IE45331B1 - Marine vessel mooring apparatus - Google Patents

Abstract

A hydraulic reactor system for minimizing mooring forces of a vessel attached to an offshore structure, by means of a ramp which enables access between the two, comprising a structural frame provided with a hydraulic piston utilizing a one-way pressure relief valve which allows inward movement with minimal resistance. Conversely, the outward movement of the hydraulic piston provides considerable resistance. The device is connected to, or involved in, a ramp structure allowing an extension or reduction in length of the radius of the geometric configuration as it pivots vertically when attached to a vessel on its outboard end. The upward movement energizes the system by compressive action with minimal resistance, while the downward movement produces considerable resistance to outboard movement of the vessel. This vertical movement of the vessel caused by the waves produces reaction forces in a direction against those elements such as wind, waves and current that would tend to cause horizontal movement of the vessel. It is this process of gently nudging the boat in an up weather direction, with each of its downward movements, the causes an interial reaction that overcomes the forces of the environment with minimal effort. The device can be pivotally attached to a mooring point, known as a "king post", allowing the device and attached ramp/vessel to "weathervane" about the "king post" responsive to changes in direction of the mooring forces.

Description

The present invention relates to the mooring of marine vessels. More particularly, the present -. invention relates to a mooring apparatus having a device which counteracts mooring forces generated by ί adverse weather and wave conditions when the marine vessel is moored to a fixed dock or other mooring structure, especially when such mooring includes the attachment of a ramp structure for the transfer of personnel and/or materials between the vessel and a fixed platform such as for example an off-shore oil platform.

One major problem facing the offshore industry today is the safe and efficient transfer of personnel and equipment between boats and the oil rig platforms.

One solution to this problem is described and claimed in British Patent Specification No. 1,525,501 which involves the mooring of a boat to the rig by means of a special ramp system. However, in mooring boats, particularly large·ones in a heavy sea, great adverse mooring forces can be created under certain conditions.

As the oil and gas industry goes further offshore, wave heights in the neighbourhood of six to eight feet are relatively commonplace and seas can become rougher. Additionally, high winds can become a negative factor. For example, in the North Sea, one can expect 45332 - 3 to be confronted with a wind blowing at twenty knots accompanied by fifteen-foot waves.

According to the invention, there is provided a marine vessel mooring apparatus comprising :a) a frame mounted for rotation about a substantially vertical axis on a mooring point: b) an extendable attachment means mounted for pivotable movement about a substantially horizontal axis on said frame for attaching a marine vessel to said frame; c) means disposed between the frame and the attachment means limiting the amount of extension of the attachment means; and d) directional force resistance means associated with said frame for resisting relative movement between said frame and said attachment means in one direction to minimize mooring forces, said directional resistance means substantially resisting relative movement in the direction of the movement which tends to increase the distance between the vessel and the mooring point but providing substantially less resistance to relative movement in the opposite direction; the action of said resistance means diminishing the adverse mooring forces created by the vessel due to the sea conditions and maintaining an at least substantially constant horizontal separation distance between the attachment means and the mooring point.

The invention also provides a method of minimising mooring forces between a marine vessel and a mooring point by means of a mooring structure which permits the vessel to move up and down in response to sea conditions while maintaining a substantially constant horizontal separation distance between the vessel and the mooring point, said method preferably comprising the steps of providing a variable length elongate mooring structure connected at one end to the mooring point, connecting the vessel to the other end of the mooring structure, the mooring structure being adapted to change its length in response to vertical movement of the vessel due to wave action, said mooring structure being adapted to lengthen against a resistive force and to shorten without said resistive force.

The present invention may be used in a ramp structure to counteract the mooring forces of heavy displacement vessels. British Patent Specification No. 1 525 501 claims an example of a ramp system which can be used with the present invention.

