Not applicable.
1. Field of the Invention
The invention relates generally to offshore structures to facilitate oil and gas production. More particularly, the invention relates to buoyant towers releasably coupled to the sea floor and configured to store and offload produced hydrocarbons.
2. Background of the Technology
Offshore structures are used to store and offload hydrocarbons (e.g., oil and gas) produced by subsea wells. Usually, the type of offshore structure employed will depend on the depth of water at the well location. For instance, in water depths less than about 300 feet, jackup platforms are commonly employed as production structures; in water depths between about 300 and 800 feet, fixed platforms are commonly employed as production structures; and in water depths greater than about 800 feet, floating systems such as semi-submersible platforms are commonly employed as production structures.
Jackup platforms can be moved between different wells and fields, and are height adjustable. However, jackup platforms are generally limited to water depths less than about 300 feet. Fixed platforms can be used in greater water depths than jackup platforms (up to about 800 feet), but are not easily moved and typically have a fixed height. Conventional floating production systems can be used in deep water, but are relatively difficult to move between different wells. In particular, most floating production systems are designed to be moored (via multiple mooring lines) at a specific location for an extended period of time. Such mooring systems typically include mooring lines that are anchored to the sea floor with relatively large piles driven into the sea bed. Such piles are difficult to handle, transport, and install at substantial water depths. Moreover, most floating productions systems are relatively expensive and cost prohibitive for smaller, marginal oil and gas fields.
Accordingly, there remains a need in the art for offshore structures and systems designed for use in water depths greater than about 800 feet and that are easily moveable between different offshore locations. Such offshore productions systems would be particularly well-received if they were economically feasible for smaller, marginal oil and gas fields.
These and other needs in the art are addressed in one embodiment by an offshore structure. In an embodiment, the offshore structure comprises a base configured to be secured to the sea floor. In addition, the offshore structure comprises an elongate stem having a longitudinal axis, a first end distal the base and a second end pivotally coupled to the base. Further, the offshore structure comprises an upper module coupled to the first end of the stem. The upper module includes a variable ballast chamber. Still further, the offshore structure comprises a first ballast control conduit in fluid communication with the variable ballast chamber of the upper module. The first ballast control conduit is configured to supply a gas to the variable ballast chamber of the upper module and vent the gas from the variable ballast chamber of the upper module. Moreover, the offshore structure comprises a deck mounted to the upper module.
These and other needs in the art are addressed in another embodiment by a method for producing one or more offshore wells. In an embodiment, the method comprises (a) transporting an elongate stem and an upper module offshore, wherein the upper module includes a variable ballast chamber. In addition, the method comprises (b) transitioning the stem from a horizontal orientation to a vertical orientation. Further, the method comprises (c) attaching the upper module to an upper end of the stem to form a tower. Still further, the method comprises (d) ballasting the tower. Moreover, the method comprises (e) pivotally coupling the tower to an anchor disposed at the sea floor at a first offshore installation site.
These and other needs in the art are addressed in another embodiment by an offshore structure. In an embodiment, the offshore structure comprises a tower having a longitudinal axis, an upper end, and a lower end opposite the upper end. The tower comprises an elongate stem extending from the lower end, an upper module coupled to the stem, and a deck mounted to the upper module at the upper end. The upper module is net buoyant. In addition, the offshore structure comprises an anchor configured to be secured to the sea floor. The anchor is pivotally and releasably coupled to the lower end of the tower.
Thus, embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings.
For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:
The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment.
Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.
In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis.
Referring now to
Structure 10 has a length L10 measured axially between ends 10a, b. In this embodiment, upper module 20 extends above the sea surface 13, and thus, length L10 is greater than the depth of water. However, in other embodiments, the upper module (e.g., upper module 20) and/or the deck (e.g., deck 60) may be disposed generally proximal but below the sea surface 13, in which case the axial length of the structure (e.g., length L10 of structure 10) is less than the depth of the water.
Referring now to
Referring now to
As best shown in
Referring again to
Referring now to
End caps 22 close off ends 20a, b of module 20, thereby preventing fluid flow through ends 20a, b into chambers 26, 27, respectively. Bulkhead 23 is disposed between chambers 26, 27, thereby preventing fluid communication between adjacent chambers 26, 27. Thus, each chamber 26, 27 is isolated from the other chamber 26, 27 in module 20.
