Abstract:
An installation vessel has upper and lower decks ( 40, 50 ) that are vertically spaced about 3 meters apart, with most of the lower deck covered by the upper one, but with a rear portion ( 60 ) of the lower deck being uncovered. The upper deck is used primarily for storage, while the lower deck is the one used for installation of anchor chains, wires, ropes, etc. that pass from a winch ( 81 ) to a stern roller ( 84 ) at the rear of the lower deck, so dangerous conditions of chains, wires, ropes, etc. moving along a deck during installation are confined to the lower deck. A main crane ( 72 ) that lies on one side of the vessel, lies forward of the rear end of the upper deck at a location wherein the crane can reach all portions of the uncovered lower deck portion and most of the upper deck.

Description:
CROSS-REFERENCE 
   Applicant claims priority from U.S. provisional patent application Ser. No. 60/774,285 filed Feb. 17, 2006. 

   BACKGROUND OF THE INVENTION 
   Installation vessels are used to install items during the setup of a hydrocarbon production system that produces hydrocarbons from the sea floor, a system where hydrocarbons are transferred to or from a shore-based installation, and other offshore systems especially for hydrocarbon transfer and well maintenance. These include installations where a floating body is held by chains extending from a turret or by spread mooring. The installation vessel installs items on the sea floor, including anchors, piles, manifolds, subsea trees (wellheads), templates and pumps, items that are to lie at a height between the sea floor and the sea surface such as buoyancy tanks that are to be attached to risers, and other in-sea items or tools including cables, chains, and underwater hammers. A conventional installation vessel has a single work deck extending from the stern of the vessel to about halfway to the bow. The limited space on the work deck limits the amount of materials, equipment and tools that can be stowed. The space is limited especially because space must be left between winches and a stern roller, between which elongated elements such as cables, chains and hoses are rapidly moved into or out of the sea. The rapidly moved elongated elements create a danger to personnel working on the deck. 
   Considerable material is usually placed at a mobilization harbor that is close to the installation site. A conventional installation vessel sails a long distance to a mobilization harbor that is located near where a system is to be installed, maintained, etc. The installation vessel has to interrupt the installation work one or more times during an installation, to sail to the mobilization harbor to pick up materials and equipment that could not be taken on board earlier because of the limited storage space on the vessel. Only after the loading and sailing back is completed, can the installation work be continued. Another ship cannot perform the transportation, because this would require the transfer of the material and equipment to the installation vessel at sea, which is normally too risky. 
   The above-described problems are especially relevant for installations in deep waters, which are normally located much further offshore and therefore at larger distances from the mobilization harbor. Also, the installation packages that include equipment and modules to be installed in deep water are usually much larger in size and weight than for waters of moderate depth. 
   SUMMARY OF THE INVENTION 
   In accordance with one embodiment of the present invention, an installation vessel is provided that has an unusually large amount of deck space for a hull of given length and width, and that confines a dangerous area where there is rapid movement of elongated members along a deck and into or out of the sea, to a limited deck area. The installation vessel includes a hull and an upper working deck at the top that has a large flat deck area where material and equipment can be easily stored and moved around. The vessel also has a lower working deck that lies at least 1.8 meters below the upper deck and that provides considerable additional deck working area. The upper deck lies directly above a portion of the lower deck, and the lower deck has an uncovered portion extending to the periphery of the vessel where chains, risers, flowlines, etc. can be moved into or out of the sea. Equipment for moving chains etc. into or out of the sea is located on the lower deck so corresponding dangers are confined to the lower deck, and the upper deck is left as a relatively safe area where materials and equipment can be stored more densely and can be more easily moved. 
