Abstract:
A sleeve, suitable for temporary use on a tendon for a tension leg platform (TLP) equipped with a bottom tendon connector, covers selected portions of the tendon and bottom tendon connector so as to provide an outer surface having fewer and/or less pronounced discontinuities in the outer diameter of the assembly. This improves the handling characteristics of the assembly in pipe tensioning devices and the like. The sleeve may be in form of split halves which may be held together around the selected portion of the tendon by band straps. The sleeve may be removed subsea by cutting the band straps using an ROV. In certain embodiments, the sleeve comprises an open frame arrangement of contoured segments held in a spaced-apart configuration by bolt-together or hinged clamps. The sleeve may provide an annular space between the tendon and the sleeve which may be used to house a flotation device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    None 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates to offshore platforms. More particularly, it relates to the tendons used to anchor tension leg platforms (TLPs). 
         [0005]    2 Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
         [0006]    A tension leg platform (TLP) is a vertically moored floating structure typically used for the offshore production of oil and/or gas, and is particularly suited for water depths greater than about 1000 ft. 
         [0007]    The platform is permanently moored by tethers or tendons grouped at each of the structure&#39;s corners. A group of tethers is called a tension leg. The tethers have relatively high axial stiffness (low elasticity) such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production wellheads on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor. This feature enables less expensive well completions and allows better control over the production from the oil or gas reservoir. 
         [0008]    A variety of TLP designs are known in the art. The following patents describe various examples. 
         [0009]    U.S. Pat. No. 7,462,000 discloses a tension leg platform that includes a deck supported on the upper ends of three or more columns interconnected at the lower ends thereof by horizontally disposed pontoons. The columns are battered inwardly and upwardly from the pontoons to the deck. Tendons connected at the columns anchor the platform to the seabed. The footprints of the base of the battered columns and the tendons are larger than the footprint of the deck supported on the upper ends of the columns. 
         [0010]    U.S. Pat. No. 4,585,373 describes a tension leg platform with exterior buoyant columns located outside the normal tension leg platform structure. The exterior columns are designed to decrease the pitch period of the tension leg platform away from the point of concentration of the largest wave spectrum energy encountered at a particular marine location. This modification of the pitch period of the tension leg platform is said to reduce the cyclic fatigue stresses in the tension legs of the platform thereby increasing the useful life of the platform structure. 
         [0011]    U.S. Pat. No. 5,558,467 describes a deep water offshore apparatus for use in oil drilling and production in which an upper buoyant hull of prismatic shape has a passage that extends longitudinally through the hull. Risers run through the passage and down to the sea floor. A frame structure connected to the hull bottom and extending downwardly comprises a plurality of vertically arranged bays defined by vertically spaced horizontal water entrapment plates providing open windows around the periphery of the frame structure. The windows provide transparency to ocean currents and to wave motion in a horizontal direction to reduce drag. The frame structure serves to modify the natural period and stability of the apparatus to minimize heave, pitch, and roll motions of the apparatus. A keel assembly at the bottom of the frame structure has ballast chambers for enabling the apparatus to float horizontally and for stabilization of the apparatus against tilting in the vertical position. 
         [0012]    U.S. Pat. No. 4,169,424 describes a tension leg buoyancy structure for use in seas exposed to wave action that includes a buoyancy section, an anchor section which rests on the sea bed, and a plurality of parallel tethers connecting the buoyancy section with the anchor section to permit the buoyancy section to move relative to the anchor section. Design parameters are selected such that the natural period of the buoyancy section for linear oscillation in the direction of wave travel, the natural period of the buoyancy section for linear oscillation in a horizontal direction perpendicular to the direction of wave travel, and the natural period of the buoyancy section for rotational oscillation about a vertical axis of the buoyancy section structure are greater than 50 seconds. 
         [0013]    U.S. Pat. No. 4,829,928 describes an ocean platform that has a negatively buoyant pontoon suspended from the balance of the platform to increase the heave resonant period. Tendons suspend the pontoon to a depth where dynamic wave forces do not materially act directly on it in seas of normally occurring periods of up to about 15 seconds but do in seas of periods above about 15 seconds. Columns and an upper pontoon provide buoyancy for the platform. 
