GB2023697A

GB2023697A - Improvements in or relating to off-shore structures

Info

Publication number: GB2023697A
Application number: GB7904580A
Authority: GB
Original assignee: John Laing Construction Ltd
Current assignee: John Laing Construction Ltd
Priority date: 1977-11-15
Filing date: 1979-02-09
Publication date: 1980-01-03

Abstract

The invention is directed to a ship for, and method of, transporting a platform deck (18) to an off- shore location. The ship (10) comprises three longitudinally aligned sections (12, 14, 16) with the platform deck (18) of an off-shore installation mounted on the centre section (14) of the ship. The three sections (12, 14, 16) are individually watertight and capable of disconnection to enable the installing of the platform deck (18) on the upper end of the legs (24) of the off-shore installation to be effected without lifting equipment. After installation of the platform deck (18) on the legs (24) the ship returns to base. <IMAGE>

Description

SPECIFICATION Improvements in or relating to off-shore structures The present invention relates to off-shore structures and is directed more particularly to a method of, and apparatus for, transporting the platform deck (as hereinbefore defined) of an off-shore structure from a shore construction yard to the legs or tower of the structure previously located off-shore and thereafter mounting the platform deck on the legs.As is well known, there are at the present time many arrangements in use for erecting a structure off-shore at a required location, but such arrangements suffer from one or both of the following disadvantages: (a) the high expenditure involved in installing individual equipments units or modules on the legs of the structure due to high labour costs, the high cost of the lifting and other marine equipment required and the comparatively long periods such equipment is re quires.

(b) the fact that adverse weather conditions exacerbates the conditions referred to in (a) above.

Throughout the following description and claims by the term "platform deck" is meant a deck having mounted thereon at least part of the modules and equipment referred to in paragraph (a) above.

The object of the present invention is to overcome the disadvantages referred to above by providing an off-shore platform deck which is initially commissioned and tested at an inshore construction site prior to transportation to its operating location.

A further object of the invention is to provide an improved method of transporting an off-shore platform deck to its operating location at a greatly reduced cost.

The present invention consists in a ship formed by three or more longitudinally aligned sections, one or more of the centre sections having the platform deck of an off-shore structure mounted thereon, all the sections being watertight and capable of disconnection to enable the installing of the platform deck on the legs of an off-shore installation to be effected without lifting equipment.

The invention further consists in a method of transporting a platform deck which comprises the steps of, forming a platform deck on the centre section or sections of a ship as referred to in the immediately preceding paragraph, interconnecting said centre section or sections to the fore and aft sections of the ship, propelling or towing the thus assembled ship to an off-shore location, positioning the platform deck over the upper end of legs already positioned on the sea bed and thereafter connecting the platform deck thereto, and then removing the fore and aft sections of the ship from the platform deck.

In the accompanying drawings:- Figure 1 is a side view of a ship having three longitudinally aligned watertight sections according to the present invention, Figure 2 is a side view of a platform deck mounted on the centre section of the ship shown in Fig. 1, Figure 3 is a side view, similar to Fig. 2, showing the centre section of the ship aligned with the fore and aft sections immediately prior to interconnection of the three sections, Figure 4 is a side view, similar to Fig. 3, but showing the three sections connected, Figure 5 is a side view, similar to Fig. 4, showing the ship alongside the legs of an offshore structure, Figure 6 is a front view of the ship positioned over the legs, and Figure 7 is a side view of Fig. 6.

In carrying the invention into effect according to one convenient mode, by way of example, a ship 10 is built (or converted) to form three separate longitudinal sections 12, 14 and 16, all three sections being independently stable, watertight and seaworthy, with the centre section 1 4 also being stable whilst supporting a platform deck 1 8 (see Fig. 2).

Initially, the centre section 1 4 is positioned in a dry dock and the platform deck 1 8 which is provided with the usual well-drilling equipment, derricks 20, living quarters 22, etc., is built on the centre section 1 4. The completed centre section assembly is then floated prior to the fore and aft sections 1 2 and 16, respectively, being connected thereto (as shown in Fig. 3) in readiness for transportation of the platform deck 1 8 to the off-shore location. It is to be noted that even after connection to the fore and aft sections 1 2 and 1 6 as shown in Fig. 4, the centre section 14 continues to bear the entire weight of the platform deck 1 8.

The thus completed ship 10 then proceeds under its own power to the off-shore location to a position alongside the upper end of the legs 24 firmly mounted on the sea bed as shown in Fig. 5 and then the weight of the platform deck 1 8 is transferred to the fore and aft sections 1 2 and 16, respectively, via spreader and cantilever beams 26 by pumping ballast water into the centre section 14 and, as the load transfers, maintaining the fore and aft sections 12, 16, respectively, level by ballasting their extreme ends.Upon completion of the load transfer, the centre section 14 is lowered and removed and the fore and aft sections 12, 16, respectively, are then manoeuvred so that the platform deck 1 8 is positioned directly over the legs 24, in which position the fore and aft sections 12, 1 6 are ballasted down so that the weight of the platform deck is transferred to the legs 24 ready for interconnection of the components as shown in Figs. 6 and 7. The fore and aft sections 1 2, 1 6 are then driven, or towed, clear of the off-shore structure and the sections of the ship are re-connected to the condition shown in Fig. 1 to enable it to return to base.

It will be seen that the above described method has the major advantages that when the three sections 12, 14 and 1 6 are interconnected, the arrangement is ship-shaped and behaves as a conventional ship in a seaway. Furthermore, this method eliminates as far as possible forces which may cause deformation of the platform deck 1 8 and, since the method involves minimum modification to the legs 24 platform deck 18, can be readily adapted to other designs of off-shore structures.

It will be appreciated that the platform deck 1 8 may be mounted on more than one centre section.

Claims

1. A ship formed by three or more longitudinally aligned sections, one or more of the centre sections having the platform deck of an off-shore structure mounted thereon, all the sections being watertight and capable of disconnection to enable the installing of the platform deck of the legs of an off-shore installation to be effected without lifting equipment.

2. A method of transporting a platform deck comprising the steps of, forming a platform deck on the centre section or sections of a chip as claimed in claim 1, interconnecting said centre section or sections to the fore and aft sections of the ship, propelling or towing the thus assembled ship to an off-shore location, positioning the platform deck over the upper end of legs already positioned on the sea bed and thereafter connecting the platform deck thereto, and then removing the fore and aft sections of the ship from the platform deck.

3. A method as claimed in claim 2, wherein after connection to the fore and aft sections of the ship, the centre section or sections continues to bear the entire weight of the platform deck.

4. A method as claimed in claim 3, wherein upon reaching said off-shore location the entire load of the platform deck is transferred to the fore and aft sections of the ship and the centre section or sections of the ship are removed.

5. A method as claimed in claim 4, wherein the load transfer is effected by spreader and cantilever beams whilst maintaining the fore and aft sections of the ship levei by ballasting their extreme ends.

6. A method as claimed in claim 4 or 5, wherein after connection of the platform deck to the legs, the removed centre section or sections of the ship are reconnected to the fore and aft sections and the ship returns to base.

7. A ship substantially as described with reference to, and as illustrated in, the accompanying drawings.

8. A method of transporting a platform deck substantially as described with reference to, and as illustrated in, the accompanying drawings.