US3279404A

US3279404A - Floating mooring system

Info

Publication number: US3279404A
Application number: US332225A
Authority: US
Inventors: George T Richardson
Original assignee: Offshore Co
Current assignee: SONAT OFFSHORE DRILLING Inc
Priority date: 1963-12-20
Filing date: 1963-12-20
Publication date: 1966-10-18
Anticipated expiration: 1983-10-18
Also published as: NL6400904A

Description

Oct. 18, 1966 s. T. RICHARDSON FLOATING MQORING SYSTEM 5 Sheets-Sheet 1 Filed Dec. 20, 1963 INVENTOR fiEORGE EYE/WW0 MW, a @4044, k/84 ATTORNEY;

Oct. 18,1966 cs. T. RICHARDSON FLOATING MOORING SYSTEM 5 Sheets-Sheet 2 Filed Dec. 20, 1963 INVENTOR 5206 GE flew/mayo ZJMJ%y MM ATTORNEY Oct. 18, 1966 Filed Dec. 20, 1963 G. T. RICHARDSON 3,279,404

FLOATING MOORING SYSTEM 5 SheetsSheet 5 INVENTOR OHGE Z'fi/c/mwswy ATTORNEYS Oct. 18, 1966 G. T. RICHARDSON 3,279,494

FLOATING MOORING SYSTEM Filed Dec. 20, 1963 5 Sheets-Sheet 4 III,

INVENTOR 62-04651 Fwy/#0504 MMJ Z MM ATTORNEYS Oct. 18, 1966 GT. RICHARDSON FLOATING MOORJVZNG SYSTEM 5 Sheets-Sheet 5 Filed Dec. 20, 1963 INVENTOR 620/? 66 I 1? MAW/2050A BYJM 1/ ATTORNEYS United States Patent 3,279,404 FLOATING MOORING SYSTEM George T. Richardson, Houston, Tex., assignor to The Offshore Company, Houston, Tex., a corporation of Delaware Filed Dec. 20, 1963. Ser. No. 332,225 9 Claims. (Cl. 114-05) This invention relates to the mooring of a floating structure in a manner which permits orientation of the structure to minimize the effects of wind and water on the structure and in particular to the mooring of floating structures from which underwater operations are conducted.

In the field of off-shore oil well drilling, it is usually desirable, where possible, to conduct the drilling operations from a platform which is supported by a steel framework from the underwater bottom. A variety of structures for this purpose have been proposed and used, including those which are permanently secured to the bottom and those which are selectively submersible, so as to be later removed from location. Obviously, in deep water, for example over 100 feet, such supports become very expensive and complex due to the major portion of the mass being supported at the upper end of a relatively limber structure. This type of structure is comparable to an inverted pendulum and is quite susceptible to earthquake damages.

In deep water, therefore, floating structures such as floating platforms or floating drilling ships, have been proposed and used to support the drilling equipment. Obviously, the difiic-ulties in drilling from floating structures are those of maintaining the structure in a fixed position under the influence of wind, waves and tide.

Conventionally, laterally extending anchor lines are utilized to restrain motion of the structure, but these lines have not been completely satisfactory in providing adequate resistance to movement, particularly movement at right angles to the drill string. It is obvious that vertical movement of a floating support structure can be compensated for by a number of simple mechanical expedients, but that horizontal movement or vertically twisting movement of the structure as a result of pitch, roll and yaw can be tolerated only in very small amounts.

Conventional systems for the deep water mooring of floating structure for supporting drilling equipment are described more in detail in, for example, United States Patent Nos. 2,512,783, 2,986,888 and 2,987,892. In each of these arrangements a plurality of mooring buoys are laterally spaced from the floating structure and are anchored to the bottom with suitable anchors and lines. The structure, generally in the center of the spaced buoys is moored fore and aft to the ring of buoys by means of cables. If the structure is a ship, its bow is generally placed in the center of the buoys. While this arrangement holds the ship over a fixed point and reduces pitch, yaw and roll caused by wind and wave action, it has not been found completely satisfactory. One disadvantage lies in the difliculty in orienting the ship with respect to wind and waves. Usually it will be desirable to point the bow of the ship into the wind to maintain roll of the ship at a minimum. With a given lateral anchor type of mooring the adjusting of ship direction, or orientation, is limited to something less than 180 degrees depending on the number and arraangement of lateral mooring lines and in addition is time consuming to effect. This is a serious disadvantage when the ship is being used for drilling operations because drilling must be discontinued during changes in orientation with serious loss of time. Since the movement of the ship in a direction transverse to the drill string should be kept at a minimum during a drilling 3,279,404 Patented Oct. 18, 1966 ice operation, any but slight changes in wave and wind direction should be corrected for by orienting the ship.

