GB2125738A - Manipulating legs of a self- elevating platform - Google Patents

Abstract

The upper section of each leg 11-13 of a self-elevating platform 5 is cut as a single portion so as to limit the height of the remaining leg portions to an admissible value for towing the platform structure in a barge configuration. After cutting, shoes 161-163 grip rack teeth on each leg and raise the cut portion by hydraulic jacks. Then one shoe 161 acts as a pivot and the portion is swung arcuately by a deck crane 34 and cable 36 for lateral displacement. This frees the space above the corresponding remaining leg portion, and the cut portion is stored on the caisson of the platform structure. The leg is reassembled after a displacement in the opposite direction of the cut portion and positioning the latter above the corresponding remaining leg portion and after joining the two leg portions. <IMAGE>

Description

SPECIFICATION Self-elevating platform legs manipulation process and device for cutting and storing leg portions and for reassembling platform legs.

The present invention concerns a process and a device for manipulating the legs of a selfelevating platform, so as to make it more easy and more economical to carry out all operations and works which are necessary before and after any trans-oceanic towing operation of the self elevating platform and so that respectively all required safety specifications and regulations for this towing operation may be satisfied and the platform may then conveniently rest in position on the seabed of a choosen site.

The invention relates more particularly to a process and a device for cutting and storing upper portions of legs and for reassembling the platform legs.

The self-elevating platforms, also called jack-up platforms, which are well known and commonly used for the exploration and drilling operations on off-shore work sites and also for production facilities, mainly comprise a floating caisson or hull which is of a basic barge construction, the upper surface of which defines a deck on which are generally mounted a derrick or rig substructure, a deck crane as well as many other tooling necessary to run the platform. Such a platform mainly comprises also several legs for resting on the seabed of the choosen work site and for anchoring the platform on this seabed. The caisson has leg wells within each of which a leg is located, and a jacking unit for lifting and lowering the corresponding leg which generally comprises at least a lifting assembly including a rack or another suitable device is located inside the caisson around each leg well.Thus the legs the longitudinal axis of which generally extends perpendicular to the deck can be raised up in relation to the caisson in such a way that the lower end of each leg is almost at the level of the underface of the caisson or hull and that the major section of each leg projects upwards above the deck. In this configuration, the jack-up platform behave like a mobile barge which can be towed onto a choosen site where the legs can be lowered in relation to the caisson in such a way that the leg lower ends rest on and/or penetrate to a certain extent into the seabed.

The caisson is then raised up on the legs which are anchored in the seabed after a preload of the platform involving ballasting of compartments provided in the caisson for receiving preload water, and the caisson is immobilized at a given height above water level so that the air gap thus obtained is large enough to keep the caisson out of reach of waves swell and tidal effects as long as work is carried out on this site.

Such jack-up platform presents the advantage that at the end of the operations carried out on a first site, and in certain conditions, the legs can be released from the seabed and raised up in relation to the floating caisson so as to give again to the platform a barge configuration which it is possible to tow on another work site where the legs are once again lowered and anchored in the seabed.

The legs of such jack-up platforms, for example in the number of three for one platform, generally have a polygonal cross-section, most of the time a triangular or square cross-section, and are truss leg with a trellis structure. This trellis structure comprises as many chords as the polygonal cross-section has apices, each chord being generally an elongated structural member substancially perpendicular to the cross-sectionnal plane of the corresponding leg. The chords of each leg are connected to each other by means of internal and external braces and gussets and the trellis structure of each leg also supports a lifting assembly, for instance a rack assembly, cooperating with the corresponding jacking unit located around the well of the caisson within which the leg is received. Further to this, the leg also comprises other members like ladders, etc.

Of course is the leg length in relation with the depth of water on the work site, the nature of the seabed and with roughest environmental and meteorological conditions that can be encountered on this site, so that the leg length is superior to the addition of the depth at this site, with the maximum penetration height of the leg lower ends in the seabed, with the height of the air gap between water level and the caisson, with the distance between the bottom of the hull and the upper guides of the jacking units and with a safety margin. For example the legs equipping a jack-up platform intented to work on sites with 300' of water have an overall length reaching 430'.In accordance With safety specifications and with regulations imposed by insurance corporations, a jack-up platform in a barge configuration (with raised up legs) must withstand angular notions in the course of which the legs incline in any direction at a maximum angle of 20 with respect to the vertical direction with a period reaching 10 seconds. This means that each leg must withstand very important shearing forces at the leval where the leg is fitted in the caisson, these shearing forces being proportionnal to the cubic value of the height of the leg section projecting above its fitted section in the caisson, with projecting section being precisely at its maximum when the platform is in a barge configuration.

As the leg structure is dimensionned in relation to the forces supported when the leg rest on the seabed, it appears that the above mentioned conditions which must be satisfied for towing are effectively satisfied when the platform legs have a length corresponding to the use of the platform on sites where the water depth does not exceed 230'. With modern jack-up platforms working on deeper sites, these conditions cannot usually been satisfied. In the last case, it is necessary, prior to a towing operation, to cut parts of the upper section of the leg and/or to disassemble this upper section, so as to shorten the height of the section of a raised up leg which projects above the caisson when the platform is in a barge configuration, and to give to this projecting section such a limited height that these safety specifications and regulations can be satisfied.And then, after the towing operation, it is necessary to reassemble the platform legs by joining the several cut parts and/or by reassembling the structure of the leg upper section on the leg section having an acceptable length.

