FR2726302A1 - METHOD OF CONSTRUCTION AND INSTALLATION OF A MARINE GRAVITY PLATFORM STRUCTURE AND PLATFORM STRUCTURE DESIGNED FOR THE IMPLEMENTATION OF THIS PROCESS - Google Patents

Abstract

The structure (1) is built dry on a surface (11) substantially horizontal and parallel to the axis of the column under construction, the structure (1) is placed in shallow water so that it floats while maintaining said axis in a substantially horizontal position, the structure (1) is straightened in deep water so that the axis of the column comes to occupy a substantially vertical position, the structure (1) is towed to the installation site, and the ballast is ballasted. structure so that it comes to rest on a seabed, in this same position.

Description

The present invention relates to a method of

  construction and installation of a platform structure

  gravity marine form and, more particularly, to such a method relating to a structure consisting of at least one support column for a bridge and a base secured to the

  column and designed to rest on a seabed.

  According to a known method of producing such a structure, the base or the base of the structure is constructed in dry dock, the structure having an axis oriented vertically, as in its final position on the site where it is to be installed. . Once the base is built, the dry dock is put in water and buoyancy means internal or external to the base ensure the buoyancy of the latter. The base is pulled out of the hold by towing and the construction of the structure continues in deep water so as to allow said structure to gradually sink into the water as it weighs down, while continuing to float. Once construction is complete, the structure is equipped with a bridge and is towed to its final installation site. Adequate ballasting of the base finally ensures its descent on the seabed on which it must rest. A final ballasting carried out with sea water or with solid materials ensures the stability of the structure

on the seabed.

  Due to the low draft of the dry dock, this known process often leads to oversizing the base so as to ensure its buoyancy under these conditions, compared to the dimensions that could be given to it to ensure its sole final function of bridge support and structural stability in deep water. In addition, this process requires that there is a well-sheltered deep water site near the dry dock, so that

  we can finish the construction of the structure.

  This last constraint can be eliminated by completely constructing the structure, base and column, in a vertical position, in dry dock. But then the oversizing of the base that must be provided to be able to float the assembly in the dry dock, necessarily with a low draft, when leaving this assembly, becomes prohibitive in cost of materials and labor - of work. Alternatively, to escape the draft requirements, one can resort to deepening the dry dock. However, this solution generally leads to

excessive costs.

  We also built a tanker loading structure called "MAUREEN articulated loading column" (see the British magazine OFFSHORE ENGINEER, pages 76-77, August 1982) for which the elements of the column and its base were assembled in dry dock so that the completed column rests horizontally on the bottom of the hold. After the hold has been filled with water, the column and the base are towed separately out of the dry dock, the column floating horizontally, and then assembled outside the hold by a cardan joint. The column is then straightened by ballasting until it occupies a vertical position perpendicular to its base, which is made possible by the joint, then it is equipped with a bridge and its equipment, towed to its site final and descended by gravity to the seabed which must

receive its base.

  When a platform structure must accommodate conductive tubes to allow the installation of wellheads on the surface, the structure described above cannot be envisaged because they would have to pass around the joint, which would singularly complicate its constitution. In addition, this concept is characterized by significant torsional forces at the joint both in the position of mounting the column horizontally with the base, and in the operational phase on the final site. Finally, the assembly of the column and the base out of the dry dock complicates and increases the cost.

construction process.

  The object of the present invention is to provide an economical process for the construction and installation of a gravity-type marine platform structure, eliminating the oversizing of the base of said structure induced by the temporary construction phases and the need to have a well sheltered deep water construction site, free of the drawbacks of articulated structures. This object of the invention is achieved, as well as other

  which will appear on reading the description which will

  follow, with a method of construction and installation of a gravity marine platform structure in reinforced or prestressed concrete, consisting of at least one support column for a bridge and a base secured to the column and designed to rest on a seabed, this process being remarkable in that the dry structure is built on a substantially horizontal surface and parallel to the axis of the column under construction, then the structure is placed in shallow water so that it floats by maintaining said axis in a substantially horizontal position, the structure is straightened in deep water so that the axis of the column comes to occupy a substantially vertical position, the structure is towed to the operating site and ballasted the structure so that it comes to rest on a

  seabed, in this same position.