The apparatus of the present invention is intended to maintain a moored vessel at a substantially constant horisontal radius about a fixed point or king post to which the apparatus is attached irrespective of the rise and fall of the vessel. The apparatus achieves this by applying a horizontal force to the moored vessel in the opposite direction to the so20 called mooring forces of wind, waves and current.

These mooring forces exert varying horizontal forces on the vessel and tend to move it in a direction away from its moored position. The horizontal forces applied by the apparatus on the vessel are applied intermitten25 tly, corresponding with the cycle time of each wave because of the directional resistance means. Thus, the apparatus applies a force horizontally to the vessel and against the mooring forces when the vessel descends but not when the vessel is rising on a wave.

As the.vessel rises from the bottom of a wave swell, the length of the apparatus reduces, so that the vessel remains at a relatively constant horizontal radius. Clearly to maintain the substantial horizontal radius during vertical movement of the vessel the length of the apparatus between the point of - 5 attachment of the frame to the king post and the point of attachment of the attachment means to the vessel must vary. Thus upward movement of the vessel causes compression of the attachment means inward towards the fixed point or king” post to which it is attached. In one embodiment of the present invention, a one way pressure release valve is located in a hydraulic piston and cylinder portion of the apparatus. The one way pressure release valve allows this inward movement with minimal resistance. Conversely, outboard movement of the hydraulic piston effects a closure of the one way valve, a further outward movement meets considerable resistance.

In operation, the apparatus having the above characteristics will gently urge the vessel in a direction towards the king post to which it is moored and against the mooring forces of wind, waves and other weather conditions which are trying to move the boat away from the king post. The vessel will meet minimal resistance from the apparatus of the present invention when the vessel is rising on a wave. In this situation the attachment means connecting the vessel to the frame will shorten, and the one-way valve will open allowing the attachment means to slide inwardly. However, as the vessel descends on a wave, the distance between the vessel and the mooring point lengthens and the attachment means tries to move outwardly away from the king post towards the vessel.

Investigation of the movement of a boat in. a sea, reveals that the vertical acceleration of the vessel, due to wave action, far exceeds its horizontal acceleration.

Vertical movements are primarily a function of buoyancy and gravity, being immediately responsive to changes in displacement.

Horizontal movements of a heavy displacement vessel are primarily a function of inertia. The greater the mass, the more resistant it is to the elements.

The operation of the directional force resistance means is based on this differential of vertical and horizontal acceleration. The upward movements will energize the system by pushing the piston inboard, due to its inertia; whereas, the downward movement exerts the necessary resistance to counteract the elements that cause horizontal movement of the vessel.

In a preferred embodiment of the present invention, the directional force resistance means comprise: a) a fluid containing chamber attached to said?: frame; j' b) a piston slidably mounted within said fluid containing chamber; and c) valve means associated with said piston for controlling the speed at which said piston slidably travels through said fluid containing chamber, said attachment means being connected to said piston. The piston may be equipped with a one-way flap type pressure release valve which allows the inward movement with minimal resistance. The extendable attachment means may be in the form of a ramp or gangway and extensions of the piston are attachable to the stringer portions of the ramp. The connections between the attachment means or ramp and the hydraulic piston may be pivotable, allowing the damping effect no matter what the arcuate position of the ramp may be.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which like parts are given like reference numerals and in which:Figure 1 is a general elevational view of an offshore marine vessel moored by an apparatus according «5331 - 7 to the invention to a king post; Figure 2 is a partial perspective view of the connection between the apparatus and the king” post; Figure 3 is a plan view of the connection shown in Figure 2; Figure 4 is a sectional view taken along the lines 4-4 of Figure 3; Figure 5 is a sectional view taken along the ι lines 5-5 of Figure 3; Figure 6 is a sectional view taken along the lines 6-6 of Figure 5; and Figure 7A and 7B are side views Illustrating the operation of the preferred embodiment of the present invention.

As shown in Figures 1 and 2 of the drawings, the preferred embodiment of the apparatus of the present invention is rotatably attached at one end portion to a relatively fixed point, in the form of a structural king post 12 which is rigidly fixed to any desired fixed platform to which a marine vessel will be moored. In Figure 1 the fixed platform is designated by the numeral 14.

