Patent Publication Number: US-6210076-B1

Title: Offshore deck installation

Description:
This application is a continuation-in-part of U.S. application Ser. No. 08/903,776, filed Jul. 31, 1997, now U.S. Pat. No. 5,988,932. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is generally related to the load out, transportation, and installation of offshore platform decks and more particularly to the installation of a deck onto a floating offshore structure. 
     2. General Background 
     There are several methods for installing decks on offshore platforms that are well known in the industry. By far, the most common method is to build the deck onshore in a fabrication yard, lift or skid the deck onto a transport barge, transport the deck to the site on a transport barge, and lift the deck from the transport barge onto the platform substructure using a derrick barge. This is the only method that has been used to install a deck onto a spar type structure. A spar type structure may be a deep draft caisson such as that described in U.S. Pat. No. 4,702,321 or an open or truss framework such as that described in U.S. Pat. No. 5,558,467. A derrick barge is a barge with a revolving crane built into its hull. Ideally, the derrick barge that is available can make a one piece lift of the deck, so that costly hook up work offshore can be avoided. Hook up involves the connection between two or more deck units of structural, piping, electrical, and control systems. If the deck is too heavy for available equipment to lift it from the fabrication yard onto the transport barge, then the deck will be skidded along skid ways onto the transport barge in an operation know as a skidded load out. 
     In an effort to avoid offshore hook up work, the industry has developed methods other than lifting to install one piece decks. One or more of the alternative methods may be considered whenever a derrick barge of sufficient capacity is not available to make a one piece deck lift. 
     One of these methods, disclosed in U.S. Pat. No. 5,403,124, includes using a vessel having one end that is U-shaped in plan view. The deck is supported on the vessel over the U-shaped end. The vessel is then moved into position such that the U-shaped end surrounds the platform and the deck is over the offshore platform. The vessel is then ballasted down to transfer the deck onto the floating offshore platform. The width of the U-shape at the end of the vessel limits the maximum size offshore platform on which a deck can be installed by this method. Such a vessel has not been built and this method has not been used. 
     For a TLP (tension leg platform), the shallow draft of the structure allows it to be brought inshore to relatively shallow and protected water. This allows the deck to be built on the structure and the structure then towed to the installation site after completion. 
     Spar structures are typically deep draft structures that are six hundred to seven hundred feet tall and thus are incapable of being brought inshore into shallow, protected waters. 
     It can be seen that for spar structures, there is a need for an alternate method and apparatus for deck installation to that presently available. This need also applies in situations where the floating offshore structure and deck structures are built at different locations and it would be impractical to transport one or both to the same inshore site for installation of the deck onto the floating offshore structure. 
     SUMMARY OF THE INVENTION 
     The invention addresses the above needs. What is provided is a method and apparatus that eliminates the need for a derrick barge to lift the deck into place on the floating offshore structure. A connector is used to connect the transport barge to the floating offshore structure. The connector allows only relative pitch motions between the transport barge and floating offshore structure in response to sea states acting on the barge and floating offshore structure. The connector also allows disconnection while large forces are acting on the connector. One or more skidding girders attached to the legs of the deck support the legs of the deck above the skidding surface of the transport barge. A skidding surface on the girders, and complementary skidding surface on the surface of the transport barge and floating offshore structure, allow the deck to be skidded from the barge to the floating offshore structure. Once the deck is in the proper position on the floating offshore structure, the deck legs are lowered into contact with the floating offshore structure by removing spacers provided below the skid girders. The girders are then detached from the legs of the deck and removed. The deck may also be transferred from the transport barge to the floating offshore structure in a manner where relative pitch between the transport barge and floating offshore structure is not allowed. This is accomplished by also using a removable knee brace between the floating offshore structure and the transport barge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further understanding of the nature and objects of the present invention reference should be made to the following description, taken in conjunction with the accompanying drawings in which like parts are given like reference numerals, and wherein: 
     FIG. 1 is an elevation view of a frame row of a typical four-legged deck. 
     FIG. 2 is a perspective view of an orthogonally framed, four legged deck. 
     FIG. 3 is a plan view of the four legged deck supported on a circular spar vessel. 
     FIG. 4 is an elevation of a transport barge connected to the spar vessel with the deck skidded to a position over the connector, with a downward kink of the skidding surface due to the relative pitch being emphasized. 