But these successive cutting and/or disassembling operations and following reassembling operations of an important number of leg parts are time consuming and costly.

The present invention is aimed at shortening the cutting and reassembling operations of the legs of jack-up platforms.

The invention is also aimed at making more easy to carry out these operations by the use of an appropriate device.

This invention makes it also possible to store on the platform itself the cut leg portions, thanks to a special structure for this purpose.

In accordance with the invention, there is provided a method of manipulating legs mounted on a floating caisson of a self elevating platform, for example prior to a towing operation of said platform towards a choosen work site, which method comprises the steps of: lowering said legs with respect to said floating caisson and anchoring said legs in the seabed of another prior choosen site; raising said caisson on said legs at a sufficient height above water level;; tutting the upper section of each leg so as to shorten the leg length until said length is short enough for towing said platform structure after having raised up the legs with respect to said caisson in a floating barge configuration while satisfiing safety regulations and specifications, wherein said upper section of each leg is cut as a single portion and said cut portion is then displaced so as to free the space above the remaining portion of the corresponding leg for allowing said remaining portion to be raised up with respect to said caisson.

In accordance with the invention, there is also provided a method of manipulating legs mounted on a floating caisson of a self elevating platform, for example on a choosen work site after a towing operation of said platform following a cutting operation of the upper section of said legs on another prior choosen site, which method comprises the steps of: lowering the corresponding remaining portions of said legs with respect to said caisson and anchoring said remaining portions in the seabed of said work site; raising up said caisson on said remaining leg portions at a sufficient height above water level; -reassembling the upper section of each leg on the corresponding remaining section, so as to give to each leg its initial length, wherein said upper section of each leg is displaced as a single portion so as to position each portion above the remaining portion of the corresponding leg, and said upper portion is then joined to said corresponding remaining portion.

Preferably, in accordance with the invention each cut leg portion is displaced as a whole by pivoting and/or translating it while maintaining it parallel to itself.

To cut the leg portions, the method in accordance with the invention further comprises the step of positionning said caisson successively with respect to the different legs choosen so that the height of the legs with respect to water level is decreasing and so that for each leg the cutting operation is carried out in a transversal plane above the deck defined by the upper face of said caisson.

When the platform is equipped with truss legs comprising each chords, internal and external gussets and a lifting assembly as constituting members, the method according to the invention for cutting and storing the portions of the upper sections of legs further comprises for each leg, the steps of: joining to each chord of said leg a transversal displacement device; tutting the leg forming members in said cutting plane preferably after having cut on at least a part of each chord perimeter a profiled member through which said cutting plane extends, so as to make it more easy to cut the leg forming members positionned inside the corresponding chord; ; raising up said transversal displacement devices joined to the cut portion of said leg with respect to said leg remaining portion by raising up said transverse displacement devices with respect to said caisson and by then raising up said caisson on said leg remaining portions, so as to have said cut portion completely disengaged from said corresponding ieg remaining portions; lowering said transversal displacement devices joined to said cut portion with respect to said caisson until a least one transversal displacement surface of each transversal displacement device engages a facing transversal displacement surface of said caisson, with a large enough remaining gap between said cut portion and said corresponding leg remaining portion; displacing said transversal displacement devices onto said transversal displacement surfaces of said caisson so as to laterally displace said corresponding cut portion until the space above the corresponding leg remaining portion is free, and positionning each cut parts of the corresponding cut portion chords on a support structure carried by said caisson; -and joining said chord cut parts of said cut portion and said support structure, preferably by interpositionning joining members having a profile corresponding to that of said profiled members cut on at least a part of each chord perimeter, so as to immobilize said corresponding cut leg portion on said platform structure in a storage position.

According to the invention, the method for reassembling the legs preferably comprises the step of positionning said caisson successively with respect to the different leg remaining portions choosen so that the height of said remaining portions above water level is increasing and so that for each leg remaining portion the joining operation is carried out in a transversal plane above the deck defined by the upper face of said caisson.

When the truss legs of the platform have been prior manipulated according to the method that is presented thereabove for cutting and storing the cut leg portions, the method according to the invention for reassembling the legs further comprises the steps of: ---disassembling the corresponding leg cut portion from the support structures carried by said caisson, preferably by cutting said joining members interpositionned between said cut chord parts of said cut portion and said support structures;; displacing said transversal displacement devices joined to said cut chord parts of said cut portion onto said transversal displacement surfaces of said caisson so as to laterally displace said cut portion and position it above the corresponding remaining leg portion in such a manner that each of said cut chord part of said cut portion stands just above the corresponding remaining chord part of said leg remaining portion; raising up said transversal displacement devices joined to said leg cut portion with respect to said caisson so as to disengage the transverse displacement surfaces in face to face relationship of said caisson and said transverse displacement devices;; lowering said caisson on said leg remaining portions so as to bring closer to each other the ends of said cut let portion and remaining leg portion which are to be joined together, and then lowering said transversal displace ment devices with respect to said caisson so as to engage said leg cut portion and said leg remaining portion in end to end relationship; -reassembling the leg by joining the ends which are to be joined of the corresponding forming members of said cut leg portion and remaining leg portion preferably by closing each chord with the joining of the corresponding profiled member on at least a part of the chord perimeter; -and disassembling said transversal displacement devices from said joined chord parts of said leg upper portion joined on said leg remaining portion.

