Patent Application
20150037103
Embodiments disclosed herein relate to a tendon system for use with tension leg platforms (TLP). More specifically, embodiments disclosed herein relate to a tendon system that includes Cellular Tendon arrangement as the main body of at least one tendon or tendon module or a portion of one tendon or tendon module, designed and configured to enable use of the tendon, and thus use of TLPs, in ultra-deep waters and/or for heavy topsides, or to improve constructability and transportation and installation and/or project cost for any water depth.
One type of offshore drilling and production platform is a tension leg platform, generally called a TLP, which utilizes tendons to support the platform. The tendons have lower terminations that connect to pilings on the sea floor. The upper ends connect to top connectors on the platform. The platform is de-ballasted after connection to the top connector, placing the tendons in tension.
One type of tendon includes a main body that is a single steel tubular formed of multiple segments connected together with mechanical connections. The pipes in the tendon segments have hollow interiors that are sealed from sea water to provide buoyancy. Bulkheads may be located within the interior, dividing the hollow interior in separate compartments sealed from each other. Use of conventional single steel pipe design for the tendon main body has technical and practical limitations in meeting the combined stiffness and the tension-collapse resistance requirements, limiting the depth and topside payload at which conventional tendon systems may be used.
U.S. Pat. No. 6,851,894 discloses tubular sections having three different wall thicknesses. The upper section has a greater diameter but lesser wall thickness than an intermediate section, and the intermediate section has a greater diameter but lesser wall thickness than the lower section.
As TLP platforms are located in deeper waters, providing steel tubular tendons that can resist the hydrostatic pressure becomes an increasingly difficult problem. For example, U.S. Pat. No. 7,422,394 discloses use of a tendon having a varied diameter, decreasing in diameter with depth, and limiting the diameter to thickness ratio at depth to overcome crushing force and hydrostatic pressure increases.
Various methods have been proposed to overcome this hindrance to use of TLPs in extremely deep water. For example, another type of tendon is a solid cable, formed of composite fibers, such as carbon fibers (often called a tether). Typically, a composite tendon, such as disclosed in U.S. Pat. No. 7,140,807, has an elastomeric jacket that encloses several bundles of fibers. A spacer or filler fills the interior space surrounding the fibers. Steel terminations are located on the ends of the separate rods or sections of a composite tendon for connecting the sections to each other.
Composite fiber tendons are generally smaller in diameter than steel tubular tendons and weigh less. However, they are less buoyant, such as their specific gravities being around 0.85 where 1.00 is considered neutral. Having solid interiors, composite fiber tendons are able to withstand high hydrostatic pressures. However, the lack of buoyancy limits the usefulness of composite fiber tendons in very deep water because a larger and more buoyant hull for the TLP is required in order to maintain the required tension at the bottom connector. Also, fatigue of the upper portion of a composite fiber tendon can be a concern because of the high bending moments caused by TLP lateral motion. U.S. Pat. No. 4,990,030 discloses use of composite fibers in the interior of a steel pipe section, which may provide the buoyancy, but as noted above, the use of the single pipe would be unsuitable for deep waters due to the high hydrostatic pressures.
In one aspect, embodiments disclosed herein relate to a cellular tendon system that may be used to moor a floating structure to a seabed. The tendon system may include a top tendon section configured to attach to a floating structure and a tendon bottom section configured to attach to a foundation. The tendon system may also include an upper transition unit connected to the top tendon section, and a bottom transition unit connected to the tendon bottom section. One or more tendon modules may be disposed intermediate the upper and bottom transition units, at least one tendon module including a tendon main body system comprising at least two pipe strings connected proximate their upper ends to a first module transition unit, which can be the same or different than the upper transition unit, and proximate their bottom ends to a second module transition unit, which can be the same or different than the bottom transition unit. A tension leg platform may be moored to seabed using such a tendon system.
In another aspect, embodiments disclosed herein relate to a cellular tendon system that may include a top tendon section configured to attach to a floating structure and a tendon bottom section configured to attach to a foundation. The tendon system may also include an upper transition unit connected to the top tendon section, and a bottom transition unit connected to the tendon bottom section. A tendon main body system including at least two pipe strings is connected proximate their upper ends to the upper transition unit and proximate their bottom ends to the bottom transition unit. The tendon system may be fully assembled onshore and transported to an offshore location, where it may be submerged and connected at its top end to a floating structure and at its bottom end to a foundation in the seabed. The floating structure may be, for example, a tension leg platform.
