GB2084228A - Mine lifting apparatus - Google Patents

Abstract

The present invention provides apparatus for lifting or stabilising marine structures, and is especially useful for positioning the top works of an oil/gas platform. Thus there is provided apparatus for supporting marine structures comprising immersible buoyancy means, upstanding lifting columns on said buoyancy means liftably engageable with the structure to be raised and lowered, and means for varying the buoyancy means to place the means in a substantially deeply immersed mode or in a buoyant rising lifting mode. The apparatus can be used in both the commissioning and decommissioning of offshore oil/gas platforms. <IMAGE>

Description

SPECIFICATION Marine support apparatus The present invention relates to apparatus for supporting, lifting or stabilising marine structures, and especially for use in positioning the top works of an oil/gas platform.

It has been the practice up till now in establishing an oil/gas platform at an offshore site to transport the platform to the site in a semi-complete state, position the deck support frame on the jacket legs piled in the sea bed, then ship aboard the various top work modules and finally carry out the necessary "hook ups" i.e. pipe installation, instrumentation etc. The installation of the modules and the hook ups have proved difficult and arduous tasks off-shore, especially where there is inaccuracy in the equipment; and it would be more satisfactory if the top works could be substantially completed at a shore building site prior to location on the jacket.However, the ship cranes or jack-up equipment available have lacked the capacity to handle substantially complete top works, and it is the principal object of the present invention to provide improved lifting apparatus for lifting oil/gas platform top works, or other structures.

According to the present invention there is provided apparatus for supporting marine structures comprising immersible buoyancy means, upstanding lifting columns on said buyoancy means liftably engageable with the structure to be raised and lowered, and means for varying the buoyancy of the buoyancy means to place the means in a substantially deeply immersed mode or in a buoyant rising lifting mode.

Preferably the buoyancy means are of compartmented form, and preferably comprises an elongate float member which is preferably of U-form or double U, or such other form as may be suitable.

The buoyancy varying means can include valve means permitting filling of the buoyancy means with water or discharge or water therefrom.

Preferably the columns have their upper ends adapted to nest with corresponding parts attached under the Structure, to facilitate lifting.

Preferably the immersible buoyancy means is associated with mechanical lifting means operable to raise or lower the buoyancy means relative to the marine structure.

According to another aspect of the present invention a method of mounting a floating marine structure on a column assembly at an off-shore location comprises positioning the floating marine structure over an immersed buoyancy means having upstanding lifting columns, connecting mechanical lifting means between the marine structure and the buoyancy means and lifting the buoyancy means until the lifting columns engage receiving elements on the marine structure, positioning the buoyancy means with the marine structure mounted thereon at the column assembly, positioning mechanical connecting means between the marine structure and the column assembly, increasing the buoyancy of the buoyancy means to at least partly support the weight of the marine structure, and securing the marine structure to the column assembly.

The buoyancy means can then be removed.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawing (Figs.

1-3) which show lifting apparatus of the present invention positioned to lift the top works of an oil platform at an offshore site.

An oil platform (1) is shown in the drawing floated to an offshore oil location and this has preferably been done by the method described in the applicants co-pending application No. utilising buoyancy floats A,A2A3,..., In particular, the top works including deck (4) and modules (5-12) are substantially complete, and at the offshore site the jack-up legs (13) are arranged to operate upwardly, from the jacket structure already piled into the sea-bed to raise the platform deck with topworks as required. This raising is achieved by the lifting apparatus (21) now described either alone or in combination with some other lifting or jacking equipment.

The apparatus (21) includes; a buoyancy float collar B1,B2,B3,. . which can be of U, W, or other form for positioning around and/or through the jacket (20), long lifting columns (22) upstanding from the float B,B2B3. These columns (22) may have increased section and buoyancy near their upper ends to allow the entire device to be stabily floated in an immersed mode with a water line W.L.1. These columns 22 also have frusto-conical upper ends (23) for nesting with corresponding parts (24) under the deck unit (4) which define stabbing guides or cups. The columns 22 also have seat-fastenings (28) at their upper ends to allow attachment to the deck unit (4) or stabbing guides (24). A similar stabbing guide arrangement 1 9 is also located between the jacks (13) and the deck unit (4).Both the stabbing guide arrangements or frusto-conical noses, can have damper or buffering facilities which may be of a hydraulic pneumatic, elastic or other nature. Means are provided for varying the buoyancy of the floats B,,2,3. .

and this may comprise air filling means or means for filling or discharging the float(s).

