are designed to move the legs 12 relative
 to the barge 10, and in particular to lower the legs then raise the barge
 up above sea level after the legs have come to rest on the bottom of the
 sea. Likewise, these lifting mechanisms 18 are designed to allow the legs
 12 and the tank 14 to be raised back up.
 Each vertical leg 12 in this particular instance has a triangular cross
 section. It consists of three vertical chords connected together by a
 lattice of metal tubes. The lower end of each leg is welded to the upper
 surface of the tank 14.
 The total mass of the legs is about 5000 t.
 The tank 14 has the overall shape of an equilateral triangle. One of the
 apexes is truncated. Thus, the longest side of the tank is 120 m long,
 whereas the two sides which lead to the truncated apex are 95 m long.
 The tank 14 is formed of a metal caisson. It has a base 20 with a surface
 area greater than that of the barge 10. This base is laterally flanked
 along two sides adjacent to the truncated apex by two upper rims 22
 forming shields. These rims 22 delimit a triangular region 20A on the base
 20, and this is for supporting the barge 10. This support region 20A has a
 surface area that slightly exceeds that of the barge 10. It is open along
 a side opposite the truncated apex of the tank.
 The base 20 is 7 m thick. It has a vertical passage 28 passing through it
 for the oil production pipework.
 The rims 22 forming shields are delimited by the metal caisson forming the
 tank. Thus, within their thickness, they delimit part of the tank 14. The
 rims 22 extend over a height of 11 m above the support region 20A. The
 tank 14 thus has, laterally along two sides, flanks which have a total
 height of 18 m. The total volume of the tank is about 60,000 m.sup.3, for
 a mass of about 7200 t.
 The underside of the tank 14 has a valve 26 selectively allowing the inside
 of the tank 14 and the surrounding marine environment to be placed in free
 communication.
 Furthermore, the inside of the tank 14 is connected by a pipe 27A passing
 along a leg 12 to an air compressor 27 installed on the barge 10.
 The soles 16 are formed of heavy solid blocks. The total mass of the soles
 is about 6000 t. The height of each sole is roughly 2 m. They are attached
 under the underside of the tank 14 in its corners. Advantageously, the
 surface area of the underside of the tank 14, covered by the soles 16, is
 less than half the total surface area of the underside of the tank. The
 soles are, for example, in the shape of equilateral triangles with sides
 30 m long.
 The soles are fixed under the tank 14 by retainers which can be released
 when the base 17 is placed on the bottom. These retainers for example
 consist of a mechanical latch or any other appropriate means, for example
 a wedge fitted through two lugs, one secured to the tank and the other
 secured to the sole that is to be fixed.
 To construct such an oil platform, the first stage is to manufacture the
 caisson that forms the tank 14. This can then be put afloat.
 The barge 10 and the legs 12 are then constructed directly on the central
 bearing region 20A of the tank while the latter is afloat. Thus, the
 manufacture of such a platform requires the use of a dry dock only for the
 initial phase of the construction.
 Once the barge 10 and the legs 12 have been completed, the soles 16 are
 ferried out to the underside of the tank 14. To achieve this, the soles 16
 are prefabricated and then submerged and kept near the surface by buoys of
 an appropriate volume. They are slipped under the corners of the tank 14
 and are secured there by the retainers.
 To install the platform according to the invention, the successive steps
 illustrated in FIGS. 2 to 7 are performed.
 In the configuration depicted in FIGS. 1 and 2, while the valve 26 is
 closed and the tank 14 is empty, the barge 10 is kept above sea level. The
 platform is thus ferried out to the site where it is to be installed.
 While it is being transported, the lateral rims 22, which form shields,
 protect the barge equipped with the production instruments from the waves.
 The shields 22 constitute protective freeboards with a height of about 11
 meters above sea level, the tank penetrating the water to a depth of about
 7 m.
 Once the platform production site is reached, the valve 26 in the underside
 of the tank 14 is opened. Thus, under the action of the total weight of
 the platform, the latter gradually sinks into the water as the tank 14
 fills.
 As the tank 14 has no opening in its upper part, the air initially
 contained therein compresses into one or more air bubbles 30 confined to
 the upper parts of the tank 14. In particular, the air compresses inside
 the rims 22 which constitute bell-shaped/inverted U-shaped cavities.
 When the pressure in the air bubbles 30 is equal to the pressure of the
 water in the tank, the platform finds itself in a position of equilibrium
 as depicted 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 rims 22 from the level of liquid in the tank 14. For
 this height B3 of air bubbles, the volume of air trapped in the tank 14 is
 about 33,000 m.sup.3.
 In this position of equilibrium, with the platform sinking into the water,
 the barge 10 is partially submerged. It thus plays a part in keeping the
 entire platform buoyant. In particular, in the example being considered,
 the barge 10 is submerged to a depth denoted C3 of about 5 m.
 In this position, the force applied to the lifting mechanisms corresponds
 to an upwards thrust on the legs under the action of the upthrust applied
 to the tank 14. This condition corresponds to the point denoted A3 in FIG.
 8, where the force is negative.
 While the platform is in this position, compressed air is injected into the
 tank 14.
 Thus, the thrust exerted by the tank 14 via the legs on the lifting
 mechanisms 18 increases and corresponds to a condition represented by the
 point A3' in FIG. 8.
 In this position, the height of the air bubble B3 is increased and the
 depth C3 to which the barge 10 is submerged is correspondingly reduced.
 The volume of air in the tank is now, for example, 55,000 m.sup.3.
 From this position, with the valve 26 kept open, a force is applied to the
 legs 12 using the lifting mechanism 18 which tends to cause the tank 14 to
 sink.
