Abstract:
A vessel with drilling and workover capability is constructed rapidly in a shipyard or dry dock by using an existing vessel that has most of the required hull. Bow and stern sections of the existing vessel are separated to leave them at opposite ends of a shipyard space, and a mid hull section is built in that shipyard space in the usual manner by welding together steel plates. The bow and stern sections are moved against opposite ends of the mid hull section and welded to it. Topside equipment is already on the bow and stern sections, and is added to the new mid hull section.

Description:
CROSS-REFERENCE 
       [0001]    Applicant claims priority from U.S. provisional patent application S. No. 61/066,610 filed 20 Feb. 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    One type of vessel used in hydrocarbon production is an FPSO (floating, production, storage and offloading) vessel, which is of moderate size and widely available. Another type which is larger and not readily available, is an FPDSO (floating, production, storage, drilling, and offloading) vessel. The FPDSO vessel has complex drilling and workover equipment in its middle which usually allows the vessel to weathervane during drilling, production and offloading operations. 
         [0003]    Shipyards have dry docks with “time slots” (continuous time periods) reserved for construction of a vessel. The required length of the time slot depends primarily on the amount of steel plate that must be welded together to produce a hull of a desired vessel. There are only a few shipyards available in the world that are capable of building very large vessels such an FPDSO, and it is often difficult to find a yard with a sufficiently long available time slot. A method for constructing a FPDSO which greatly reduced the required length of a shipyard time slot for such construction, would be of value. 
       SUMMARY OF THE INVENTION 
       [0004]    In accordance with one embodiment of the invention, a method is provided for the construction of a very large vessel such as an FPDSO, using only a short time slot in a large shipyard. Applicant first obtains an existing, or initial, FPSO (floating, production, storage and offloading) vessel and sails it into a shipyard. Applicant then divides the FPSO hull into bow and stern sections and separates them to leave a yard space between them. Applicant then constructs a mid hull section in the yard space. The mid hull section usually provides drilling and workover capability as well as providing a turret for weathervaning capability. When mid hull section construction is finished, the bow and stern sections are moved against the front and rear ends of the mid hull section and welded to it. Most equipment for the bow and stern ends is usually already mounted on them. Equipment for the mid hull section is usually prefabricated so it can be rapidly mounted on the mid hull section, in the shipyard or at a harbor side or quay or dock. 
         [0005]    Beams are preferably mounted on opposite sides of the FPDSO hull. The beams extend along the entire length of the mid hull section and along portions of the bow and stern hull sections. The beams strengthen the hull and account for the fact that the original bow and stern sections were constructed for a lighter vessel. The beams preferably extend along a majority of the height of the hull up to the deck, and add to the usable deck space. 
         [0006]    The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a sectional side view of a FPDSO vessel, constructed in accordance with the present invention. 
           [0008]      FIG. 2  is a simplified side elevation view of a FPSO vessel, from which applicant constructs the vessel of  FIG. 1 . 
           [0009]      FIG. 3  is a simplified plan view showing how the vessel of  FIG. 1  is constructed in a shipyard. 
           [0010]      FIG. 4  is a side elevation view of a vessel similar to that of  FIG. 1 , but with side beams. 
           [0011]      FIG. 5  is a simplified plan view of the vessel of  FIG. 4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]      FIG. 1  shows a FPDSO (floating, production, drilling, storage and offloading) vessel  10 , which has bow and stern ends  12 ,  14 , and which can be considered to have a bow hull section  20 , a stern hull section  22 , and a mid hull section  24 . The mid hull section  24  is used to hold complex equipment for drilling undersea wells and for the workover of wells, and usually holds a turret  26  to allow the vessel to weathervane (turn without limit about a vertical axis), or the vessel is spread moored.  FIG. 1  shows a drilling rig  27  and a pipe string storage locker  28 , a riser  30 , fluid swivel  32  and anchor lines  34 , and an offloading facility  35 . 
         [0013]      FIG. 2  shows a FPSO (floating, production, storage and offloading) vessel  36  which is similar to the vessel  10  except that it does not include the mid hull section  24  of  FIG. 1  and the complex equipment on the mid hull section. The vessel has multiple tanks  54 ,  56  for storing produced oil (or other hydrocarbons). The simpler FPSO vessel  36  is available or can be built, but the more complex FPDSO vessels  10  are difficult to build because they require a large slot of time in a large shipyard. The amount of time to build a vessel depends primarily on the amount of steel plate that must be welded (inspected and ground, etc.) to produce the hull. There are only a few shipyard in the world large enough to hold the FPDSO and most of them do not have a long enough time slot available at any given time, to build one. The present invention comprises a method for building a large vessel such as a FPDSO  10 , which requires only a short time slot in a large ship yard. 
