Patent Grant
7168889
Not applicable.
Not applicable.
The present invention relates to spoolable tethers for anchoring a floating platform or vessel, and in particular a tension leg platform (TLP), to the ocean floor in deepwater and a method for installing the tether. More particularly, the present invention relates to mounting spools of composite tethers directly on a TLP and subsequently unspooling and installing the tethers to anchor the TLP.
Tethers (also referred to as cables, tendons, support lines, mooring lines, and the like) are useful for securing floating structures such as TLPs in deepwater. Particularly in depths over about 4000 feet, composite tethers offer significant economic and technical advantages and reliability over steel tethers. Composites such as carbon fibers embedded in a polymer matrix material are lightweight and have high specific strength and stiffness and excellent corrosion and fatigue resistance, which make them attractive for water depth sensitive components such as tethers and risers or umbilicals, which transport hydrocarbons from a wellhead on the ocean floor. Furthermore, composites are easily outfitted with instrumentation such as fiber optics integrated into the composite for load and integrity monitoring.
Conventional composite TLP tethers comprise top and bottom end connectors for connection to the TLP and a foundation on the ocean floor, respectively, and a tether body having a plurality of parallel twisted strands. The twisted strands herein referred to are formed from a twisted assemblage of small, parallel rods having a diameter of about 3–6 mm, and typically comprise in the range of about 50 to 200 rods per strand, wherein the assemblage of rods is subjected to a helical twist, typically about 2 to 3° on the outer rods. The plurality of parallel twisted strands, wherein each strand is typically about 50 to 75 mm in diameter, are also twisted slightly to achieve a helix in the conventional tether, also referred to herein as a twisted tether. The size of the conventional tether is determined by the number of twisted strands, which is dictated by the strength and axial stiffness requirements for a given tether service (e.g., size of the TLP, water depth, ocean currents, storm history, etc.). The number of twisted strands per conventional tether is typically from about 8 to 30 twisted strands per assembled conventional tether. Conventional tethers are twisted as described previously so that they may be wound upon tether spools, typically having a diameter of greater than about 4.0 meters, and preferably from about 4 to 8 meters. In order for the conventional composite tethers to be spoolable, small diameter rods having a diameter of no greater than about 6 mm are required, otherwise the size of the required spool becomes impractical, as described below. The spooled tethers are transported upon reel ships or barges for installation and anchoring of the TLP to the ocean floor.
The manufacturing process of a conventional, spoolable composite tether includes the following steps: fabrication of small diameter composite rods, assembly of the rods into twisted strands, assembly of the twisted tether from multiple twisted strands (including addition of filler and profile material as needed), and termination of the twisted strands in top and bottom end connectors of the tether. The manufacturing of conventional, spoolable composite tethers is described in the following conference paper, which is incorporated by reference herein in its entirety: Composite Carbon Fiber Tether for Deepwater TLP Applications, presented at the Deep Offshore Technology Conference held in Stavanger, Norway on Oct. 19–21, 1999.
Composite materials for rod manufacture consist of small diameter fibers (from about 6 to about 10 microns) of high strength and modulus, preferably carbon fibers, embedded in a polymer matrix material, e.g., resins or glues. Commonly known thermoset or thermoplastic polymeric matrices may be used. Preferred matrix materials include vinylesters and epoxies. The resin materials have bonded interfaces which capture the desirable characteristics of both the carbon fibers and the matrix. The carbon fibers carry the main load in the composite material while the matrix maintains the fibers in the preferred orientation. The matrix also acts to transfer load into the carbon fibers and protects the fibers from the surrounding environment. Carbon fibers incorporated in the matrix may be spun in long continuous lengths; however, short (from about 25 to about 100 mm) discontinuous fibers may also be used. The carbon fibers may be medium and/or high modulus carbon fibers. Low cost, medium modulus (from about 32 msi to about 35 msi) carbon fibers include polyacrylonitrile (PAN) carbon fibers such as those available from Grafil Inc., Toray Industries, Inc, Akzo Nobel, and ZOLTEK, among others. Low cost, high modulus (from about 55 msi to about 80 msi) carbon fibers include those available from Conoco Inc. and Mitsubishi Corp.
