Patent Publication Number: US-11377174-B2

Title: Mooring system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a national stage application, filed under 35 U.S.C. § 371, of International Patent Application No. PCT/NO2018/050306, filed Dec. 7, 2018, which claims priority to Norwegian Patent Application No. 20171967, filed Dec. 8, 2017, both of which are incorporated by reference herein in their entirety. 
     FIELD OF THE INVENTION 
     The invention concerns a mooring system as set out by the preamble of claim  1 . 
     BACKGROUND OF THE INVENTION 
     There is a need for a mooring system suited for shallow waters which is simpler and easier to install than the present systems, while at the same time fulfills operational requirements. 
     In shallow waters, e.g. down to less than 20 metres, and where wind and waves are relatively benign and directional, the well-known turret option is too elaborate and unnecessary. Typical data may be: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Waves 
                 H s  3.3 m/T p  12-18 s (swell dominated) 
               
               
                   
                 Wind 
                 20 m/s wind (1 hour mean) 
               
               
                   
                 Current 
                 0.5-1 m/s surface currents. 
               
               
                   
                   
               
            
           
         
       
     
     A mooring system that keeps the vessel (e.g. a Floating Storage and Regasification Unit; FSRU) orientated with a fixed heading towards the dominating wave direction, and maintained on heading by stern holdback lines, is considered adequate. However, the mooring system must allow a cargo vessel, e.g. an LNG carrier (LNGC) to be moored alongside the FSRU during LNGC loading operations. A problem with known catenary mooring systems is that the mooring lines tend to interfere with large vessels (such as an LNGC) that intend to moor alongside the FSRU. 
     SUMMARY OF THE INVENTION 
     The invention is set forth and characterized in the main claim, while the dependent claims describe other characteristics of the invention. 
     It is thus provided a mooring system, comprising a pair of first mooring elements configured at first ends for connection to a floating vessel; a pair of second mooring elements configured at first ends for connection to a seabed below a water surface; characterized by a restrictor member connected between said pair of first mooring lines&#39; second ends. 
     In one embodiment, the restrictor member is connected between said pair of second mooring elements&#39; second ends. The restrictor member may be an inelastic member and the pairs of first and second mooring elements are connected to the restrictor member at opposite ends of the restrictor member. The restrictor member has a length dimension between said ends that is less than or equal to the width of the vessel to which the mooring system is connected, whereby the a pair of first mooring lines are pulled together and will not interfere with other vessels moored alongside said vessel. The restrictor member may be a stiff member, such as a steel rod, pipe or bar, or one or more chains or ropes. 
     A mooring system may comprise a floating vessel, a mooring system according to the invention connected to the bow portion of the floating vessel; and a mooring system according to the invention connected to the stern portion of the floating vessel. 
     The invented mooring system, referred to as a Restricted Catenary Mooring (RCM) system is particularly suited for shallow water application in benign areas with directional waves (swell). The RCM system is an effective mooring system which together with holdback mooring lines act as a spread mooring system. 
     The invented mooring system contributes to holding the mooring lines together and away from side-by-side moored LNGC. 
     The invented mooring system shall not be limited to use on an FSRU, but may be used on other vessels, such as FPSO and FSO vessels, and other ships. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other characteristics of the invention will become clear from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached schematic drawings, wherein: 
         FIGS. 1 and 2  are perspective views of an embodiment of the invented mooring system, in use for mooring a floating vessel; 
         FIG. 3  is a perspective schematic sketch corresponding to  FIGS. 1 and 2 ; 
         FIG. 4  is a perspective view of an embodiment of a bow mooring restrictor; 
         FIG. 5  is a perspective view of an embodiment of a stern mooring restrictor; 
         FIG. 6  is a side view schematic sketch illustrating the invented mooring system in a mooring state in equilibrium; 
         FIG. 7  is a side view schematic sketch illustrating the invented mooring system in a mooring state with maximum backward offset; 
         FIG. 8  is a side view schematic sketch illustrating the invented mooring system in a mooring state with maximum forward offset; 
         FIG. 9  is a side view schematic sketch illustrating the invented mooring system in a mooring state in maximum sideways tension; 
         FIG. 10  is a front view of an embodiment of the invented mooring system, in use for mooring a floating vessel, on a sideways (to starboard) offset state; 
         FIG. 11  is a plan view of an embodiment of the invented mooring system, in use for mooring a floating vessel, and also showing another vessel moored alongside the floating vessel; and 
