Patent Application
20190284912
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The disclosure generally relates to the production of hydrocarbons from subsea formations. More particularly, the disclosure relates to floating platform and disconnectable assemblies of risers and related support structures used in such production.
In producing hydrocarbons from subsea formations, a number of wells are typically drilled into the seabed in positions that are not directly below or substantially within the outline of an offshore floating platform, such as a floating offshore production platform. The produced hydrocarbons are subsequently exported via subsea pipelines or other means. Current engineering practice links the offset wells with the floating platform through risers that generally have a catenary curve between the platform and the seabed. Wave motion, water currents, and wind cause movement of the floating offshore structure and/or risers themselves with corresponding flex and longitudinal stress in the risers.
The current state of the art has accommodated the flex in the risers by incorporating flexible risers. However, the flexible risers are generally more expensive and less reliable long-term than rigid pipe segments that are welded together.
Several types of risers are designed to be coupled to the floating offshore structure through guide tubes extending from the lower keel of the offshore structure to the upper part of the offshore structure. A guide tube is generally an elongated conduit that forms a guide through which the riser is pulled or otherwise moved from the seafloor and coupled to the offshore structure. The guide tube is attached to the offshore structure generally at an angle from the vertical, so as to be in line with a natural catenary angle that the installed riser would assume on a calm day or in line to form a lazy wave shape. As the offshore structure shifts laterally and vertically, the guide tube helps reduce stresses on the riser.
Typically, a tapered stress joint is placed near a lower exit location of the guide tube adjacent to one of the attachment points and is sized to control the riser stress. The main function of a guide tube stress joint is to provide flexible support for the riser.
Another option to bending flexibility through a tapered stress joint is to use a flexible joint on the riser. However, such a joint is still expensive and can cost as much or more as the tapered stress joints.
A further complication arises in some hostile environments, such as in locations prone to ice bergs and other locations prone to hurricanes or typhoons. To avoid a potentially catastrophic damage to a floating platform, the floating platform sometimes has sufficient time to be moved out of the way of an approaching iceberg or hurricane/typhoons. However, because the floating platform is typically connected to multiple risers, the time to disconnect and the large expense renders such operations difficult and expensive.
Some efforts have previously been done to address a more quickly disconnectable assembly from a floating platform. A subsea module can support risers below the water surface, while the remainder of the floating platform can be disconnected and temporarily moved to another location. For example, U.S. Pat. No. 7,197,999, entitled “Spar Disconnect System” illustrates a spar-type floating platform with a disconnectable subsea mooring buoy module. The disconnectable subsea module can facilitate separating the risers into an upper portion that remains with the floating platform, and a lower portion that remains with the subsea module, while the floating platform is disconnected and moved to a temporary location. FIGS. 11 and 12 of this patent with original reference numbers are copied for illustrative purposes as
Another example is illustrated in U.S. Pat. No. 8,881,826, entitled “Installation For The Extraction Of Fluid From An Expanse Of Water, And Associated Method”. FIGS. 1 and 4 of this patent with original reference numbers are copied for illustration as
A further example is illustrated in U.S. Pat. No. 7,669,660, entitled “Riser Disconnect And Support Mechanism”. A series of risers that flexibly bend around a laterally extended support arch of a main subsea body can be lowered and raised between the main body portion and the arch. A floating platform can be connected to the subsea main body and the risers raised from the main body to be connected to the topside of the floating platform for production, and disconnected and lowered from the floating platform so that the floating platform moved when needed. FIGS. 1, 2 and 3 of this patent with original reference numbers are copied for illustrative purposes as
While the above examples address the persistent challenge of providing a structure that can be disconnected from the floating platform for reconnection later, each has challenges. The expense of arch supports, complexity of multiple connections between the endpoints of the subsea well to the working deck of the floating platform, and stress on the risers in the repetitive bending from lowering during disconnection times are some of the challenges. A less expensive and easier solution is needed.
