Abstract:
A method of lowering a floating body of a floating foundation and carrying a structure projecting above the water surface to an end position located beneath the water level, includes threading, through each of the upwardly projecting tubes, which are secured to the floating body above-through-openings formed in the floating body, a cable or a rod and connecting the cable or rod to a set-in-ground anchor, lowering the floating body, with the uptake devices provided at the free ends of the tubes, to the end position of the floating body beneath the water surface against buoyancy of the floating body, and fixidly securing the cables or rods, in the end position of the floating body, to the floating body and cutting off sections of the cables and rods above fixation points of the cables and rods with the floating body and removing the tubes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of lowering a floating body of a floating foundation and carrying a structure projecting above the water surface to an end position located beneath a water level and of securing the floating body in the end position with cables or rods connected with anchors set in the ground. 
     2. Description of the Prior Art 
     Floating foundation, e.g., which are used in oil rigs, are known. Such floating foundations include a floating body that supports the weight of a structure projecting above the water surface. The structure has a portion located beneath the water surface and a portion located above the water surface and which, in case of an oil rig, includes a derrick. The sum of the buoyancy of the floating body of the floating foundation and of the buoyancy of the underwater portion of the structure correspond exactly to the weight of the entire construction, i.e., the entire construction floats in water, with the floating body of the floating foundation being located beneath the water surface. In order for the construction to retain its position above the seabed, the floating foundation is retained against a horizontal displacement relative to the seabed by cables, rods, or hawsers connected with the set in the seabed, anchors. The cables, ropes or hawsers extend at an angle to a horizontal of, e.g., 45°. 
     The cable, rope, hawser retaining anchors are formed dependent on the ground, e.g., as rock anchors for rock beds, as stay-bars for gravel beds . . . Conventionally, the cable-forming strings remain open (hanging loose). The end of a cable is inserted in a bore formed in the ground, and a mortar is pressed through an injection tube that extend up to the front end of the cable, providing for anchoring the cable. Other anchoring methods, e.g., using a pure mechanical clamps and the like, are also known. 
     A floating foundation with a floating body located beneath a water surface and retained by cables or hawsers connected with the set in the ground, anchors is disclosed in U.S. Pat. No. 5,964,550. Japanese Publication JP 06255573 discloses a construction the base of which lies beneath the water surface, with the structure projecting above the water surface, and which is held by cables or hawsers in its position, with the cables or hawsers being connected with set in the ground, anchors. 
     European Application EP 02 015395.3 discloses a floating foundation for a projecting above the water surface, structure which is retained in its end position beneath the water surface against a buoyancy force of at least one buoyancy body by locking devices anchored in the ground. The buoyancy force is greater than it would be necessary for retaining the floating condition of the entire construction in the end position of the floating body, i.e., the floating body, because of the amount of its buoyancy, is preloaded with respect to the locking devices. This increases the stability of the floating foundation. The displacement of the floating foundation, together with the supported structure to a predetermined site, is effected by floating it over the water surface. However, the lowering of the floating foundation to its end position is associated with a considerable underwater work, which is associated with increased costs. 
     An object of the present invention is to provide a method of lowering a floating foundation, which floats above the water surface, to its end position, beneath the water surface against the buoyancy of the floating body, with the underwater work being substantially reduced. 
     SUMMARY OF THE INVENTION 
     This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a method of lowering a floating body of a floating foundation and carrying a structure projecting above a water surface to an end position located beneath the water level, and of securing the floating body in the end position. The method includes forming, in the floating body which floats above the water surface, at least three through-openings in corner points of an imaginary triangle, securing, to the floating body above each of the through-openings, an upwardly projecting tube, and threading, through each of the upwardly projecting tubes, a cable or a rod and connecting the cable or the rod with a set-in-ground anchor. 