In another aspect, embodiments disclosed herein relate to a method of assembling a tendon system. The method may include assembling a tendon main body section having two or more pipe strings. The two or more pipe strings may be connected at their top end and bottom ends to a tendon top segment and a tendon bottom segment, which may be, for example, configured to connect to a floating structure, a foundation, or another tendon module.
In another aspect, embodiments disclosed herein relate to a hybrid tendon system, including: an upper tendon module and a lowermost tendon module. The upper tendon module includes a top tendon section configured to attach to a floating structure. The lowermost tendon module includes a bottom transition unit configured to attach to a foundation. At least one of the tendon modules include a tendon main body system comprising at least two pipe strings connected proximate their upper ends to an upper transition unit and proximate their bottom ends to a lower transition unit. A tension leg platform may be moored to a seabed using such a tendon system.
In another aspect, embodiments disclosed herein relate to a method of mooring a floating structure. The method may include: transporting a tendon system according to embodiments herein from an onshore assembly location to an offshore location; submerging the tendon system; connecting the tendon system to a foundation and connecting the tendon system to the floating structure.
Other aspects and advantages will be apparent from the following description and the appended claims.
Embodiments disclosed herein relate to a tendon system for use with a tension leg platform (TLP). More specifically, embodiments disclosed herein relate to a tendon system having at least one cellular tendon main body portion designed and configured to enable use of the tendon, and thus use of TLPs, in ultra-deep waters and/or with heavy topside payload.
In a Tension Leg Platform (TLP), the tendons maintain the platform position and control the TLP dynamic motions in various environmental conditions to meet the operational requirements. An individual tendon consists of three major parts: a tendon top segment (TTS), or top section, for top interface at the platform, a tendon bottom segment (TBS), or bottom section, to connect to the tendon foundation at the seafloor, and a main body that links between the two. The main body may be formed, according to embodiments herein, using one or more tendon modules.
Tendons according to embodiments herein include one or more tendon modules including a tendon main body section that has multiple metallic or composite tubular members or strings, bundled together, acting as the main body of the respective module of a tendon system for a Tension Leg Platform (TLP). These “cellular” tendons have unique features which enable the use of TLP technology and application in hydrocarbon production in ultra-deep water and/or with heavy topside payload. Where the use of conventional tendons is technically and practically possible, the advantages of the cellular tendon over conventional tendons reside in the technical robustness, and, depending on platform geographical and economical characteristics, local fabrication, and cost savings.
The cellular tendon system includes the tendon main body, and for a single-module tendon system, an upper transition unit to interface with the tendon top interface, and a bottom transition unit to interface with the tendon bottom connection. Unlike the single carbon steel pipe used as the main body in a conventional tendon design, a cellular tendon consists of multiple metallic or composite tubular strings that are bundled together acting as one single body. Each individual string consists of multiple tubes or pipe segments connected to each other at the ends of the tubes, such as by welding or mechanical connections. The strings may be arranged in parallel and assembled on-shore.
Centralizers or frames may be used to bundle the strings together. Buoyancy modules are used partially or fully along the cellular tendon length to provide buoyancy necessary for the installation of the tendons. The TTS and TBS are assembled onshore and connected to the cellular tendon top transition unit and bottom transition unit, respectively. The cellular tendons, once fully assembled, may then be towed out to the site, upended, and installed.
A tendon system according to embodiments herein, such as a single-module tendon system described further below with respect to
A tendon system according to embodiments herein, such as a multiple-module tendon system described further below with respect to
The pipe strings are connected to the upper and bottom transition units such that the pipe strings are mechanically coupled. The coupling (connection) may be made by welding or other means of mechanical coupling such as treated connections. For example, the pipe strings may be welded to the upper and bottom transition units at the upper and lower ends of the individual pipe strings, respectively. Alternatively, the upper and lower ends of the pipe strings may be mechanically coupled to the upper and bottom transition units, respectively.
