US20040161984A1 - Subsurface buoy and methods of installing, tying and dynamically stabilizing the same - Google Patents

Abstract

The present invention relates to equipment installed in an intermediary region between a floating production unit at the ocean surface and a wellhead of an oil reservoir on the ocean floor. The equipment, known as a subsurface buoy, is designed to support rigid and flexible production tubes used for the transport of reservoir fluids from the oil well and/or fluids used in support systems for the oil reservoir.

Four cylindrical bodies1, 2, 3, and4, connected at their extremities, form the body of the subsurface buoy. In each of the vertices of the subsurface buoy, components of a tying and dynamically stabilizing system13are rigidly connected. This tying and dynamically stabilizing system13is configured to control the positioning of the subsurface buoy, the tension of installation chains15and anchoring tendons21, thereby promoting a stabilization of the entire assembly against large-amplitude rotations or angular changes even after rigid tubes10and flexible tubes11are coupled to the body of the subsurface buoy.

In addition, a method to install the subsurface buoy is also presented.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention[0001]
• The present invention relates to the field of oil-well equipment used in an intermediary region between a floating production unit at the ocean surface and a wellhead of an oil reservoir, or well, on the ocean floor. The equipment is designed to support rigid and flexible production tubes, or pipes, used for the transport of reservoir fluids from the oil well and/or fluids used in support systems for the oil reservoir. In addition, a method of installing the equipment at its designed operating depth is also disclosed.[0002]
• 2. State of the Art[0003]
• Offshore exploration of hydrocarbons in deep or ultra-deep waters has always presented a technological challenge to companies involved in such an undertaking and represents a significant boundary, or limit, to applicable scientific knowledge. This challenge, when overcome, will take us to the forefront of this technology.[0004]
• Given the large depths associated with deep-water exploration and, consequently, the exposure to a harsh environment, interconnection between the wellhead on the ocean floor and a flow system for extracting the oil production is a great technological challenge. In addition, the installation and support of ascending production tubes, also known as “risers” in a configuration known as suspended catenaries, adds to the technological challenge.[0005]
• In the environment of ultra-deep water, it has been known that hybrid concepts, if not considered the only alternative due to limited field experience, should be evaluated with much care. This need of evaluation exists due to the use of flexible risers as an interconnection between the wellhead on the ocean floor and the production unit. Flexible risers in suspended-catenary configuration, i.e., a direct connection between the wellhead and the floating production unit for depths greater than 1000 meters, present dynamic problems caused by movement of the floating production unit itself. These movements may provoke a compression and/or excessive curvature at the location where the catenary, formed by the riser, contacts the ocean floor or creates an additional load at the top of the riser due to angular motion and/or horizontal displacement that may provoke rupture in the connections at the surface. In the case of the flowing unit being a Floating Production Storage and Offloading (FPSO) ship, the above mentioned problems exist in a much more critical scale.[0006]
• Another type of riser which is used is a substantially rigid riser, known as Steel Catenary Riser (SCR). The use of SCRs, connected directly to the floating production units, has limitations, as far as their configuration when used in ships converted to production (FPSO), because the ship movements provoked by waves are more accentuated compared to semi-submersed platforms. SCRs may have desirable characteristics associated with the capacity to support, along their extremities, higher tension as compared to flexible risers; however, the flexible risers have longer life in terms of resisting fatigue even taking into account their interaction with ocean waves and currents.[0007]
• It has been, therefore, proposed to combine the two types of risers in an assembly in order to take advantage of the best characteristics of each type: namely the resistance to tension and the higher economic viability associated with SCRs; and a significantly higher resistance to fatigue associated with flexible risers.[0008]
• In an assembly, an intermediary system would be necessary, having sufficient floatation to support the weight of the rigid production tubes, while, at the same time, exhibiting only small displacements in response to the horizontal loading of SCRs and the environmental working loads. The system, or assembly, would also need to be submersed sufficiently to protect it from the effects of waves at the ocean surface, be capable to connect the SCR to the flexible riser, and be reasonably easy to install. With these necessary prerequisites, the concept of a subsurface buoy, or float, developed naturally, permitting a considerable reduction in the weight of the production tubes on the floating unit, improving undersea arrangement, and thus making possible the use of a hybrid system of risers.[0009]
• Hybrid systems of risers, based in a subsurface buoy, have recently been recognized as an alternative to the limitations found in petroleum production activities in deep-sea water.[0010]
• There exist in the art, and more specifically in the field of introduction and connection of production tubes, concepts of intermediary support systems to promote the connection between a vertical riser and flexible tubes or even concepts to reduce the loads on equipment and tubes.[0011]
• Examples of these systems may be found in the documents BR/PI 9202379-A, belonging to Bechtel Limited, in which a system to deploy, or unfold, cable used at an intermediate floating level and the associated flexible rising tubes are disclosed. U.S. Pat. No. 4,423,984, assigned to Mobil Oil Corporation, discloses a way to interconnect the flexible tubes originating at a surface unit and a rigid vertical riser with a buoy connected at the upper extremity of the vertical riser. U.S. Pat. No. 5,007,482, assigned to British Petroleum Company, similarly discloses another concept to make the connection between the wellhead and a floating unit using a buoy as an intermediary support for the flexible tubes.[0012]
