Abstract:
The method of supporting and lowering a subsea package load on an umbilical from the deck of an offshore service vessel to a subsea work location including providing a reel to store the umbilical on the deck which is not capable of sustaining the maximum load, providing a supporting tractor with dual chains which have multiple dogs mounted on skewed surfaces which amplify a spring load support against the umbilical for frictional support of the umbilical and therefore the subsea package.

Description:
TECHNICAL FIELD 
     This invention relates to the method of lowering and raising payloads into ocean depths using a winch system. 
     BACKGROUND OF THE INVENTION 
     Conventional lowering and lifting in subsea environments using an armored umbilical (lowering/communication cable) is by using a winch with the load rating suitable to the task. When lowering a load to extreme depth such as 10,000 feet, the weight of the armored umbilical in water will often exceed the weight of the payload. In the case of a remotely operated vehicle (ROV), the objective is to make the ROV as near neutrally buoyant for ease of operations with only enough weight to allow it to be lowered to the desired depth. The net weight of the ROV plus a handling cage or top hat will be in the range of 1000 lbs., and the armored umbilical getting it to the bottom can exceed 20,000 lbs. Some ROVs are lowered subsea in a heavy cage and swim out as a neutrally buoyant assembly on a short flexible lead. Some ROVs are lowered below a heavy top hat and are released when at the working depth with a short umbilical from a small reel mounted in the top hat. 
     The armored cable must have substantial capacity as the ROV plus cage or top hat will weigh 1000 lbs. in water, but may well weigh 30,000 lbs. when being lifted through the air/water interface and onto the deck. The winch system at the surface sees its maximum load condition either when it is being lifted through the air/water interface or when it is at its lowest operational depth. Although the ROV plus Top Hat will be only a smaller load such as 1000 lbs., the steel armored umbilical when fully deployed will represent a major load. 
     With the requirement for 10,000 feet or more in armored cable under tensions up to 30,000 lbs., the crushing load on the drum and the loading on the end flanges which acts similar to pressure, requiring the winch spool to be relatively heavy and expensive to manufacture. The winch torsional requirements for lifting the ROV system out of the water at the air/water interface mandate a substantial gear box to be provided. 
     An additional difficulty with the conventional winch arrangement is that the cable must be loaded onto the spool with tensions in the range of 12,000 lbs., or when a 30,000 lb. tension load is imparted the current outer wrap of the umbilical will “knife” into the inner wraps and damage in the cable. In some cases the clients insist that the pre-wrapping is at the full 30,000 lbs. tension for added safety. In addition to general difficulties, when a cable is to be replaced, it means it must be taken to shore to be reloaded with equipment which can hold a back tension of 12,000 lbs. (or 30,000 lbs.) tension as it is being spooled. 
     Some loads similar to this have been handled by coiled tubing injector heads such as the Beta Coiled Tubing Units manufactured by the Beta Division of Brown Oil Tools in the 1970 time frame (U.S. Pat. No. 4,265,304) and is contemporarily done with traction winches on offshore pipe laying vessels. Characteristically, the friction loading against the cable, coiled tubing, or pipeline is from two opposite directions, tending to squash the cable, coiled tubing, or pipeline to an out of round condition which tends to reduce the service life of the components. 
     Coiled tubing units have sought to engage the coiled tubing from two sides since the 1960s with the resulting loss in service life of the armored umbilical, coiled tubing, and pipeline. This has not been a detriment to pipe line installation as they are installed one time and left in place. However, coiled tubing and armored umbilicals are characteristically service tools deployed and retrieved repeatedly and the added stress of being deformed reduces their usable service life. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of this invention is to provide a method of lowering a subsea package system through the air/water interface and down to a working depth without requiring a winch drum which will sustain the loads inherent in the tension associated with the operations. 
     A second objective of the present invention is to have the gripping forces on the umbilical to be failsafe due to the mechanical storage of energy rather than depending on hydraulic force to generate the load. 
     A third object of this invention is to amplify the normal force provided by the failsafe mechanical loading such that the normal force against the umbilical or cable will exceed the normal force provided by the failsafe mechanical loading to a sufficient amount to allow the usage of smooth faced slip inserts rather than slip inserts with sharp teeth which will damage the umbilical. 