The pipe strings may be formed from steel, aluminum, or composite materials. The inner and outer diameter of the pipe in the pipe strings may be constant throughout the length of the pipe string in some embodiments. In other embodiments, the inner diameter, the outer diameter, or both, may vary along the length of the pipe string. For example, in some embodiments, the outer diameter of the pipes in the pipe string may remain constant, while the inner diameter decreases with depth, the thickness of the pipe thereby increasing with depth. Other variations in pipe diameter and thickness are also possible. The outer diameter and wall thickness may be selected for each point along the length of a tendon to carry tension from the buoyant and partially submerged TLP (which consists of a nominal tension plus tension variations due to functional and environmental loads), to maintain a necessary tendon stiffness, to achieve a desired buoyancy, and to withstand the crushing forces of the surrounding sea.
The tendon main body system may include one or more centralizers disposed along a length of the pipe strings. The centralizers may be, for example, metallic or non-metallic plates configured to space the pipe strings and arrange each the pipe strings in a parallel configuration (substantially parallel, in some embodiments, as manufacturing process tolerances may provide for some minor deviation from parallel). The centralizers may also be connected to the pipe strings or otherwise configured such that the pipe strings are mechanically coupled. In this manner, the individual pipe strings form a single operative unit. In some embodiments, the centralizers may include a non-ferric material associated with a metallic guide frame, described further below.
To reduce consequences of flooding of the pipe strings, a plurality of bulkheads may be mounted in the pipe strings. The bulkheads may form sealed compartments so that leakage at any point along the length of a pipe section will flood only one compartment. The remaining sealed compartments would maintain sufficient buoyancy to support the weight of the tendon. Bulkheads may be placed according to the choice of the designer. The bulkheads may be located at the end of a designated joint of pipe, or at selected intervals, for example. Bulkheads may be secured in a variety of manners, and in some embodiments are secured by welding or mechanic locks.
The tendon main body system may also include one or more buoyancy modules disposed around at least a portion of the pipe strings. In some embodiments, buoyancy modules may be disposed around the individual pipe strings in the tendon main body; in other embodiments, the buoyancy modules may be disposed around the set of pipe strings forming the tendon main body. In some embodiments, the buoyancy modules may be formed from two or more sections disposed around respective portions of the pipe strings, and may be fastened to the pipe strings via non-ferric or metallic straps. For example, the buoyancy modules may be bundled to the pipe strings using metal straps or straps made of high-strength synthetic fiber or fabric, with a mechanical lock connecting the ends.
The tendon main body system may also include one or more buoyancy arrest collars connected to the pipe strings. The buoyancy arrest collars may be provided to constrain the buoyancy modules to the cellular tendon. In some embodiments, the buoyancy arrest collar includes one or more mechanic parts configured to engage corresponding profiles in the buoyancy module.
The physical requirements of the tendons may vary with depth. Thus, in some embodiments, the tendon may be formed by two or more modules. In some embodiments, for example, the tendon may include an upper tendon module and a lower tendon module; in other embodiments, the tendon may include an upper tendon module, a lower tendon module, and one or more intermediate tendon modules. As the design requirements (crush strength, etc.) of the shallower tendon modules may be lower than those used at depth, a hybrid tendon according to embodiments herein may include, for example, an upper tendon module, which may be a conventional tendon, such as a single-string tendon, and a lower tendon module or modules, which may be cellular tendons as described herein. In other embodiments, the tendon may include multiple cellular tendon modules. Where multiple cellular tendon modules are used along the length of the tendon, the design and physical requirements of the segments may be adjusted for operating depth.
A hybrid tendon system according to embodiments herein, for example, may include an upper tendon module and one or more lower tendon modules, including a lowermost tendon module. The upper tendon module may include a top tendon section configured to attach to a floating structure. The upper tendon module may also include a lower tendon section configured to attach to one of the lower tendon modules, such as to an upper portion of an intermediate tendon module or the lowermost tendon module.
The lower tendon modules may be configured to attach at their upper ends via an upper transition unit to either (a) the lower tendon section of the upper tendon module or (b) a lower tendon section of an axially higher lower tendon module. The lower tendon modules may also be configured to attach at their lower ends via a lower transition unit to either (a) a foundation or (b) an upper tendon section of an axially lower tendon module. The one or more lower tendon modules may be formed of a cellular tendon system, similar to that as described above, having a tendon main body system including at least two pipe strings connected proximate their upper ends to the upper transition unit and proximate their bottom ends to the lower transition unit.
