Abstract:
Mooring systems for floating vessels positioned off-shore. A rigid column extends from the sea floor and provides a single-point mooring for a barge or other floating structure which includes facilities for storing oil or liquified gases. The barge may also include facilities for liquefying gases. A fluid line for oil or gas extends from below the water line inside the vertical column to the mooring point above the water line. A pair of mooring arms is pivotally attached to a table pivoted on the mooring point, and the arms diverge with their other ends attached to the barge at zones spaced from each other. The arms are extensible and each of them has a compensator which tends to hold each arm to a predetermined length. Each compensator also restrains the rate of movement during extension and contraction of its arm. A fluid-flow line extends from the mooring point to the barge with flexible interconnections between the fluid-flow lines above the water line. A third arm is provided in certain embodiments of the invention and a fluid line is provided from a sea flow upwardly through the vertical column and then along one of the arms to the barge or other floating structure.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 116,846 filed Jan. 30, 1980, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to mooring systems for ships, barges and other floating structures which are referred to herein as &#34;vessels&#34;. The invention relates particularly to mooring systems for vessels which receive or discharge fluid cargo while they are moored, for example, barges and ships which receive or discharge liquid or gas, such as LNG (liquified natural gas), LPG (liquified petroleum gas), oil or fuel gas. 
     An object of this invention is to provide mooring systems of the above type which are free of certain of the deficiencies of the prior art. A further object is to provide such systems which can be used for barges which are stationed at a particular location for accumulating or storing LNG. These and other objects will be in part obvious and in part pointed out below. 
     2. Description of the Prior Art 
     The increased importance of LNG has resulted in the development of systems for mooring vessels such as barges where the gas is received and liquified, and such installations which provide regasification facilities. A publication of Moss Rosenberg Verft A/S entitled &#34;The Moss Rosenberg Marine LNG System&#34;, September 1978, discloses a marine LNG system with a mooring arrangement for barges having LNG storage tanks, liquefaction facilities for gas, and facilities for transferring the gas to the barge and the LNG cargo to tankers. A hinged yoke on the barge is attached to an upright leg or &#34;tension leg&#34; which is anchored to the sea bed and which has a large buoyancy container which tends to urge the tension leg toward an upright position. The tension leg is hinged at its bottom end to the anchoring means and at its top end to the yoke. Transfer lines for fluid cargo extend down the tension leg and have movable connecting structures or couplings which permit the lines extending from the barge to have free movement with respect to stationary lines extending from the shore or elsewhere to the anchoring position. Those connections or couplings are below the surface of the water, and that prevents ready inspection and servicing. That mooring system may also be subject to damage by collision, for example, by a tanker moving to or from a position alongside the barge. 
     A similar mooring system is disclosed in &#34;OCEAN INDUSTRY&#34;, November 1978, and is identified as incorporating a special triaxial swivel universal joint in the piping of the cargo transfer lines. That system also has an underwater upright leg which is connected by a universal joint at its lower end to anchoring means on the sea bed. In each of those systems, the upright leg moves about its lower end, and the barge moored to it can swing generally around the mooring point formed by the pivot provided by the attachment of the bottom end of the upright leg to the anchor. 