     FIG. 5 is an elevation of the transport barge connected to the spar vessel with the deck skidded to a position over the connector, with an upward kink of the skidding surface due to the relative pitch being emphasized. 
     FIG. 6 is a section view through the skid girder taken along lines  6 — 6  of FIG.  5 . 
     FIG. 7 is an elevation of the deck and its support system while being fabricated onshore. 
     FIG. 8 is an elevation of the deck in the fabrication yard after the skid girders have been installed between the deck legs. 
     FIG. 9 is an elevation view of the deck in the fabrication yard after the deck has been lowered onto the skid girders. 
     FIG. 10 is an elevation view of the deck during a skidded loadout showing the deck partially on the transport barge. 
     FIG. 11 is a plan view of the transport barge moored to the spar vessel in preparation for docking and connection. 
     FIG. 12 is a plan view of the transport barge and the spar vessel docked, just before connection. 
     FIGS. 13A, B are elevation views detailing the lowering of the deck from its resilient skid girder runners onto the permanent deck leg supports built into the spar vessel. 
     FIG. 14 is an elevation view of an alternate embodiment of the invention. 
     FIG. 15 is a plan view of the alternate embodiment of the invention. 
     FIG. 16 is a section view through the axes of the swivel receiver for the swivel seen in FIG.  15 . 
     FIG. 17 is a section view through the axes of the swivel receiver and the swivel, showing the swivel seated in the receiver. 
     FIG. 18 is a plan view showing the rigging at the beginning of the brace installation onto the transport barge. 
     FIG. 19 is a plan view showing the rigging with the brace partly through its installation onto the transport barge. 
     FIG. 20 is a plan view of the brace installed on the transport barge. 
     FIG. 21 is an elevation of the brace attached to the transport barge at the swivels with the brace floating in a horizontal attitude. 
     FIG. 22 is an enlarged view of the area indicated by numeral  22  in FIG.  21 . 
     FIG. 23 is an elevation of the brace attached to the transport barge at the swivels with the brace lifted and supported near the stern in the tow attitude. 
     FIG. 24 is an elevation view of the transport barge approaching the spar vessel with the brace in an attitude too low for connection. 
     FIG. 25 is an elevation view of the transport barge joined to the spar vessel at the top connector. 
     FIG. 26 is an elevation view of the transport barge with the brace attached to the spar vessel. 
     FIGS. 27-29 illustrate an alternate embodiment of the invention wherein jacks are used instead of a wood runner. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Typical orthogonal framing for a four-legged deck  10  is shown in FIGS. 1, and  2 . The framing system for a spar vessel  12  delivers all of the deck load to four legs  14  located at the corners of a square  16  inscribed in the spar vessel  12  cylindrical shell as seen in FIG.  3 . Thus, the spar vessel diameter determines the deck leg spacing  17 . 
     FIGS. 4 and 5 show a transport barge  18  joined to a spar vessel  12  with a marine connector  20  such as that described in U.S. application filed on Jul. 31, 1997 and assigned Ser. No. 08/903,776. The transport barge  18  may be a launch barge and is provided with a wedge  23  on the stern rake that replaces the rocker arms normally present on a launch barge. The toggle nose  24  of the marine connector is built into the aft end of the wedge  23 . The toggle nose receiver  26  is attached to the spar vessel  12  with connection plates  28 . It should be understood that the toggle nose  24  may be mounted on the bow of the transport barge and that a launch barge is not necessarily indicated since any barge with sufficient stability and strength may be used. 
     FIG. 6 is a cross section through the skid girder  30  seen in FIGS. 4 and 5. Attached to the bottom of the skid girder  30  is a resilient runner  32  which may be formed from a piece of solid rubber  34  with steel plates  36  bonded to the top and bottom sides of the rubber. Sandwiched between the resilient runner  32  and the bottom of the skid girder bottom flange is a wood runner  38 . The function of the wood will be explained below. The bottom steel plate is the actual skidding surface. 