Composite rods are typically manufactured by pultruding the composite material comprising the carbon fibers and the polymer matrix material. Pultrusion is the pulling of the resin wetted fibers through a die rather than pushing it through the die as in extrusion processes used for metal manufacturing. The die size and shape control the final size and shape of the pultruded composite product. There are several commercial pultruders such as Glasforms, Inc., DFI Pultruded Composites Inc., Exel Oyj, Strongwell Corp., Spencer Composites Corp., and others that are capable of producing the composite rods. Rods used in conventional spoolable tethers are typically round in cross-section. The composite rods produced typically have a weight which is approximately ⅙ that of required for an equivalent steel rod. As discussed previously, rods for use in conventional composite tethers typically are from about 3 to about 6 mm in diameter and are often wound onto rod spools, for example a 1.8 or 2.2 m diameter rod spool, for transportation to a strand and/or tether manufacturing facility.
In general, it is desirable to increase the stiffness of rods used in a tether, and the stiffness of a rod may be calculated according to the following equation:
where E=axial stiffness of a rod (Pa); A=cross sectional area of 1 tether (m2); L=water depth (m); n=number of tethers; T=heave natural periods (s), typically from about 5 to about 5.5 seconds; vertical mass=mass of the platform (kg); and added mass=mass of the water that moves when the platform moves (kg). Typically, a stiffer rod cannot be bent as much as a less stiff rod. Given that the rods typically must be wound onto a rod spool for transportation, the bending stiffness of the rod is proportional to the diameter of the rod (d) raised to the fourth power (i.e., d4). Thus, it is necessary to use a small diameter composite rod (i.e., from about 3 to about 6 mm) in order for the resulting rod spool diameter to be a practical size for handling and transport and the force necessary to spool the rod and maintain it on the spool be practical. More specifically, in sizing the rod spool, the strain in the spooled rod is equal to the diameter of the composite rod divided by the diameter of the rod spool. In a properly sized spool, the rod strain is less than 50% of the ultimate strain to failure of the rod. Thus, if the composite rod has 1% strain to failure, then the diameter of the rod spool then needs to be larger than 200 times the diameter of the rod to be able to spool the rod onto the rod spool without damaging the rod. If the composite rod has ½% strain to failure, then the diameter of the rod spool has to be larger than 400 times the diameter of the rod. The diameter of a spool refers to the hub or core of the spool. In sum, where the rod itself must be spooled (or a strand or tether incorporating the rod must be spooled, as discussed below), the diameter and/or the stiffness of the rod must be engineered accordingly.
In a conventional, spoolable composite tether, the rods are assembled into bundles to form twisted strands. The twisted strands can be manufactured using typical wire rope stranding methods. Specifically, the rods are uncoiled from the rod spools and pulled through a guide plate for bundling. When the required number of rods per strand are laid out, the guide plates are rotated to impart a slight helical twist, typically 2 to 3° on the outer rods. Twisting the strand provides sufficient coherence to the strand for handling, coiling and transportation without significantly affecting the axial strength and stiffness. The rods in the twisted strands are fixed into a position by wrapping with tape or other securing device, cut to length and spooled onto strand spools for use in the assembly of the tether body. Generally, twisted strand spools include 1.8 or 2.2 m diameter spools such as those used for rod spools.
The twisted strands are assembled to form a conventional, spoolable composite tether [205] (i.e., a twisted tether) as shown in
The twisted strands [215] are free to move individually in the length direction, allowing individual adjustment and hence a better distribution of axial loads. The composite rods [210] and twisted strands [215] are free to act or move independently in the twisted tether [205]. In other words, there is relative axial movement between adjacent composite rods [210] within a twisted strand [215] and between adjacent composite twisted strands [215] within a twisted tether [205]. Otherwise the entire diameter of the conventional, spoolable tether [205] must be considered in calculating the diameter of the tether spool, since strain relates to the diameter of the body that is being spooled divided by the diameter of the spool, as described previously. By putting a twist in the composite rods [210] (via twisted strands [215] and twisted tether [205]) and keeping them separate and independent, the diameter of the individual composite rods [210] can roughly be used to calculate the diameter of the tether spool rather than the entire diameter of the twisted tether [205]. Typically, however, the tether spool is made somewhat larger to account for the friction between adjacent composite rods [210] as the conventional tether is spooled onto the tether spool.