• Although the inventions just summarized may appear as viable solutions, they may become problematic when considering their economic viability, installation difficulties, and behavior when exposed to the harsh environmental conditions such as, for example, the effect of the waves.[0013]
SUMMARY OF THE INVENTION
• The present invention relates to equipment designed to support and interconnect rigid and flexible tubes used in the production and transport of oil from an underwater well and/or fluids used in support systems for the reservoir. Such equipment is usually known as a subsurface buoy. A system to tie and to dynamically stabilize the buoy is also in the scope of the present invention. Additionally, a method is also disclosed to install the subsurface buoy at its location of operation.[0014]
• The subsurface buoy, one of the objects of the present invention, comprises four interconnecting cylindrical units forming a single unit, each one of these cylindrical units having in its interior a plurality of draining compartments for the purpose of ballasting. Such a floating body resembles a quadrilateral frame defining a whole therethrough, having a plurality of fixed connections for interconnecting the rigid and flexible tubes thereto. A tying and dynamically stabilizing system, rigidly connected at each of the vertices of the subsurface buoy, is another object of the present invention. The tying and dynamically stabilizing system is designed to control the float position and the traction and the tension in the anchoring tendons, providing stabilization for the entire unit against large-amplitude rotation or changes in angular position even after the rigid and flexible tubes are couple or connected to the floating body, or buoy. Hereinafter, chained sections formed by links will be referred to as “chains” and steel or polyester cables will be designated as “tendons.”[0015]
• Another object of the present invention is a method of installation of the subsurface buoy at its operating location with the fluid transport tubes connected thereto and the use of a system to tie and dynamically stabilize the subsurface float.[0016]
BRIEF SUMMARY OF THE DRAWINGS
• The characteristics of the subsurface buoy, of the tying and dynamically stabilizing system, and of the method of installation of the subsurface buoy, all objects of the present invention, will be better understood by the detailed description to be later presented, as a mere example, together with the drawings summarized below, which drawings are integral with the present application and comprise:[0017]
• FIG. 1 illustrates schematically a top view of the subsurface buoy according to the present invention;[0018]
• FIG. 2 illustrates schematically a longitudinal cut of the top view of the subsurface buoy of FIG. 1;[0019]
• FIG. 2[0020]aillustrates schematically the details of a longitudinal cut of the ballast or draining compartments inside the cylindrical, or tubular, bodies that form the subsurface buoy of FIG. 1;
• FIG. 3 illustrates schematically a top view of the subsurface buoy of FIG. 1 showing the rigid tubes, flexible tubes, intermediary tubes, and the tying and dynamically stabilizing system;[0021]
• FIG. 4 illustrates a frontal view of the subsurface buoy of FIG. 1, detailing the tying and dynamically stabilizing system for the subsurface buoy of the present invention;[0022]
• FIG. 4[0023]aillustrates a passive system, which is one of the integral parts of the tying and dynamically stabilizing system for the subsurface buoy of the present invention;
• FIG. 4[0024]billustrates a tension equalizing system, which is another integral part of the tying and dynamically stabilizing system for the subsurface buoy of the present invention; and
• FIG. 5 illustrates a side view of the subsurface buoy of FIG. 1 with the tying and dynamically stabilizing system, the rigid tubes, the flexible tubes connected.[0025]
DETAILED DESCRIPTION OF THE INVENTION
• The detailed description of the subsurface buoy, the tying and dynamically stabilizing system, and the method of installation of the subsurface buoy, all being objects of the present invention, will be presented utilizing the identification of the respective components as illustrated in the above-summarized drawing figures.[0026]
• A top view schematically representing the subsurface buoy of the present invention is illustrated in FIG. 1. The subsurface buoy comprises four cylindrical bodies, a first cylindrical body[0027]1 and a secondcylindrical body2, having equal lengths and diameters The first and second cylindrical bodies are positioned longitudinally parallel and separate relative to one another. In order to facilitate the description, the side of the subsurface buoy where the first cylindrical body1 is located will be referred to as the “starboard side.” Similarly, the side where the secondcylindrical body2 is located will be referred to as the “portside.” To one of the extremities of the longitudinalcylindrical bodies1 and2, a thirdcylindrical body3 is connected, in a transversal position, having a larger diameter than the other two cylindrical bodies. The other extremities of the longitudinalcylindrical bodies1 and2 are also connected transversally to a fourthcylindrical body4, having a length equal to that of the thirdcylindrical body3, and having a diameter equal to those of the longitudinalcylindrical bodies1 and2. The side of the subsurface buoy where the thirdcylindrical body3 is located will be referred to hereinafter as the “stern side.” Similarly, the side of the subsurface buoy where the forthcylindrical body4 located it will be referred to as the “bow or nose side.” The interconnection between thecylindrical bodies1,2,3 and4 forms the body of the subsurface buoy, giving it a quadrilateral appearance defining a hole therethrough. A descendingstabilizer5 in the form of a blade, for example, is connected by its extremities to the longitudinalcylindrical bodies1 and2 parallel to the fourthcylindrical body4 and next to the bow side in order to stabilize the positioning of the subsurface buoy during the descending operation.
• FIG. 2 illustrates a horizontal cut of a top view of the subsurface buoy of the present invention, wherein a plurality of draining compartments[0028]6 are shown located internally in each of thecylindrical bodies1,2,3, and4. FIG. 2aschematically illustrates a longitudinal cut of anyone of the draining compartments6 illustrated in FIG. 2. Connected to the top of each draining compartment6 is afirst valve7 for the introduction or removal of compressed air; Toward the bottom of each draining compartment6, a second valve8 is connected for the ballasting purposes, allowing seawater in and out of each draining compartment6.