  As we will see later, we can then put the structure in shallow water without being obliged to oversize its base for this, as is the case when we build such a structure in a vertical position. As the concrete structure is also entirely dry-built, it is no longer necessary to have a sheltered site in deep water to complete it. The base being integral with the column and forming a block with it, the structure is also free from the drawbacks of the

  articulated structure described above.

  According to a first embodiment of the invention, the quay structure is constructed, the said structure is then loaded onto a ballast barge and the structure is impounded by releasing the barge from the structure, by ballasting or by launching as for the metallic structures. According to another embodiment of the invention, we

  built the structure in dry dock.

  For the implementation of the method according to the invention, the invention provides a gravity platform structure consisting of at least one support column for a bridge and a base secured to the column, this structure being remarkable in that the column and the base are located on the same side of a substantially tangent common plane

parallel to the axis of the column.

  Thanks to this geometry, it becomes possible to construct or assemble the structure according to the invention on a horizontal surface substantially coinciding with

  said tangent plane, on a quay or in a dry dock.

  Other features and advantages of this

  invention will appear on reading the description which follows

  follow and examine the appended drawing in which: - Figures 1A to 1H schematically illustrate a first embodiment of the method according to the invention, - Figures 2A to 2C are three sectional views of an embodiment of the platform structure according to the invention, - Figure 3 shows a detail of a variant of said structure, - Figures 4A to 4C illustrate a variant of the construction and installation process according to the invention, - Figures 5A and 5B show another embodiment of the structure according to the invention, and - Figures 6A and 6B show yet another

  embodiment of the structure according to the invention.

  Reference is made to FIGS. 1A to 1H of the appended drawing, which schematically illustrate the successive stages of the method according to the invention, aiming at the installation of a gravity platform structure 1 on a predetermined site of a seabed 2 (see Figure 1H), so that the head of the structure carries a bridge 3 above the surface 4 of the water. The bridge is suitable for receiving all kinds of equipment for operating an "offshore" hydrocarbon production field, such as well heads, production equipment, living quarters or loading stations

to tank, for example.

  The structure 1 is shown in more detail in FIGS. 2A, 2B and 2C which are sections of this structure along the section lines A, B and C, respectively. In these views it appears that the structure, according to the embodiment shown, comprises a cylindrical column 5 of revolution, carried by a base 6 designed to rest on the seabed 2 and forming with the column a one-piece assembly. The base 6 includes ballast compartments 7, 7.0 capable of receiving materials

  heavy such as sand, gravel, iron ores, etc ...

  Some of these compartments can, as is well known, be closed and provided with sealing means so as to temporarily transform them into floats for towing the structure to its final installation site. Column 5 is hollow and can also be closed to form a float, before receiving

  water or other ballasting materials at the final site.

  The conductors or "risers" fitted to the structure preferably pass through the column, away from the swell, between the seabed and the deck 3 installed out of water on the head 9 of the column, as is will explain later. Buttresses 10 ,, 10, 103 ,, ... stiffen the anchoring of the

column 5 on the base 6.

  The structure is designed to be made of reinforced and / or prestressed concrete. After ballasting, it rests by

gravity on the seabed.

  According to a characteristic of the structure according to the invention, the column 5 and its base 6 are located on the same side of a common tangent plane P (see FIG. 2B), substantially parallel to the axis X of the column. The structure thus has an absence of axial symmetry which allows its manufacture by the method according to the invention, as explained below in conjunction with FIGS. 1A to 1H. According to a first embodiment of the method according to the invention, the structure 1 is formed or assembled on a platform 11 (see FIG. 1A) having a substantially horizontal surface. This construction then takes place conveniently from the plane P, then substantially merged with the surface of the quay, by conventional techniques in the art of reinforced concrete and / or prestressed concrete construction of works to be installed at sea. The column can itself be produced by assembling sections of cast columns

  vertically then assembled horizontally.

  The completed structure is then loaded onto a barge 12 (FIG. 1B) in shallow water, immediately adjacent to the quay 11. Such loading is possible for structures of medium dimensions, with a total height of 120 meters for example and of a total weight of around 15,000 tonnes (later fitted with a bridge of around 2,000 tonnes) intended for marginal or satellite fields which it is desired to equip with economically constituted platforms. In this regard, it should be noted that a conventional vertical construction of this structure, in a dry dock, would require limiting the height of the dry-constructed structure to around 30 meters, so that buoyancy can be ensured in shallow waters. deep in this structure, near the dry dock. It is then necessary to tow the part of the structure thus constructed in a site sheltered in deeper waters to complete the construction, which makes the costs excessive. The other classic solution consists in oversizing the base largely to reduce the draft of the part built in dry dock, solution

  also very costly in materials and labor.

