Abstract:
An offshore well platform is towed in an upright condition to a well site by the use of a temporary flotation device. The flotation device mounts to a portion of the platform and is partially submerged, increasing the ability and buoyancy of the platform. At the site, the engagement of the flotation device shifts to a deploying position. In the deploying position, the ballast of the platform is increased to cause it to more deeply submerge. The flotation device remains at least partially surrounding the platform and floating while the platform moves downwardly relative to it. This provides lateral support if needed to prevent heeling while being submerged. The flotation device is disengageable from the platform when the platform is fully submerged.

Description:
This invention is a continuation-in-part application of Ser. No. 09/303,078, filed Apr. 30, 1999, now U.S. Pat. No. 6,371,697, entitled Floating Vessel for Deep Water Drilling and Production. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to floating vessels used for offshore drilling and production of petroleum. 
     2. Description of the Related Art 
     Petroleum production often requires the placement of rig in an offshore location. In shallower waters, the rigs and production facilities can be placed on freestanding offshore platforms. As the water becomes deeper, however, these become impractical, and it is necessary to have a floating platform, or support vessel, upon which the rigs and production facilities can be placed. 
     One type of deepwater support vessel is a tension leg platform (TLP). The TLP is a buoyant platform that is secured to the seabed using generally vertically-oriented rigid tethers or rods that restrain the platform against vertical and horizontal motion relative to the well in the seabed below. These platforms have a very short period in response to wave action. 
     An alternative to the TLP is the deep draft caisson vessel (DDCV). The DDCV is a free floating vessel that is moored to the seabed using flexible tethers so that vertical and horizontal motion of the vessel is restrained, although not eliminated . Examples of DDCVs are found in U.S. Pat. No. 4,702,321. 
     Methods for restraining the DDCVs attempt to slow, rather than eliminate, the natural response period of the vessel to wave effects. Current DDCV arrangements “decouple” the vessel from the individual wells being supported so that the wells are not subject to the same induced motions as the vessel. Decoupling is typically accomplished by using buoyant means to make the wells separately freestanding and using flexible hoses to interconnect the vertical risers from the well to the production facilities. 
     A common variety of DDCV is the type shown in U.S. Pat. No. 4,702,321, which utilizes a long cylindrical structure and is commonly known as a spar. The long cylindrical shape of the spar provides a very stable structure when the vessel is in its installed position, exhibiting very slow pitch, surge and heave motions. Heave motion, however, is not totally eliminated, allowing the structure to bob up and down vertically in the sea. Recently, attempts have been made to add a number of horizontally extending plates along the length of the spar in order to help the spar be more resistant to heave. 
     Regardless of the presence of the plates, the spar must be assembled and transported in a horizontal position and then installed by being upended at or near the final site using a large crane that must also be transported to the installation site. As these caisson structures are often around 650 ft. in length, transport and upending of the structure are risky. Further, it is only after a successful upending of the structure has occurred, and the lower portion of the structure has been successfully moored, that components of the rig can be placed atop the spar. 
     SUMMARY OF THE INVENTION 
     In this invention, a platform is provided that has a variable ballast. A flotation device is coupled to the platform to increase the buoyancy of the platform. The flotation device causes the platform to float in a towing position with the platform and the flotation device partially submerged. The flotation device is fixed to the platform while in the towing position, and the platform is towed upright. When at the site, the flotation device is moved to a deploying position. In the deploying position, the flotation device remains in close proximity with a portion of the platform, but is not fixed to it vertically. As ballast is increased in the platform, the platform moves downward relative to the flotation device. The flotation device remains floating closely spaced to a portion of the platform. If the platform heels while lowering, it will contact the flotation device, which provides lateral stability against heeling. Once the platform has been submerged sufficiently so that it is stable, the flotation device is released from the platform. 
     In the preferred embodiment, the platform has an upper elongated tower section and a lower base section. The base section has a greater cross-sectional dimension than the tower section. The flotation device is preferably annular and fits on top of the base section, surrounding a lower portion of the tower section. Preferably the flotation device is formed in circumferentially extending segments. The segments are separable from each other. The flotation device is disengaged from the platform by uncoupling the segments from each other and pulling them laterally outward from the platform. 
