Title:
Production and flare caisson system
United States Patent 3881549


Abstract:
A production and flare caisson system in which the caissons are anchored in the ocean floor by conductor pipes or piles extending into the ocean floor and rigidly attached to a template at the mud line. A spud rigidly attached to the template is imbedded into the floor through an opening generally centrally of the template, with the caisson being lowered into the spud and clamped thereto. In the production caisson a production separator and the production strings and flow lines are housed within the caisson thereby reducing the equipment normally stored on deck, or exposed to the marine environment. All components are prefabricated, preassembled, tested, and disassembled prior to being reassembled at the site. Installation can be effected by employing a drilling rig thereby eliminating the need for costly derrick barges.



Inventors:
THOMAS JOHN P
Application Number:
05/354949
Publication Date:
05/06/1975
Filing Date:
04/27/1973
Assignee:
INTERSEAS ASSOCIATES
Primary Class:
Other Classes:
166/366, 166/367, 405/203
International Classes:
E02B17/00; E21B41/00; E21B43/01; (IPC1-7): E21B43/01
Field of Search:
166
View Patent Images:
US Patent References:
3638720METHOD AND APPARATUS FOR PRODUCING OIL FROM UNDERWATER WELLS1972-02-01Thomas
3612177DEEP WATER PRODUCTION SYSTEM1971-10-12Gassett
3601189N/A1971-08-24Weiss
3504741UNDERWATER PRODUCTION SATELLITE1970-04-07Baker
3380520Drilling and production platform1968-04-30Pease
3366173Subsea production system1968-01-30McIntosh
3224204Method of anchoring an offshore structure1965-12-21Siebenhausen



Primary Examiner:
Purser, Ernest R.
Assistant Examiner:
Favreau, Richard E.
Attorney, Agent or Firm:
Jeffery, Donald D.
Claims:
I claim

1. A caisson system for offshore oil production comprising:

2. The system of claim 1 wherein said template is prefabricated and is comprised of a series of radially extending ribs and interconnecting support members, said radially extending ribs terminating centrally to form an opening for receiving said spud, the diameter of said spud being slightly larger than the outside diameter of said caisson, with the annulus between such members being grouted to prevent wobbling of the caisson and to evenly distribute the moment and shear load from the caisson to the spud and thus to the template and conductor pipes.

3. The system of claim 1 wherein wellheads are operatively connected at approximately the mud line to the top of said conductor pipes.

4. The system of claim 1 further including production risers extending upwardly from said conductor pipes, and wellheads operatively connected to said risers, the elevation of said wellheads above said ocean floor serving to facilitate maintenance and service of said wellheads.

5. The system of claim 1 further including a production separator positioned vertically in said caisson, wellheads operatively connected to said conductor pipes at the mud line, and flow lines from said wellheads extending upwardly within said caisson to a production manifold and to said production separator for conveying the recovered oil to said separator.

6. The system of claim 5 wherein said caisson is positively buoyant when preassembled with said production separator to facilitate transportation and installation thereof.

7. A flare caisson system for offshore oil production comprising:

8. The system of claim 7 further including a knockout tank in said caisson, and wherein said caisson is positively buoyant when preassembled with said knockout tank to facilitate transportation, and installation thereof.

9. The method of installing an offshore production caisson comprising the steps of:

10. The method of claim 7 further including the step of preassembling the caisson and a production separator in prearranged structural relationship so as to provide an assembly having positive buoyancy, and transporting said assembly by towing to the site.

Description:
BACKGROUND OF THE INVENTION

The present invention relates as indicated to a production and flare caisson system adapted to be anchored to the ocean floor for use in the production of offshore oil and gas.

In the drilling and production of offshore wells, relatively large structures are typically employed which are anchored in the ocean floor and which extend upwardly above the water surface to a distance above the maximum wave forces encountered in that particular area. Typical installations include standard tubular truss type templates which extend above the water surface where the wellheads and production equipment are located. The installation costs of these structures are high for several reasons; the steel tonnage requirements are high, complex and costly fabrication is necessary to form such structures owing to difficult welding and coping operations; the towing and loadout charges for transporting the structure to the site are likewise very costly, and the mobilization/demobilization charges coupled with high day rates for derrick barges required to install the structures are similarly very costly. Those skilled in the art will also readily recognize the time-consuming and thus costly procedures required in the installation of truss type templates heretofore used in offshore operations of the type concerned here.