The invention also provides a device for carrying out the methods of the invention on a self elevating platform having a floating caisson with wells within which the legs are lowered jammed or raised up with respect to said caisson by means of a lifting unit cooperating with a lifting assembly integral with each leg comprising chords linked together by means of internal and external gussets and braces, and with each chord preferably equipped with a lifting assembly, which device comprises:: -at least a transversal displacement device for each leg chord, each transversal displacement device being a massive member for being joined to the corresponding chord and having at least a transversal displacement surface, one of said massive members having a lifting element for cooperating with the lifting assembly of said corresponding leg when said corresponding massive member(s) are joined to the corresponding chords; -at least a transversal displacement box located on the deck defined by the upper face of said caisson and having at least a transversal displacement surface for cooperating with the surface(s) of said massive member(s) so that said latter transversaly move onto said box(es);; -a raising and lowering mechanism for lowering and raising said massive members with respect of said transversal displacement box; -a transversal displacement mechanism of a leg cut portion onto at least one of the corresponding boxes; -and a support structure for each said cut chord part of a corresponding cut leg portion, each support structure being carried on said caisson for receiving and being joined to said corresponding cut chord part on a storage position on said caisson, after transversal displacement of said corresponding cut leg portion.

Preferably the device according to the invention further comprises joining members for being interpositionned each between a sup port structure and the lower end of said corresponding cut chord part and for being joined to said support structure and said lower end, the lower surface of each joining mem ber having a shape complementary to that of the upper surface of said corresponding sup port structure and the upper surface of each joining member having a shape complementary to that of the lower surface of said corresponding chord part, which lower surface is partially complementary to the shape of at least a profiled member cut on at least a part of the chord perimeter when said corresponding leg upper portion is being cut and joined in its initial position when said leg is being reassembled.

In a preferred embodiment, the raising and lowering mechanism comprises at least a jack for each of said massive members joined to said chords of a leg, sais jacks being located inside said transversal displacement boxes so that the jack piston rods pass through the upper face of said boxes which defines said transversal displacement surfaces of said caisson.

Preferably the transversal displacement devices, boxes and surfaces are skidding and/or rotating shoes, boxes and surfaces, and the lifting assemblies and elements are respectively racks and rack elements having complementary shapes so as to engage each other.

In a preferred embodiment the device comprises, for each leg, a rotation box onto which a shoe joined to one chord part is received for rotation and a skidding box having an arcuate shape onto which the shoes joined to the other chord parts of said cut leg portion are received for skidding when said cut leg portion is rotating around said rotation box.

It is preferable that, for each leg, abutments limiting said skidding and/or rotation movements are integral with said skidding and/or rotation boxes so as to help the transversal positionning of said cut leg portion at the end of its displacements respectively after the cutting operation of said leg portion and prior to the reassembling operation of said corresponding leg.

In an embodiment having a simple structure the transversal displacement mechanism of a cut leg portion comprises a system of cables and blocks attached to fixed points of said caisson and of at least one of said transversal displacement device of said corresponding cut leg portion.

It is also an advantageous feature of the device according to the invention that it comprises a horizontal and vertical positionning assembly for positionning each said cut leg portion onto said corresponding remaining leg portion, said positionning assembly comprising for each leg chord a vertical centering axis with a conical end and integral with one oz said two portions of said corresponding leg by means of a support member having a flat face received in the cutting and reassembling plane and fixed inside said corresponding chord so that said conical end extends beyond said cutting and reassembling plane, and said positionning assembly comprising further a vertical cylindrical housing for receiving said centering axis and integral with the other of said two portions of said corresponding leg by means of another support member having a flat face received in said cutting and reassembling plane and fixed inside said corresponding chord, so that the end of said housing which is closest to said cutting and reassembling plane is flush with this plane in such a manner that the vertical positionning is achieved when the flat faces of said two support members engage each other and that the horizontal positionning is achieved when the centering axis penetrates into the corresponding housing.

Further features and advantages of the invention will emerge from the following description of an embodiment of the invention illustrated in the drawings, wherein: Figure 1 illustrates schematically a jack-up platform with three legs, Figure 2 is a schematic elevationnal view partly in cross-section of a device cooperating with a leg chord at the beginning of the cutting operation of the corresponding portion, Figure 3 is a plan view of the assembly represented on Fig. 2, Figure 4 is a view similar to Fig. 2 in a different position of the device, corresponding to a following step of the cutting operation, Figure 5 is a cross-section along line V-V of Fig. 4, Figures 6 and 7 are views similar to Fig. 4 corresponding to different positions of the device in following steps of the cutting sequence, Figure 8 is a partial plan view of the platform showing the manipulating device associated to one leg, Figure 9 is a shematic elevationnal view partly in cross-section of the support structure on which a chord cut part of a leg portion rests with interposition of a joining member, Figures 10 to 13 are views similar to Figs.

2, 4, 6 and 7 corresponding to successive positions of the device in the sequence of a reassembly operation of a corresponding leg.

As shown more particularly on Figs. 1 and 8, the jack-up platform comprises three substancially vertical legs 1, 2 and 3, each of which is positionned at an apex of a triangle to provide a good rest position of the platform on the seabed. Each leg is located in a leg well 4 of a water-tight caisson 5 or hull of basic barge construction with compartments for receiving preload water, and the upper surface of which defines a deck 6. The caisson 5 is equipped with a lifting unit generally indicated in 7 for lifting jamming and lowering the corresponding leg received in each well 4 with respect to the caisson 5.Each leg is made of a metallic trellis framework attached at its lower end to a foot 8 for resting on or for partially or totally penetrating in the seabed 9 when the platform is positionned on a choosen site, i.e. when the legs 1, 2 and 3 initially in a raised up position in relation to the caisson 5 in afloat condition and in a barge configuration, as represented with dotted lines in 5' are lowered with respect to the floating caisson 5 and then when the caisson 5 is raised up at a certain height above the water level with the help of the lifting units 7.

The metallic framework of each leg has a cross section in shape of an equilateral triangle and comprises three identical chords 11 each of which is a rigid tubular structural member of square and/or diamond shape cross-section. The three chords 11 are each located in an apex of the triangular cross section and are connected to each other by internal braces 12, external braces 1 3 and gussets.

Further each chord 11 is integral with a rack 14 facing outwards of the corresponding leg and clearly shown on Fig. 2. This rack 14 cooperates with one of the three members 71, 72 and 73 positionned at the three apices of the triangular opening of the corresponding well 4 and being parts of the lifting unit 7 for lowering raising up or jamming the corresponding leg with respect to the caisson 5.

For example, this jack-up platform is intented to be position ned on sites having a water depth of 300' approximately, and the length of the leg reaches 430'. To satisfy the safety regulations and specifications which have been presented thereabove, the height of the legs must be limited to 340' approximately for towing operations when the platform is in a barge configuration with the legs raised up.After the platform has left the shipyard where it has been constructed, it is first towed on a first site choosen for carrying out the cutting operations of the upper leg sections as a single portion 1 5 having a length of approximately 90'. Preferably this first site is so choosen that the cutting operations can be carried out on the three legs of the platform, i.e. the water depth on this site is such that with the addition of the maximum penetration height of the legs in the seabed 9 and with the addition of the minimum air gap height between the water level 10 and the caisson 5, it is inferior to the maximum admissible height for towing minor the height above the bottom of the hull of a special structure described thereafter and which is mounted on the deck 6 of the caisson 5, for receiving the single portion 1 5 which is cut at the upper section of each leg 1, 2 and 3.

After positionning the platform on the first choosen site and preloading involving ballasting of the caisson 5 for providing a good anchoring in the seabed 9, as shown on Fig.

1, the deck 6 is positionned in relation to the highest leg with respect to the water level 10, which leg is leg 1 on Fig. 1, so that the transversal cutting plane of the leg is at a predetermined height above the deck 6. This predetermined height is ajusted in relation to the geometry and machining characteristics of three shoes 1 6 provides for being each joined to one of the three leg chords and for being displaced or pivoted on surfaces of the caisson 5, so that a gap remains between the shoes 1 6 and the surfaces. As it is shown on Fig. 2, each shoe 1 6 is a massive member a part of which is formed as a rack element 1 7 which has a shape complementary to that of the rack 14 integral with the corresponding leg chord.Each shoe also has at its underface a skidding and/or rotation surface 18 with a cylindrical recess 1 9 for receiving the upper end of a piston rod 20 of a hydraulic jack.

The cylinder 21 of this jack is located inside a box 22 mounted on the deck 6. The upper surface 23 of the box 22 is a skidding and/or rotation surface with an orifice allowing the rod 20 to pass through this surface 23, and the jack lifts the shoe 1 6 so that a gap of approximately 1" remains between the skidding and/or rotation surfaces 1 8 and 23 in face to face relationship respectively under the shoe 1 6 and over the box 22. This gap makes it possible to adjust the rack element 1 7 of shoe 1 6 in engagement with the rack 1 4 of the chord 11.The piston rods 20 of the three jacks which are associated to a same leg are blocked under the respective shoes 1 6 with a pressure in each jack balanced at 1/3 of the total weight of the portion 1 5 of the corresponding leg. Each shoe 1 6 is then joined to the corresponding chord 11 by welding on the rack element 1 7 and on this chord 11 two upper lateral rigid plates 24 and two lower lateral rigid plates 25. Given the diamondshaped section of chord 11, these plates 24 and 25 have each a general triangular shape and are positionned in pairs on either side of the rack element 17, so as to form a V in which the two external faces of the chord 11 are received, said external faces being those faces welded to the plates 24 and 25.

It is then possible to carry on the cutting operation of the three chords 11, the racks 14, the braces and gussets, the ladders etc. . . in the transversal cutting plane indicated by numeral 26 on Fig. 2, so as to cut the portion 1 5 from the remaining portion of the leg. As this is shown on Figs. 4 and 5, two profiled members 27 through which the cutting plane 26 extends are first cut out of the external faces of chord 11, so as to facilitate the access towards the inside of the chord 11 for cutting the rack 14 and the gussets.