         [0014]    Applicant constructs the FPDSO  10  of  FIG. 1  by starting with a smaller vessel such as a FPSO  36  such as shown in  FIG. 2 . The FPSO  36  is sailed into a large shipyard  40  ( FIG. 3 ), and its bow and stern sections  20 ,  22  are spit or separated, such as along line  42  ( FIG. 2 ).  FIG. 3  shows the bow and stern section  20 ,  22  moved to opposite ends of the shipyard  40 , to leave a yard space  44  between them. The mid hull section  24  is then built from “scratch” in the yard space, by welding together steel plates, until the mid hull section is completed. 
         [0015]    Applicant welds steel plates  52  over the front end  50  of the stern section  22  and welds steel plates  54  over the rear end  56  of the bow section, to keep out water and to isolate the hydrocarbon tanks on the bow and stern ends from sparks produced by drilling and other equipment on the mid hull section. However, since the bow and stern sections  20 ,  22  do not have to sail out of the shipyard  40  before being joined to the mid hull section, the bow and stern sections do not have to be independently sea worthy. Then, the bow and stern hull sections  20 ,  22  are moved against the front  46  and rear  48  ends of the mid hull section and welded to the mid hull section to produce a tandem-connected hull  49  ( FIG. 1 ). Topside equipment is installed, primarily on the mid hull section. 
         [0016]    The FPSO  36  ( FIG. 2 ) which is converted to a FPDSO, was initially constructed to provide sufficient strength to withstand the weight and forces encountered by the FPSO, but not those encountered by the larger FPDSO. Applicant strengthens opposite sides of the hull of the FPDSO by welding beams, or sponsons, to opposite sides of the hull of the FPDSO.  FIGS. 4 and 5  show a pair of beams  55 ,  57  at the port and starboard sides of the hull, and which extend along a majority of the height of the hull and that are each welded to all of the hull sections. Each beam has a length that is a majority of the length of the FPDSO between its ends  12 ,  14  with opposite beam ends  60 ,  62  that each lies a plurality of meters short of the closest hull end. To weld each beam to the bow or stern of the vessel generally would require properly bending the beam, which would be expensive and not necessary. Also, regions close to the bow and stern do not require reinforcement by such beams. The beams preferably extend along a majority, and preferably at least 80% of the height of the hull, and preferably extend up to the deck  64  of the FPDSO hull to provide additional deck space, as for drilling and workover equipment. The beams also enhance resistance to roll of the vessel, and essentially provide a partial double hull to avoid spillage of hydrocarbons. 
         [0017]    In most cases the FPSO which is to be converted to a FPDSO, will be a plurality of years old. A brief examination of the FPDSO vessel will show that the bow and stern sections are a plurality of years older than the mid hull section, so the vessel was constructed in the manner described above. 
         [0018]    The bow and stern sections each have a plurality of hydrocarbon storage tanks  70 ,  72  ( FIG. 1 ), each having a volume of a plurality of cubic meters. Hydrocarbons produced from the sea floor are stored in these tanks and regularly offloaded to a carrier that takes them to a distant location. The mid hull section  24  does not store over a cubic meter of hydrocarbons and preferably does not store any hydrocarbons. This eliminates hydrocarbons on the mid hull section that could be ignited by sparks etc produced by the drilling/workover equipment on the mid hull section. The steel plates welded to the rear end of the bow hull section and to the front end of the bow hull section, provide further isolation of stored hydrocarbons from sparks. 
         [0019]    Applicant has designed the conversion of a FPSO vessel  36  having a length of 260 meters and a width of 45 meters, into a FPDSO vessel having a length of 340 meters and a width of 53 meters (including the beams). It is expected that a time slot of at least 8 months would be required in a large ship yard to build the hull of such FPDSO. However, a time slot of only 4 months would be required to split an existing FPSO and weld plates over their ends, build a mid hull section, and weld the bow and stern hull sections to the mid hull section. 
         [0020]    Thus, the invention provides a method for constructing a large vessel such as a FPDSO, which uses an existing, or initial, smaller vessel such as a FPSO which is similar to the FPDSO except for a mid hull section. The initial vessel is moved into a ship yard and is split into bow and stern hull sections, which are separated but remain in the ship yard. A mid hull section is constructed in the yard space between the bow and stern hull sections. The bow and stern hull sections are moved against the mid hull section and welded thereto. Where bow and/or stern hull sections have hydrocarbon-holding tanks, and the mid hull section is designed to contain drilling equipment, which produces sparks, a plate is preferably welded across the rear end of the bow hull section and across the front end of the stern hull section. Beams are preferably welded to opposite sides of the hull at all three of the sections, with the beams preferably extending up to the deck to enlarge deck space. 
         [0021]    Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.