When the wellheads and all production equipment are located above maximum wave height, the resulting system has the further disadvantage of having excessive loading on the deck and template structure thereby increasing the forces and stresses on the system which the anchored structure must be designed to withstand.

SUMMARY OF THE INVENTION

The present invention provides an offshore system which greatly reduces steel requirements, simplifies fabrication, substantially reduces towing charges, and obviates the need for derrick barges for the installation. In accordance with the present invention, the drilling rig, which is normally required at the site, can be utilized to install the entire system.

A further, more specific object of the present invention includes the provision of a prefabricated template which is positioned on the ocean floor and which is firmly anchored to the floor by means of conductor pipes which serve not only as structural members but also as well conductors during drilling and production operations. After anchoring the template to the sea floor by means of the conductor pipes, a spud section is drilled into the sea floor and fastened to the template for receiving the production caisson, with the latter being firmly clamped to the spud section, with the spud, template, and conductors thereby providing a system which efficiently distributes the load imparted by the caisson to the subsoil.

A further object of the present invention is to provide a system wherein subsea wellheads are employed which are located either on the sea floor or at positions intermediate the water depth. The caisson system in accordance with the present invention is specifically designed to operate in relatively shallow water depths, for example, 150 feet or less, whereby the subsea wellheads can be installed and serviced inexpensively and conveniently by a diver.

Another object of the present invention is to provide a caisson system in which certain of the production equipment normally stored on deck or exposed to the marine environment is housed within the caisson. In accordance with the invention, the production separator is positioned vertically within the caisson, along with all production strings and flow lines, which arrangement not only eliminates the weight on the deck which normally supports these members, but also avoids the dangers associated with outside installation and exposure of these members, for example, accelerated corrosion and surface hazards caused by contact with towboats, work boats, crew boats, supply barges, workover rigs and the like.

A still further object of the present invention is to permit all components of the caisson system to be prefabricated, preassembled and tested thereby serving not only to reduce costs but to ascertain that each section component and subsystem is compatible with its mating section before reassembly and installation. The installation time is thus considerably reduced and utilized more efficiently.

Another object of the invention is to design a caisson and template system in such a way so as to preclude the necessity of employing heavy lift equipment thereby permitting the drilling rig to effect installation by use of its own crane and drilling equipment. Eliminating the need of derrick barges results in great cost savings inasmuch as the mobilization/demobilization charges are eliminated as well as the day rates for this equipment, which run generally double the rates for drilling rigs. It is therefore unnecessary to duplicate installation capability.

A still further object of the invention is to provide a system such that the transportation charges such as load out, barges, launching and towing are substantially reduced. This is accomplished by designing the system into subsystems and modular subsystems such as caissons, templates, fenders, deck sections and wellheads. In this way the caisson can be towed without a barge because it has sufficient positive buoyancy to float by its inherent buoyancy.

BRIEF DESCRIPTION OF THE APPLICATION DRAWINGS

FIG. 1 is a perspective view of the production and flare caisson system in accordance with the present invention, with certain areas being broken away to expose the full structure in accordance with the invention;

FIG. 2 is a partially sectioned side elevational view of the production caisson showing more particularly the manner in which the same is anchored in the sea floor through the spud, template, and conductor pipes. An alternative positioning of the subsea wellheads is shown in dash lines in this figure;

FIG. 3 is a sectional view taken on line 3--3 of FIG. 2;

FIG. 4 is a plan view of the deck of the caisson which extends above the water line;

FIG. 5 is a partially sectioned side elevational view of the flare caisson in accordance with the present invention, showing more particularly the manner in which the flare caisson is anchored to the sea floor; FIG. 6 is a sectional view taken on line 6--6 of FIG. 5;

FIG. 7 is a top plan view of the deck of the flare caisson;

FIG. 8a shows a sequence in the installation of the system, this figure schematically showing the lowering of the template by a crane on the drilling rig;

