NO155128B - STRETCH SHOCK DEVICE. - Google Patents

Description

The invention generally relates to ship accessories and in particular a strain-absorbing device that can be used to absorb the sudden changes in tensile forces that a steel rope or other flexible line that secures a vessel to its mooring is subjected to when the vessel moves in relation to the mooring.

When a boat or other vessel is moored to a pier or other mooring by means of a wire rope or other flexible but relatively inelastic line, it is common to provide the line with some type of device to keep the line taut while a predetermined degree of spring extension is allowed to allow for relative movement between the vessel and its mooring.

US patent 4,022,450 entitled "Line tensioning device" is representative of one such prior art device which utilizes a pair of tension springs to maintain . a springy, Z-shaped bend in the line that is stretched into a more elongated shape when a predetermined tensile force is applied to the line.

The US patents 546 788 and 86 9 130 with the respective titles "Cable buffer" and "Resilient boat clamp" show previously known mechanical devices in which one or more spiral springs are compressed when excessive tension or loads are applied to the cable, cable or rope.

US Patents 143,993 and 107,917 with the respective titles "Gear Spills" and "Improvements in Wave Attenuators" show other devices in which shocks in the cable caused by the movement of the vessel are absorbed by means of a number of rubber springs.

It will be noted that the aforementioned previously known strain absorption devices utilize spring elements which are compressed or extended in a linear direction, and there is consequently an approximately linear relationship between the load applied to the rope and the resulting increase in its effective length.

US patent 4 273 066 entitled "Oil storage vessel, mooring device and oil delivery for offshore production of oil" proposes various devices for mooring a vessel at sea to a deep-water submerged mooring which includes both a mooring line (as in of the particular.application can be either a flotation rope or a heavy chain) and a flexible length of an oil transfer hose. The patent document mentions a manually or automatically controlled, tension-relieving (self-running) mooring winch, and a mechanical damping device (damping roll) in columns 5 and 8. In column 15 of the patent document, a mechanism called "The damper" is described which said to be designed and manufactured to absorb some of the shock that cyclic loads place on the mooring line. The damper is described as comprising a concave roller with thick ends over which the mooring line will pass, the roller being held at a certain height above and behind the wire rope winch by means of two arms which swing about an axis, the arms in turn being held in position by means of springs or hydraulically controlled pressure pistons. The patent also mentions the use of a mooring line to provide a resilient connection between a floating storage vessel and a service tanker, the necessary tension being provided by placing the propeller on the service tanker in its reverse operating position to provide a predetermined degree of tension on the line, and the oil slan -gen is arranged in such a way that a relative movement between the vessels of up to approx. 45 meters can be allowed (column 12).

Mechanical tensioners have also been used to maintain a more or less constant tension in endless drive belts, e.g. such as are used to drive the various auxiliary subsystems that are linked to a modern, self-powered machine. Such belt tensioning devices are intended to compensate for the changes in the belt's length during its lifetime while ensuring that a proper degree of tension is maintained in the belt to eliminate any slippage between the belt and the various pulleys around which the belt travels, and at the same time time without exerting excessive tensile force on the belt.

US patent 4 285 6 76 and Reissue patent 30 842 with

the respective titles "Mechanical Belt Tensioner Construction" and "Tensioning Device" show typical prior art endless drive belt tensioners where a torsion spring is provided

at one end of a lever arm to pivotably move a conductor.ull at the other end of the lever arm in a direction essentially normal to the longitudinal orientation of the belt as it extends between adjacent, fixed sheaves, so as to compensate for changes in the length of the belt by increasing the effective distance between the fixed discs. The torsion spring can comprise a number of spiral springs (as shown in patent no. 4 285 6 76) or alternatively be a hollow cylinder of elastomeric material which exerts the necessary rotational force as a result of the elastomer being subjected to a torsional shear force (as shown in the Reissue patent).

US patent 3 817 113 entitled "Chain drive with intermediate wheel tensioning device pre-tensioned by means of elongated buffers" relates in particular to a chain drive for textile machines which must be stopped and restarted repeatedly, so that the chain is thereby exposed to considerable stress. A torsional effect occurs when a number of rubber buffers are deformed in cross-section as the stretching device is rotated.

It will be realized that dampers which have been adapted for use with continuous belt chains and other similar drive systems will never have to deal with the particularly large forces and constant pounding associated with many applications at sea. Consequently, the reaction force resulting from the application of an unbalanced torque (instead of a balanced torque-force pair) will not reasonably significantly affect the reliability of such devices when used for their intended application.

