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spring 19 may be inside a flexible cover. This may allow each
                                                              spring to function independently of its neighboring springs,
                                                              while making sure the total spring pack extends no farther
                                                              than the length of the cover.
                                                               A series of springs may be concentric with the cable 18
                                                              and be designed to safely slow a participant to a stop. Upon
                                                              impact, these springs compress along the axis of the cable,
                                                              but also deflect laterally. This lateral deflection causes the
                                                              spring to rub against the cable, creating friction. This acts as
                                                              a dampening force on the system, and the two forces (spring
                                                              force and friction) together create a smooth deceleration for
                                                              a large variety of participant weights.
                                                              Hoisting systems with heave compensation
                                                              Pat. 9,567,814 U.S. class E21B 19/09 Int. class E21B 19/09
         Figure 10: Perspective view of a trailer.            Inventor:  Erling  Tambs,  Kristiansand,  NO.,  Harald  Lan-
         may match up to an anchor track 60 on a trailer 14. When   genes,  Songe,  NO.,  Hakon  F.  Bergan,  Kristiansand,  NO.,
         a temporary anchor 16 is being loaded onto a trailer 14 for   Baard H. Kaasim, Gvarv, NO.
         transport, the temporary anchor 16 may be put into a trans-  Assignee: Cameron Sense AS., Kristiansan D, NO.
         port position. An anchor frame 68 may have anchor skids 70   Various hoisting systems with heave compensation are
         connected to the anchor frame 68, generally on the under-  provided.  In one embodiment,  an apparatus  includes a
         side of the anchor frame 68. Anchor skids 70 may be made of   hoisting system having a crown block and a drawworks. The
         rubber and provide a high-friction surface.          drawworks includes a rotatable drum for reeling in and reel-
           An anchor frame 68 may include an anchor extension 72   ing out a hoisting line that is wound on the rotatable drum
         extendable from one side of the anchor frame 68. An anchor   and  reeved over  the  crown  block.  The  hoisting  system  in-
         extension  72 may include at least one extension  jack  74,   cludes active heave control at the drawworks and a passive
         generally connected to a corner of the anchor extension 72.   heave  compensation  system.  Additional systems,  devices,
         Counterweights 80 may be placed on the anchor frame 68,   and methods are also disclosed.
         an anchor extension 72, or both. Counterweights 80 may in-  A system 10 is illustrated in figure 12 in accordance with
         clude sand bags, concrete blocks, water barrels, or the like.   one embodiment. In this example, the system 10 is an off-
         Counterweights may not be required in other configurations.  shore drilling rig in the form of a floating vessel 12. More
           A temporary anchor 16 may include at least one anchor   specifically, the floating vessel 12 is generally depicted as
         arm 76, an anchor strut 78, and an anchor beam 82. An an-  a drillship in figure 12, but the floating vessel could be pro-
         chor beam 82, an anchor strut 78, and two anchor arms 76   vided in another form, such as a semi-submersible drilling
         may be connected in the configuration of a square or rect-
         angle. An anchor arm 76, an anchor strut 78, and an anchor
         beam 82 structure may be pivotally connected to the anchor
         frame 68 by means of an anchor clevis 84. At least one hy-
         draulic ram 88 may be pivotally connected to the anchor
         frame 68 and an anchor arm 76 by means of a ram clevis 90
         connected to both the anchor frame 68 and the anchor arm
         76 and located at each end of a hydraulic ram 88. A tempo-
         rary anchor 16 may be raised hydraulically. An anchor strut
         78 may include at least one anchor brace 79.
           The cable 18 may include at least one spring 19 on the
         cable 18 and near or proximate an anchor brace 79. A spring
         19 may be made from steel, but that is not integral to the de-  Figure 12: Floating drilling rig with a hoisting system.
         sign and it could be modified to use polymer or air springs. A
                                                              rig, in other embodiments.
                                                               The vessel 12 includes a hoisting system for raising and
                                                              lowering equipment with respect to a drill floor of the vessel,
                                                              which facilitates well drilling and completion operations. The
                                                              depicted hoisting system includes a derrick 14 constructed
                                                              on the drill floor of the vessel 12. Various equipment and
                                                              other loads can be supported by one or more hoisting lines
                                                              20 of the hoisting system. In figure 12, the supported load
                                                              includes a top drive 16 and a drill string 18 suspended from
                                                              the top drive 16. The drill string 18 extends through a hole
                                                              in the drill floor of the vessel 12 and can be rotated by the
                                                              top drive 16 to facilitate drilling of a subsea well. It will be
                                                              appreciated that the hoisting system could be used for hoist-
                                                              ing other loads, such as casing strings, wellhead equipment,
                                                              and other subsea well components.
                                                               The hoisting system includes a drawworks 22, which can
                                                              be provided on the drill floor with the derrick 14, as shown in
         Figure 11: Perspective view of a temporary anchor.                                   continued on next page

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