     There is provided, in a region of an upper end of each of the upwardly projecting tube, an uptake device connectable with the respective cable or rod. Then, the floating body, is lowered with the uptake devices, to its end position beneath the water surface against buoyancy of-the floating body, and the cables or rods, in its end position of the floating body, are fixedly secured thereto. Thereafter, sections of the cables or rods above fixation points of the cables or rods with the floating body are cut off, and the upwardly projecting tubes, which are located above the respective through-openings, are removed. 
     The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however both as its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings show: 
     FIG. 1. a schematic view showing a first phase of lowering and securing a floating foundation in accordance with the method according to the present invention; 
     FIG. 2. a schematic view showing a second phase of lowering and securing a floating foundation in accordance with the method according to the present invention; 
     FIG. 3. a schematic view showing a third phase of lowering and securing a floating foundation in accordance with the method according to the present invention; 
     FIG. 4. a schematic view showing a fourth phase of lowering and securing a floating foundation in accordance with the method according to the present invention; 
     FIG. 5. a schematic bottom view of the floating foundation; 
     FIG. 6. a detail “A” of FIG. 2 at an increased scale; 
     FIG. 7. a detail “B” of FIG. 2 at an increased scale; 
     FIG. 8. a detail “C” of FIG. 2 at an increased scale; 
     FIG. 9. a side view of a tube for forming a through-opening to a buoyancy chamber and having a closable connection opening; 
     FIG. 10. a perspective view of a tube for forming a through-opening to a buoyancy chamber and having a closable connection opening; and 
     FIG. 11. a schematic cross-sectional view of a portion of the floating body in the region of the connection opening of the through-opening. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to bring a floating body  1  of a floating foundation with a structure  2  erected thereon to its operational site, the floating body  1  floats on a water surface, as shown in FIG.  1 . The floating body  1  has a plurality of buoyancy chambers  3  which are open, in the embodiment shown in the drawings, downwardly. The use of closed buoyancy chambers is also possible. In the shown embodiment, the floating body  1  has a deck plate  4  having a circular shape and along the outer edge of which an annular side wall  5  extends. Separate buoyancy chambers  3  are separated from each other by radially extending intermediate walls  6  and inner, annularly extending, intermediate walls  7 . Other shapes of the deck plate and/or of the buoyancy chambers are also possible. The floating body shown is the drawings, is shown only symbolically and the shown wall thicknesses are not to the scale. 
     The structure  2 , which is supported on the floating foundation, can represent, e.g., a wind tower of a wind energy plant and of which only the lower portion is shown in the drawings. 
     A plurality of through-openings  8 , which pass through the side wall  5 , are provided along the circumference of the floating body  1 . A plurality of drilling tools  9 , which are arranged on the floating body  1 , are used for drilling bores, with the drills  10  that extend through the respective through-openings  8 , in the ground  11 , in particular in the seabed. Subsequently, anchors  12 , e.g., conventional rock anchors or tie rods will be secured in these bores. 
     A flexible cable  13  (e.g., a hawser) or a more of less rigid rod, which is shown in FIGS. 2-4 with dash lines, can be secured to each of the anchors  12 . The cable  13  can be, if needed, surrounded with a flexible sheath. 
     Instead of using the drilling tools ( 9 ) mounted on the floating body  1  for forming the bores for the anchors  12 , the anchors can be secured in the seabed independently of the floating body  1 , e.g., before the floating body  1  is brought to its site of use. 
     Generally, above each through-opening  8 , an upwardly extending tube  14  is secured on the floating body  1 . This can be effected, e.g., in a manner schematically shown in FIG.  7 . Above each through-opening  8  (which can be limited by a tube  15 ), mounting brackets  16  are secured on the deck plate  4 . The mounting brackets  16  have each a flange plate  18  which is screwed to the deck plate  4  with screws  17 , providing for securing the mounting bracket  16  to the deck plate  4 . An upwardly extending riser  19  is welded to the flange plate  18 . A flange disc  20  is secured to the upper end of the riser  19 . The tube  14 , together with a base plate  21  secured to its lower end, is secured to the flange disc  20 , with a screw bolt  22  that extends through the base plate  21  and the flange disc  20 . Between the risers  19 , mounting openings  23  are provided. 