The above described tendons may be used to secure any variety of floating platform to a seabed.
Upper tendon supports 12 are mounted to platform 1 at each column 4 for attachment to an upper end 13 of each tendon 14. A minimum of two tendons 14 may be used to support at each tendon support 12, and thus a platform 1 with four corners would have at least eight separate tendons 14. The lower end 16 of each tendon 14 is secured to a piling or foundation 18.
The top of tendon main body section 22 is axially connected to the bottom of tendon top segment 20, and the bottom of tendon main body section 22 is axially connected to the top of tendon bottom segment 24. The bottom of the tendon bottom segment 24 is terminated with a connector assembly 27, commonly referred to as a bottom latch assembly that is arranged and designed to be received and locked into a piling 18 or other foundation structure on the seabed.
Tendon main body section 22 includes multiple (i.e., two or more, small quantities for large diameter seam pipes and large quantities for small diameter seamless pipes) metallic pipe strings 38. Each individual pipe string is formed from multiple pipe sections (segments, stands, or joints) welded or mechanically coupled end-to-end. Alternatively, tendon main body section 22 may include multiple composite pipe sections.
To facilitate connection of tendon main body section 22 to top and bottom segments 20, 24, an upper transition unit 26 and a bottom transition unit 28 may be used. Transition units 26, 28 may include tapered sections 30, 32 expanding in diameter from the diameter of the tendon top segment and the tendon bottom segment, respectively, terminating at a large diameter section 34, 36. Large diameter sections 34, 36 are sized appropriately for connection with the main body section 22. The top and bottom of pipe strings 38 forming main body section 22 may be welded or mechanically connected to transition units 26, 28, respectively.
For example, pipe strings 38 may be attached to the upper transition unit 26 that interfaces with the tendon top segment 20. The pipes in the pipe strings 38 are welded to, or mechanically connected to, the upper transition unit 26. A short taper 30 may or may not be used at the pipe up ends where the transition is made. At the top, the upper transition unit 26 may include a weld profile to be welded at the bottom of the tendon top segment 20. The pipe strings 38 may also be attached to the bottom transition unit 28 that interfaces with the tendon bottom segment 24. The pipes in the strings 38 are welded to, or mechanically connected to, the bottom transition unit 28. A short taper 32 may or may not be used at the pipe bottom ends where the transition is made. At the bottom, the bottom transition unit 28 may have a weld profile to be welded to the top of the tendon bottom segment 24.
The physical arrangement of pipe sections 38 may vary, and may depend on the depth of service, the expected environmental conditions, and other factors. In some embodiments, the pipe strings 38 may be arranged such that the pipe strings run alongside each other. In other embodiments, the pipe strings 38 may be spaced apart from each other.
For example, in some embodiments, the pipe strings 38 may be spaced apart using a centralizer 40, a guide frame 42, or a combination of the two. Centralizers 40 may be made of non-ferric material, with or without the metallic guide frames 42, and may be used to bundle the pipe strings together and keep all the strings acting as a mechanically composite unit to meet the tendon strength and fatigue requirements.
Compartmentalization of the pipes may be used to reduce the in-water weight of the tendon when a leak occurs in one pipe string 38. For example, as shown in
Buoyancy modules 50 may be used partially or fully along the Cellular Tendon length to provide buoyancy necessary for installation of the tendons. Material used to form the buoyancy modules may be synthetic foams or other suitable light material. Open-bottom air cans or other types of buoyancy devices may also be used. Further, the buoyancy of the buoyancy modules 50 may vary along the length of pipe strings 38.
The buoyancy modules 50 may also be used as a fabrication aid when the pipe strings 38 are assembled on shore. For example, using a centralizer 40, internal to a pipe segment group, encapsulation of multiple pipe segments with a buoyancy module 50 may facilitate disposition of the ends of the pipe segment for connection of guide frames 42, bulkheads 44, and/or connection of the pipe segment to the subsequent pipe segment. In some embodiments, the buoyancy modules 50 may be fastened to the pipe strings 38 by non-ferric or metallic straps 52, which may be recessed within a buoyancy module or may be placed at the surface around a buoyancy module.
