Abstract:
A self-tensioning tendon for pulling a tension leg platform (TLP) down to a desired draft position. The self-tensioning tendon can be utilized with either a co-installation process or a pre-installation process and connects to an assembled tendon joint. The self-tensioning tendon comprises a hydraulically controlled Length Adjustment Joint (LAJ) that further comprises an integral cylinder with external threads and a piston rod. The self-tensioning apparatus has features that advantageously reduce the time required to ballast the hull of the TLP. A hydraulic source actuates the integral cylinder to pull the TLP down to the target draft position, significantly reducing the time needed to ballast the hull of the TLP with millions of gallons of water. A Top Tendon Connector (TTC) ratchets down along the cylinder to lock the TLP at the final draft position.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to provisional application 61/106,024, filed Oct. 16, 2008. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to tension leg platforms (TLPs) used in offshore oil production, and in particular, to reducing the time and expense associated with achieving the required draft level of the platform via the use of self-tensioning tendons that reduce ballasting time. 
     BACKGROUND OF THE INVENTION 
     A tension leg platform (TLP) is a vertically moored floating structure typically used for the offshore production of oil or gas. The platform is permanently moored by means of tethers or tendons connected to the structure of the TLP. A group of tethers is called a tension leg. 
     Generally the installation process of a TLP is lengthy and extremely expensive. In general, there are two different methods to install a TLP, referred to as pre-installation and co-installation. Conventionally, the pre-installation method initiates by locking the mooring systems, called tendons, into the existing foundation piles. The tendons are held in stable tension using a temporary air can or buoyancy module. The platform is ballasted down onto the tendons and locked into place at the proper draft, then deballasted to the proper tendon tension. The buoyancy cans are then removed, resulting in a stable structure. 
     The other conventional method of installing a TLP is called co-installation. This method involves assembling the mooring system, also referred to as tendons, prior to TLP installation. All the tendons are hung from the platform and centered over the existing foundation piles. The platform is ballasted down to lock the tendons into to the foundation piles. The platform is then de-ballasted to the proper draft and tendon tension. 
     The operation of installing a TLP, whether pre-installation or co-installation, is very time consuming and costly. This is especially true for the process of ballasting and de-ballasting the hull. Hull ballasting requires the movement of millions of gallons of seawater. This process typically takes up to 24 to 48 hours to complete. A need exists for a technique to reduce ballasting time while reducing unnecessary and costly installation equipment for both types of TLP installation methods. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present technique, a self-tensioning apparatus for installing a tension leg platform (TLP) is provided that can connect to a tendon string. Thus the self-tensioning apparatus could be utilized during the installation of a TLP with each of a plurality of tendon strings. The self-tensioning apparatus has features that advantageously reduce the time required to ballast the hull of the TLP. The self-tensioning apparatus can have a Length Adjustment Joint (LAJ) comprising an integral cylinder with external threads along its length, and a piston rod with a piston that can stroke nearly the length of the cylinder in response to pressurization of the cylinder during installation of the TLP. The bottom of the cylinder can connect to a tendon string and a soft line can connect to the top of the piston rod. The soft line can be, for example, an adjustable chain or steel cable attached to a clevis located at the top end of the piston rod. 
     In the illustrated embodiments, the self-tensioning apparatus can be utilized in either the pre-installation or co-installation procedures for installing a TLP. In the co-installation process, an entire tendon string is assembled out of sections of steel pipe and placed in the water in proximity to the TLP. A Bottom Tendon Connector (BTC) is provided at the bottom end of the tendon string. A Top Tendon Connector (TTC) is also provided and engages the external threads of the cylinder. The entire tendon string assembly can then be hung onto a hull porch with the TTC. 
     Hydraulic lines can be run from a hydraulic source down to the top and bottom ends of the LAJ&#39;s integral cylinder to provide pressurization of the cylinder and thereby stroke the piston rod in or out. The soft line can be anchored to the TLP and pulled tight as required. 
     In the illustrated embodiments of the co-installation process, the TTC is disengaged from the threaded integral cylinder and the soft line is pulled tight by the weight of the tendon string. The hydraulic line feeding the hydraulic fluid into the bottom of the LAJ, causes the integral cylinder  18  to extend until the bottom portion of the tendon string is locked into an existing foundation pile via a BTC. The TTC can again engage the threaded integral cylinder, and the top of the LAJ is pressurized to retract the integral cylinder. The retraction of the LAJ&#39;s integral cylinder pulls the TLP down to the target draft position without the need of costly and time-consuming ballasting. The target draft position is maintained by the TTC, which ratchets down the length of the threaded integral cylinder and prevents the cylinder from moving down with respect to the TTC. The target tendon tension can then be achieved by minimal de-ballasting of the hull using seawater and the soft line can be removed. Further, the hydraulic lines and the piston rod can be removed from the integral cylinder. 