  After loading the structure on barge 12, a

  tugboat 13 pulls the assembly in deep water (Figure 1C).

  After ensuring the horizontal buoyancy of the platform by guaranteeing the tightness of the column or part of the column and, possibly, of certain compartments of the base provided for this purpose, the barge is released of the structure by appropriate ballasting or any other means of launching (Figure 1D). The impoundment of certain ballasting compartments of the structure then allows a gradual recovery of the latter (FIG. 1E) to a substantially vertical position (FIG. 1F). The buoyancy of the platform is ensured in this position

  vertical if necessary using floats or counter

  additional weights (not shown) and the bridge 3 is deposited on the column head 9 using one or more loading cranes 14 carried for example by a pontoon 15. The structural connection between the bridge 3 and the column head 9. The structure is then towed (FIG. 1G) to the final site (FIG. 1H). On this site, the assembly is lowered slowly towards the seabed by an appropriate command for the progressive impoundment of column 5. The base of the structure then comes to rest on the bottom. Optionally, as shown in FIG. 2A,

  this base can be fitted with long spades 16 ,, 162 ...

  etc provided to sink into a soft seabed to ensure lateral stability of the structure, as taught in French patent FR-A-2 492 429. Adequate ballasting of the compartments 7, of the base with a

  ensures the stability of the structure on the seabed.

  A complete structure of gravity platform was thus installed in deep water, of 100 meters or more for example, having a bridge out of water suitable for receiving the planned equipment. Thanks to the horizontal construction method according to the invention, which allows transport of the complete structure on shallow depths, an economical construction structure is provided since it can be constructed entirely "dry" and that the base 6 is dimensioned as a function of its sole final foundation function and not as a function of the need to float a heavy structure with a low draft. The base can then have smaller dimensions, which is advantageous both from the point of view of the cost of the materials used and of the labor costs. FIG. 3 shows part of a variant of the structure shown in FIG. 2A. In this variant, the 5 'concrete column has a height such

  that it is completely submerged on its final site.

  It is then extended to above the surface 4 of the water by a wire mesh support 5 '' of height suitable for supporting the bridge 3 out of water. This support can be assembled to the column horizontally, during dockside construction, or on site using floating cranes. Such support greatly reduces

  stresses on the structure due to swell.

  FIGS. 4A to 4C show a variant of the method illustrated in FIGS. 1A to 1H. Instead of building the dock structure, it is built, still horizontal, in dry dock (Figure 4A). Once construction is complete, the wedge is put in water (Figure 4B) after ensuring the buoyancy of the horizontal structure. The structure is then towed out of the hold (Figure 4C). The installation process then continues as illustrated in FIGS. 1E to 1H. Such a dry dock construction makes it possible to save the barge 12 used in the process illustrated in FIGS. 1A to 1H in the case where the use of such a dry dock

  becomes more economical than using the barge.

  In the above, the bridge is installed while the structure floats in a vertical position. As a variant, this installation could obviously be provided once the

  structure set up by gravity on the seabed.

  The column of the structure could not be cylindrical of revolution and have a square or polygonal section. It could also have a circular section of variable diameter, so as to have a frustoconical appearance. The column section will also comply with the functional conditions of use such as the minimum interior diameter or dimensions, to allow the passage of the conductive tubes to be arranged by

example.

  FIGS. 5A and 5B show sectional views of another embodiment of the structure according to the invention, taken along the section lines A and B respectively. The structure comprises two columns 5, and 52 secured to the same base 6 'defining ballast compartments 7', to 7 ',. The cohesion of the structure can be reinforced by compartments 8, and 82 forming an intermediate spacer for the columns. Such a structure with two columns allows the installation, for example, of an elongated rectangular bridge, or else the use of the columns for two different functions,

  for example drilling and development of services.

