FR2774967A1 - Barge for the development and exploitation of an oilfield at sea - Google Patents

Abstract

The submerged base (2) of a floatable sub-structure (1) contains oil storage compartments and a number of columns (3) extend upwards from the base to the water surface. A number of bridges (5), at least two of which are separately mounted on the columns above the water surface, contain equipment for different development stages of the oilfield. Process for installing the barge is also claimed. The bridges are installed as a function of the equipment used during successive phases of development and exploitation of the oilfield. At least one of the bridges is loaded with drilling equipment and at least one other is loaded with production equipment. The barge forms an elongated shape in plan. At least the top faces of the columns support equipment. The bridges can float and include means of directing themselves over the length of the columns during ballasting or unballasting together with temporary locking screws at the top of the columns. The columns contain floatability and ballast chambers. The barge is anchored to the sea bed by tethering lines. Process of installing such a barge, where (a) the sub- structure is loaded whilst floating with at least one bridge brought by transport barges; (b) the sub- structure is ballasted to lower it so the bridge floats on the water; (c) the ballasting is stopped when the base reaches a predetermined depth and at least one bridge is fixed to the base columns; and (d) the substructure is deballasted until the bridge reaches a predetermined height above the water surface, are further claimed.

Description

 The present invention relates to a barge for the development and exploitation of an oil field at sea and to a method for the installation of such a barge on such a field.

 There is described in French patent application No. 97 15774, filed on December 12, 1997 by the applicant, a semi-submersible platform for the exploitation of an oil field at sea and a method of installing this platform. It is a platform designed for oil fields located on very deep seabed, several hundreds of meters to 1500 m or more, for example. It includes a buoyant substructure consisting of a base of substantially square plan and of several columns projecting upwards from this base and carrying, above the water surface, a buoyant hull, property which is used to mount said bridge on the following substructure the installation process described in this patent application. This platform has a deep draft and means for storing petroleum products in its submerged base. In combination with an adapted anchoring, this arrangement minimizes the s pounding movements of the platform under the effect of the swell, which facilitates the installation of well heads on the hull deck.

 When it must have large dimensions, such a substantially square plane platform cannot be built in current shipbuilding installations designed for elongated ships. They must then be built in specially fitted dry docks, which increases manufacturing costs. In addition, the storage capacity of such a platform is, given the current state of the art, limited to a few hundred thousand barrels.

 In addition, such a substantially square or circular platform does not offer optimal hydrodynamic stability in marine environments where the swell develops in a preferred direction. In this case, the stability of a ship is optimized by orienting it facing the swell, its length then being aligned with the direction of the latter.

 In addition, the development and exploitation of an offshore oil field takes place in several successive phases, notably a first phase of exploration and drilling, followed by a second phase of production requiring larger surface facilities than the exploration and drilling phase.

 The installation, ab initio, of a platform with a surface suitable for both the initial drilling phase and the subsequent production phase, leads to a very significant initial investment.

 The present invention therefore aims to achieve a semi-submersible structure for the exploitation of such fields, of medium or great depth, capable of being built in particular in existing installations of shipyards, offering a storage capacity for petroleum products. from one to several million barrels and having good hydrodynamic stability with respect to a swell site of preferential direction.

 The present invention also aims to achieve such a structure designed to ensure optimal adaptation of the means to be installed on site to the operations carried out on it, during the various successive phases of operation, so as to reduce costs by rational spreading of the necessary investments.

 These objects of the invention are achieved, as well as others which will appear on reading the description which follows, with a barge for the development and exploitation of oil fields at sea, this barge being remarkable in that it comprises a) a buoyant substructure consisting of a fully submerged base comprising compartments for storing petroleum products extracted from an oil field, and a plurality of columns projecting upwards from said base to above the water surface and, b) a plurality of bridges, at least two of said bridges being separately mountable on said columns, above the water surface.

 Advantageously, according to the invention, said bridges are installed according to the equipment used during successive phases of the development and exploitation of an oil field.

 According to a preferred embodiment of the invention, at least one of the bridges is loaded with equipment used in the well drilling phase and at least one other bridge is loaded with equipment and other installations used only in the product production phase tankers.