US patent 2,087,253 entitled "Tilt mechanism, particularly for chairs" illustrates the use of a cylindrical body of rubber which is strongly compressed and confined between concentric, tubular parts so that it is in effect bonded to their surfaces, so that relative rotational movement between the tubular parts put the rubber body under torsional stress. An adjustment mechanism is provided to vary the torsional load applied to the rubber body, and thus the force required to tilt the seat of a chair supported by such a resilient rubber arrangement. Such a construction form is believed to be representative of a number of different applications where a cylindrical rubber bushing is utilized to accommodate a limited rotational movement between adjacent structural elements, while biasing the mechanism towards its normal position. It should be noted that such a bushing, although cylindrical in shape, does not use what is hereafter referred to as "longitudinal torsional shear stress", and that furthermore the "radiating" torsional shear stress in the rubber is concentrated in its central part rather than to be uniformly distributed.

From US Patents 1,744,760 and 3,648,948 are

there are known tightening devices which involve either a winding action or a continuous action where threads constantly move from a spool to a winding device. For example, US patent 1 744 760 involves a thread winding operation where threads are continuously unwound from supply spools, twisted and then wound on a winding mechanism. US patent 3 648 948 involves a continuous unwinding of threads from spools to a point of use which is suggested as reinforcements for sheets of plastic materials. The problem of maintaining threads that are in continuous motion, under the correct tension or tension, is fundamentally different from the problem that is faced when a ship is to be kept moored safely. In a typical mooring situation, the length of the mooring line and the line itself are more or less fixed during the changes that occur from wind or wave action. This is in contrast to the effects achieved with the devices according to the two above-mentioned patents.

According to the invention, there is provided a strain relief device for a steel rope or other relatively inelastic, flexible line for securing a vessel to its mooring, comprising a stationary frame which is lor-tied with a rotatable arm with a first axis of rotation, the arm rotating from a zero load position to a position under load, a first disc rotatably supported at one end of the arm about a second axis of rotation parallel to the first axis of rotation of the arm, and a second disc rotatably supported at the opposite end of the arm about a third axis of rotation parallel to the arm's first axis of rotation, the first disk and the second disk guiding the rope from a substantially S-shaped configuration to a relatively straight configuration under increasing tension as the arm is brought to rotate about the first axis of rotation, which device is characterized by the comprises an elastomeric torsion spring that is oriented about the arm's first axis of rotation and beyond between the arm and the frame, the spring providing a counter-twisting moment which is approximately proportional to the angular displacement of the arm from the aforementioned zero-load position to a position under load,

and an arm adjustment device for adjusting the rotational position of the arm relative to the frame from a zero-load position to a position under load, so that the rate at which the effective length of the rope changes relative to a predetermined increase in tension in the rope varies.

An advantageous embodiment of the device according to the invention is characterized by the fact that the torsion spring comprises a cylindrical body in which the torsional displacement is distributed in the longitudinal direction of the torsion element to cause a longitudinal torsional displacement effect, the cylinder having a first end which is arranged to rotate with the arm, and a other end which is prevented from rotating relative to the frame.

In a preferred embodiment of the device according to the invention, the elastomeric torsion spring comprises at least two cylindrical, elastomeric torsion elements, wherein at least one of the elements is hollow and arranged to accommodate another of the elements in a concentric, stacked manner, wherein a first end of the hollow, cylindrical element is rigidly attached to the frame, a second end of the hollow cylindrical element being rigidly attached to a first end of the second cylindrical element, and a second end of the second cylindrical element being connected to the arm. Since each of the two stacked cylindrical torsion elements needs to accommodate only part of the arm's rotation relative to the stationary frame, the corresponding length of each of the cylinders will be shorter for a given maximum shear angle within the elastomer, allowing that a more compact design can be constructed

for a given set of operating parameters.

When an increase in the tension in the rope rotates the arm and lengthens its S-shaped configuration, as a result of the construction geometry, a correspondingly smaller weight-rod ratio is associated with the torque-force pair applied to the arm due to the tension in the rope. Since the elastomeric torsion spring provides a counter-twisting moment that is approximately proportional to the angular displacement of the arm, the device will always maintain at least a slight bend in the rope or line and thus be able to absorb further instantaneous increases in the tensile force applied to the vessel or rope. By suitable initial setting of the orientation of the rotatable arm under conditions of zero torsional load in relation to the orientation of the rope as it enters and exits the device, it is further possible to vary the operating characteristics of the device and in particular the speed at which the effective length of the rope changes with respect to a predetermined increase of the stretch.

The invention shall be described in more detail below

in connection with a number of design examples with reference to the drawings, where fig. 1 shows a schematic representation of a rope tension damper device according to the invention in use on board a ship to absorb sudden changes in tension in the rope used to secure the ship to a mooring buoy, fig. 2 shows an isometric view of the damper device in fig. 1 together with a part of the rope which is connected to this, fig. 3 shows a partially cut side view of the damper device in fig. 2, fig. 4 shows two isometric diagrams of a cylinder of elastomeric material and shows the geometric relationship between the shear stress and the torsion before and after the cylinder has been subjected to a torsional shear stress, fig. 5 shows an isometric view of a second embodiment of a rope tension damper device constructed in accordance with the invention, in which the elastomeric torsion elements are stacked so that a more compact unit is obtained, fig. 6 shows a partially cut side view of the embodiment in fig. 5, fig. 7 shows an isometric view of a third embodiment of a rope tension damper device,

fig. 8 shows a partially cut side view of the embodiment in fig. 7, fig. 9 - 14 are geometric diagrams which schematically show a rope tension damper device according to the invention where the angle between the arm and the external orientation of the rope is respectively minus 30 degrees, 0 degrees, 15 degrees, 45 degrees, 90 degrees and 120 degrees, and fig. 15 is a set of curves showing the relationship between the tension applied to the rope as a function of the displacement when the original zero load orientation of the arm relative to the rope is respectively 45 degrees, 15 degrees, 0 degrees and minus 30 degrees.