     Pre-installation is an alternate method similar to the co-installation process described above. As described in the co-installation method above, the entire tendon string is similarly assembled and placed in the water in proximity to the TLP. As in the co-installation method, the tendon string is made up of sections of steel pipe and includes a BTC at the bottom end of the tendon string. The tendon string also includes an LAJ comprising an integral cylinder and a piston rod that can stroke nearly the length of the cylinder. The integral cylinder is controlled by hydraulic power and has external threads along its length. Unlike the co-installation process, the tendon string is first locked to the existing foundation pile via the BTC and air cans or buoyancy modules are used to keep the tendon string buoyant and under stable tension. 
     Further unlike the co-installation process where the TTC is part of the tendon string, in the pre-installation process the TTC is installed at the hull porch on the TLP. The TTC on the porch can be aligned approximately above the buoyed tendon string. An adjustable soft line is first attached at an anchor point on the TLP and lowered down through the TTC and secured to the end of the piston rod. This is further unlike the co-installation process where the soft line is first attached to the piston rod and then to the anchor point on the TLP. 
     In the illustrated embodiments of the pre-installation process, the hydraulic lines are also lowered through the TTC on the hull porch and connected to the top and bottom ends of the LAJ&#39;s integral cylinder. The hydraulic line feeding the hydraulic fluid into the top of the LAJ, causes the integral cylinder  18  to partially retract until the top of the cylinder is approximately up through the TTC on the hull porch. The tightened soft line creates a reaction point at the anchor point to allow the TLP to be pulled down. 
     The TTC engages the threaded integral cylinder and the top hydraulic line feeds hydraulic fluid into the top of the LAJ&#39;s integral cylinder, causing the integral cylinder to retract and causing the TLP to move down until the target draft position is reached. The TTC ratchets down the length of the threaded cylinder as the cylinder retracts and prevents the cylinder from moving down with respect to the TTC. As in the co-installation process, the target tendon tension is achieved by minimal de-ballasting of the hull using seawater. At this point the TLP is at the final draft position and the soft line can be removed. Further, the hydraulic lines and the piston rod can be removed from the cylinder. 
     The TTC along with the LAJ comprising the integral cylinder and the piston rod, allow the TLP to be pulled down to a target draft position in a timely and cost effective manner due to the significant decrease in hull ballasting requirements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A through 1E , are illustrations of a TLP co-installation process, in accordance with an exemplary embodiment of the present technique; 
         FIGS. 2A through 2E  are illustrations of a TLP pre-installation process, in accordance with an exemplary embodiment of the present technique; 
         FIG. 3A  is an illustration showing the LAJ with cylinder in the retracted position, in accordance with an exemplary embodiment of the present technique; 
         FIG. 3B  is an illustration showing the LAJ with cylinder in the extended position, in accordance with an exemplary embodiment of the present technique. 
         FIG. 4  is a sectional view illustrating a portion of an alternate embodiment of the LAJ. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1A through 1E , the co-installation process of a Tension Leg Platform (TLP)  2  is illustrated. Entire tendon string  10  is assembled and placed in the water in proximity to the TLP  2 . The tendon string  10  is made up of sections of steel pipe  14  connected to each other at their ends and includes a Bottom Tendon Connector (BTC)  12  at the bottom end of the tendon string  10 . The tendon string  10  also includes a Top Tendon Connector (TTC)  16  and a Length Adjustment Joint (LAJ)  17 . The LAJ is made up of an integral cylinder  18  and a piston rod  20  that can move axially within the cylinder  18  and can stroke nearly the length of the cylinder  18 . For example, an LAJ  17  design may accommodate in excess of 30 feet of stroke and 1,250 kips of pulling force. The integral cylinder is controlled by a hydraulic power unit (not shown) installed on the platform. Referring to  FIG. 3A , cylinder  18  has external threads  28  or grooves along its length. The TTC  16  is engaged to the external threads  28  on the cylinder  18 . A soft line  22  is attached to the end of the piston rod  20 . As illustrated in  FIG. 1A , the integral cylinder is initially set to the retracted position ( FIG. 3A ). BTC  12  is located a short distance above foundation pile  40 . The distance is less than the length of LAJ  17  with the cylinder  18  in the retracted position. 
