Abstract:
A driven pile anchor suitable for securing a tendon of a tension leg platform to the seafloor is supplemented with added weight. A load frame is added to the individual pile to accommodate ballast weights. Pre-manufactured ballast weights are placed on the frame to increase the holding power of the pile anchor system. In a second embodiment, the pile is intentionally plugged and installed with the plug intact. Pre-manufactured ballast weights are then placed inside the pile and may be held in place by gravity. In a third embodiment, the pile is a conventional driven friction pile installed with an underwater pile hammer. The pile is initially open but subsequently evacuated and intentionally plugged near its pile tip. Pre-manufactured ballast weights are placed inside the pile to increase its holding capacity. The first embodiment may be retrofitted to existing, driven-pile anchor systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 12/207,337 filed Sep. 9, 2008. 
     
    
     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 subsea, driven-pile anchor systems. More particularly, it relates to subsea piles used to anchor the tendons of tension leg platforms or other vertically-moored structures to the seafloor. 
         [0005]    2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98 
         [0006]    A Tension-Leg Platform or TLP is a vertically moored floating structure normally used for the offshore production of oil or gas, and is particularly suited for water depths greater than 300 meters (about 1000 ft). The design has also been proposed for wind turbines. 
         [0007]    The platform is permanently moored by means of tethers or tendons grouped at each of the structure&#39;s corners. The tendons have relatively high axial stiffness (low elasticity) such that virtually all vertical motion of the platform is eliminated. This feature 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 makes for a less expensive well completion and provides better control over the production from the oil or gas reservoir. 
         [0008]    Conventionally, the tendons of a TLP are secured to pile foundations comprising tubular piles driven into the seafloor by mechanical hammers. Such anchoring means provides high resistance to the tensile load applied by the floating TLP through the tendons. 
         [0009]    U.S. Pat. No. 5,551,804 discloses a method of driving a pile wherein a pile is plugged at its lower end or tip region so as to make it easier to drive. Additionally, such a plugged pile facilitates the transportation and handling of the pile prior to its driving. This invention comprises the use of a plug at or near the bottom or end of a pile during handling, assembly, and lowering of the pile to self support in the sea bottom, followed by the driving of the pile with the plug intact. General consensus in the industry is that the plug will make the driving of the pile more difficult since it presents more of a profile that must be moved through the soil. However, this was not found to be the case in highly sensitive clays; the driving of a plugged pile is considerably easier than the driving of an open pile. While plugs have been employed in the past to facilitate certain aspects of pile transportation, handling, assembly, and lowering, they were always removed prior to pile driving because of the belief that the plug would make the pile more difficult or impossible to drive. Previously, the advantages derived from using plugs did not compensate for the costs related to installing and removing the plugs. 
         [0010]    U.S. Pat. Nos. 6,318,933 and 6,142,709 describe a foundation system for tension leg platforms without a foundation template, wherein each tendon is directly connected to a socket inside the pile, the piles being positioned for driving purpose by means of a pile-driving template which is employed as a spacing device is described. The pile-driving template is positioned with the aid of pins that slot into guides built into the well template. After the groups of piles needed to anchor a corner of the platform have been driven in, the pile-driving template is withdrawn and repositioned so as to enable the piles for the other group of legs to be driven; this process continues until all of the pile-driving is finished. Alternatively one single pile-driving template may be employed to guide the driving of all the piles thus doing away with the need to reposition the template every time. The bottom ends of the piles are conical in shape, and after the piles have been driven they are filled with a high specific gravity material. 
         [0011]    U.S. Pat. No. 3,984,991 describes an anchor which includes a tubular body, a top closure and a bottom closure secured to opposite ends of the tubular body, a plurality of drilling cutters mounted on the bottom closure, a neck having an external groove therein secured to the top closure, an opening in both the top and bottom closures, means for co-acting with a drill string extending into the closures for sealing to maintain the interior of the body substantially free of water when submerged, a ratchet collar adapted to co-act with a mating ratchet collar of the drill string for rotating the anchor to cause it to drill into the bottom of a body of water, a swivel adapted to be lowered onto the neck of the top closure when it has been set, latching dogs engaging in the external groove to secure the swivel to the body, and floatation means for signaling the surface that the swivel is set. The method of setting an anchor assembly including the steps of lowering an anchor body having cutters on the bottom thereof on the end of a drill string extending through the top of the anchor body and into the bottom for circulation of drilling fluid onto the face of the formation being drilled, the drill string being sealed to the anchor body to prevent entry of water therein, rotating the drill string and anchor body to drill the hole and lower the anchor body into the hole simultaneously, cementing around the exterior of the anchor body, then cementing the interior of the anchor body, lowering a swivel onto the top of the anchor body and signaling the seating of the swivel. 