     In the region of the upper ends of the base body  1 , a production platform  24  is secured on respective tubes  14  (FIG.  6 ). The cables (hawsers or ropes)  13  or rods, which are connected to the anchors  12 , extend through the tubes  14  up to above the upper ends of the tubes  14 . E.g., the cables  13  or the rods can be threaded into the tubes before the tubes  14  are secured on the floating body  1 . On the upper end of the tube  14 , there is arranged an uptake apparatus  25  that can be operated, among others, by a hydraulic device  26 . Such uptake apparatuses are known as “tendon presses” or “lifting hydraulic apparatuses” and are commercially available. With these uptake apparatuses  25 , cable  13  or rods can be pulled out of the tubes  14 . In effect, with a simultaneous pull of the cables  13 , the floating body  1  is pulled downwardly by beneath the water surface  27  and against the buoyancy of the floating body  1  until the floating body  1  reaches its end position shown in FIG.  3 . 
     To facilitate pulling of the floating body  1  downwardly, the buoyancy of the floating body  1  can be changed during this process, by reducing the air volume in the buoyancy chambers  3 . However, lowing is always effected against the buoyancy of the floating body  1  which exceeds the weight of the structure  2  carried by the floating body  1  (subsequently, the lowering takes place in connection with a portion of the structure located beneath the water surface). The water level in the buoyancy chambers is shown in the drawings with a line  28 . Because lowering of the floating body  1  is effected against its buoyancy, a certain stabilization of the floating body  1  during the lowering is achieved. 
     In the position shown in FIG. 3, the end position of the floating body  1 , the upper ends of the tubes  14  always project above the water surface  27 , so that lowering of the floating body  1  from the production platform  24  above the water surface  27  can be effected. 
     Subsequently, the cables  13  or the rods are fixed with respect to the floating body  1 , with clamping devices  29  which are shown schematically in FIG.  7  and can be closed through the mounting openings  23  in the mounting brackets  16  for clamping the cables  13 . The cables  13  or the rods are cut off only above the clamping devices  29 , and the tubes  14  are removed from the mounting brackets. In order to water-tight seal the ends of the cables  13 , subsequently, the mounting openings  23  and the through-openings in the flange discs  20  of the mounting brackets  16  can be closed, and the mounting brackets  16  are filled with foam. These works are done by divers. 
     In order to further increase the stability of the floating foundation, in addition to substantially vertically extending rods or cables  13  (which extend to a vertical at angle of less than 10°), further rods or cables should be provided which would extend to or be inclined to the vertical at an angle between 25 and 45°. In order to secure anchors  30  for the inclined rods or cables  31  (hawsers), in the lower region of the structure  2 , a water-free region, of a caisson type, is formed. To this end, the circumferential wall  32  of the structure  2  is circumferentially closed and water-sealed at least in its lower region, and further, a water and/or air-tight intermediate deck  33  is provided. Into this space  34 , through-bores, which pass through the floating body  1  and are inclined to a vertical, open. These through-bores can be formed, e.g., by tubes  35  extending through the floating body  1  and inserted, advantageously, in the intermediate walls  6  of the floating body  1 . Upon lowering of the floating body beneath the water surface, these tubes can be sealed to prevent penetration of water when the space  34  is not yet completely sealed or the production apparatuses are lowered through an opening in the intermediate deck  33 . In the lowered position, the tubes  35  are open, and the air-tight space  34  remains free of water due to the available air pressure. 
     On the deck plate  4 , within the space  34 , one or more drilling tools can be located for forming bores in the ground  11  with corresponding drills extending through the tubes  35  for setting the anchors  30 . Cables  31  or appropriate rods are connected with the anchors  30 . The cables  31  or rods extend through the tubes  35  and are secured to the floating body  1 . Thus, the works for setting the anchors  30  and for securing the cables  31  or rods need not to be effected by divers, and the works done by divers, can be reduced to a minimum. 
