The invention provides a way of obtaining flexibility and energy absorption in the anchoring system that is not ordinarily available from catenary chain or wire rope systems in shallow water. The invention also provides a system for mooring in deep and very deep water.

BACKGROUND ART In recent years a number of systems have been developed to moor vessels offshore in high sea states. These systems generally provide for the mooring of oil tankers and oil production and storage vessels such that they are moored to a single point (such as a single point mooring or a turret mooring). However the present invention is also applicable to vessels moored by the so-called spread mooring system in which a vessel is held in a fixed position and with a fixed heading by several mooring legs deployed in a radial pattern.

The anchor legs in the prior art are typically made from chain, steel wire rope, or a combination thereof. These anchor legs depend on the catenary action to provide flexibility to resist the dynamic forces from wind and waves. In shallow water the legs touch the seabed and for this reason do not form a complete catenary. Therefore flexibility is difficult to obtain in shallow water.

Auxiliary buoys or pendulums are often used in shallow

water to obtain flexibility.

Synthetic rope systems have been proposed to solve this problem. In this case the anchoring system would rely on the elasticity of the ropes to provide flexibility.

However, synthetic rope is easily damaged, particularly when dragged across the seabed under stress. For this reason classification societies are reluctant to approve the use of synthetic rope for this application.

SUMMARY OF THE INVENTION It is the object of the present invention to provide chain or wire rope mooring svstems that do not rely on vulnerable components and which obtain their flexibility by lifting or lowering heavy weights.

Another object of the invention is to provide mooring systems for vessels in deep water using heavy weights to provide flexibility of the system, thereby limiting the horizontal excursion of the vessel and also limiting the vertical loads imparted to the vessel from the mooring.

These and other objects are met by placing heavy weights made from steel, lead, or other heavy materials inside prismatic hollow piles embedded into the seabed. The weights are connected through a fairlead in the top of the pile to the mooring buoy or the vessel.

Alternatively in deeper water heavy weights are placed inside hollow prismatic structural members supported by a tripod or a similar structure secured to the seabed. The weights are connected through a fairlead in the top of the structural member to a mooring buoy or the vessel. Two or more such structures separated by some distance may be connected to a single mooring buoy or a single point of

the vessel to restrict the horizontal excursion of the vessel.

A particular advantage of the system is that the anchor leg may be vertical when the mooring buoy or the vessel are not subjected to horizontal forces. In consequence the mooring leg is always taut and not subject to impact loads when subjected to slowly varying dynamic loads.

BRIEF DESCRIPTIONS OF THE DRAWINGS Figure 1 shows a side view of a first embodiment mooring a submerged mooring element, which in turn moors a vessel to the anchoring system.

Figure 2 shows a first embodiment mooring a submerged mooring element an in idle condition.

Figure 3 shows a second embodiment mooring a submerged mooring element in a mooring system adapted for Arctic conditions.

Figure 4 shows a third embodiment in which a vessel is permanently moored by the system in a single point mooring configuration.

Figure 5 shows a fourth embodiment in which a vessel is moored by the system in a multi-point mooring configuration.

Figure 6 shows a fifth embodiment of the invention in which the vessel is moored to a mooring system placed above the seabed.

Figure 7 shows a sixth embodiment in which the vessel is moored to multiple moorings of the fifth embodiment.

Figure 8 shows a deep water version of the sixth embodiment.

Figure 9 shows a plan view of a plate which serves as a component of the anchor weight Figure 10 shows a side view of a possible arrangement of plates to make up the anchor weight Figure 11 show a side view of a possible way of suspending the weight shown in figure 10 Figure 12 shows the first embodiment applied to mooring pleasure craft in a marina Figure 13 shows a variation of the fifth embodiment in which the multiple weights are suspended within inclining structural members.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS.

Figure 1 shows a first embodiment of the invention applied to a single point mooring of the type which is moored to the keel of the vessel such as for example described in US Patent 5,305, 703. A vessel 12 floating on the surface of the sea 11 is anchored to a single hollow stake pile 14 embedded in the seabed 10 vertically below the neutral position 18 of the vessel 12. The anchor leg 16 is attached to a submerged mooring element 13 through chain swivel 17. The anchor leg 16 is attached to a suspended weight 15 inside stake pile 14. The weight may have, for example, a submerged weight of 300 Mg. The anchor leg 16 enters the stake pile 14 through fairlead 21.

When the vessel 12 is subjected to environmental forces it may deflect from the neutral position 18 to the deflected position 19 shown in dotted lines. In this case the anchor

leg 16 deflects to the position 22 and the weight 15 is lifted to position 20 inside the stake pile 14. This action provides the restoring force to return the vessel to the neutral position 18 and to resist the environmental forces. The stake pile 14 will normally have its bottom 23 well below the vertical position of anchor weight 15 for any loading condition of the vessel 12 such that the anchor weight 15 never touches the bottom, with the result that the anchor line 16 is always taut.

The anchor leg 16 incorporates a chain swivel 17. For single stake piles 14 as shown in figure 1 this may be omitted. However it would often be convenient to use multiple stake piles 14 each with a weight 15 and each with a chain 16. In this case the chains 16 must join into a single chain below the chain swivel 17.

Figure 2 shows the first embodiment in a stored condition.

The submerged mooring element is pulled further below the sea surface 11 by the anchor weight 15. The position of the submerged mooring element 13 is determined by the anchor weight 15 resting on the bottom 23 of the stake pile 14.

Figure 3 shows a second embodiment similar to the first embodiment where it is applied to moor the submerged mooring element described in figure 3 of US Patent 5,647,295. As described in US Patent 5,647,295 this embodiment is particularly adapted to moor a vessel in Arctic conditions. The mooring system is placed within a well 31 in the seabed 10. Within the well there is a buoyant roof 32 with variable buoyancy. The roof 32 may be in the stored summertime position 33, as shown, or in the floating winter time position 34 in which it slidably engages the submerged mooring element 35.

In the summertime, as shown, the submerged mooring element

is moored by the anchor weight 36 suspended within the stake pile 37 through the vertical anchor line 42.

Fluid is transferred between the pipe line end manifold 38 at the bottom of well 31 and the submerged mooring element 35 though a helical riser as described in US patent 5,553,976. The fluid connection is a flexible pipe 39 supported in a helical shape by elastic cords 40. The anchor line 42, the cords 40, and the flexible riser 39 are all terminated at the sleeve 41. The sleeve 41 is rotatably mounted in the submerged mooring element 35 with a low friction bearing (not shown). The cords 40 provide the torque to keep the sleeve 41 rotationally stationary with respect to the seabed, whenever the vessel 30 weather vanes. The fluid piping in sleeve 41 is connected by riser 52 to fluid swivel 50 which in turn is connected to the vessel piping 51.

The vessel may be displaced by the wind, current, and wave forces as illustrated in Figure 1. Fluid connection between the seabed and the vessel may be established in all examples as shown on Figure 3.

Figure 4 shows a third embodiment of the invention in which the anchoring system moors a vessel permanently.

This embodiment shows the vessel moored to two stake piles 61 each with an anchor weight 62. Each anchor weight 62 is held by a chain or rope 68. The chains or ropes 68 are joined at fishplate 63 to a single anchor line 64 fitted with a chain swivel 65 and connected to the vessel 60 at the mooring point 66. The mooring point 66 is illustrated to be at the keel of vessel 60, however, it may be located at any suitable location on the hull of vessel 60.

The embodiment in Figure 4 is illustrated with two stake piles 61. Any number of stake piles is possible from one and up in all the embodiments.

Figure 5 shows a side view of a fourth embodiment of the invention. A vessel 70 is moored at a forward fairlead 72 by cable 76 to an anchor weight 75 that is suspended inside a stake pile 73. The stern of vessel 70 is moored at fairlead 71 by anchor cable 76 to anchor weight 75 suspended inside stake pile 74. In this embodiment a minimum of two stake piles 73 and 74 would be employed, however more commonly three, four, or more such anchors would be used.

Figure 6 shows a side view of a fifth embodiment of the invention particularly applicable to deep water moorings. a vessel 12 is anchored to tripod 75 anchored to the seabed 10 vertically below the neutral position 18 of the vessel 12. The action of the mooring in this embodiment is equivalent in all respects to the actions of the embodiment shown in figure 1. The anchor leg 16 is attached to a submerged mooring element 13 through chain swivel 17. The anchor leg 16 is attached to a suspended weight 15 inside tube 70. The weight may have, for example, a submerged weight of 200 Mg. The anchor leg 16 enters the tube 70 through fairlead 74.