         [0014]    U.S. Pat. No. 5,707,178 describes a tension base for a tension leg platform. A buoyant base is submerged below the water surface and is retained with base tendons to a foundation on the sea floor. The buoyant base is attachable to the mooring tendons of a tension leg vessel positioned above the buoyant base. The buoyant base can be selectively ballasted to control the tension in the base tendons. Additional buoyant bases and connecting tendons can extend the depth of the total structure. Mooring lines can be connected between the buoyant base and the sea floor to limit lateral movement of the buoyant base. The buoyant base creates a submerged foundation which is said to reduce the required length of a conventional tension leg platform. The tension leg platform can be detached from the buoyant base and moved to another location. 
         [0015]    U.S. Pat. No. 4,585,373 describes a pitch period reduction apparatus for tension leg platforms. A tension leg platform is provided with exterior buoyant columns located outside the normal tension leg platform structure. The exterior columns decrease the pitch period of the tension leg platform away from the point of concentration of the largest wave spectrum energy encountered at a particular marine location. Modification of the pitch period of the tension leg platform in this manner is said to reduce the cyclic fatigue stresses in the tension legs of the platform, and thereby increase the useful life of the platform structure. 
         [0016]    U.S. Pat. No. 7,854,570 discloses a pontoonless tension leg platform (TLP) that has a plurality of buoyant columns connected by an above-water deck support structure. The design eliminates the need for subsea pontoons extending between the surface-piercing columns. In certain embodiments, the buoyancy of the columns is increased by the addition of subsea sections of increased diameter (and/or cross-sectional area) to provide the buoyancy furnished by the pontoons of the TLPs of the prior art. A pontoonless TLP has a smaller subsea projected area in both the horizontal and vertical planes than a conventional multi-column TLP of equivalent load-bearing capacity having pontoons between the columns. This reduction in surface area produces a corresponding reduction in the platform&#39;s response to ocean currents and wave action and consequently allows the use of smaller and/or less costly mooring systems. Moreover, the smaller vertical projected area results in a shorter natural period which enables a pontoonless TLP according to the invention to be used in water depths where conventional TLPs cannot be used due to their longer natural periods. The absence of pontoons in a multi-column TLP also has the added benefit of providing an unobstructed path for risers to connect with the deck of the platform. 
         [0017]    U.S. Pat. No. 6,447,208 describes an extended-base tension leg substructure for supporting an offshore platform where the substructure includes a plurality of support columns disposed about a central axis of the substructure and interconnected by at least one pontoon. Each column comprises an above water and submerged portion. The substructure also includes a plurality of wings or arms radiating from the columns and/or the pontoons, each wing securing at least one tendon extending from a wing to an anchor on the seabed. It is said that the wings minimize translational movement and rotational flex in the substructure reducing fatigue in the tendons and their connections. 
         [0018]    U.S. Pat. No. 7,621,698 describes a rotating lock ring bottom tendon connector. The tendon bottom connector assembly has a receptacle with a bore having an annular locking profile divided into segments by axially extending slots. The connector has a housing that inserts into and locks in the receptacle. The housing and receptacle have mating anti-rotation elements. The lock ring has an outer surface with an annular locking profile divided into segments by axially extending slots. The lock ring is carried by the housing initially in the installation position with its segments aligned with the slots of the receptacle. This position allows the housing to be fully inserted into the receptacle. An ROV then rotates the ring from the installation position to a locked position, with the segments of the lock ring engaging the segments of the receptacle. Alternatively, a split ring with one end fixed rotates the lock ring. 
         [0019]    In the past, the tendons of a TLP were either assembled vertically at an offshore location by joining segments together as the tendon was lowered towards the seafloor or assembled horizontally onshore and subsequently towed to the TLP&#39;s installation site where they were up-ended and connected to piles driven into the seafloor. Both of these prior art methods are difficult to perform and may expose the tendon assembly to potentially damaging conditions—e.g., bending loads and uncontrolled sinking. 