The present invention provides a buoyant system and method for a hydrocarbon offshore floating platform to be coupled and decoupled from a subsea buoyant extension with risers slidably coupled thereto. The buoyant system can allow rigid or flexible risers to be coupled and decoupled and alternatively move between a first elevation below the offshore floating platform, such as at the buoyant extension, and a higher second elevation at the offshore floating platform independent of a spool piece, arch support, and flexible joint for the risers. The buoyant system can reduce riser stress by reducing bending required for the riser to form a catenary or other curved shape even as a rigid riser.
The disclosure provides a buoyant system configured to be detachable from an offshore floating platform, comprising: a buoyant extension having a buoyancy and configured to be detachably coupled with the offshore floating platform; and a first guide tube coupled to the buoyant extension and configured to allow a rigid portion of a riser to pass through the guide tube alternatively between a first elevation below the offshore floating platform and a higher second elevation at the offshore floating platform.
The disclosure further provides a method of using an offshore floating platform with a buoyant system having a buoyant extension and a first guide tube coupled to the buoyant extension and configured to allow a rigid portion of a riser to be moved through the guide tube alternatively between a first elevation below the offshore floating platform and a higher second elevation at the offshore floating platform, and wherein the riser has been coupled through the guide tube on the buoyant extension to the floating platform, the method comprising: disconnecting the riser from the floating platform; allowing the rigid portion of the riser to descend to the first elevation below the offshore floating platform while sliding through the first guide tube on the buoyant extension; disconnecting the floating platform from the buoyant extension with the riser remaining on the buoyant extension; and moving the floating platform to a location other than a location of the buoyant extension.
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 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 disclosure 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 or location, or with 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. Further, the various methods and embodiments of the system 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 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 term “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 terms “top”, “up’, “upward’, “bottom”, “down”, “downwardly”, and like directional terms are used to indicate the direction relative to the figures and their illustrated orientation and are not absolute in commercial use but can vary as the assembly varies its orientation. 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. Some elements are nominated by a device name for simplicity and would be understood to include a system of related components that are known to those with ordinary skill in the art and may not be specifically described. Various examples are provided in the description and figures that perform various functions and are non-limiting in shape, size, description, but serve as illustrative structures that can be varied as would be known to one with ordinary skill in the art given the teachings contained herein. As such, the use of the term “exemplary” is the adjective form of the noun “example” and likewise refers to an illustrative structure, and not necessarily a preferred embodiment.
The present invention provides a buoyant system and method for a hydrocarbon offshore floating platform to be coupled and decoupled from a subsea buoyant extension with risers slidably coupled thereto. The buoyant system can allow rigid risers to be coupled and decoupled and alternatively move between a first elevation below the offshore floating platform, such as at the buoyant extension, and a higher second elevation at the offshore floating platform independent of a spool piece, arch support, and flexible joint for the risers. The buoyant system can reduce riser stress by reducing bending required for the riser to form a catenary or other curved shape even as a rigid riser.
The floating platform 102 can be detachably coupled with a buoyant extension 110. In general, an extension upper portion 108 of the buoyant extension 110 can form a transition for the buoyant extension 110 with the floating platform 102, and some such embodiments are illustrated herein. However, it is expressly stated that the buoyant extension 110 does not require the extension upper portion 108 for the buoyant extension 110 to be coupled to the floating platform 102. The buoyant extension 110 generally is at least partially buoyant. The buoyancy is generally sufficient for the weight of the structure with the risers and mooring lines coupled to the structure to maintain the risers above a seabed and reduce riser stress by reducing bending required for the riser to form a catenary or other curved shape even as a rigid riser. In some embodiments, the extension upper portion 108 can have sufficient buoyancy for the remaining portion of the buoyant extension 110. In other embodiments, the extension upper portion 108 as well as other structure of the buoyant extension 110 can both contribute to the buoyancy. For example and without limitation, tubular components of the buoyant extension 110 may be at least partially sealed to create buoyancy. Other chambers, buoyant fill material, or structure can be provided for the buoyant extension 110 to have buoyancy independent of the extension upper portion 108. In other embodiments, the extension upper portion 108 may not contribute a significant amount, if any, to the buoyancy of the buoyant extension 110 and the buoyancy can be designed into other portions of the structure of the buoyant extension 110. In yet other embodiments, the extension upper portion 108 may not be present, so that the buoyant extension 110 is designed for buoyancy without the extension upper portion 108.
