AU2001244879A1 - Load transfer system - Google Patents

Description

Load transfer system

The invention relates to a system foi lifting and moving heavy loads, especially for use in the installation or removal of offshoi e platforms, comprising a number of lifting devices which are arranged for mounting on a floating structure which during use floats on the surface of a volume of watei beside the load which has to be lifted

Various types of systems and devices are known the pπoi art for lifting heavy sections during the installation or removal of offshore platforms Conventional methods used in this connection have usually been based on the use of offshore crane ships or heavy lift vessels In a typical operation of this kind, a crane ship will be positioned close to a platfoim and lift the various sections of the platform in a predetermined sequence

Systems or devices are also known which ai e designed to lift the entire uppei part or deck of a platform a single operation Such a system, which is of the type mentioned at the beginning, is the so-called "Vei sati uss" lifting system

This system is a twm-barge lifting system which is based on two barges which are located at a suitable distance apart on opposite sides of the structure which has to be lifted, and which are mtei connected and can be pulled towards each other with great force by means of winch devices On each barge aie placed a numbei of lifting beams which ai e tilted inwai ds and upwai ds in the dn ection of the load, and which are bi ought into engagement with the load The two barges are then pulled towai ds each other, thus causing the angles of inclination of the lifting beams to inci ease as the distance between the bai ges decreases, thereby causing the load to be lifted up in the area between the barges which then forms a catamaran configuration

This known system is based on custom-built lifting beams which therefore have to be specially made oi adapted for each lifting operation Furthermore, the system is restricted to use in l elatively calm watei s, without paiticularly large waves, since very heavy lifts of up to 20 000 tons ai e involved heie, and thus very gi eat forces, with the l esult that large waves can easily cause senous damage to oi desti uction of the equipment, since the system does not possess any kind of heave compensation

Another known device in this field is the so-called "offshoie shuttle" This is a U-shaped, unmanned offshore vessel based on tugboats foi operational assistance During installation oi l emoval of platform sti uctui es the vessel is ballasted so that it is submerged and surrounds the upper part of the platform or the deck, and then deballasted so that the structure is lifted A typical "offshore shuttle" may be 150 m long, 80 m wide and 60 in high, with a weight of around 12 000 tons A large structure is therefore involved here The sti ucture has the advantage of being extremely stable in a submerged condition, in the same way as a semi-submei sible structure oi "semi-sub" However, this principle l equπes a corresponding depth at the application site

In practice the hitherto known concepts, which are based on the semi-sub principle or on two interconnected vessels in a catamaran configuration, will be faced with senous problems In the case of a semi-sub, one of the main problems is the time it takes to evacuate the ballast to a point where the semi- submerged structure makes contact with the object which has to be lifted, and the time it takes to peifoim the lift The problems involved ai e naturally associated with the heaving motion experienced by the floating objects as a result of wave action

As mentioned above, a semi-sub will be l elatively stable and relatively little affected by waves Assuming, however, that the waves have an influence, the mass forces which are set in motion will have to be absorbed when the floating structui e encounters the object which has to be lifted The senous consequences involved can easily be imagined if such a floating structure ( 12000 tons) should experience a heaving motion of 1 m and hit a platform deck on the way up from a wave ti ough

The second problem which may be encountei ed is when thei e is a failui e to perform a lift to a safe height within a wave period when using a semi-sub solution oi a two-vessel system The consequence theieof can be that the load (the platform deck) is l eplaced on the foundation from which it was lifted, thereby causing damage oi possibly losing the entπe lift, or that an accelei ation is experienced during installation as a lesult of faulty "timing"

A fuither pi oblem in connection with such lifting operations is the unceitamty which l eigns concerning the disti ibution of weights on a platfoi m deck In earlier times there was no adequate documentation and conti ol of the building process, nor were the subsequent modifications to the platforms completely documented This can lead to ignoi ance of the platform deck's centre of gravity, with the result that a controlled deballasting of the floating structures cannot be prepared in order to take this factoi into account The consequences can be a tilted lift, or in the worst case failuie to peifoim the lift

In view of this, it is an object of the invention to provide a system which has substantial lifting powei and lnhei ent heave compensation, whei e the system can conti ol the forces tiansfen ed to the lift object dunng the entn e lifting opei ation, and a lift to a safe height can be accomplished within a wave penod, with the l esult that the system is suitable foi lifting exti emely heavy loads while being reliable in opei ation

A second object of the invention is to pi ovide a system of this kind which is cost-effective and module-based, thus enabling sevei al lifting devices to be connected together as requπ ed