Upper module 20 has a length L20 measured axially between ends 20a, b, and section 21a has a diameter D21a and length L21a measured axially between end 20a and section 21b. For an exemplary structure 10 deployed in 1,000 ft. of water and having a length L10 of 1,000 ft., length L20 is 250 ft., diameter D21a is 25 ft., and length L21a is 200 ft. However, depending on the particular installation location and desired dynamics for structure 10, lengths L20, L21a, and diameter D21a may be varied and adjusted as appropriate.
Chamber 27 is filled with a gas 16 and sealed from the surrounding environment (e.g., water 11), and thus, provide buoyancy to upper module 20 during offshore transport and installation of module 20, as well as during operation of structure 10. Accordingly, chamber 27 may also be referred to as a buoyant chamber. In this embodiment, gas 16 is air, and thus, may also be referred to as air 16. As will be described in more detail below, during offshore transport of upper module 20, variable ballast chamber 26 is also filled with air 16, thereby contributing to the buoyancy of module 20. However, during installation of module 20 and operation of structure 10, variable ballast 18 may be controllably added to ballast adjustable chamber 26 to decrease the buoyancy of module 20 and structure 10. In this embodiment, variable ballast 18 is water 11, and thus, variable ballast 18 may also be referred to as water 18.
Although module 20 includes two chambers 26, 27 in this embodiment, in general, module 20 may include any suitable number of chambers. Preferably, at least one chamber is an empty buoyant chamber and one chamber is a ballast adjustable chamber. Further, although end caps 22 and bulkhead 23 are described as providing fluid tight seals at the ends of chambers 26, 27, it should be appreciated that one or more end caps 22 and/or bulkhead 23 may include a closeable and sealable access port (e.g., man hole cover) that allows controlled access to one or more chambers 26, 27 for maintenance, repair, and/or service.
Referring still to
Ballast control system 80 includes an air conduit 82, an air supply line 83, an air compressor or pump 84 connected to supply line 83, a first valve 85 along line 83 and a second valve 86 along conduit 82. Conduit 82 extends subsea into chamber 26, and has a venting end 82a above the sea surface 13 external chamber 26 and an open end 82b disposed within chamber 26 proximal upper cap 22. Valve 86 controls the flow of air 16 through conduit 82 between ends 112a, b, and valve 85 controls the flow of air 16 from compressor 84 to chamber 26. Control system 80 allows the relative volumes of air 16 and water 11, 18 in chamber 26 to be controlled and varied, thereby enabling the buoyancy of chamber 26 and associated module 20 to be controlled and varied. In particular, with valve 86 open and valve 85 closed, air 16 is exhausted from chamber 26, and with valve 85 open and valve 86 closed, air 16 is pumped from compressor 84 into chamber 26. Thus, end 82a functions as an air outlet, whereas end 82b functions as both an air inlet and outlet. With valve 85 closed, air 16 cannot be pumped into chamber 26, and with valves 85, 86 closed, air 16 cannot be exhausted from chamber 26.
In this embodiment, open end 82b is disposed proximal the upper end of chamber 26 and port 81 is positioned proximal the lower end of chamber 26. This positioning of open end 82b enables air 16 to be exhausted from chamber 26 when column is in a generally vertical, upright position (e.g., following installation). In particular, since buoyancy control air 16 (e.g., air) is less dense than water 11, any buoyancy control air 16 in chamber 26 will naturally rise to the upper portion of chamber 26 above any water 11, 18 in chamber 26 when module 20 is upright. Accordingly, positioning end 82b at or proximal the upper end of chamber 26 allows direct access to any air 16 therein. Further, since water 11, 18 in chamber 26 will be disposed below any air 16 therein, positioning port 81 proximal the lower end of chamber 26 allows ingress and egress of water 11, 18, while limiting and/or preventing the loss of any air 16 through port 81. In general, air 16 will only exit chamber 26 through port 81 when chamber 26 is filled with air 16 from the upper end of chamber 26 to port 81. Positioning of port 81 proximal the lower end of chamber 26 also enables a sufficient volume of air 16 to be pumped into chamber 26. In particular, as the volume of air 16 in chamber 26 is increased, the interface between water 11, 18 and the air 16 will move downward within chamber 26 as the increased volume of air 16 in chamber 26 displaces water 11, 18 in chamber 26, which is allowed to exit chamber through port 81. However, once the interface of water 11, 18 and the air 16 reaches port 81, the volume of air 16 in chamber 26 cannot be increased further as any additional air 16 will simply exit chamber 26 through port 81. Thus, the closer port 81 to the lower end of chamber 26, the greater the volume of air 16 that can be pumped into chamber 26, and the further port 81 from the lower end of chamber 26, the lesser the volume of air 16 that can be pumped into chamber 26. Thus, the axial position of port 81 along chamber 26 is preferably selected to enable the maximum desired buoyancy for chamber 26.