As shown in
The assembly of the conventional, spoolable tether [205] is performed using a conventional umbilical closing machine. Spools containing the twisted strands [215] and the profiled members [220] are lifted onto the closing machine. The twisted strands [215] and the profiled members [220] are then pulled through closing plates. During this process, the machine rotates to impart a helical twist in the twisted strands [215] to form the twisted tether [205]. A yarn or other securing device is then applied to hold the assembly together prior to extrusion of the protective, outer jacket [245] such as high density polyethylene (HDPE), nylon, or the like over the twisted strands to hold the twisted strands in place and protect the tether during handling. Conventional, spoolable tethers may be manufactured as a single continuous body that is spooled onto a spool. Alternatively, the tether body may be manufactured as a plurality of body lengths or segments that are spooled onto a spool. The segments are connected with connectors (e.g., couples or collars) to create a continuous tether. Segmenting the tether is helpful in accommodating production of rods, strands, and tethers, in limiting spool size, and in readjusting tether length for re-installations.
The final step of manufacturing a conventional, spoolable composite tether is the termination process that includes connecting the twisted tether to top and bottom end connectors. Termination using resin-potted cones has been extensively used in the wire rope industry. Resin terminations have been proven to be successful for terminating composite twisted strands as well. The twisted strands are fastened to a steel end connector using a potted cone technique similar to that used for termination of steel wires. The twisted strands are spread with a specific angle in the steel cones, and the cone is then filled with epoxy resin. A vacuum injection method is used in this process to avoid air gaps and to ensure consistent molding. Use of a flexible cone and cylindrical metal connector with spacers can minimize the effect of termination bending and provide better rod distribution inside the end connector. Alternatively, the twisted strands may be individually terminated and then assembled into a tether. After termination, the tether is spooled onto an appropriately sized conventional tether spool having a drum diameter of from about 4 to 8 m and a width of about 5 m, for transportation and installation offshore. An appropriately sized tether spool should be selected based upon the characteristics of the composite rods as described previously.
Heretofore spooled tethers have been transported upon reel ships or barges for installation and anchoring of a platform such as a TLP to the ocean floor. For example, a reel ship is used to unwind and lay down a spooled composite tether and another support vessel is typically required to assist the connection of the tether to the foundation and TLP. The process using reel ships is similar to that used to lay down and install umbilicals. Use of support vessels such as barges and reel ships is expensive, requires complicated logistical scheduling, and results in longer installation times, thereby subjecting the installation to an increased risk for complications due to bad weather. The present invention eliminates these problems and simplifies the installation process by placing the spooled composite tether directly upon the TLP for transportation and installation at a designated site.
The present invention includes a process for installing a floating platform to the ocean floor using a spoolable composite tether, comprising: installing the spooled tether onto the floating platform; towing the floating platform to a site for installation; unspooling the tether; connecting a bottom end connector of the tether to a foundation on the ocean floor; and connecting a top end connector of the tether to the floating platform. A preferred floating platform is a tension leg platform. The spooled tether installed on the floating platform may further comprise a spool holder, a spool (also referred to as a reel or carousal) having a tether wound thereon installed on the spool holder, and a means for winding and unwinding the spool connected to and driving the spool. The spooled tether may further comprise a constant tension winch having a wire line connected to the top end connector of the tether.
In a preferred embodiment the foundation is a suction anchor, and the suction anchor is connected to the bottom end connector prior to unspooling the tether and attached to the ocean floor after unspooling the tether. The tether may be divided into a plurality of lengths, each length being spooled onto one of a corresponding plurality of spools, the tether being connected together with connectors as the lengths thereof are unspooled.