A buoyancy arrest collar 54 may be used, such as with the metallic guide frames 42 as shown in
Referring now to
Similar to the cellular tendon system described above, upper tendon supports 12 are mounted to platform 1 at each column 4 for attachment to an upper end 13 of each tendon 14. Minimum two tendons 14 may be supported at each tendon support 12, and thus a platform 1 with four corners would have minimum eight separate tendons 14. The lower end 16 of each tendon 14 is secured to a piling or foundation 18.
Upper tendon module 72 may include a tendon module top segment 81, a tendon module main body section 82, and a tendon module bottom segment 84. The top of the uppermost pipe segment in tendon top segment 81 may be terminated with a connector assembly 25, commonly referred to as a length adjustment joint (LAJ) that is arranged and designed to connect to the TLP hull. Conventional tendon module 72 main body section 82 may be formed from a single pipe string 86 including one or more segments 88 of pipe axially connected end-to-end.
The top of tendon main body section 82 is axially connected to the bottom of tendon top segment 81, and the bottom of tendon main body section 82 is axially connected to the top of tendon bottom segment 84. The bottom of the tendon bottom segment 84 is terminated with a connector assembly 87, which may be similar to a bottom latch assembly.
The connector 87 may be designed to be received and locked into a receiver assembly 90, having a receptacle 92, which may be designed similar to connector assemblies for attachment of a tendon to a piling, for example. The receiver assembly 90 may be connected, directly or indirectly, to bottom tendon module 74. For example, receiver assembly 90 may be part of a transition module (not shown) for latching connector assembly 87 and likewise latching to a connector assembly (not shown) forming an upper terminal end of bottom tendon module 74. In other embodiments, such as illustrated in
Bottom tendon module 74 may be similar to that as described above with respect to
Upper tendon module 78 may include a tendon module top segment 101, a tendon module main body section 102, and a tendon module bottom segment 104. The top of the uppermost pipe segment in tendon top segment 101 may be terminated with a connector assembly 25, commonly referred to as a length adjustment joint (LAJ) that is arranged and designed to connect to the TLP hull.
The top of tendon main body section 102 is axially connected to the bottom of tendon top segment 101, and the bottom of tendon main body section 102 is axially connected to the top of tendon bottom segment 104. The bottom of the tendon bottom segment 104 is terminated with a connector assembly 107, which may be similar to a bottom latch assembly.
The connector 107 may be designed to be received and locked into a receiver assembly 110, having a receptacle 112, which may be designed similar to connector assemblies for attachment of a tendon to a piling, for example. The receiver assembly 110 may be connected, directly or indirectly, to bottom tendon module 80. For example, receiver assembly 112 may be part of a transition module (not shown) for latching connector assembly 107 and likewise latching to a connector assembly (not shown) forming an upper terminal end of bottom tendon module 80. In other embodiments, such as illustrated in
The top of bottom tendon main body section 122 is axially connected to the bottom of upper tendon module 72, as described above, via connector assembly 107, and the bottom of tendon main body section 122 is axially connected to the top of a tendon bottom segment 124. The bottom of the tendon bottom segment 124 is terminated with a connector assembly 127, commonly referred to as a bottom latch assembly that is arranged and designed to be received and locked into a piling 18 or other foundation structure on the seabed.
Upper tendon module 78 and bottom tendon module 80 may otherwise be similar to the cellular tendons as shown and described above with respect to
In the various tendon modules 78, 80, as well as for intermediate modules that may be placed between and connecting modules 78, 80 for embodiments including more than two tendon modules, the size, number, and physical arrangement of pipe sections 38 may vary, and may depend on the depth of service, the expected environmental conditions, and other factors. In some embodiments, the pipe strings 38 may be arranged such that the pipe strings run alongside each other. In other embodiments, the pipe strings 38 may be spaced apart from each other. For example, as described above with respect to
The upper tendon modules, such as modules 72, 78, as well as the bottom tendon modules, such as modules 74, 80, may include transition units 130, similar to transition units 26, 28, connecting the respective tendon main body segments 22, 82, 102, 122 to the tendon top segments and tendon bottom segments, as well as any intermediate connector segments used along the length of the hybrid tendon system.