Reference is now made to the drawings, and in particular to fig.

1 which, as will be remembered, shows a schematic representation of a device according to the invention which is used to absorb changes in tension in a ship's mooring rope. It will be realized that the ship 10 is attached to a mooring buoy 20 by means of a mooring rope 30. The mooring rope 30 will typically be a twisted cable of steel parts with a total diameter of approx. 5 cm and will be able to withstand a considerable tensile force, but it can also be any other suitable type of rope or chain. Thus, the cable will normally be more than sufficient to secure a relatively large vessel (e.g. a 100 tonne oil tanker) in relation to its mooring 20. In high seas, however, the action of the waves will cause the vessel 10 to pitch and roll , which in turn will result in relative movement between the vessel 10 and its mooring 20. As the vessel 10 moves towards its mooring, there will consequently be a tendency for slack to occur in the mooring rope 30. When the vessel 10 moves away from the mooring 20 , conversely, any slack in the rope 30 will be taken up, and unless a suitable device is provided to prevent the build-up of excessive tension, the rope 30 will suddenly have to withstand a sudden, possibly large force impulse when the vessel's 10 momentum is transferred to its mooring 20 via the now taut rope 30. The effects of such an increase in the stretch to which the rope 10 is subjected can be avoided to some extent by means of previously known, spring-loaded the shock absorber devices as they have been described above. Furthermore, the winch 40 can be constructed and operated so that it provides only a limited resistance to increases in the tension in the rope 30 before automatically allowing the rope to be quadrupled. However, one must be aware that such aids are functional only within a limited range of operating parameters, and there still remains a need for a suitable device that will reliably dampen and absorb the effects of increases in rope tension prior to the tensile forces have built up to the point where they can equal or exceed the tensile strength of the rope or its termination.

As shown in fig. 1, is the vessel's mooring

a buoy 20 which is also exposed to the effects of waves. However, the buoy 20 will normally be anchored to the seabed by means of a system of anchors and/or pipes (not shown in the figure) which are held in tension by means of the buoyancy effect of the buoy 20. Although it is possible to minimize the effects of wave action on the buoy 20 by constructing it so that the majority of its volume lies sufficiently deep to be reasonably free from severe wave action, it will nevertheless be realized that the buoy 20 will also a certain movement which will be out of phase with the pitching and rolling of the vessel 10 and will thus further contribute to the relative movement between the vessel 10 and its mooring 20.

A first end of the rope or cable 30 is wound around a winch 40 on the deck of the vessel 10, while the other end of the rope 30 is attached to the buoy 20 by means of a suitable rope fastening 50. The winch 40 thus serves to secure - albeit in an adjustable way - the rope 30 in relation to the vessel 10. A new strain relief device 60 is installed on the vessel

10 so that it is in an intermediate position between the winch 40 and a cable guide 70 which can be arranged at the vessel's rail to control the orientation of the outgoing part 80 of the rope 30 in relation to the strain relief device 60. The part 90 of the rope 30 which enters on the strain relief device 60, will normally have the correct orientation in relation to both the winch 40 and the strain relief device 60 as a result of the rope 30 extending directly from the winch 40 to the device 60 and both being mounted together on the vessel's 10 deck. The incoming part 90 of the rope 30 and the outgoing part 80 are further oriented more or less parallel to each other, but separated from each other so that when tension is applied to the rope 30, a torque-force pair of a magnitude will be applied to the tension damper device 60 which is determined not only by the tension in the rope 30, but also by the vertical displacement of the input part 90 in relation to the output part 80.

It should be noted here that although the strain relief device 60 can be particularly adapted for use in securing an oil tanker to a buoy which is arranged at the upper end of a riser leading from an undersea oil well, so that the vessel is held in position in relation to the mooring the buoy while oil is pumped from the well to the tanker by means of a flexible oil line (not shown in the figure), the strain relief device 60 can advantageously also be used when the vessel is to be secured to fixed moorings, e.g. provided by a pier or a fixed platform mounted on piles that penetrate the seabed (for example in shallower areas such as can be found along the continental shelf), as well as when a first vessel is to be moored to a second vessel ( for example a tug to a barge, or a floating storage vessel moored near a subsea well, to a service tanker while the service tanker is loading oil from the storage vessel).

