GB2311042A

GB2311042A - Transportation system used to install a deck or modular assembly for an offshore platform

Info

Publication number: GB2311042A
Application number: GB9705127A
Authority: GB
Inventors: Poul-Eric Christiansen; Earl J Piermattei
Original assignee: Kvaerner RJ Brown Pty Ltd; Kvaerner Oil and Gas Ltd
Current assignee: Kvaerner RJ Brown Pty Ltd; Kvaerner Oil and Gas Ltd
Priority date: 1996-03-12
Filing date: 1997-03-12
Publication date: 1997-09-17
Anticipated expiration: 2017-03-12
Also published as: AU1933497A; GB2311042B; AU1624297A; GB9705127D0; WO1997033788A1; AU719838B2

Abstract

A seagoing transportation system for a deck assembly 24 to form part of an offshore platform, and comprising two elongate pontoons 20 21 fixed together in aligned side by side relationship with a gap between them, in combination with a framework 22 arranged to act as a structural link between the pontoons so that the pontoons act together as a catamaran, and in which the same framework also acts as a support frame for the deck. The framework may form an integral part of the structure of the offshore platform when it is installed on a substructure 29. A hydraulic pull-down system to install the deck is also described.

Description

TRANSPORTATION SYSTEM AND INSTALLATION METHOD The invention relates to a seagoing transportation system, and to the use of that system to install a deck or modular assembly for an offshore plafform.

In particular the invention relates to buoyant pontoons which are intended to be used in pairs for transporting decks to their field locations. To this end a deck is carried on a support frame which spans between the pontoons, so to form a catamaran.

Heretofore it has been known to transport integrated decks to their intended sites on barges. These decks have then been installed on offshore plafforms by direct lifts. This can be an effective method for the installation of small and medium sized decks. However, for large decks - over say 12,000Tonnes - there is currently no cranage capable of carrying out a direct lift.

It has been proposed to use a large barge to transport and install a heavy and bulky integrated deck for an offshore platform. The barge is first used to transport the deck from its place of fabrication to its intended location; and the barge is then used to install the deck on the substrncture of the offshore platform. An example of such a proposal is shown in US Patent Specification No. 4,242,011. In this case the barge must be located between legs of the substructure, which enforces constraints on the configuration of the legs. This is because a clear path must be left between the legs to accept the barge at installation.

Recently, floatover methods have been implemented in relatively calm conditions offshore Malaysia. A description of two such installations is included on pages 27 to 29 of offshore Engineer" dated October 1996. This proved to be a very effective installation method. However, a consequence of this method was that the transitory phase of installation dictated the permanent configuration of the jacket. In some circumstances it may not be appropriate to have a large docking slot in the jacket.

For very large decks, float over installation methods have been proposed (e.g. in UK Patent Specification No. 2,165,188) using specially modified semisubmersible vessels.

It has also been proposed to use a pair of tanker hulls fixed together to transport and install a deck. An example of such a proposal is shown in UK Patent Specification 2,199,791.

In this case the installation could require sophisticated and expensive installation equipment to be on stand-by, while waiting for a suitable weather window.

There remains a need for a cost effective transportation and installation system which will not dictate the configuration of the basic structure of the plafform.

The invention is concerned with the use of pontoons which are joined together by a support frame to form a catamaran. The pontoons are designed to limit the wave loading on the catamaran. This allows the pontoons, support frame and deck to be towed in open seas and harsh environments. The use of the pontoons also allows the mating of the deck with a pre-installed substructure to take place in relatively severe sea states at the offshore field location.

The invention is also concerned with an offshore float-over deck mating method that involves using a hydraulic pull-down system which is mounted on the deck using hydraulic cylinders located above each of the deck stubs and substructure legs. The cylinders are connected to a support frame which allows manipulation of tendons which are used to pull the float-over deck and substructure legs together with minimal impact loads.

The invention provides a seagoing transportation system for a deck or modular assembly to form part of an offshore plafform, and comprising two elongate pontoons fixed together in aligned side by side relationship with a gap between them, in combination with a framework arranged to act as a structural link between the pontoons so that the pontoons act together as a catamaran, and in which the same framework also acts as a support frame for the deck.