Tube 70 is secured in a vertical or near-vertical position by structural members 71 and 72 that in turn are connected to the anchors 73 securing the tripod 75 to the seabed 10.

In figure 5 is illustrated an anchor 73 known as a suction anchor because it relies on the hydrostatic pressure at the seabed to avoid pull-out. However all known methods to secure structures on soil against shear, tension, and compression may be employed to secure anchors 73 to the seabed 10. Examples are piles (not shown), heavy weights (not shown), and deep embedment anchors (not shown). The tube 70 is shown in a vertical position, however, in this embodiment a tilt of, for example, 20 degrees may be more advantageous in order to enhance the hysteresis of the mooring system by the friction developed between the

weight 15 and the tube 70.

In this embodiment the tube 70 and the members 72 may be omitted and the weight 15 may hang freely in the water column. In this case it is a requirement that the submerged mooring element 13 in a disconnected position (not shown) have adequate buoyancy to prevent the toppling of weight 15 when it rests on the seabed 10. The structure 75 is shown as a tripod; however, any structure that would serve the purpose of securing tube 70 and fairlead 74 against the mooring forces from anchor leg 16 such as a submerged caisson (not shown) may be used.

Figure 7 shows a plan view of the fifth embodiment in which three anchoring structures 75 are deployed in a radial pattern to moor vessel 12. Three structures 75 are shown, however, any number from two and up may be employed. The deployment of multiple moorings 75 may serve the purpose of limiting the weight of the anchor weight (not shown) or of limiting the horizontal excursion of vessel 12 when subjected to forces from wind, current, and waves or for both purposes.

Figure 8 shows side view of the fifth embodiment particularly suited for the mooring of vessels in very deep water in the range of 300 m to 3000 m, or even deeper. The anchor leg 16 is comprised of two parts 78 and 77. Part 78 is a short chain passing through fairlead 74 and lifting the anchor weight (not shown). Part 78 is a chain in order to resist the wear that results from the anchor leg 16 being pulled out through fairlead 74 and let back in repeatedly. Part 77 is a nearly neutrally buoyant rope preferably with high stiffness. The anchor legs 16 secure a submerged mooring element 13 which is shown in a disconnected mode. Parts 78 and 77 are joined by a connector 79 which, for example, may consist of an ordinary shackle and thimble connection. Connector 78 may

include an in-line swivel (not shown).-- Figures 9 through 11 show a detailed view of one method of making up the anchor weight 15 such that it exhibits the force characteristics of an elastic spring until fully suspended, after which point it delivers a constant force.

The anchor weight should not be made such that it fits snugly inside the tube. Such a tolerance requirement would lead to high costs and there is a risk that sediments and debris could cause the anchor weight to seize inside the tube. Consequently the anchor weight would ordinarily have dimensions that are a few percent less than the interior dimensions of the tube within which it is confined. If the anchor weight is solid it may swing as a pendulum within the confining tube and deliver large impacts to the tube.

A solid weight also has the disadvantage of delivering full force when it is suspended and zero force when resting on the bottom.

Figure 9 shows a plan view of a plate 80 from which the anchor weight 15 may be made. A plate 80 is shown that fits within a circular tube 81. Figures 10 and 11 show how the plate 80 is supported and lifted within tube 81. The plate 80 is designed to be supported by two chains (not shown) that engage slots 82 in plate 80. The anchor weight is in this case made up of two sets of plates 80 placed in a staggered condition vertically, thus the plate 80 has two cut-outs 83 permitting the chains that support the other set of plates (not shown) to pass the plate 80 without engaging it. The supporting chain (not shown) has a wire diameter denoted d in the following. The width of slot 82 would ordinarily be in the range of 1.05 d to 1.15 d. The plate as shown has an axis of symmetry 86 shown pointing in the direction 87. The confining tube 81 is shown to be circular, however it may have any other suitable prismatic shape provided plate 80 is shaped to fit loosely inside the confining tube 81.

-9- SUBSTITUT SKEET (RULE 26)

Figure 10 shows a section taken at section 85 as shown on figure 9, of a stack of plates supported inside tube 81 The anchor weight can include two sets of plates, one set of plates 80 which are supported by two chains 89, of which only one is shown, and another set of plates 88 supported by two chains, not shown, that are hidden by the wall of tube 81. The chain 89 is shown with links having the normal standard length of 6 d and a width of 4 d. The plates 80 and 88 are shown with a thickness of 1.5 d.