Reference is now made to fig. 2 and 3 which respectively show an isometric perspective view and a partially cut-away side view of the tension damper 60 shown in fig. 1. The rope 30 is controlled or guided by an upper sheave 102 from its essentially horizontal orientation where it extends between the winch 40 and the tension damper 60, over the front part 104 of the upper sheave 102 to the rear part 106 of a lower sheave 108, and then finally to the lower, horizontal part of the rope 30 where it extends from the tension damper 60 in the direction towards the cable guide 70 (fig. 1).

The upper sheave 102 and the lower sheave 108 are rotatably supported at the upper and lower ends, respectively, of a rotatable arm assembly 110. As shown in the figure, the longitudinal orientation of the arm assembly 110 when at rest (zero tension in the cable), is more or less vertical and thus more or less perpendicular to the horizontal orientation of the respective incoming and outgoing parts 90, 80 of the rope 30.

In the one in fig. 2 and 3, the arm assembly 110 comprises two separate plates, namely a left plate 112 and a right plate 114, which are connected to each other at their respective upper ends by means of an upper disc pin 116 and at their respective lower ends by means of a lower disc pin 118. The upper disc 102 comprises a hub sleeve 120 to allow the upper disc 102 to rotate coaxially about the upper pin 116, while the lower disc 108 comprises a similar hub sleeve 122 so that this is similarly rotatably stored about the lower pin 118. Each of the two disc pins 116, 118 is rigidly attached to only one of the two side plates 114 which make up the arm assembly 110. The free end of each pin slides in and out of a sleeve which is arranged as part of the opposite plate 112. With the help of this device, a certain degree of variation in the distance between the left plate 112 and the right plate 114 can be allowed.

The left arm plate 112 is connected to a left outer frame part 124 by means of a left elastomer torsion spring 126. A right elastomer torsion spring 128 similarly connects the right plate 114 to a right outer frame part 130. With particular reference to fig. 3, it can be realized that the left torsion spring 126 and the right torsion spring 128 are essentially identical to each other, each being in the form of a cylinder and each being concentric about a common axis 132 which crosses the arm assembly 110 at an angle of approx. 90 degrees u/2 radians) .

Referring now to the left elastomeric torsion spring 126 which is shown in cross-section in FIG. 3, it will be realized that the torsion spring assembly comprises a relatively thick, outer end plate 134, an intermediate part 136 and an inner end plate 138 which is somewhat thinner than the outer end plate 134. The intermediate spring part 136 is molded from a suitable, elastomeric material, e.g. e.g. neoprene or other rubber-like material resistant to the effects of sunlight, salt water and other related adverse environmental conditions as may be encountered in a marine environment. The intermediate part 136 also comprises a number of rigid reinforcing discs or reinforcing plates 140, each of which is thin, relatively flat and oriented mainly normally on the axis 132 about which the arm assembly 110 rotates. The elastomeric portion 142 of the elastomeric springs 126, 128

is bound to the reinforcement plates 140 and to the two end plates 134, 138, so that when the inner end plate 138 is rotated about the axis 132 and the outer end plate 134 is kept stationary, a torsional displacement effect is transferred to the elastomeric part 142 which in turn produces a counter-twisting moment about the axis 132 with a size that is essentially proportional to the size of the inner end plate 138's rotation in relation to the outer end plate 134.

The task of the reinforcement plates 140 is to impart anisotropic properties to the elastomeric spring, and in particular to stiffen the elastomer against unwanted bending without reducing the torsion spring's ability to allow for winding and unwinding; of one end in relation to the other about the central axis 132 of the cylinder 136 (see also the subsequent description of fig. 4). This is achieved by orienting the reinforcement plates parallel to the direction of the intended shear stress.

It will also be appreciated that the reinforcement plate 140 has the further effect of minimizing the elastomeric assembly's tendency to undergo changes in length or diameter in response to simple external compressive and tensile forces.

Although it will be appreciated that when a torque-force couple is applied to the reinforced elastomeric assembly 136 to produce a shear stress in a direction normal to the central axis 132, the individual reinforcing plate will have no appreciable effect thereon.

In the embodiment shown in fig. 2 and 3, each of the outer ends 134 is fixed in position in relation to its corresponding outer frame part 124, 130 by means of a number of fastening devices 144. The fastening devices 144 engage in a corresponding number of holes 146 in the outer end plate 134. As the fasteners 144 and the respective holes 146 are spaced at regular intervals (e.g. 15°) around the circumference of a circle, it will be appreciated that it will be possible to adjust the original angular orientation of the arm assembly 110 and the two washers 102 , 108 in relation to the orientation of the rope where it exits and enters the strain relief device 60, by choosing which particular peg 144 is to be inserted into which particular hole 146. The minimum increment of such an adjustment will be determined by the angular distance between adjacent fastening devices 144. If a finer adjustment is required, it will however be obvious to a person skilled in the art that such a need can be satisfied by providing a further adjustment device for to reduce the size of a single growth.