     As illustrated in  FIG. 1B , the tendon string  10  assembly is hung onto the hull porch  32  with the TTC  16  connected to the porch  32 . The porch  32  has a c-shaped side opening that allows the tendon string  10  to enter the porch  32  from the side. Hydraulic lines  19  ( FIG. 3A ) are connected from a supply (not shown) on the TLP  2  to the end cap  21  ( FIG. 3A ) at the top of the integral cylinder  18 , and also connected to the bottom end of the cylinder  18 . The soft line  22  at the end of the piston rod  20  is anchored to the TLP  2  hull at an anchor point  34  and guided onto a pulley  36 . The soft line  22  preferably has some slack at this stage. The soft line  22  could be an adjustable chain or cable. 
     Alternatively, as illustrated in  FIG. 4 , the piston rod  20  could be hollow and the hydraulic line  19  could be connected to the top of the piston rod  20 , with rod  20  having a communication port  23  extending just above the piston rod  20 . The communication port  23  runs down the hollow portion of piston rod  20  and through the piston  31  via a port  25 . The ports  23 ,  25  feed hydraulic fluid to the cylinder  18  chamber under the piston for rod  20  stroke out. Further, hydraulic line  19  could be connected to a port  29  on the side of the upper part of piston rod  20  to feed hydraulic fluid through the hollow part of piston rod  20 . A port  26  on the side of the lower part of piston rod  20  and just above the piston communicates hydraulic fluid from the hollow rod  20  to the cylinder  18  chamber above the piston for rod  20  stroke in. 
     As illustrated in  FIG. 1C , the TTC  16  is disengaged from the threaded integral cylinder  18  and the soft line  22  is pulled tight by the weight of the tendon string  10 . The bottom hydraulic line  19  ( FIG. 3B ) feeds hydraulic fluid into the bottom of the LAJ  17 , causing the integral cylinder  18  to extend ( FIG. 3B ). The cylinder  18  is extended until the tendon  10  is locked into the existing foundation pile  40  via a BTC  12  as illustrated in  FIG. 1D . 
     The TTC  16  again engages the threaded integral cylinder  18 , and the top hydraulic line  19  ( FIG. 3A ) feeds hydraulic fluid into the top of the LAJ  17 , causing the integral cylinder  18  to retract. As the cylinder  18  is retracted ( FIG. 3A ), it exerts a force on the TLP  2  that pulls TLP  2  down to the target draft position as illustrated in  FIG. 1E . This target draft position is achieved without ballasting. In addition, the TTC  16  ratchets down the length of the threaded cylinder  18  as the cylinder  18  retracts and prevents the cylinder  18  from moving down with respect to the TTC  16 . Target tendon tension is achieved by minimal de-ballasting of the hull using seawater  38 . At this point the TLP  2  is at the final draft position and the soft line  22  can be removed. Further, the hydraulic lines  19  and the piston rod  20  can be removed from the cylinder. Piston rod  20  may alternatively be stroked in and left in place. 
     Pre-installation is an alternate method to the co-installation process of  FIGS. 1A to 1E . Referring to  FIGS. 2A through 2E , the pre-installation process of a TLP  2  is illustrated. As described in the co-installation method above, the entire tendon string  10  is similarly assembled and placed in the water in proximity to the TLP  2 . The tendon string  10  is made up of sections of steel pipe  14  connected to each other at their ends and includes a BTC  12  at the bottom end of the tendon string  10 . The tendon string  10  also includes an LAJ  17 . The LAJ is made up of an integral cylinder  18  and a piston rod  20  that can move axially within the cylinder  18  and can stroke nearly the length of the cylinder  18 . For example, an LAJ  17  design may accommodate in excess of 30 feet of stroke and 1,250 kips of pulling force. The integral cylinder is controlled by a hydraulic power unit (not shown) installed on the platform. Cylinder  18  has external threads or grooves along its length. Unlike the co-installation process, the pre-installation process uses air cans or buoyancy modules  15  to keep the tendon string  10  buoyant and under stable tension once the tendon  10  is locked into the existing foundation pile  40  via a BTC  12  as illustrated in  FIG. 2A . 