     A fourth objective of this invention is to provide a method of gripping the umbilical in a way which does not tend to squash it to an out of round condition and potentially damage the internal communication links. 
     Another objective of this invention is to provide a system which allow umbilical to be reinstalled in the field without the need for back tension as it is being installed. 
     Another objective of this invention is to eliminate the need of a high load sheave to change the direction of the umbilical from vertical to proximately horizontal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an ROV Launch and Recovery System (LARS) as an example of a handling system for a subsea package. 
         FIG. 2  is a perspective view similar to  FIG. 1 , except the mast boom is raised to the ROV deploying position. 
         FIG. 3  is a view of the LARS as would be seen from the ocean. 
         FIG. 4  is a half section of the tractor which embodies this invention. 
         FIG. 5  is a partial section of  FIG. 4  taken along lines “ 5 - 5 ”. 
         FIG. 6  is a partial section of  FIG. 4  taken along lines “ 6 - 6 ”. 
         FIG. 7  is a perspective view of a section of chain with a chain dog separated from the chain block. 
         FIG. 8  is the same view as  FIG. 6  showing the forces vectors and amplification of the forces. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a launch and recovery system (LARS)  10  is shown on an offshore vessel  12  in a laid down mode for travelling. The mast  14  comprises a base  16 , a boom  18 , lifting cylinders  20 , and a tractor  22 . A remotely operated vehicle (ROV)  24  is shown with the top hat  26  landed on the top of it. Umbilical  28  is shown coming from the top of the top hat  26 , going through the tractor  22 , and to a reel  30 . 
     The top hat  26  is a heavy member which will assist the near neutrally buoyant ROV  24  in being lowered to ocean  32  and includes a small reel with a short neutrally buoyant umbilical inside which will allow the ROV  24  to swim away from the top hat  26  to do subsea service operations. 
     Reel  30  is not a heavy duty winch as is normally associated with LARS systems, but is rather a light duty reel similar to the one as described in U.S. Pat. No. 5,959,953. The distinction between a winch and a reel in this context is that for a reel the load is carried by something else and the reel simply rolls the cable up. In the case of the reel as seen in U.S. Pat. No. 5,950,953, the umbilical is strapped to the blowout preventer drilling riser which carries its weight. As the blowout preventer drilling riser is pulled back to the surface, the reel simply rolls the umbilical up for storage. In contrast, a winch is intended to pick up a load. 
     Referring now to  FIG. 2 , the same equipment is seen as was seen in  FIG. 1  except the lifting cylinders  20  have been stroked out and the boom  18  has lifted the ROV  24  and top hat  26  overboard and is lowering them into the ocean  32  or is recovering them from the ocean  32 . 
     Referring now to  FIG. 3 , it can be seen that the boom  18  needs to be wide enough to pass the tractor  22 , ROV  24 , and the top hat  26  as the components are deployed and recovered. As the conditions are relatively tight and the ROV can come up in any orientation, the tractor  22  must be able to rotate the ROV to a desired orientation before the boom  18  can be raised to recover the ROV. Within the tractor  22  there are rotational motors (see  FIG. 4 ) to accomplish this. 
     Referring now to  FIG. 4 , tractor  22  is shown with pulling section  40  and latch and rotate section  42 . Pulling section  40  has an inner chain  44  and an outer chain  46  to grip the umbilical  28 . Inner chain  44  has track support  50 , hardened track race  52 , drive sprocket  54 , motor  56 , chain tension adjuster  58 , and chain support  60 . Outer chain  46  has load cylinders  64 - 74 , drive sprocket  76 , motor  78 , chain tensioner  80 , upper chain guide  82 , and lower chain guide  84 . Load cylinders  64 - 74  put a failsafe mechanical spring load on the umbilical  28  for friction gripping, as will be seen in  FIGS. 6 and 7 . Umbilical  28  enters the tractor  22  on the upper left side and naturally has considerable vertical flexibility. Rollers  86  are added on each side of the umbilical  28  to guide the umbilical horizontally to make sure it aligns with the inner and outer chains  44  and  46 . Pulling section  40  has a bottom plate  88  which the latch and rotate section  42  is attached with bolts  90 . 