A number of bolts 148 are provided to clamp the respective left and right plates 112, 114

forming the arm assembly 110, to the respective inner end plates 138 of the respective left and right elastomeric torsion spring assemblies 126, 128. In the embodiment shown, the respective surfaces of the inner end plates 138 facing the arm assembly 110 are provided with a suitable groove in which the respective arm plate 112,

114 is partially recessed, so that any rotational slip between the end plate 138 and its respective arm plate 112, 114 is prevented. Accordingly, the individual bolts 148 attaching the two end plates 138 to their respective arm plates 112, 114 are not subjected to excessive lateral shear stress as a result of the torque transmitted from the elastomeric springs 126, 128 to the arm assembly 110 and vice versa. In the embodiment shown, it is also possible to adjust the height of the arm assembly 110 in relation to the elastomeric springs 126, 128 and the outer frames 124, 130 by simply loosening the arm fastening bolts 148 and pushing the arm 110 along the aforementioned grooves in the inner end plate 138 to the desired position, after which the bolts 148 can be tightened again. In this respect, it will normally be preferable to center the arm assembly 110 in relation to it

cylindrical torsion element 136's central axis 132 and to orient the winch 40 and the cable guide 70 (see Fig. 1) in relation to the input and output parts 90, 80 of the rope 30 in a symmetrical manner, so that only a pure torque-force pair is applied to the elastomeric springs 126, 128 and these are not subjected to excessively large translational forces.

The twisting moment generated by an elastomeric torsion element can be expressed by the equation

where a is the angular displacement of one end plate in relation to the other in radians, G is the shear modulus of the elastic mtaretegrheiatlsme omi enkit lo i mpert. ekr vai drfajetmrede tepr ote(kngs /m (m 24 ) , ), I ^and L is the length of the two-mer in the axial direction expressed in meters (m). (For a massive cylinder, the polar moment of inertia will be; equal to l/4irR 4where R is the radius of the cylinder in metres.)

Since one degree is equal to 0.0174 5 radians and a typical shear modulus is approx. 1.6 x 10 5 kg/m <2>, it can therefore be calculated that two elastomeric cylinders as they . is shown in fig. 2 and 3 and which has a radius of approx. 0.61 meters and a length of approx. 1.17 metres, will produce a combined counter-twisting torque of approx. 520 meter-kilogram per degree of rotation.

Referring now to fig. 4 which, as will be remembered, is intended to show the geometric relationship between the shear stress and the twist, it will be appreciated that a square element 150 may be defined on the outer circumference of an elastomeric cylinder prior to the application of a torsional shear force such that the square will lie towards a straight line A-B connecting points A and B on the outer and inner ends 154, 156 respectively of the cylinder 152A and parallel to the axis 158 of the cylinder. However, when one end 154 is rotated relative to the other end 156 by an angle a ( the cylinder 152A), so that the line connecting the point A with the point B is no longer a straight line parallel to the axis 158, but instead is a helical line 160, the square element 150 will be deformed (displaced by the angle a) into a rhombus 162 whose respective vertices will be (l/2ir - a) and (1/2tt + a). For a typical elastomeric material, the maximum shear angle to which the material can be subjected without any loss of its elastic properties can be limited to 45 degrees (tt/4 radians). When using the typical values specified above (L equal to 1.17 meters, R equal to 0.61 meters), it can be calculated that the corresponding maximum rotation & of one end plate 134 in relation to the other 138 will be approx. 1.9 radians (109 degrees).

Reference is now made to fig. 5 and 6 which show another embodiment of the invention where a more compact unit is provided than that shown in fig. 1, as a result of the elastomeric torsion elements being stacked within each other

Based on the preceding discussion of fig. 4 you want

remember that careful design practice makes it desirable that no part of the elastomer be subjected to a shear load greater than 45 degrees. If the arm assembly 110 in the embodiment of fig. 2 and 3 must be given the ability to deviate 109

degrees in one direction or the other from its rest position, it is therefore necessary that the diameter of the cylinder is approximately equal to the length of the cylinder. Based on the above formula for the torque generated by an elastomeric element when it is twisted about an axis, one will further notice that the torque is a function of the polar moment of inertia about one axis. The interior of the cylinder thus makes a relatively small contribution to the total torque. Now that both of the aforementioned factors are taken into account, it can be readily realized that by stacking two hollow elastomeric cylinders one inside the other, with a first end of the outer cylinder firmly connected to a first end of the inner cylinder, the torque generated by the inner elastomeric cylinder and the torque generated by the outer elastomeric cylinder are combined in such a way that only a portion of the rotation of the arm assembly 110 relative to the outer frame needs to be absorbed by each of the two stacked cylinders. The embodiment shown in fig. 5 and 6, leaving forde! of these considerations. In these figures, the various reference numbers that have been used previously in connection with the embodiment in fig. 1 and 2, retained, but the mark symbol has been added to distinguish between otherwise similar elements in the two versions. (A similar convention will be applied to

the embodiment in fig. 7 and 8 which use the double tick symbol (")).