In this embodiment, conduit 82 extends radially through tubular 21. However, in general, the conduit (e.g., conduit 82) may extend through other portions of the module (e.g., module 20). For example, the conduit may extend axially through the module (e.g., through cap 22 at upper end 20a or bulkhead 23) in route to the ballast adjustable chamber (e.g., chamber 26). Any passages extending through a bulkhead or cap are preferably completely sealed.
It should be appreciated that air 16 will automatically vent from chamber 26 when ends 82a, b are in fluid communication. In particular, the air 16 in chamber 26 is compressed due to the hydrostatic pressure of water 11, 18. End 82b is positioned at the surface 13 (i.e., at about 1 atmosphere of pressure). Thus, when end 82b is in fluid communication with compressed air 16 in chamber 26, the compressed air 16 will inherently flow from the high pressure region (chamber 26) to the lower pressure region (end 82b), thereby allowing water 11, 18 to flood chamber 26 through port 81.
Without being limited by this or any particular theory, the flow of water 11, 18 through port 81 will depend on the depth of chamber 26 and associated hydrostatic pressure of water 11 at that depth, and the pressure of air 16 in chamber 26 (if any). If the pressure of air 16 is less than the pressure of water 11, 18 in chamber 26, then the air 16 will be compressed and additional water 11, 18 will flow into chamber 26 through port 81. However, if the pressure of air 16 in chamber 26 is greater than the pressure of water 11, 18 in chamber 26, then the air 16 will expand and push water 11, 18 out of chamber 26 through port 81. Thus, air 16 within chamber 26 will compress and expand based on any pressure differential between the air 16 and water 11, 18 in chamber 26.
In this embodiment, conduit 82 has been described as supplying air 16 to chamber 26 and venting air 16 from chamber 26. However, if conduit 82 is exclusively filled with air 16 at all times, a subsea crack or puncture in conduit 82 may result in the compressed air 16 in chamber 26 uncontrollably venting through the crack or puncture in conduit 82, thereby decreasing the buoyancy of upper module 20 and potentially impacting the overall stability of structure 10. Consequently, when air 16 is not intentionally being pumped into chamber 26 or vented from chamber 26 through valve 86 and end 82b, conduit 82 is preferably filled with water up to end 82b. The column of water in conduit 82 is pressure balanced with the compressed air 16 in chamber 16. Without being limited by this or any particular theory, the hydrostatic pressure of the column of water in conduit 82 will be the same or substantially the same as the hydrostatic pressure of water 11, 18 at port 81 and in chamber 26. As previously described, the hydrostatic pressure of water 11, 18 in chamber 26 is balanced by the pressure of air 16 in chamber 26. Thus, the hydrostatic pressure of the column of water in conduit 82 is also balanced by the pressure of air 16 in chamber 26. If the pressure of air 16 in chamber 26 is less than the hydrostatic pressure of the water in conduit 82, and hence, less than the hydrostatic pressure of water 11 at port 81, then the air 16 will be compressed, the height of the column of water in conduit 82 lengthen, and water 11 will flow into chamber 26 through port 81. However, if the pressure of air 16 in chamber 26 is greater than the hydrostatic pressure of the water in conduit 82, and hence, greater than the hydrostatic pressure of water 11 at port 81, then the air 16 will expand and push water 11, 18 out of chamber 26 through port 81 and push the column of water in conduit 82 upward. Thus, when water is in conduit 82, it functions similar to a U-tube manometer. In addition, the hydrostatic pressure of the column of water in conduit 82 is the same or substantially the same as the water 11 surrounding conduit 82 at a given depth. Thus, a crack or puncture in conduit 82 placing the water within conduit 82 in fluid communication with water 11 outside conduit 82 will not result in a net influx or outflux of water within conduit 82, and thus, will not upset the height of the column of water in conduit 82. Since the height of the water column in conduit 82 will remain the same, even in the event of a subsea crack or puncture in conduit 82, the balance of the hydrostatic pressure of the water column in conduit 82 with the air 16 in chamber 26 is maintained, thereby restricting and/or preventing the air 16 in chamber 26 from venting through conduit 82. To remove the water from conduit 82 to controllably supply air 16 to chamber 26 or vent air 16 from chamber 26 via conduit 82, the water in conduit 82 may simply be blown out into chamber 26 by pumping air 16 down conduit 82 via pump 84, or alternatively, a water pump may be used to pump the water out of conduit 82.