     FIGS. 4 and 5 show the deck  10  skidded partway between the transport barge  18  and the spar vessel  12 . As shown, the mid-span region of the skid girder  30  is located over the transverse pin of the marine connector  20 . Relative pitch between the vessels causes the top surface of the marine connector  20  to kink down, as seen in FIG. 4, and to kink up, as seen in FIG.  5 . The magnitude of the pitch is exaggerated to illustrate the problem solved by the resilient runner. The kinking would alternately crush the ends and then the middle of the skidding surface on the skid girder leading edge after a few cycles to failure. The resilient runner can distort to accommodate the cyclical kinking without damage. 
     The platform deck  10  is fabricated onshore in one piece as shown in FIG.  7 . The highly concentrated loading from the four deck legs  14  require a significant foundation system to fabricate and load out the deck  10 , indicated as pile supported caps  40  and load out ways  42 . During fabrication the deck legs  14  are supported on cups  44  that bear on the pile caps  40 . “Cups” are a term of art in the offshore construction industry used to indicate short sections of steel pipe with a diameter approximately equal to the deck legs  14 . The cups  44  support the deck  10  during most of the fabrication period. Near the end of the fabrication period, cambered skid girders  30  with the timber and resilient runners already attached beneath the skid girders  30 , are welded to the deck legs  14  as shown in FIG. 8. A predetermined gap less than the height of the cups  44  is left between the resilient runners  32  on the bottom of the skid girders  30 . 
     As seen in FIG. 9, immediately prior to load out onto a transport barge, the cups  44  are removed to lower the deck  10  onto the skid girders  30 , which are preferably cambered. The cups  44  are cut with a cutting torch in repeated circumferential passes. Each pass causes the cup to be shortened and the deck lowered by the kerf of the cut. The weight of the deck will straighten the cambered skid girder  30 , resulting in a uniform compression of the resilient runners  32  along their lengths. After the cups  44  are removed, a predetermined gap is left between the bottom of the deck legs and the top of the load out ways  42 . 
     FIG. 10 illustrates the transport barge  18  in position next to the load out ways  42 , with the deck  10  partly skidded onto the barge. The stern of the transport barge  18  may be grounded as shown so that only barge trim needs to be considered during the skid transfer to the barge. Alternately, a floating load out can be utilized. 
     FIG. 11 shows the transport barge  18  rigged to the spar vessel  12  with mooring lines  46  in preparation for docking. FIG. 12 shows the transport barge  18  and the spar vessel  12  docked, just before connection. The marine connector is engaged as described in the co-pending application referred to above. This illustrates that conventional mooring systems can be utilized to dock the marine connector  20  without any special effort. 
     Once the transport barge  18  and spar vessel  12  are connected, the platform deck  10  is skidded on the spar vessel  12 . After it is skidded to its final location the deck legs are located over receiving legs  48  that are built into the spar vessel  12 . FIG. 13A illustrates the situation before the lowering of the deck  10 . In order to lower the deck  10  onto the spar legs  48  and to recover the resilient runners  32 , the wood runner  38  is cut away in a series of passes with a beam chain saw or a hydro-blaster. A hydro-blaster is a device that produces a fine, high-pressure jet of water that is able to cut through steel plate or pipe. After several passes the wood will be reduced enough in thickness to unload the resilient runner  32 , let the camber back into the skid girders  30 , and lower the deck  10  by the gap thickness. FIG. 13B illustrates the situation after the lowering of the deck  10 . The resilient runner  32  may be recovered after the deck  10  has been lowered onto the spar vessel. 
     FIGS. 14 and 15 illustrate an alternate embodiment of the invention where a brace  50  is installed between the transport barge  18  and the spar vessel  12  to eliminate relative pitch between the two vessels. The brace  50  has two arms that extend from midship of the transport barge  18  to about mid depth on the spar vessel  12 . Since the spar vessel depth is about six hundred feet and the transport barge length is also about six hundred feet, the brace  50  is a large structure that requires special features for transport and connection. 
     The brace  50  has a first end  52  adapted to be connected to the spar vessel  12  and a second end  54  with each arm adapted to be connected to the transport barge  18 . Connectors are provided on the spar vessel  12  and transport barge  18  and will be described below. 
     The first end  52  tapers to a closed end having a T-shaped connector  53  constructed of large diameter pipes as seen in FIG.  15 . The transverse pipe forms the toggle nose for a marine connector such as that described in U.S. application filed Jul. 31, 1997 and assigned Ser. No. 08/903,776. The transverse pins, toggle mechanism, and hydraulic ram of the marine connector fit inside the “T”. The toggle nose receiver  55  of the marine connector is joined to the spar vessel  12  while the vessel is under construction in the shipyard. 