     With both of the systems just discussed, the general approach is to provide a riser projecting generally upwardly from the anchor and pivoted at its bottom end, and to a mooring yoke. The pivoting action at the sea bottom can create problems because of the rotatable connections in the line or lines for transferring liquid or gas to or from the moored vessel. Also, the upright leg is often completely submerged so that its location cannot always be determined by observation from a vessel approaching or passing the barge which is moored. It is a further object of the present invention to provide mooring systems which overcome difficulties which are present with prior mooring systems such as those discussed above. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a mooring system for vessels is provided which includes a rigid anchor structure which is securely mounted on the sea floor and which provides a stationary mooring point above the water line. Fluid cargo transfer lines extend along the sea floor to the mooring base from the shore or from an off-shore source or storage facility. The cargo lines extend upwardly through the anchor structure to the mooring point above the water line. There are no underwater movable connections in the fluid flow path. The anchor ends of the mooring arms are connected to the top of the anchor structure by separate hinge or pivot structures. Other important features include the normal positioning of the moored vessel in a predetermined relationship with respect to the single mooring point. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings: 
     FIG. 1 is a somewhat schematic perspective view of a floating structure and a mooring system constituting one embodiment of the invention; 
     FIG. 2 is a plan view of the mooring system of FIG. 1; 
     FIG. 3 is a somewhat schematic side view of the mooring system of FIG. 2; 
     FIG. 4 is a side view of the central arm in the mooring system of FIGS. 1 to 3; 
     FIGS. 5 and 6 are views similar to FIGS. 3 and 2, respectively, but showing another embodiment of the invention; 
     FIG. 7 is a vertical section on the line 7--7 of FIG. 6; 
     FIG. 8 is a view similar to FIG. 6 and showing a modified form of the embodiment of FIGS. 5 and 6; and, 
     FIG. 9 is a schematic view of one of the three compensators for the mooring arms of the embodiments of FIGS. 1 to 8. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 of the drawings, a barge 1 has built upon it a gas treatment plant 2 and an LNG storage plant 3. Barge 1 is moored by a mooring system which includes an anchor structure 4 formed by a rigid column 4&#39; of concrete (see FIG. 3) which is fixed to the sea floor rock, and the base of which is surrounded by a layer of mud 27. Column 4&#39; projects above the surface of the water and is of substantial weight and size. Column 4&#39; can be fixed to a concrete slab, and there may be no mud around the column. As shown best in FIG. 2, three telescoped extensible arms 5, 6 and 7 are connected to the top of column 4&#39; and to barge 1. Each of the arms is formed by two arm sections which telescope together to provide the extensible characteristics of the arms, arm sections 5a, 6a and 7a being connected to the end 1a of barge 1, and arm sections 5b, 6b and 7b being connected to the anchor structure 4. The connections between arm sections 5a and 6a and barge 1 are by ball joints 18 and 16, respectively. Arms sections 5a, 6a and 7a are connected to the respective arm sections 5b, 6b and 7b by compensators 9, 10 and 11, respectively. The connections between arm sections 5b and 6b and the anchor structure 4 are through a rotatable table 8 (see also FIG. 4) which is mounted on a center pivot construction (not shown) with the vertical axis 8&#39; of column 4&#39;. That axis constitutes the single mooring point upon which barge 1 is moored. Arm sections 5b and 6b are connected to table 8 by ball joints 19 and 17, respectively. Arms section 7a is connected to barge 1 by ball joint 20 (FIG. 4), and arm section 7b is connected to the top of table 8 by a horizontal axis hinge 21. It is thus seen that when barge 1 moves in any of its linear or rotational directions or in several motion modes simultaneously, so that its end 1a moves, the attached ends of the arms are permitted to swing around pivots with respect to the barge. 
     Barge 1 and arms 5, 6 and 7 (FIG. 2) are shown in the drawings in the &#34;desired attitude&#34; or neutral position in which the vertical longitudinal center plane of the barge, as represented by the center line 1&#39;, extends along a radius line 7&#39; from axis 8&#39; of column 4&#39;, and none of the compensators 9, 10, 11 is extended or further telescoped. The vertical longitudinal center plane of the barge extends through axis 8&#39;, and the barge is a fixed distance from the mooring point when the barge is in its neutral horizontal position. Each of the compensators is enclosed within the arm sections of its arm and it operates somewhat in the manner of commonly-known shock absorbers to limit the rate of the relative movement between the arm sections from the neutral position where the arms are being extended or further telescoped. Each of the compensators also limits the extension of its arm and provides a force urging the arm sections toward the neutral or home position. Each of the compensators 9, 10 and 11 acts to hold its arm 5, 6 and 7, respectively, in that neutral position until a force greater than a predetermined value is exerted upon the arm to draw the arm sections apart or to telescope them further. The compensator then produces a force urging the return of the arm to its neutral position when the arm sections have been moved from that position. Each of the compensators also restrains or limits the rate of movement of the arm sections from and to the partially-telescoped neutral position. 