The present invention includes a process for uninstalling, moving and re-installing a floating platform to the ocean floor using a spoolable composite tether, comprising: disconnecting a top end connector of the tether from the floating platform; disconnecting a bottom end connector of the tether from a foundation on the ocean floor; spooling the tether onto a spool installed on the floating platform; towing the floating platform to a different site for re-installation; and unspooling the tether and re-installing the tether to the ocean floor and the floating platform. In a preferred embodiment of the reinstallation process, the foundation is a suction anchor, the bottom end connection remains connected to the suction anchor, and the suction anchor is released from the ocean floor and raised to the surface as the tether is spooled onto the spool.
The invention includes a novel tension leg platform (TLP) comprising a spooled composite tether installed thereon. The spooled composite tether may further comprise a spool holder, a spool having a tether wound thereon installed on the spool holder, and a means for winding and unwinding the spool connected to and driving the spool. The spooled composite tether may further comprise a constant tension winch having a wire line connected to a top end connector of the tether. The spooled composite tether may be mounted above or below the waterline, for example temporarily mounted to the TLP hull, permanently mounted to the TLP hull, temporarily mounted to the TLP deck, or permanently mounted to the TLP deck.
According to the present invention as embodied in
As shown in
Upon reaching the installation site, the TLP [20] can be positioned and held in place by the towing vessels, while the twisted tethers [15] are unwound from the spools [10] and [12] and the bottom end connectors [45] of the twisted tethers [15] are lowered to the ocean floor [70], as shown in
Before connecting the top end connectors [55] of the twisted tethers [15] to the tether porches [50], the constant tension winches [65] are activated in combination with the addition of ballast, and the TLP [20] sinks lower in the water (i.e., the draft of the TLP is increased). The top end connectors [55] of the twisted tethers [15] are connected to the tether porches [50], and the draft is reduced through deballasting until the correct draft of the TLP [20] and tension on the tethers are maintained. Typically, a plurality of twisted tethers [15] are installed to hold the floating platform securely in position. Upon installation of the twisted tethers [15], the wire lines [60] may be disconnected from the top end connectors [55], and the spools [10] and [12], spool holders [25], and constant tension winches [65] may be removed to free up space on the TLP. Alternatively, this equipment may remain installed upon the TLP, for example for use in moving and reinstalling the TLP at another site.
Helpful background information regarding subsea installation of tethers is disclosed in the following articles each of which is incorporated by reference herein in its entirety: OTC 8100: The Heidrun Field—Heidrun TLP Tether System, presented at the Offshore Technology Conference held in Houston, Tex. on May 6–9, 1996 (P. 677–688); OTC 8101: The Heidrun Field—Marine Operations, presented at the Offshore Technology Conference held in Houston, Tex. on May 6–9, 1996 (P. 689–717); OTC 6361: Materials, Welding, and Fabrication for the Jolliet Project, presented at the Offshore Technology Conference held in Houston, Tex. on May 7–10, 1990 (P. 159–166); and OTC 6362: Installation of the Jolliet Field TLWP, presented at the Offshore Technology Conference held in Houston, Tex. on May 7–10, 1990 (p. 167–180).
Upon reaching the installation site, the TLP can be positioned and held in place by the towing or other vessels, while the twisted tethers [15] are unwound from the spools [10] and [12] and the bottom end connectors [45] and the suction anchors [100] are lowered to the ocean floor [70]. As shown in
The installation process described previously may be reversed to uninstall the twisted tethers, freeing the TLP to be towed to another site and reinstalled by re-performing the installation processes described previously. A different water depth can be accommodated by changing spools, tethers, or both, or by using different segment lengths if the tether body is segmented. More specifically, the spools [10] and [12], spool holders [25], and constant tension winches [65] are reinstalled, if previously removed. The wire lines [60] are reconnected to the top end connectors [55], and the draft of the TLP is increased by adding ballast. The top end connectors [55] of the twisted tethers [15] are disconnected from the tether porches [50]. The wire lines [60] hold the top ends of the twisted tethers [15] during the uninstallation process, thereby ensuring that the twisted tethers [15] remains relatively stable and do not fall to the ocean floor [70]. The ballast is removed and the wire lines [60] loosened to allow the TLP to float on the surface. The wire lines [60] are slackened further and the bottom end connectors [45] (or optionally the suction anchors [100]) are detached from the ocean floor [70]. The twisted tethers [15] are winched up and the top ends thereof threaded onto the spools [10] and [12]. The motors for winding the spools [10] and [12] are engaged, and the twisted tethers [15] are wound up and out of the ocean and onto the spools. Preferably, but not necessarily, the twisted tethers [15] are wound completely or substantially completely out of the water to reduce drag and risk of damage during towing to another location. After all of the twisted tethers [15] have been uninstalled, the floating platform can be towed to another location and reinstalled. Where reuse and/or reinstallation of the tethers is desired, preferably composite, twisted tethers are used as such are typically more corrosion resistant as compared to steel tethers.