The length of the remaining portion of the buoyant extension 110 can be for example and without limitation at least the length of the extension upper portion 108 and longer such as at least twice the length of the extension upper portion. In at least one embodiment, the buoyant extension can be an “open structure”. An “open structure” is intended to mean a structure that allows water to pass through laterally, that is, the structure is not a sealed container with closed sides and closed bottom. An open structure buoyant extension 110 can be formed as a truss structure with vertical legs, horizontal legs, and cross bracing. Alternatively, the buoyant extension can be an at least partially closed container that restricts water passing through laterally, such as having sides and/or a bottom that allow at least some water to pass therethrough where the sides could be closed. The shape of the buoyant extension can vary and can include various geometrical shapes, including cylindrical, cubic, conical and frustoconical (such as without limitation having a larger base cross section than an upper cross section), and cross sections of square, rectangular, circular, elliptical, rhombus, and other polygons.
The buoyant extension 110 can be moored to a subsurface structure, such as a seabed, by one or more mooring lines 114 that for purposes herein can include traditional mooring lines or tendons. In some embodiments, the extension upper portion can be temporarily dynamically positioned, such as with thrusters, prior to being moored at a given location, particularly if the extension upper portion is wholly subsea, and reduce the need for mooring lines. In the embodiment shown in
At least one riser 113 and generally a plurality of risers are slidably coupled through the buoyant extension 110 and the extension upper portion 108 if present between a first elevation below the offshore floating platform, such as at the buoyant extension 110, and a generally higher second elevation at the offshore floating platform 102, as explained below. The term “riser” is broadly used herein and includes Steel Catenary Risers (SCRs), Steel Lazy Wave Risers (SLWRs), rigid risers, flexible risers, and umbilical lines. Advantageously, the riser can be rigid or at least partially rigid for reduced cost.
The extension upper portion 108 and buoyant extension 110 are configured to support and guide the riser 113 independent of a spool piece, arch support, and flexible joint that prior efforts relied on. While such structures may be optionally present, the system is configured to depend on the buoyant extension without needing such structures. While flexible risers can be used, the system can also use rigid risers. In at least one embodiment, the risers can be continuous between subsea production equipment and production equipment on the floating platform, where at least an upper portion of the riser could pass through the buoyant extension and the extension upper portion, if present (as illustrated in
When appropriate, the floating platform 102 can be brought back to the extension upper portion 108 and coupled thereto. Equipment, such as winches and the like, can raise the upper end 124 of the riser 113 and fluidicly couple the upper end to production equipment on the floating platform, such as a well tree 126 and/or valve 128.
In some embodiments, it may be advantageous to attach one or more buoyancy modules 134 to the riser 113. For example, the depth of the buoyant extension may be too shallow to allow the riser 113 to maintain a normal catenary shape or other appropriate curve that may overstress the riser when the riser is lowered from the floating platform to the buoyant extension. In other scenarios, the floating platform could have a large offset response that could overstress the riser. One or more such buoyancy modules 134 may be advantageously used to elevate the riser 113 above the seabed 132 to help avoid sharp bends and overstressing the riser as it curves to the subsea production equipment.
The system can include the mooring lines 114 coupled to the buoyant extension (or buoyant extension and turret), as described above. Risers 113 can be slidably coupled in guide tubes 112 and extend from the platform through the buoyant extension to subsea equipment below the platform.
Other and further embodiments utilizing one or more aspects of the inventions described above can be devised without departing from the disclosed invention as defined in the claims. For example, various shapes of extension upper portions and buoyant extensions and other variations can occur in keeping within the scope of the claims, and other variations.
The invention has been described in the context of preferred and other embodiments, and not every embodiment of the invention has been described. Obvious modifications and alterations to the described embodiments are available to those of ordinary skill in the art. 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 equivalents of the following claims.