In another type of mooring system, disclosed in United States Patent 2,699,321, a floating oil storage vessel is provided with a vertical well through which a rigid shaft structure extends into anchored engagement with the submerged bottom. The floating structure is not connected to the shaft and is thereby free to move both vertically and rotatably with respect to the shaft structure. Drilling is performed through the shaft structure from a drilling rig mounted on top of the shaft structure. While this mooring arrangement permits the floating vessel to be oriented with respect to the action of wind and Water Without moving the anchor means it requires a rigid structure extending to the submerged bottom and thus possesses the disadvantages of high cost, complexity and instability already referred to.

The primary object of the present invention is to overcome, to the large extent, the above-mentioned disadvantages inherent in the rigid, bottom-engaging type of support and in the conventional anchored type of support. Broadly, the invention accomplishes this object by providing a rigid mooring member which is supported and carried by a floating vessel in a vertical well through the vessel. The mooring member is restrained against vertical movement relative to the vessel, but is freely rotatable about its own vertical axis. In use, the mooring member is held in a fixed position above a desired point on the submerged bottom by attaching it to a plurality of generally radical mooring lines which extend downwardly and outwardly to anchor points on the bottom. The vessel is then rotated about the fixed mooring member by means by propeller-type thrusters at the ends of the vessel to a position in which wind and wave action on the vessel are at a minimum. Preferably; the mooring member has a vertical how so that underwater operations of any nature, such as diving, salvage, drilling and well completion, can be conducted from the vessel through the bore. Equipment for performing these operations, such as a drilling rig, is mounted on the vessel above the mooring member.

A further object of the invention is to provide a float-- able vessel with a vertical well, a mooring member carried by the vessel within the well and mounting means for the mooring member for permitting relative rotation between the member and the vessel about a vertical axis through the member so that the vessel may be readily oriented 360 degrees about the member.

It is a further object to provide a method of mooring a floating vessel of the above type by a novel combination of steps for establishing a pattern of mooring lines extending outwardly and downwardly from the mooring member.

The invention will be further understood from the following detailed description in conjunction with the drawings in which:

FIGURE 1 is a schematic prespective view of a drilling ship which embodies the principles of the present invention;

. FIGURE 2 is a schematic top plan view, on a reduced scale, of the drilling ship of FIGURE 1;

FIGURE 3 is a fragmentary elevational view, taken generally along the line 33 of FIGURE 2 of the mooring member and its arrangement within the drilling ship;

FIGURE 4 is a top plan view, partly broken away, of the mooring member and its arrangement in the drilling shi l IGURE 5 is a fragmentary elevational view, on an enlarged scale, of part of the mounting arrangement for the mooring member looking in the direction of the arrows 5-5 in FIGURE 3;

FIGURE 6 is an elevational view on an enlarged scale and partly broken away, of one of the fairleads on the mooring member; and

FIGURES 7-13 are schematic views showing the steps of anchoring the ship of FIGURE 1.

Referring to FIGURES l and 2, there is shown a drilling ship 10 having an elongated hull 12 of conventional construction and carrying a derrick 14 and conventional hoisting equipment 16 for transferring supplies and equipment to and from the ship. The center portion 18 of the top deck is provided with various handling equipment and storage facilities (not shown) for pipe and other materials employed in under water drilling and well-completion operations. The stern portion of the top deck is provided with a landing platform 21 for helicopters.

A derrick support structure is mounted on the center deck portion 18 over a hollow, cylindrical mooring member which is constructed and arranged according to the principles of the invention and which is illustrated in detail in FIGURES 3-6. The support structure, which also forms part of the mooring arrangement, includes four upright support columns 22 disposed at the corners of a square and firmly secured to the deck 18 of the ship 10. The upper ends of the support columns 22 are secured to a horizontal support frame on top of which is slidably mounted a derrick platform 24 of conventional construction. The derrick 14 is secured to the derrick platform 22 and is movable therewith in a horizontal plane between a position over'the mooring member 20 and a position over one side of the ship 10.