The apparatus comprises a frame 10 and an extendible attachment means 16 in the form of a ramp or gangway structure which is pivotally attached by one end to the frame 10 so as to allow relative pivotal movement of the ramp 16 and an attached vessel 22. The opposite end portion of the ramp structure 16 is secured by a connection means 20 to a vessel designated by the numeral 22 in Figure 1.

Pivotal attachment of the frame 10 to the post 12 allows the frame 10 and the vessel 22 to swing or weathervane about post 12 with changing wind and wave action. For example, if the post 12 is at a corner of the platform, the vessel 22 could weathervane through a horizontal arc of approximately two hundred seventy degrees (270°). , Figure 2 illustrates the attachment of the apparatus of the present invention to a king post structure 12 which is structurally and integrally connected to the offshore oil platform 14 or the like.

In Figure 2 a ramp or gangway 15 _ is pivotally connected by pin 25 to the end portions of the frame 10. This pivotal connection by means of the pin 26 is also a slidable connection, with connecting pin 26 being slidably retained in a slot 28 by Outer retaining rings 27. As can best be seen by the arrows in Figures 1 and 2, ramp 16 can thus translate with respect to the king post 12 by the sliding of shaft 26 in slot 28.

The body portion of the frame 10 of the present invention is preferably constructed of a heavy structural metal, such as for example, structural steel. The upper portion, as is shown in Figures 2, 3 and 4, provides a deck 11 on which individuals can walk after leaving ramp 16. Deck portion 11 is provided with any type of non-skid surface to prevent slipping. In the preferred embodiment a check plate deck 11 is provided, which is of non-skid nature. At the junction of ramp 16 and the frame 10, a sliding deck 30 is provided for the transition between ramp 16 and frame 10. Sliding deck 30 is pivotally connected to ramp 16 by pin 26 and allowed to slide upon the surface of deck 11 as will be more fully described hereinafter.

Figures 3 to 6 illustrate the frame lo in greater detail. As can best be seen by Figure 3, the frame 10 has an outer body 32 which can be pivotally attached to a supporting member 12, hereinafter referred to as a king post. Body portion 32 has an upper deck 11 and a bottom 13. Structural side walls 15 are also provided. 5331 - 9 Side walls 15, as can best be seen in Figure 5, can be for example conventional I beams of the size and strength necessary to sustain the mooring forces and bearing loads which may develop. Side walls 15 can be further reinforced using a plurality of stiffeners 17 (Figure 4) at sufficient intervals as is known in the art.

The frame 10 includes a chamber 40 provided with an inner hydraulic piston 41 sliding in the chamber 40.

As can best be seen by Figures 4 to 6, piston 41 is preferably rectangular having a plurality of inner connecting rods 42 to which the end portion of ramp 16 is pivotally connected by means of hinge pin 26. Connecting rods 42 are connected (at their opposite end portions from the hinged connection) to piston web 43, Piston 41, as can best be seen by Figure 4, is dimensioned to slidably travel with attached rods 42 on. the inner surface of piston chamber 40. The chamber 40 can be filled with any conventional hydraulic fluid as needed to damp the sliding effect of the piston 41 within piston chamber 40. Piston web 43 is provided with a flap valve 45, as shown in Figures 5 and 6. The action of flap valve 45 controls the reciprocating movement of piston 41. Flap valve 45 is pivotally connected, as can best be seen in Figure 6, to piston web 43 by a hinge pin 55. From Figures 3 to 6, it can be seen that inward (i.e., towards the king post) movement of ramp 16 will meet minimal resistance, while outward movement of ramp 16 causes a closure of flap 45 and increased resistance to this outward movement.

Piston 41 is so dimensioned as to form a fit with clearance between the piston and the inner walls of piston chamber 40, as can best be seen by Figure 5.