         [0012]    U.S. Pat. No. 5,582,491 describes a system to increase the tension capacity of pipe piles driven into the ocean floor. A pile cap is attached to a pipe pile. A partition is installed below the pile cap creating an air chamber between them that is at surface atmospheric air pressure. An external conduit containing a valve that is closed connects the pile&#39;s interiors above and below the partition. The pile is driven into the ocean floor filled with entrapped sea water below the partition so that little or no soil core is generated. After the clay soils adjacent to the pile have regained their strength, the valve is opened. A small amount of sea water expands into the air chamber. The pressure on both sides of the partition and the bottom of the pile cap is now slightly above surface atmospheric air pressure. It is the that the downward force of hydrostatic pressure on top of the pile cap increases the tension capacity of the driven pile. 
         [0013]    U.S. Pat. No. 6,536,993 describes an apparatus for providing a mooring anchorage and a method of drilling and installing a pile in ground comprising the steps of: providing a pile, providing a drill bit at an end of the pile rotatable relative to the pile, engaging the ground with the drill bit, and rotating the drill bit relative to the ground and the pile generating a hole into which the pile is received. 
         [0014]    U.S. Pat. No. 6,312,195 describes a method of installing a foundation for a tension leg platform that eliminates the foundation template as a permanent, load bearing part of the foundation. Piles are installed by, for example, being driven into the ocean floor so that each pile is secured to the ocean floor, but is unsecured to any other structure that is on the ocean floor. A tension leg platform is coupled via tendon structures to the piles so that anchoring load paths are defined from the tension leg platform to the ocean floor in a plurality of generally vertical paths extending in axial alignment through the tendon structures to the pile and the ocean floor. Each of the tendon structure to pile anchoring systems is said to be substantially independent of one another. 
         [0015]    U.S. Pat. No. 5,020,764 describes a pole ballasting device adapted to be positioned about the lower end of one or more poles of a pole-using temporary structure for the purpose of holding the poles. The device includes at least two discrete blocks having, in their assembled condition, one or more common vertically extending through holes for receiving the poles therethrough. The blocks are arranged along planes passing through the through holes. A joining mechanism is provided for separably joining the blocks into a unit. A container is formed in at least one of the blocks and adapted to contain a fluidic load therein. 
       BRIEF SUMMARY OF THE INVENTION 
       [0016]    The present invention comprises a method for improving the performance of Single Piece Tension Piles such as those conventionally used to anchor TLP&#39;s. The piles may be configured one per tendon and driven to design penetration depth with an underwater pile hammer. Subsequent to being driven, pre-manufactured ballast weights are added to the pile to improve its tension capacity beyond what is achieved from skin friction and the weight of the pile itself. The ballast can be added either internally, externally on a load frame or a combination thereof. 
         [0017]    The practice of the invention requires no mechanical connection between the pile (or load frame) and the ballast weights other than that provided by gravitational forces. This greatly simplifies the installation process. The ballast weights may offer other improvements to the foundation&#39;s performance beyond an increase in tension capacity. In certain preferred embodiments, there are provided certain unique features on the driven pile that allow the ballast weights to transfer gravity loads. These features include, but are not limited to, load shoulders, shear keys, forged lugs, and other fabricated apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0018]      FIG. 1A  is a cross-sectional view of a first embodiment of the invention taken along line A-A in  FIG. 1B . 
           [0019]      FIG. 1B  is a top plan view of the embodiment shown in  FIG. 1A . 
           [0020]      FIG. 1C  is an exploded, perspective view showing the embodiment of  FIG. 1A  with the driven pile installed in the seafloor. 
           [0021]      FIG. 2A  is a cross-sectional view of a second embodiment of the invention taken along line A-A in  FIG. 2B . 
           [0022]      FIG. 2B  is a top plan view of the embodiment shown in  FIG. 2A . 
           [0023]      FIG. 2C  is a perspective view, partially in phantom, showing the embodiment of  FIG. 2A  installed in the seafloor. 
           [0024]      FIG. 3A  is a cross-sectional view of a third embodiment of the invention taken along line A-A in  FIG. 3B . 
           [0025]      FIG. 3B  is a top plan view of the embodiment shown in  FIG. 3A . 
           [0026]      FIG. 3C  is a perspective view, partially in phantom, showing the embodiment of  FIG. 3A  installed in the seafloor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    The invention may best be understood by reference to certain illustrative embodiments which are shown in the drawing figures. 
