GB2064469A

GB2064469A - Transhipping at sea

Info

Publication number: GB2064469A
Application number: GB8038697A
Authority: GB
Original assignee: Akers Mek Verksted AS
Current assignee: Akers Mek Verksted AS
Priority date: 1979-12-06
Filing date: 1980-12-03
Publication date: 1981-06-17
Also published as: NO793977L

Abstract

A vessel unloading arrangement wherein the vessel has a load- carrying platform (4) provided on the deck (2) which is movable up and down relative to the deck, and which is moved up to a middle position through a distance which exceeds half of the total periodic motion A of the vessel before the tension is applied to the hoisting wire 3. The platform (4) can move against resilient opposition (19, 20) as the tension is increased in the hoisting wire, and the platform is then lowered when the load is borne entirely by the hoisting wire. <IMAGE>

Description

SPECIFICATION A load transferring device The present invention relates to a method and a device for transferring a load from a vessel which moves up and down due to waves to the support by a crane which is substantially at rest and whose hoisting wire is connected to the load.

When unloading vessels outside sheltered waters, the vessel and load will move up and down due to waves. Even on calm days there will be ground swell offshore which imparts a periodical vertical motion to the vessel. When a load unit on such a vessel is to be unloaded by means of a crane which may be situated f.inst. on a fixed oil platform, jerks can easily occur in the hoisting wire when it is tightened due to the unavoidable motions of the vessel, so that the wire and the derrick of the crane may be subjected to shock loads. Such shock loads may often be tolerated due to the elasticity in the hoisting wire and the crane derrick.

However, particularly when hoisting heavier load units, other problems may easily arise. In such cases the lifting speed of the crane is usually considerably lower than the velocity by which the vessel moves up and down due to the waves.

When shortening the hoisting wire, it will often become tight and lift the load for the first time while the vessel is at a throw of the waves. Since the lifting speed of the crane is lower than the average speed of the vertical motion of the vessel, the vessel will catch up with the load and impact against the bottom side of the load on its way up on the next wave crest. Such impacts can be very strong and give rise to heavy dynamic loads which easily may damage both the load and the vessel.

Thus, it is the purpose of the invention to provide a method and a device which make it possible to avoid such strong impacts between the load and the vessel. This is obtained according to the invention by a method of the type mentioned by way of introduction, where the characteristic features are that the load is placed on a frame, platform or the like on the vessel, that the frame, platform or the like is raised with the load to a middle position a distance which preferably exceeds half the total vertical periodical motion of the vessel, the frame, platform or the like preferably being arranged to be movable resiliently up and down around said middle position, that the hoisting wire of the crane is tightened in order to take over the weight of the load, and that the frame, platform or the like is lowered when the weight of the load is substantially taken over by the crane.

Further advantageous features of the method according to the invention and a device for carrying out the method are indicated in the subclaims.

For a better understanding of the invention, it will be described with reference to the exemplifying embodiment shown in the appended drawing.

In the drawing 1 designates a load, which is to be lifted from the deck 2 of a vessel by means of a crane (not shown), the hoisting wire of which being designated 3. The load 1 rests on a frame 4 which has quadrangular shape and which at the bottom side near the corners is provided with depending legs or columns. The columns 5 are slidably arranged in guides 6 which penetrate the deck 2 of the vessel. At their lower end, the columns 5 are provided with a wire sheave 7, and a wire 8 extends from an attachment 9 at the underside of the deck 2 down over the wire sheave 7 and onwards up over an idler wheel 10, which likewise is attachedvto the underside of the deck.From here the wire continues to one end of a connection element 11, which in the horizontal plane has quadrangular form and is slidably supported between two parallel guiding rails 12, so that it cannot pivot in the horizontal plane.

From the other end of the connection element, the wire 13 runs over an idler wheel 14 attached to the deck and from there over a wire sheave 1 5 supported at the free end of the piston rod 1 6 of a cylinder device 1 7. From the wire sheave 1 5 the wire 13 runs to a winch 18.

The space 1 9 closed by the piston 20 in the cylinder device 1 7 contains a gas under pressure, the cylinder device 17 thus forming a spring means. The space 1 9 may be attached, f.inst. via a line 21 , to a larger, possibly variable gas volume in order to give the cylinder device a suitable low spring constant. The line 21 may also be connected to a gas pressure source, so that the pressure in the cylinder device may be adjusted to the weight of the load 1. The cylinder device 17 and the winch 1 8 thus form a winch known per se providing generally constant tension.

When the invention is to be used, the load 1 is firstly placed on the frame 4 while the latter is in the lower position resting on the deck 1. The hoisting wire 3 from the crane is attached.

Thereafter, the frame 4 is hoisted by puliiny in on the winch 18, while concurrently a sufficient pressure is created, resp. maintained in the space 1 9 in the cylinder 1 7 for the piston 20 to stay substantially in the middle position. In this way the frame 4 and the load is raised a distance h, which preferably is somewhat larger than half A of the total vertical, periodic wave generated motion 2 A of the vessel. If the load 1 is very heavy compared to the displacement of the vessel, it is also necessary to take into consideration changes in the draft of the vessel when the load is removed.

When the load has arrived at the desired height h above the deck 2, the winch 1 8 is stopped and the hoisting wire 3 is tightened. Concurrently the gas pressure in the cylinder space 1 9 may be reduced somewhat, so that the frame 4 no longer is able to support the total weight of the load 1.

The difference between the weight of the load and the lifting capacity of the frame is transferred to the hoisting wire 3.

In this condition the load 1 and the frame 4 will be generally at rest while the deck 2 of the vessel moves up and down with an amplitude A to both sides of the middle position shown in the drawing.

The piston 20 performs a corresponding motion in the cylinder device 17.