Preferably, the twisted tether is a continuous length wound upon a single spool. However, the twisted tether may be divided into a plurality of lengths or segments, wound upon a corresponding plurality of spools, and connected as the tether lengths are unwound during installation to form a continuous twisted tether. Mechanical connectors or collars may be used to connect the twisted tether lengths without adversely impacting the strength or performance of the twisted tether.
In this example, a plurality of spooled tethers, preferably composite twisted tethers, are mounted on the top of a TLP hull at deck level and installed as follows. Similar installation procedures may be used when the spool is mounted at other locations on the TLP, including underwater.
Preferably, the spool assemblies [105] are mounted on the TLP prior to beginning the installation process, for example while the TLP is still at dock. Preferably, the BCAs [109] are pre-installed while the twisted tether [106] is in a retracted position, for example while the TLP is at dock or during transport. When the TLP has been towed to the installation site and is ready to be installed, the draft of the TLP is increased by adding ballast (e.g., seawater). The additional draft preferably is more than the maximum heave of the TLP during installation, for example about 2 m below normal draft.
To deploy and install the twisted tether, the twisted tether [106] is slowly unwound from the spool [104] via the hydraulic drive machinery, thereby lowering the BCA [109] down toward the tether porch [50]. The tether porch [50] comprises a guide funnel [122] and a bore [124] having a diameter large enough to allow the BCA [109] and bottom end connector [108] to pass through. As the twisted tether [106] is unwound, the BCA [109] is lowered and comes into contact with the guide funnel [122], whereby the BCA [109] and bottom end connector [108] are guided into and pass through bore [124], as shown in
Referring to
Preferably, at least one twisted tether [106] at each corner has an integral tether tension monitoring system (TTMS) that is connected prior to tensioning the twisted tethers. The TTMS may be connected temporarily, for example by a cable clamped to the lowering wire [121], or permanently, for example by a cable extending down from the deck to the tether porch [50] and connected to a wet mate connector on the TCA [126]. The twisted tethers [106] are tensioned by deballasting the TLP (i.e., pumping out seawater), and deballasting continues until the BCAs [109] are properly engaged in their foundations [75] and about normal tether tension is obtained in the twisted tethers [106], as indicated by the TTMS. As shown in
Upon completion of deballasting, the twisted tether tension may be further adjusted or “fine-tuned” through use of the length adjusting joint [128] and split nut [129] on TCA [126]. A tension adjustment tool (TAT), as is known in the art, is launched from an installation support vessel and maneuvered by WROV onto the TCA [126]. The TAT is used to adjust the position of the split nut [129] up or down on the length adjusting joint [128], thereby adjusting the tension in the twisted tether [106], which is monitored via the TTMS. The process is continued until all twisted tethers [106] have been adjusted, as needed, and the TAT is returned to the support vessel.
The twisted tethers [106] having been deployed, installed, and tensioned, the remaining elements of the spool assemblies [105] are removed from column [37], for example by use of a crane on a support vessel. The crane lifts the spool assembly [105] vertically until the spool holder [25] exits the pins [102] on top of column [37].
While preferred embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations, combinations, and modifications of the invention disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is defined by the claims which follow, that scope including all equivalents of the subject matter of the claims.
This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 60/286,885, filed Apr. 27, 2001, which is hereby incorporated herein by reference.
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