At least a part of the framework may form an integral part of the structure of the offshore platform when the deck is installed on a substructure for that offshore plafform.

It is preferred that both of the pontoons have elongate lower hull portions and elongate upper portions mounted on top of the lower hull portions and narrower than the lower hull portions.

It is also preferred that at least one of the pontoons has an upstanding caisson at or near at least one of its ends.

Preferably both of the pontoons have at least one controllably ballastable compartment fitted with rip out diaphragms or burstable discs to allow rapid flooding.

In one preferred form, the framework (or means securely attached thereto) has at least three downwardly extending tendons arranged to engage corresponding latch means on a substructure for the offshore plafform, and there is hydraulic means to tension the tendons, and so to pull the framework down onto the substructure.

In this form it is further preferred that the hydraulic means to tension the tendons includes non return valves which open to allow the deck to move down in response to downward wavelswell action, but close to restrict upward movement of the deck in response to upward wavelswell action.

It is still further preferred that the hydraulic means to tension the tendons includes pressure relief valves which open under excess pressure.

The invention also provides a method of transporting a deck or modular assembly for an offshore plafform which comprises the steps of arranging two pontoons in aligned side by side relationship with a gap between them, fixing them together with a framework, setting the deck on the framework, and then moving the pontoons, framework and deck to the site of the offshore plafform.

It is preferred that the method described above is followed by a method of installing a deck which includes the steps of locating the deck over a substructure, and then drawing the deck down onto the substructure using hydraulic means attached to at least three tendons arranged to engage latch means on the substructure.

The invention includes an offshore platform in which a deck or modular assembly has been set upon a substructure (e.g. jacket structure or lower part of a catenary anchored floater or tension leg platform or spar buoy) using the system or method described above.

A specific embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a side elevation of a transportation system showing a deck for an offshore platform mounted on that transportation system; Figure 2 is a plan view showing the framing of a pontoon forming part of the transportation system; Figure 3 is a section on the line Ill-Ill in Figure 2; Figure 4 is a section on the line IV-IV in Figure 2; Figure 5 is an end view of the transportation system as it is about to set the deck down onto a substructure of the offshore platform; Figure 6 is a diagram showing the use of tugs to maneuver the deck towards the top of a substructure; Figure 7 is a diagram of a later stage in maneuvering of the deck; Figures 8 to 11 are diagrammatic end elevations showing four stages in the installation of the deck over the substructure to form an offshore platform; Figure 12 is a detailed part section of the region designated VII in Figure 8; Figure 13 is a detailed part section of the region designated XIII in Figure 9; Figure 14 is detailed part section of the region designated XIV in Figure 10; and Figure 15 is an hydraulic circuit diagram.

As shown in Figures 1 and 5, a seagoing transportation system has two elongate pontoons 20 and 21. A support frame 22 fixes the pontoons together in aligned side by side relationship with a gap between them to form a catamaran as shown particularly in Figure 5. A deck 24 for an offshore platform rests on the support frame 22. (The deck consists of several modular assemblies 24A to 24E.) The pontoon 20 will now be described in more detail with reference to Figures 1 to 4.

The pontoon has a lower hull portion 30, which is full width throughout its length. The lower hull portion is constructed using conventional shipbuilding techniques, and is divided into watertight compartments with a single longitudinal bulkhead, and several transverse bulkheads. Advantageously, some of the watertight compartments have rip out diaphragms or burstable discs to allow rapid flooding.

The pontoon has an upper hull portion 31, which is narrower than the lower hull portion 30, and is fixed to that lower hull portion in watertight relationship along its whole length.

At the ends of the pontoon 20 there are upstanding caissons 32 and 33. These caissons have rounded ends, and are taller than the upper hull portion 31.

The support frame 22 has vertical columns 25 and 26 which rest on reinforced parts of the upper hull portions of the pontoons 20 and 21 respectively. The support frame 22 also has vertical leg stubs 27 and 28. The columns 25 and 26 and the leg stubs 27 and 28 are best seen in Figure 5. It is apparent from Figure 1 that there are in fact four rows (A to D) of columns and leg stubs, giving a total of eight columns and eight leg stubs in all.