• FIG. 3 illustrates the positioning of[0029]supports9aand9b, known in the art as “goose necks,” that serve to support the two types of tubes used in the transport of oil from the ocean floor to the surface, namelyrigid tubes10 andflexible tubes11.Supports9aand9bare fixed to the transversalcylindrical bodies3 and4, respectively. To the thirdcylindrical body3, supports9aare installed to which, at one of their extremities, rigid tubes, or “SCRs,” are installed connecting the wellhead at the ocean floor to the subsurface buoy.Flexible tubes11, or “Jumpers,” are installed on one of the extremities ofsupports9binstalled to the fourthcylindrical body4, connecting the subsurface buoy to the floating production unit.Intermediary tubes12, being aligned to the longitudinalcylindrical bodies1 and2 and interconnected to therigid tubes10 andflexible tubes11, are connected to the other extremities ofsupports9aand9b. At the vertices formed by the union ofcylindrical bodies1,2,3, and4, fixation points for the tying and dynamically stabilizingsystem13, also used to install the subsurface buoy at its depth of operation.
• FIG. 4 illustrates in more detail the characteristics of the tying and dynamically stabilizing[0030]system13, which comprises a passive system A and a tension equalization system B.
• The passive system A is passive because it does not use any source of energy (electric, hydraulic, pneumatic, or otherwise) to operate, serving only to install the subsurface buoy at its place of operation. Each one of the passive systems A is rigidly connected to the vertices of the subsurface buoy located at the upper extremities and sides of[0031]cylindrical bodies3 or4. The details of passive systems A are illustrated in FIG. 4aand comprise:
• pulleys, or runners,[0032]14 connected to one of the upper extremities of thecylindrical bodies3 or4;
• an installation chain, or cable,[0033]15, passing overpulley14, used as a counterweight during the float's descending operation to its depth of operation;
• a[0034]chain stopper16, through the interior of which theinstallation chain15 passes, rigidly connected to the extremities of each one of the tubular, or cylindrical,bodies3 and4, and used to limit the travel ofinstallation chain15 during the installation of the subsurface buoy;
• an[0035]actuator link17 for thechain stopper16, interposed between links of theinstallation chain15;
• an end-of-[0036]travel device18, interposed between links of the installation chain135 and located below thechain stopper16, used to interrupt or suspend the travel of theinstallation chain15 during the installation of the subsurface buoy;
• a[0037]coupling plate19, joined to one of the ends ofinstallation chain15, used to connect the passsive system A to the tension equalizing system B and to the anchoringtendon21; and
• a lower connection[0038]20, connecting theinstallation chain15 to the anchoringtendon21.
• The tension equalizing system B, after installation of the subsurface buoy at its operating position, is configured to maintain the tension in each anchoring[0039]tendon21 equally distributed, connect the subsurface buoy to the ocean floor, and maintain the subsurface buoy in stable conditions, i.e., preventing unwanted variations in inclination caused by ocean currents or the weight of the tubes, or pipes, connected to the subsurface buoy. The details of tension equalizing system B are illustrated in FIG. 4band comprise:
• major-base pulleys[0040]22 connected to lower extremities of thecylindrical bodies3 and4 one major-base pulley22 being connected to one extremity ofcylindrical body3 and another to a diagonally opposed extremity ofcylindrical body4;
• minor-base pulleys[0041]23 connected to the other lower extremities of thecylindrical bodies3 and4, one minor-base pulley23 being connected to one extremity ofcylindrical body3 and another to a diagonally opposed extremity ofcylindrical body4;
• a first supporting[0042]component24, passing over the major-base pulleys22, connected by its extremities to the diagonally opposedcoupling plates19 on the anchoringtendons21; and
• a second supporting[0043]component25, passing over the minor-base pulleys23, connected by its extremities to the diagonally opposedcoupling plates19 on the anchoringtendons21.
• FIG. 5 illustrates a side view of a complete assembly of the subsurface buoy with the tying and dynamically stabilizing[0044]system13 connected to the anchoringtendons21 and to therigid tubes10 andflexible tubes11, properly connected tosupports9aand9b.
• The scope of the present invention also comprises methods of installation of the subsurface buoy. Such methods will now be described, providing a better understanding of the function of each of the components of the present invention. The method of installation of the subsurface buoy comprises:[0045]
• driving anchoring stakes (not illustrated) into the ocean floor at predetermined installation locations, connecting anchoring[0046]tendons21 to driving anchoring stakes, and connectingcoupling plates19 to anchoringtendons21;
• connecting[0047]coupling plates19 to the tension equalizing systems B;
• feeding each one of the[0048]installation chains15 through points in each one of the passive systems A;
• connecting[0049]installation chains15 tocoupling plates19 of the tension equalizing system B;
• connecting a supply line to each one of the[0050]valves7 on draining compartments6 located insidecylindrical bodies1,2,3, and4 in order to supply compressed air thereto;
• opening[0051]first valve7 and second valve8 from draining compartments6 located insidecylindrical bodies1,2,3, and4 to allow flow of sea water therein;
• lowering slowly the subsurface buoy until the end-of-[0052]travel device18 comes in contact withchain stopper16;
• injecting compressed air into each of the draining compartments[0053]6 through each one of thevalves7, to expel sea water therefrom and to cause a thrust towards the ocean surface;
• removing the ballast until actuator links[0054]17 contact the upper parts ofchain stoppers16, thus tensioning anchoringtendons21 and installing the subsurface buoy at its depth of operation;
• coupling[0055]flexible tubes11 tosupports9blocated on the cylindrical body at the bow side of the subsurface buoy; and
• coupling[0056]rigid tubes10 tosupports9a, located on the cylindrical body at the stern side of the subsurface buoy.
• The description presented above of the subsurface buoy, the tying and dynamically stabilizing[0057]system13, and the method of installing the subsurface buoy, all objects of the disclosed invention, should be considered only exemplary embodiments. Any particular characteristics introduced in these examples should be understood only as being presented or described to facilitate an understanding of the invention to one of ordinary skill in the art. Therefore, these particular characteristics should not be considered as limiting the scope of the present invention, which scope is limited solely by the appended claims.