         FIGS. 12 a  and 12 b    are perspective views illustrating alternative embodiments of the mooring restrictor. The dotted lines indicate that the mooring restrictor chains are interconnected and may have a length that suit the applicable configuration. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERENTIAL EMBODIMENT 
     The following description will use terms such as “horizontal”, “vertical”, “lateral”, “back and forth”, “up and down”, “upper”, “lower”, “inner”, “outer”, “forward”, “rear”, etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader&#39;s convenience only and shall not be limiting. 
       FIG. 1  shows an embodiment of the invented mooring system, in use for mooring a floating vessel  1 , for example a Floating Storage and Regasification Unit (FSRU) ship  1 . An export gas riser  2  is routed from a balcony along one side of the ship  1 . An umbilical (not shown) may be included for control and operation of a seabed Pipeline End Manifold (PLEM; not shown). The umbilical may be stand-alone or piggy-backed to the riser (preferred). 
     In the illustrated embodiment, the ship  1  is moored via a bow mooring system  3  and a stern (hold-back) mooring system  4 . The bow mooring system  3  and the stern mooring system  4  may in principle be similar (and thus generally be referred to as a “mooring system”). 
     The bow mooring system  3  comprises in the illustrated embodiment two upper mooring elements  31   a ,  31   b . Each of the upper mooring elements is at one end connected to the ship&#39;s  1  bow portion in a manner which is well known in the art (e.g. to winches on the ship&#39;s deck). The upper mooring elements  31   a ,  31   b  may comprise wires, ropes, lines, chains, or any other flexible mooring element known in the art. In the embodiment illustrated in  FIG. 4 , each upper mooring element  31   a ,  31   b  comprises three mooring chains, but the invention shall not be limited to this number or type of mooring element. 
     Each upper mooring element  31   a ,  31   b  is at its other end connected to generally opposite ends of an elongate element  34  which in the following will be referred to as a mooring restrictor. Also connected to (and generally at opposite ends of) the mooring restrictor  34 , are two lower mooring elements  35   a ,  35   b . These lower mooring elements are connected between the mooring restrictor  34  and respective seabed anchors (not shown) and extend in a catenary fashion in the body of water. The mooring elements  35   a ,  35   b  may comprise wires, ropes, lines, chains, or any other flexible mooring element known in the art. 
     In the illustrated embodiment, each lower mooring element  35   a ,  35   b  comprises a first section  32   a ,  32   b  that may comprise synthetic fibre ropes (e.g. polyester) and a second section  33   a ,  33   b  that comprises chains and extend to the seabed anchors.  FIGS. 1-3  show that each lower mooring element  35   a ,  35   b  comprises three elements (lines, ropes, chains, etc.), but the invention shall not be limited to this number or type of mooring element. 
     The stern mooring system  4  comprises in the illustrated embodiment two upper mooring elements  41   a ,  41   b . Each of the upper mooring elements is at one end connected to the ship&#39;s  1  stern portion in a manner which is well known in the art (e.g. to winches on the ship&#39;s deck). The upper mooring elements  41   a ,  41   b  may comprise wires, ropes, lines, chains, or any other flexible mooring element known in the art. In the embodiment illustrated in  FIG. 5 , each upper mooring element  41   a ,  41   b  comprises two mooring chains, but the invention shall not be limited to this number or type of mooring element. 
     Each upper mooring element  41   a ,  41   b  is at its other end connected to generally opposite ends of an elongate element  44  which in the following will be referred to as a mooring restrictor. Also connected to (and generally at opposite ends of) the mooring restrictor  44 , are two lower mooring elements  45   a ,  45   b . These lower mooring elements are connected between the mooring restrictor  44  and respective seabed anchors (not shown) and extend in a catenary fashion in the body of water. The mooring elements  45   a ,  45   b  may comprise wires, ropes, lines, chains, or any other flexible mooring element known in the art. 
     In the illustrated embodiment, each lower mooring element  45   a ,  45   b  comprises a first section  42   a ,  42   b  that may comprise synthetic fibre ropes (e.g. polyester) and a second section  43   a ,  43   b  that comprises chains and extend to the seabed anchors.  FIGS. 1-3  show that each lower mooring element  45   a ,  45   b  comprises three elements (lines, ropes, chains, etc.), but the invention shall not be limited to this number or type of mooring element. 
       FIG. 4  illustrates an embodiment of the mooring restrictor  34 , which serves to hold the mooring lines together, in order to avoid interference with any ship moored alongside the ship  1  (See e.g.  FIG. 11 , described below). The system comprises a restrictor triangle having a bar (the mooring restrictor) with the leeward lines relocated to point forward, resulting in a 3+3 bow mooring line configuration in the illustrated embodiment. The horizontal motions at the mooring restrictor are small. The Restricted Catenary Mooring (RCM) may be split in the following key components:
         Mooring hang-off at deck level   Handling equipment   Upper mooring hang-off   The mooring, 3+3 lines assumed with fluke anchors.   The mooring restriction (The Restrictor Bar) close to the bow/stern.   A riser with PLEM for gas pipeline to shore and an umbilical for PLEM control       