Referring now to
Module 41 has a length L41 measured axially between ends 41a, b, and a diameter D41 that is less than D21a. For an exemplary structure 10 deployed in 2,000 ft. of water and having a length L10 of 2,000 ft., upper module 20 has a length L20 of 250 ft., and stem 40 is comprised of twenty modules 41, each module 41 having a length L41 of 87.5 ft. and a diameter D41 of 6 to 10 ft. However, depending on the particular installation location and desired dynamics for structure 10, the number of modules 41, length L41 and diameter D41 of each module 41 may be varied and adjusted as appropriate. Although this example is designed for deployment in 2,000 ft. of water, in general, structure 10 may be lengthened for deployment in greater depths of water (e.g., 5,000 ft.) depending on environmental conditions and the load of deck 60.
During offshore transport of modules 41, variable ballast chambers 44 are filled with air 16, thereby contributing to the buoyancy of each module 41. However, during installation of stem 40 and operation of structure 10, ballast 18 may be controllably added to any one or more ballast adjustable chambers 44 to decrease the buoyancy of the corresponding module 41, stem 40, and structure 10.
Referring still to
Ballast control system 100 includes an air conduit 102 mounted on a reel 103, an air line 104 extending from reel 103, an air compressor or pump 105 coupled to line 103 with an air supply conduit 106, a first valve 107 along line 104, and a second valve 108 along conduit 106. Line 104 is in fluid communication with conduit 102 and has an open or venting end 104b. Valve 107 controls the flow of air 16 between conduit 102 and end 104b, and valve 108 controls the flow of air 16 from compressor 104 through lines 106, 104 into conduit 102. Conduit 102 extends subsea from reel 103 along structure 10 and has an opening or port 109 proximal its lower or subsea end 112a. In this embodiment, conduit 102 is a semi-rigid hose or line capable of being bowed or flexed while simultaneously withstanding compressional and tensile loads such as coiled tubing. Conduit 102 is moveably coupled to modules 41 with conduit coupling members 110. In other embodiments where the conduit (e.g., conduit 102) does not need to flex or bend, the conduit may be a pipe string comprising a plurality of rigid pipe joints. One conduit coupling member 110 extends radially from each module 41, guides conduit 102 as it moves up and down along structure 10, and enables conduit 102 to provide gas to chambers 44.
Referring now to
A pair of annular seals 116 extend radially inward from guide tubular 112 on opposite sides of port 114—one seal 116 is positioned above port 114 and the other seal 116 is positions below port 114. Seals 116 sealingly engage tubular 112, and sealingly engage conduit 102 as it extends through guide tubular 112. In particular, seals 116 form an annular static seal with tubular 112 and an annular dynamic seal with conduit 102. To ensure conduit 102 is centered in tubular 112 within annular seals 116 as conduit 102 moves through tubular 112, a pair annular ramps 117 having a frustoconical guide or camming surface 118 is disposed within tubular 112 on opposite sides of seals 116—one ramp 117 is positioned axially adjacent and above the upper seal 116 and the other ramp 117 is positioned axially adjacent and below the lower seal 116.
Port 109 in conduit 102 may be positioned within tubular 112 to place conduit 102 in fluid communication with chamber 44 via port 114, conduit 113, and line 115. In particular, conduit 102 is axially advanced through or retracted from tubular 112 to axially position conduit port 109 between annular seals 116, thereby placing conduit 102 in fluid communication with chamber 44 via port 114, conduit 113, and line 115.
Control system 100 allows the relative volumes of air 16 and water 11, 18 in chamber 44 to be controlled and varied, thereby enabling the buoyancy of chamber 44 and associated module 41 to be adjusted. In particular, with valve 107 open and valve 108 closed, air 16 may be vented from chamber 44, thereby allowing water 11, 18 to flow into chamber 44 via port 101 (i.e., decreasing the volume of air 16 and increasing the volume of water 11, 18 in chamber 44); and with valve 108 open and valve 107 closed, air 16 may be pumped from compressor 105 into chamber 44, thereby forcing air 16 into chamber 44 and pushing water 11, 18 out of chamber 44 via port 101 (i.e., increasing the volume of air 16 and decreasing the volume of water 11, 18 in chamber 44). Thus, end 104b functions as an air outlet, whereas end 115b functions as both an air inlet and outlet. With valve 108 closed, air 16 cannot be pumped into chamber 44, and with valves 107, 108 closed, air 16 cannot be vented from chamber 44.