FIG. 8b shows a subsequent step in the installation sequence, illustrating the drilling rig positioned over the site and the conductor pipes installed through the template;

FIG. 8c shows schematically a further step in the installation sequence, with the caisson being shown towed to the site and transferred to the crane boom which is located over the previously installed conductor piles, template and spud, and

FIG. 8d illustrates schematically a further step in the installation sequence, with the caisson shown lowered by the crane boom into the spud for rigid connection thereto.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now in detail to the application drawings, wherein like parts are indicated by like reference numerals, and initially to the entire system shown in perspective view in FIG. 1, the system includes a production caisson generally indicated at 10 and a flare caisson generally indicated at 12, both of which are anchored to the sea floor in accordance with the novel concepts of the present invention. The caisson has mounted at the top thereof a deck generally indicated at 14 which can be substantially reduced in size when compared with prior art systems by virtue of the housing of a production separator, generally indicated at 16, inside the caisson 10 as clearly shown in FIG. 1. Mounted around the caisson at slightly above water line is a fender and walkway generally indicated at 18 for servicing the system.

The caisson 10 is anchored to the sea floor 20 by means of a template generally indicated at 22 which is in turn rigidly connected to conductor pipes commonly designated at 24 which are installed into the sea floor and thereafter rigidly connected to the template to form an integral structural unit. A spud 26 is similarly installed into the subsoil at generally the center of the template, with the caisson 10 being lowered into the spud and rigidly locked thereto so as to effectively distribute the moment and shear load from the caisson to the spud and to the template and conductor pipes. The conductor pipes in accordance with the present invention perform the dual function of both serving as supporting piles for the entire structure and for reservoir drilling and production. Thus, wellheads commonly indicated at 28 are shown installed on the production casing 29 positioned within the conductor pipes for controlling production from the reservoir. It will be understood that the standard and usual drilling and completion methods employed in subsea operations will be followed. The details of this structure have accordingly not been illustrated in the drawings.

It should be noted that in the system illustrated, three conductor pipes are employed substantially equally spaced around the template, but it will be apparent to those skilled in the art that a greater or lesser number of conductor pipes could be employed in accordance with the invention concepts.

A gas flare line 30 communicates the production caisson 10 with the flare caisson 12, and flow line 32 conveys the oil produced by the system to a tanker T moored adjacent the production system. A power and control line 34 is provided between the tanker T and the system for supplying the necessary power and control functions to the system. The construction and function of the tanker T is conventional and forms no part of the present invention, with the tanker being illustrated only for a more clear understanding of the present invention.

Before proceeding directly to a description of the structural details of the invention as illustrated in FIGS. 2-7, a better understanding of the unique concepts of the present invention can be had by brief reference to the installation sequence of the system. The template, conductor pipes, spud, and production caisson, and the comparable components in the flare caisson assembly, are conveyed to the preselected site at which a drilling rig is located. An important feature of the present invention is to permit the complete installation of the entire system by means of such drilling rig thereby eliminating the excessively high costs of employing derrick barges with their attandent high mobilization charges and day rates.

To install the system, referring to FIG. 8a, the template 22 of the production caisson is lowered to the mud line ML by means of a crane C on the drilling rig R. The drilling rig is then moved to a position over the template, referring to FIG. 8b, and the conductor pipes lowered by the rig to the template and through the preformed openings in the template. Each such opening in the template for the conductors is provided with a stabbing funnel to facilitate location of the conductor in the opening therefor in the template.

The installation of the conductor pipes can be accomplished by drilling, driving or other standard techniques so as to firmly imbed the conductor pipes in the soil. Any annulus formed between the soil and the conductor pipe during installation thereof is preferably filled with grout so as to achieve a positive bond between the soil and the conductor pipe.

During the installation of the conductor pipes, they are firmly attached or fastened to the template by standard oil field mechanical connectors such as breech blocks, rings, clamps, flanges or the like for transferring the load from the template to the conductor pipes. A satisfactory type of mechanical connector is the breech block connector commerically sold by Rockwell Manufacturing Company for large diameter conductor pipe, casing and piling. Such connector is shown in the brochure of Rockwell Manufacturing Company entitled UV-421. The breech block connector when employed in the present invention includes a female part welded to the sleeved openings in the template receiving the conductor pipes, and a male part welded to the top of the conductor pipe and adapted to be locked to the female part when the parts are adjacently disposed and fully engaged. Other types of mechanical fasteners that can be employed are the connecting rings manufactured by Ventura Manufacturing Company and OCT Company.