     In the first embodiment, the upper deck structure of the platform is mounted to the platform before the platform is towed to the desired location. In the second embodiment, the upper deck structure is installed at the location. This is handled by mounting the upper deck structure on a buoyant member and towing the buoyant member to the location. The buoyant member has two spaced-apart arms, resulting in a slot. The arms are spaced apart from each other sufficiently to allow the arms of the buoyant member to float on opposite sides of the platform after the platform has been fully deployed and the flotation device removed. The arms support the upper deck structure at a distance above the upper end of the platform. Once in place over the platform, the platform buoyancy is increased, allowing the platform to rise up into contact with the upper deck structure. The deck structure is then secured to the platform, and the buoyant member is then moved laterally away from the structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic sectional view of a platform and flotation device constructed in accordance with this invention. 
     FIG. 2 is a schematic side elevational view of the platform and flotation device of FIG. 1, taken along the line  2 — 2  of FIG.  1 . 
     FIG. 3 is a schematic side elevational view illustrating the platform and flotation device of FIG. 1 being towed to a site. 
     FIG. 4 is a schematic side elevational view of the flotation device and platform of FIG. 1, showing the platform being lowered relative to the flotation device. 
     FIG. 5 is a schematic top view of the platform and flotation device of FIG. 1, showing the segments of the flotation device separated from each other and being towed away from the platform. 
     FIG. 6 is a schematic side elevational view of the platform of FIG. 1 in its fully installed position. 
     FIG. 7 illustrates an alternate method, wherein the platform of FIG. 1 is towed to the site without its upper deck structure. 
     FIG. 8 is a schematic side elevational view of the platform of FIG. 7, showing it being lowered further into the sea relative to the flotation device. 
     FIG. 9 is a schematic side elevational view of the platform of FIG. 7, shown in a submerged position, and showing the upper deck structure being floated over it by means of a buoyant member. 
     FIG. 10 is a sectional view of the buoyant member of FIG. 9, taken along the line  10 — 10  of FIG.  9 . 
     FIG. 11 is a schematic side elevational view of the platform of FIG. 8, showing the buoyant member around the upper end of the platform, with the platform raised up into contact with the upper deck structure. 
     FIG. 12 is a schematic side elevational view of the platform of FIG. 7, shown fully installed with the upper deck structure. 
     FIG. 13 is a graph illustrating an example of a righting arm and a heel arm of the platform and flotation device of FIG. 1 being towed under selective wind conditions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, platform  11  has a base section  13  and a tower section  15 . Base section  13  has a greater horizontal cross-sectional area than the cross-sectional area of tower section  15 . In the preferred embodiment, both base section  13  and tower section  15  are cylindrical. Base section  13  has a vertical height that is much less than the vertical height of tower section  15 . 
     An upper deck structure  17  is schematically shown mounted on the upper end of tower section  15 . Upper deck structure  17  may in some instances comprise drilling equipment, including a derrick, living quarters and associated machinery. Upper deck structure  17  may also comprise production equipment for separating gas and water from well fluids and processing the oil or gas. Alternately, upper deck structure  17  could be a much simpler structure, such a deck for helicopter landing. In the latter instance, tower section  15  and base section  13  could be employed for storing chemicals and the like, in which case platform  11  serves as a tender to a production or drilling vessel. 
     Preferably base section  13  has a section of fixed ballast  19  such as heavy metal. Additionally, base section  13  has at least one ballast chamber  21 , which is a watertight chamber that can be flooded selectively with water to increase the ballast or pumped free of water to decrease the ballast. Tower section  15  also has a number of ballast chambers  23 , each of which may be selectively filled with water or pumped free of water. In this embodiment, a central vertical passage  24  extends downward through tower section  15  and base section  13 . Central passage  24  allows drilling tools to be lowered from upper deck structure  17  into the sea. If platform  11  is employed as a tender, the lower end of base section  13  would preferably be closed against sea water, and central passage  24  would be used for transporting materials and personnel from base section  13  to upper deck structure  17 . 
     A flotation device  25  is shown mounted on platform  11 . Flotation device  25  is a buoyant member, preferably a tank that is filled with air and sealed from water to provide a buoyant chamber. In this embodiment, flotation device  25  is annular and secured to platform  11  by a set of fasteners  27 , shown by dotted lines. Fasteners  27  are illustrated to be located on an inner diameter  29  of flotation device  25  for engaging the top of base section  13 . Fasteners  27  could alternately engage tower section  15  or both tower section  15  and base section  13 . Fasteners  27  may be a variety of types of clamps or locking members either mechanically or hydraulically actuated. 
     Flotation device  25  in the embodiment of FIG. 1 has an outer diameter  31  that is greater than the outer diameter of base section  13 . A lower portion of the outer diameter  31  surrounds the outer diameter of base section  13 . This results in an outer lower portion  33  that extends downward flush with the lower end of base section  13 . Fixed ballast such as ballast  19  may optionally be located in the lower end of outer lower portion  33 . Outer lower portion  33  is not essential and in some cases, the lower end of flotation device  25  could be flush with the top of base section  13 . In that case, outer diameter  31  of flotation device  25  could be the same or even less than the outer diameter of base section  13 . 