 During this sinking, the upper part of the rims 22 break through the
 surface of the water, so that the entire tank 14 is submerged, as depicted
 in FIG. 4. In this position, the height B4 of the air bubbles 30 is
 reduced accordingly. Likewise, the depth C4 to which the barge 10 is
 submerged is decreased. The latter is, for example, about 1 m. For the
 depth considered here of the tank 14, the force applied by the lifting
 means 18 corresponds to the point A4 in FIG. 8.
 The tank 14 is lowered slowly enough to guarantee that the pressures
 outside and inside the tank 14 will reach equilibrium, through water
 entering the tank 14 through the valve 26. In particular, pauses are
 observed at regular intervals during the descent.
 The force applied by the lifting means 18 to the legs 12 to lower the tank
 14 continues as depicted in FIG. 5. Thus, as the tank 14 gradually sinks,
 the height B5 of the air bubbles 30 trapped in the tank 14 decreases. At
 the same time, the height, denoted C5, by which the barge 10 is submerged,
 increases.
 In FIG. 8, this initial phase of lowering the tank 14 corresponds to the
 curve portion denoted A5. As depicted in this figure, as the tank 14
 gradually sinks, the force applied by the lifting mechanism decreases in
 terms of absolute value. This is because, since the hydrostatic pressure
 of the water increases with depth, the volume of air contained in the air
 bubbles 30 decreases, thus reducing the buoyancy of the tank 14.
 For a given depth of submersion, denoted Pi which, in the example in
 question, is roughly equal to 40 m, the force applied on the lifting
 mechanisms is cancelled out.
 For depths in excess of this depth of submersion Pi, the air bubbles 30
 confined in the tank 14 are not enough to make this tank buoyant. So, the
 tank 14 exerts a pulling force on the legs 12. This pulling force is
 transmitted to the lifting mechanisms 18 which are then subjected to a
 positive force, as depicted in FIG. 8 by the curve portion A6.
 This later phase of lowering the tank 14 corresponds to the phase depicted
 in FIG. 6, where the height B6 of the air bubbles is reduced. Furthermore,
 as the legs 12 are exerting a pulling force on the barge 10, the depth,
 denoted C6, to which this barge is submerged, increases. It is then, for
 example, on the order of 7 m, when the tank 14 is in close proximity to
 the floor.
 Throughout the phase corresponding to curve portion A6, the barge 10
 restrains the tank 14 to prevent the latter from descending too fast. In
 particular, the lifting mechanisms 18 are released slowly enough to allow
 the pressures inside and outside the tank 14 to reach equilibrium.
 Finally, once the tank 14 has been placed on the bottom via the soles 16,
 the barge 10 is raised up above sea level. The air contained in the tank
 is extracted, for example through the vertical pipework installed in the
 chords of the legs.
 The force exerted on the lifting means then corresponds to the point A7 in
 FIG. 8. In this position, the legs 12 are locked directly to the barge,
 avoiding prolonged strain on the lifting mechanisms 18.
 Once the platform has been installed as indicated earlier, the chamber
 formed by the tank 14 is used to store the oil extracted by the platform.
 It will be understood that throughout the lowering of the tank 14, the
 pressures inside and outside this tank are more or less always equal,
 which avoids high stress being applied to the walls of the tank. Thus,
 these walls can be made of relatively low-thickness plate.
 It order to re-site the oil platform, it needs to be returned to its
 transport position depicted in FIG. 2.
 To do this, the steps followed for lowering the tank 14 are reproduced in
 the reverse order.
 In particular, an air bubble with a height B7 is first of all injected into
 the tank.
 A pulling force is then exerted on the legs 12 using the lifting mechanisms
 18. This pulling force is exerted until such time as the tank 14 reaches
 the submersion depth Pi.
 Next, the tank 14 tends to rise to the surface of its own accord. To limit
 the speed at which it rises, the lifting mechanisms 18 exert thrust on the
 legs 12 so that they force the tank downwards. In this configuration, the
 barge 10 weighs down on the tank.
 Thus, the raising speed can be reduced so that at every moment the pressure
 difference between the inside and the outside of the tank is practically
 zero.
 Observing decompression steps allows water to flow satisfactorily through
 the valve 26 so that the pressure in the tank 14 is equal to the
 hydrostatic pressure at the depth to which the tank is submerged.
 Thus, in every circumstance, be this during installation, operation or
 dismantling, the tank is in pressure equilibrium.
 Furthermore, with a platform as described here, the force applied to the
 lifting mechanisms is relatively low for installing a large-capacity tank.
 What is more, when lowering and raising the tank, this tank acts as a
 stabilizer for the barge floating on the water.
 During manufacture, the special shape of the tank allows it to act as a
 floating dock which makes it possible for the barge and the legs to be
 manufactured directly on the floating tank.
 Under certain particularly arduous conditions, particularly when the sea
 bed is unstable, there is a risk that the underside of the base 17 may
 stick to the bottom.
 In this case, before raising the tank 14, the retainers 19 that secure the
 tank 14 to at least some soles 16 are released. This release may be
 performed, for example, by a diver. Thus, some of the soles 16 remain on
 the bottom, allowing the tank 14 to be raised.
 The situation is that, because of the thickness of the soles 16, when the
 base 17 is placed on the bottom, the central region of the tank 14 is
 spaced away from the bottom by a space I (FIG. 7) about 2 m tall. Thus,
 the surface of the underside of the tank 14 is not held firmly against the
 bottom by sticking and the tank can therefore be raised.
 Such soles may be used with any type of tank or submerged element intended
 to rest on the bottom of the sea and then be raised back up to the
 surface.