Common dimensions may be d=75mm and the diameter of tube 81 of 3000 mm, while the diameter of plates 80 and 88 may be 2950 mm. The plates 80 have orientations 87 as shown.

The plates 88 have orientations 87 perpendicular to the paper. The plates 80 (and 88) are staggered such that the chain 89 is forced to take a shape such that the orientation 91 of the links 92 deviate by the angle a from the orientation of the axis 90 of tube 81. The deviation angle a forces the plates 80 (and 88) onto the interior wall of tube 81 in the opposite direction of 87 with consequent frictional contact. When the chain 89 moves there is a moment imbalance that will force all plates 80 to tilt and contact adjacent plates 88 and all plates 88 to tilt and contact adjacent plates 80, not shown.

When plates 88 tilt they contact plates 80 in the vicinity of chain 89, which then counteracts the tilting force. The maximum force on pullout can be shown to be approximately R 1 (1-f *tm (a)) where F is the force in mooring line 16 w is the submerged weight (in force terms) of plates 80 and 88 f is the friction coefficient between plates 80

and 88 and the tube 81 a is the angle between 90 and 91 n is the number of plates 80 lifted by chain 89 If the product of tan (a) and f is more than one the anchor weight is self locking and cannot move within the tube 81.

Normally tan (a) is on the order of 0.1 to 0.2 and f on the order of 0.1.

The denominator in the equation above consequently deviates little from unity. For each additior-'plate a force is added in which the denominator is raised to the power of n which is equal to the number of plates 80 or plates 88. Therefore each additional plate becomes progressively more effective in providing mooring force.

This factor can be used to prevent pull-out of the anchor weight from the tube 81 by changing the shape of the lower plates 80 and 88 to increase the angle a for these lower plates. In this way, the force provided by the anchor weight can be increased to the breaking strength of the anchor line 16 (drawings 1 through 8). When the mooring line 16 lowers the anchor weight the force is governed by: Thus there is an appreciable reduction in force when anchor line 16 retracts with consequent hysteresis in the mooring.

With standard chain 89 and plate thickness 1.5 d of plates 80 and 88 there is a space between each plate of 0.5 d when in compressed and relaxed state whereas in the extended state shown on figure 10 there is a space between adjacent plates 80 and 88 of 3.25 d. Thus the anchor line must move 2.75 d to engage or disengage another plate 80

with damping.

Figure 11 shows a side cut-away of the weight shown in figure 10. The anchor line 16 passes through fairlead 21 and connects to a fish plate 91 that connects to four chains 89 and 90. The chains 89 support plates 80 and the chains 90 support plates 88. The figure shows a total of only 6 plates 80 and 88; normally a mooring is fitted with many more such plates 88 and 80.

If non-standard chain is applied to chains 89 and 90 with long links, a compact design with 0 distance between the plates in relaxed mode may be achieved. For example a link length of 7 d and a plate thickness of 2.5 d results in this condition.

Figure 12 shows another application for the first embodiment, namely to moor pleasure craft 95 in marinas on swing moorings. In this application the mooring 96 replaces the commonly used mushroom anchor. The application of this mooring 96 in marinas would enhance the mooring security and permit closer placement of the mooring points by reducing the swing circle of craft 95.

This mooring could also have general application to vessels moored at single points such as military vessels and barges at fleeting points. In the case shown on figure 12 the diameter of the stake pile 14 may be 200 mm and the weight of the anchor weight as low 100 kg or even lower.

Figure 13 shows a variation of the fifth embodiment in which a tripod 100 is founded on the seabed 10 through foundations 102. The tripod 100 is composed of three hollow prismatic legs 101 each of which has a weight 103 inside suspended by the chain 104. Each of the three chains 104 pass through a common fairlead 108 to be joined at fish plate 105 to the anchor leg 106. The anchor leg

106 contains an in-line swivel 107 and is connected to the buoy 113 on the surface of the sea 10. The vessel 110 is moored at the bow 111 by hawser 114 connecting to buoy 113. The fairlead 108 is shown as one common fairlead but may also consist of separate fairleads 108 spaced some distance apart.

In the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Improvements, changes and modifications within the skill of the art are intended to be covered by the claims.