The support frame 22 is designed to form a permanent part of the offshore platform for which the deck 24 has been constructed.

The support frame can be designed to suit a deck with a wide range of external dimensions, and for any specific substructure. The substructure may be a fixed jacket structure, or the lower part of a catenary anchored floater or tension leg platform, or a spar buoy. An upper part of a particular fixed substructure is designated 29 in Figure 5.

In use the two pontoons 20 and 21 are aligned with each other and then joined together with the support frame 22, so to form a catamaran. The deck 24 can be lowered onto the support frame, or the deck can be constructed on top of the support frame, or the deck can be lifted from construction plinths by de-ballasting the pontoons.

The pontoons 20 and 21, support frame 22 and deck 24 can then be towed to the offshore location of the plafform. The system could be used for a large modular assembly as well as for a complete deck.

Key features of the transportation system will now be described in the following paragraphs.

The pontoons are specifically designed to be used in a catamaran configuration with the deck straddling between them. The deck is elevated above its final in-place position, thereby allowing it to be mated with a pre-installed substructure by floating it over the substructure followed by a vertical downwards movement.

The deck is kept elevated during sea transportation by the support frame which is the main structural link between the catamaran pontoons. Part of the support frame may be maintained as an integral part of the permanent structure, after the deck has been installed.

The pontoons are designed with a small water plane area to reduce wave loads during transportation as much as possible, thereby reducing the loads (and hence the steel required) in the support frame.

Each pontoon comprises a lower hull portion which is deep, long and slender to achieve the necessary buoyancy and at the same time to maintain a small water plane area to minimize sea transportation wave loads. The upper hull portion hull may be stepped to reduce the water plane area further, and to provide additional hydrodynamic damping when the whole assembly rolls and pitches in the sea, thereby reducing the dynamic loading on the deck. The bow and stem are rounded to reduce the tow resistance as much as possible. The pontoon is fitted with upstanding caissons near its extremities which provide additional buoyancy when the whole assembly is trimmed or heeled, thereby improving the stability range for the catamaran.

The pontoons may be fitted with ballast compartments which can be rapidly flooded to shorten the deck mating operation. To this end the flooding arrangements may use rip-out diaphragms or burstable discs.

Figures 6 and 7 show how tugs are used to manoeuvre the deck 24 towards the preinstalled substructure (29 in Figure 5). Initially a main tug 40 and two auxiliary tugs 41 and 42 are disposed around the deck in a triangular array. A work boat 43 is used to deploy cables 44 between the substructure 29 and the deck 24. In all six cables are deployed as shown in Figure 7. The auxiliary tugs 41 and 42 are then moved to the end of the deck away from the substructure, and the deck is drawn towards the substructure by hauling in on the cables 44.

The deck is then maneuvered over the substructure. By rapidly de-ballasting the pontoons (andlor other means), the deck 24 can be installed by lowering it onto the substructure. A particular installation technique is described in more detail below.

Figure 8 shows a simplified version of the substructure 29 having legs 36 which extend to just above the sea surface. A simplified version of the deck 24 is shown supported on frame 22 of transportation steelwork. The support frame 22 is carried on top of and between two transportation pontoons 20 and 21. The legs 36 have fenders 37 to locate the pontoons 20 and 21 in aligned relationship on either side of the substructure 29. The support frame 22 is located on the pontoons with transportation/load out grillage 34, incorporating jacking support points.

The situation illustrated in Figure 8 is that following the setting down and piling of the substructure 29, and the arrival of the pontoons 20 and 21, support frame 22 and deck 24 directly over that substructure. At this stage the deck 24 is not connected to the substructure.

The method for installation of the deck onto the substructure features a tendon and latching arrangement shown in Figure 12. The deck 24 has downwardly extending leg stubs 27 and 28 which act as deck supports. Within the tubular column forming the leg stub there is a tendon 52. The tendons are connected to double acting hydraulic cylinders 53 mounted on the deck at the top of each support stub (as shown in Figure 13). These cylinders are operable to extend or retract the tendons downwardly or upwardly. Referring again to Figure 12, the lower end of the tendon 52 has a roto-latch 54 which is engageable with a detent 55 extending around the intemal periphery of the leg.

