Patent Grant
10053929
Not applicable.
Not applicable.
Embodiments disclosed herein generally relate to offshore oil and gas production operations. More particularly, embodiments disclosed herein relate to systems and methods for coupling risers to floating offshore production vessels.
During offshore oil and gas production operations, risers are coupled to a floating offshore platform (e.g., semi-submersible platform) and extend subsea to a production fluid source disposed at or proximal the sea floor (e.g., a subsea well, a manifold, a subsea pipeline, etc.). In some circumstances, particularly in deep water applications, the weight of the riser results in a significant amount of tension in the upper section of the riser disposed above the surface of the water and coupled to the platform. For steel catenary risers (SCRs), such tension can induce significant bending moments at the connection point(s) between the riser and offshore platform. Movement of the floating platform in response to dynamic loads (e.g., movements caused by wind, waves, and other phenomena) can cause additional tension and bending in the riser which is borne at these connection point(s).
Some embodiments disclosed herein are directed to an extension member for coupling a tapered stress joint to a basket coupled to a porch extending from an offshore platform. In an embodiment, the extension member includes a central axis, a first end, and a second end opposite the first end. In addition, the extension member includes a radially inner surface extending axially from the first end to the second end. The inner surface comprises a first mating profile proximate the first end that is configured to engage a radially outer surface of the tapered stress joint. Further, the extension member includes a radially outer surface extending axially from the first end to the second end. The outer surface comprises a second mating profile proximate the second end that is configured to engage a mating profile within the basket.
Other embodiments are directed to a system for supporting a riser from an offshore platform. In an embodiment, the system includes a basket configured to be coupled to the offshore platform. In addition, the system includes a tapered stress joint coupled to the riser. The tapered stress joint includes a central axis, a first end, a second end opposite the first end, and a radially outer surface that tapers radially inward from the first end toward the second end. Further, the system includes an extension member coupled to each of the basket and the tapered stress joint. The extension member includes a first end and a second end opposite the first end. The extension member is coupled to the tapered stress joint proximate the first end of the extension member. The extension member is coupled to the basket proximate the second end of the extension member.
Still other embodiments are directed to a system for supporting a riser from an offshore platform. In an embodiment, the system includes a connection assembly coupled to the offshore platform. In addition, the system includes a tapered stress joint coupled to the riser. Further, the system includes an extension member coupled to each of the connection assembly and tapered stress joint. The extension member is a hollow tubular member that includes a central axis, a first end, and a second end opposite the first end. In addition, the extension member includes a radially inner surface extending axially between the first end and the second end. Further, the extension member includes a radially outer surface extending axially between the first end and the second end. The extension member is coupled to the connection assembly along the radially outer surface proximate the second end. The extension member is coupled to the tapered stress joint along the radially inner surface proximate the first end.
Embodiments described herein comprise a combination of features and characteristics intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical characteristics of the disclosed embodiments in order that the detailed description that follows may be better understood. The various characteristics and features described above, as well as others, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes as the disclosed embodiments. It should also be realized that such equivalent constructions do not depart from the spirit and scope of the principles disclosed herein.
For a detailed description of various exemplary embodiments, reference will now be made to the accompanying drawings in which:
The following discussion is directed to various exemplary embodiments. However, one of ordinary skill in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.
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 of the two devices, or through an indirect connection that is established via other devices, components, nodes, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a given axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the given axis. For instance, an axial distance refers to a distance measured along or parallel to the axis, and a radial distance means a distance measured perpendicular to the axis.
As previously described, the weight of a riser induces tension in the riser and dynamic movement of the offshore platform to which the riser is coupled (e.g., due to weather, waves or other phenomena) induces bending moments that are borne by the connection point(s) between the platform and the riser. If the bending moments become sufficiently large, they can lead to undesirable fatigue and/or failure at the connection between the riser and platform. Conventionally, the induced bending moments are accommodated by an elastomeric flex joint that allows limited pivoting of the riser relative to the offshore platform. However, as production fluid conditions (e.g., temperature, pressure, etc.) become more extreme, the use of elastomeric flex joints is less feasible. In particular, contact with higher temperature fluids and/or higher pressure fluids weaken the elastomers making up the flex joint, thereby leading the possibility of a leak or other failure. All metal tapered stress joints offer an alternative to elastomeric flex joints, and exhibit increased resistance to the above described harsh operating conditions. However, tapered stress joints are significantly more rigid then elastomeric flex joints, and as a result, tend to transfer much higher bending moments to the support structure on the offshore platform (e.g., the porch and basket). In some cases, a tapered stress joint can transfer a moment that is between four times (4×) and thirty times (30×) greater than the moment transferred by an elastomeric flex joint for a similar tension load on the riser. Most offshore platforms do not include sufficient structures to withstand the high bending moments associated with tapered stress joints. Thus, embodiments disclosed herein include structures for coupling a tapered stress joint to a floating offshore platform that offer the potential to reduce the magnitude of the bending moments experienced at the connection point between the tapered stress joint and the offshore platform during production operations. Accordingly, embodiments described herein can be retrofit for use in connection with existing offshore platforms in place of the more traditional elastomeric flex joint.
Referring now to
Referring now to
Referring still to
Stress joint 60 is generally frustoconical in shape, and thus, radially outer surface 60c tapers radially inward moving axially from upper end 60a toward lower end 60b. In other words, the outer diameter of stress joint 60 decreases moving from upper end 60a to lower end 60b. As a result, stress joint 60 has an increasing degree of flexibility moving axially from upper end 60a toward lower end 60b. During operations, stress joint 60 is inserted within basket 24 such that radially outer surface 60c slidingly engages profile 26, thereby coupling stress joint 60 and riser 50 to platform 20. In this embodiment, stress joint 60 is secured within basket 24 via a friction fit between radially outer surface 60c and a shoulder 27 defined profile 26; however, any other suitable engagement may be used. In addition, in this embodiment, basket 24 of connection assembly 30 is oriented such that when stress joint 60 is inserted axially therein, the central axis 65 of joint 60 forms an angle α with the vertical direction. As shown in
Referring now to
M=(H)(T sin θ).
Depending on the operating conditions (e.g., weight of the riser 50, height of the ocean waves, strength of the ocean current, etc.), the tension T may increase such that the resulting moment M overcomes the strength of basket 24 and/or porch 22, thereby damaging connection assembly 30 by either extreme loading or cyclic overutilization (fatigue). In addition, during operation, the angularity between the riser 50 and platform 20 may also greatly contribute to the magnitude of moment M. Depending on the severity of the damage, the riser 50 may become completely disconnected from platform 20. Simply increasing the load bearing capacity of connection assembly 30 (e.g., basket 24) may not be economically feasible for existing platforms 20 due to the costs of such mechanical modifications to the supporting structure (which may be located under water). Therefore, embodiments disclosed herein are directed to connection assemblies to reduce the bending moments transferred to the basket 24 and porch 22 by the riser 50 and stress joint 60 during such offshore production operations.
Referring now to
Risers 50 extend downward from platform 72 to a production fluid source site (not shown) proximal or at the sea floor. In this embodiment, the risers 50 are steel catenary risers (SCRs), and thus, risers 50 take on a curved shape between platform 72 and the sea floor (not shown). Each riser 50 is coupled to platform 72 with one connection assembly 130. As a result, movements and loads (e.g., tension, torque, etc.) experienced by risers 50 are transferred to platform 72 through the corresponding connection assemblies 130. Conversely, movements and loads experienced by platform 72 are transferred through connection assemblies 130 to risers 50. In general, risers 50 transfer production fluids from the subsea source to platform 72. Thus, during production operations, production fluids are routed from the subsea production site to platform 72 through risers 50.
Referring now to
Upper riser assembly 152 includes a spool 54, a tapered stress joint 60, and an extension member 100. Spool 54 and stress joint 60 are each as previously described. Namely, tapered stress joint 60 includes a central axis 65, a first or upper end 60a, a second or lower end 60b opposite upper end 60a, and a frustoconical radially outer surface 60c extending between ends 60a, 60b. Upper end 60a of stress joint 60 is coupled to spool 54 with a first or upper connection flange 62 and lower end 60b of stress joint 60 is coupled to riser 50 with a second or lower connection flange 64. Spool 54 extends from upper end 60a of stress joint 60 to additional piping 56 on platform 20. In this embodiment, the central axis 65 of joint 60 forms an angle α with the vertical direction. In general, L100 angle between 0° and 90°. As shown in
Referring still to
Extension member 100 includes a total length L100 measured axially (relative to axis 105) between ends 100a, 100b. In some embodiments, length L100 ranges from 10 to 20 feet. In this embodiment, length L100 is 15 feet. Further, extension member 100 has an extension length L110-120 measured axially (relative to axis 105) between mating profiles 110, 120. Extension length L110-120 represents the minimum distance between the region or point of engagement of upper profile 110 and radially outer surface 60c of stress joint 60 and the region or point of engagement of lower profile 120 and profile 26 of basket 24. In embodiments described herein, extension length L110-120 ranges from 5 to 25 ft. In this embodiment, extension length L110-120 is 15 ft.
As will be described in more detail below, extension length L110-120 generally represents the axial displacement of the stress joint 60 from basket 24 as compared to the connection assembly 30 shown in
To couple riser 50 to platform 20, lower end 100b of extension member 100 is inserted within basket 24 until lower profile 120 slidingly engages mating profile 26 and lower end 100b engages or abuts shoulder 27 of basket 24 as previously described. Thereafter, stress joint 60 is inserted axially through extension member 100 until frustoconical outer surface 60c of stress joint 60 slidingly engages and is seated on the frustoconical surface of upper profile 110 of extension member 100 as previously described. Upper end 60a of stress joint 60 is then coupled to spool 54 at connection flange 62 and lower end 60b is coupled to riser 50 at connection flange 64.
Referring now to
M1=(H1)(T sin θ).
The height H1 is approximately the same (or at least similar) to the height H shown in
M2=(H2)(T sin θ).
As is evident from
In addition to reducing the bending moment exerted on basket 24 and porch 22, extension member 100 may also provide additional flexibility to upper riser assembly 152 such that the amount or degree of bending of stress joint 60 may be reduced during operations. Such a reduction in the required bending or curvature in stress joint 60 increases the service life of stress joint 60 and allows for the use of smaller and more cost effective stress joints for connecting riser 50 to platform 20. In some embodiments, it is preferable that extension member 100 be ⅕th or less as flexible as stress joint 60 to ensure the desired bending and performance thereof. In addition, in some embodiments, it is preferable that the extension member 100 have a bending stiffness within +/−20% of the bending stiffness of the tapered stress joint 60 proximate upper end 60a. Accordingly, in some embodiments, extension member 100 may also include one or more material selection and/or design features that increase the flexibility of extension member 100 about axis 105.
For example, referring now to
Also for example, referring now to
As shown in
Without being limited to this or any other theory, slots 130 effectively reduce the amount of material making up extension member 100 (particularly the second moment area) such that extension member 100 is more flexible about central axis 105. In other words, slots 130 allow extension member 100 to more easily bend or flex relative to axis 105 such that extension member 100 may reduce the amount of bending or flexing that is required of stress joint 60 during operations (e.g., as a result of tension T).
While the embodiment of
Referring now to
Each aperture 140 is circular in shape and extends between surfaces 100c, 100d of extension member 100 (i.e., apertures 140 extend completely through the wall of extension member 100). In addition, in this embodiment, each aperture 140 includes a maximum inner diameter D140 that may range from ⅛ to 3 in., and preferably equals ½ in.
Without being limited to this or any other theory, apertures 140 effectively reduce the amount of material making up extension member 100 such that extension member 100 is more flexible about central axis 105. In other words, apertures 140 allow extension member 100 to bend or flex relative to axis 105 such that extension member 100 may reduce the amount of bending or flexing that is required of stress joint 60 during operations (e.g., as a result of tension T).