Claims (6)

What is claimed is:

1. A subsurface buoy used in the production and transport of oil from an underwater well to a production unit at the ocean surface, the subsurface buoy comprising:

a first cylindrical body1 and a second cylindrical body2, longitudinally positioned parallel and separate from each other and having equal lengths and diameters;

a third cylindrical body3, having a diameter larger than the diameter of said first and second cylindrical bodies1 and2, connected transversally to one of the extremities of the longitudinally positioned cylindrical bodies1 and2;

a fourth cylindrical body4, having a length equal to the length of cylindrical body3 and a diameter equal to the diameter of the longitudinally positioned cylindrical bodies1 and2, connected transversally to the other extremities of the longitudinally positioned cylindrical bodies1 and2;

a descending stabilizer5 connected by its extremities to the longitudinally positioned cylindrical bodies1 and2, parallel and close to the fourth cylindrical body4, configured to stabilize the positioning of the subsurface buoy during the descending and installation operations;

supports9a, connected to the third cylindrical body3, the extremities of which have a plurality of rigid tubes10 connected thereto;

supports9b, connected to the fourth cylindrical body4, the extremities of which having a plurality of flexible tubes111 connected thereto;

intermediary tubes12, connected to supports9aand9band aligned to the longitudinal cylindrical bodies1 and2, the intermediary tubes12 interconnecting to the rigid tubes10 and flexible tubes11; and

a tying and dynamically stabilizing system13 configured to install the subsurface buoy at its depth of operation being connected to vertices formed by the union of cylindrical bodies1,2,3, and4.

