Abstract:
A mooring method for a moored vessel that comprises a first chain stopper that is fixed to the vessel and is operable to selectively couple an upper chain to the vessel. A chain tensioner is coupled to the vessel and is operable to pull the upper chain. A second chain stopper is fixed to the vessel and is operable to selectively couple the upper chain to the vessel. The chain tensioner engages the upper chain at a location between the first chain stopper and the second chain stopper.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    Embodiments of the present invention relate to methods and apparatus for mooring floating platforms. More specifically, certain embodiments of the present invention relate to systems for tensioning mooring lines that are connected to a floating vessel. 
         [0004]    Multi-leg mooring systems generally include a plurality of mooring lines extending outward from a floating platform to a plurality of anchors set into the seafloor. Each mooring line comprises a short length of chain extending from the vessel and connected to a long length of mooring line running to the anchor. Common procedures for anchoring the vessel include extending (pay out) the chain segment from the vessel so as to connect the chain segment to the mooring line that is connected to the anchor, which has been previously set at the seafloor. The chain segment is then pulled into (pull in) the vessel until the proper tension is generated in the mooring line. 
         [0005]    Once the mooring line has been tensioned, the chain is fixed to the vessel so as to maintain the tension in the mooring line. This procedure is repeated for each mooring line until the vessel is securely moored. In order to tension the next mooring line, the equipment used to handle the chain is relocated to the next line and set up for use. One key component of the handling equipment is the large pull-in winches that are used to pull the chain segments and generate the proper tension in the mooring line. Many vessels are equipped with fewer pull-in winches than mooring lines and therefore the winches have to be moved between lines during the mooring procedure. Moving the pull-in winches increases the time needed for mooring as well as increases the difficulty in making later adjustments of the mooring system, such as relocating the vessel within its watch circle. When relocating a vessel within its watch circle, separate mooring lines are payed out or pulled in to change the position of the vessel. This process can be time consuming when the mooring lines can only be adjusted one line at a time. 
         [0006]    Thus, the embodiments of the present invention are directed to mooring systems that seek to overcome these and other limitations of the prior art. 
       SUMMARY OF THE PREFERRED EMBODIMENTS 
       [0007]    Embodiments of the present invention include a mooring method for a moored vessel that comprises a first chain stopper that is fixed to the vessel and is operable to selectively couple an upper chain to the vessel. A chain tensioner is coupled to the vessel and is operable to pull the upper chain. A second chain stopper is fixed to the vessel and is operable to selectively couple the upper chain to the vessel. The chain tensioner engages the upper chain at a location between the first chain stopper and the second chain stopper. 
         [0008]    Thus, the embodiments of present invention comprise a combination of features and advantages that enable substantial enhancement of mooring a floating vessel. These and various other characteristics and advantages of the present invention will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention and by referring to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]    For a more detailed understanding of the present invention, reference is made to the accompanying Figures, wherein: 
           [0010]      FIG. 1  is a partial schematic view of a vessel having a mooring system designed in accordance with embodiments of the invention; 
           [0011]      FIG. 2  is more detailed view of the vessel of  FIG. 1 ; 
           [0012]      FIG. 3  illustrates a first phase in a mooring operation using a mooring system designed in accordance with embodiments of the invention; 
           [0013]      FIG. 4  illustrates a second phase in the mooring operation of  FIG. 3 ; 
           [0014]      FIG. 5  illustrates a third phase in the mooring operation of  FIG. 3 ; and 
           [0015]      FIG. 6  illustrates a fourth phase in the mooring operation of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    In the description that follows, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawing figures are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. 
         [0017]    Referring now to  FIG. 1 , mooring system  10  for vessel  12  comprises a plurality of mooring lines  14  coupled to subsea anchors  15 . Each mooring line  14  comprises lower chain  16 , main mooring line  18 , and upper chain  20 . Lower chain  16  of each mooring line  14  is coupled to a subsea anchor  15 . Main mooring line  18  is coupled to lower chain  16  and extends upward through the water to upper chain  20 , which connects to vessel  12 . 
         [0018]    Referring now to  FIG. 2 , vessel  12  comprises fairleads  22 A-B, chain stoppers  24 A-B, chain lockers  26 A-B, and chain tensioner  28 . Chain tensioner  28  and chain lockers  26 A-B are located close to a centerline of vessel  12 . Chain tensioner  28  can be any device that can apply tension to the chain, such as a chain jacks, chain windlasses, and chain capstans. Fairleads  22 A-B and chain stoppers  24 A-B are disposed on or near the sides of vessel  12 . Fairleads  22 A-B redirect and laterally secure upper chain  20  as it comes in contact with vessel  12 . Chain stoppers  24 A-B releasably engage the upper chain  20   
         [0019]    Upper chain  20  is a continuous length of chain that is connected to each mooring line  14 A,  14 B. From mooring line  14 A, upper chain  20  is guided by fairlead  22 A upward to chain stopper  24 A, which releasably engages one or more links of upper chain  20  so as to effectively fix the chain relative to vessel  12 . Upper chain  20  passes from chain stopper  24 A to chain tensioner  28  and then to chain stopper  24 B. Upper chain  20  passes from chain stopper  24 B through fairlead  22 B and connects to the opposite mooring line  14 B. 