  When the circumstances require it (for example an increased depth or special foundation conditions), the present invention also makes it possible to install on a seabed structures with symmetrical base, despite the asymmetry of the structures of FIGS. 2A, 2B and 5A, 5B. FIGS. 6A, 6B show a platform structure according to the invention having this characteristic. In the elevation view of FIG. 6A and in the plan view of FIG. 6B, it appears that the structure comprises a structure 1 such as that of FIGS. 2A, 2B, shown in dotted lines, associated with a complementary base element 6 '' shown in solid lines, consisting of ballast compartments 7 "1 'to 7"' 6 and symmetrical buttresses 10 'of the corresponding elements

of the base 6 of the structure 1.

  The additional 6 '' element is produced in the dry together with the structure. It is launched after its buoyancy has been ensured by any known means or after being attached to floating support pontoons. It is assembled to the structure 1 while the latter floats in a horizontal position. The complementary element is winched in the assembly position and this assembly is made permanent using prestressing cables. The assembly is then straightened and installed on site as illustrated in

Figures 1E to 1H.

  It now appears that the present invention makes it possible to construct and install gravity platform structures at sea in a particularly economical manner. Such structures are particularly, but not exclusively, intended for the exploitation of marginal or satellite oil fields. They join

  also to the concept of uninhabited platforms.

Claims (17)

  1. Method of construction and installation of a gravity marine platform structure (1) in reinforced concrete, consisting of at least one column (5; 51,52) supporting a bridge and a base ( 6; 6 ') integral with the column and designed to rest on a seabed, characterized in that: - the structure (1) is constructed dry on a substantially horizontal surface and parallel to the axis (X) of the column (5; 51, 52) under construction, - the structure is then placed in shallow water so that it floats while maintaining said axis in a substantially horizontal position, - the structure is raised in deep water so that the axis of the column comes to occupy a substantially vertical position, - the structure (1) is towed to the operating site, and the structure is ballasted so that it comes to rest   on a seabed (2), in this same position.   2. Method according to claim 1, characterized in that, before towing the structure to the installation site, a bridge (3) is installed on the column (5; 51,52) 3. Method according to claim 1, characterized in that a bridge (3) is installed on the column (5; 51,52) after the establishment of the structure (1) on the seabed (2). 4. Process according to any one of   claims 1 to 3, characterized in that the   structure (1) at the quay, the structure is then loaded onto a ballast barge (12) and the structure (1) is filled with water   by releasing the barge (12) from the structure (1).   5. Process according to any one of   claims 1 to 3, characterized in that the structure (1) in dry dock.   6. Process according to any one of   claims 1 to 5, characterized in that one builds   dry an additional element (6 '') of the base (6) and in that this element (6 '') is assembled to the rest of the base, after   the impoundment of the structure (1) and this element (6 '').   7. Process according to any one of   claims 2 to 6, characterized in that one builds   dry a structure (1) having a column (5 ′) of height such that this column is fully submerged on the final installation site, and in that a support (5 ″) is mounted on a lattice on said column ( 5 ') to support the bridge (3) out of water.   8. Gravity platform structure designed for the implementation of the method according to claim 1, consisting of at least one support column (5; 51,52) of a bridge (3) and a base (6; 6 ') integral with the column, characterized in that the column and the base are located on the same side of a common tangent plane (P) substantially   parallel to the axis (X) of the column.   9. Structure according to claim 8, characterized in that the base (6 ') supports at least two columns (51.52).   10. Structure in accordance with any of   claims 8 and 9, characterized in that the cross-section of   the column varies along the axis (X) of this column.   11. Structure in accordance with any of   claims 8 and 9, characterized in that the column is cylindrical in shape.   12. Structure in accordance with any of   Claims 8 to 10, characterized in that the column is frustoconical.   13. Structure in accordance with any of   Claims 8 to 10, characterized in that the cross-section of the column is polygonal.   14. Structure in accordance with any of   Claims 8 to 13, characterized in that an element   additional base (6 '') is designed to be assembled to the base (6) built with the column, according to the plan   tangent (P) to the column and the base.   15. Structure in accordance with any of   Claims 8 to 14, characterized in that the column   (5 ') is dimensioned to be fully submerged in the final position, the upper part of the column then being designed to receive a support (5' ') in trellis   metal which itself supports a bridge (3), out of water.   16. Structure in accordance with any of   Claims 8 to 15, characterized in that the base (6)   is provided with long spades (16,) for its anchoring in the seabed (2).   17. Structure in accordance with any of   Claims 8 to 16, characterized in that at least one   part of it is made of prestressed reinforced concrete.