To support the derrick 14 and derrick platform 22 over the side of the ship 10 there is provided a hinged cantilever structure which can be swung downward to jut over one side of the ship. As seen schematically in FIGURE 1, this structure includes a pair of spaced generally triangular support members 26 which are pivoted to two of the support columns 22 at 28 for rotation about a horizontal axis extending longitudinally of the ship 10. In the position shown in FIGURE 1 the support members 26 have been rotated downwardly over one side of the ship into engagement with a pair of fixed upstanding frames 30 which are secured to the deck 18 of the ship. The upper edges of the members 26, together with cross members 32 provide a fiat supporting frame onto which the derrick platform 24 may he slid so as to be in a position to service Well heads which are located above water level.

Referring now to FIGURES 3 and 4, the mooring member 20 which constitutes the primary feature of the invention, is shown in its operative position in a vertical well 33 which is located on the longitudinal axis of the ship 10. The well 33, which extends completely through the hull 12, is defined by a vertical cylindrical side wall 34 joined at its upper end to the top deck 18 and at its lower end to the ships bottom 36. Intermediate its ends the wall 34 is joined to various framing members 37 (FIGURE 4) inside the hull 12.

4 The mooring member 20 includes a vertical cylindrical sleeve 38 which extends completely through the well 33 in spaced relationship with the wall 34 and an outwardly extending upper flange portion 40 which overlies an annular portion of the deck 18 adjacent the well 3.3. The flange portion 40 includes three horizontal vertically spaced

annular plates

44, 46 and 48 which are disposed around the upper portion of the sleeve 38, the upper and lower plates 44 and 48 being secured to the sleeve 38 as by welds 50. The intermediate plate 46 which has an internal diameter greater than the other two plates is suspended from the upper plate 44 by three concentric cylindrical plates 52, 54 and 56.

The mooring member 20 is mounted for rotation about its own axis and supported against downward movement by means of a plurality of circumferentially spaced double-flanged rollers 58 disposed between the flange 40 and the deck 18. The rollers 58 are vertically mounted for rotation about horizontal axes within a cage formed of inner and outer vertical

concentric plates

60 and 62 which are coaxial with the mooring sleeve 38. As seen best in FIGURE 5, the

plates

60 and 62 are held in concentric relationship by means of a plurality of spacer sleeves 64 radially disposed between the plates and a plurality of bolts 66 passing through the sleeves 64 and the plates.

Each roller 58 is journalled on a fixed axle 68 which extends in a radial direction through a hole in each

plate

60, 62. An annular reinforcing plate 70 is welded to the exterior of each plate around each axle hole. The axles 68 are retained in place by means of rectangular keeper plates 72 bolted to the exterior of the reinforcing plates '70 and residing in slots in the peripheries of the axles 68.

The tops of the vertical rollers 58 are engaged by a downwardly facing circular rail 74 which is fixed to the lower surface of the mooring member flange 40. As seen in FIGURES 3 andS, the rail 74 is secured to the intermediate horizontal plate 46 at a location intermediate its inner and outer edges. The rail 74 is generally I-shaped and consists of a running flange 76 engaging the rollers 58, a base flange 78 and a web 80 integral with the

flanges

76, 78. The base flange 78 is clamped to the plate 46 in any suitable manner as by means of a plurality of spaced brackets 82. In the construction illustrated, each bracket 82 includes a vertical bracket plate 84 of generally inverted U-sha-pe which extends through slots in the horizontal plate 46 and straddles the rail 74. Each bracket plate 84 has welded thereto a T-shaped member 86 which tightly engages the web 80 and base flange 78 of the rail 74 to hold the same in place.

The vertical rollers 58 ride on top of a horizontal circular rail 88 which is fixed to the deck 18 in any suitable manner. As shown, the rail 88 rests on top of a support ring 90 which is T-shaped in cross section. The ring 90 is welded to the deck 18 and the rail 88 is clamped to the ring 90 by means of brackets 92 which are similar to the brackets 82 employed with the upper rail 74.