As reference is now made in particular to fig. 5 and 6, where fig. 5 shows an isometric view (partially broken away to better show the arm assembly 110) and fig. 6 shows a partially cut side view along the line 6 - 6 in fig. 5, it will be appreciated that this embodiment is much more compact in the direction of the longitudinal axis 132, although its overall height for a given set of operating parameters is determined by the vertical displacement between the input rope portion 90 and the output rope portion 80 and thus will be in essentially identical to the corresponding vertical dimensions of the embodiment described above. The function of the solid cylindrical elastomeric torsion element 126

(Figs. 2 and 3) is now performed by a stacked pair of hollow torsional members, namely an inner hollow cylindrical member 170 and an outer hollow cylindrical member 172. An outer tubular jacket 174 rigidly mounted to a hollow base member 176 corresponds functionally to the left outer frame 124 in the previous embodiment.

Based on the cross-sectional drawing in fig. 6, it can be seen that an inboard annular edge 178 is attached to an inboard annular end plate 180 of the outer elastomeric member 172 by means of a suitable mechanical fastener 182. Although the fastener in the drawing is symbolized by a bolt 182, be aware that other fastening means, such as welding and/or riveting, can also be used to provide the necessary, relatively rigid connection between the outer jacket 174 and the inboard end plate 180. The outboard end of the outer, elastomeric element 172 is terminated by a outer, annular end plate 184 which, by means of a number of bolts 188 (or other suitable fastening devices), is rigidly attached to the outboard, annular end plate 186 of the inner, elastomeric element 170. The inboard end of the inner, elastomeric element 170 is terminated by means of a inboard end plate 138' to which the left arm plate 112' of the rotatable arm assembly 110' is anchored. It can also be seen from the figure that the radial thickness of the inner hollow cylinder 170 is somewhat greater than the thickness of the outer hollow cylinder 172. As previously noted, the torque produced by such an elastomeric element for a given torsional twist is the longitudinal axis, a linear function of the polar moment of inertia, and in reality increases with the fourth power of the radius. As the outer torsion element 172 should share the arm assembly 110' torque and rotational movement equally with the inner torsion element 170, it should consequently have approximately the same stiffness as the inner element, and thus must be somewhat thinner than this.

Both the inner and outer elastomeric elements 170, 172 use the same reinforced, one-piece molded construction as the elastomeric element 136 shown in FIG. 3, and in particular they are preferably cast from a rubber-like material (so-

such as neoprene) capable of withstanding the adverse effects of the maritime environment, the rubber being reinforced by a number of plates of a suitable reinforcing material such as stainless steel, aluminum or Kevlar which are embedded in the elastomer during the molding process and are oriented normally on the device's main axis of rotation 132. However, the individual reinforcing elements are not disk- or plate-shaped, but are instead in the form of outer rings 190 and inner rings 192, respectively. (In this respect, it can be mentioned that the design in Fig. 2 and 3 can optionally be modified by inserting an annular reinforcement element instead of the disc-shaped element 140 shown in Fig. 3, as the disc or plate 140 gain function is more necessary in the direction towards the circumference of the cylinder than in its middle region).

It will be appreciated that the right elastomeric spring of the second embodiment 60' inside the right tubular jacket 191

which is supported by the right base part 193, is the mirror image of the corresponding construction discussed above with respect to the left part of the device. In order to provide additional rigidity, an upper connecting member 194 may advantageously be provided to connect the corresponding upper portions of the left tubular jacket 174 and the right tubular jacket 191. Such a connecting member 194 will also provide a means by means of whereby the entire device 60' can be suitably lifted, and it will serve as a guide to keep the rope entry portion 90 in its proper connection with respect to the upper sheave 102' when the arm mon-

the rope 110' is in its resting position and there will consequently be a possibility of slack in the rope 30.

Reference is now made to fig. 7 and 8 which show another possible embodiment of the invention which is substantially different from the two previously described embodiments which utilize the elastomeric damping effect of a cylindrical body in which the torsional displacement is distributed along the torsion element's axial (longitudinal) direction, one end of the torsion element being attached to a rotatable arm and the other end of the element is attached to a fixed frame. In contrast, the embodiment of fig. 7 and 8 a different design of an elastomeric torsion element in the form of a tapering or conical disk in which the shear load is distributed radially from a hub portion near the axis of the disk to a circumferential portion at its outer circumference, with a significant taper in the thickness of the disk from the hub to the circumference, so that displacement

or the shear action will be substantially uniform throughout the elastomer. The former type of displacement can be considered "longitudinal torsional displacement", while the latter can be termed "radiating torsional displacement".