In this embodiment, open end 115b is disposed proximal the upper end of chamber 44 and port 101 is positioned proximal the lower end of chamber 44. This positioning of open end 115b enables air 16 to be vented from chamber 44 when column is in a generally vertical, upright position. In particular, since buoyancy control gas 16 (e.g., air) is less dense than water 11, 18, any air 16 in chamber 44 will naturally rise to the upper portion of chamber 44 above any water 11, 18 in chamber 44 when module 41 is generally upright. Accordingly, positioning end 115b at or proximal the upper end of chamber 44 allows direct access to any air 16 therein. Further, since water 11, 18 in chamber 44 will be disposed below any air 16 therein, positioning port 101 proximal the lower end of chamber 44 allows ingress and egress of water 11, 18, while limiting and/or preventing the loss of any air 16 through port 101. In general, air 16 will only exit chamber 44 through port 101 when chamber 44 is filled with air 16 from the upper end of chamber 44 to port 101. Positioning of port 101 proximal the lower end of chamber 44 also enables a sufficient volume of air 16 to be pumped into chamber 26. In particular, as the volume of air 16 in chamber 44 is increased, the interface between water 11, 18 and the air 16 will move downward within chamber 44 as the increased volume of air 16 in chamber 44 displaces water 11, 18 in chamber 26, which is allowed to exit chamber through port 101. However, once the interface of water 11, 18 and the air 16 reaches port 101, the volume of air 16 in chamber 44 cannot be increased further as any additional air 16 pumped into chamber 44 will simply exit chamber 44 through port 101. Thus, the closer port 101 to the lower end of chamber 44, the greater the maximum volume of air 16 that can be pumped into chamber 44, and the further port 101 from the lower end of chamber 44, the lower the maximum volume of air 16 that can be pumped into chamber 44. Thus, the axial position of port 101 along chamber 44 is preferably selected to achieve the desired maximum volume of air 16 in chamber 44 and associated buoyancy of chamber 44.
In this embodiment, flowline 115 extends radially through tubular 42. However, in general, the flowing extending into the chamber (e.g., flowline 115) may extend through other portions of the module (e.g., module 41). For example, the flowline may extend axially through the module (e.g., through cap 43 at upper end 41a) in route to the ballast adjustable chamber (e.g., chamber 44). Any passages extending through a bulkhead or cap are preferably completely sealed.
Without being limited by this or any particular theory, the flow of water 11, 18 through port 101 will depend on the depth of chamber 44 and associated hydrostatic pressure of water 11 at that depth, and the pressure of air 16 in chamber 44 (if any). If the pressure of air 16 is less than the pressure of water 11, 18 in chamber 44, then the air 16 will be compressed and additional water 11, 18 will flow into chamber 44 through port 101. However, if the pressure of air 16 in chamber 44 is greater than the pressure of water 11, 18 in chamber 44, then the air 16 will expand and push water 11, 18 out of chamber 44 through port 101. Thus, air 16 within chamber 26 will compress and expand based on any pressure differential between the air 16 and water 11, 18 in chamber 44.
It should be appreciated that air 16 will automatically vent from chamber 44 when ends 104b, 115b are in fluid communication. In particular, the air 16 in chamber 44 is compressed due to the hydrostatic pressure of water 11, 18 in chamber 44. End 104b is positioned at the surface 13 (i.e., at about 1 atmosphere of pressure). Thus, when end 104b is in fluid communication with compressed air 16 in chamber 44, the compressed air 16 will inherently flow from the high pressure region (chamber 44) to the lower pressure region (end 104b), thereby allowing water 11, 18 to flood chamber 44 through port 101.
Although only one module 41 and associated chamber 44 is shown and described in
As conduit 102 is moved axially along stem 40, it may be completely removed from select coupling members 110, thereby placing the corresponding flowline 115 in fluid communication with the surrounding environment via conduit 113, port 114, and tubular 112. However, for a given module 41, port 114, conduit 113 and end 115a are disposed at the same axial position as port 101 (at or proximal lower end 41b), and thus, the hydrostatic pressure of water 11 at ports 101, 114 is the same. Since the air 16 in chamber 44 is compressed to the hydrostatic pressure of water 11 at port 101, it is also compressed to the hydrostatic pressure of water 11 at port 114. Therefore, the relative volumes of air 16 and water 11, 18 within a given chamber 44 will remain the same or substantially the same when conduit 102 is completely removed from the corresponding coupling member 110.
