Abstract:
A mooring arrangement for a semi-submersible offshore structure having an outer hull with a lower ( 2 ) and an upper ( 1 ) water line, comprises a mooring line ( 4 ) extending from a fixed point on the seabed to an attachment on the off-shore structure, at least a portion of the mooring line closest to the attachment being a chain. The attachment comprises a hawse ( 10 ) and a chain stopper ( 22 ) located below the lower water line. The chain stopper ( 22 ) is located in a dry space inside the outer hull and is sealingly attached at one end to the hawse and at the opposite end to a chain pipe ( 9 ) which extends inside the hull to above the upper water line. The hawse extends outwards from the chain stopper and is sealed to an opening in the outer hull.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a mooring arrangement for a semi-submersible offshore structure, such as a platform, specifically designed for operation in Arctic conditions. 
       BACKGROUND 
       [0002]    Operations in Artie waters put large demands on crew and equipment due to the harsh climate. Large icebergs that are adrift in open waters, or multi-year ice ridges of substantial mass and stiffness that are drifting with the surface ice, could pose threats to moored floating installations. This makes it necessary to be able to disconnect and move the installations out of danger on short notice, and to be able to re-connect in an effective manner. 
       SUMMARY OF THE INVENTION 
       [0003]    The object of the invention is to provide a mooring arrangement that allows a moored structure to be disconnected and quickly relocated in case of danger. 
         [0004]    This is achieved in an arrangement according to the appended claims. In particular a mooring arrangement for a semi-submersible offshore structure having an outer hull with a lower and an upper water line, said mooring arrangement comprising a mooring line, extending from a fixed point on the seabed to an attachment on the offshore structure, at least a portion of the mooring line closest to the attachment being a chain, and the attachment comprising a hawse and chain stopper located below said lower water line, characterized in that the chain stopper is located in a dry space inside the outer hull, in that the chain stopper is sealingly attached at one end to the hawse and at the opposite end to a chain pipe which extends inside said hull to above said upper water line and in that the hawse extends outwards from the chain stopper to, and sealingly with, an opening in said outer hull. 
         [0005]    Advantages of the inventive mooring arrangement include the ability of a safe quick release of the mooring lines. It also allows for easy retrieval, connection and maintenance of the mooring system. Mooring lines can be released under full tension up to mooring line breaking load. The arrangement allows for simple connection and maintenance procedures. The arrangement has sound anti-chaffing and low chain fatigue design features. The arrangement can be operated by stored energy independently of the platform energy supply. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention will now be described in detail in reference to the appended drawings, in which: 
           [0007]      FIG. 1  is an overview picture showing a floating platform moored using the inventive arrangement, 
           [0008]      FIG. 2  is a picture of the in-hull parts of the inventive arrangement, 
           [0009]      FIG. 3  is a detailed view of the chain stopper arrangement 
           [0010]      FIG. 4  is a view of the platform when released from the mooring, 
           [0011]      FIG. 5  illustrates how the chain stopper cavity may be sealed off for maintenance, 
           [0012]      FIG. 6  shows the hawsehole arrangement in detail, 
           [0013]      FIGS. 7 and 8  shows the chain stopper arrangement holding the chain, 
           [0014]      FIGS. 9 and 10  shows the chain stopper arrangement when the chain is released, 
           [0015]      FIGS. 11 and 12  shows the chain stopper arrangement when the chain has been fully released, 
           [0016]      FIGS. 13 and 14  shows the chain stopper arrangement when the mooring line is being pulled into the stopper, 
           [0017]      FIGS. 15 and 16  shows the chain stopper when again holding the chain, 
           [0018]      FIG. 17  shows the retrieval buoy in detail. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows a semi-submersible platform,  7 , floating in the sea. The platform,  7 , is moored to the seabed,  8 , with a number of mooring lines,  4 . The mooring lines,  4 , enter the platform,  7 , hull through hawseholes with flared guides well below the water line,  1 ,  2  (the winter and summer water line, respectively). Each mooring line,  4 , is made fast by a quick release chain stopper,  22 , placed directly inboard of the hawsehole,  10 . 
         [0020]    The mooring lines,  4 , are equipped with chain link top end sections,  23 . The rest of the mooring line,  4 , may consist of several different line types and attachments. The mooring line,  4 , may be equipped with a retrieval buoy,  3 , fitted with a grab handle to facilitate picking up the mooring line,  4 , off the seabed,  8 , after a release. The mooring line,  4 , may also be equipped with a mid-line spring buoy,  5 , which during release pulls the top end chain and upper mooring line,  4 , section horizontally towards the anchor,  6 , position. This reduces the chance of the mooring lines,  4 , entangling and protects any subsea installations. This is illustrated in  FIG. 4  which shows the situation when a mooring line,  4 , has been released. The mooring line,  4 , is now lying on the seabed,  8 , except that the mid part of the line is pulled upward by the spring buoy,  5 , which also lifts the retrieval buoy,  3 , a little distance off the seabed,  8 . 
         [0021]      FIG. 2  shows the mooring arrangement on the platform,  7 . The mooring line,  4 , enters the platform,  7 , hull through the hawsehole and is made fast in the chain stopper which has a flooded inner cavity and a water tight casing. The hawsehole and chain stopper have a sealed connection. All mechanical operating components and electrical sensors are on the dry side of the water tight casing and easily accessible from within the hull of the platform,  7 . The chain stopper has a through-bore and opening upwards to which is connected a J-shaped chain pipe,  9 , rising vertically to above water line. The chain stopper and the chain pipe,  9 , have a sealed connection. The mooring lines,  4 , are tensioned by pull-in winches,  24 , operating from a height above the top of the chain pipes. 
