Patent Application
20120201611
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1. Field of the Invention
The invention relates to generally to the field of floating offshore platforms or vessels for the exploitation of undersea deposits of petroleum and natural gas. More specifically, it relates to a system and apparatus for tensioning risers that extend from a subsea wellhead or subsurface structure to a floating platform or vessel.
2. Description of the Related Art
Offshore platforms for the exploitation of undersea petroleum and natural gas deposits typically support production risers that extend to the platform from one or more wellheads or structures on the seabed. In deep water applications, floating platforms (such as spars, tension leg platforms, extended draft platforms, and semi-submersible platforms) are typically used. These platforms are subject to motion due to wind, waves, and currents. Consequently, the risers employed with such platforms must be tensioned to permit the platform to move relative to the risers. Also, riser tension must be maintained, so that the riser does not buckle under its own weight. Accordingly, the tensioning mechanism must exert a substantially continuous tension force to the riser within a well-defined range. One broad class of risers is the category called ‘Top Tensioned Risers’ or TTRs. Such risers extend from the subsea wellheads below the hull of the platform substantially vertically to the deck area of the platform, where they are supported by a tensioning mechanism. Such risers are termed “Top Tensioned Riser.” Each TTR typically extends from a riser tension point up into the production deck levels of the platform with the use of a heavy wall conduit or stem joint. At the top of the conduit or stem joint is an upper riser termination, where a surface wellhead and a production tree or flow control device are mounted. (Platforms with such an arrangement are called ‘dry tree’ platforms.) A flexible jumper attached to the production tree enables the produced well fluids to be transferred to the topside processing facilities.
Passive buoyancy cans are a well-known type of riser tensioning mechanism that is used primarily on spars. The buoyancy cans independently support each TTR, which allows the platform to move up and down relative to the riser. This ability to move isolates the risers from the heave motion of the platform and eliminates any increased riser tension caused by the horizontal offset of the platform in response to the marine environment.
Hydro-pneumatic tensioner systems are another form of a riser tensioning mechanism used to support TTRs on various dry tree platforms. Hydro-pneumatic riser tensioning has its origins in the support of drilling risers of MODUs (mobile offshore drilling units).
A plurality of active hydraulic cylinders with pneumatic accumulators can be connected between the platform and the riser to provide and maintain the necessary riser tension. Platform responses to environmental conditions, mainly heave and horizontal motions causing hull set-down, necessitate changes in riser length relative to the platform, which causes the tensioning cylinders to stroke in and out. The spring effect caused by the gas compression or expansion during riser stroke partially isolates the riser from the low heave platform motions, while maintaining a nearly constant riser tension. However, when the platform takes a significant horizontal offset, the compression of the gas in the cylinders causes increased cylinder pressure and thus increased riser tension. The magnitude of this increased riser tension is a function of the stiffness of the riser and the tensioning system. Such tensioning systems are designed, so that each system operates independently on a single riser. Further, several risers can be individually supported by a movable deck, which in turn is supported by winches or hydraulic cylinders to the floating platform. The movable deck is constrained from horizontal movement by guides, rails, and other structures but allowed to move in the vertical direction. The winches or hydraulic cylinders appear to allow macro adjustments to an overall elevation of the deck relative to the platform, while the individual tensioning systems operate as heave and horizontal motion compensators upon the individual risers. Examples of such solutions are disclosed in U.S. Pat. Nos. 4,934,870, 6,431,284 and 6,6691,784.
While the benefits of a vertically movable deck with multiple risers are disclosed in systems of such patents, such systems retain the complexities of individual tensioning systems for each riser. There remains a need for a different system and method for an offshore platform suspending multiple risers therefrom.
The disclosure provides a deck tensioning system, supporting multiple risers, that is coupled to an offshore floating platform and compensated for heave and horizontal movement. Such a tensioning system provides simplified access to the production trees on the risers. The tensioning system provides tensioning for a tensioning deck to which the multiple risers are coupled, while reducing the tensioning cylinders for individual risers. The deck tensioning system further provides a simplified tension flex connector (“TFC”) with an intermediate movable member for each riser. The TFC assists in absorbing and/or adjusting for forces that may cause the tensioning deck to pitch and roll from the interactions between the multiple risers coupled to the tensioning deck as a unitary structure. If a fire or other event causes damage to the tension flex connector, the tension flex connector is designed to maintain a supporting connection with the riser without the intermediate movable member.