         [0028]      FIG. 1  depicts a single-piece, open-ended pile designed as a tension pile which may be lowered to the seafloor and set to self-penetration depth in the seafloor—i.e., the pile may sink in the mud at the bottom of the sea to a substantial depth under its own weight. The pile may then be driven to design penetration with an underwater hammer using equipment and methods conventional in the art. 
         [0029]    A load frame may then be added to the installed pile. Pre-manufactured ballast weights are placed on the load frame thereby increasing the foundation pile&#39;s holding capacity, particularly in the vertical direction. 
         [0030]    Referring particularly to  FIG. 1B  wherein the assembly is shown installed in seafloor S, driven pile  10  is a tubular structure having open end  16  at a first end and a section of reduced diameter (or “neck”)  14  at an opposing second end. Shoulder  12  joins the two sections and provides a bearing surface for load frame  20 . 
         [0031]    In the particular embodiment shown in  FIG. 1 , load frame  20  comprises center member  21  sized to fit over the upper portion of pile  10 . Center member  21  has central opening  28  for receiving pile  10  and may comprise sections of differing diameters joined by transition section  23 . Transition section  23  may be sized and spaced to bear against shoulder  12  of pile  10  when installed, thereby providing a load path between pile  10  and load frame  20 . Center member  21  may also comprise angled flange  29  at its lower end. Angled flange  29  helps to center load frame  20  on pile  10  when it is lowered into place during the installation procedure. Flange  29  will cam against the upper end of pile neck  14 , thereby correcting for slight misalignments when the two components are joined. 
         [0032]    Surrounding center member  21  are a plurality of equally-spaced receivers  26  which are joined to center member  21  by radial arms  22  and angled braces  24 . The upper end of each receiver  26  may be provided with flared portion  27  which acts to align corresponding weight  30  as it is lowered into weight receiver  26  and also provides a stop for weight  30  when fully installed in load frame  20 . 
         [0033]    Each weight  30  has a lifting eye  32  at a first end and a conical or frusto-conical tip  36  at an opposing second end. Tip  36  may assist the centering of weight  30  in receiver  26  during installation and assist the penetration of weight  30  in seafloor S. As shown in  FIG. 1C , the installation of weights  30  in load frame  20  creates penetrations  18  in seafloor S. Weight  30  may have a flange  34  at its upper end which may have under-surface  35  configured to rest on flared portion  27  of weight receiver  26 . 
         [0034]    Weight  30  may be solid or, alternatively, may comprise a tubular member having wall  39  defining a central cavity which may be filled with ballast  38 . Ballast  38  may comprise any suitable, high-density material, such as concrete, iron, iron ore or lead. One particular iron ore which may be used in this application is hematite (or haematite) which is the mineral form of Iron (III) oxide (Fe 2 O 3 ), one of several iron oxides. It typically exhibits a specific gravity of between 4.9 and 5.3. Hematite is a mineral, colored black to steel or silver-gray, brown to reddish brown, or red. It is mined as the main ore of iron. Hematite is harder than pure iron, but much more brittle. 
         [0035]    Another example of a suitable ballast material for weight  30  is Barite (or Baryte), a mineral consisting of barium sulfate (BaSO 4 ). It is generally white or colorless, and is the main source of barium. The mineral is also called “heavy spar” or “tiff.” The radiating form is sometimes referred to as Bologna Stone. Its Mohs hardness is 3, and it has a specific gravity of 4.3-5. Its crystal structure is orthorhombic. 
         [0036]      FIG. 2  depicts another embodiment of the invention which employs a single-piece, intentionally plugged (closed ended) pile designed as a tension pile. During installation, the pile is lowered to the seafloor with the plug intact. The pile is then set to self-penetration depth in the seafloor and subsequently driven to design penetration using an underwater hammer or other conventional means. 
         [0037]    Pre-manufactured ballast weights are then placed internally in the central cavity of the pile to increase the vertical holding capacity of the foundation pile. 
         [0038]    Referring to the cross-sectional view of  FIG. 2A , driven plugged pile  110  is shown installed in seafloor S. As is conventional, pile  110  may be a tubular member having sections of differing diameter. For example, the upper end of plugged pile  110  may comprise neck  114  joined to the lower section of pile  110  by shoulder  112 . Neck  114  may have a smaller o.d. than the lower section of pile  110  to facilitate connection to tendon connectors (not shown). 
         [0039]    The opposing, lower end of pile  110  may be closed, either by a metal cap member or, as shown in  FIG. 2B , by permanent grout plug  117  which may be installed in pile  110  prior to its installation in the seafloor. 