   Applicant prefers to locate the uncovered, or open portion of the lower deck at the rear of the vessel, with the upper deck having a rear end located a plurality of meters and preferably at least ten meters forward of the vessel stern. This locates the region where chains etc. are dropped or pulled up, at the stern where there is less likely to be danger to the vessel. The upper and lower decks preferably extend across the entire width of the hull. The lower deck is preferably devoid of columns to support the upper deck, to avoid interruptions in the wide space over the lower deck. A large crane for lifting heavy items, is located a short distance forward of the upper deck rear end, and can extend to lift item anywhere on the uncovered lower deck portion and on most of the upper deck. The vessel has a moon pool that extends upward though the bottom of the hull and the lower deck. The upper deck has a removable deck portion so items such as riser sections, can be deployed though the upper deck and down though the moon pool into the sea. 
   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 
       FIG. 1  is a rear, left side, and top isometric view of an installation vessel of the present invention. 
       FIG. 2  is a right side sectional view of the vessel of  FIG. 1 , with the crane having been moved to a stowed position. 
       FIG. 3  is a rear elevation view of the vessel of  FIG. 2 , showing only the A-frame for the sake of clarity. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates an installation vessel  10  which includes a hull  12  having a bow  14 , a stern  16 , and port and starboard sides  20 ,  22  that form the periphery of the vessel. The front portion  30  of the vessel is occupied by a helicopter deck, a navigation bridge, control rooms and crew quarters, ROV (remotely operated vehicles), heavy duty winches  34  for ROV handling and control rooms for them, and other heavy equipment. Below-deck portions  36  of the vessel are occupied by fuel tanks, engines, chain lockers and other heavy equipment. All of the foregoing equipment is stored in the installation vessel when it is outfitted to ready it to sail what may be a long distance, to a mobilization harbor that is located near the site where the installation will occur. At the mobilization harbor, supplies that will be used up in the installation, and specialized installation equipment for the particular site, are loaded onto the vessel, and the vessel sails to the installation site. The installation site may be where a hydrocarbon production system, a hydrocarbon transfer system, or other major system is to be installed. 
   The vessel has two decks with parallel flat deck surfaces, instead of the usual one deck. An upper deck  40  lies at the top of the rear half of the hull, and is used primarily to store heavy equipment including reels  42  that hold mooring wires or hoses, suction anchors, or tall wrappings  44  that hold long rigid elements such as piles or pipe sections, and other supplies. In accordance with the present invention, the vessel has a lower deck  50  that lies below the level of the upper deck  40 . Both decks are horizontal in a quiescent vessel orientation. The upper deck  40  has a front end  52  lying at about the middle of the vessel length in a longitudinal direction M, and has a rear end  54  lying a plurality of meters, and generally more than ten meters, forward of the stern  16  of the vessel. The lower deck  50  has a rear end  56  lying at about (within 4 meters of) the stern of the vessel. The lower deck  50  has a region extending from the vessel middle at  54  to the rear end  56 , with a majority of such region being covered by the upper deck  40 . As a result, the lower deck has an uncovered rear portion  60  with a large open space so equipment can be lowered directly onto the lower deck rear portion and lifted up off the rear portion, and there is almost no limit to the height of equipment on the rear portion of the lower deck. The upper deck has a flat surface extending over most of its area so equipment can be easily moved onto and along it, in the same manner as prior art decks. The difference from prior art decks is that the upper deck is somewhat shorter than prior art decks for a vessel hull of the same size. 
   The lower deck has an entrance  62  that lies under the rear end  54  of the upper deck and that leads to a covered lower deck portion  64 . The clear height (H,  FIG. 2 ) above the lower deck, that is, the height of equipment that can lie on and be moved along the lower deck covered portion without hitting beams holding up the upper deck, should be at least 1.8 meters. This allows a man of about average height (wearing shoes and a helmet) of 1.8 meters (6 feet) to walk on the front covered portion  64  of the lower deck that lies under the upper deck, without stooping. Actually, applicant prefers to leave a space of at least 2.6 meters height above the flat walking surface, so materials of up to 2.5 meters height can be moved on the lower deck, with a preferred height of about 3 meters. There is a standard height of a maximum of 2.5 meters for most equipment and supplies that will be stowed or shipped in a vessel. Providing a height of at least 2.6 meters allows such equipment and supplies, such as shown at  66  in  FIG. 2 , to be moved around and stored on the lower deck. 