The disclosure provides a deck tensioning system for an offshore floating platform having a plurality of risers extending downward from the platform and suspended from the platform, comprising: a tensioning deck having a frame, the frame defining a plurality of openings adapted to receive the plurality of risers; a plurality of flexibly mounted control cylinders adapted to suspend the tensioning deck to the offshore platform, the control cylinders coupled with flexible connections to the tensioning deck, the platform, or a combination thereof, the flexible connections adapted to allow the tensioning deck to move in at least one horizontal direction relative to the offshore floating platform; and a plurality of tension flex connectors coupled between the tensioning deck and the risers, the tension flex connectors disposed in the openings of the frame, the tension flex connectors adapted to allow the risers to move at angles to the tensioning deck while the tension flex connectors support the risers to the tensioning deck.
The disclosure also provides a method of supporting a riser from an offshore floating platform, comprising: supporting a tensioning deck from the offshore platform, the tensioning deck having a frame; allowing the frame to move in at least one horizontal direction and a vertical direction relative to the platform; supporting a plurality of risers from the tensioning deck; and allowing the risers to move angularly relative to the tensioning deck.
The Figures described above and the written description of specific structures and functions below are not presented to limit the scope of what Applicant has invented or the scope of the appended claims. Rather, the Figures and written description are provided to teach any person skilled in the art how to make and use the inventions for which patent protection is sought. Those skilled in the art will appreciate that not all features of a commercial embodiment of the inventions are described or shown for the sake of clarity and understanding. Persons of skill in this art will also appreciate that the development of an actual commercial embodiment incorporating aspects of the present inventions will require numerous implementation-specific decisions to achieve the developer's ultimate goal for the commercial embodiment. Such implementation-specific decisions may include, and likely are not limited to, compliance with system-related, business-related, government-related and other constraints, which may vary by specific implementation, location, and from time to time. While a developer's efforts might be complex and time-consuming in an absolute sense, such efforts would be, nevertheless, a routine undertaking for those of ordinary skill in this art having benefit of this disclosure. It must be understood that the inventions disclosed and taught herein are susceptible to numerous and various modifications and alternative forms. The use of a singular term, such as, but not limited to, “a,” is not intended as limiting of the number of items. Also, the use of relational terms, such as, but not limited to, “top,” “bottom,” “left,” “right,” “upper,” “lower,” “down,” “up,” “side,” and the like are used in the written description for clarity in specific reference to the Figures and are not intended to limit the scope of the invention or the appended claims. Where appropriate, some elements have been labeled with an alphabetic character after a number to reference a specific member of the numbered element to aid in describing the structures in relation to the Figures, but is not limiting in the claims unless specifically stated. When referring generally to such members, the number without the letter is used. Further, such designations do not limit the number of members that can be used for that function.
The disclosure provides a deck tensioning system, supporting multiple risers, that is coupled to an offshore platform and compensated for heave and horizontal movement. Such a tensioning system provides simplified access to the production trees on the risers. The tensioning system provides tensioning for a tensioning deck to which the multiple risers are coupled, while reducing the tensioning cylinders for individual risers. The deck tensioning system further provides a simplified tension flex connector (“TFC”) with an intermediate movable member for each riser. The TFC assists in absorbing and/or adjusting for forces that may cause the tensioning deck to pitch and roll from the interactions between the multiple risers coupled to the tensioning deck as a unitary structure. If a fire or other event causes damage to the tension flex connector, the tension flex connector is designed to maintain a supporting connection with the riser without the intermediate movable member.