2. The subsurface buoy according toclaim 1 characterized by any one of the cylindrical bodies1,2,3, or4, forming the subsurface buoy, having in the interior thereof a plurality of draining compartments6, each one of the plurality of draining compartments6 having a first valve7, for the introduction or removal of compressed air, connected to the top thereof and a second valve8, for the introduction or removal of a ballast or seawater, connected to the bottom thereof.

3. The subsurface buoy ofclaim 1 further including a tying and dynamically stabilizing system13 comprising:

a passive system A configured to install the subsurface buoy to its depth of operation; and

a tension equalizing system B configured to maintain the tension equally distributed to anchoring tendons21.

4. The tying and dynamically stabilizing system13 for a subsurface buoy according toclaim 3 wherein the passive system A comprises:

a pulley14 connected to one of the upper extremities of one of the cylindrical bodies3 or4;

an installation chain15, passing over pulley14, used as a counterweight during the subsurface buoy's descending operation to its depth of operation;

a chain stopper16, through the interior of which the installation chain15 passes, rigidly connected to the extremities of each one of the cylindrical bodies3 and4 and used to limit the travel of the installation chain15 during the installation of the subsurface buoy;

an actuator link17 for the chain stopper16, interposed between links of the installation chain15, having dimensions larger than the links of the installation chain15, located above the chain stopper16, and configured to interrupt the travel of the installation chain15 during the installation of the subsurface buoy;

an end-of-travel device18, interposed between links of the installation chain15 and located below the chain stopper16, configured to interrupt or suspend the travel of the installation chain15 during the installation of the subsurface buoy;

a coupling plate19, joined to one of the ends of installation chain15, configured to connect the passive system A to the tension equalizing system B and to the anchoring tendon21; and

a lower connection20 configured to install installation chain15 to the anchoring tendon21.

5. The tying and dynamically stabilizing system13 for a subsurface buoy according toclaim 3 wherein the tension equalizing system B comprises:

at least two major-base pulleys22 connected to lower extremities of the cylindrical bodies3 and4, one major-base pulley22 being connected to one extremity of cylindrical body3 and the other major-base pulley22 being connected to a diagonally opposed extremity of cylindrical body4;

at least two minor-base pulleys23 connected to the other lower extremities of the cylindrical bodies3 and4, one minor-base pulley23 being connected to one extremity of cylindrical body3 and the other minor-base pulley23 being connected to a diagonally opposed extremity of cylindrical body4;

a first supporting component24, passing over the major-base pulleys22 and being connected by its extremities to the diagonally opposed coupling plates19 on the anchoring tendons21; and

a second supporting component25, passing over the minor-base pulleys23 and being connected by its extremities to the diagonally opposed coupling plates19 on the anchoring tendons21.

6. A method to install a subsurface buoy according toclaim 1, said method comprising:

driving anchoring stakes into the ocean floor at predetermined installation locations, connecting anchoring tendons21 to driving anchoring stakes, and connecting coupling plates19 to anchoring tendons21;

connecting coupling plates19 to the tension equalizing systems B;

feeding each one of the installation chains15 through each one of the points of the passive system A;

connecting installation chains15 to coupling plates19 of the tension equalizing system B;

connecting a supply line to each one of the valves7 on draining compartments6 located inside cylindrical bodies1,2,3, and4 in order to supply compressed air thereto;

opening first valve7 and second valve8 from draining compartments6 located inside cylindrical bodies1,2,3, and4 to allow flow of sea water therein;

lowering slowly the subsurface buoy until the end-of-travel device18 comes in contact with the chain stopper16;

injecting compressed air into each of the draining compartments6 through each one of the first valves7, to expel sea water therefrom and to cause a thrust towards the ocean surface;

removing the ballast until actuator links17 touch the upper parts of chain stoppers16, thus tensioning anchoring tendons21 and installing the subsurface buoy at its depth of operation;

coupling flexible tubes11 to supports9blocated on the cylindrical body at the bow side of the subsurface buoy; and

coupling rigid tubes10 to supports9alocated on the cylindrical body at the stern side of the subsurface buoy.

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