     FIG. 1  shows the possibility of a closeable and sealable door  70  of a height of about that of the lower deck, though which equipment and material can be moved onto and off the covered portion of the lower deck. Of course such a door is not necessary for an upper deck, but is useful to load and unload supplies that are stored near the front end of the covered portion of the lower deck. 
   The vessel contains several cranes. A main crane  72 , with the largest lifting capacity (e.g. 250 tons) and longest boom  74  (when fully extended), lies near the rear end of the upper deck. The crane can lift items anywhere on the uncovered rear portion  60  of the lower deck and on most of the upper deck. Because of the large load that the main crane can lift, it extends down through the lower deck. Otherwise, the covered portion of the lower deck is free of columns that would interrupt it. The upper and lower decks each extends between the opposite sides of the vessel. The lower deck may hold a large amount of stores at its sides, in the covered section of the lower deck. 
     FIG. 1  shows an anchor  80  lying at the end of a line or chain  81  and held by an A-frame  82  whose upper end lies over the sea behind the stern of the vessel. The anchor is not used for the installation vessel, but for a floating body that will be anchored to the sea floor. A stern roller  84  that lies at the rear of the lower deck is used to facilitate movement of elongated members such as chains, hoses, wires, etc. that are moved between the lower deck and the sea. A main winch  86  ( FIG. 2 ) is used to move elongated members  81  ( FIG. 1 ) such as chains, wires, etc. between itself and the stern roller  84 . Hoses and wires are normally stored on rolls, while chains are normally stored in a chain locker in the vessel, and all can be controlled during deployment from the vessel to the sea by the main winch. The upper rotating surface of the stern roller lies within a meter of the height of the lower working deck. There is a clear space extending along the lower working deck to allow chains, hoses, wires and other elongated members to extend from the winch  86  to the stern roller without interference. It can be seen in  FIG. 2  that a majority of the moving member  81  ( FIG. 1 ) that moves between the main winch  86  and the stern at stern roller  84 , lies below the upper deck  40 . 
   The vessel has a moon pool  90  ( FIG. 1 ) that extends though the bottom of the hull and through the lower deck. A front part  92  of the upper deck has removable floor parts  94 , which forms an even surface and that can be removed to gain access to the moon pool though the upper deck and to gain vertical access to the front of the covered deck portion. 
   Applicant has designed a vessel of the construction illustrated, which had a length between bow  14  and stern  16  of one hundred twenty meters and a maximum lateral L width between its opposite sides  20 ,  22  of twenty-eight meters. The upper deck  40  had a length in a longitudinal direction M of forty-five meters and a lateral L width of twenty-eight meters. The lower deck covered portion  64  had a length of forty-five meters, and the lower deck uncovered portion  60  had a length of twenty meters, with both having a lateral L width of twenty-eight meters. Since the front end  52  of the upper deck lay at the middle of the vessel length, the upper deck had a length of 45 meters, and the open lower deck portion had a length of 80 meters, the upper deck therefore extended along a majority of the length of the rear half of the vessel. The upper deck had an area of 1300 m 2  while the lower working deck had an area of about 1700 m 2  (not including the moon pool). The vessel is useful for deep water installations and field maintenance.  FIG. 2  shows propellers  96  and thrusters  98  for propelling the vessel and holding its position without the need for an anchor and anchor chain or for mooring lines. 
   Thus, the invention provides an installation vessel of given hull size, that can hold more equipment and materials than a previous vessel with that size of hull, including heavy items that are best raised and lowered by a heavy crane and moved along a flat deck space. The vessel also has a clear space through which chains, wires, etc. can move into the sea. This is accomplished by constructing the vessel with upper and lower decks having parallel flat deck surfaces, with the upper deck having a rear end lying a plurality of meters and preferably more than 10 meters forward of the vessel stern. As a result, the lower deck has an uncovered area at the rear of the vessel where heavy and tall items can be lowered into the sea floor and raised therefrom and onto which heavy items can be lowered and raised by a crane. 
   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.