When piston 41 is urged outwardly towards the ramp thereby shutting flap valve 45 and causing maximum 2 3 31 .-10resistance of piston 41 vzith the damping fluid in piston chamber 40, fluid flow can only take place around the outer edges of piston 41. The spacing allows limited fluid movement between the top and bottom flanges 53,54 respectively, of piston 41 as well as its ends.

The outer wall 35 of piston chamber 40 provides openings through which rods 42 pass by means of conventional seals 36 which ensure a substantially fluid tight connection between connecting rods 42 and piston chamber 40.

The end portion of each rod 42 opposite piston 41 is pivotally attached to the hinge pin 26 and thus to the respective stringers, designated by the numerals 38 in Figure 3, of ramp 16. Hinge pin 26 is provided at its ends with retainers 27 to fixedly retain pin 26 within slot 28. Slot 28 is an opening cut in each side wall 15 and has a thickness which preferably corresponds to the diameter of hinge pin 26. Slot 28 can be provided with a stiffening outer rim 31 about its periphery to give slot 28 the necessary structural rigidity to withstand forces transmitted to it through hinge pin 26. The peripheral stiffener rim 31 for slot 28 can best be seen in Figures 2 and 4.

When the push rods 42 and the piston 41 reciprocate, corresponding to reciprocation of pin 26 in slot 28, the sliding deck 30 will slide upon deck 11, since it is fixed on the tubular spacer 60 which can rotate on pin 26 between stringers 38 (see Figure 3). Thus, a safe walkway is provided between ramp 16 and the deck portion 11 of frame 10, notwithstanding the reciprocation of hinge pin 26 and tubular spacer 60.

Figures 7A and 7B best illustrate the method of operation of the preferred embodiment of the present invention. In Figures 77$ and 7B a*conventional offshore S 3 3 1 - 11 vessel 22 is moored to a fixed platform 14 by means of a ramp 16, connected at its opposite end portions to boat connection 20 and frame 10. The vessel 22 bobs upwardly and downwardly in response to wave action, causing an oscillation of the vessel and the end portion of ramp 16 at the connection 20.

Figure 7A illustrates the relative positions of vessel 22, ramp 16 and hinge pin 26 at the crest of a wave. In this position, the ramp connects the shortest distance between vessel 22 at connection 20 and frame 10 at pin 26. In Figure 7A, it can be seen that this distance is substantially a horizontal straight line. However, as the end portion of vessel 22 at connection 20 begins to fall towards the bottom of a wave or swell, connection 20 and the end portion of the boat will move downwardly and cause the end portion of ramp 16 to follow, thereby creating a greater distance between connection 20 and frame 10. It can be seen from Figure 7B that the extra distance necessary causes ramp 16 to pull on pin 26 and slide it to an outward position in slot 28. The cycle of the bobbing vessel is completed when the next sequential wave urges the vessel upwardly as the crest of the wave passes along the vessel, again shortening the distance between connection 20 and reactor 10, causing pin 26 to slide to an inward position within slot 28.

In the absence of other forces, this phenomenon will continue, as the mass of the boat would hold it in the same position in a constant horizontal radius about king post 12.

However, other forces also act on vessel 22 besides the gravitational and buoyant forces as described above. These are the forces which act on the vessel in a primarily horizontal direction and include all the forces such as wind, waves, current, and the like. These forces, collectively referred to hereinafter as mooring forces, tend to move the vessel in a direction awayifrom its desired spot adjacent the fixed platform.

While the vessel has such a large mass which tends to cause it to be moved very slowly, a great problem occurs when the vessel finally reaches the end of its rope and begins to exert a tremendous force component on the fixed platform itself vzhen the vessel descends into the trough of a wave.

The frame 10 produces a horizontal reactive component of force created when the vessel proceeds downwardly responsive to wave action. As can best be seen in Figure 7B, when the vessel moves downwardly at the bottom or trough portion of a wave, its weight pulls the ramp 16 downwardly and since the horizontal distance of the boat at connection 20 from king post 12 remains substantially constant, the ramp must slide outwardly to compensate for the increased diagonal distance between frame 10 and boat 22 at connection 20.

