Abstract:
A submersible mooring buoy for a floating vessel comprises a compartmented hull; a plurality of non-buoyant, catenary anchor lines each connected to the hull and a spring buoy such that the hull may be submerged without sinking to the seafloor by flooding selected compartments; and, a plurality of mooring legs anchoring each spring buoy to the seafloor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/029,817 filed Feb. 19, 2008, and entitled: “Submersible Mooring System for Tankers.” 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     NOT APPLICABLE 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to mooring a floating vessel. More particularly, it relates to a mooring buoy which may be submersed to protect the buoy and associated submarine hose from extreme weather or sea conditions. 
     2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
     Certain mooring systems of the prior art comprise anchored, floating buoys to which vessels may connect. However, such buoys are subject to whatever metocean conditions may exist at their location and are susceptible to damage from wind and wave action. 
     U.S. Pat. No. 5,431,589 describes a single-point submersible mooring buoy for use in ice-infested and severe sea state anchorages for transferring fluids between marine tankers and fluid handling facilities. The device includes a buoy body having a rotatable turntable disposed thereon for supporting a hose reel and a mooring hawser and associated winch. The buoy includes onboard anchor chain windlasses for maintaining anchor chain tension in the surfaced as well as submerged positions. Ballast tanks, compressed air reservoirs, hydraulic pumps and motors are disposed on the buoy and may be operated by remote control to move the buoy between floating and submerged positions and to provide for connecting and disconnecting the fluid transfer hose and mooring hawser with respect to a vessel such as a tanker. 
     U.S. Pat. No. 4,892,495 describes a subsurface buoy loading system for offshore oil gas production from production wells in the sea bottom. One or more flexible risers lead from the respective production wells to a submersible buoy adapted to be fitted to a complementary loading vessel in order to transfer petroleum products from the production wells via the loading buoy to the cargo space in the vessel. When not carrying out loading operations, the loading buoy may be descended down into an equilibrium position in the body of water. In the loading vessel, the buoy is fitted into a rotatable turret seat located in a downwardly open tunnel in the ship hull. The rotatable turret seat is positioned at such a level that the top side of the buoy, when in the loading position, is situated above sea level. The riser or risers extend through a vertical shaft in the buoy and are attached to the top of the buoy. The rotatable turret seat supports pipes communicating between the risers and a receiving pipeline system in the vessel. 
     U.S. Pat. No. 4,266,500 describes a compressed fluid hover control system for a submersible buoy in which the water level in a buoyancy chamber is controlled in accordance with external water pressure and predetermined levels of water in the buoyancy chamber. More specifically, a submersible buoy having a fluid-containing chamber containing a compressed fluid is connected to a buoyancy chamber by a gas inlet valve. A gas exhaust valve connects an upper portion of the buoyancy chamber to the surrounding water and a relief duct connects a lower portion of the buoyancy chamber to the surrounding water. Both the gas inlet and gas exhaust valves are controlled by a valve control circuit which opens and closes the valves in accordance with predetermined criteria related to water levels within the buoyancy chamber and the depth of the buoy as determined by a water pressure transducer. The valve control circuit thus causes the buoy to oscillate between predetermined depth levels, those levels changing as the compressed fluid is expended in order to maximize the operating life of the buoy. 
     U.S. Patent Publication No. 2004/0161303 describes a catenary anchor leg mooring system that includes a cylindrical hull portion which reduces the tendency of the cylindrical hull to roll and pitch in response to the action of winds and waves. This motion reduction is said to come from changing the diameter and draft of the cylindrical hull. 
     U.S. Pat. No. 6,062,769 describes a steel catenary riser system for use in an arrangement which includes a floating vessel such as a Floating Production, Storage and Offloading (FPSO) vessel floating on the sea surface and secured to the seabed by means of anchor legs which substantially prevent rotation of a turret which is rotationally supported on the vessel. The vessel is thus capable of weathervaning about the stationary turret under forces of wind, currents and waves. Steel Catenary Risers (SCR) run from the seabed sources of hydrocarbons to a Steel Catenary Riser Interface Buoy, called a “SCRIB”. A flexible riser hose, which may be suspended in a double catenary configuration, is coupled to each SCR at a SCRIB. The upper end of each flexible riser runs to the turret and connects to a fluid coupling (i.e., a swivel) and then via a pipe to a vessel holding tank. A cross-link is placed between two or more steel tubular lines in order to enhance the stability of the riser system. Devices are coupled to the steel tubular lines for increasing their tension in order to increase the natural frequency of vibration in order to reduce sensitivity to vortex induced vibration. 
     BRIEF SUMMARY OF THE INVENTION 
     A submersible mooring buoy for a floating vessel comprises a compartmented hull; a plurality of non-buoyant, catenary anchor lines each connected to the hull and a spring buoy such that the hull may be submerged without sinking to the seafloor by flooding selected compartments; and, a plurality of mooring legs anchoring each spring buoy to the seafloor. In this way, the buoy may be submerged below the surface and remain at a controlled depth during periods of extreme and potentially damaging wind and wave conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         FIG. 1  shows a buoy of the present invention in an operating condition wherein a tanker is moored to the buoy by a hawser, the buoy is anchored by a system comprising spring buoys, and the buoy is in fluid communication with both an offshore production platform and a tanker, FPSO or similar vessel. 
         FIG. 2  shows the system illustrated in  FIG. 1  with the addition of a tanker in fluid communication with an FPSO moored to the submersible buoy. 
         FIG. 3  illustrates the buoy of the present invention in its submersed condition for protection from the environmental conditions existing on the surface. 
         FIG. 4  is a larger-scale view of the buoy system shown in  FIG. 3 . 