         [0020]    U.S. patent application Ser. No. 13/095,597 filed Apr. 27, 2011, and entitled “Method for assembling tendons” (the disclosure of which is hereby incorporated by reference in its entirety) describes a novel method wherein a tendon is assembled in a horizontal orientation at or near the installation site of a TLP using connectors or weld stations on a barge or other vessel. During assembly, the tendon is pulled away from the assembly vessel and tensioned by a tug or offshore work vessel. When fully assembled, the tendon is up-ended (in a manner similar to a wet-towed tendon) and then either preinstalled using floats or passed over to the TLP which is on-site and ready to receive tendons. 
       SUMMARY OF THE INVENTION 
       [0021]    An apparatus according to the invention comprises a sleeve that is configured for temporary use on a tension leg platform (TLP) tendon equipped with a bottom tendon connector. The sleeve is designed to cover selected portions of the tendon and its bottom tendon connector so as to provide an outer surface having fewer and/or less pronounced discontinuities in the outer diameter of the assembly. This improves the handling characteristics of the tendon assembly in pipe tensioning devices and the like. The sleeve may be in form of split halves which may be held together around the selected portion of the tendon by band straps. The sleeve may be removed subsea by cutting the band straps using an ROV. In certain embodiments, the sleeve comprises an open frame arrangement of contoured segments that are held in a spaced-apart configuration by bolt-together or hinged clamps. The sleeve may provide an annular space between the tendon and the sleeve which may be used to house a supplementary flotation device. 
         [0022]    A tendon assembly method according to the invention may comprise attaching a sleeve around a portion of the tendon bottom connector and assembling a tendon by joining tendon segments together in a horizontal orientation on a first floating vessel to produce a tendon assembly while simultaneously applying a tension load via the tendon sleeve to the tendon assembly with a second vessel that is remote from the first vessel. Alternatively, the sleeve may be installed on an opposing, second end of the tendon assembly and used to apply a tension load to the tendon assembly while it is floating in the sea so as to maintain it in substantially linear alignment. 
         [0023]    BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0024]      FIG. 1  is a top plan view of a tendon being assembled on a tendon assembly vessel while the tendon is tensioned by a tug. 
         [0025]      FIG. 1A  is an enlargement of the portion indicated in  FIG. 1 . 
         [0026]      FIG. 2  is a side view of the tendon assembly apparatus illustrated in  FIG. 1 . 
         [0027]      FIG. 2A  is a side view of a prior art pipe tensioning device. 
         [0028]      FIG. 3  is a side view of a tendon sleeve according to a first embodiment of the invention installed on the bottom portion of a tendon, the covered portions of which are shown in phantom. Also shown in phantom are associated flotation devices within the sleeve 
         [0029]      FIG. 4  is an exploded view of a tendon sleeve and its associated floatation device according to a first embodiment of the invention. 
         [0030]      FIG. 5  is a cross-sectional view of the tendon sleeve illustrated in  FIG. 3 . 
         [0031]      FIG. 6A  is a partial cross-sectional view of a tendon sleeve according to a second embodiment of the invention shown installed on a tendon bottom connector. 
         [0032]      FIG. 6B  is a partial cross-sectional view of a tendon sleeve according to a third embodiment of the invention shown installed on a tendon bottom connector. 
         [0033]      FIG. 6C  is a partial side view of a tendon sleeve according to a fourth embodiment of the invention shown installed on a tendon bottom connector. 
         [0034]      FIG. 7A  is an isometric view of a tendon sleeve according to a fifth embodiment of the invention. 
         [0035]      FIG. 7B  is a cross-sectional view taken along the line indicated in  FIG. 7A . 
         [0036]      FIG. 7C  is an isometric view of an alternative clamp for use in connection with the embodiment shown in  FIG. 7A . 
         [0037]      FIG. 8  is a partial side view of a sixth embodiment of the invention shown installed on a bottom tendon connector of different design. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The invention may best be understood by reference to certain illustrative embodiments. Referring now to  FIGS. 1 and 2 , tendon assembly vessel  10  may have a barge-type hull and may be secured at a desired location using anchor lines  20 . Alternatively, a dynamic positioning system (not shown) may be used for station keeping. 