The boat descending has a great vertical force component, but also a lesser component is exerted on the boat when its downward movement in an absolute vertical direction is prevented. The one-way valve in the frame 10 provides resistance to the outward movement of the ramp. The frame thus exerts a horizontal force component on the vessel which tends to urge or nudge it towards the fixed platform.

In the ideal situation, this reactive force would be designed to exactly offset the force which is exerted on the Vessel by wind, vzave action, current, and the like, referred to as the mooring forces. It is these mooring forces designated by arrow 70 in Figures 7A and '7B which try to urge the vessel In a direction away from the fixed platform.

It would only be a matter of straightforward engineering design to construct frame 10 to be able to -45332 - 13 cope with any mooring force situation and maintain vessel 22 in a relatively constant horizontal radius about king post 12. Additionally, the mounting of frame 10 in a rotational arrangement on king post 12 allows the vessel, ramp, and reactor to weathervane about king post 12 when the direction of mooring forces changes.

A cycle of the physical interactions which take place on the complete rise and fall of vessel 22 will be described hereinafter. Beginning with Figure 7A as an illustration, the end portion of vessel 22 at connection 20 will be at its highest position, as can best be seen in Figure 7A on the crest of a wave. The pin portion 26 of reactor 10 slides within slot 28 to an innermost position. By the design of flap valve 45 (see Figures 5 and 6), it can be seen that this inward movement of pin 26 within slot 28 is met with minimal resistance, corresponding to the opening of flap valve 45 which allows damping fluid in cylinder 40 to flow freely as piston 41 moves inwardly.

As the crest of a wave passes and the trough approaches, vessel 22 begins to fall towards the bottom of the trough as does connection 20 and the end portion of ramp 16. While the horizontal radius remains constant, between connection 20 and king post 12, the distance diagonally between connection 20 and frame 10 increases, the vessel tries to pull on ramp 16 pulling pin 26 towards an outward position within slot 28. As the ramp urges the pin 26 outwardly, flap valve 45 closes and maximum resistance is met, as the damping fluid within chamber 40 is only able to flow around the outer edges of piston 41, a much smaller area than the area provided when flap valve 45 is in an open position. Due to the mass of the vessel, it is of course able to pull pin 26 close to the outermost position. However, there is a substantial amount of force exerted on vessel - 14 22 which tends to nudge or urge vessel 22 in an upweather direction a slight distance. This distance would then be lost when the vessel rises on the next sequential wave crest, as the frame 10 exerts minimal force on vessel 22 when the vessel rises and pin 26 moves to the innermost position (position A) in slot 28. Flap valve 45 opens and fluid freely flows through it with minimal resistance. During this inward movement of pin 26, and minimal force application to vessel 22 from frame 10, the mooring forces in the direction of arrow 70 will urge the vessel away from king post 12 a small distance until it is re-nudged back to its original position on the next downward fall of the vessel with the next wave trough.

It can be seen from the above, that an idealized situation could possibly be created through engineering design whereby the vessel is maintained in approximately the same horizontal radius from king post 12.

In fact, frame 10 could be provided with an adjustability which would allow it to be controlled and the vessel moved gradually towards or away from the king post as needed with different weather conditions, wave heights, tides, wind, and the like.

This adjustability could be in the form of, for example, a change in volume of hydraulic fluid contained within dampening cylinder 40.

Another method of adjusting the resisting effect of frame 10 could be by providing a variable orifice to vary the volume of fluid which could flow through piston 43 in either the inward or outward direction.

In fact, an operator could merely examine the net distance which the vessel was slowly moving and adjust the variable orifice to overcome the slow encroachment of the vessel in any given direction.