As best shown in
Although a single ballast control system 100 and conduit 102 are employed to selectively control and adjust the relative volumes of air 16 and water 11, 18 in each chamber 44 in this embodiment, in other embodiments, each chamber 44 may have its own dedicated ballast control system. For example, each chamber 44 may have a ballast control system configured the same as ballast control system 80 previously described. As another example, conduit 102 may be completely eliminated and each chamber 44 may be selectively deballasted by injecting air using a subsea ROV.
Referring now to
As will be described in more detail below, during installation of structure 10, skirt 31 is urged axially downward into the sea floor 12, and during decoupling of structure 10 from the sea floor 12 for transport to a different offshore location, skirt 31 may pulled axially upward from the sea floor 12. To facilitate the insertion and removal of anchor 30 into and from the sea floor 12, this embodiment includes a suction/injection control system 120.
Referring now to
Pump 123 is configured to pump fluid (e.g., water 101) into cavity 32 and pump fluid (e.g., water 101, mud, silt, etc.) from cavity 32 via line 122 and conduit 121. A valve 125 is disposed along line 122 and controls the flow of fluid through line 122—when valve 125 is open, pump 123 may pump fluid into cavity 32 via line 122 and conduit 121, or pump fluid from cavity 32 via conduit 121 and line 122; and when valve 125 is closed, fluid communication between pump 123 and cavity 32 is restricted and/or prevented.
In this embodiment, pump 123, line 122, and valves 124, 125 are positioned axially above stem 40 and module 20, and may be accessed from deck 60. However, in general, the injection/suction pump (e.g., pump 123), the suction/supply line (e.g., line 122), and valves (e.g., valves 124, 125) may be disposed at any suitable location. For example, the pump and valves may be disposed subsea and/or remotely actuated.
Referring now to
To pull and remove anchor 30 from the sea floor 12 (e.g., to move tower 100 to a different location), valve 124 may be opened and valve 125 closed to vent cavity 32 and reduce the hydraulic lock between skirt 31 and the sea floor 12. Skirt 31 may also be removed from sea floor 12 by pumping fluid (e.g., water 11) into cavity 32 via pump 123, conduit 121 and line 122. In particular, valve 125 may be opened and valve 124 closed to allow pump 123 to inject fluid into cavity 32 through conduit 121 and line 122, thereby increasing the pressure in cavity 32 and urging anchor 30 upward and out of the sea floor 12.
As previously described, in this embodiment, anchor 30 is a suction pile. However, in other embodiments, the anchor (e.g., anchor 30) for coupling the productions structure (e.g., structure 10) to the sea floor may comprise other suitable anchoring devices or system including, without limitation, a driven pile or a gravity anchor. Any of the embodiments for releasably and pivotally coupling structure 10 to anchor 30 described below may be employed with such driven piles or gravity anchors.
Referring now to
To pivotally couple structure 10 and anchor 30, locking blocks 48 are radially withdrawn by actuators 49 as shown in
During offshore operations, systems 80, 100 are employed to adjust the ballast in chambers 26, 44 such that structure 10 remains generally vertical and upright. For example, structure 10 may be configured to be net buoyant (i.e., the total buoyancy of structure 10 exceeds the total weight of structure 10), thereby placing stem 40 and coupling 90 in tension. As another example, structure 10 may not be configured to be net buoyant (i.e., the total buoyancy of structure 10 is less than the total weight of structure 10), with upper module 20 and/or select upper modules 41 configured to be net buoyant to maintain the generally vertical upright orientation of structure 10. In such embodiments, an upper portion of stem 40 is in tension, whereas a lower portion of stem 40 and coupling 90 is in compression. Accordingly, embodiments of couplings between structure 10 and anchor 30 (e.g., coupling 90) are preferably configured to releasably and pivotally couple structure 10 under both tensile and compressional loads. Surfaces 48a of blocks 48 extending along an upper portion and lower portion of mating surface 38 of ball 37 enables coupling 90 to sustain compressional and tensile loads while simultaneously allowing structure 10 to pivot relative to anchor 30. Whether coupling 90 is in tension or compression, anchor 30 maintains engagement with the sea floor 12 and prevents structure 10 from moving translationally relative to anchor 30, while allowing structure 10 to pivot relative to base 30.