When all the constituting members of the leg through which plane 26 extends, except three centering axes described thereafter, have been cut, the cut portion 14 thus obtained is lifted about 2" with respect to the deck 6 with the help of the three jacks 20, 21 to provide a progressive disengagement of the three centering axes 28 of the corresponding leg, as this is indicated in dotted lines on Fig.

6. Then the deck is lifted by raising the caisson 5 on the leg remaining portions which are anchored in the seabed. Thus, the lower end of the portion 15 is separated from the upper end of the leg remaining portion, so that the centering axes 28 are entirely disengaged.

These centering axes 28 are parts of a horizontal and vertical positionning assembly which is mounted for each leg when the platform is constructed. Each of these centering axes 28 is fixed inside the lower section of a chord 11 in relation to the cutting plane 26 on a transversal wall 30 of this chord 11 and with the help of a massive support member 31 which surrounds it.This support member 31 has a square cross-section and a flat upper surface flush with the plane of the upper end of the corresponding leg remaining portion, whereas the conical end 29 of this centering axis 28 projects beyond and above this plane for being received in a cylindrical housing 32 in another massive support member 33 or in a sleeve integral with this other support member, which also has a square section, is fixed to this part of chord 11 that is intended to be cut with the portion 15, i.e. the upper part of this chord 11 with respect to the cutting plane 26. This other support member 33 also has a lower flat surface flush with the lower end plane of portion 1 5.

The support members 31 and 33, which are shown in cross-section on Fig. 12, thus have facing surfaces for providing a vertical positionning of portion 1 5 in relation to the corresponding leg remaining portion, when the reassembling operation of the leg is carried out, as is explained thereafter, whereas the centering axes 28 and housings 32 provide the required transversal or horizontal positionning of same two leg portions.

After a complete disengagement of centering axes 28 from housings 32 as is shown on Fig. 7, portion 1 5 is lowered in relation to the deck 6 by means of the three jacks the rods 20 of which disappear inside the box 22, so that the undersurfaces 18 of shoes 1 6 rest on upper surfaces 23 of box 22. The gap between the lower end of portion 15 on one hand, and the upper end of the corresponding leg remaining portion and conical ends 29 of axes 28 on the other hand remains large enough so that nothing opposes to a transversal displacement of the whole formed whith portion 1 5 and the three corresponding shoes 16.

If this is not the case, this gap can be increased thanks to a further lifting movement of the deck 6 on the leg remaining portions.

As shown on Fig. 8, one of the three shoes 1 6 for each leg is a rotation shoe 1 61 provided for pivoting substancially around the vertical axis of the corresponding jack 20, 21 on a part of box 22 which constitutes a rotation box 221 mounted on the deck 6 outside the corresponding leg cross section, whereas the other two shoes 1 6 correspond ing to the portion 1 5 of this leg are skidding shoes 162 and 1 63 provided for skidding horizontally on an arcuate part of box 22 which builts an arcuate skidding box 222, which extends along a sufficient portion of a circle so that by pivoting as a whole around the rotation center of the rotation box 221, the portion 15 and the shoes 161, 162 and 1 63 which carry it can be displaced side wards, from the position shown on full lines on Fig. 8 and corresponding to the position just above the leg remaining portion, to the position shown in dotted lines, in which the cut portion 1 5 is completely out of the space above the leg remaining portion, so that this latter can be later raised up in relation to the caisson 5 in view of a towing operation. Prior to this pivoting, the skidding surface 23 over the arcuate box 222 is coated with an appro priate grease to lower the friction factor.This sidewards displacement is made more easy thanks to a deck crane 34 mounted on the deck 6 and which provides for vertical traction on the shoes 1 6 by means of cables and attachement padeyes 35 integral with the upper face of the shoes 1 6 as is shown on Fig. 2, and this sidewards displacement is controlled by means of a transversal displace ment mechanism comprising a system of cables and blocks.This system comprises for example a cable 36 attached by one of its ends to a first fixed point 37 on an end of the arcuate skidding box 222 and runs succes sively around a first block 38 borne by the skidding shoe 163, around a second block 39 attached to a second fixed point near the first fixe point 37, and around a third block 40 attached near the rotation box 221, so that a traction on cable 36 in the direction of the arrow by means of a winch (not shown) displaces the shoe 163 until it reaches the end of the arcuate skidding box 222 which is close to the block 39 and to the fixed point 37, with the skidding shoe 162 occupying substancially the initial position of the other skidding shoe 163.

Conversely, the displacement from the posi tion indicated in dotted lines, or storage posi tion of the portion 1 5 on the deck 6 of the platform, to the position indicated in full lines, just above the leg remaining portion, for reas sembling the leg and which is more precisely described thereafter, is obtained by a traction in the direction of the arrow on a second cable 41 attached by one of its ends to another fixed point 42 at the other end of the arcuate skidding box 222 and running succes sively around a fourth block 43 fixed to the skidding shoe 162 and around a fifth block 44 attached near the fixed point 42.

Two abutments (not shown) for limiting the displacements are mounted on the arcuate skidding box 222 in order to facilitate the lateral positionning of the portion 1 5 in each of its extreme positions. In the position indicated with dotted lines, each of the cut parts of the portion chords 11 is just above a support structure mounted on the caisson 5, one of these support structures 45 being in fact not above the deck but supported by two cantilever beams 46 and 47 extending sideways from the caisson 5. After having checked the good position of the portion 1 5 just above the corresponding support structures, each cut part of the chords 11 is joined to its support structure like structure 48 as is shown on Fig. 9, with the interposition of a welded joining member 49.Each joining member 49 is a machined and bevelled metal plate the lower face of which has a shape corresponding to that of upper face of the support structure 48 (for example a flat surface), and the upper face of which has a shape corresponding to that of the lower end of the cut part of the corresponding chord 11 of portion 15, this shape matching particularly the shape of the upper face of the profiled members 27 which are cut out of this chord 11 as this has been described with reference to Figs. 4 and 5.

After a positive check-up operation of the weld lines indicating that the portion 1 5 is correctly fixed in its storage position and towing position, the same sequence of operations can be reproduced by means of similar devices for the other two legs, the highest leg with respect to the water level being choosen first.

Referring to Fig. 1, the cutting operations are carried out successively on legs 1, 3 and 2. The three portions 1 5 having been cut, laterally displaced and fixed in their storage position, it is then possible to lower the caisson 5 in afloat condition and to raise up the leg remaining portions in view of a transoceanic towing operation which leads the platform onto a second convenient site choosen for carrying out the reassembling operation of the legs and close to the work site of the platform if not identical with it.

After having positionned the platform on this second site and after having preloaded the platform, the deck 6 is position ned in relation to the lowest leg remaining portion with respect to the water level 10, which is the remaining portion of leg 2 with reference to Fig. 1, so that the joining or reassembling plane corresponding to the cutting plane and defined by the upper end plane of the leg remaining portion is situated above the deck 6 at a certain height which is adjusted after having checked the height of the shoes 1 6 so that nothing opposes to the coming back of the corresponding portion 1 5 just above the leg remaining portion, the relative position of these elements being substancially the position shown on Fig. 7.

The portion 1 5 is then separated from the caisson 5 by cutting the joining members 49 linking the chords parts to the support structure 48 along cutting planes indicated by thick lines 50 on Fig. 9. Then, the different constituting members of the leg which need to be joined are prepared. For example the lower ends of the cut parts of chords 11, of rack 14, gussets and the upper ends of chords 11, rack 14, gussets of the leg remaining portion are bevelled and ground.

Then the portion 1 5 is pivoted from its storage position to its position just above the leg remaining portion (see Fig. 8) by pivoting the shoe 161 on box 221 and skidding the shoes 162 and 163 on the arcuate skidding box 222, with the help of a traction diven by the deck crane 34 and thanks to the system of cables 41 and blocks 43 and 44 as already described thereabove. The skidding surface 23 of box 222 is prior coated with grease to reduce the friction factor. One of the abutments for limiting the displacements helps to laterally position the three cut parts of chords 11 of this portion 1 5 just above the corresponding chord lower parts of the leg remaining portion.

With the help of the three jacks 20, 21 and as shown on Fig. 10, the shoes 1 6 and the portion 1 5 which these shoes carry are then raised up at a distance of approximately 3" with respect to the deck 6, and the deck 6 is then lowered by lowering the caisson 5 along the leg remaining portions so that the gap between the lower end of portion 1 5 and the upper end of the corresponding leg remaining portion is approximately 1" less than the gap between the underfaces 1 8 of shoes 1 6 and the upper surfaces 23 of the boxes 22, as is shown on Fig. 11.

After the deck 6 has stopped in a correct position, the three jacks 20, 21 lower the portion 1 5 and the shoes 1 6 carrying it and the portion 1 5 rests on the upper end of the leg remaining portion. The horizontal positionning of the three centering axes 28 with conical ends 29 penetrating the housings 32, and the vertical positionning is achieved by the engagement of the facing flat faces of the support members 31 and 33, as is shown on Fig. 12.

The leg is then reassembled by welding the members in end to end relationship, and particularly by welding the gussets, the chords 11 and the racks 14. Then, after cooling of the welded members and check up of the weld lines, each chord 11 is closed by welding the profiled members 27 in their initial position. The taking down operation of the shoes 1 6 may be carried out as soon as welding of portion 1 5 on the leg remaining portion has begun.

After a partial welding of the first portion 1 5 and taking down of the corresponding shoes 16, it is possible to start the same reassembling sequence of operations for the other two legs, but the lowest leg remaining portion with respect to the water level is always choosen at first. With reference to Fig.

1, the successive reassembled legs are legs 2, 3 and 1.

Preferably unevenness of the legs should be minimal so that the join of a partly welded leg is always above the level of the upper surfaces of the boxes.

It will be understood that, without departing from the ambit of the present invention, the forms of construction that have been described can be modified in particular by substitution of equivalent technical means.

The invention is not limited to jack-up platforms having three legs, each of which having a triangular cross-section and being realized with a trellis structure.

The platform may have more or less than three legs, and at least one leg.

The leg cross-section may be a square, diamond shaped or polygonal section, or even a circular section.

The trellis structure may be replaced by a tubular structure.

Further, the lifting unit of each leg is not limited to a rack but may be a jack unit.

Finally, the transversal displacement device for displacing a leg cut portion is not limited to skidding and/or rotation shoes, but may comprise a system with rollers rolling on special surfaces of rolling boxes or even an air cushioning arrangement.