The slot 28 along which pin 26 reciprocates should be dimensioned so as to allow sufficiently for «5331 - 15 the reciprocation of ramp 16 and its attached pin 26 therein. This would be a function of the length of the ramp, and the anticipated size of the waves at a given time. A well designed unit would of course provide for any weather conditions which might be expected in a given area.

Additionally, although the preferred embodiment has been described with respect to a vessel being moored about a point on a fixed structure, the present invention could be applied to a mooring system having multiple mooring points and reactors or to mooring a vessel to something other than a fixed structure such as for example mooring one vessel to another.

Claims (9)

1. A marine vessel mooring apparatus comprising:a) a frame mounted for rotation about a substanti ally vertical axis on a mooring point; 5 b) an extendible attachment means mounted for pivotable movement about a substantially horizontal axis on said frame for attaching a marine vessel to said frame; c) means disposed between the frame and the 10 attachment means limiting the amount of extension of the attachment means; and d) directional force resistance means associated with said frame for resisting relative movement between said frame and said attachment means in one direction 15 to minimize mooring forces, said directional resistance means substantially resisting relative movement in the direction of the movement which tends to increase the distance between the vessel and the mooring point but providing substantially less resistance to relative 20 movement in the opposite direction; the action of said resistance means diminishing the adverse mooring forces created by the vessel due to the sea conditions and maintaining an at least substantially constant horizontal separation distance between the attachment means 25 and the mooring point.

2. Apparatus according to claim 1, wherein said attachment means is an elongate structure t one end of said structure being pivotably connectable to the Vessel and the other end portion being connected to said frame. 30

3. Apparatus according to claim 1 or claim 2 wherein said directional force resistance means is comprised of: a) a fluid containing chamber attached to said •32331 - 17 framej b) a piston slidably mounted within said fluid containing chamber; and c) valve means associated with said piston for 5 controlling the speed at which said piston slidably travels through said fluid containing chamber, said attachment means being connected to said piston.

4. Apparatus according to claim 3, wherein said valve means is a one-way valve mounted in said piston. 10 5. Apparatus according to claim 4, wherein said one-way valve is a flap valve. 6. Apparatus according to any one of claims 3 to 5, wherein said fluid is a liquid. 7. An apparatus according to any one of claims 1 15 to 6 wherein the extension limiting means includes means for limiting the maximum and minimum extension comprising a rigid member movable and between two fixed stop members, said directional force resistance means at least partially serving to prevent said rigid member 20 from contacting the fixed member in the direction of maximum extension and tending to move said rigid member . towards the fixed member in the direction of minimum extension. 8. An apparatus according to any one of claims 1 25 to 7 wherein the extendible attachment means comprises an extendible ramp. 9. An apparatus according to claim 7 or claim 8 wherein the extension limiting means comprises a pin member movable in a slot, the pin defining said rigid 30 member and the ends of said slot forming the two fixed stop members. ι - 18 10. A method of minimizing mooring forces between a vessel and mooring point comprising the steps of: a) providing a variable length elongate mooring structure connected at one end to the mooring point?

5. B) connecting the vessel to the distal end of said elongate mooring structure; c) said elongated mooring structure being adapted to lengthen and shorten under the actions of the vessel being moved up and down by wave movement; and

6. 10 d) resisting by a substantial force the lengthening of said mooring structure but allowing said mooring structure to shorten without such substantial resistance in order to maintain the horizontal separation distance between the distal end of the mooring structure and the 15 mooring point substantially constant.

7. 11. Method according to claim 10 wherein said mooring structure includes a piston moving within a fluid, and wherein the force resistance is applied by increasing the resistance of a piston moving through 20 said fluid as the mooring structure lengthens and is diminished by decreasing the resistance of said piston moving through said fluid as the mooring structure shortens.

8. 12. A marine vessel mooring apparatus substanti25 ally as hereinbefore described with reference to the accompanying drawings.

9. 13. A method of minimising mooring forces acting between a marine vessel and a mooring point, substantially as hereinbefore described with reference to the 30 accompanying drawings.