Since structure 10 is secured to the sea floor 12 and held in place relative to the sea floor 12 at a single point (via coupling 90), structure 10 may be described as a “single-moored” structure. Structure 10 may be released and decoupled from stabbing member 36 and anchor 30 by radially withdrawing locking blocks 48 with actuators 49, and then lifting or floating structure 10 upward thereby allowing ball 37 to exit cavity 46. Once decoupled from anchor 30, tower 10 may be floated to a different offshore site and installed at the new site with an anchor 30 in the same manner as previously described.
Other examples of suitable pivotable couplings include, without limitation, stabbing connections, U-joints, gimbles, or chain or shackle systems known in the art. Such connections may be configured to be releasable by any means or mechanism known in the art including, without limitation, a J-slot connector, a ball grab, or other remotely actuated releasable connection. Moreover, pivotable and releasable couplings used in conjunction with subsea risers and tendons such as the SCR FlexJoint® Receptacle and Pull-In Connectors available from Oil States International, Inc. of Houston, Tex., FlexJoint® Tendon Bearing available from Oil States International, Inc. of Houston, or H-4 Subsea Connectors available from VetcoGray of Houston, Tex. may also be used in place of coupling 90 previously described.
Referring again to
Structure 10 may be assembled and installed at the desired offshore location in a variety of different manners. For example, structure 10 may be completely assembled on shore or nearshore, transported to the offshore installation site, and coupled to anchor 30. Another exemplary embodiment of a method for assembling and installing structure 10 is schematically illustrated in
Moving now to
Referring now to
As shown in
As previously described, anchor 30 secures structure 10 to the sea floor 12. In general, anchor 30 may be installed at the offshore installation site before, after, or during assembly of structure 10. Thus, anchor 30 may be lowered subsea and secured to the sea floor 12 followed by coupling of structure 10 to anchor 30. For example, anchor 30 may be installed in a similar manner as a conventional driven pile with the exception that system 120 may be employed as previously described to facilitate the insertion of suction skirt 31 into the sea floor 12. In embodiments where anchor 30 is installed in the sea floor 12 prior to coupling structure 10 to anchor 30, structure 10 may be moved laterally over anchor 30, ballasted to advance stabbing member 36 into cavity 46, and then transitioning locking blocks 48 to the radially advanced position, thereby capturing ball 37 within cavity 46. Alternatively, anchor 30 may be coupled to structure 10 and then secured to the sea floor 12 using structure 10. For example, anchor 30 may be coupled to lower end 40b of stem 40 and urged into the sea floor 12 by deballasting structure 10 and employing system 120 as previously described. With structure 10 coupled to anchor 30, and anchor 30 embedded in the sea floor 12, select chambers 26, 44 may be ballasted and/or deballasted to achieve the desired overall buoyancy and orientation of structure 10.
Although not shown in
Referring now to
In this embodiment, the components of structure 10 are assembled piece-by-piece in a vertical stack extending subsea from vessel 200. Assembly stabilizer 230 and lifting apparatus 220 work together to align the axially adjacent components one-above-the-other for subsequent coupling. Specifically, as best shown in
As will be understood by one skilled in the art, vessel 200 may list and rock with the waves at the sea surface 13 during offshore assembly. However, stem modules 41 are preferably coaxially aligned such that they may be coupled together end-to-end to form stem 40. In this embodiment, the stem module 41 supported by lifting apparatus 220 generally maintains its vertical orientation since it is hung from lifting apparatus 220 and is free to move relative to vessel 100 under its own weight. Likewise, stem modules 41 supported by stabilizer 230 generally maintain their vertical orientations. In particular, as best shown in
Referring briefly to
Referring now to
With stem modules 41′, 41″ substantially coaxially aligned, upper stem module 41′ is lowered axially onto lower module 41″ such that lower end 41b of stem module 41′ engages upper end 41a of stem module 41″. A plurality of circumferentially spaced alignment assemblies 180 function to aid in the alignment of modules 41′, 41″ during an after assembly of modules 41′, 41″. In particular, assemblies 180 are preferably positioned to circumferentially align coupling members 110 and riser guides 72 on adjacent modules 41. For purposes of clarity, coupling members 110 and riser guides 72 are not shown in
In this embodiment, each alignment assembly 180 is disposed on the inner surface of tubular 42 and comprise a plurality of circumferentially-spaced male alignment members 181 extending axially downward from lower end 41b of upper stem module 41′, and a plurality of circumferentially-spaced mating female alignment receptacles 182 along upper end 41a of lower stem module 41″. Alignment members 181 and alignment receptacles 182 are sized and configured to matingly engage. In this embodiment, members 181 and receptacles 182 are generally V-shaped—alignment members 181 and alignment receptacles 182 include mating sloped guide surfaces 181a, 182a, respectively, that slidingly engage to guide and funnel members 181 into corresponding receptacles 182. Thus, upper module 41′ is positioned above module 41″ with riser guides 72 substantially circumferentially aligned and coupling members 110 substantially circumferentially aligned. Next, module 41′ is lowered onto module 41″, and sliding engagement of surfaces 181a, 182a guides module 41′ to the desired rotational orientation relative to module 41″ and ensures proper alignment of riser guides 72 and coupling members 110.