It is also important to note that the device described thereabove may be used for the initial building of the legs and/or for a dismantling operation of the legs in a shipyard having but too small cranes with respect to the overall leg length, and in which the platform is build or disassembled. Such platform legs are made by successively raising leg portions over one portion or several portions already joined over oneanother, and by joining the last raised portion to the upper end of the prior raised, stacked and joined portions.

With the device of the invention, it is possible to join successively each portion on the corresponding support structures of the device which are borne on the caisson when said caisson is on a lower position in the shipyard, then to raise up the caisson on the already joined leg portions, then to cut said portion from said support structures and to displace it laterally and position it over the already joined leg portions, and finally to join it to these portions. Thereafter, the caisson is lowered to its lower position to have a next leg portion joined to the support structures, and another similar sequence is carried out, until the legs have their final length.

When dismantling the legs, the upper leg portions are cut and laterally displaced and joined onto said support structures, the caisson is lowered to its lower position, and the portions are discharged from the caisson, which is then raised up for cutting the upper portions of the remaining leg sections, and another similar sequence is carried out.

Claims (14)

1. A method of manipulating legs mounted on a floating caisson of a self elevating platform, for example prior to a towing operation of said platform towards a choosen work site, which method comprises the steps of: lowering said legs with respect to said floating caisson and anchoring said legs in the sea bed of another prior choosen site, raising said caisson on said legs at a sufficient height above water level, cutting the upper section of each leg so as to shorten the leg length until said length is short enough for towing said platform after having raised up the legs with respect to said caisson in a floating barge configuration while satisfiing safety regulations and specifications, wherein said upper section of each leg is cut as a single portion and said cut portion is then displaced so as to free the space above the remaining portion of the corresponding leg for allowing said remaining portion to be raised up with respect to said caisson.

2. A method of manipulating legs mounted on a floating caisson of a self elevating platform, for example on a choosen work site after a towing operation of said platform structure following a cutting operation of the upper section of said legs on another prior choosen site, which method comprises the steps of:: lowering the corresponding remaining portions of said legs with respect to said caisson and anchoring said remaining portions in the seabed of said work site, raising up said caisson on said remaining leg portions at a sufficient height above water level, -reassembling the upper section of each leg on the corresponding remaining section, so as to give to each leg its initial length, wherein said upper section of each leg is displaced as a single portion so as to position each portion above the remaining portion of the corresponding leg, and said upper portion is then joined to said corresponding remaining portion.

3. A method according to claim 1 or claim 2, further comprising the step of displacing aach cut leg portion by pivoting and/or translating it as a whole while maintaining it parallel to itself.

4. A method according to claim 1, claim 2 or claim 3, further comprising the steps of positionning said caisson successively with respect to the different legs choosen so that the height of the legs with respect to water level is decreasing and so that for each leg the cutting operation is carried out in a transversal plane above the deck defined by the upper face of said caisson.

5. A method according to claim 4, for a platform structure having truss legs the forming members of which comprise chords, internal and external gussets, and a lifting assembly, which method further comprises for each leg the steps of: joining to each chord of said leg a transversal displacement device, cutting the leg forming members in said cutting plane preferably after having cut on at least a part of each chord perimeter a profiled member through which said cutting plane extends, so as to make it more easy to cut the leg forming members positionned inside the corresponding chord, raising up said transversal displacement devices joined to the cut portion of said leg with respect to said leg remaining portion by raising up said transverse displacement devices with respect to said caisson and by then raising up said caisson on said leg remaining portions, so as to have said cut portion completely disengaged from said corresponding leg remaining portion, lowering said transversal displacement devices joined to said cut portion with respect to said caisson until at least one transversal displacement surface of each transversal displacement device engages a facing transversal displacement surface of said caisson, with a large enough remaining gap between said cut portion and said corresponding leg remaining portion, displacing said transversal displacement devices onto said transversal displacement surfaces of said caisson, so as to laterally displace said corresponding cut portion until the space above the corresponding leg remaining portion is free, and positionning each cut part of the corresponding cut portion chords on a support structure carried by said caisson, -and joining said chord cut parts of said cut portion and said support structure, preferably by interpositionning joining members having a profile corresponding to that of said profiled members cut on at least a part of each chord perimeter, so as to immobilize said corresponding cut leg portion on said platform structure in a storage position.

6. A method according to claim 2 or claim 3, further comprising the step of positionning said caisson successively with respect to the different leg remaining portions choosen so that the height of said remaining portions above water level is increasing and so that for each leg remaining portion the joining oper ation is carried out in a transversal plane above the deck defined by the upper face of said caisson.

7. A method according to claim 6, for a platform having truss legs which have been prior manipulated according to a method as claimed in claim 5, further comprising for each leg the steps of: -disassembling the corresponding leg cut portion from the support structures carried by said caisson, preferably by cutting said joining members interpositionned between said cut chord parts of said cut portion and said support structures, displacing said transversal displacement devices joined to said cut chord parts of said cut portion onto said transversal displacement surfaces of said caisson so as to laterally displace said cut portion and position it above the corresponding remaining leg portion, in such a manner that each of said cut chord part of said cut portion stands just above the corresponding remaining chord part of said leg remaining portion, raising up said transversal displacement devices joined to said leg cut portion with respect to said caisson so as to disengage the transverse displacement surfaces on face to face relationship of said caisson and said transverse displacement devices, lowering said caisson on said leg remaining portions so as to bring closer to each other the ends of said cut leg portion and remaining leg portion which are to be joined together, and then lowering said transversal displacement devices with respect to said caisson so as to engage said leg cut portion and said leg remaining portion on end to end relationship, -reassembling the leg by joining the ends which are to be joined of the corresponding forming members of said cut leg portion and remaining leg portion preferably by closing each chord with the joining of the corresponding profiled member on at least a part of the chord perimeter, -and disassembling said transversal dis placement devices from said joined chord parts of said leg upper portion joined on said leg remaining portion.