     As illustrated in  FIG. 2B , a TTC  16  is installed at the porch  32 . This is unlike the co-installation process where the TTC  16  is part of the tendon string  10 . The TLP  2  is located in the water such that the porch  32  and TTC  16  are approximately above the buoyed tendon string  10 . A soft line  22  is attached at an anchor point  34  on the TLP  2  and guided onto a pulley  36 . The soft line  22  could be an adjustable chain or cable. 
     As illustrated in  FIG. 2C , the soft line  22  is lowered through the TTC  16  on the porch  32  and secured to the end of the piston rod  20 . The TTC  16  is preferably in the disengaged position at this stage and the soft line  22  has some slack. Hydraulic lines  19  ( FIG. 3A ) connected to a supply (not shown) on the TLP  2 , are also lowered through the TTC  16  on the porch  32  and are connected to the end cap  21  ( FIG. 3A ) at the top of the integral cylinder  18 , or end of piston rod  20  as previously described, and connected to the bottom end of the cylinder  18 . 
     As illustrated in  FIGS. 2C and 2D , the top hydraulic line  19  ( FIG. 3A ) feeds hydraulic fluid into the top of the LAJ  17 , causing the integral cylinder  18  to partially retract. The cylinder  18  is extended until the top of the cylinder is approximately up through the TTC  16 . At the point illustrated by  FIG. 2D , the soft line  22  is pulled tight. The soft line  22  can also be connected to the anchor point  34  at this stage rather than earlier in the process as described in the previous section. 
     The TTC  16  engages the threaded integral cylinder  18  and the top hydraulic line  19  ( FIG. 3A ) feeds hydraulic fluid into the top of the LAJ  17 , causing the integral cylinder  18  to retract. As the cylinder  18  is retracted ( FIG. 3A ), the TLP  2  begins to be pulled down ( FIG. 2D ) until the target draft position is reached as illustrated in  FIG. 2E . In addition, the TTC  16  ratchets down the length of the threaded cylinder  18  as the cylinder  18  retracts and prevents the cylinder  18  from moving down with respect to the TTC  16 . Target tendon tension is achieved by minimal de-ballasting of the hull using seawater  38 . At this point the TLP  2  is at the final draft position and the soft line  22  can be removed. Further, the hydraulic lines  19  and the piston rod  20  can be removed from the cylinder. 
     In this embodiment, the hydraulic supply is located on the platform. However, the hydraulic supply may be located elsewhere, such as on the porch  32 . Alternatively, the hydraulic supply could be located on a structure independent from the TLP  2 . 
     In an additional embodiment (not shown), a motor moves the internal cylinder  18  up or down through the TTC  16  to achieve the desired draft position for the TLP  2 . In another alternative embodiment (not shown), the internal cylinder  18  moves up or down through the TTC  16  via a screw-like drive to achieve the desired draft position for the TLP  2 . A plurality of tendon strings  10  will utilize the self-tensioning apparatus and undergo the installation methods described above to bring the TLP  2  to the desired draft position. 
     In yet another embodiment, either of the hydraulic cylinder  18  or the piston rod  20  can be adapted to be coupled to a tendon  10  that is coupled to the subsea structure. Further, one of the hydraulic cylinder  18  or the piston rod  20  is coupleable to the TLP  2  to enable the hydraulic cylinder  18  assembly to draw the TLP  2  toward the subsea structure by retracting the piston  20  into the hydraulic cylinder  18 . 
     An LAJ  17  design may accommodate in excess of 30 feet of stroke and 1,250 kips of pulling force. The integral cylinder is controlled by a hydraulic power unit (not shown) installed on the platform. The cylinder pulls the TLP down to the desired draft position with minimal ballasting and achieves desired tension and draft tuning with minimal de-ballasting. The system eliminates the need to ballast the TLP down onto the tendons in the pre-installation process or to lock the tendons onto the foundation piles in the co-installation process. Thus the system reduces the time and expense associated with traditional ballasting and deballasting by drastically reducing the amount of seawater that must be pumped in and out of the hull. 
     Other advantages of the system is that platform stability is ensured by not exceeding the minimum and maximum draft limits of the platform, and allows for the use of a stroke indicator to make fine adjustments during installation. The cylinder can also be used in conjunction with the active ratcheting feature of the TTC to perform flawlessly during platform heave. Further, the components are integrated to reduce additional installation equipment and the steps of the method can simply be reversed to easily decommission the platform. Finally the cost of an auxiliary cylinder barrel is not required. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. These embodiments are not intended to limit the scope of the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.