     As shown in FIG. 2, flotation device  25  is preferably constructed in a plurality of separate circumferentially extending segments  35 . Four segments  35  are shown, although this number could be more or less. Segments  35  are assembled and coupled to each other in the annular configuration shown in FIG. 2 by fasteners  37 . Fasteners  37 , similar to fasteners  27 , could be of many different types, such as clamps or locks, either hydraulically or mechanically actuated. Each segment  35  is a separate sealed, watertight member so that each is independently buoyant. 
     Flotation device  25  is employed to provide additional buoyancy to platform  11  to increase the stability of platform  11  while it is being towed upright to a desired location, shown in FIG. 3, and also to stabilize platform  11  while it is being submerged to the desired position as illustrated in FIG.  4 . The dimensions of flotation device  25  are developed by known principles. Once installed on base section  13 , base section  13  will be fully submerged and flotation device  25  will be partly submerged. Lower outer portion  33  of flotation device  25  will be fully submerged. The horizontal cross-sectional area of flotation device  25  significantly increases the water plan of platform  11  while being towed, the water plan being the surface area of platform  11  and flotation device  25  measured at the waterline. The increased water plan greatly increases the stability of platform  11  while being towed. 
     Referring to FIG. 13, the graph is representative of a righting arm curve  39  and a heeling arm curve  41  of platform  11  when assembled with flotation device  25 . Righting arm curve  39  represents the ability of the assembled platform  11  and flotation device  25  to right itself if it is being heeled due to strong winds. In the example of FIG. 13, the wind is assumed to be 70 knots. As the amount of heel increases to around 25°, righting arm curve  39  increases, and therefore the ability of platform  11  to right itself also increases. The heeling arm  41  decreases slightly as the heel increases because as the platform  11  heels more, it presents less structure normal to the wind. The area A 1  under righting arm curve  39  and above heeling arm curve  41  should be greater than the area A 2 . The area A 2  is the area under heeling arm curve  41  and above righting arm curve  39  to the first point where they cross, which is about 7° in the example shown. For stability, the ratio of A 1  over A 2  in many cases should be at least 1.4. In the example shown, it is 2.53, presenting a stable configuration for towing even in a 70 knot wind. 
     The graph of FIG. 13 will change for the same structure at different wind speeds. Also, the graph of FIG. 13 changes as tower section  15  is more deeply submerged. At the fully installed depth, there will be no point at which the righting arm curve  39  crosses the heeling arm curve  41  because of its extensive depth. That is, once installed, even if heeled to 40°, the righting arm will be greater than the heeling arm, preventing capsizing. 
     If a graph such as FIG. 13 is plotted for the platform  11  without flotation device  25 , the area A 1  would still be greater than the area A 2 , but the ratio would be much less than 2.53. Adding flotation device  25  improves the righting ability because it adds buoyancy and also creates a greater water plan. Without flotation device  25 , the water plan would only be the cross-sectional area of tower section  15 , considerably less than if combined with the water plan of flotation device  25 . Flotation device  25  also lowers the vertical center of gravity. 
     In one example, the overall height from the lower end of base section  13  to upper deck structure  17  is 200 ft. Base section  13  has a diameter of 108 ft. and a height of 30 ft. Tower section  15  is cylindrical with an outer diameter of 50 ft. and an inner diameter of 20 ft. Flotation device  25  has an outer diameter  31  of 136 ft and an inner diameter  29  of 64 feet. In this example, the water plan of flotation device  25  is much greater than the water plan of tower section  15 . The water plan of tower section  15  is pi times the square of the radius, approximately 1962 square feet, and the water plan of flotation device  25  is pi times outer diameter  31  divided by two and squared less inner diameter  29  divided by two and squared, approximately 11,304 square feet. The height of the portion of flotation device  25  extending above base section  13  is 20 ft, resulting in an overall height at outer diameter  31  of 50 feet. This produces a draft while towing of 29.50 ft. and a vertical center of gravity of 45.47 ft. Of course, platform  11  and flotation device  25  may have different dimensions than those listed above. 
     Referring again to FIG. 3, in operation, flotation device  25  will be assembled and secured to platform  11  by fasteners  27  (FIG.  1 ). A tow vessel  43  will be secured to base section  13  for towing platform  11  to a desired location. Once at the desired location, as shown in FIG. 4, moorings  45  will be attached to the sea floor. Fasteners  27  (FIG. 1) will be released to place flotation device  25  in the deploying mode. Platform  11  is now free to move downward relative to flotation device  25 , although flotation device  25  is retained with tower section  15  because it still surrounds it. Because inner diameter  29  of flotation device  25  is greater than the outer diameter of tower section  13  by a clearance on a side of seven feet, flotation device  25  will not initially be in physical contact with tower section  13 . Water is pumped into ballast chambers  21  and  23  (FIG.  1 ), causing platform  11  to move downward. As it moves downward, flotation device  25  provides lateral stability by remaining in place surrounding platform tower section  15 . That is, should platform  11  begin to heel, tower section  15  would contact part of inner diameter  29  of flotation device  25 , which would add stability. Prior to reaching a certain depth, platform  11  will still be unstable, therefore flotation device  25  adds stability during this deploying movement. 
     Once platform  11  has been submerged to a depth in which it is stable, such as about 120 ft. in the above example, there will be no degree of heel in which the righting arm curve  39  (FIG.  13 ), drops below the heeling arm curve  41 . At this point, if desired, flotation device  25  could be disengaged from tower section  15 . Alternately, the operator may wish to completely deploy platform  11  to its final depth before detaching flotation device  25 . In the above example of dimensions for platform  11 , the draft while fully deployed is about 160 ft. 
     Flotation device  25  is disengaged from tower section  15  as illustrated in FIG.  5 . Fasteners  37  (FIG. 2) are released to enable segments  35  to separate and segments  35  are pulled radially outward from platform  11 . Flotation device  25  may be reassembled, towed back to land and reused. FIG. 6 shows platform  11  at its fully deployed depth with flotation device  25  removed. 
     FIG. 7 illustrates an alternate method for deploying platform  11 . In FIG. 7, upper deck structure  17  is left off initially. This reduces the amount of weight at the upper end of platform  11 . Flotation device  25  is assembled on base section  13  and towed to the site by vessel  43 . Then, as illustrated in FIG. 8, platform  11  is moored by moorings  45  and fasteners  27  (FIG. 1) are moved to the deploying position. The ballast of platform  11  is increased by pumping water into it, causing it to lower as shown in FIG. 8 while flotation device  25  remains floating. Once platform  11  is stable, flotation device  25  is removed. 
     Referring to FIG. 9, preferably, platform  11  is over-ballasted to a depth somewhat deeper than its desired draft when fully installed. Upper deck structure  17  is towed separately to the site on a buoyant member  47 . Buoyant member  47  has the shape of a horseshoe, as shown in FIG.  10 . It has vertical columns  49  that support upper deck structure  17  above buoyant member  47 . Columns  49  are located on two spaced-apart buoyant arms  51 . Arms  51  are parallel to each other and join each other at a base  53 . The end opposite base  53  is open, defining a slot  55  between the free ends of arms  51 . Slot  55  has a width greater than the width or diameter of tower section  15 . This enables buoyant member  47  to be towed and pushed around the upper portion of tower section  15 , as shown in FIG. 11, with arms  51  on opposite sides of tower section  15 . 
     Initially, the lower ends or legs  57  of upper deck structure  17  are spaced above the upper end of tower section  15 . Then, the buoyancy in platform  11  is increased, causing the upper end of tower section  15  to come up into engagement with legs  57 . Tower section  15  will lift upper deck structure  17  from buoyant member  47 , and legs  57  will be secured to the upper end of tower section  15 . Then, as illustrated in FIG. 12, buoyant member  47  is removed along with columns  49 . This is done by towing buoyant member  47  laterally outward from tower section  15 . 
     The invention has significant advantages. The flotation device increases the stability while towing of the platform, enabling the platform to be towed in an upright condition. The platform therefore does not need to be towed horizontally, then upended for deploying. The flotation device also adds stability while the vessel is being deployed at the site, resisting heeling by encircling the tower section. The flotation device is readily removed from the tower once it is submerged to a depth of stability. This allows the flotation device to be reused or recycled. 
     While the invention has been shown in only two of its forms, it should be apparent to those skilled in the art that it is not so limited but susceptible to various changes without departing from the scope of the invention. For example, the platform may be configured in other shapes other than cylindrical. Although, preferred, the platform need not have larger diameter base section and a smaller diameter tower section. Also, the flotation device could be configured in other shapes rather than annular. Additionally, devices such as rollers could be mounted to the inner diameter of the flotation device to contact the tower section while the platform is being submerged.