After the conductor pipes have been installed, a spud section is installed in the soil under the central region of the template for receiving the caisson. The spud is installed to its designed depth by either jetting, driving, drilling, augering, dredging, or any other known method, and the spud is thereafter installed in such opening and grouted to the soil and rigidly attached to the template.

The soil is removed from inside the spud to a depth sufficient to receive the lower section of the caisson during installation of the same.

Wells are then drilled through the conductors, with all casing, tubing, blowout preventers and wellheads being supported by the casing within the conductors in accordance with standard production techniques.

The production caisson is then towed to location in a horizontal position, referring to FIG. 8c, righted by controlled flooding and lowered downwardly by the crane toward the previously prepared spud. The spud, like the template openings, is provided with a stabbing funnel to faciliate location of the caisson in the spud section. The caisson is then lowered, referring to FIG. 8d, in the spud section, locked into the spud or template, and thereafter the annulus between the spud and caisson is grouted in order to prevent wobbling of the caisson and to evenly distribute the moment and shear load from the caisson to the spud section and thus to the template and the conductor pipes and ultimately into the soil.

After the caisson is installed, the fender, walkway and work deck are installed on top of the caisson and fastened into position. The vertical production separator previously positioned within the caisson together with all production strings and flow lines are then connected to the respective piping on the template and work deck. Extending the production strings and flow lines inside the caisson completely avoids the danger of damage to the otherwise normally exteriorly exposed lines.

Additional needed equipment is included in a prepackaged modular deck section which is mounted on the deck of the production caisson. The deck section can be made considerably smaller than in prior art systems by virtue of the placement of the production separator within the caisson.

The installation of the flare caisson is substantially the same, except that piles are employed rather than conductor pipes. However, the installation sequence is similar, and following installation of the flare caisson, the flare line is installed from the production caisson to the flare caisson.

Having described the general installation sequence of the system, reference will now be made to the structural details of the invention, with initial reference being made to FIGS. 2-4 which illustrate the production caisson.

The template 22 transfers the loads imparted to the caisson into the conductors and comprises a central hub 40 from which extend radial rib plates commonly designated at 42, with the ribs tapering from a full height at the hub to a reduced height at the periphery of the template. Additional truss or bracing members commonly designated at 44 are provided and interconnected by welding with other such bracing members and the radial webs 42 to form the completed template structure, with openings being provided in the resulting framework in circumferentially spaced locations for receiving the conductor pipes 24. Cylindrical sleeves 45 are secured in the openings and form part of the prefabricated template. The template can be fabricated of any suitable metal material such as structural steel. The template is also pre-equipped with piping and control lines necessary for connecting marine flow lines to the caisson flow lines and to wellhead equipment.

The spud 26 is likewise preferably of structural steel and is slightly greater in diameter than the external diameter of the caisson 10 and fastened to the template, with the annulus therebetween being grouted as above described to further prevent its movement.

The conductor pipes are likewise preferably formed of structural steel and are fastened to the template by means of locking devices schematically shown at 46. As above noted, the locking members are standard oil field mechanical connectors, and have accordingly been shown only schematically in the application drawings. The spud section 26 is likewise rigidly connected to the template through locking devices 48 which can be similar in structure and function to the locking device 46.

The annulus between the conductor pipes 24 and the soil is filled with grout 50 to rigidify to the extend possible the conductor pipe installation. It will thus be seen that the template 22 is rigidly secured to the sea bottom through the conductor pipes 24, with the spud section 26 being rigidly connected both to the template 22 and the sea floor thereby providing a rigid structure for receiving the caisson 10 and transferring all loads into the surrounding soil foundation.

After the caisson 10 has been lowered into the spud section 26 as above described, the caisson is locked to the spud section by mechanical connectors such as the fastening devices previously described so as to prevent motion between the caisson, the spud and the template. The clamps 48 can be employed for locking both the spud and the caisson to the template.

