Patent Application
20100061810
NONE
NOT APPLICABLE
1. Field of the Invention
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.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
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.
The platform is permanently moored by means of tethers or tendons grouped at each of the structure'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.
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.
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.
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.
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.
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'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.
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.
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.
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.
The present invention comprises a method for improving the performance of Single Piece Tension Piles such as those conventionally used to anchor TLP'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.
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'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.
The invention may best be understood by reference to certain illustrative embodiments which are shown in the drawing figures.
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's holding capacity, particularly in the vertical direction.
Referring particularly to
In the particular embodiment shown in
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.
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
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 (Fe2O3), 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.
Another example of a suitable ballast material for weight 30 is Barite (or Baryte), a mineral consisting of barium sulfate (BaSO4). 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.
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.
Referring to the cross-sectional view of
The opposing, lower end of pile 110 may be closed, either by a metal cap member or, as shown in
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.
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 (Fe2O3), 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.
Another example of a suitable ballast material for weight 30 is Barite (or Baryte), a mineral consisting of barium sulfate (BaSO4). 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.
Yet a third embodiment of the invention is illustrated in
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.
Pre-manufactured ballast weights are placed in the central internal cavity of the pile to increase the foundation pile's vertical holding capacity.
Referring in particular to the cross-sectional view of
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.
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.
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 (Fe2O3), 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.
Another example of a suitable ballast material for weight 30 is Barite (or Baryte), a mineral consisting of barium sulfate (BaSO4). 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.
It will be appreciated by those skilled in the art that the embodiments of the invention illustrated in
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.