As the hoisting wire 3 is tightened, more and more of the weight of the load is transferred thereto. If the amount of gas trapped in the cylinder space 19 is not changed, the middle position of the frame 4 will concurrently be raised with respect to the deck 2, and the piston 20 in the cylinder device 17 will also have its middle position displaced towards the left in the drawing.

However, the middle position of the frame 4 may be kept substantially constant by letting out part of the gas in the cylinder space 19. During this operation the frame 4 will move up and down with respect to the vessel, and keep in contact with the load 1.

When a sufficient part of the weight of the load 1 has been transferred to the hoisting wire 3, the frame 4 is quickly lowered, preferably by paying out wire 13 from the winch 18. Concurrently, further amounts of gas may be let out of the cylinder space 19, or the piston rod 16 may be blocked, preferably while the piston 20 is situated towards the right in the drawing. The entire weight of the load 1 is thereby taken over by the hoisting wire 3, and a considerable distance between the load and the frame 4, resp. deck 2 is obtained, so that no part of the vessel will any longer be able to impact against the load during the wave induced motion of the vessel.

The frame 4 may advantageously be lowered when the vessel is at the crest of a wave, i.e. when the frame 4 is in its lower position. Thereby the frame 4 does not have to move much before it reaches its original position in contact with the deck 2 and, furthermore, the following motion of the vessel will be downwards. In addition, the vessel will have to go through an entire period of its wave induced motion before it again is in an upper position. This period is usually sufficiently long for the crane to have lifted the load 1 to a safe distance from the vessel. Of course, the frame 4 may be lowered when the vessel is in other positions, but in that case it will be advantageous to lower the frame with a speed exceeding the difference between the speed of the vertical motion of the vessel and the hoisting speed of the crane.Better still, the lowering speed should be higher than the vertical speed of the vessel. Thus, better security against any impacts against the load is obtained, f.inst. should the load be lowered somewhat because of elasticity in the hoisting wire and crane derrick, when the support by the frame ceases.

It will be understood that the connection element 11 which is introduced between the wires 8 for the columns 5 and the wire 13 leading to the winch 18, functions to ensure that the wires 8 are tensioned, resp. slackened equally so that the frame 4 is displaced in parallel with the deck 2 and jamming betwee the columns 5 and guides 6 is avoided.

Furthermore, it will be understood that the invention may be practised by means of other devices than the one shown in the drawing. As an example, the columns 5 may be moved by other means than the wires, the cylinder device 1 7 and the winch 18. F.inst. one may envision the columns 5 constituting piston rods in hydropneumatic cylinders which concurrently form both lifting means and spring means.

Furthermore, it is not necessary to have a column in each corner of the frame 4. One may use other forms for effecting parallel motion of the frame, scissor arms as an example, and the lifting resp.

spring means may be constituted by a centrally placed hydropneumatic cylinder device.

To the skilled person it will also be clear that the invention advantageously can be used when placing a load on a support moving more or less periodically up and down. In addition, the invention may be used with certain modifications where the load by means of a crane in motion is to be placed on or removed from a support which is substantially at rest.

Claims

1. A method for transferring a load from a vessel which moves up and down due to waves, to the support by a crane which is substantially at rest and whose hoisting wire is connected to the load, characterized in that the load is placed on a frarne, platform or the like on the vessel, that the frame, platform or the like is raised with the load to a middle position a distance which preferably exceeds half of the total periodical motion of the vessel, the frame, platform or the like preferably being arranged to be movable resiliently up and down about said middle position, that the hoisting wire of the crane is tightened in order to take over the weight of the load, and that the frame, platform or the like is lowered when the weight of the load is substantially taken over by the crane.

2. A method according to claim 1, characterized in that the frame, platform or the like is lowered with a speed whidh exceeds the difference between the speed of the vertical motion of the vessel and the hoisting speed of the crane.

3. A method according to claim 1 or 2, characterized in that the frame, platform or the like is lowered with a speed exceeding the speed of the vertical motion of the vessel.

4. A method according to a preceding claim, characterized in that the lowering of the frame, platform or the like is started approximately when the vessel passes the highest point during its periodical motion.

5. A device for carrying out the method according to a preceding claim, characterized in that it comprises a load carrying frame, platform or the like (4) for installation in a vessel (2), means (5, 6) for parallel motion of the frame, platform or the like (4) in substantially vertical direction with respect to the vessel (2), and means (2-21) for raising and lowering the frame, platform or the like (4) with respect to the vessel (2).

6. A device according to claim 5, characterized in that the said means (7-21) for raising and lowering the frame, platform or the like (4) are resilient.

7. A device according to claim 5 or 6, characterized in that the frame, platform or the like (4) has the form of a polygon at the corners of which are placed depending columns (5) which are slidably arranged in guides (6) in the deck (2) of the vessel, that the columns at their lower end are provided with a wire sheave (7), and that a wire (8) runs from an attachment (9) in the vessel down over the wire sheave (7) and up therefrom over an indler wheel (10) in the vessel, wherefrom the wire continues to a winch means (17, 18).

8. A device according to claim 7, characterized in that the winch means (17, 18) is arranged to give substantially constant tension.

9. A device according to claim 8, characterized in that the substantially constant tension of the winch means (17, 18) is variable.

10. A device according to claim 7, 8 or 9, characterized in that between the idler-wheels (10) and the winch means (17, 18) the wires (8) for the individual columns (5) are connected to means (1 1, 12) providing that the wires (8) are equally tensioned resp. slackened.

11. A method according to claim 1 and substantially as hereinbefdre described with reference to, and as shown in the accompanying drawing.

12. A device according to claim 5 and substantially as hereinbefore described with reference to, and as shown in the accompanying drawing.