         [0039]    Assembly vessel  10  may have one or more cranes  14  on deck  12  for lifting and transferring tendon segments  18  from supply vessel  16  (shown moored alongside vessel  10 ) to tendon supports  22  on deck  12 . Tendon supports  22  are generally aligned with welding station  26 , tensioning device  28  and stinger  24 . Stinger  24  may be supported by gantry  25  and may project from the aft end of vessel  10 . Gantry  25  may be used to adjust the angle of stinger  24  relative to deck  12  (or the plane of supports  22 ). As will be appreciated by those skilled in the art, the horizontal plane of supports  22  is above the water line of vessel  10 . Accordingly, that portion of tendon assembly  30  which is floating at or near the water surface will be at a different elevation than that portion which is supported on supports  22  on deck  12  of vessel  10 . Stinger  24  may be used to minimize the bend radius of tendon  30  as it transitions from assembly vessel  10  into its horizontal floating position in the water. 
         [0040]    Also shown in  FIGS. 1 and 2  is tug  40  equipped with winch  36  which may be a constant-tension winch. Tensioning line  34  is attached to winch  36  and first end  38  of tendon  30 . Tendon tensioning vessel  40  need not be a tugboat, per se, but rather any suitable vessel capable of tensioning tendon  30  via line  34  such that tendon  30  remains substantially aligned with tendon supports  22  on vessel  10  during the assembly of tendon segments  18 . Tensioning device  28  transfers the tension load in tendon segment  18 ′ to vessel  10  and feeds the assembled portion of the tendon into the sea via stinger  24 . In certain embodiments, the propulsion system of tensioning vessel  40  and/or winch  36  may form a part of a dynamic positioning system for vessel  10 . 
         [0041]    As may best be seen in  FIG. 1A , one or more floatation jackets  32  may be installed on selected segment(s)  18 ′ of tendon  30  during tendon assembly to ensure the desired buoyancy while the tendon undergoing assembly is floating in the sea in a generally horizontal orientation. 
         [0042]    Tendons may be assembled from tendon segments  18  by any suitable method. Most commonly, tendon segments  18  will be joined together by welding at station  26 . Alternatively, mechanical tendon connectors may be installed and clamped to tendon sections at station  26 . In certain embodiments, floatation devices  32  may be installed at station  26 . 
         [0043]    A pipe tensioning device  28  according to the prior art is shown in  FIG. 2A . It will be appreciated that tendon segments  18  have the same configuration as thick-walled pipe and may generally be manipulated using equipment designed for pipe handling. 
         [0044]    Tensioner  28  comprises base  102  which may be attached to deck  12  of the tendon assembly vessel to thereby transfer the tension load applied to the tendon under assembly to that vessel. Lower drive unit  108  which comprises a drive motor  110  and a drive belt or conveyor track  112  is mounted on base  102 . Upper drive unit  106  which also comprises a drive motor  110  and a drive belt or conveyor track  112  is mounted on frame  104 . Tensioner frame  104  is hinged to base  102  to permit the pipe tensioning device to be opened in order to load pipe segment  18  therein. 
         [0045]    It will be appreciated by those skilled in the art that the juncture between the tendon bottom connector and the tubular segments which comprise the tendon body may constitute a discontinuity that may adversely affect the operation of the pipe tensioning device. A pipe tensioning device of the type shown in  FIG. 2A  is generally configured for pipe of a certain diameter (or pipes having a diameter which falls within a certain range of diameters). Moreover, currently available pipe tensioning devices are designed to interface with pipe—i.e., generally cylindrical tubular members. Non-cylindrical elements (such as bottom tendon connectors) may obstruct, cause slippage in, or jam a conventional pipe tensioning device. The present invention solves this problem by providing a smooth transition in outer diameter from the tendon body to the bottom tendon connector. 