In ol der to achieve the above-mentioned objects, a system is pi ovided of the type mentioned in the mti oduction which accoi ding to the invention is chai actenzed in that each lifting device compnses a level aim unit with a first and a second aim projecting in opposite dn ections fi om a common mounting point, the fu st arm having a lifting point at its fi ee end foi engaging with the load, at least one first containei which is connected to the fu st aim at a point near the said lifting point, and which is ananged to leceive and discharge a flowable medium and to be submeiged in the volume of watei , and at least one second containei which is suspended at the fi ee end of the second arm, the teπoi of the container being connected via a pipeline device, and a device is pi ovided foi fast tiansfei of medium in the fu st containei via the pipeline device to the second containei

The system according to the invention is cost-effective since it is based on the use of floating structuies in the foπn of existing baiges oi other suitable vessels which can be lined The lifting devices will be pi efabncated and modulat, thus enabling the system to be easily ti anspoited to the site wheie a lifting opeiation has to be peifoimed A typical system foi lifting a platfoim will be compiised of two baiges in a catamai an configuiation with two oi moi e lifting devices located on each baige It may be expedient to pi ovide a hydiaulic auxiliary system with In di auhc cylmdei s which ai e connected between lespective levei aim units and the floating stiuctuie. in order to ensui e that lntei acting levei arms ai e lifted in pai allel and unifoimly. and thus enabling the lift can be earned out in a conti olled mannei By combining a floating structure with substantial load capacity with one or more partly submerged containers in the manner indicated, the advantage of both stability and lifting power is obtained, thus piovidmg minimal movement in the water and maximum lifting power By transferring the force in the manner indicated by means of weight transfer from the partly submerged containers at one end of the lever arm unit, a passive heave compensation is obtained and particularly a progressive heave compensation as the containers ai e increasingly submerged

The invention will now be described in greater detail m connection with an embodiment with refei ence to the di awings. in which

fig 1 is a schematic side view of a system according to the invention,

figs 2 and 3 are a side view and a top view respectively of an embodiment of the system according to the invention, and fig 4 is a perspective view of the system in figs 2-3, whei e the lifting device has been brought into position beside a part of a platform which has to be lifted by means of the system

In the drawings, coπ esponding parts and elements in the different figui es are indicated by identical l eference numerals

The schematic view in fig 1 illustrates a system accoi d g to the invention where a lifting device 1 is mounted on a floating sti uctui e in the form of a bai ge 2 floating on the sui face 3 of a volume of watei 4 The lifting device comprises a lever arm unit 5 with a first arm 6 and a second arm 7 pi ojecting opposite directions from a common mounting point 8 a suppoit structure 9 which is advantageously located in the centre of the bai ge's 2 deck 10, prefeiably with the mounting point's 8 axis of l otation located in the vertical plane through the barge's longitudinal centre line The system will thereby convey the load down into the centre of the barge, or generally in the centre of the floating structure employed, without causing any rolling motion on the floating structure The first arm 6 is provided at its fiee end with a lifting point 1 1 for engaging with a corresponding or complementary lifting point on the structure which has to be lifted The device further comprises a first container or tank 12 which is connected via a load-transmitting part 13 with the first arm 6 at a point 14 near the lifting point 1 1 at the aim's free end, or possibly more or less coinciding with the lifting point The containei 12 has an internal volume for receiving a desired amount of a flowable medium, such as water, or possibly a suitable sludge

The container is provided with an upper inlet device 15 for supply of medium As illustrated, the container is partly submerged m the volume of water 4, with the result that it is influenced by a corresponding buoyancy The container is vertically movable along the adjacent outer side of the barge 2 For control of the container's movement, between the container and the said outer side of the baige there is mounted a guide device which is shown in the form of guide i ails 1 6, 1 7 and intermediate l ollei s 1 8

The containei 12 may advantageously be designed with an upper portion with reduced cross section, with the l esult that in its submerged state it has l elatively little waterhne ai ea in the wave zone This will help to increase the stability of movement in the watei in a similar manner to a partly submerged object (semi-sub)

The device further comprises a second containei or tank 1 9 which is suspended in a suspension point 20 at the free end of the second arm 7 The interior of the second container 19 is connected to the liiteπor of the first container 12 via a pipeline device 21 for transfer of medium in the first containei 12 to the second container 19, or vice versa The system demands fast transfer of the medium oi liquid volume concerned, and this can advantageously be achieved by means of compressed air, oi another suitable pi essui e gas For this puipose the container 12 is connected at its upper end with a compressor unit 22 with associated storage tanks for pressuiising the intei ior of the containei with compressed air (or pressure gas) The compressor unit must have sufficient capacity to ensure transfei of the volume of medium concerned (e g approximately 400 tons) in the course of a few seconds If the medium employed is water, the water volume, which is ti an sf erred to the second container 19 during a lifting operation, is di a ed out of the container via a suitable outlet 23 Instead of a compressor system a suitable pumping system may be employed