     When forces greater than the predetermined value act to swing barge 1 out of the neutral position shown with respect to radius line 7&#39;, two or all of arms 5, 6 and 7 are moved from the neutral position against the action of their compensators, and the rate of such swinging movement of the barge is restricted to an acceptable value by the compensators. With the barge moored as shown, if forces act to turn the barge counterclockwise, arm sections 5a, 5b will be extended, or arms sections 6a, 6b will be further telescoped, or there can be a combination of both of those actions, and the rate of each such movement is controlled. Contrarywise, clockwise movement of the barge either draws arm sections 6a, 6b apart or telescopes arm sections 5a, 5b apart, each at a controlled rate, or there can be a combination of both of those sections. The amount of telescoping or expanding of arm 7 depends upon the directions and and magnitudes of forces and the effective resistance exerted by arms 5 and 6 and their compensators. For example, if one of arms 5 or 6 remains in its neutral condition, and the barge turns clockwise or counterclockwise, arm 7 will expand or telescope substantially one-half of the change in the length of the other arm 6 or 5. Arms sections 7a and 7b are telescoped or extended when the vessel is moved, respectively, directly toward or directly away from the mooring point. Also, arm 7 always maintains a length which is of the order of the difference between its length in the neutral position and the averages of the amounts by which arms 5 and 6 have moved from their respective neutral positions. That is, arm 7 can remain in its neutral position when the vessel swings and one of arms 5 and 6 telescopies and the other extends the same amount. The angles between the arms change when barge 1 moves from the desired attitude either by turning or by moving toward or away from mooring axis 8&#39;, and the arms are free to pivot at their ball joints. When the forces acting on barge 1 to move it from its home position are reduced, the compensators draw the barge back to the neutral position or &#34;desired attitude&#34;. It should be noted that barge 1 can swing around axis 8&#39; while maintaining the desired orientation with respect to that axis. Although a turning motion of barge 1 with respect to radius line 7&#39; is described above, it must be understood that the compensators also allow and restrain (control) motions of the barge in all six of its degrees of motion (three rotations and three lineal, tri-axial motions). It is thus seen that the arms permit a wide range of relative movement of the vessel so that the vessel can accommodate itself to changing conditions of wind, wave action and current flow. 
     Referring now to FIG. 9 of the drawings, compensator 9 has a double cylinder 49 formed by cylinders 50 and 51 which are interconnected in tandem and have pistons 52 and 53, respectively. Each of the pistons has an extended sleeve portion 52&#39; and 53&#39;, respectively, which extend through sealed bores in the respective cylinder heads 50&#39; and 51&#39;. The is a common piston rod assembly comprising a piston rod 54, a centrally positioned cylindrical impeller block 55 and a piston rod 56. When the arm sections are in the neutral position, block 55 is positioned as shown in broken lines with its reduced end portions 57 and 58 snugly received in the respective central bores 59 and 60 of pistons 52 and 53. Double cylinder 49 is mounted upon arm section 5a and piston rod 54 is connected to arm section 5b. When the arm sections are in their neutral position, impeller block 55 and piston 53 are in the broken line position. When the arm sections are extended from the neutral position, piston rod 54 is pulled to the right, and impeller block 55 moves away from piston 52 and pulls piston 53 to the right in cylinder 51 toward the position shown in the drawing. Piston 52 is held in the position shown. When the arm sections are telescoped from the neutral position, piston rod 54 is moved to the left and impeller block 55 moves away from piston 53 and pushes piston 52 to the left in cylinder 50 and piston 53 remains in the broken line position. At the juncture of cylinders 50 and 51 there is a stop flange 64 which projects radially inwardly from the cylinder walls, and against which pistons 52 and 53 rest when the arms are in the neutral position. A vent hole 66 through the cylinder wall permits air to enter and to be discharged from the space between the pistons. Hydraulic liquid lines 68 and 70 are connected to the opposite ends of cylinders 50 and 52, and a common line 72 extends from the juncture of lines 68 and 70 to an accumulator tank 74 for the liquid which is oil. An adjustable restricter valve 76 is positioned in line 72 and can be adjusted to control the rate of flow of oil between tank 74 and the cylinders. Tank 74 has a body of compressed air in the space 80 above the level 78 of the body of oil in the tank, and a pressure gauge 82 is connected to the top of the tank. Compressed air is supplied to space 80 in the tank through a line 84 having a shut-off valve 86 therein. An air discharge line is connected to space 80, and has a shut-off valve 88 therein. Oil is supplied to tank 74 by a pump 90 from an oil reservoir 92, and a shut-off valve 94 is closed when the desired quantity of liquid has been supplied to the tank and the pump is stopped. An oil drain line 91 extends from tank 74 to oil reservoir 92, and has a shut-off valve 93. 