         [0046]    It will also be appreciated by those skilled in the art that the practice of the above-described and illustrated method of assembling a tendon horizontally in the sea requires means for attaching tug  40  to end  38  of tendon  30 . It is advantageous to attach line  34  to an element of tendon  30  that is designed to withstand a tensile load. One such element is the bottom tendon connector—more particularly, the locking lugs of the bottom tendon connector which are designed to engage the anchor pile in the seafloor. The present invention provides means for transferring the tension load of line  34  to tension-bearing members of the bottom tendon connector. First end  38  of tendon  30  may be either the end having the bottom tendon connector or the end having the top tendon connector. When the bottom tendon connector is on the end of tendon  30  which is opposite first end  38 , it is still advantageous to have tension application means attached to the second end—e.g., a winch on barge  10 —so that tendon  30  can be kept substantially linearly aligned while floating in the sea by applying tension to both ends. 
         [0047]    Referring now to  FIGS. 3 and 5 , a first embodiment of the invention is shown installed on the lower portion of a tendon  118  that is equipped with a tendon bottom connector  122  of the type supplied by Oil States Industries, Inc. (Arlington, Tex.) as the ROTOLATCH™ tendon bottom connector. Sleeve  120  may surround a portion of the lowest tendon section, tapered transition piece  126 , spool piece  154  (see  FIG. 5 ) and the upper portion of tendon bottom connector  122  so as to provide a smooth, tapered transition among these pieces. Tendon sleeve  120  has smaller diameter end  128  oriented towards the upper end of tendon  118  and opposing larger diameter end  130  which may be configured to fit over a portion of bottom connector  122 . 
         [0048]    One or more grooves may be provided in the outer surface of sleeve  120  to locate bands or straps  134  and  134 ′ which may hold the two halves of sleeve  120  together. Connector  135  which may, in some embodiments, be a band strap cinch, secures band  134  around the tendon sleeve. Bands or straps  134  and  134 ′ may be cut to remove sleeve  120  from tendon  118 . In certain applications, banding straps  134  and  134 ′ may be cut by a subsea Remotely Operated Vehicle (ROV) after the tendon is upended at the TLP installation site. 
         [0049]    The particular type of bottom tendon connector illustrated in  FIGS. 3 ,  5  and  6  has radially projecting lugs  124  which are designed to be tension-bearing elements. Bell portion  148  of tendon sleeve  120  may have a shoulder  138  configured to bear against lugs  124  when the sleeve  120  is installed on tendon  118 . Towing or tensioning bridle  132  may be attached to sleeve  120  at any convenient location proximate larger diameter end  130 . Alternatively, as shown in phantom in  FIG. 3 , bridle  132  may be looped through water entry holes  150  in bottom tendon connector  122 . 
         [0050]    It will be appreciated that the installation of sleeve  120  on tendon  118  creates an annular chamber, the major portion of which is defined by the inner surface of sleeve  120  and the outer surface of spool piece  154 . This chamber may be advantageously used to add supplemental flotation to the lower end of tendon  118 . This may be of particular utility when tendon  118  is floating horizontally in the sea during the assembly operation and prior to being upended during the installation procedure. 
         [0051]    The supplemental flotation may take any conventional form—e.g., molded foam pieces, air cans, air bags and the like. In the illustrated embodiment, flotation material  144  is a solid material configured in two halves which fit together around spool piece  154  within the above-described annulus. Flotation piece  144  may have one or more grooves  146  and  146 ′ in its outer surface to locate banding straps  136  and  136 ′ which may hold the two halves of flotation piece  144  together. As in the case of bandings straps  134 , connector  137  may, in some embodiments, be a band strap cinch which acts to secure band  136 . Bands or straps  136  and  136 ′ may be cut to remove flotation device  144  from tendon  118 . In certain applications, banding straps  136  and  136 ′ may be cut by a subsea Remotely Operated Vehicle (ROV) after the tendon is upended at the TLP installation site and sleeve  120  is removed. In certain embodiments, tendon  118  may be provided with indicator bands  152  and/or  152 ′ (see  FIG. 5 ) to assist an ROV in positioning itself relative to tendon  118  and tendon sleeve  120 . 
         [0052]      FIG. 4  is an exploded view of the tendon sleeve  120  and flotation device  144  shown in  FIG. 3 . Tendon sleeve  120  comprises shell halves  140  and  140 ′ which, when assembled, abut one another at edges  142  and  142 ′. In the illustrated embodiment, edges  142  and  142 ′ comprise a butt joint. In other embodiments, edges  142  and  142 ′ may be beveled, tongue-and-groove, splined or otherwise configured for joining by methods well-known in the art. If, as illustrated in  FIG. 4 , flotation material  144  is likewise configured as split halves, the adjoining edges of the mating halves may also be similarly profiled or otherwise equipped for aligned joining by any of the above-mentioned methods. 
         [0053]      FIGS. 6A ,  6 B and  6 C show alternative embodiments of the invention, each configured for use on a tendon equipped with the same type of bottom tendon connector as that shown in  FIGS. 3 and 5 . The embodiment illustrated in  FIG. 6A  has radial flange  156  configured to bear against the adjacent surface of bottom tendon connector  122 . Flange  156  may be a tension-load bearing member and may thus serve to avoid any damage to lugs  124  which might occur during tendon towing or tensioning operations by providing a greater contact area. 
         [0054]    The embodiment illustrated in  FIG. 6B  includes individual lug pockets  158  in end  130  of tendon sleeve  120 . Lug pockets  158  both cover lugs  124  (thereby protecting them from mechanical damage) and prevent the rotation of sleeve  120  relative to tendon  118 . 
         [0055]      FIG. 6C  shows an embodiment which has individual lug notches  160  in end  130  of tendon sleeve  120 . Lug notches  160  allow the outside diameter of sleeve  120  to be somewhat smaller than that of the embodiments illustrated in  FIGS. 3 ,  5 ,  6 A and  6 B and also act to prevent the rotation of sleeve  120  relative to tendon  118 . 
         [0056]      FIG. 8  illustrates how a tendon sleeve according to the invention may be adapted to fit tendons having other types of bottom tendon connectors. 
         [0057]      FIG. 8  shows the lower end of a tendon  218  having a bottom tendon connector  222  of the type described in U.S. Pat. No. 7,621,698 to Pallini et al. and assigned to Vetco Gray, Inc. (Houston, Tex.). Bell portion  248  is configured to fit the upper end (as installed) of tendon connector  222 . The illustrated embodiment comprises tension load-bearing radial flange  256  in contact with the cap ring  223  of the tendon connector  222 . In this way, the tensioning load from bridle  232  may be transferred to tendon  218  during its assembly and installation operations. 
         [0058]      FIGS. 7A and 7B  show yet another embodiment of the invention. In this embodiment, open-frame sleeve  170  comprises a plurality of contoured segments  172  which are held in a spaced-apart configuration by clamps  178  and  180 . Optional recesses  174  and  176  in segments  172  may be provided for locating clamps  178  and  180 , respectively. As may be best seen in the cross-sectional view of  FIG. 7B , the inner surface of clamp  178  may have recesses for locating segments  172  which, in certain embodiments, may be welded (at  184 ) to clamp  178 . The two halves of clamp  178  may have flanges which permit the two halves to be bolted together with bolts  182 . Sleeve  170  may be removed from a tendon on which it is installed by removing bolts  182 —e.g., using an ROV—and allowing the two halves of the open-frame sleeve to fall away. Clamp  180  may be similarly configured. 
         [0059]    An alternative hinged clamp  190  is shown in  FIG. 7C . Each half of hinged clamp  190  has a hinge plate  192  with a thru-hole  193 . A bolt or the like (not shown) may secure the opposing hinge plates  192  together and act as a hinge pin. The opposite side of clamp  190  may have a flange  194  on each half with a thru-hole  196 . A bolt (not shown) may be used to secure the opposing flanges  194  together. Optional recesses  198  on the inner surface of clamp  198  may be provided for locating sleeve segments  172 . 
         [0060]    A tendon sleeve equipped with hinged clamps  190  may be removed from a tendon by removing the bolt (or other such fastener) which secures flanges  194  together and opening the sleeve by pivoting the two halves of clamp  190  on hinge plates  192 . This may be accomplished with an ROV after the tendon is upended at the installation site or by divers when the tendon is floating near the surface. 
         [0061]    Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.