As illustrated in fig 1 , a hydraulic cylindei/piston unit 24 is connected between the barge's 2 deck 10 and the levei arm unit's second arm 7 The unit 24 represents an auxiliary system which may be applied paiticularly when several lifting devices are airanged beside one another on the barge, working in paiallel By means of suitable activation of the units concerned 24 during a lifting operation, a corresponding downwardly-directed additional force can be achieved on the second arm 7, and a corresponding additional lifting force m the lifting point 1 1 This may be necessary in the case of unequal weight distribution of the load which has to be lifted, in order to achieve parallel lifting movement and a controlled lift

The hydraulic auxiliaiy system 24 may also be arranged to determine the lever arms' turning angle, and thereby the suitable lifting height for the lifting point 1 1 duπng the performance of a lifting operation As an alternative, the auxiliary system may be connected between the first arm 6 and the barge deck 10. in oi dei to exert a desn ed additional foi ce on the arm

When a lifting operation is caiπed out with the piesent system, the barge 2 is positioned in such a manner that the lifting point 1 1 remains located under the coπ esponding lifting point on the load which has to be lifted To begin with, the container 12 will be filled with a suitable medium with a weight corresponding to the volume of water in the watei 3 which is displaced by the container in the submerged position, with the result that the lever arm unit 5 is in balance When the barge is in the correct position, at least a part of the medium, foi example water, in the container 12 is rapidly transferred to the container 19 at the levei ai m unit's second end If a water volume of, e g , 400 tons is transferred, this will l esult in a corresponding upwardly-dnected force on the lifting point 1 1. assuming a level arm ratio of 1 1 Since the containei 12 will still essentially be submeiged as befoie, despite the lifting movement achieved, the buoyancy foi ce comes in addition to the said lifting foi ce of 400 tons, thus giving a total lifting force of 800 tons

By employing a suitable choice of lever aim ratio other than one, a coiiespondmg increase in lifting foice can be obtained This can be achieved by means of a suitable extension of the second aim 7, i e without a reduction in the length of the first aim 6, and theieby the same lifting height is achieved as before

On account of the levei arm's lotational movement about the axis of l otation 8, the lifting point 1 1 at the end of the arm 6 will move along a circular path about the axis of lotation Even though this involves a circle with a relatively laige tadius of appioximately 20-30 m. it ma\ be necessary to compensate foi the small deviation from vertical movement of the lifting point 1 1 This can be accomplished in various ways, e g by means of sufficient tolerance in the engagement between the lifting point and the complementary lifting point on the load which has to be lifted The lifting point 1 1 may be provided with limited movement (longitudinal or rotatable) on the arm 6

An embodiment of the system according to the invention, as it may be constructed in practice, is illustrated in figs 2-4

As illustrated in the figures, in this embodiment the lever arm unit 5 is composed of a truss constiuction which extends substantially outside the barge 2 on each side thereof, with the levei arm unit's arms 6, 7 extending across the barge's longitudinal direction In this embodiment a group of three first containers 12 is provided, the containers being attached to a support frame 25 which is connected to the load-transmitting pait 13 A guide fi ame 26 is attached to the adjacent outer side of the barge 2, and is connected to suitable guide elements, for example rollei s (not lllusti ated in more detail) which are engaged with respective vertical guide rails 27 which are attached to and extend along respective containers 12, to permit the necessary, vertical movement of the containers

Heie, the second containei 19 is in the foim of a large, square tank whose vertical movement is controlled by a guide device in a similar manner to the containers 12 The guide device here consists of a frame 28 which is attached to the adjacent outer side of the barge 2. and which is attached to vertical guide rails 29 foi contiol of suitable guide elements 30 foi vertical movement of the tank 19 As illustrated in figures 2 and 4. the pipeline device 21 between the containers 12 and the tank 19 is provided with a flexible poition 3 1 , with a view to the vertical movement of the containers 12

With regard to the dimensions of the barges which are suitable for use in the system according to the invention, these noimally have a length of 97 m, a width of 27 m and a height of 6 m Even though the illustrated embodiment of the system a barge is employed, in other cases it may be appropriate to use another type of vessel, a rig, or possibly a semi-submersible structure

A typical configuration for the system according to the invention may comprise two lifting devices 1 which are arranged side by side on a barge 2 which is suitable positioned at one side of a platfoim section 32 which has to be lifted by means of the system, as indicated in fig 4 An additional barge, which is not illustrated in fig 4, will be located in a similai manner on the opposite side of the platfoim section 26, and will be pi ovided with two lifting devices 1 which are arranged in a similar mannei side by side on the barge The lifting operation conceined will be performed in a similai manner to that descnbed above, the lifting devices' lifting points being connected to coiiespondmg lifting points (not shown) at the coinei s of the platfoim Similai hydi aulic auxiliaiy systems to the above-mentioned auxiliaiy system 24 may be pi ovided, and will be activated as l equiied, foi example in case the platform section should have an unequal weight distnbution and thei eby a diffei ent weight in the comei ai eas