     The end of each arm of the second end  54  of the brace  50  is connected to a swivel  56  mounted in a swivel receiver  58  in the transport barge  18 , seen in FIGS. 15-17. The swivel receiver  58  is built into the side shell and one of the longitudinal bulkheads of the transport barge  18 . Each swivel  56  is provided with a reduced diameter or saddle-shaped section  57 . The swivels are readily attached and removed to allow normal barge operation when the swivels are not needed. 
     FIGS. 27-29 illustrate an alternate embodiment of the invention where a jack  66  is provided in each leg  14  of the deck  10 . This eliminates the need for the wood runner  38  described above. In this embodiment, the deck  10  is skidded into its final position on the spar vessel  12  on resilient runners  32  as described above. As seen in FIG. 27, jacks  66 , seen in the cutaway section of the deck leg  14 , are mounted on support plates  68  in the legs  14  so that the axes of the jacks and deck legs are coincident. As seen in FIG. 28, the jacks  66  are actuated to cause the jack rams  70  to lift the deck high enough to unload the resilient runner  32  and open a gap  72  between the bottom of the resilient runner  32  and the spar vessel  12 . The resilient runner  32  and skid girder  30  are removed and then the jacks  66  are used to lower the deck onto the spar vessel  12 , as shown in FIG.  29 . The jacks  66  are not recovered. 
     In operation, the deck  10  is skidded onto the transport barge  18  and tied down. If the brace  50  is to be used, the transport barge  18  travels to protected water for installation of the brace. FIGS. 18 and 19 illustrate the installation of the brace  50  on the transport barge  18 . The brace  50  is designed to float horizontally at the waterline. Winches pull the brace into position while tugs maintain back tension on the lines. FIG. 18 shows the brace in position to be pulled along side the transport barge. FIG. 19 shows the brace partway along side the transport barge. FIGS. 20 and 21 show the horseshoe shaped brace connector  60  received around the saddle-shaped section  57  on the swivel  56 . FIG. 22 shows the brace docked on the swivels and connected by lowering a stake  59  through the eyes on the ends of the horseshoe shaped brace connector  60 . 
     After the brace  50  is connected to the swivels  56  and is still floating horizontally, winch lines  62  are rigged to the brace  50  from the cantilever  64  provided on the barge  18 , as seen in FIGS. 21 and 23. The winch lines  62  are used to lift the brace  50  until it seats on the bottom of the cantilever  64 . Once lifted, a support  65  (one on each side of the barge) is swung out, and the winches lower the brace  50  a short distance onto the supports as seen in FIG.  23 . The brace  50  is then tied down on the supports  65  to secure it for transport to the installation site. 
     Once at the installation site the brace  50  is lifted slightly by the winches, the brace supports  65  are swung out of the way, and the winches lower the brace  50  into the water. Flooding chambers in the brace  50  are opened to cause the buoyancy of the brace  50  to change from neutral to slightly negative. As seen in FIG. 24, the winches and winch lines  62  then lower the brace  50  to a position lower than the position at which it will be connected to the spar vessel  12 . With the brace  50  in this out of the way position the top connection at the water line is made as seen in FIG.  25 . The top connection is made using the same procedures described above for the relative pitch option. The winches and winch lines  62  are used to pull the brace  50  upward until the T-shaped connector, which forms the toggle nose of the marine connector, docks in the toggle nose receiver  55  on the spar vessel  12  as seen in FIG.  26 . The connection is made by operating the toggle with hydraulic lines running up the brace  50  to the transport barge  18 . After the connection is made the winch lines  62  can be slacked off, leaving the configuration ready for skid on operations, as seen in FIG.  26 . 
     The skid on operation and deck lowering operation using the brace  50  are essentially the same as the operation conducted without the brace  50  where relative pitch is allowed between the transport barge  18  and the spar vessel  12 . After the skid on is completed, the brace  50  and barge  18  are disconnected by reversing the operations described above. 
     Although the description above refers to a spar vessel for installing a deck, it should be understood that the spar vessel is merely used as an example of a floating offshore structure and that the invention is applicable to other floating offshore structures. 
     Because many varying and differing embodiments may be made within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.