Abstract:
A method of dynamic positioning of a vessel ( 16, 32 ), wherein a positioning system controls at least one positioning machinery ( 18 ), and where two or more buoys ( 1 ), each comprising at least one positioning machinery ( 18 ), are coupled to the vessel ( 16, 32 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to, and claims priority from, PCT/NO2005/000177 filed May 31, 2005, and Norway Patent Application No. 20042362 filed on Jun. 3, 2004, the entire contents of which are hereby incorporated by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention regards a method of dynamic positioning of a vessel. More particularly, it regards a method that involves two or more floating buoys being coupled to a vessel by means of buoyancy, wherein the buoys are provided with positioning machinery. The buoys form part of a dynamic positioning system and are preferably provided with energy from a generator placed on the vessel. 
     2. Description of Related Art 
     When operating in relatively shallow waters offshore it is customary to use anchor based mooring methods. Anchor systems of this type reveal substantial flaws when it comes to anchoring a vessel at greater depths. The main reason for this is that the length of chain or steel cable required causes a weight-induced strain on the mooring that takes up a considerable share of the tensile strength of the mooring. 
     Thus it has become more and more common to equip vessels operating in deep waters with so-called dynamic positioning. Vessels such as drilling vessels, supply vessels, accommodation vessels or cargo vessels are to an increasing extent being equipped with this kind of equipment. 
     In addition to navigation and control systems, a dynamic positioning system also includes positioning machinery. It is essential that the vessel be equipped with positioning machinery that is arranged to exert a pushing force in at least two directions and in two positions on the vessel. Positioning machinery of this type typically includes several positioning thrusters that are generally arranged for rotation in the horizontal plane, so as to allow them to exert a pushing force in any horizontal direction. 
     For vessels originally built to use conventional anchor system, the conversion required to allow them to use dynamic positioning has proven to be a relatively complex and costly process. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to remedy the disadvantages of prior art. 
     The object is achieved in accordance with the invention, by the characteristics stated in the description below and in the following claims. 
     A buoy is equipped with positioning machinery and is arranged to form part of a dynamic positioning system. The buoy is constructed so as to be relatively easy to connect a vessel without requiring any significant adaptation or conversion of the vessel. 
     Cost factors demand that buoys of this type be designed with relatively small dimensions. As a result it is normally necessary to fit a vessel with two buoys to maintain an accurate position and directional orientation. If so required, for reasons of security or other reasons, the vessel may be equipped with two additional buoys. 
     In a preferred embodiment the buoy comprises a submersible coupling portion arranged to stick to the bottom of a vessel by means of buoyancy forces. Advantageously the coupling portion is provided with friction material. If so desired, the contact pressure between the coupling portion and the vessel can be increased further by draining off the water between said devices in order to reduce the pressure in this area. 
     Preferably the propulsion machinery is supplied with energy from the vessel or an energy supply placed on the deck of the vessel. 
     For installation the buoy is lowered partway into the sea until the coupling portion is at a deeper level than the bottom of the vessel. Then the buoy is displaced towards the vessel so as to bring the coupling portion in under the vessel, whereupon ballast is pumped out of the buoy. 
     By doing so, the buoy lifts in the sea until the coupling portion contacts the bottom of the vessel, pushing and sticking against the bottom. 
     The buoy may be installed on the vessel prior to the vessel sailing to the site where the dynamic positioning is to be used, or it may be coupled to the vessel on site, the buoy being self-propelled when supplied with propulsive power. 
     If the vessel in question is no longer to be used in deep waters and therefore no longer requires the use of dynamic positioning equipment, the buoy may easily be transferred to another vessel. 
     The method according to the invention, which allows the use of dynamic positioning on a vessel not equipped with the necessary propulsion machinery, makes it possible to use a vessel not originally intended for such positioning, also in deeper waters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following describes a non-limiting example of a preferred method illustrated in the accompanying drawings, in which: 
         FIG. 1  shows a vessel equipped with two buoys according to the invention; 
         FIG. 2  is a section taken along I-I in  FIG. 1 , on a larger scale; 
         FIG. 3  is a plan view of a buoy; and 
         FIG. 4  shows two buoys connected to a semi submersible platform. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawings, reference number  1  denotes a buoy comprising machinery housing  2 , a flat coupling portion  4  projecting sideways from the machinery housing  2  and two vertical columns  6  projecting upwards from the machinery housing  2 . 
     The machinery housing  2  and the coupling portion  4  are submerged when the buoy  1  is in the operative position, while the columns  6  protrude, at least partly, above the sea level  8 . 
     The upward facing side of the coupling portion  4 , which is equipped with ballast tanks  10 , is fitted with a flexible friction material  12 . Surrounding the friction material  12  is a seal  14  arranged to sealingly abut the bottom of a vessel  16  when the buoy  1  is coupled to the vessel  16 . 
     In the machinery housing  2  there is arranged positioning machinery in the form of two positioning thrusters  18  that may be rotated about their own vertical axes. Each thruster comprises a submerged angular gear  20  with a propeller  22  driven by an electric motor  24 . A generator  26  placed on the deck of the vessel  16  delivers energy to the motor  24  via a cable  28 . 
     The machinery housing  2  holds ballast pumps (not shown) and also the necessary piping and valves (not shown). 
     The columns  6  have fenders  30  facing the coupling portion  4 . The upper portion of the columns  6  is always located above the surface  8  of the sea, and therefore the columns  6  constitute a connection from the surface down to the machinery housing  2 , for access and conveyance of the cable  28  plus control cables and pipe connections (not shown). 
     The positioning machinery  18  of the buoy  1  is controlled in a manner that is known per se, by a control system for dynamic positioning (not shown). 
     In order to couple the buoy  1  to the vessel  16 , the buoy  1  is lowered into the sea by regulating the ballast, among other things in the ballast tanks  10 , until the coupling portion  4  is somewhat deeper in the sea than the bottom of the vessel  16 . 
     The buoy  1  is then moved in towards the vessel  16 , preferably until the fenders  30  stops against the vessel  16 . Pumping out ballast raises the buoy  1  until the friction material  12  stops against the bottom of the vessel  16 . Pumping out additional ballast from the buoy  1  increases the contact pressure between the friction material  12  and the bottom of the vessel. 
     The seal  14  that sealingly abuts the bottom of the vessel  16  prevents inflow of water to the friction material  12  when water is drained from the space between the coupling portion and the bottom of the vessel  16 . 
       FIG. 4  shows two buoys coupled to a semi submersible platform  32 .