Such as in the previously described embodiments, is

an inner arm assembly 110" provided with an upper washer 102" and a lower washer 108", each of the two washers being rotatable at opposite ends of the arm assembly by means of an upper pivot pin 116" and a lower pivot pin 118", so that they two discs 102", .108" will be free to rotate about their respective axes which are parallel to a central axis 132" crossing the arm assembly 110" in the area between the two discs.

A left, elastomeric, conical disk element 200 is attached to the left plate 112" of the rotatable arm 110" so that the hub portion 202 of the elastomeric element 200 will rotate about the main axis 132" of the device in accordance with the rotation of the arm assembly 110". The circumferential portion 204 of the elastomeric element 200 is clamped by means of an outer circumferential band 208 which, by means of suitable bolts or other fastening devices 210, is attached to the corresponding left frame 124".

Similarly, a substantially identical, elastomeric, conical disc member 206 is disposed on the right side of the device, with the member's hub portion fixed relative to the arm assembly's right arm plate 114" and with its peripheral portion secured to the right frame member 130" by means of a second fastening band 208. With particular reference to the cross-sectional view of the left elastomeric element 200 which is visible on the left side of fig. 8, it will be noted that each of the elastomeric elements 200, 206 is provided with a number of tubular reinforcing elements 212, each of which is arranged concentrically about the axis of the device 132".

The operating characteristics of a rope tension damper device which is constructed in accordance with the invention will now be discussed, with particular reference to the geometric diagrams in fig. 9 - 14 and the curves in fig. 15.

As will be remembered, FIG. 9-14 meant schematically

to show a typical rope tension damper device 60, 60' or 60" according to the invention where the angle between the arm assembly 110, 110' or 110" and the outer orientation of the rope 80, 90 is respectively minus 30 degrees (fig. 9), 0 degrees (fig. 10), 15 degrees (fig. 11), 45 degrees (fig. 12), 90 degrees (fig. 13) and 120 degrees (fig. 14).

Referring now to fig. 9, it will be appreciated that the figure symbolically shows the upper sheave 102, the lower sheave 108, the arm 110 and a portion of the rope 30 (including the upper portion 90 where it enters the device from the winch, and a lower exit portion 80 where out of the device on its way towards the cable guide 70). Both the upper disc 102 and the lower disc 108 have the same radius "R" and are mounted apart from each other along the rotatable arm 110 at a fixed distance "A". As the rope's 30 entry portion 90 and exit portion 80 will be oriented more or less horizontally during the intended use of the device (see Fig. 1), it can be realized that the stretch will exert a torque-force couple on the strain relief device with a magnitude equal to the stretch "T " times the vertical displacement "V" between the entry and exit sections of the rope. Fig. 9-14 correspond to the tension damper device according to the invention when used with the arm 110 at different angles of inclination in relation to the vertical. In the general case, however, the tensile force T is not necessarily applied to the device in the horizontal direction, and the angle of interest will then be measured in relation to a line normal to the orientation of the incoming rope section 90 and the outgoing rope section 80 (or a line normal to the bisector of the angle between these rope parts in the event that they are not parallel).

Since reference is now made to fig. 9, which is intended to show the geometric characteristics of the device when the arm is inclined backwards at an angle of 30 degrees with the vertical, it will be realized that the vertical distance V is somewhat less than that shown in fig. 10 when the arm 110 is oriented vertically. A simple geometric calculation will show that the vertical distance V in fig. 10 is equal to 2R + A, while the one in fig. 9 is equal to 2 R + A cos 30°." The effective length of the rope 30 will further increase by an amount S=S1+S2=2Re+

A sin & between the position at minus 6 radians which is shown in fig. 9 and the 0 position shown in fig. 10 (or between 0 and plus © radians).

Referring now to fig. 11, showing the arm 110 oriented at an angle of plus 15 degrees to the vertical, it will be appreciated that the vertical displacement V

is now equal to 2R + A cos 15°, while the effective length of the rope 30 will have been further increased by an amount ttR/6.

The corresponding calculations for fig. 12 and 13, which show the arm 110 rotated further as a result of successive increases in the tensile force T, will show that the corresponding vertical distance V will be A cos 4 5° + 2R and 2R respectively, while the corresponding extension S will be A sin 45° + 1/2ttR and A + irR respectively.

Referring now to fig. 14, it will be realized that the tension in the rope 30 has been increased to the point where the angle has been increased to 120 degrees, a value which will normally be greater than the normal operating range of the elastomeric torsion element which, as will be remembered (at least with regard to the numerical examples given in connection with the embodiment in Figs. 2 and 3), is constructed with a normal, maximum rotational deflection of 109 degrees. In all cases, the vertical displacement V will be reduced to an amount equal to 2R - A sin 30°, and the extension S will be (4/3)ttR.