Referring again to
Although lifting apparatus 220 and stabilizer 230 are shown and described as being employed during assembly of stem 40, it should be appreciated that lifting apparatus 220 and stabilizer 230 may also be employed to couple upper module 20 to stem 40. Moreover, although assemblies 180 have been shown and described as being used to coaxially align and rotationally orient exemplary modules 41′, 41″ during assembly of stem 40, and assemblies 190 have been shown and described as coupling exemplary modules 41′, 41″ during assembly of stem 40, the remaining modules 41 of structure 10 may be assembled in the same manner, and further, upper module 20 may be coupled to stem 40 in the same manner. For example, upper module 20 may be coupled to upper end 40a of stem 40 using lifting apparatus 220, stabilizer 230, alignment assemblies 180, and coupling assemblies 190 as previously described. Alternatively, after stem 40 is formed, upper module 20, with deck 60 mounted thereto, may be floated over and aligned with stem 40 as previously described and then coupled to stem 40 using alignment assemblies 180 and coupling assemblies 190. It should be appreciated that adjacent modules 41 coupled together with assemblies 190, as well as upper module 20 coupled to stem 40 with assemblies 190, may be decoupled by simply removing each member 193 from is corresponding toothed racks 191, 192. Accordingly, modules 41 may be described as being releasably coupled, and upper module 20 may be described as being releasably coupled to stem 40.
With stem 40 coupled to upper module 20 (with deck mounted thereto and control system 80 installed), buoyancy control gas conduit 102 is installed and advanced through circumferentially aligned coupling members 110. Next, structure 10 is coupled to anchor 30 and secured to the sea floor as previously described, and systems 80, 100 are employed to adjust the buoyancy of modules 20, 41 to achieve the desired net positive buoyancy for structure 10.
In the manners described above, structure 10 is assembled and coupled to base 30 and the sea floor 12 for subsequent production operations. When production ceases or there is a desire to move structure 10 to a new location, structure 10 may released from base 30 by transitioning locking blocks 48 to the radially withdrawn position with actuators 49, deballasting structure 10 and lifting it from stabbing member 36. Structure 10 may then be floated to the new location. At the new location, structure 10 is coupled to an anchor 30 and the sea floor 12 as previously described. If the depth at the new location is different than that of the previous location, stem modules 41 may be added or removed from stem 40 to adjust the overall height of structure 10 as desired.
In the embodiment of structure 10 previously described, buoyancy is primarily provided by upper module 20 (e.g., air 16 in chambers 26, 27). Some buoyancy is also provided by modules 41 (e.g., air 16 in chambers 44). However, in other embodiments, buoyancy may be provided by a plurality of circumferentially spaced buoyancy cans coupled to the upper portion of the structure (e.g., module 20 of structure 10). In yet other embodiments, stem 40 may be replaced with an elongate truss frame. Such a truss frame is generally transparent to currents and waves, and thus, reduces loads on the production structure, but adds weight and does not provide any buoyancy. Accordingly, in such embodiments, the upper module (e.g., module 20) and/or buoyancy cans are relied on to provide sufficient buoyancy to the production structure.
In the manner described, embodiments described herein provide a height adjustable offshore structure 10 that may be used in depths greater than those to which jackup platforms and fixed platforms may be used. Further, since embodiments of structure 10 described herein include a single point mooring and adjustable buoyancy, they may be moved from location-to-location with relative ease and low expense.
While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simply subsequent reference to such steps.
This application claims benefit of U.S. provisional patent application Ser. No. 61/389,577 filed Oct. 4, 2010, and entitled “Tension Buoyant Tower,” which is hereby incorporated herein by reference in its entirety.
Number | Date | Country | |
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61389577 | Oct 2010 | US |