     FIG. 8 shows an advantageous formation of a through-opening  8  in the region of its lower end section. In order to be able to move the floating body  1  with the cables  13 , the lower end sections of the through-openings expand conically outwardly, with the conically expanding regions  36  adjoining portions of the respective openings  8 , which lie above the regions  36  by respective radii  37 . In its mouth region, the conically expanding region  36  has a further radius  38 . 
     In order to increase the buoyancy of the floating body  1  and thereby, the preload of the rods or cables  13 ,  31 , air is pumped into the buoyancy chambers  3  in the lower end position of the floating body. Preferably, the buoyancy of the floating body in the end position is by 10% greater, advantageously by 20% greater, than the buoyancy required for maintaining the floating condition of the floating body  1  with the, supported thereon, structure  2  that projects above the water surface. In this way, a very stable floating foundation is formed. 
     In other embodiments, the number and position of the through-openings  8  can be different. Anyway, at least three through-openings  8 , which are formed in the corner point of an imaginary triangle, are provided. This imaginary triangle preferably crosses the central region of the floating body  1 . Also, besides the openings associated with the tubes  14 , four or six more through-openings can be provided for cables  13  and/or rods. The number of the through-opening  8  depends on the size of a floating body. Also, a different number of the inclined, to a vertical, cables  31 , which are connected with anchors  30  and extend through the tubes  35 , can be provided. Preferably, at least three such cables  31  or rods are provided which extend along side edges of an imaginary pyramid. Also, six, eight or more of such, inclined to a vertical, cables or rods can be provided. 
     A modified embodiment of the invention will be described based on FIGS. 9-11. FIGS. 9 and 10 show a portion of a tube  39  for forming a through-opening  8  and which is sealingly secured in the floating body  1 . In the tube  39 , a connection shaft  40  is formed which forms a connection opening  41  between the opening  8  and a respective buoyancy chamber  3 . At the free end of the connection shaft  40 , a connection flange  42  is provided in which there are formed a plurality of openings so that the shaft  40  can be closed with a locking plate  43 . 
     For servicing the cables  13 , hawsers or rods, in the end position of the floating body beneath the water surface, the through-openings  8  are sealed above the connection openings  41 , (e.g., with a plastic foam). Subsequently, the water level in the respective buoyancy chambers  3 , from which the following works should be done, if needed, is lowered so far that it lies beneath the connection opening(s)  41 . The water level in the adjacent buoyancy chambers  3  can correspondingly be elevated. The connection opening  41  is open by removing the locking plate  43 , so that the cable  13 , which extends through a respective through-opening  8 , becomes accessible from a respective buoyancy chamber  3 . 
     A clamping device, not shown in FIGS. 9-11, which has already been described above, can be used for clamping the cable  13  or the rod which is cut off above the clamping device. This region can be water-sealed, e.g., by a plastic foam. 
     The clamping can be effected under dry conditions, so that no salt water would penetrate in the cut-off, tensioned cable. This facilitates the works and reduces the danger of the corrosion. 
     The through-openings  8  can also be sealed beneath the connection openings  41 . After the works are finished, the connection openings  41  can again be closed with locking plates  43 . 
     In the same manner, it is also possible to effect clamping of the inclined cables  31  or rods through connection openings in the tubes  35  from the buoyancy chamber  3 . 
     Other modification are also possible. E.g., the floating body  1  can be formed of several, connected with each other sections. 
     Though the present invention was shown and described with references to the preferred embodiments, such are merely illustrative of the present invention and are not to be construed as a limitation thereof and various modifications of the present invention will be apparent to those skilled in the art. It is therefore not intended that the present invention be limited to the disclosed embodiments or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims.