GB1577619A - Assembly of massive components - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE ASSEMBLY OF MASSIVE COMPONENTS (71) We, CONOCO INC., formerly CONTINENTAL OIL COMPANY, a State of Delaware Corporation of 1000, South Pine Street, Ponca City, Oklahoma, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the assembly together of massive components of a structure for example the assembly together of decks of an offshore platform.

Offshore platforms, of the type used for example in oil or gas production, frequently have a deck construction comprising a lower deck and one or more upper decks. The complete deck construction is often fabricated as an integrated structure and then transported to a suitable site for joining to the remainder of a platform.

It is frequently the case that equipment needs to be installed in the lower deck before an upper deck is fabricated on top. It will be appreciated therefore that if there are even short delays in obtaining such equipment, fabrication of the upper deck must be suspended, and this can result in very substantial delays in the fabrication of the complete deck construction. In the case of platforms which are to be used for example in the North Sea, transport to final site of the deck should be during the annual "weather-window", and if this is missed one year because of delays in obtaining equipment, a whole year can be wasted.

The individual decks are very massive, being for example 12 m high 80 m long 70 m wide, and made of steel, weighing 10,000 tonnes. For this reason fabrication of the decks completely separately and subsequently placing the upper deck on the lower deck would heretofore have presented substantial problems.

According to the invention there is provided a method of assembling two massive components of a structure to be subsequently transported from the point of assembly, wherein a first component has the bottom thereof at substantially the same level as the top of the second component, the first component is slid in a substantially horizontal direction onto the second component whilst the level thereof is maintained substantially constant, the two components are joined together, and the assembled structure is transported from the point of assembly by flotation means supporting both components.

The components could conceivably both be fabricated on a solid foundation, the second component being subsequently slid onto one or more barges or like flotation means. Advantageously however, the second component is fabricated in position on the flotation means, or in a position such that the flotation means may be positioned underneath it.

The second component may thus be fabricated whilst supported on one or more barges, and although it could then be moved so as to be adjacent the first component, advantageously the components are fabricated adjacent one another, for example the first component being fabricated at the side of a dock holding the barge or barges so that assembly can be immediately carried out. In one preferred embodiment the second component is supported at substantially the same height as the first component during fabrication and is subsequently lowered to enable sliding of the first component onto the second component.

The second component may be supported during fabrication by a raised solid foundation in an area which can be flooded with water and in which the flotation means may be positioned. The solid foundation could for example be an island in a dock. Advan tageously the island could be made of such a height that the finished second component would be at the correct level when resting upon it. The flotation means may serve to support necessary parts of the second component which the island does not, and to provide variable reaction forces to give the correct support whilst the first component is slid on top of the second, which results in a changing weight distribution. Once the two components are joined, the flotation means is used to lift the complete structure off of the island so that it can be moved away.

The flotation means can be adjusted so that the level of the second component or the reaction forces applied to it by the flotation means can be varied, either by changing the ballast of the flotation means or in the event that a floodable dock is employed, by altering the level of water in the dock.

The first component may be slid onto the second in any known manner for example by using winches, or jacks for pulling or pushing the first component. Advantageously the first component is fabricated in position on skid beams or similar to facilitate its movement. The second component could conceivably have skid beams incorporated in its structure to assist in movement of the first component over it, such skid beams comprising for example modified structural beams of the component.

The invention is particularly applicable to the construction of offshore platform components, and is of use in the joining together of upper and lower decks for such platforms.

Two embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which Fig 1 shows a plan view of a dock with the lower deck of an offshore platform supported on barges, and an upper deck in position to be slid onto the lower deck; Fig 2 shows a section on the line II-II of Fig 1; Fig 3 shows a section on the line Ill-Ill of Fig 1; Fig 4 shows a side elevation of the upper and lower decks in position on each other; Fig 5 shows the decks in section; Fig 6 shows a second embodiment in which the lower deck is supported on an island in a dock; and Figs 7, 8 and 9 show subsequent stages in the assembly of the two decks.

Referring now to the drawings, Fig 1 shows the lower deck 1 of an offshore platform supported on three barges 2 in a dock 3, which can be closed by a gate 4. The barges could for example be converted tankers. The lower deck 1 is steel, and of plate girder construction, as can be better seen in Figs 4 and 5, and has four stub columns 5 for mating with the legs of an offshore platform.

This lower deck 1 has been fabricated in the dock 3 whilst supported on the barges 2.

On the side of the dock, a steel upper deck 6 is supported on skid rails 7, arranged to be aligned with main girders 8 of the lower deck. The upper deck 6 is of trussed girder construction, and has been fabricated in situ, and simultaneously with the lower deck 1. The two decks are suitably installed with equipment and ready for joining together.

The level of the lower deck is adjusted by altering the buoyancy of the barges and/or the level of water in the dock, so that the top of the main girders 8 are at the same level as skid rails 7. It may not be possible to provide complete alignment, but certainly the girders 8 should be no more than a few inches lower than rails 7, and of course no higher.

The lower deck may be fabricated whilst at substantially the correct level for sliding the upper deck onto it, or alternatively at a level corresponding to that of the upper deck. The latter procedure involves the use of a dock of greater draught, but has the advantage that all deck outfitting work can be carried out at one level, that of the dockside, and equipment can be skidded or rolled into the lower deck, not lifted down into it.

The upper deck 6 is now skidded as a whole over and onto the lower deck 1. Hydraulic gripper jacks as known in the art are used to push the upper deck 6 along the said skid rails 7. These jacks engage with beams 9 of the upper deck. If a pulling action is required, then the top flanges of the main beams 8 of the lower deck can be adapted to accept the jack gripping jaws. Should horizontal forces on the lower deck become excessive, temporary compression members could be introduced between the lower deck and the dock wall, for example as indicated at 10.

The changing weight and distribution of weight to be supported by the barges can be accommodated by varying the buoyancy of the barges, so as to keep the lower deck at the correct level and on an even keel.

When the upper deck is correctly positioned on the lower deck, the two are joined by welding, and at the same time facilities interconnection between the decks can be made. Side cladding can also be added to the upper deck at this stage if appropriate. When the deck is complete the dock is flooded completely and the gate 4 opened to enable the complete deck to be towed away on the barges.

Fig. 6 shows an alternative embodiment, in which a dock 11 includes a central island 12 which forms two sections 13 and 14 which may be separately floodable. Fabrication of the lower deck 1, identical with that of the previous embodiment commences on the island 12, the floor beams of the deck being supported by this island. The ends of the floor beams are supported by barges 2 in the sections 13 and 14. As fabrication of the lower deck 1 progresses and the weight increases, the reaction forces provided by the barges are adjusted accordingly. This can be done by either changing the ballasting of the barges or by altering the level of the water in the sections 13 and 14, or both.

When the lower deck 1 is completed, and its beams 8 aligned with the skid rails 7 on the side of the dock, the upper deck 6, which has been fabricated simultaneously is slid onto it in a manner corresponding to the previous embodiment, as shown in Fig 7.

During sliding, the reaction forces supplied by the barges 2 are varied to take into account the changing weight and weight distribution. Thus at the stage shown in Fig. 7, section 14 is flooded to a greater extent than section 13, and in Fig 8 when the upper deck is in position, both sections 13 and 14 are flooded more than when only the lower deck needed support. Alternatively the barges could be differentially de-ballasted Compression members like those 10 may be used.

Following welding together of the decks, the dock 11 is completely flooded to lift the entire deck structure off of the island so that it can be towed away. It will be appreciated that in certain circumstances it would be possible to support the lower deck solely by the central island 12 the barges being employed only when it is required to move the deck structure. They could if necessary be moved into the dock only at this stage.

A completed deck, floating on its barges, can be towed to sheltered deep water for mating with platform legs in a known manner. Alternatively the whole deck could be mated on site to a previously installed platform by controlled ballasting of the barges, which could be suitably designed.

A second alternative would be to tow the deck to sheltered waters for attaching to and lifting by a purpose-built deck-installation vessel, for example of the type described in an article by V. G. Patrick in "Offshore Services" October 1976, page 55.

Depending on the procedure used, the positioning of the barges must be suitably arranged, and they must be capable of performing the required functions.

Regarding structural differences between the deck constructed in accordance with the above embodiment and a conventional integrated deck, the only substantial changes are the addition of the beams 9 which extend the length of the upper deck and not only provide additional support but also provide attachment points for the hydraulic gripper jacks, should these be used.

The lower deck, although as shown being of plate girder construction, could be of box beam construction. In this event a single web skid beam is preferably attached to the flange of the box-beam.

Alternatively, both the upper and lower decks could be of the plate girder type. This would give a structural beam over 20 m deep, and some weight savings could be achieved.

Although in the embodiments described, only two decks have been assembled, it will be appreciated that the procedure could be repeated and other decks slid on to the structure as required, providing the correct levels can be obtained.

It will be seen that, at least in the preferred embodiments the invention permits the simu!taneous fabrication of upper and lower decks for an offshore platform, and their subsequent assembly, thereby saving time and expense which might otherwise result e.g. from the late delivery of certain items of equipment. The sliding of the upper deck onto the lower deck is quite feasible with present techniques, for example those used in another context by Hydranautics Inc, who employ jacks and plastics coated skid rails giving a low coefficient of friction.

WHAT WE CLAIM IS: 1. A method of assembling two massive components of a structure to be subsequently transported from the point of assembly, wherein a first component has the bottom thereof at substantially the same level as the top of the second component, the first component is slid in a substantially horizontal direction onto the second component whilst the level thereof is maintained substantially constant, the two components are joined together, and the assembled structure is transported from the point of assembly by flotation means supporting both components.

2. A method as claimed in claim 1, wherein the second component is supported during fabrication by the flotation means.

3. A method as claimed in claim 2 wherein the second component is supported at substantially the same height as the first component during fabrication and is subsequently lowered to enable sliding of the first component onto the second component.

4. A method as claimed in claim 1, wherein the second component is supported during fabrication by a raised solid foundation in an area which can be flooded with water and in which the flotation means can be positioned.

5. A method as claimed in claim 4, wherein the height of the solid foundation is such that the top of the second component when fabricated is at substantially the same level as the bottom of the first component.