         [0040]    Following its installation in the seafloor, one or more pre-manufactured ballast weights  130  may be placed in the central cavity of plugged pile  110 . Ballast weights  130  may comprise lifting padeye  132  at a first end and a recess  133  at an opposing second end which is sized and shaped to accommodate the padeye  132  of the ballast weight  130  stacked below it. 
         [0041]    Weight  130  may be solid or, alternatively, may comprise a tubular member having wall  139  defining a central cavity which may be filled with ballast  138 . Ballast  138  may comprise any suitable, high-density material, such as concrete, iron, iron ore or lead. One particular iron ore which may be used in this application is hematite (or haematite) which is the mineral form of Iron (III) oxide (Fe 2 O 3 ), one of several iron oxides. It typically exhibits a specific gravity of between 4.9 and 5.3. Hematite is a mineral, colored black to steel or silver-gray, brown to reddish brown, or red. It is mined as the main ore of iron. Hematite is harder than pure iron, but much more brittle. 
         [0042]    Another example of a suitable ballast material for weight  30  is Barite (or Baryte), a mineral consisting of barium sulfate (BaSO 4 ). It is generally white or colorless, and is the main source of barium. The mineral is also called “heavy spar” or “tiff.” The radiating form is sometimes referred to as Bologna Stone. Its Mohs hardness is 3, and it has a specific gravity of 4.3-5. Its crystal structure is orthorhombic. 
         [0043]    Yet a third embodiment of the invention is illustrated in  FIG. 3 .  FIGS. 3A ,  3 B and  3 C show three views of a single-piece, open-ended pile designed as a tension pile. During installation, the pile is lowered to the seafloor and allowed to set to self-penetration depth in the seafloor. It may then be driven to design penetration with an underwater hammer or other means well-known in the art. Following driving, the interior of the pile may be jetted out or otherwise treated to remove the soil plug. 
         [0044]    A permanent plug that is capable of supporting ballast weights and transferring shear loads to the driven pile may then be set near the pile tip. 
         [0045]    Pre-manufactured ballast weights are placed in the central internal cavity of the pile to increase the foundation pile&#39;s vertical holding capacity. 
         [0046]    Referring in particular to the cross-sectional view of  FIG. 3A , driven pile  210  is shown installed in seafloor S. As is conventional, pile  210  may be a tubular member having sections of differing diameter. For example, the upper end of plugged pile  210  may comprise neck  214  joined to the lower section of pile  210  by shoulder  212 . Neck  214  may have a smaller o.d. than the lower section of pile  210  to facilitate connection to tendon connectors (not shown). 
         [0047]    The opposing, lower end of pile  210  may be closed subsequent to its installation in the seafloor by removing the resulting soil plug from its central cavity either by jetting or other suitable means known in the art. Grout plug  217  may then be installed at the lower terminus of pile  210 . 
         [0048]    Following its installation in the seafloor, jetting and plugging, one or more pre-manufactured ballast weights  130  may be placed in the central cavity of plugged pile  210 . Ballast weights  130  may comprise lifting padeye  132  at a first end and a recess  133  at an opposing second end which is sized and shaped to accommodate the padeye  132  of the ballast weight  130  stacked below it. 
         [0049]    Weight  130  may be solid or, alternatively, may comprise a tubular member having wall  139  defining a central cavity which may be filled with ballast  138 . Ballast  138  may comprise any suitable, high-density material, such as concrete, iron, iron ore or lead. One particular iron ore which may be used in this application is hematite (or haematite) which is the mineral form of Iron (III) oxide (Fe 2 O 3 ), one of several iron oxides. It typically exhibits a specific gravity of between 4.9 and 5.3. Hematite is a mineral, colored black to steel or silver-gray, brown to reddish brown, or red. It is mined as the main ore of iron. Hematite is harder than pure iron, but much more brittle. 
         [0050]    Another example of a suitable ballast material for weight  30  is Barite (or Baryte), a mineral consisting of barium sulfate (BaSO 4 ). It is generally white or colorless, and is the main source of barium. The mineral is also called “heavy spar” or “tiff.” The radiating form is sometimes referred to as Bologna Stone. Its Mohs hardness is 3, and it has a specific gravity of 4.3-5. Its crystal structure is orthorhombic. 
         [0051]    It will be appreciated by those skilled in the art that the embodiments of the invention illustrated in  FIGS. 2 and 3  may be combined with the embodiment shown in FIG.  1 —i.e., ballast weights  130  may be added to the central driven pile  10  to further increase its holding power. 
         [0052]    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.