Assembly of the cellular tendons according to embodiments herein may be performed in any number of manners. The main body section 22 may be assembled pipe segment by pipe segment. In some embodiments, the top and bottom transition units and tendon top and bottom segments may be connected in order of depth or height (top-to-bottom or bottom-to-top) along with the main tendon body pipe strings, or these components may be individually assembled and connected to the main tendon body pipe strings following completion of the main tendon body pipe strings.
Methods of assembling a tendon system according to embodiments herein may thus include assembling a tendon main body section including two or more pipe strings. The two or more pipe strings forming the tendon main body may be individually assembled by axially connecting two or more pipe segments to form a pipe string. The two or more pipe strings, having roughly equivalent length, may be axially connected at the respective ends to a tendon top segment and a tendon bottom segment. An upper transition unit and a bottom transition unit may be used to facilitate connection of the tendon main body section with the tendon top segment and the tendon bottom segment. As noted above, the tendon bottom segment and the tendon top segment may each be terminated with a connector assembly.
As noted above, it may be desired to space the pipe strings forming the tendon main body section apart from one another. Guide frames, centralizers, and/or a buoyancy module may be used to achieve the desired spacing, either during collective manufacture of the individual pipe strings or following manufacture of the individual pipe strings individually. A bulkhead may periodically be disposed in the pipe strings during assembly, and buoyancy arrest collars may be disposed along the length of the pipe strings, either during collective assembly of the individual pipe strings or following manufacture of the individual pipe strings individually.
In some embodiments, the main tendon body pipe strings may be assembled collectively as follows. A buoyancy module section, such as a semi-circular section, may be placed horizontally, with the interior of the module facing upward. The buoyancy module section may be configured, similar to that as illustrated in
One possible arrangement for buoyancy modules 50 is thus as illustrated and described with respect to
As illustrated in
As illustrated in
As illustrated in
The buoyancy modules as illustrated in
Similar to the embodiments for buoyancy modules illustrated in FIGS. 3 and 8-10, guide frames may also be formed from a variety of shapes so as to properly space strings 38 and facilitate fabrication of the cellular tendons or tendon modules.
After assembly on shore, the cellular tendon system 14 as described above may be transported, or towed out, in single units, pairs, or other convenient connection numbers, to the offshore site. The towing can be carried out as surface tow or submerged tow. At the offshore site, the cellular tendon system 14 is upended and attached to the foundation 18. The tendon top segment 20 is pulled in and locked at the tension leg platform 1 tendon porch 12 via the length adjustment joint (LAJ) 25. After the tendon pretension reaches the design value, the tendon is ready for the in-place services.
To add flexibility in the TLP construction schedule, the Cellular Tendon system 14 can be pre-installed. In the pre-installed condition, a temporary tendon supporting buoy may be used to maintain the near vertical position of the tendons and meet the strength and fatigue requirements from the wave and current loads, similar to those used in conventional tendon systems.
When multiple tendon modules are used to form a single tendon, such as illustrated in
As described above, tendons according to embodiments herein include a tendon main body section that includes multiple metallic or composite tubular members, bundled together acting as the main body of a tendon system for a Tension Leg Platform (TLP). These “cellular” tendons have unique features which enable the TLP technology and application in hydrocarbon production in ultra-deep water and/or with heavy topsides. In addition, the advantages of the cellular tendon over the conventional tendons reside in the technical robustness, local fabrication, and cost savings.
The cellular tendon design has technical merits for actual field developments using TLPs with representative large, medium or small payloads in water depths between 1500 meters and 3000 meters, and possibly deeper. The cellular tendon system has the following advantages over the conventional tendons: provides sufficient vertical and lateral stiffness, while also ensuring sufficient resistance to tension-collapse; provides more neutrally buoyant tendons that reduce TLP displacement and temporary buoyancy requirement during tendon installation; provides more structural redundancy for the tendon main body where the multiple tube construction in the Cellular Tendon warrants additional structural redundancy in the event of one string damaged; enhances (and in some cases enables) the local fabrication content, where applicable; provides material cost savings as a result of eliminating mechanical couplings; and provides installation cost savings as a result of eliminating the need for heavy lift vessels.
While the disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the attached claims.
| Number | Date | Country | |
|---|---|---|---|
| 61868760 | Aug 2013 | US | |
| 61860409 | Jul 2013 | US |