6. A method as claimed in claim 4 or 5

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. of the previous embodiment commences on the island 12, the floor beams of the deck being supported by this island. The ends of the floor beams are supported by barges 2 in the sections 13 and 14. As fabrication of the lower deck 1 progresses and the weight increases, the reaction forces provided by the barges are adjusted accordingly. This can be done by either changing the ballasting of the barges or by altering the level of the water in the sections 13 and 14, or both. When the lower deck 1 is completed, and its beams 8 aligned with the skid rails 7 on the side of the dock, the upper deck 6, which has been fabricated simultaneously is slid onto it in a manner corresponding to the previous embodiment, as shown in Fig 7. During sliding, the reaction forces supplied by the barges 2 are varied to take into account the changing weight and weight distribution. Thus at the stage shown in Fig. 7, section 14 is flooded to a greater extent than section 13, and in Fig 8 when the upper deck is in position, both sections 13 and 14 are flooded more than when only the lower deck needed support. Alternatively the barges could be differentially de-ballasted Compression members like those 10 may be used. Following welding together of the decks, the dock 11 is completely flooded to lift the entire deck structure off of the island so that it can be towed away. It will be appreciated that in certain circumstances it would be possible to support the lower deck solely by the central island 12 the barges being employed only when it is required to move the deck structure. They could if necessary be moved into the dock only at this stage. A completed deck, floating on its barges, can be towed to sheltered deep water for mating with platform legs in a known manner. Alternatively the whole deck could be mated on site to a previously installed platform by controlled ballasting of the barges, which could be suitably designed. A second alternative would be to tow the deck to sheltered waters for attaching to and lifting by a purpose-built deck-installation vessel, for example of the type described in an article by V. G. Patrick in "Offshore Services" October 1976, page 55. Depending on the procedure used, the positioning of the barges must be suitably arranged, and they must be capable of performing the required functions. Regarding structural differences between the deck constructed in accordance with the above embodiment and a conventional integrated deck, the only substantial changes are the addition of the beams 9 which extend the length of the upper deck and not only provide additional support but also provide attachment points for the hydraulic gripper jacks, should these be used. The lower deck, although as shown being of plate girder construction, could be of box beam construction. In this event a single web skid beam is preferably attached to the flange of the box-beam. Alternatively, both the upper and lower decks could be of the plate girder type. This would give a structural beam over 20 m deep, and some weight savings could be achieved. Although in the embodiments described, only two decks have been assembled, it will be appreciated that the procedure could be repeated and other decks slid on to the structure as required, providing the correct levels can be obtained. It will be seen that, at least in the preferred embodiments the invention permits the simu!taneous fabrication of upper and lower decks for an offshore platform, and their subsequent assembly, thereby saving time and expense which might otherwise result e.g. from the late delivery of certain items of equipment. The sliding of the upper deck onto the lower deck is quite feasible with present techniques, for example those used in another context by Hydranautics Inc, who employ jacks and plastics coated skid rails giving a low coefficient of friction. WHAT WE CLAIM IS:

1. A method of assembling two massive components of a structure to be subsequently transported from the point of assembly, wherein a first component has the bottom thereof at substantially the same level as the top of the second component, the first component is slid in a substantially horizontal direction onto the second component whilst the level thereof is maintained substantially constant, the two components are joined together, and the assembled structure is transported from the point of assembly by flotation means supporting both components.

2. A method as claimed in claim 1, wherein the second component is supported during fabrication by the flotation means.

3. A method as claimed in claim 2 wherein the second component is supported at substantially the same height as the first component during fabrication and is subsequently lowered to enable sliding of the first component onto the second component.

4. A method as claimed in claim 1, wherein the second component is supported during fabrication by a raised solid foundation in an area which can be flooded with water and in which the flotation means can be positioned.

5. A method as claimed in claim 4, wherein the height of the solid foundation is such that the top of the second component when fabricated is at substantially the same level as the bottom of the first component.

6. A method as claimed in claim 4 or 5

wherein the flotation means are employed to provide additional support for the second component during fabrication.

7. A method as claimed in claim 2, 3, or 6 wherein the flotation means are adjusted during sliding of the first component onto the second component.

8. A method as claimed in any preceding claim wherein the first component is fabricated on a solid foundation.

9. A method as claimed in claim 8 wherein the first component is supported on skid beams to facilitate sliding of the first component onto the second component.

10. A method as claimed in any preceding claim wherein structural beams of the second component serve as skid beams to facilitate sliding of the first component onto the second component.

11. A method as claimed in any preceding claim wherein the two components are fabricated substantially simultaneously.

12. A method as claimed in any preceding claim wherein the structure is part of an offshore platform.

13. A method as claimed in claim 12 wherein the components are upper and lower decks respectively of the offshore platform.

14. A method as claimed in claim 13 wherein equipment is installed in the lower deck before the upper deck is slid onto the lower deck and joined thereto.

15. A method as claimed in claim 12, 13 or 14 wherein the flotation means are used to transport the structure to a site for joining to the remainder of the offshore platform.

16. A method of a assembling two massive components of a structure substantially as hereinbefore described with reference to the accompanying drawings.

17. A structure including two massive components assembled together by a method as claimed in any precediiig claim.