While apertures 140 have been shown and described as being circular in shape, it should be appreciated that in other embodiments, apertures 140 may be formed in various other shapes. For example, in some embodiments, apertures 140 may be elliptical, rectangular, square, polygonal, triangular, etc. Also, regardless of the shape of apertures 140, each aperture 140 may include fillets and/or radiused surfaces to avoid the formation of stress concentrations and to avoid the manufacturing expense of recessed corners. In addition, while apertures 140 have been shown and described as extending with in a region of extension member 100 that extends axially between mating profiles 110, 120, in other embodiments, apertures 140 may extend in other or additional regions of extension member 100. Further, while apertures 140 have been shown and described as being disposed in axially extending columns 142 and helically extending rows 144, it should be appreciated that the number and arrangement of apertures 140 may be greatly varied in other embodiments. For example, in some embodiments, apertures 140 may be disposed in a plurality of axially extending columns and circumferentially extending rows (i.e., adjacent axial columns are not axially offset from one another as shown in
Referring now to
As shown in
Without being limited to this or any other theory, ribs 150 provide additional structural support and rigidity to extension member 100 such that the wall thickness of extension member 100 between ribs 150 (e.g., the radial distance between surfaces 100c, 100d) can be reduced to thereby result in a desired amount of flexibility of extension member 100 relative to axis 105. In other words, the reduced wall thickness of extension member 100 between ribs 150 allows extension member 100 to bend or flex relative to axis 105 such that extension member 100 may reduce the amount of bending or flexing that is required of stress joint 60 during operations (e.g., as a result of tension T).
In some embodiments, the thickness T150 and width W150 of each rib 150 may taper along length L150 between ends 150a, 150b. For example, in some embodiments, the thickness T150 and/or width W150 of each rib 150 may taper from larger values at one end (e.g., end 150a or end 150b) to smaller values at the other end (e.g., end 150b or end 150a). The tapering of thickness T150 and/or width W150 may be gradual (e.g., linear) or thickness T150 and/or width W150 may include one or more step changes between ends 150a, 150b. In addition, while ribs 150 are shown and described herein as being rectangular shaped projections, it should be appreciated that ribs 150 may be formed in a wide variety of shapes (e.g., elliptical, triangular, etc.).
Referring now to
Without being limited to this or any other theory, the reduced wall thickness (e.g., thickness T160) of region 160 increases the flexibility of extension member 100 about central axis 105. In other words, region 160 allows extension member 100 to bend or flex relative to axis 105 such that extension member 100 may reduce the amount of bending or flexing that is required of stress joint 60 during operations (e.g., as a result of tension T).
While only a single region 160 is shown in the embodiment of
In addition to the particular embodiment of the extension member shown in
Similarly, it is not necessary for lower mating profile 120 to be frustoconical in shape. The important function of this feature is to provide a lower end 100b that engages or abuts shoulder 27, thereby securing extension member 100 within basket 24. In addition, lower mating profile 120 may generate friction between profile 26 and lower mating profile 120, to further secure lower end 100b of extension member 100 within basket 24. In order to accomplish that, however, it is not necessary for lower mating profile to be frustoconical in shape. Thus, to achieve the optional purpose of generating such friction, lower mating profile 110 could have a surface that is stepped or curvilinear or any other shape, as long as the outer diameter at the top end of the lower mating profile is larger than the outer diameter at the bottom end of the lower mating profile.
In the manner described, by coupling a stress joint (e.g., stress joint 60) to a basket (e.g., basket 24) of an offshore platform (e.g., platform 20) with an extension member in accordance with the embodiments disclosed herein (e.g., extension member 100), the bending moment experienced by the basket and adjacent support structures (e.g., porch 22) as a result of tension in the riser may be reduced. As a result, the basket may be utilized with a metallic tapered stress joint even when higher bending loads (e.g., caused by environmental conditions) are expected. In addition, through use of an extension member in accordance with the embodiments disclosed herein, the amount of bending typically experienced by the stress joint may be reduced due to the additional bending of the extension member during operations. As a result, the life of the stress joint may be increased and the operating requirements for the stress joint may be reduced.
While exemplary 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, in some embodiments, the slots 130 may be tapered such that each slot 130 is wider at one end (e.g., an upper end) and narrower at an opposite end (e.g., a lower end). As another embodiment, in some embodiments, the wall thickness of extension member 100 may be tapered between the ends 100a, 100b.
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 simplify subsequent reference to such steps.
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