 As will be seen below, by successively mounting these bridges, and the associated equipment, on the barge substructure, the cost of the equipment installed on it is adjusted to actual needs, throughout the successive phases of development and production of an oil field.

 According to an advantageous characteristic of the barge according to the invention, it has, in plan, an elongated shape. This elongated shape, analogous to that of a ship, allows the construction of the barge in existing shipyard installations, improves the hydrodynamic stability of the barge for preferential swell directions, and makes it possible to spare in the substructure of the petroleum product storage compartments with a capacity of one to several million barrels.

 According to the invention, to install such a barge on an oil field at sea, a) the substructure fitted with at least one bridge supporting the equipment used during a first phase of development and operation is installed on this field of said field and b) at the end of said first phase, at least one other bridge supporting said equipment and other installations used during a second phase of development and operation of the field is installed on said structure.

Other characteristics and advantages of the present invention will appear on reading the description which follows and on examining the appended drawing in which
FIGS. 1 and 2 are views in elevation and in plan, respectively, of a first embodiment of the barge according to the invention,
FIG. 3 is an elevation view of a variant of the barge of FIGS. 1 and 2,
FIGS. 4A to 4H illustrate various successive stages of an embodiment of the method for installing the platform of FIGS. 1 and 2,
FIGS. 5A to 5G illustrate various successive stages of another embodiment of this method, suitable for installing the platform for a drilling phase,
FIGS. 6A to 6D illustrate various successive stages of the conversion of the platform according to the invention, from a drilling phase to a production phase, and
- Figures A to 7E illustrate a variant of the conversion illustrated in Figure 6.

 Referring to Figures 1 and 2 of the accompanying drawing where it appears that the barge shown comprises a semi-submersible substructure 1, concrete or steel for example, consisting of a base 2 fully submerged and a plurality of columns 3i (i from 1 to 8) projecting upwards from the base 2, up to the surface 4 of the water. The barge differs from the semi-submersible platform described in the aforementioned patent application first in that it comprises, not a single bridge mounted on the columns 3i, but a plurality of bridges 51,52,53 capable of '' be assembled in a variable manner to constitute, possibly with the upper faces of the columns 3i (see Figure 1), a modular operating surface according to needs, throughout the various successive phases of drilling and production of a field submarine tanker on which the barge is installed. The advantage of this characteristic of the barge according to the invention will be detailed below, in connection with FIGS. 5A to 7E, during the description of the method for installing the latter.

 The bridges 51, 2, 53 are of the buoyant "hull-bridge" type capable of sliding along the columns, to the top of the latter where locking means (not shown) fix the bridges to these columns, so that after a suitable ballasting / deballasting sequence of the substructure 1, these bridges are each fixed on four columns above the surface 4 of the water, with a suitable predetermined freeboard. Such a sequence is described in the aforementioned French patent application. It will be recalled later in connection with the examination of FIGS. 4 to 7.

 The columns 3i are regularly distributed over the periphery of the base 2, at the ends of the latter as well as one third and two thirds of its length. This is how the columns 31, 32, 37 and 38 support the bridge 51, the columns 32, 33, 36, 37 support the central bridge 52, and the columns 33, 34, 35, 36 support the bridge 53.

 The deck of the barge according to the invention supports the various equipment necessary during the successive phases of drilling the oil field for the production of products extracted from it. These pieces of equipment are conventional and thus comprise, by way of non-limiting example, a derrick 6 placed above a group 7 of wells extending to the seabed, these pieces of equipment being installed on the central deck 52 with the well heads and a system 8 for discharging petroleum products.

 The bridge 53 can carry, as shown, means used in the production phase: water injection means, separation means, etc., as well as a handling crane 9.

 The bridge 51 can receive various electrical equipment 10, an electrical energy generator 11, another handling crane 9 ', etc.

 Other equipment can be mounted on the columns 3i themselves, for example a helicopter landing deck 12, living quarters 13 on the column 38, a storage area on the column 37, a flare 14 on column 34, etc., etc.

 All the equipment mentioned above is conventional and does not require a detailed description.

 Footbridges (not shown) allow the movement of personnel between different equipment carried by columns 3i and bridges 51,52,53.

 The substructure 1 of the barge according to the invention, constituted by the base 2 and the columns 3i, ensures the flotation and ballasting of the barge, the storage of petroleum products and the like and accommodates various auxiliary equipment.

 Mooring lines 15d (d from 1 to 12) anchor the barge on the seabed.

 The storage of petroleum products operates according to the principle of water-oil displacement which is known in the art and in particular used for structures for the storage of petroleum products under water, so as to eliminate the differential in hydrostatic pressures and perform thus saving materials.

 The flotation of the barge is ensured by buoyancy compartments (not shown) filled with air, distributed in the various columns 3i. These columns can also accommodate ballast compartments (not shown) loaded with seawater. This makes it possible, in particular, in cooperation with appropriate pumps and control systems, to ensure "dynamic" ballasting of the barge, so as to maintain the freeboard of the deck at a predetermined value when the quantities of water and petroleum products contained in the base vary, in order to take into account the difference in density of the liquids stored.

 Columns 3i can also accommodate, in special compartments, part of the production of the barge. However, this production is stored, essentially, in the base 2. It is understood that this can reach a very large volume suitable for storing large quantities of petroleum products, one or two million barrels or more for example , this in accordance with one of the aims of the present invention.

 In addition, this large quantity of petroleum products can be stored at a sufficient depth for a floating structure, at 40 m or more for example, which is very favorable to the stability of the barge.

 In this respect, the plan view of FIG. 2 shows that the barge according to the invention has an elongated rectangular shape, by virtue of the on-line arrangement of the bridges 51,52,3 By way of illustrative and nonlimiting example , the barge can have a length of 250 m and a width of 60 m, with a draft of around 40 m. It is then possible to orient the longitudinal axis of the barge parallel to the preferential direction of the swell, the elongated shape of the barge giving it characteristics of hydrodynamic behavior much superior to those of a platform with a square plane. or circular for example, this in accordance with another object of the present invention.

 It will also be noted that the elongated shape of the barge according to the invention makes it possible to build it in existing basins designed for conventional ships of the same elongated shape, unlike a square or circular platform of large capacity whose space would then exceed the width of the basin, this in accordance with yet another object of the invention.

 Of course, the invention is not limited to a barge comprising three bridges. It could, depending on the needs of a particular oil field, include a different number of bridges, arranged in one or more parallel lines. Thus, FIG. 3 represents another embodiment of the present invention, comprising two buoyant bridges only 5'1.5'2, mounted on a structure 1 'with base 2' and six columns 3'1 to 3 Here again, as will be seen below, equipment used during the drilling phase is advantageously combined on one bridge and production equipment on another. This equipment is moreover conventional and is referenced in FIG. 3 as the equipment of the same type carried by the barge of FIG. 1. It will however be noted that it is then possible to pass the group of wells 7 into a column 3 ' 2 of the substructure 1 ′, this to reduce the environmental stresses on the wells.

 Reference is now made to FIGS. 4A to 4G of the appended drawing to describe an embodiment of the method for installing the barge according to the invention.

This is analogous to the method of installing the platform described in the aforementioned French patent application, to which reference may be made for more details.

 After construction of the substructure 1 in a dry basin (FIG. 4A), it is put in water and the base is towed (FIG. 4B) to a site where it is loaded with the bridges 51, 52 , 53, unloaded on the base 2 by shifting, from transport barges such as that referenced 20 in FIG. 4C.

 The bridges 51, 52, 53 thus resting on the base (FIG. 4D), the barge is towed to its final installation site (FIG. 4E). Ballasting of the substructure then makes it descend into the water while the bridges, thanks to their buoyancy, remain on the surface of the water, the columns 3i of the substructure sliding along the bridges 51r52r53 thanks to lateral guide means (not shown) such as those described in the aforementioned French patent application. The latter also describes specific locking means for temporarily fixing the bridges at the top of the columns, in the position shown in FIG. 4F. We then proceed as necessary to the lifting and installation of equipment and other additional installations on the fitted bridges. The substructure is then deballasted to raise the bridges 51, 52, 53 above the surface of the water (FIG. 4G) with a predetermined freeboard.

 According to another embodiment, the bridges and other equipment are installed in a sheltered site and the fully equipped barge is towed to its final operating site.

 After laying the mooring lines, the wells of group 7 are drilled (Figure 4H). The equipment installed on deck 53 and intended for the production phase only is partially used, some remaining inactive until the end of the drilling phase.

 It is understood that the immobilization of the capital necessary for the production of this inactive production equipment, throughout the duration of the drilling phase, is disadvantageous from the economic point of view.

In addition, the supply times for this equipment delay the start-up of the platform.

 According to an implementation of the method of installing the barge according to the present invention, which will be described in connection with the examination of FIGS. 5A to 7E, this drawback is eliminated by spreading out over time the installation of the various bridges on their final site.

 FIGS. 5A to 5G, which correspond substantially to FIGS. 4A to 4G, illustrate the construction and installation of a barge equipped with the only bridge 52 carrying the materials necessary in the drilling phase by which the development and operation of the oil field located on this final site.

 It is only after the drilling of the wells which, possibly, can be done from several positions on the oil field, that the barge receives the bridges 51 and 53, on the final site where it is installed. Two alternative procedures, illustrated respectively in FIGS. 6A to 6D and in FIGS. 7A to 7E can be used, according to the present invention, for mounting and fixing the bridges 51 and 53 on the barge.

 According to the procedure shown in FIGS. 6A to 6D, this consists in reballasting the barge to bring the top of the columns close to the surface 4 of the water. Two transport barges such as those referenced 21 in FIG. 6A bring the bridges 51 and 53 against the barge.

These transport barges are then engaged between the columns intended to support the bridges. The barges 21 are ballasted, which makes it possible to place the bridges on the columns (FIG. 6B). The barge 1 is deballasted to take up the load on the bridges and to raise all of the bridges above the water, with a predetermined freeboard (FIG. 6C). The anchor lines and the wells, which had been disconnected before the step illustrated in FIG. 6A, are then reconnected to the barge.

 According to the alternative procedure described in FIGS. 7A to 7E, the barge carrying the bridge 52 is deballasted so as to bring the upper surface of the base 2 of the substructure 1 in the vicinity of the surface 4 of the water, to a level making it possible to transfer by bridging the bridges 51, 53 to this upper surface from barges, such as the transport barge 21, brought near this surface (FIG. 7A).

 The substructure is then reballasted to put the bridges 51, 53 in waterline (FIG. 7B), these then progressively rise to the level of the bridge 52 during the descent of the substructure 1 (FIG. 7C).

The locking means of the bridges 51, 53 are then actuated, to fix them on their respective columns 3i, at the same level.

 The substructure 1 is then deballasted to raise all of the bridges above the surface of the water, with a predetermined freeboard (FIG. 7D). The anchor lines 15j and the wells of group 7 are then reconnected (Figure 7E).

 Whatever the procedure used to separately mount the bridges 51,53 on the substructure, it is understood that these bridges can be delivered after the drilling phase. Their construction can therefore be delayed compared to that of the bridges installed according to the procedure illustrated in FIGS. 4A to 4H, a procedure which requires that the three bridges be available even before the start of the drilling phase. The investments thus delayed provide the procedures described in connection with FIGS. 5A to 7E with a significant economic advantage.

 Of course, the invention is not limited to the installation methods described above which are given only by way of example. The flexibility of the invention allows the installation of the equipment according to the most diverse uses in an order which best corresponds to the needs of the users. In particular, it is perfectly possible to use the solution of interchangeable bridges, according to which one replaces, for example, a production bridge by another differently equipped.

Claims (14)

 1. Barge for the development and exploitation of offshore oil fields, characterized in that it comprises  a) a buoyant substructure (1; 1 ') consisting of a fully submerged base (2; 2') comprising compartments for storing petroleum products extracted from an oil field, and a plurality of columns (3i ; 3'i) projecting upwards from said base (2; 2 ') up to the surface (4) of the water and,  b) a plurality of bridges (51,52t53), at least two of said bridges (51,52,53; 51,52) being separately mountable on said columns (3i; 3'i) above the surface of 1 ' water.  2. Barge according to claim 1, characterized in that said bridges (51, 52, 53; 51, 52) are installed according to the equipment used during successive phases of the development and exploitation of an oil field.  3. Barge according to claim 2, characterized in that at least one (52) of said bridges is loaded with equipment used in the well drilling phase and in that at least one other bridge (51,53) is loaded with equipment and installations used only in the production of petroleum products.  4. Barge according to any one of claims 1 to 3, characterized in that it has, in plan, an elongated shape.  5. Barge according to any one of claims 1 to 4, characterized in that at least one of the upper faces of the columns (3i) supports equipment.  6. Barge according to any one of claims 1 to 5, characterized in that said bridges (51,52,53; 5'1,5'2) are buoyant and in that it comprises means for guiding the bridges (51,52,53; 5'1,5'2) along their support columns (3i; 3'i) during ballasting or deballasting of the substructure (1; 1 '), and temporary locking means of said bridges (51t52.53; 5'1.5 '2) at the top of said columns (3i; 3'i) -  7. Barge according to any one of claims 1 to 6, characterized in that said columns (3i; 3'i) include buoyancy compartments and ballast compartments.  8. Barge according to any one of claims 1 to 7, characterized in that it is anchored on the seabed by mooring lines (15d).  9. A method of installing a barge according to any one of the preceding claims, according to which  a) the substructure (1; 1 ') is loaded in flotation with at least one of said bridges (51,52,53; 5'1,5'2) provided by transport barges (20; 21),  b) the substructure (1; 1 ') is ballasted to bring it down to the bottom while said at least one bridge (51,52,53; 5'1,5'2) floats on the surface of the water,  c) the ballasting of the substructure (1; 1 ') is stopped when its base (2; 2') reaches a predetermined depth and the said at least one bridge is then blocked on columns (3i; 3'i) of said base, and  d) deballasting said substructure (1; 1 ') until said at least bridge is raised to a predetermined height above the surface of the water.  10. The method of claim 9, wherein  a) the substructure (1) fitted with at least one bridge (52) supporting the equipment used during a first phase of development and operation of said field is installed on an oil field, and  b) at the end of said first phase, at least one other bridge (51, 53) is installed on said structure supporting equipment used during a second phase of development and operation of the field.  11. Method according to claim 10, characterized in that said first and second phases are phases of drilling wells and producing petroleum products, respectively.  12. Method according to claim 10, characterized in that, in step a), the buoyant substructure (1) is towed at sea, a bridge (52) is loaded on the base (2) of said sub- structure (1) by shifting said bridge from a barge (20) for transporting this bridge (52), the substructure (1) is ballasted which sinks into the water until the bridge (52), floating on the surface (4) of the water, is at the level of the upper face of the columns (3i) of the substructure, the bridge (52) is fixed on the adjacent columns, and the ballast is unpacked. substructure (1) for raising the deck (52) above the surface (4) of the water, with a predetermined freeboard (FIGS. 5A to 5G).  13. Method according to claim 10 or 12, characterized in that, in step (b), the substructure (1) is ballasted to bring the top of the columns near the surface (4) of the water , one engages on said substructure (1) at least one bridge (51,53) between columns of the substructure, one fixes said bridge (51,53) on said columns and one deballasts the substructure to raise the set of bridges (51t52,53) above the surface (4) of the water, with said predetermined freeboard (FIGS. 6A to 6D).  14. Method according to claim 10 or 12, characterized in that, in step (b), the substructure is unballasted to bring the upper face of the base (2) of the substructure in the vicinity of the surface (4) of the water, said sub-structure is loaded with at least one bridge (51, 53) deposited on said base (2) between columns of the sub-structure (1), the said sub-structure is reballasted to bringing said bridge (51,53) by flotation to the level of the upper face of the columns which surrounds it, said other bridge (51,53) is fixed on said columns and the substructure (1) is de-ballasted to raise the set of decks (51, 52, 53) above the surface (4) of the water, with said predetermined freeboard (FIG. 7A to 7E).