As mentioned above, a production separator 16 is preassembled in the caisson 10 and an annulus 60 separates the same from the inside of the caisson 10, and all wellhead production strings and flow lines are located in the annulus. In the application drawings, production strings commonly designated at 62 are shown connecting the wellheads 28 with the production manifold on deck, with flow lines 70 extending from the manifold to the separator 16 inside the caisson 10. As previously explained, the wellheads 28 are conventional and have been illustrated only schematically in the application drawings.

The fender and walkway 18 secured to the exterior of the caisson slightly above the water line (WL) is provided for servicing the caisson, and access doors or openings can be provided in the caisson wall for maintenance and repair of the separator. A ladder 64 extends between the walkway 18 and deck 14 for access to the deck 14 from the walkway. Routine maintenance of the system is normally performed by personnel transported to and from the system by work boats which transfer the personnel to the walkway or fender so they are able to perform the necessary operations on the work deck.

The equipment mounted on deck 14 is typical of that ordinarily used in offshore facilities of this type, such as a production manifold 80, test separator 82, pig launcher 84, wire line unit 86, and navigational aides 88 necessary for this type of above water equipment. All of these components, being conventional, are shown schematically in the application drawings, including FIG. 4. The conductor pipes 24 appear in dash lines in FIG. 4 to indicate the spacing of the same relative to the size of the deck 14, and flare risers 90 are shown in the annulus 60 between the production separator 16 and the caisson 10. The walkway 18 is also shown in dash lines in FIG. 4, as are bumpers 92 which extend outwardly from the walkway for preventing damage to the caisson from work boats, crew boats, etc.

Referring to FIG. 3, there is illustrated in the annulus 60 between the separator and caisson 10 pairs of production strings commonly designated at 90 which communicate with the associated wellhead through spool pipe sections 92. A production riser 94 is also positioned in the annulus 60, to which the template production flow line 96 is connected by a spool pipe section. Separate offgas and fluid flow lines extend from the separator 16 to the deck mounted pig launcher, then down the annulus 60 and through the caisson wall where they are connected by spool pipe sections to the template piping, then to their respective marine lines and terminal pig traps. A flare riser 98 is located in the annulus and connected by a spool pipe section to the template flare line 30 for conveying gas to the flare caisson 12 via a marine flow line.

Extending around the periphery of the template are control piping and wiring shown commonly at 100, FIG. 3.

It will thus be seen that the production caisson and template can be prefabricated so as to accommodate a substantial part of the control pipes and lines for the system. Such prefabrication and preassembly not only reduces costs but insures that each section is compatible with other sections of the system before installation. The location of the production separator within the caisson achieves the important advantage of substantially reducing the deck space previously required with installations of this type. It will be noted in this regard that the production separator is vertically disposed in the caisson thereby permitting the gas to be separated from the oil in a relatively confined space. The minimizing of space requirement thus reduces the forces on the caisson and yet permits the design of an efficient piping system without the necessity of building the standard production template which is required in prior systems of this general type to provide the surface area necessary for the normally employed horizontal production separator. Those skilled in the art here concerned will recognize the significant advantage of not having to carry all separating equipment on the caisson deck.

It should be noted that the present invention contemplates positioning the subsea wellheads approximately intermediate the water depth as well as on the sea floor as illustrated in solid lines in FIG. 2. The wellheads 28 have been shown in dash lines in FIG. 2 in such intermediate position, with production risers in such alternative form of the invention being illustrated at 110, extending from the conductor pipes 24. The supporting platform 112 is employed in such alternative arrangement, suitably rigidly connected to the caisson 10. The advantage of the intermediate positioning of the wellheads is that servicing by divers is greatly facilitated thus further reducing maintenance costs for the system. The anchoring of the caisson to the sea floor is identical whether the wellheads are positioned intermediate the water depth or at the sea floor, as shown in full lines in FIG. 2.