Referring again to FIGURES 3 and 4, it will be seen that the mooring member 20 carries an outwardly facing circular rail 94 which engages a plurality of deck-supported horizontal rollers 96 in a manner to prevent lateral movement of the mooring member 20 relative to the hull 12. The rail 94 is suspended from the lower plate 48 of the mooring member flange 40 by means of suitable brackets 88. The horizontal rollers 96 are carried in a cage which includes upper and lower annular plates 100 and 102 welded along their outer edges to a vertical cylindrical plate 104, which in turn is mounted on the deck 18 by means of a plurality of upstanding triangular bracket plates 106. The inner edges of the annular plates 100, 102 are welded to an inner cylindrical plate 108, and radially disposed vertical reinforcing plates are Welded between the upper edges of the inner and outer

cylindrical plates

104 and 108. Each roller 96 is rotatable on a fixed vertical axle 112 which extends through holes in the annular plates 100, 102 and which is held in place by keeper plates not shown) in the same manner as the axles 68 for the rollers 58.

The top plate 44 of the mooring member flange 40 carries a circular hold-down rail 114 which is secured to the upper surface of the plate 44 near its outer edge in any convenient manner. Each of the upstanding support columns 22 carries four spaced vertical hold-down rollers 116 which engage the top of the rail 114 and thereby prevent any upward movement of the mooring member 20. The rollers 116 are mounted on fixed axles 120 which are secured in suitable cages in generally the same manner as are the axles 68 for the rollers 58.

The top plate 44 of the mooring member flange 40 is also provided with four double-drum electrically operated anchor winches 122 which include a cable traversing mechanism 124 synchronized with the winch drive. Each winch is secured to the plate 44 adjacent a pair of holes 126 through which anchor lines 128 from each drum pass downwardly between the sleeve 38 and the side wall 34 of the well 33.

The lower end of the mooring member sleeve 38 is provided with eight equi-spaced fairleads 130, one for guiding each anchor line 128. Each fairlead 130 includes a flanged pulley wheel 132 mounted on a horizontal axle 134 between two vertical bracket plates 136. As seen in FIGURES 3, 4 and 6, the vertical plates 136 are secured to the lower end of a sleeve assembly which permits the pulley wheel to swivel in a horizontal plane. The sleeve assembly includes an inner sleeve 138 rotatably mounted within an outer sleeve 140 by means of upper and

lower bushing elements

142 and 144 and upper and

lower end plates

146 and 148.

The outer sleeve 140 is rigidly mounted between two

annular plates

150 and 152 which are secured along their inner edges to the mooring member sleeve 38 near enough to its lower end that the pulley wheels 132 are disposed below the ship bottom 36. A plurality of upstanding triangular plates 154 are welded to the top of the upper annular plate 150 and to the mooring member sleeve 38 to reinforce the fairlead mounting. The anchor line 128 for each fairlead 130 passes through the bore of the inner sleeve 138 and between the pulley wheel plates 136 where it engages the pulley wheel 132 along the portion of the periphery which faces the mooring member sleeve 38. A small roller 156 mounted on a horizontal axle 158 between the plates 136 holds the anchor line 128 in engagement with the pulley wheel 132.

FIGURES 7-13 illustrate schematic-ally the procedures and equipment which are employed in running and setting anchors for the mooring member 20. The eight anchor lines 128 which may be, for example, 2-inch diameter cable, remain threaded through the mooring member flange 40 and the fairleads 130, once the assembly of the parts described above have been completed. When the lines 128 are not in use their free ends, usually with buoys 160 attached, are looped around the bottom of the ship and carried on the deck 18 as seen in FIGURE 7. In this position, of course, almost the entire length of the lines 128 is carried on the drums of the winches 122. For simplicity of illustration only, one anchor line 128 is shown in FIGURES 7-13, but it will be understood that all eight lines are handled in the same manner.

When the ship 10 has arrived at a site where it is to be moored, the free ends of the anchor lines 128, with their buoys 160 attached by means of a shack-1e 161, are taken from the deck 18 by means of the hoisting equipment 16 (FIGURE 1) and placed in the water, as seen in FIG- URE 8. Each line 128 is then handled in sequence by a supply boat 162 in the manner illustrated in FIGURES 913. The deck of the supply boat carries an A-frame 164, a horizontal stern roll 166, and a double-drum winch 168. An anchor 170, having a shackle 172 and a short length of anchor chain 174 connected thereto, is also stowed on the deck of the supply boat 162. The shackle 172 connects with the free end of a trip wire 175 which is carried on drum No. 2 of the winch 168.

