CROSS REFERENCE TO RELATED APPLICATION 0001 This application concerns a method of construction related to the invention disclosed in co-pending U.S. non-provisional application Ser. No. 10/325,122 filed on Dec. 19, 2002 and hereby incorporates by reference the disclosure of that prior application. This application is a Continuation-In-Part application of U.S. non-provisional application Ser. No. 10/350,729 filed on Jan. 24, 2003 which was based upon prior-filed provisional application No. 60/351,786 filed on Jan. 25, 2002 the priority of which is claimed.

BACKGROUND OF THE INVENTION 0002 1. Field of the Invention 0003 This invention relates to turret mooring systems in general and in particular to a method for constructing a very large turret in a moonpool of a vessel while at drydock. 0004 2. Description of the Prior Art 0005 Prior turrets for turret mooring systems have been built as an integral unit and installed with cranes into a moonpool of a vessel while the vessel is in drydock. With an extremely large turret, the turret is too large for most shipyards, because they do not have lifting cranes large enough so that the turret can be constructed externally of the vessel and then lifted into the moonpool for installation. An extremely large turret is one which is capable of supporting up to sixty risers or more. 0006 3. Identification of Objects of the Invention 0007 A primary object of this invention is to provide a method for installing a Very Large Turret in a vessel for a turret mooring system. 0008 Another object of the invention is to provide a method of installing a Very Large Turret in a moonpool of a vessel while the vessel is in drydock. 0009 Another object of the invention is to provide a method of installing a lower portion of a Very Large Turret in a moonpool of a vessel while the vessel is in drydock and after the vessel has been moved to the water and at dockside, lifting a top deck onto the lower portion of the turret followed by final construction steps.

SUMMARY OF THE INVENTION 0010 The objects identified above and other features and advantages are incorporated in a method by which a Very Large Turret (one that can support from 60 to 120 risers from sea bed wells) is constructed at least in part or entirely in the moonpool of a vessel while the vessel is in drydock. According to one embodiment of the invention, substantially the entire turret including the chain table, column and main deck is constructed while the vessel is in dry dock. A large surface machining tool is employed to machine upper rail surfaces on the moonpool and upper and lower and radial surfaces of the main deck while it is being assembled in place in the moonpool. The vessel is not floated out of drydock into the water from drydock until the turret construction is substantially complete in order to avoid using massive lifting cranes during construction of such a large turret. 0011 According to an alternative embodiment, the chain table and columns and an intermediate deck are constructed in the vessel's moonpool while the vessel is in drydock, but the main deck is fabricated at a fabrication yard and lifted onto the column or columns of the partially completed turret after the vessel has been floated out of drydock into the water and is dockside. A large lifting crane is used to lift the main deck in place.

BRIEF DESCRIPTION OF THE DRAWINGS 0012 FIG. 1 illustrates in a cut-away view (partially in cross-section) a preferred Very Large Turret installed in a moonpool of a vessel according to the method of the invention. The phrase, Very Large Turret, as applied to the turret of FIG. 1 implies a structural frame weight of, for example, 2,300 metric tons comprising a main deck (1,100 tons), six columns (660 tons 110 tons/column), and a chain table of 440 tons. The welding operations of the method include distortion control and thermal stress relief. Machining is performed with portable machining equipment to obtain large diameter machining accuracy. The turret is massive by conventional standards, capable of supporting from 60 to 120 risers. 0013 FIG. 2 shows a vessel in drydock with a moonpool installed and blocks on the drydock floor set and leveled. Vessel blocks are shorter than turret support blocks. A T-flange is installed at the top of the moonpool for mounting of rails for an axial bearing comprising a roller/rail arrangement. 0014 FIG. 3 shows the chain table being constructed in three 120 segments on the turret support blocks. 0015 FIG. 4 shows a surface machining tool placed on a frame support for machining rail surfaces on a T-flange of the moonpool. 0016 FIG. 4A illustrates the T-flange and roller rail surfaces for axial and radial bearing arrangements for the main deck. 0017 FIG. 5 shows columns extending upwardly from the chain table and shows the machined rails installed on the T-flange of the moonpool. 0018 FIG. 6 illustrates a first access platform installed at mid-height of the columns. 0019 FIG. 7 illustrates bracing members installed between the columns and the moonpool for aligning and bracing the columns. 0020 FIG. 8 illustrates installing a second access platform with bracing on the columns. 0021 FIG. 9 illustrates installation of alignment jacks on a shelf of the moonpool. 0022 FIGS. 10-13 illustrate assembly of the main deck which includes a hub with six spokes (see especially FIG. 13) and mounting of the main deck to the support columns. 0023 FIG. 14 illustrates machining of three main deck bearing surfaces with fine alignment with jacks between the main deck and moonpool shelf after initial bracing has been removed and with vertical machining space provided by turret support blocks being higher than vessel support docks. 0024 FIG. 15 illustrates using jacks to raise the entire turret from the drydock floor. 0025 FIG. 16 illustrates the removal of the turret support blocks from the drydock floor, installation of the vertical bearing wheel assembly, and assembly of a bearing support structure about the main deck. 0026 FIG. 17 illustrates installation of the radial load bearing assemblies and installation of I tubes for the risers. 0027 FIG. 18 illustrates, after the vessel has been floated from drydock to dockside, the installation of the swivel stack assembly, the torque tube assembly and pull-in deck support columns mounted on top of the main deck. 0028 FIG. 19 illustrates installing skid-mounted manifold systems on the main deck. 0029 FIG. 20 illustrates installing a pull-in deck between the vessel and the torque tube assembly. 0030 FIGS. 21-24 depict steps of an alternative construction method incorporating use of a higher lift capacity crane. In this alternative, the vessel support blocks are taller than the turret support blocks. After the chain table, support columns and intermediate deck have been assembled in drydock, lift jacks and lifting rods are used to support the partially completed turret from the moonpool shelf, after which the vessel is floated out of drydock into the water and a pre-constructed main deck is lifted at dockside onto the columns of the partially completed turret.

DESCRIPTION OF PREFERRED EMBODIMENTS 0031 Embodiment with Main Deck of Turret Constructed while the Vessel is in Drydock 0032 The preferred arrangement of the very large turret 100, to be constructed according to the methods described herein, is described in detail in the above-mentioned U.S. application Ser. No. 10/325,122 and incorporated by reference herein. FIG. 1 shows a side view of one representative embodiment with a cut-away cross-section of a vessel 30 in which the turret 100 is rotatably supported on bearing surfaces within a moon pool 5 of the vessel 30 by a vertical bearing arrangement 12 and a radial bearing arrangement 13 comprising rail-roller arrangements. The vertical bearing rollers 12 and radial bearing rollers 13 are seen more clearly in FIG. 4A. 0033 The main deck 1, to which the vertical roller bearings 12 and radial roller bearings 13 provide rotatable support with respect to the vessel 30, is designed to have a predetermined flexibility as described in the above-mentioned U.S. application Ser. No. 10/325,122 so as to accommodate vessel sagging and hogging caused by rough seas. The main deck 1 carries chain table 3 by means of support columns 2. As explained in the above-mentioned patent application, a single column could be substituted for the six support columns as illustrated herein. Chain table 3 supports three sets of anchor legs 20 via three sets of chain supports 19. Risers 18 run from sea bed locations (e.g. subsea wells) through bend stiffeners 17 and through riser or I tubes 16 which extend through chain table 3 and access platform 6 to the main deck 1. Riser hang-off devices 24 are carried by main deck 1. Piping (not shown) from risers 18 extends via stations 17 for manifolding and pig launching/receiving and ultimately to a swivel stack and torque tube 11. A pull-in deck 8 is supported by columns 4 from main deck 1. A winch and sheave arrangement (not shown) is mounted on deck 8 for pulling in anchor legs 20 and risers 18. A torque arm 6 is provided between the vessel 30 and torque tube 11. 0034 The turret 1 of FIG. 1 is very large. For example; its structural frame weight for one design is 2,300 metric tons comprising a main deck 1 of 1,100 tons, six columns 2 each weighing 110 tons each for a total of 660 tons and a chain table 3 of 440 tons. Such size makes it practically and commercially impossible to build the entire turret at a shipyard and later lift it onto a floating vessel. The difficulty faced is that most shipwards of the world do not have lifting cranes with the capacity to lift an assembly of such massive weight. The first method described here provides steps for constructing the turret in the moonpool of the vessel while at drydock in a shipyard. 