Referring to FIGS. 5-7, there is illustrated therein the flare caisson assembly in accordance with the present invention. The anchoring of the flare caisson 12 is the same in essential respects as the anchoring of the production caisson 10, except that piles commonly designated at 130 are employed rather than the conductor pipes in accordance with the production caisson assembly. The piles 130 are drilled into the sea floor and fastened to the template, with the annulus therearound being grouted as shown at 132 to rigidify the installation.

A template generally indicated at 134 similar to the template 22 in the production caisson system is anchored to the piles 130, and a spud 136 is drilled into the floor in the manner previously described. Components of the template previously described in reference to template 22 have been reapplied with an attached prime in FIGS. 5-7.

The template 134 is formed with stabbing funnels to guide the piles and spud vertically through the preformed and sleeved openings in the template, and the spud 136 is similarly provided with a stabbing funnel to facilitate location of the caisson in the spud. The template 134 is connected to the piles 130 and the caisson 12 is connected to the spud 136 through fastening devices of conventional construction of the type and in the manner previously described.

The caisson 12 includes a personnel platform 140 equipped with a pig trap 141, ignition system 142, navigational aids 143, and manifold 144. A landing walkway 145 is located above the water line (WL). Bumpers 146 are mounted around the caisson in the vicinity of the walkway, with a ladder 147 permitting access to the deck from the walkway. All piping is directed up through the template piping which is connected to the marine control and flow lines.

Extending upwardly from the deck 140 is a flare stack 150 which is rigidly connected to a truss structure generally indicated at 152 for centering and rigidifying the flare stack relative to the flare caisson. A latter 153 is mounted on the flare stack to permit access to the top of the flare stack from the deck 140. Flare ignition control lines 154 extend upwardly adjacent the stack for controlling the ignition thereof.

The flare line 30 which extends from the production caisson communicates with a flare riser 160 which in turn communicates with the flare stack 150 for flaring off the gas produced. A vertical knockout tank 149 is mounted within the flare caisson and functions in a known manner to collect fluids which drop out of the flare gas when the temperature and pressure thereof are reduced. A knockout loading line 164 is connected to the knockout tank 149 for removing the collected fluids.

It will thus be seen that in both the production and flare caissons, the caisson is firmly anchored in the template which is in turn firmly anchored in the subsoil through the conductor pipes in the case of the production caisson system or by means of piles in the case of the flare caisson system. The template provides the caisson with a point of fixity at the interconnection to the template so that the movement of the caisson is greatly restricted when compared with caissons imbedded directly into the soil. The caisson, template and conductor pipes or piles all act as an integral structure with a solid foundation in the subsoil whereby loads are efficiently distributed to the subsoil. An important feature of the present invention is the utilization of the conductor pipes in the production caisson both as structural members and as well conductors.

It will be understood that the number of conductors and the size of the caisson will vary with field characteristics, water depth and wave conditions. Although not intended to be limiting, a satisfactory system designed for a water depth of 80 feet, for example, includes the penetration of the three conductor pipes approximately 70 feet into the subsoil, with the conductor pipes being 30 inches in diameter and located symetrically around the caisson as described. The caisson can be 5 feet in diameter and extends approximately 33 feet above the water line so as to elevate the deck above the wave forces. Although the outside diameter of the caisson can be constant, alternatively the outside diameter can taper, taking into account the increased forces resulting from bending moments on the caisson in the lower regions thereof toward the point of fixity at the mud line. The thickness of the caisson wall can also vary, for example, a preferred caisson design includes a wall diameter of three-fourths inch in the upper region of the caisson, for example, in the top approximately 75 feet thereof, and 11/2 inch in wall diameter in the remaining, lower portion thereof. By varying the wall thickness, the imposed forces and moments on the caisson will remain constant throughout the length of the caisson. The lower end of the caisson extends into the spud approximately 10 feet below the mud line. As noted, the dimensions referred to are illustrative only, and it will be understood that other diameters and thicknesses could alternatively be employed without departing from the concepts of the invention. It will also be understood that all structure in the water zone should preferably be provided with cathodic protection, and that structure in the atmospheric zone will be provided with suitable protective coatings and in the splash zone provided with caisson wraps and/or suitable protective coatings. Conventional vortex shedding members such as helical spoilers can also form part of the completed installation, in accordance with known technology.