         [0022]      FIG. 6  shows the hawsehole in detail. The mooring line,  4 , top chain enters the hull through an underwater hawsehole with flared guide,  15 , to avoid chain contact with side walls under normal operating conditions. The inboard edges of the flared guide are welded to the outboard face of a foundation plate,  16 . 
         [0023]      FIG. 3  shows the chain stopper arrangement,  22 , in further detail. The flared guide,  10 , of the hawsehole ensured that there are sufficient clearances around the top chain outwards of the chain stopper such that no contact with other surfaces occurs from movements under normal operating conditions. In the chain stopper cavity,  25 , the chain is held between a chain guide,  11 , and a chain stopping device,  26 . The chain stopping device,  26 , is an elongated member with a first end that is fastened to rotating stopper shaft,  27 . The second end,  20 , is held fixed (i.e. as shown in the figure) in a releasable locking device. The locking device includes a rotating locking shaft,  28 , with a semi-circular cut out (actually the cut out is a little more than 180°). Due to this cut out, a locking surface is formed on which the second end,  20 , of the chain stopping device will rest when in the locked position as shown in the figure. By rotating the locking shaft, to  28 , the chain stopping device,  26 , is released. The said shafts are penetrating the watertight housing,  12 , of the chain stopper. Seals ensure that the mechanism operating the chain stopping device and locking device inside the chain stopper cavity may be mounted in dry space within the hull. 
         [0024]      FIGS. 7 and 8  shows the chain stopper from the outside ( FIG. 7 ) and the inside ( FIG. 8 ) _ 15  of the sealed cavity,  25 , when in a locked stand by (for quick release) position. The mechanism on the outside of the cavity includes a stopper rocker arm,  29 , mounted on the stopper shaft,  27 . On the distant end of the stopper rocker arm,  29 , there is a fork,  30 , with a pin,  31 , sliding in an elongated slotted arm,  32 , mounted at one end of a hydraulic stopping device cylinder,  33 . The stopping device cylinder is pivotable mounted onto the cavity housing in its other end. The locking mechanism includes a locking rocker arm,  34 , mounted on the locking shaft,  28 . The locking rocker arm,  28 , is in its distant end pivotable connected with the first end of a first link arm,  35 . This first link arm,  35 , is in its second end connected further to the first end of a second link arm,  36 . The second link arm,  36 , is pivotable mounted onto the cavity housing in its second end. In the middle of the second link arm,  36 , there is a pin,  37 , sliding in an elongated slotted member,  38 , mounted at one end of a locking device cylinder,  39 . The other end of the locking device cylinder is pivotable mounted to the cavity housing,  12 . 
         [0025]      FIGS. 9 and 10  shows the mechanism inside and outside the cavity housing when the chain is released. Chain release under tension is accomplished by locking device cylinder,  39 , contracting to buckle the locking device link arms,  35 ,  36 , allowing the locking device shaft,  28 , to rotate freeing the end,  20 , of the chain stopping device,  26 . The tension in the chain causes the chain stopping device to rotate. The slotted member,  32 , on the chain stopping device cylinder allows for this rotation without extension of the cylinder. 
         [0026]      FIGS. 11 and 12  show the chain stopper in full release. Then, the chain is rushing out through the hawsehole to rest on the seabed,  8 . 
         [0027]    The next step is to fish up the chain from the sea. This is made by gripping the retrieval buoy, bring it up to the surface and fasten a pull-in line to the end of the top chain link. Then, the chain may be hauled through the hawsehole and up to the winch. This is illustrated in  FIGS. 13 and 14 . The chain stopping device cylinder,  33 , is actively in use to hold the device in open position when the mooring line,  4 , is being pulled into the stopper. 
         [0028]    The chain stopping device cylinder is also used to hold the stopping device in closed position when the chain has entered the stopper and until the locking device has been activated. See  FIGS. 15 and 16 . The last step will be to turn the locking device shaft to lock the stopping device. 
         [0029]    Components of the chain stopping arrangement; retrieval buoy and mid-line spring buoy. 
         [0030]    To facilitate pick up of the mooring line,  4 , from the sea bed a retrieval buoy,  3 , may be fitted between the upper rope section of the mooring line,  4 , and the top chain,  23 . The buoy has enough reserve buoyancy to ensure that it comes to rest above the sea bed. The buoy is equipped with a grab handle,  40 , which orients in a favourable way to ease pick up. See  FIGS. 4 and 17 . 
         [0031]      FIG. 5  shows an embodiment of how the chain stopper cavity may be sealed off for maintenance. The chain stopper and the chain pipe have a sealed connection sealing of hawsehole guide with a tailored wedge shaped plug,  13 . 
         [0032]    The inner cavity of the chain stopper can be accessed, or the complete chain stopper can be removed by means of the following procedure:
       1. Sealing of hawsehole guide with a tailored wedge shaped plug,  13 .   2. Draining,  14 , the chain stopper and chain pipe of sea water.   3. Removal of chain stopper side plate or;       
 
         [0036]    4. Removal of the lower section of chain pipe, 
         [0037]    5. Removal of chain stopper, 
         [0038]    6. Secure the opening in Foundation Plate with a covering plate. 
         [0039]    The function of the mid-line spring buoy,  5 , is during a quick release to pull the top end chain,  23 , and upper mooring line,  4 , section horizontally towards the anchor position reducing the chance of the mooring lines,  4 , entangling and to protect any sub sea installations. 
         [0040]    It needs to be designed with sufficient buoyancy to support the mooring line,  4 , section from itself and upwards to the retrieval buoy,  3 , and be placed along the length of the mooring line,  4 , to ensure functionality.