The floats can comprise several water compartments. The float B1,2,3. . can be provided with steering and/or propulsion means, (not shown). Winch systems are provided to assist mating the deck (4) with the lifting device (21), and these winch systems may be mounted either on the deck, or as shown upon the lifting device (21). Preferably these systems are similar to drill string compensator equipment used onboard floating drill rigs or drill ships. Such drill string compensating equipment is manufactured by NL Rig Equipment of 12950 West Little York Road, Houston, Texas, U.S.A. and others.

The apparatus operates in the following stages.

Stage 1 Referring to Fig. 1, the platform's topsides and deck is brought near to the already installed and piled jacket, and the platform can be supported upon transportation float A,2,3, a 8 b.. As described in the applicants co-pending application.

Stage 2 The lifting device B1,2,3. . having been ballasted to float with water-line W.L.1 is manoevered below the floating deck unit (4) and is suitably fendered to prevent collision with the unit. Lines from motion compensation winches (29) are attached to deck (4)or, if winches are mounted on the deck lines, are attached to the lifting device (21) which is then drawn up above its natural water line (W.L.1) by the tension in the winch lines which will gradually eliminate the differences in motion of the two units created by the seastate. When all differential motion has been minimised the lifting device may be winched hard up and sea-fastenings (28) made good, with hydraulic, pneumatic or elastic shock absorbers mounted within or behind stabbing guide cups (24) to accommodate any remaining misalignment, thereby preventing any overstressing at the moment of contact.

Stage 3 The combined unit i.e. Deck (4) with lifting device (21) firmly attached but still in a water filled mode, all supported by transport floats A122, a,b) is fed over, around and through the jacket structure which depending upon its design may require to have some structural braces temporarily removed to enable positioning of the combined unit. Again fenders are preferably used to prevent colision between the floating unit and the fixed structure. This combined floating unit would have a considerably reduced wave inspired motion than would a similar deck supported upon a conventional surface riding displacement barge.The motion would be reduced due to three features; Firstly because the transport floats A123. . would be to some degree semi-submerged having lesser water-plane area and water-line length than a comparable barge, secondly because the attached lifting device B1 23 would act as a water filled keel, thus giving the combined unit a much lower centre of gravity than the structure comprised by the deck (4) on floats Al 23 a 8 b alone, while the effective centre of buoyancy would be less effected, thirdly the combined unit would have a mass and inertia nearly double that of deck (4) with floats A123 a a b.

Additionally wave inspired motion would be reduced due to the physical size and configuration of deep combined unit since the chance of many hydrodynamic and/or hydrostatic forces acting in concert is less, while of course the unit would need to be physically sufficient to withstand any differential forces.

Stage 4 Once the entire floating unit is located over the jacket it is attached to the jacket by wires, chains or rods which are connected to motion compensating type winches and preferably the same winches (29) as used in stage 2 would be connected to pad eyes (30) and used again. In this operation wave inspired motion of the combined floating unit would be reduced by gradually increasing the tension in the winch lines, which may be achieved either by winching in on the winches or by reducing the ballast of the floating unit or by a combination of these two methods. In the preferred method once the motion of the floating unit has been sufficiently constrained the unit may then be positively located by jacks.In Fig. 2, the jacks (13) are shown operating upwards from the jacket (20) and locating in stabbing cups (19) under the deck (4), these stabbing cups or jacks may also have shock absorbing or buffering facilities incorporated in a similar manner as had stabbing cups (24). The jacking mechanisms may be arranged to give a controlled load sharing to reduce the chance of overstressing any structure portion.

Further load is now placed upon the jacks to remove any possibility of wave inspired motion of the deck unit, thereafter ballast may be transferred from lifting device B123.. To transport floats Al 23 a and b.. Until they no longer support the deck, thereupon sea fastenings (17 and 18) be released and the floats A1,2,3, a 8 b, with attendant fenders, removed for future use.

Stage 5 The deck unit (4), now no longer floating can be raised by the jacks (13), or by the lifting device (21), or as preferred by a combination of jacks and lifting device. As shown in Fig. 3, the deck (4) may be raised above its final required position and permanent legs (25) may be installed between the jacket and the deck onto which the deck may then be lowered, or by other design the jacks (13) may become incorporated as permanent legs, or such other arrangement as may be suitable. The design shown in Figs. 1-5 has been adapted, as close as reasonably possible, around an existing platform elevation with the object of showing how simply it might be incorporated. If a structure were being designed afresh to use this lifting/stabilising device it is suggested to make the upper portions of the jacket legs vertical so that jacks (13) could extend telescopically from within operated either hydraulically or me chanically. Once the deck (4) is adequately supported the sea-fastenings (28) may be released and the lifting device again ballasted with water to allow lowering thereof from the installed deck, and removal for use again.

Depending upon the platform final design it may now be necessary to replace some structural members such as item (26) in Fig. 3, and if the design calls for sway bracing between jacket and deck this may also be fitted at this stage.

All other connections between jacket and deck may now be made which might include for example:-- ladders, pipeline risers, fuel lines, drain lines, seawater risers, cathodic protection cables, sump pump and fire pump cables and others. As many as possible of these services could be pre-established and the connections made of quick-fit manner wherever possible. Stabbing cups (19) and (24) with buffers may be removed for use again.

Using this method of installing a complete pre-tested platform topsides on its jacket it is considered that the offshore hook-up time could be considerably reduced, for example, to between 4 days and 4 weeks depending upon platform complexity, pre-readiness and of course weather conditions. With adequate standardisation this same system and equipment could be adapted to many future platforms of different sizes and functions, with considerable cost savings.

The same or similar equipment and system may be used in reverse for the removal of platform topsides at the end of their useful life. This would not be limited only to platform decks which had been originally installed by this means. Equipment could also be adopted to remove topsides complete which had originally been installed by other means.

Claims (9)

1. Apparatus for supporting marine structures comprising immersible buoyancy means, upstanding lifting columns on said buoyancy means liftably engageable with the structure to be raised and lowered, and means for varying the buoyancy of the buoyancy means to place the means in a substantially deeply immersed mode or in a buoyant rising lifting mode.

2. Apparatus as claimed in claim 1, wherein the buoyancy means are of compartmented form.

3. Apparatus as claimed in claim 1 or 2, wherein the buoyancy means comprises an elongate float member.

4. Apparatus as claimed in claim 3, wherein the buoyancy means is of U-form or double U-form.

5. Apparatus as claimed in any one of the preceding claims, wherein the buoyancy varying means comprises valve means permitting filling of the buoyancy means with water or discharge of water therefrom.

6. Apparatus as claimed in any one of the preceding claims, wherein the columns have their upper ends adapted to nest with corresponding parts attached under the structure, to facilitate lifting.

7. Apparatus as claimed in any one of the preceding claims, wherein the immersible buoyancy means is associated with mechanical lifting means operable to raise the buoyancy means relative to the marine structure.

8. A method of mounting a floating marine structure on a column assembly at an offshore location comprising positioning the floating marine structure over an immersed buoyancy means having upstanding lifting columns, connecting mechanical lifting means between the marine structure and the buoyancy means and lifting the buoyancy means until the lifting columns engage receiving elements on the marine structure, positioning the buoyancy means with the marine structure mounted thereon at the column assembly, positioning mechanical connecting means between the marine structure and the column assembly, increasing the buoyancy of the buoyancy means to at least partly support the weight of the marine structure, and securing the marine structure to the column assembly.

9. Apparatus for supporting marine structures substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

1 0. A method of mounting a floating marine structure on or removing the structure from a column assembly substantially as hereinbefore described.