         [0020]    Chain lockers  26  are disposed on either side of chain tensioner  28  to store any slack lengths upper chain  20  between the chain tensioner and either chain stopper  24 A-B. In certain embodiments, chain lockers  26  may be receptacles built into the hull of vessel. In other embodiments, excess chain may simply be suspended below the decks of the vessel in catenary loops or on a support vessel adjacent to the vessel. 
         [0021]      FIGS. 3-6  show a mooring operation using mooring system  10  as described in  FIGS. 1 and 2 . A first phase of the mooring operation is shown in  FIG. 3 . Anchors  15  are set in the seafloor at predetermined locations. Lower chains  16  and main mooring lines  18  are connected to anchors  15  and are suspended by buoys  30 . The installations of anchors  15  and the pre-positioned portions of the mooring lines  14 A-B are often performed by a specially designed installation vessel  32  or other service vessel before the arrival of floating vessel  12 . 
         [0022]    Once floating vessel  12  is on location, upper chain  20  is payed-out from the vessel by releasing chain stopper  24 A and pulling the chain with installation vessel  32 . Upper chain  20  is pulled from chain locker  26 A until the end of the chain can be coupled to mooring line  14 A by installation vessel  32 , as is shown in  FIG. 4 . Chain tensioner  28  is then activated so as to pull-in upper chain  20  until the desired tension is achieved in mooring line  14 A. Chain stopper  24 A is then engaged with upper chain  20  to fix the chain relative to vessel  12  and maintain the tension in mooring line  14 A. 
         [0023]    During tensioning, excess chain can be disposed in chain locker  26 B or may be simultaneously payed-out to a second mooring line  14 B by installation vessel  32 , as is shown in  FIG. 5 . Chain tensioner  28  is then activated in the opposite direction so as to pull in upper chain  20  until the desired tension is achieved in mooring line  14 B. Chain stopper  24 B is then engaged with upper chain  20  to fix the chain relative to vessel  12  and maintain the tension in mooring line  14 B. 
         [0024]    Referring now to  FIG. 6 , once mooring lines  14 A-B have been fully tensioned, chain tensioner  28  can be relocated to another pair of mooring lines  14 C-D. The procedure is then repeated until each pair of mooring lines is fully tensioned. In some embodiments, each pair of mooring lines may have its own chain tensioner therefore eliminating the need to relocate the chain tensioner. 
         [0025]    Thus, the mooring system  10  described above provides faster installation than conventional systems. Mooring system  10  allows upper chain  20  of one mooring line to be pulled-in while simultaneously paying-out the upper chain that connects to another mooring line. This can eliminate the need for a special pull-in winch and saves time because as one line is being tensioned, the chain for the next mooring line is being payed-out. The system may be further enhanced by using a multi-speed chain tensioner that allows the initial low tension pull-in to be carried out faster. 
         [0026]    In certain embodiments, vessel  12  may also be able to relocate by adjusting the tension in mooring lines  14 . By pulling-in and paying-out select mooring lines  14 , vessel  12  can be repositioned at any location within its watch circle. A vessel may be relocated within its watch circle in order to drill multiple wells. With conventional mooring systems and vessels, this operation is done by pulling in and paying out separate mooring chains on separate windlasses. Mooring system  10  simplifies this relocation process by enabling the simultaneous pulling-in and paying-out of at least two mooring lines at one time. Thus, when one end of an upper chain is pulled in, the slack portion is fed into a chain locker, a catenary loop, or onto a support vessel, and then payed out on the other side of the vessel. 
         [0027]    The preferred embodiments of the present invention relate to apparatus and systems for mooring a floating vessel. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. In particular, various embodiments of the present invention provide mooring systems for floating vessels. Reference is made to the application of the concepts of the present invention to mooring a semi-submersible vessel, but the use of the concepts of the present invention is not limited to these applications, and can be used for any other applications including other moored floating vessels. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. 
         [0028]    The embodiments set forth herein are merely illustrative and do not limit the scope of the invention or the details therein. It will be appreciated that many other modifications and improvements to the disclosure herein may be made without departing from the scope of the invention or the inventive concepts herein disclosed. Because many varying and different embodiments may be made within the scope of the inventive concept herein taught, including equivalent structures or materials hereafter thought of, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.