     Latch and rotate section  42  includes slip assembly  92 , latch assembly  94 , and cushion assembly  96 . Slip assembly  92  has internal smooth faced dogs (not shown) to provide failsafe support for the umbilical without scratching it as is illustrated in U.S. Pat. No. 6,820,705. 
     Latch assembly  94  provides dogs  100  to engage a profile on the top of the top hat  26  for support of the top hat  26  and the ROV  24  when parked at the surface. Dogs  100  are operated by cylinders  102  and linkages  104 . Latch assembly  94  also includes a large gear  106 , motor  108 , and bearings  110  to rotate the top hat  26  and ROV  24  to the proper orientation for landing on the vessel as seen in  FIG. 3 . 
     Cushion assembly  96  includes a ring  112  with a lower surface  114  for contacting the upper surface of the top hat  26 , and dampening means  116  to slow the upward movement of the top hat  26  and the ROV  24  they approach the upper end of their travel to prevent damage. 
     Referring now to  FIG. 5 , a partial section of the tractor  22  taken along lines “ 5 - 5 ” showing the inner chain  44 , the outer chain  46 , sprockets  54  and  76 , and motors  56  and  78 . 
     Referring now to  FIG. 6 , a partial section of tractor  22  taken along lines “ 6 - 6 ” of  FIG. 4  is shown. Load cylinder  70  provides cylinder  120 , piston  122 , cap  124 , load shoe  126 , bolt  128 , retract port  130 , load port  132 , retaining ring  134 , seals  136 - 140 , upper spring washers  142 , middle spring washers  144 , and lower spring washers  146 . Bolts  150  and  152  connect load cylinder  70  and support track  50  to side plates  154  and  156  respectively. Outer chain  46  is shown with rollers  160  and  162  connected to axle  164  (not shown) by bolts  186  and  168 , chain block  170 , chain dogs  172  and  174 , and leaf springs  176  and  178 . Inner chain  44  is made of similar components. 
     Referring now to  FIG. 7 , a perspective view of chain block  170  is shown with chain dog  172  displaced upwardly for clarity. T-slot profiles  180  on chain block  170  and  182  on chain dog  172  are prepared to allow movement in one direction along the t-slots  180  and  182 , but not along the direction of the chain itself. Leaf spring  176  holds chain dog  172  in a desired initial position, but allows it to be moved along the direction of the t-slots  180  and  182  for purposes to be discussed. After the chain dog  172  is assembled on the chain block  170  similarly to how chain dog  174  is shown, the leaf spring  176  is inserted into the end of the then aligned slots  184  and  188  and spring pin  188  is inserted into hole  190 . In this way spring pin  188  retains the leaf spring  178 , and the leaf spring  176  retains the chain dog  172 . A similar leaf spring  178  and roll pin are inserted in the opposite end of chain dog  174 . 
     Referring now to  FIG. 8 , the same partial section of  FIG. 6  is seen again with force vector arrows illustrated. Load cylinder  70  provides a force  200  on load shoe  126  which is imparted to rollers  160  and  162  and then to chain block  170 . The force  200  against chain block  170  of outer chain  46  is imparted to chain dogs  172  and  174  through angled surfaces  202  and  204  which are at an angle relative to the direction of force  200 , resulting in a wedging amplification of the force  200  yielding the sum of the resulting forces  206  and  208  being greater than the force  200 . The forces  206  and  208  pass through chain dogs  172  and  174 , respectively, and provide a frictional force against the umbilical  28 . In like manner the forces are transmitted through umbilical  28  and load against chain dogs  210  and  212  yielding forces  214  and  216  against angled surfaces  218  and  220 , down through inner chain  44  and onto harden track race  52  and track support  50  as reaction force  222  which equals force  200 . 
     As load cylinder  70  is capable of putting up a force  200  which may not provide enough friction causing load to support the umbilical, the reaction force  222  effectively doubles the friction causing load available and the wedging action caused by angled surfaces  202 ,  204 ,  218 , and  220  enhances force  200  and reaction force  222  to an even greater extent thereby providing sufficient frictional support to safely support the umbilical  28 . 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims.