GB2402422A

GB2402422A - Buoyancy body transfer of an offshore structure

Info

Publication number: GB2402422A
Application number: GB0412294A
Authority: GB
Inventors: Rolf Eide
Original assignee: Aker Marine Contractors AS
Current assignee: Aker Marine Contractors AS
Priority date: 2003-06-02
Filing date: 2004-06-02
Publication date: 2004-12-08
Anticipated expiration: 2024-06-02
Also published as: NO20032485D0; GB2402422B; GB0412294D0

Abstract

A method for lifting and transporting a tubular jacket structure for a stationary offshore structure utilizes elongate buoyancy bodies which are towed in horizontal position to the jacket structure and ballasted to an upright position before they are attached to the jacket above and under the sea surface (3,Fig 1) whereupon the buoyancy bodies (6) are deballasted in order to lift the jacket structure and permit towing thereof in the upright position. In order to simplify and assure safe lifting force transmission between the buoyancy bodies and the jacket structure (1), each buoyancy body (6) is attached to a leg (4) of the tubular jacket, and substantially all the lifting force from the buoyancy body is transmitted to a force receiving bracket (13) welded to the respective leg (4) at a location above the sea surface (3, Fig 1), from a force transmitting device (11 and 12, Fig 11) at the upper end of the upright buoyancy body.

Description

. 1 2402422 A method and system for moving an offshore structure The

present invention relates to a method for lifting and transporting a supporting structure for a stationary off- shore structure, said supporting structure having a tubular truss which comprises legs and brace members, said legs ex- tending from the seabed and having a portion located above the water surface, wherein elongate buoyancy bodies are ballasted to a substantially upright position and connected to the supporting structure below and above the water sur o face, whereupon the buoyancy bodies are deballasted in or- der to lift the supporting structure and permit towing thereof in the upright position.

Such a method is known from GB 2120606. This prior art method uses buoyancy bodies which are attached to the sup porting structure through upwardly open hooks that engage under horizontal braces of the supporting structure. How- ever, when deballasting the buoyancy bodies in order to lift the supporting structure, the braces are subjected to transverse loads for which they are not designed. Such forces may jeopardize the structural integrity of the sup- porting structure. In any case, the limited strength of the horizontal braces prevents lifting the supporting structure before all the topside equipment has been removed.

An object of the present invention is to avoid the draw backs and deficiencies mentioned above.

This is obtained according to the invention through a method as recited in the first paragraph above, said method being characterized in that each buoyancy body in the up- right position is attached to one of the legs of the sup o porting structure under water by means of at least one connection capable of transmitting substantially horizontal forces, whereupon the buoyancy body is deballasted to pro- vide a lifting force to the supporting structure substan- tially via the upper end of the buoyancy body and a force i receiving device arranged on said leg above the water sur- face.

Since all the lifting force from the buoyancy bodies is transferred to the legs of the supporting structure, the forces are acting on structural members designed to take high vertical loads. Since the force receiving device on the legs is located above the sea surface, these devices may be installed on the legs prior to the lifting operation in a dry and easily accessible location, thus permitting to the necessary welding work and controls thereof to be per- formed in a highly dependable manner. These features may permit lifting and transporting the supporting structure with some or all the top side equipment in place.

The invention also relates to a system for use in the method according to the invention, comprising a plurality of elongate buoyancy bodies having means for attachment to a supporting structure for a stationary offshore structure, said structure having a tubular truss comprising legs and brace members, said legs extending from the seabed and hav o ing a portion located above the sea surface, said buoyancy bodies being ballastable to an upright position before at- tachment to the supporting structure and deballastable in order to subject the supporting structure to a lifting force, characterized in that each buoyancy body at one end iS provided with a force transmitting device for cooperat- ing with a force receiving device arranged on at least some of said legs above the water surface.

The above and other features and advantages of the method and system according to the invention are described in more so detail below with reference to the appended drawings, in which Fig. 1 is a side elevation view of an offshore supporting structure ready for lifting in accordance with an embodi- ment of the present invention, Fig. 2 is an isometric view of the supporting structure in fig. 1 at a smaller scale, Fig. 3 is a side elevation view of a buoyancy body accord- ing to one embodiment of the present invention, Fig. 4 is a schematic plan view illustrating a preparation stage for the method according to the invention, Figures 5-8 are schematic views illustrating various steps in a method according to the invention, Fig. 9 shows cross-sections through legs and buoyancy bod o ies according to the invention illustrating the clamping of the buoyancy bodies to the respective legs, Fig. 10 shows cross-sections through legs and buoyancy bod- ies illustrating abutment means between the buoyancy bodies and the adjacent supporting structure, Fig. 11 is a side elevation view of a force absorbing de- vice and force transmitting device according to the inven- tion, Fig. 12 is a plan view of the elements in Fig. 11, Fig. 13 is a view somewhat similar to Fig. 6 and illus zo bates a step in an alternative embodiment of the method according to the invention.

With reference to the drawings, in Fig. 1 a supporting structure 1 is shown resting on the seabed 2 and extending somewhat above the sea level 3. The supporting structure is made like a truss comprising tubular legs 4 and brace mem- bers 5. Such a supporting structure is often referred to as a jacket. r

Attached to the legs 4 of the supporting structure are buoyancy bodies 6 in the form of elongate cylindrical tanks, which are attached to the respective leg 4 above the sea surface 3 by means of a force transmitting device 7 at the upper end of the buoyancy body 6 and a force receiving device 8 arranged on the jacket leg 4. The buoyancy body 6 is also attached to the leg 4 below the water at two loca- tions 9, the details of which will be explained in some de- tail below.

lo Fig. 2 is an asymmetric view of the jacket in Fig. 1 and shows that all the legs 4 of the jacket 1 have been pro- vided with a buoyancy body 6. It will be understood that the number of legs that need to be provided with a buoyancy body will depend upon the weight of the jacket and any top I5 side structure (not shown) and the size of the buoyancy bodies. Furthermore, two or three buoyancy tanks may be welded together and operate as one unit. Stability require- ments will also have to be considered when deciding the number and distribution of the buoyancy bodies.

so One embodiment of the buoyancy body 6 is shown in Fig. 3.

The buoyancy body has a cylindrical buoyancy tank 10 which may be divided into a plurality of compartments provided with piping and valving well known to the skilled person for filling the compartments with water and expelling the water by means of pressurized air. At the top the buoyancy tank 10 is provided with a transition piece 11 which have a pointed shape and forms a closed body in order to provide buoyancy and a smaller water plane area when the buoyancy body is in the maximum submerged condition. The transition piece 11 acts as a force transmitting device which transfers the buoyancy force from the buoyancy body 4 through a transverse pin 12 at the apex of the transition piece, to a force receiving device or bracket 13 welded to the respec- tive leg 4 of the supporting structure 1 above the sea level 3.

The buoyancy tank in the present embodiment is provided with two bolting clamps 14 to be firmly attached to the re- spective jacket leg 4 before the lifting operation starts.

Further details of these clamps will be described below in connection with Fig. 9.

Abutment means 15 also acting as roll stoppers extend on both sides of the bolting clamps 14 along the length of the buoyancy tank 10. Further details will be described below in connection with Fig. 10. The roll stoppers are provided lo with mating hooks 16 for gripping behind transverse braces (not shown) of,the supporting structure 1 for holding the buoyancy tank in position along the jacket leg 4 while the buoyancy body 6 is raised slightly in order for the pin 12 of the transition piece 11 to make force transmitting con tact with the force receiving bracket 13, whereupon the bolting clamps 14 can be tightened to take over the holding action. The mating hooks 16 are supposed to provide a hori- zontal holding force only, i.e. the respective horizontal braces are not supposed to come into contact with the bot zo tom of the openings of the mating hooks 16 during this pro- cedure. The roll stoppers 15 may also act like bilge keels during the horizontal towing of the buoyancy body 6 to the installation site. The roll stoppers 15 do not have to ex- tend along the entire length of the buoyancy tank 4, but may be provided as two or more separate sections as shown at the attachment points 9 in the Fig. 1 embodiment.

The mating hooks 16 may be slideable along the roll stop- pers 15 and fixable at any desired location in order to be accurately adjustable to the bracing location of different jackets for which the buoyancy bodies 6 may be used. Such longitudinal adjustability is normally not required in the bolting clamps 14 since they have the advantage of being able to grip the leg 4 at any place between the braces and therefore can be located so as to fit several different jacket structures. t

A method according to the invention will be described below with reference to Figures 4-8.

Fig. 4 shows schematically how a buoyancy body 6 is towed in the horizontal position by means of tugs 17 and pulling lines 18 to the vicinity of a supporting structure 1. Next, the tank 10 is ballasted by letting water in through a re- mote controlled valve 19 operated through a cable 20 from one of the tugs 17. This is shown in Fig. 5.

When the buoyancy body 6 has reached a near vertical posi o Lion with only part of the transition piece 11 extending above the sea surface 3, thus providing a small water plane area and correspondingly low dynamic forces in the buoyancy body due to surface waves, the buoyancy body 6 is pulled towards the jacket by means of winch wires 21 running over fairleads 22 to a winch 23 located in the topside of the offshore structure. The tugs 17 and their pulling lines 18 are used to control the movement of the buoyancy body 6 to- wards the jacket 1. The topside 24 also contains a hydrau- lic power unit 25 for providing hydraulic power to control zo the valving on the buoyancy body.

In Fig. 7 the buoyancy body 6 has been brought into contact with the jacket and has been raised somewhat from the sub mergence shown in Fig. 6 in order for the force transmit ting pin 12 on the transmission piece to come into firm contact with the force receiving bracket 13 on the jacket leg 4. In this position the bolting clamps 14 will be set by means of an ROV, and control of the valving on the buoy- ancy body will be transferred from the assisting tug or support vessel 17 to a control manifold connected to the so hydraulic power unit 25 on the top side 24. An air supply on the tug provides pressurized air through a hose to the buoyancy body.

When all the buoyancy bodies needed for the lifting opera- tion have been installed on other legs 4 of the jacket 1, some or all of the equipment modules on the topside 24 may be removed commensurate with the lifting capacity of the buoyancy bodies and the stability requirements of the subsequent tow of the offshore structure to an inshore shel tered location, where dismantling of the offshore structure may take place.

Once the offshore structure is ready for lifting, the piles of the jacket legs are cut, and water expelled from the buoyancy bodies to raise the offshore structure as shown in ro Fig. 8. For simplicity, only one buoyancy body 6 is shown in this figure, but it will of course be understood that i similar buoyancy bodies have been arranged in a preferably symmetrical relationship around the jacket 1.

Figure 9 shows further details of the bolting clamps 14 in I5 dicated in Fig. 3. Figure 9 shows three situations, the rightmost situation showing a buoyancy body 6 clamped to a corner leg 4, while the other two figures illustrate clamp- ing of the buoyancy body to a leg in the side of the jacket. The center figure illustrates various open post so Lions of the clamp 14, while the left and right figures show the clamp closed and tightened by means of bolts 26.

It will be understood that the closing of the clamp 14 and insertion and tightening of the bolts 26 may be performed by an ROV.

Fig. 10 illustrate further details of the roll stoppers 15 shown in Fig. 3. The two embodiments illustrate the roll stopper arrangements for side legs and corner legs, respec- tively, of the jacket. The roll stoppers, which engage braces 5 of the jacket, may be provided with bolted shim so plates 27 for easy adjustment to varying brace dimensions.

The abutment surface of the roll stoppers may also be pro- vided with a compressive rubber compound 27 to act like shock absorbers since they will be the first points on the buoyancy bodies to make contact with the jacket. During the transportation phase of the offshore structure, the roll stoppers will have constant contact with the jacket at sev- eral places and prevent the buoyancy bodies from wobbling.

Figures 11 and 12 show further details of the force trans- mitting and receiving devices shown in Fig. 3. The force transmitting device comprises two triangular brackets 29 i forming the transition piece 11. These brackets are made from plate material and form closed entities providing some buoyancy. The brackets 29 may also form the side walls of a larger volume for increasing the buoyancy of the transition lo piece 11. At the apex of the brackets 29 or transition piece 11 a force, transmitting pin 12 is located. The pin 12 i is received in a downwardly facing opening of a hook-like bracket 13 which constitute the force receiving device and is welded to the jacket leg 4, e.g. in the vicinity of the so-called spider deck level, typically 6-7 meters above the sea level 3. The bracket 13 may include a damping device (not shown) for reducing shock loads when the force trans- mitting pin 12 first makes engagement with the bracket 13.

A somewhat simplified method for installing the buoyancy so bodies 6 on the jacket may be used when weather and swell conditions are favorable. Instead of using two winch wires to pull the buoyancy body towards the jacket leg, of which at least one of the wires would have to pass through a fairlead at a substantial water depth, a single winch wire iS attached to the force transmitting pin 12 and run through the force receiving bracket 13, thus pulling the pin 12 into contact with the bracket 13 before the buoyancy body is fully submerged and letting the hinge action be- tween the pin 12 and bracket 13 perform the guidance of the so buoyancy body as it swings down into contact with the jacket leg upon further ballasting. This simplified method may benefit from a V-shaped guiding arrangement extending laterally from a lower part of the buoyancy body and acting as an alignment means as the buoyancy body approaches the j acket leg.

The methods described above were developed for removal of offshore jacket structures. It has become evident, however, that the method could also be used for installation. Usu- ally, the jacket structure is fabricated horizontally in a yard and skidded into a barge for transport offshore, with subsequent launching and upending by use of buoyancy tanks connected to the legs. Lift installation offshore using a large offshore crane vessel is another common method.

New alternatives for installation can be: lo Alternative 1: , À At an offshore deep draft location, the jacket is lifted off the launch- barge by means of an inshore crane vessel and lowered into water and upended by! means of the buoyancy tanks connected to the jacket legs.

À In order to ease the inshore crane requirements, a submersible barge type can be used for transportation to the inshore location. The barge deck can be sub merged and part of the jacket can be made buoyant; lo prior to lift-off.

À In principle, topside modules can be installed on the jacket by an inshore crane and follow the transport to the offshore location. The amount of topside weight capacity must be within the stability capacity. The L buoyancy tanks may be increased in size to take neces sary topside weight.

À The jacket can then be towed to the offshore location in vertical position and installed by ballasting the tanks, with subsequent removal of tanks, and piling can commence.

Alternative 2: t À Another way is to assemble the jacket sections verti cally in a floating phase at the inshore site. Section by section (horizontally cut) could be put out in the water in vertical condition, and held by means of barges and strand jacks or similar. The section(s) will step by step be lowered in water to make space for the next section(s) above. The new higher sections are lifted over the previous section(s) by floating crane barge(s), or floated over and mated by means of lo barge(s).

Comment to both alternatives above is: À Before tow-out, buoyancy tanks have been mounted in the sheltered inshore location.

À The jacket and module supporting frame can be made as one unit.

À When the tanks are attached and the jacket is properly moored, the topside modules can be lifted in and com missioned.

À When this is completed, the entire structure can be towed to the field and ballasted into contact with the seabed. The tanks are removed.

À The piles will then be installed. This can either be done traditionally, or the piles can be driven from the deck, in which case the method is independent of a heavy lifting vessel.

À Drilling can start.

À When the reservoir is empty, the tanks can be in- stalled again and the entire structure removed by this method or re- located.

It will be understood that the present invention is not limited to the exemplifying embodiments shown in the draw- ings and discussed above, but may be varied and modified by the skilled person within the scope of the invention de- fined by the appended claims.

Claims

C 1 a i m s 1. A method for lifting and transporting a supporting

structure for a stationary offshore structure, said sup- porting structure having a tubular truss which comprises legs and brace members, said legs extending from the seabed and having a portion located above the water surface, wherein elongate buoyancy bodies are ballasted to a substantially upright position and connected to the supporting structure below and above the water surface, whereupon the lo buoyancy bodies are deballasted in order to lift the sup- porting structure and permit towing thereof in the upright position, characterized in that each buoyancy body in the upright position is attached to one of the legs of the sup- porting structure under water by means of at least one con nection capable of transmitting substantially horizontal forces, whereupon the buoyancy body is deballasted to pro- vide a lifting force to the supporting structure substan- tially via the upper end of the buoyancy body and a force receiving device arranged on said leg above the water sur face.
2. A method according to claim 1, wherein the buoyancy body is brought into contact with the force receiving de- vice on said leg after having been brought into said up- right position.
3. A method according to claim 1, wherein the buoyancy body is brought into contact with the force transmitting device on said leg before it is completely ballasted to the upright position, the upper end of the buoyancy body being pulled into contact with a hinge point in the force receiv ao ing device by means of a winch, whereupon the buoyancy body is made to pivot into contact with said leg during the fi- nal ballasting.
4. A method according to claim 1, 2 or 3, wherein the force receiving device comprises brackets which are welded to the respective leg prior to the lifting operation.
5. A method according to claim 2, wherein the buoyancy body, before being attached to said leg, is pulled towards the leg with winch wires, at least one of which is below the water surface, while floating with a water plane area which is substantially smaller than that of the major part of the buoyancy body.

lo
6. A method according to one of the preceding claims, wherein the buoyancy body is attached to said leg underwa- ter by means of a clamping device.
7. A system for use in the method according to one of the preceding claims, comprising a plurality of elongate buoy ancy bodies having means for attachment to a supporting structure for a stationary offshore structure, said struc- ture having a tubular truss comprising legs and brace mem- bers, said legs extending from the seabed and having a portion located above the sea surface, said buoyancy bodies so being ballastable to an upright position before attachment to the supporting structure and deballastable in order to subject the supporting structure to a lifting force, char acterised in that each buoyancy body at one end is provided with a force transmitting device for cooperating with a force receiving device arranged on at least some of said legs above the water surface.
8. A system according to claim 7, wherein the force re- ceiving device comprises a bracket having the shape of a hook with the opening facing downwards.

so
9. A system according to claim 8, wherein the force trans- mitting device comprises an upwardly tapering transition piece which at the top is provided with a transverse pin fitting into the opening of said bracket. L; s,
10. A system according to claim 8 or 9, wherein the force receiving device is provided with shock absorbing means.
11. A system according to one of claims 7-10, wherein the buoyancy body is provided with clamps for gripping around the respective leg, and abutment means extending lengthwise on either side of the clamps for abutment against braces of the supporting structure for preventing rotation of the buoyancy body.
12. A system according to claim 11, wherein the abutment lo means are provided with hooks having upwardly facing open- ings for gripping behind braces of the supporting structure and holding the buoyancy body against said leg while the buoyancy body is raised for bringing the force transmitting device into contact with the force receiving device.