As will be l ea sed fi om the foi egomg description, the system accoi dmg to the invention has a numbei of unique and advantageous chaiactenstics These can be summaπsed as follows

The system does not ti ansfei gi eatei loads than those peimitted (depending on the volume of liquid in the fu st and second containei s, and the buoyancy of the submeiged containei s) to the object which has to be lifted This applies fi om the fu st contact until the lift is completed The system is self-i egulating since the lift object is gnpped by a levei aim which will be balanced at all times If the floating sti uctui e expenences an upwai dly-dn ected movement as a lesult of wave influence, the levei aim at the point of application will give way while still being capable of ti ansfei nng foi ces coii espondmg to the weight/buoyancy ratio between the fu st and second containei s Heave compensation is achieved as a lesult of the above, and as a lesult of the paitly submeiged containei s

A lift can be earned out to a safe height in the couise of a wave penod (A safe height will be deteimmed by the l atio between the lifting speed and the wave penod, thus avoiding conflict between the lift ob]ect and the suppoiting base aftei the lift is initiated) The l eason foi the system being able to achieve this is the dual effect obtained b moving the liquid between the containei s, and the fact that the container oi containei s which initially contain the liquid aie paitly submeiged in watei. and that by means of gas oi pumps a sufficient volume of liquid is moved within a given penod of time The system takes account of the fact that the lift object does not necessarily have its centre of gravity in the centre, and there is therefore a risk of tilting during the lift This results from the fact that by means of the volume of liquid in the containers the force can be deteimmed by which the arms should grip the lifting points on the lift object, m order to keep the lift object in a horizontal position

The system will further be able to keep the lift object m an approximately horizontal position even though the effect of a wave influences the floating sti uctui e This is due to the fact that at all times a balance will be maintained of the forces influencing the lever arms from the foi ces, and the forces influencing the arms from the lifted object thi ough the lever ai ms

The conti ol of the liquid transfer between the containei s on the various levei arms can be regulated by load cells placed at sti ategic points between the object which has to be lifted and the base away fi om which the object has to be lifted, thus providing a continuous measurement of the forces applied to the lifting object via the arms, and thereby also controlling the lifting of the object in a horizontal position

Claims (1)

1. PATENT CLAIMS

   1 A system for lifting and moving heavy loads, especially for use in the installation or removal of entire or paits of offshoie installations, compnsing a number of lifting devices (1) which aie arranged foi mounting on a floating structure (2) which during use is located beside the load (26) which has to be lifted, characterized in that

   - each lifting device (1) comprises a lever arm unit (5) with a first (6) and a second (7) arm projecting in opposite directions from a common mounting point (8), the fust arm (6) having a lifting point ( 11) at its free end for engaging with the load (26).

   - at least one first containei (12) which is connected to the fust aim (6) at a point near the said lifting point (11), and which is arianged to receive and discharge a flowable medium and to be submerged in a volume of watei (4), - at least one second containei (19) which is suspended at the fiee end of the second arm (7),

   - the interiors of the containers (12, 19) aie mtei connected via a tiansferπng device (21)

   2 A system accoiding to claim 1, characterized in that a device (22) is piovided foi transfening medium between the first containei (12) via the tiansfei" device (21) to the second containei (19)

   3 A system accoiding to claim 2, chaiacteπzed in that tiansfei of medium is undeitaken by a compressoi unit (22) for pressurising the interioi of the first containei (12) with compiessed an

   4 A system according to claim 1 oi 3, characterized in that a guide device (16, 17, 18) foi veitical movement of the containei (12) is arranged between the containei and an adjacent outer side of the floating structuie concerned (2)

   5 A system according to one of the claims 1-4, charactenzed in that between the levei arm unit's (5) second arm (7) and the floating stiuctuie (2) there is arranged a hydiaulic cylindei/piston unit (24) which is arraimed to exeit a force on the second aim (7) 6 A system according to claim 5, characterized m that the hydraulic cylinder/piston unit (24) is arranged to determine the lever arms' (6, 7) turning angle, and thereby the lifting point's ( 1 1 ) lifting height

   7 A system according to one or more of the preceding claims, charactenzed in that it comprises a number of floating structures (2) which during use are each located on one of two opposite sides of the load (26) which has to be lifted

   8 A system according to claim 7, characterized that it comprises foui lifting devices ( 1 ) which are mounted two by two beside one another on the l espective baige (2), the device's lifting points ( 1 1 ) being adapted for engagement with a respective cornei poi tion of the load (26) which has to be lifted