0035 FIG. 2 shows the vessel 30 in drydock supported on the drydock floor 105 by blocks 110. A moonpool 5 is constructed in the vessel 30, and a T-flange 27 is installed on an upper peripheral surface of a cylindrical extension of the moonpool 5. Refer to FIG. 4A for a larger view of rail 27 and its structural relationship to moonpool 5. Blocks 120 are set and leveled on the floor 105 of the drydock. Blocks 120 are higher than are blocks 110 so that when the turret 100 is completed, sufficient space exists above T-flange 27 to machine surfaces on the main deck 1, as will be explained in more detail below. 0036 FIG. 3 shows the first, second and third sections 131, 132, 133 of the chain table 3 lowered onto blocks 120 by a hook by means of a crane (not shown). The chain support blocks 19 are provided for each section. The bases 2B for columns 2 are mounted on each chaintable section. Bell mouths and bend stiffeners for passage of risers are provided for each chain table sector. 0037 Next, as illustrated in FIGS. 4 and 4A, the rails 26B of flange 27 of the moonpool 5 are machined with a surface machining tool 200 which is supported on a truss frame support 220 secured to a shelf of moonpool 5. The surface machining tool 200 is an extra large circular self-leveling mill such as manufactured and operated by Self Leveling Machines, Inc. FIG. 4A illustrates more clearly the rails 26B which are machined by machine tool 220. (Alternatively, the T-rail flange can be machined prior to installation of rails 26B). 0038 In FIG. 5, the machined rails 26B are shown on the T-flange 27, and columns 2 are erected from the bases 2B as illustrated in FIGS. 3 and 4. 0039 As illustrated in FIG. 6, an access platform 6 is installed on columns 2, and as shown in FIG. 7, bracing members 76 are provided between the column 2 and the moonpool 5 to provide construction stability. Such bracing members 76 are removed after the main deck is later constructed and supported with respect to the moonpool by the bearing arrangement. FIG. 8 shows an upper access platform 6A being installed with column to column bracing 77 providing alignment. A support member 230 is installed which will become part of the hub of the main deck. The support member 230 is installed on deck 6A prior to its installation onto the turret. 0040 FIG. 9 shows alignment jacks 78 installed about an upper shelf of moonpool 5. FIG. 10 shows sections of main deck 1 being lowered for welding on top of columns 2. Jacks 78 assist in the leveling of main deck section 1A, 1B and 1C on columns 2. The main deck 1 is being constructed in sections atop the vessel while the vessel remains in drydock in order to obviate the need for massive lifting cranes. FIG. 11 is a top view of main deck 1 with sections 1A, 1B and 1C in place on top of column 2. FIG. 12 shows a hub 32 and spokes 31 installed, and FIG. 13 shows three more spokes installed to complete the construction of main deck 1. Riser hangoffs 24 are installed in the outer ring of main deck 1. Such hangoffs 24 are aligned with corresponding openings 24A, 24B in deck 6 and chain table 3. During installation, the sections 1A, 1B, 1C are leveled, welded together, and machined before being lowered onto the columns 2 and welded thereto. 0041 FIG. 14 illustrates that the surface machining tool 200 is used again, this time to machine an outer rail 26R of main deck 1 and vertical bearing rail surfaces 26U and 26B (See FIG. 4A) with vertical clearance of main deck 1 achieved, because turret support blocks 120 are vertically taller than are vessel support blocks 105. (Alternatively, the support surfaces for the rails 26U, 26B and 26R may be machined prior to installation of such rails on the support surfaces). In other words, the support blocks 120 provide sufficient vertical clearance beneath main deck 1 to allow tool 200 to reach below deck 1 and above rails 26B for milling operations. The entire turret 100 weight continues to rest on blocks 120 on drydock floor 105. 0042 Next, as shown in FIG. 15, alignment jacks 78 raise the entire turret 100 (maindeck 1, column 2, and chain table 3) enough to provide vertical clearance below chain table 3 to remove blocks 120. 0043 FIGS. 16 and 17 illustrate installation of vertical rollers 12 between main deck 1 rails 26B and assembly of bearing support structure 4 between the vessel moonpool and the outer ring of main deck 1 and installation of radial load rollers 13. FIG. 4A shows more detail concerning the placement of bearing support structure 4 and the rollers of the vertical and radial bearing assemblies. After the bearing support structure 4 is installed, uplift rollers 14 and radial rollers 13 are installed. The jacks 78 are now removed, and the entire turret 100 is rotatably supported on the moonpool 5 by vertical and bearings 12. I tubes 16 are installed via riser hangoffs 24 and corresponding openings in deck 6 and chain deck 3. FIG. 17 shows the riser I tubes 16 installation complete. At this stage, the vessel can be floated out of drydock into the water 500 as shown in FIG. 18 and the remainder of the turret constructed at dockside with lifting cranes suited for the purpose. 0044 FIG. 18 shows the installation of the swivel stack 11A and torque tube 11B of the swivel stack and torque tube assembly 11 (See FIG. 1) and pull-in deck support columns 24. FIG. 19 illustrates skid-mounted manifold systems 17 mounted on main deck 1 for connection to risers where the risers are pulled into the riser I tubes 16 after the vessel moves to a mooring location in the sea. 0045 FIG. 20 illustrates the construction of pull-in deck 8 on support columns 2A, and as illustrated in FIG. 1A torque arm 6 is later installed between the vessel 30 and the torque tube 11B. 0046 The installation of the turret 100 is now complete while at dockside in the water. The vessel can be readied for sailing to an operative location in the sea where the turret serves to moor the vessel and to transfer hydrocarbon products to the vessel while the vessel is free to weathervane with respect to the turret. 0047 Alternative Construction ProcedurePartial Turret Construction with the Vessel at Drydock 0048 An alternative construction procedure for installing a very large turret in a vessel provides for partial construction of the turret (the lower part) with the vessel at drydock and fabricating the main deck at a separate fabrication yard. The vessel is floated out of drydock into the water, and while at dockside, the main deck is lifted with a large capacity crane and assembled to the lower part of the turret. 0049 FIGS. 1-8 as described above are generally identical for this alternative procedure except that the blocks 120 beneath the chain table 3 are lower in height than the vessel support blocks 110. For this alternative construction procedure, different construction steps follow thereafter and are described first by reference to FIG. 21. Lift jacks 300 are installed in moonpool 5 with lifting rods 310 connected to chain table 3 so that, as shown in FIG. 22, the lower portion 100 of the turret 100 can be supported by the lift jacks 300 and lifting rods 310, and the support blocks 120 removed. The vessel 30 can be floated from drydock and into the water 500 where crane lifting capacity is large enough to lift the main deck in one piece (as described below). It is generally advantageous to move the vessel 30 out of drydock as quickly as possible to minimize drydock charges for vessel fabrication and modification. 0050 As shown in FIG. 23, the main deck 1 has been assembled in a fabrication yard, and there the two vertical facing surfaces 26U, 26B and the radial surface 26R have been machined on it. (Alternatively, the support surfaces for such rails have been machined and the rails installed on such surfaces.) Such surfaces are described by reference to FIG. 4A and in the above-mentioned U.S. patent application Ser. No. 10/325,122 incorporated by reference herein. The same or similar surface machining tool 200 (See FIGS. 4 and 14) is used to machine the moonpool rails and the deck bearing rails. (Alternatively, the tool 200 is used to machine the T-flange surfaces 27.) As shown in FIGS. 23 and 4A, the vertical load rollers 12 are installed beneath the vertical facing machined surfaces and rails 26U on the main deck 1 and the machined surfaces and rails 26B of the flange 27 of moonpool 5. The main deck 1 is now rotatably supported by vertical load rollers on the rails 26B of the moonpool 5. The lower turret 100 continues to be supported independently of main deck, because it is supported by jacks 300 and lift rods 310 to chain table 3. Sufficient vertical height exists for the installation of main deck 1 on the moonpool 5, because turret support blocks 120 are lower in height than vessel support blocks 120, thereby placing the lower turret 100 lower in the moonpool during initial construction. 0051 As shown in FIG. 24, the lower turret 100 is lifted up by the lift jacks 300 and lift rods 310 until the columns 2 are in contact with bottom surfaces of the spokes of main deck where they are welded thereto. The entire turret is now supported vertically by the load rollers 12 on upward facing rails 26B of the moonpool 5. The lifting jacks and lifting rods are then removed. 0052 The final assembly steps for this alternative method are identical to those described above by reference to FIGS. 16-20, but they are done with the vessel out of drydock and in the water at dockside.