         FIG. 5  shows a step in the installation process of a buoy according to the invention wherein the buoy has been connected to one spring buoy. 
         FIG. 6  shows a step in the installation process of a buoy according to the invention wherein the buoy is connected to one spring buoy and a second spring buoy is positioned for connection to the buoy. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention may best be understood by reference to the representative embodiment shown in the drawing figures wherein an offshore production platform P is in fluid communication with submersible mooring buoy  10 . Buoy  10 , in its normal operating condition, floats on water surface W. 
     As shown in  FIGS. 1 and 2 , the present invention comprises a tanker mooring system which has certain features that are similar to conventional Catenary Anchor Leg Mooring (CALM) systems used for loading and unloading moored tanker vessels. A tanker or other vessel such as a Floating Production, Storage and Offloading vessel (FPSO) may be moored to the CALM buoy  10  by a hawser  14 . CALM buoy  10  may comprise turntable  12  which is adapted to rotate relative to the hull of buoy  10 . Hawser  14  may be attached to turntable  12  on buoy  10  so as to permit a vessel moored to buoy  10  to weathervane freely around CALM buoy  10 . The product may be transferred between the tanker and the CALM buoy  10  by a floating hose  16  connected to turntable  12  on buoy  10  via a swivel fitting, as is conventional in the art. The CALM buoy  10  may be connected to the source of the product or to the storage facility by a submerged pipeline consisting at least partially of a flexible underwater hose  18  or a riser system. 
       FIG. 2  depicts a typical operating configuration in which hydrocarbon product flows from production platform P to buoy  10  via submarine hose  18 . The product then flows via floating hose  16  to FPSO vessel F which is moored to buoy  10  by hawser  14 . Product may be transferred from FPSO vessel F to tanker T moored by line  15  via floating hose  17 . 
     The present invention allows a CALM buoy to be submerged before the onset of extreme environmental events such as hurricanes or ice conditions. The submerged condition during severe storms protects the buoy from wave loading and reduces the motions of the buoy. The reduced motions result in reduced loads and motions on the connected submarine hose or riser system. 
     This invention allows the CALM buoy  10  to be submerged to a desired depth by flooding selected compartments of the buoy. Compartments in the lower part of buoy  10  may be flooded such that the center of buoyancy of buoy  10  remains above its center of gravity so as to provide stability of the buoy  10  in its submerged configuration. The buoy  10  is stable at the desired depth due to the transfer of the weight of the upper mooring legs  28  to the submerged “spring” buoys  20  that are part of the mooring legs. As buoy  10  descends, a greater portion of the weight of upper mooring legs  28  is borne by spring buoys  20  and buoy  10  will therefore descend to and maintain an equilibrium depth. 
     In one particular preferred embodiment, upper anchor lines  28  comprise chain. The chain is sized to provide not only sufficient holding strength, but also sufficient weight to stabilize buoy  10  at the desired depth when submerged. As shown in  FIGS. 3 and 4 , the anchor system comprised of spring buoy  20 , lower leg  22  and upper leg  28  may assume an inverted or double catenary configuration when buoy  10  is submerged. It will be appreciated by those skilled in the art that other materials having a density greater than sea water may be used in this application. 
     Following submersion, mooring buoy  10  may be re-floated by de-ballasting the flooded compartments of buoy  10  by means of an air hose (or other conduit) from a workboat or other vessel, by self-contained compressed gas, by on-board gas generation means or by a hose from the platform P. After refloating the buoy, the hawser  14  and floating hose  16  (if removed prior to submerging) can be reconnected at the surface using workboats or other means conventional in the art. 
     The system may have three or more mooring legs. In one particular preferred embodiment, the mooring legs are made up of: an anchor  26 , a bottom chain length  24  (with a portion on seafloor S), a polyester section  22  (for deep water) and a spring buoy  20 . The upper mooring leg  28  from the CALM buoy  10  to the spring buoy  20  may comprise chain to provide stabilizing weight when buoy  10  is submerged. 
     In an alternative embodiment, spring buoys  20  may be moored to anchors  26  in a taut configuration—i.e., anchor lines  22  are substantially straight and do not assume a catenary configuration. In such embodiments, portion  24  of the anchor line is eliminated and no portion of the anchor line lies on the seafloor when CALM buoy  10  is either floating on the surface or submerged. 
     The spring buoys  20  and the upper mooring legs  28  may be eliminated and the mooring legs connected directly to buoy  10  if mooring legs  22  have the proper weight to result in the desired submerged buoy depth when the buoy is flooded (but still positively buoyant). As buoy  10  descends, the portion  24  of anchor leg  22  resting on seafloor S becomes a greater percentage of the total anchor length and the effective weight of the anchor leg supported by buoy  10  decreases. Hence, buoy  10  will reach an equilibrium depth where its (reduced) buoyancy equals the total combined effective weight of anchor legs  22  and submarine hose  18  and where it will remain until such time as it is deballasted. 
     The anchors  26  and mooring legs  22  can be preinstalled. One particular preferred installation method is illustrated in  FIGS. 5 and 6 . The mooring leg  22  may be supported by a spring buoy  20  floating on the water surface W, or below the surface. The connection of the CALM buoy  10  to the mooring system and subsequent connection of the submerged hose  18 , floating hose  16  and hawser  14  can be accomplished on the surface in the same manner as done for conventional CALM systems. As shown in  FIG. 5 , buoy  10  may be initially connected to a first anchored spring buoy  20  and subsequently to a second anchored spring buoy  20 ′. Additional spring buoys may be connected in similar fashion. Mooring and cargo transfer operations using the system may be accomplished by conventional procedures used at terminals. 
     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.