       FIG. 5  shows an embodiment of a mooring restrictor  44  for the stern holdback lines in a stern RCM system  4 . These lines may be similar to the bow mooring, but with e.g. 2+2 lines if weather permits. See also  FIG. 11 . The upper part (above the restrictor) is chains, while the first portion of the lower lines are suggested to be buoyant polyester ropes in order to avoid interference with the existing pipelines and/or provide flexibility into the lines. 
     The mooring restrictors  34 ,  44  may be made from a reinforced pipe structure with a pad eye structure at each end. The mooring lines are attached with H-links. 
       FIG. 6  shows the ship  1  and the mooring system according to the invention in a state of equilibrium. Reference letter S denotes the water surface. Note that the mooring restrictors  34 ,  44  are located below and near the water surface S. 
       FIG. 7  illustrates a maximum backward offset, and shows how the bow mooring restrictor  34  has been elevated above the water surface S, while the stern mooring restrictor  44  has sunk deeper into the water. 
       FIG. 8  illustrates a maximum forward offset, and shows how the stern mooring restrictor  44  has been elevated above the water surface S, while the bow mooring restrictor  34  has sunk deeper into the water. 
       FIG. 9  and  FIG. 10  illustrates the ship in a state of maximum sideways tension, but in equilibrium in the fore-aft direction. 
       FIG. 11  shows a ship (e.g. an FRSU ship)  1  moored with a bow mooring system  3  and a (stern) hold-back system  4 , and a ship  5  (e.g. an LNGC) moored alongside. 
     Referring to  FIGS. 12 a  and 12 b   , the mooring restrictors (bow and/or stern) may in other embodiments comprise one or more chains, wires or ropes, but a stiff connection which is considered to be more durable. Here, the mooring system  6  comprises upper mooring elements  61   a,b  that at one end are connected to the ship&#39;s bow portion or stern portion and at the other end are connected to a mooring restrictor  64 . Lower mooring elements  62   a,b  are connected between the mooring restrictor  64  and respective seabed anchors (not shown) and extend in a catenary fashion in the body of water. It should be understood that this mooring system  6  may be used at the bow or at the stern, and that the number or chains and ropes may be dimensioned according to the applicable situation. The major difference between the mooring restrictors  34 ,  44  described above, and the alternative mooring restrictor  64 , it that the latter comprises one or more chains (two shown in  FIG. 12 ), which are rigid in tension. The mooring restrictor chains are at each end connected to end links  65   a,b , which in turn are connected to respective mooring links  66   a,b . The upper  61   a,b  and lower  62   a,b  mooring elements are connected to respective mooring links  66   a,b.