The deck tensioning system 1 tensions multiple risers with compensated tensioning. The deck tensioning system 1 is coupled to an offshore floating platform 24, generally having a hull deck 25, such as shown in
The tensioning deck 2 is coupled below one or more control cylinders 3 (such as hydraulic or pneumatic cylinders or combinations thereof) on an attachment 4 via a connection 3b on the lower end of the cylinder to allow for horizontal movement of the tensioning deck from the relative heave, pitch, yaw, and roll of the floating platform 24, in contrast to other suspended decks such as disclosed in the above background section. The upper end 3a of each cylinder 3 is coupled to the floating platform 24, such as to the hull deck 25. One or both of the connections 3a, 3b may be a flexible connection that allows movement in at least one horizontal direction and advantageously in multiple horizontal directions (that is, in orthogonal “x” and “y” directions, including combinations thereof). Such a flexible connection can include a moment-free connection that allows movement at multiple angles in at least two horizontal directions. Example of such a moment-free connection could be a spherical bearing, a flex joint, or simply two shackles coupled together.
A plurality of risers 7 are coupled to the tensioning deck 2, which may pitch, roll and/or stroke up or down. In some embodiments, an optional lower structure 9 coupled to the platform 24 below the tensioning deck 2 may further support movement of the risers 7 and would depend on a riser analysis and hull analysis based on a riser loading to the hull. In such embodiments, an optional roller 10 can be used to assist the risers to move vertically relative to the platform as the platform moves. These motions occur due to the floating platform's offset from a pattern center, heave, pitch, or roll, causing a change in a geometric distance between the riser's null elevation at a rest state on the vessel and the subsea wellhead. The change in distance results in a tightening or relaxing of the supported risers. The change may be different between the risers 7 attached to the same tensioning deck 2 and result in unequal loading of the tensioning deck. The unequal loading may cause the tensioning deck 2 to pitch, roll, and stroke as the hydraulic cylinders passively equalize the moment and forces imparted to the tensioning deck by both the cylinders 3 and risers 7 when the floating platform heaves, pitches, yaws, and rolls relative to the tensioning deck. Due to the pitch/roll of the tensioning deck 2, a new connection between the riser (tension joint) and the tensioning deck is desirable.
A flexible connection is provided between the risers 7 and the tensioning deck 2. As more particularly seen in
Referring particularly to
The TFC 8 minimizes the forces transmitted to the risers due to pitch/roll of the tensioning deck 2. Using a tensioning deck 2 with TFCs 8 for a deck tensioning system 1 provides the advantages of simplified access of the production trees, reduction in the number of tensioning cylinders required, and ultimately a reduction in weight and cost of the tensioning system.
Other and further embodiments utilizing one or more aspects of the inventions described above can be devised without departing from the spirit of the invention. For example, different connections and equipment can be used for heave and horizontal movement of the tensioning deck and for the riser tension flex connector. Other variations in the system are possible.
Further, the various methods and embodiments described herein can be included in combination with each other to produce variations of the disclosed methods and embodiments. Discussion of singular elements can include plural elements and vice-versa. References to at least one item followed by a reference to the item may include one or more items. Also, various aspects of the embodiments could be used in conjunction with each other to accomplish the understood goals of the disclosure. Unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising,” should be understood to imply the inclusion of at least the stated element or step or group of elements or steps or equivalents thereof, and not the exclusion of a greater numerical quantity or any other element or step or group of elements or steps or equivalents thereof. The device or system may be used in a number of directions and orientations. The term “coupled,” “coupling,” “coupler,” and like terms are used broadly herein and may include any method or device for securing, binding, bonding, fastening, attaching, joining, inserting therein, forming thereon or therein, communicating, or otherwise associating, for example, mechanically, magnetically, electrically, chemically, operably, directly or indirectly with intermediate elements, one or more pieces of members together and may further include without limitation integrally forming one functional member with another in a unitary fashion. The coupling may occur in any direction, including rotationally.
The order of steps can occur in a variety of sequences unless otherwise specifically limited. The various steps described herein can be combined with other steps, interlineated with the stated steps, and/or split into multiple steps. Similarly, elements have been described functionally and can be embodied as separate components or can be combined into components having multiple functions.
The invention has been described in the context of preferred and other embodiments and not every embodiment of the invention has been described. Apparent modifications and alterations to the described embodiments are available to those of ordinary skill in the art given the disclosure contained herein. The disclosed and undisclosed embodiments are not intended to limit or restrict the scope or applicability of the invention conceived of by the Applicant, but rather, in conformity with the patent laws, Applicant intends to protect fully all such modifications and improvements that come within the scope or range of equivalent of the following claims.