8. A device for carrying out a method according to any one of claims 1 to 7 on a self-lifting platform structure, the floating cais son of which has wells within which legs are lowered, jammed or raised up with respect to said caisson by means of a lifting unit cooperating with a lifting assembly integral with each leg comprising chords linked together by means of internal and external gussets and braces, and with each chord preferably equipped with a lifting assembly, which device comprises: : -at least a transversal displacement device for each leg chord, each transversal displace ment device being a massive member for being joined to the corresponding chord and having at least a transversal displacement surface, one of said massive members having a lifting element for cooperating with the lifting assembly of said corresponding leg when said corresponding massive member(s) are joined to the corresponding chords, -at least a transversal displacement box located on the deck defined by the upper face of said caisson and having at least a transversal displacement surface for cooperating with the surface(s) of said massive member(s) so that said latter transversely move onto said box(es), -a raising and lowering mechanism for lowering and raising said massive members with respect of said transversal displacement box, -a transversal displacement mechanism of a leg cut portion onto at least one of the corresponding boxes, -and a support structure for each said cut chord part of a corresponding cut leg portion, each support structure being carried on said caisson for receiving and being joined to said corresponding cut chord part on a storage position on said caisson, after transversal displacement of said corresponding cut leg portion.

9. A device according to claim 8, which further comprises joining members for being interpositionned each between a support structure and the lower end of said corresponding cut chord part and for being joined to said support structure and said lower end, the lower surface of each joining member having a shape complementary to that of the upper surface of said corresponding support structure and the upper surface of each joining member having a shape complementary to that of the lower surface of said corresponding chord part, which lower surface is partially complementary to the shape of at least a profiled member cut on at least a part of the chord perimeter when said corresponding leg upper portion is being cut and joined in its initial position when said leg is being reassembled.

10. A device according to claim 8 or claim 9, wherein said raising and lowering mechanism comprises at least a jack for each of said massive members joined to said chords of a leg, said jacks being located inside said transversal displacement boxes so that the jack piston rods pass through the upper face of said boxes which defines said transversal displacement surfaces of said caisson.

11. A device according to any one of claims 8 to 10, wherein each said lifting assembly integral with said corresponding leg and preferably with each said chord of said corresponding leg is a rack, each transversal displacement device is a skidding and/or rotating shoe for being joined to said corresponding chord by means of at least an upper plate and/or at least a lower plate, each transversal displacement box is a skidding and/or rotation box, each transversal displacement surface respectively of said transversal devices and boxes is a skidding and/or rotation surface, said lifting element of at least one of said transversal displacement devices is a rack element having a shape complementary to that of said rack of said leg for engaging said rack of said leg when said corresponding shoes are joined to said corresponding chords, so that the shoe transversal displacements are skidding and/or rotation movements.

1 2. A device according to claim 11, further comprising for each leg a rotation box onto which a shoe joined to one chord part is received for rotation and a skidding box having an arcuate shape onto which the shoes joined to the other chord parts of said cut leg portion are received for skidding when said cut leg portion is rotating around said rotation box.

13. A device according to claim 12, wherein, for each leg abutments limiting said skidding and/or rotation movements are integral with said skidding and/or rotation boxes so as to help the transversal positionning of said cut leg portion at the end of its displacements respectively after the cutting operation of said leg portion and prior to the reassembling operation of said corresponding leg.

14. A device according to one of claims 8 to 13, wherein said transversal displacement mechanism of a cut leg portion comprises a system of cables and blocks attached to fixed points of said caisson and of at least one of said transversal displacement device of said corresponding cut leg portion.

1 5. A device according to any one of claims 8 to 14, which further comprises a horizontal and vertical positionning assembly for positionning each said cut leg portion onto said corresponding remaining leg portion, said positionning assembly comprising, for each leg chord a vertical centering axis with a conical end and integral with one of said two portions of said corresponding leg by means of a support member having a flat face received in the cutting and reassembling plane and fixed inside said corresponding chord so that said conical end extends beyond said cutting and reassembling plane, and said positionning assembly comprising further a vertical cylindrical housing for receiving said centering axis and integral with the other of said two portions of said corresponding leg by means of another support member having a flat face received in said cutting and reassembling plane and fixed inside said corresponding chord, so that the end of said housing which is closest to said cutting and reassembling plane is flush with this plane in such a manner that the vertical positionning is achieved when the flat faces of said two support members engage each other and that the horizontal positionning is achieved when the centering axis penetrates into the corresponding housing.

1 6. A method of manipulating a platform substantially as hereinbefore described.

1 7. A device substantially as hereinbefore described with reference to the accompanying drawings.