Reference is now made to fig. 15 which is a graphical representation showing the operating characteristics of a rope tension damper device using an elastomeric torsional element subjected to longitudinal torsional displacement, the four curves corresponding to the operating characteristics of the device 60 when the arm 110 has been set to a rest condition (ie, zero tensile force T in the rope 30) with an initial slope of respectively minus 30 degrees (lower curve 30 0), 0 degrees (curve 302), plus 15 degrees (curve 304) and plus 45 degrees (curve 306). These curves correspond to the geometric configurations shown respectively in fig. 9, 10, 11 and 12. It will be appreciated that the first curve 300 is nearly linear and provides a displacement S of about 4 meters when a tensile force of approx. 50 metric tons are applied to the rope 30 (or conversely, a tensile force T of about 50 tons will result in a displacement S of about 4 meters). In contrast, the 0 degree curve 302 starts to rise steeply when the displacement S is over 3 meters and/or the tensile force T is over 40 tons. The 15 degree curve 304 begins to rise steeply at an even smaller displacement S and/or tensile force T. The 4 5 degree curve 306 is even more extreme. The horizontal dotted line 308 which corresponds to approx. 65 tons, and the vertical dashed line 310 corresponding to slightly more than 3.0 meters indicates a typical set of design parameters. From the various curves, it can be seen that an initial orientation of the arm 110 at an angle between 0 degrees and plus 15 degrees will result in a smooth transition that produces a noticeable increase in resistance when the design parameters are achieved, but will allow some additional displacement if the operating parameters are limited, without resulting in an excessively high rate of increase for the tensile force T.

Claims (9)

1. Strain relief device for a steel rope or other relatively inelastic, flexible line for securing a vessel to its mooring, comprising a stationary frame (124, 130) which is connected to a rotatable arm (110) with a first axis of rotation (132). , as the arm rotates from a zero-load position to a position under load, a first disk (102) which is rotatably supported at one end of the arm about a second axis of rotation (116) which is parallel to the first axis of rotation (132) of the arm, and a second disk (108) which is rotatably supported at the opposite end of the arm about a third axis of rotation (118) which is parallel to the first axis of rotation of the arm, the first sheave (102) and the second sheave (108) guiding the rope from a substantially S-shaped configuration to a relatively straight configuration under increasing tension as the arm is brought to to rotate about the first axis of rotation, characterized in that it comprises an elastomeric torsion spring (126) which is oriented about the first axis of rotation (132) of the arm (110) and connects the arm to the frame (124, 130), the spring providing a counter-twisting moment which is approximately proportional to the arm's angular displacement from the aforementioned zero-load position to a position under load, and an arm adjustment device (144, 146) for adjusting the pivot position of the arm (110) in relation to the frame one from a zero-load position to a position under load, so that the rate at which the rope's (30) effective length changes in relation to a predetermined increase in tension in the rope varies. 2. Tension damper device according to claim 1, characterized in that the elastomeric torsion spring (126) comprises a cylindrical body (136) in which the torsional displacement is distributed in the longitudinal direction of the torsion element to cause a longitudinal torsional displacement effect, the cylinder having a first end (112) which is aligned to to rotate with the arm (110), and a second end (134) which is prevented from rotating relative to the frame (124).. 3. Tension damper device according to claim 2, characterized in that it comprises a second frame (130) and a second elastomeric torsion spring (128), the second elastomeric torsion spring being essentially identical to the first elastomeric torsion spring (126) with respect to operating characteristics, one end (114) of the second elastomeric torsion spring is adapted to rotate with the arm (110), and the other end of the second elastomeric torsion spring is prevented from rotating relative to the second frame (130), the second frame ( 130) is separated from the first frame (124) and is separated from this by means of the arm (110). 4. Tension damper device according to claim 1, characterized in that the elastomeric torsion spring (126) comprises at least two cylindrical, elastomeric torsion elements (170, 172), with at least one (172) of the elements being hollow and arranged to accommodate a second (170) of the elements in a concentric, stacked manner, a first end of the hollow cylindrical element being rigidly attached to the frame (176), a second end of the hollow cylindrical element (172) being rigidly attached to a first end of the second , cylindrical member (170), and a second end of the second cylindrical member is connected to the arm (112'). 5. Strain damper device according to claim 4, characterized in that the connection between the other end of the cylindrical element (170) and the arm (112') is a rigid connection (138'). 6. Tension damper device according to claim 4, characterized in that a second such stacked arrangement is provided to secure the arm (110) to a second frame (193) which is separated from the first frame (176) and separated from this by means of the arm. 7. Tension damper device according to claim 1, characterized in that the elastomeric torsion spring (200) has a hub part (202) and a peripheral part (204) so that when the hub part is rotated in relation to the peripheral part, the elastomer in the said element is exposed to a radiating torsional displacement effect which is essentially uniform throughout the elastomer, the hub portion (202) being connected to said arm (110") and the peripheral portion (204) being connected to the frame (124"). 8. Tension damper device according to claim 1, characterized in that the elastomeric torsion spring (126) is reinforced by means of a number of rigid, annular plates (140). 9. Tension damper device according to claim 1, characterized in that the elastomeric torsion spring (200) is reinforced by means of a number of rigid tubes (212).