The supply boat 162 first picks up the buoy 160 and hauls it on deck, as seen in FIGURE 9. The supply boat winch 168 is provided with a cab-1e 176 for this purpose. Next, a runner 178 from drum No. 1 of the winch 168 is attached to the anchor shackle 172 and by means of the A-frame 164 lifts the anchor 170 over the stern of the boat, as seen in FIGURE 10. At the same time the men on the boat 162 disconnect the buoy 160 from the shackle 161 and attach the free end of the anchor chain 174 thereto. When the anchor 170 is hanging free of the boat 162, as seen in FIGURE 10, the runner 178 is disconnected from the shackle 172. The anchor 170 is then lowered to the submerged bottom 180 by drum No. 2, the

6 trip wire 175 and the stern roller 166, as seen in FIG- URE 11.

The end of the trip wire 175 opposite the anchor carries a shackle 182 which, as seen in FIGURE 12, is connected by the men on the boat 162 to the buoy 160 when the trip wire has been unwound from drum No. 2. This is accomplished in a manner known in the art with the aid of

auxiliary runners

184 and 186 which are employed to hold the trip wire on board while the connection is made. The buoy 160 is then cast free of the boat 162, as seen in FIGURE 13.

The running of seven more anchors, one for each of the other seven anchor lines 128 is carried out in sequence in the same manner as that just described. Usually, the anchors will be equally spaced along the circumference of a large circle of, for example, a diameter of 5000 feet. After all of the anchors have been placed, the winches 122 on the mooring member 20 in the ship 10 are operated Y to wind in their cables 128 to a predetermined tension. This positions the mooring member 20 at the center of the circle of anchors and restrains it against displacement in both horizontal and vertical planes.

In operation of the mooring system the hull 12 of the ship 10 is freely rotatable about the mooring member 20 which is held stationary by its anchor lines 128. Pull on the lines 128 is measured by strain-gage recorders which receive signals from gages on pawls in the drive for the winches 122. The mooring forces, as well as the ship motion, are minimized by keeping the ship 10 headed into the sea. This is accomplished with two electric propellertype thrusters, one located at each end of the ship. The stern thruster, illustrated at 184 in FIGURE 2, occupies about the same position as an outboard motor. The bow thruster, illustrated at 186, is located in a tranverse tunnel 188 through the bow.

The direction and speed of rotation of the

thrusters

184 and 186 are controlled automatically from an instrument console which continuously receives direction signals from a gyroscope system. The console is designed to produce output signals which are proportional to the difference between the direction signals and an input signal which is set manually for a desired heading. The output signals govern the output of diesel-driven direct current generators located in the power plant room of the ship. The current from the generators is conducted to the motors of the

thrusters

184 and 186. The system also includes means for sensing and controlling rate of change of direction and feedbacks thus preventing the hull from overswinging or hunting.

A very flexible system is achieved by controlling the thruster motors by excitation of the generators, because the motors can then be run in either direction at any speed from zero to maximum. In addition, the stern thruster can be operated manually and rotated 360 about its vertical axis.

After the mooring member 20 has been anchored above a desired site, the

thrusters

184 and 186 are operated to rotate the hull 12 of the ship 10 to a position in which the effects of wind and water on the ship are minimized. The automatic control system for the thrusters then maintains this heading which has been set on the control console. The system also includes instruments which measure the amount and direction of any lateral displacement of the mooring member 20 from the dead center position. When this displacement is not correctable by changing the heading of the hull 12, the winches 122 may be operated to pull the mooring member 20 back to its original position. While the hull 12 is rotatable 360 about the mooring member 20 without being obstructed, the electric, air and any other lines leading to equipment which may be used on the member 20 will require unplugging and replugging about every 120 rotation.

When the mooring member 20 has been anchored at a location in the manner described above, drilling into the submerged bottom 180 and other underwater operations may be conducted from the deck 18 of the ship through the bore of the mooring member 20. It will be appreciated that the draw works hook is suspended by the derrick 14 along the vertical axis of mooring member so that the hull, which carries the derrick, may be rotated to any position without affecting alignment of the drilling and completion equipment with the well hole.

It will thus be appreciated that the invention provides a floating mooring system which accurately and positively locates a vessel above a predetermined submerged location and which permits the vessel to be oriented with respect to the wind and wave Without disturbing the anchor pattern. The system is particularly applicable to floating drilling vessels because underwater operations conducted through the mooring member 20 need not be discontinued when the hull 12 is rotated. While a specific embodiment of the invention has been described and illustrated it is not intended that the details thereof be limiting except as they appear in the appended claims.

This application describes and claims an improvement on the apparatus of application Serial No. 184,466, filed April 2, 1962.