     The predetermined air pressure in space 80 acts through the body of liquid in tank 74 and lines 72, 68 and 70 provide substantially that pressure in cylinders 50 and 51. Hence, each of the pistons is urged toward the center of the double cylinder against flange 64. That predetermined liquid pressure determines the force which must be overcome to move arm sections 5a and 5b from their neutral position. That force of the liquid also acts to return the arms to the neutral position when the forces exerted by the vessel are reduced. 
     The rate of flow of liquid through valve 76 controls the rate at which one of pistons 52 and 53 can be moved away from or toward the neutral position. In this embodiment, the limit on the rate of movement is the same when the arm sections are being telescoped as when they are being extended. The invention contemplates that separate liquid lines with separate control valves can be provided between tank 74 and the respective cylinders 50 and 51. It is also contemplated that the movement of either of the pistons away from the neutral position can be made different from the return movement of that piston to the neutral position. That may be accomplished by separate one-way control valves in parallel lines with the flow to the cylinder through one line and from the cylinder through the other line. It should be noted that piston rod 56 has no function in the construction shown. 
     As indicated above, the compensators are identical, each biasing its arm sections to the neutral position and limiting the rate of relative movement between the arm sections. The &#34;Moss Rosenberg System&#34; and that disclosed in the &#34;Ocean Industry&#34; article have buoyancy chambers and other components which are positioned in the water and which act to restrain the rate of movement of the vessel. The compensators of the present invention act directly on their arm sections, and the limitation on the rate of movement is inherent in the arm assembly. The arm sections and their connections with the vessel and the mooring point are not subjected to excessive forces when the vessel is being moved to the neutral position. That is because the rate of such movement is limited by the rate of flow of the hydraulic fluid to the cylinder of the compensator (or compensators). That flow is under the pressure of the air in the accumulator. The action is somewhat the same as that of a spring positioned to act on the piston, except that additional advantageous features are provided. When the arm sections are being returned to the neutral position, the force is sufficient to effect the return movement, but the rate of movement is reliably controlled by the rate at which the hydraulic fluid is permitted to flow to the cylinder. Hence, the arm exerts the desired force between the mooring point and the vessel with there being a &#34;time factor&#34; which is determined by the rate of flow of the hydraulic fluid to the cylinder. 
     Referring to FIG. 3, a fluid line 22 extends from a shore station or from another source or storage facility for the fluid cargo which can be a gas or a liquid. Line 22 extends upwardly within column 4&#39; to a movable connection 23 and to a line 22a which extends along arm 7 toward the barge. Line 22a is connected through movable pipe connection 24a to one end of a pipe assembly connection 24. Pipe assembly connection 24 is formed by two pipe sections 24d and 24e which are interconnected by a movable connection 24b. Section 24d is connected through connection 24a to arm section 7a (See FIG. 1), and section 24e is connected through a movable connection 24c to a line on the barge. When barge 1 moves from the position shown so as to extend arm 7, the pipe sections swing downwardly from the position shown (FIG. 3), and at all times provide a continuous flow path while permitting arm 7 to contract or to extend to its maximum length subject to the action of compensator 11. 
     A fender 25 is mounted upon the top of column 4 and provides protection from damage to the column by collision from vessels. Arms 5 and 6 also provide collision protection in that the barge is secured to column 4 even if there is serious damage to one or both of the arms. If there is serious damage to either or both of arms 5 and 6, arm 7 maintains a secure connection which can cooperate with either of the arms or can act alone within certain limits. The movable connections for the fluid flow path are of known types. The compensators can be adjusted to provide the desired range of resistance to movement of the barge from the desired orientation, and also to provide the desired maximum rate of movement of the barge for a given unbalanced force causing the movement. 
     The embodiment of the invention shown in FIGS. 5, 6 and 7, is the same as that of FIGS. 1 to 4, except as is pointed out below. There are only two mooring arms 31 and 32 which connect barge 1 to column 4, the center arm being omitted. Also, fender 42 is in the shape of an open &#34;C&#34;, connected at its ends to barge 1 by horizontal axis hinges 43 and 44. A semi-submersible buoyancy tank 45 is mounted at the center of fender 42 by two vertical arms 45a. Tank 45 supports the fender in the horizontal position above the water line, as shown. Referring to FIG. 6, arms 31 and 32 have arm sections 31a and 32a connected to the barge by universal joints 37 and 38, respectively. Arm section 31b is connected to a rotatable table 30 by a horizontal axis hinge 35, and arm section 32b is connected to table 30 by a universal joint 36. A gas or liquid cargo transfer line 40 (FIG. 5) corresponds to line 22 in FIGS. 2 to 4, and extends upwardly within column 4 to a movable connection adjacent table 30. That movable connection provides the fluid flow path to a line 40&#39; which extends along arm section 31b and is connected at its other end to a line 41 which extends to the barge. Line 41 is supported by a tower structure 41a (FIG. 5) mounted on the barge and extending upwardly and outwardly. Fender 42 is swung upwardly above the level shown to permit the barge to move to or from column 4. 
     Referring now to FIG. 7, table 30 is rockably mounted upon the top of column 4 by a horizontal axis hinge 39. Hence, table 30 can rock with respect to the column, and that provides additional freedom of movement for arms 31 and 32 and the barge is still securely moored. 
     The embodiment of FIG. 8 differs from that of FIGS. 5, 6 and 7, only in that fender 42 is replaced by a rigid fender 46 extending from the barge. Fender 46 comprises two arms which extend in parallel relationship from the barge, with end portions 46a and 46b which extend at right angles toward each other. The ends of portions 46a and 46b are separated by a gap 47 which permits the barge to approach column 4 and pass the ends of the fender arms upon opposite sides of the column. 
     It is thus seen that single-point mooring arrangements are provided which are superior to those of the prior art. All three of the embodiments incorporate the extensible arms which converge toward the mooring point and which include compensators for urging the barge toward the desired attitude with respect to the mooring point. The rate of movement from and to that desired attitude is controlled by the compensators. The system promotes a stable relationship between the barge and a tanker moored alongside. As discussed above, the fluid cargo lines have movable couplings or connections only above the surface of the water. That permits inspection and servicing of the couplings and it also permits the ready changing of the couplings to provide for multiple lines when that is desirable. 
     The compensators are so constructed and arranged as to prevent objectionable surging or cyclic movements of barge 1. Each compensator operates to urge its arm to the predetermined length in which the barge is in the desired orientation, as discussed above, and the rates of expansion and contraction of the arm are controlled so as to dampen any tendency for vibration or relative cyclic movement of the arm sections. It should be noted that barge 1 has six &#34;degrees of motion&#34;, that is, three linear tri-axial motions and three rotational motions. The mooring system of the present invention restrains those various movements while urging barge 1 toward the desired &#34;home&#34; position at all times. 
     The term &#34;desired attitude&#34; has been discussed above and is the home portion or neutral position of barge 1 or another floating structure which is moored to column 4. It is understood that the floating structure can swing around the mooring axis formed by column 4, as illustrated, while maintaining the desired attitude as discussed above. In each embodiment, the fender is an energy-absorbing construction. Hence, the fender acts to stop a floating vessel which collides with it. The mooring arms also act as energy-absorbing constructions when they are involved in a collision. 
     It is understood that modifications can be made in the illustrative embodiment of the invention and that there may be other embodiments all within the scope of the claims.