What is claimed is:

1. In combination with a floatable structure having a vertical well therein open through the bottom of said structure: a vertical cylindrical member supported by said structure within said well; first vertical roller means supporting said cylindrical member on said structure for rotation about the vertical axis of said cylindrical member; second vertical roller means restraining said cylindrical member from upward movement relative to said floatable structure and horizontal roller means restraining said cylindrical member from lateral movement relative to said floatable structure.

2. Apparatus as in claim 1 wherein each of said roller means includes a plurality of rollers disposed in a circular pattern and in engagement with a circular rail.

3. Apparatus as in claim 1 wherein said cylindrical member has a vertical bore for permitting underwater operations to be performed from said floatable structure through said bore.

4. Apparatus as in claim 1 wherein said cylindrical member has an outwardly extending horizontal flange which carries a first rail on its lower surface and a second rail on its upper surface, said rails engaging rollers supported by said floatable structure to thereby form said first and second roller means.

5. 'In combination with a floatable structure having a vertical Well therein open through the bottom of said structure: a vertical hollow cylindrical member within said well in spaced relationship from the sides of the Well; an outwardly extending horizontal flange on said member; means supporting said cylindrical member for rotation about its vertical axis relative to said floatable structure and preventing Vertical and lateral displacement relative to said floatable structure, said means including first roller means below said flange restraining downward displacement of said cylindrical member, second roller means above said flange restraining upward displacement of said cylindrical member and third roller means laterally of said cylindrical member for restraining lateral displacement thereof; a plurality of mooring line Winding means carried by said flange; mooring line guiding means carried by said member below and in substantial alignment with said winding means; and a plurality of mooring lines extending from said winding means through the space between said member and the sides of the well to said guiding means.

6. The method of mooring a structure floating in a body of water, said structure having a vertical well therein which comprises: mounting a vertical cylindrical member within said well for rotation about its vertical axis; attaching one end of a mooring line to said member and passing the other end downwardly through said well and up to the surface of the water; placing an anchor on a second floating structure and moving the latter into proximity with said other end of said mooring line attaching the anchor to said other end; lowering said anchor from said second floating structure to the submerged bottom; hauling in on said line from said member to thereby tension said line and restrain said member against movement; and rotating said floating structure about said member to a position at which wind and water forces on said structure are minimized.

7. Apparatus as in claim 5 wherein said first roller means includes a plurality of vertical rollers disposed in a circular pattern and journalled in a circular cage, the lower surface of said flange resting on top of said rollers and the lower surface of said rollers resting on a support surface carried by said floatable structure.

8. Apparatus as in claim 7 including a derrick-supporting platform disposed above said flange, said platform being supported by a plurality of legs extending upwardly from the deck of said floatable structure, said second roller means including rollers carried in fixed positions by said legs and engaging a circular supporting surface on the top of said flange.

9. Apparatus as in claim 7 wherein said winding means are carried on top of said flange, said flange being provided with a hole adjacent each of said winding means for the passage of one of said mooring lines.

References Cited by the Examiner UNITED STATES PATENTS 2,808,229 10/1957 Bauer et al 1140.5 2,986,888 6/1961 Borrmann et al. 114206 X 3,031,997 5/ 1962 Nesbitt 114-0.5 3,919,201 6/1965 Richardson et al 114-05 FOREIGN PATENTS 1,235,615 5/1960 France.

MILTON BUCHLER, Primary Examiner.

T. M. BLIX, Assistant Examiner.

Claims

1.IN COMBINATION WITH A FLOATABLE STRUCTURE HAVING A VERTICAL WELL THEREIN OPEN THROUGH THE BOTTOM OF SAID STRUCTURE: A VERTICAL CYLINDRICAL MEMBER SUPPORTED BY SAID STRUCTURE WITHIN SAID WELL; FIRST VERTICAL ROLLER MEANS SUPPORTING SAID CYLINDRICAL MEMBER ON SAID STRUCTURE FOR ROTATION ABOUT THE VERTICAL AXIS OF SAID CYLINDRICAL MEMBER; SECOND VERTICAL ROLLER MEANS RESTRAINING SAID CYLINDRICAL MEMBER FROM UPWARD MOVEMENT RELATIVE TO SAID FLOATABLE STRUCTURE AND HORIZONTAL ROLLER MEANS RESTRAINING SAID CYLINDRICAL MEMBER FROM LATERAL MOVEMENT RELATIVE TO SAID FLOATABLE STRUCTURE.