As shown in Figure 9, the pontoons 20 and 21 are ballasted down so that the leg stubs 27 and 28 are close to the tops of the legs 36. The hydraulic cylinders 53 then lower the tendons 52 so that the roto-latches 54 engage the detents 55 as shown in Figure 14. The hydraulic cylinders are actuated to tension the tendons, so to reduce relative movement between the pontoons 20 and 21, and the deck 24; and the legs 36 of the substructure 29. In the case of a floating substructure, the tendons will draw the deck and that substructure together.

As shown in Figure 10, the hydraulic cylinders 53 continue to tension the tendons 52 until the leg stubs 27 and 28 are in full contact with the tops of the legs 36. The hydraulic cylinders are then used to maintain constant bearing between the stubs 27 and 28 and the tops of the legs 36, while jacks 56 between the grillage 34 and the pontoons 20 and 21 are extended and then unloaded (and the pontoons further ballasted) until the grillage 34 is clear of the pontoons. The jacks 56 are rapidly retracted so that the pontoons can be removed.

Finally, as shown in Figure 11, the surplus steel work can be cut off the support frame 22 as scrap (cut lines shown dotted as 57 and 58) and removed to shore on the pontoons.

If the surplus steelwork was allowed to remain on the support frame, a pair of pontoons might be used to remove the deck from the substructure at the end of the life of the plafform.

In that case the installation procedure could be reversed.

Indeed, if the substructure was very weight sensitive - e.g. in the case of a spar buoy the pontoons could be used to install a drilling module; then remove the drilling module; and then replace it with a production module.

A line diagram for a hydraulic pull down system is presented as Figure 15. This system is applicable to one leg stub (e.g. 28) of the deck, so eight such systems would be required for the deck shown in Figures 1 and 5.

A bifurcated circuit is fed from a reservoir 60, through filter 61, pump 62 and check valve 63, so that each side of the circuit supplies two hydraulic cylinders (53A and 53B). Hydraulic fluid can retum to the reservoir 60 via a spring release valve 64.

The pump 62 may be by-passed through a further check valve 65, so that pistons in the cylinders can stroke upwards rapidly but not open again unless there is an overload. A further spring release valve 66 is included to protect the cylinders against overload. A manual controlled valve 67 divides the two halves of the system.

Key features of the installation technique will now be described in the following paragraphs.

The deck is elevated above its final in-place elevation, thereby allowing it to be mated with a pre-installed substructure by floating it over the substructure followed by a vertical downwards movement. The mating procedure is commenced by stabbing deck mounted tendons into stab-in connectors located in the top of each of the substructure legs. These tendons use roto-latch connectors or similar to connect to the substructure.

Once the tendons are connected to the substructure legs they can be tensioned using deck mounted hydraulic cylinders. Continued tensioning of the tendons pulls the deck down, in the process sinking the pontoons into the water. Once full pre-tension is mobilised, the pontoons are ballasted while the gap between the deck coiumn and the substructure leg is carefully controlled by use of the hydraulic system.

The hydraulic system is designed such that any motion caused by waves during tendon pre-tensioning can be accommodated and will assist the operation; i.e. during downward movement a check valve will ensure free flow of oil into the hydraulic cylinders and thereby allow them to extend with the motion. This will allow a faster build-up of tendon tension than that which could be achieved by relying on the hydraulic pump only.

A safety release valve in the hydraulic system will ensure that no excessive loading can develop at any one deck column.

The roto-latch connectors permit removal of the tendons after the pontoons are removed.

Specific advantages of the system and method described above are set out in the following paragraphs.

The weight and CG position of a deck or modular assembly may change, as the design progresses. For instance, some plafform foundation options could result in long term settlements of up to 3m. If it proved necessary to allow for this amount of settlement, the deck would need to be 3m higher above water, when first installed. The pontoon configuration is designed to have superior performance for both transportation and deck mating. In particular, the ability to modify the size and shape of the caissons 32 and 33 can be used to accommodate late deck weight and centre of gravity changes without compromising the stability criteria.

The motions and accelerations for the deck on the pontoons are approximately half of those which a deck module would experience on a cargo barge. Thus there is potential for reduction in deck steel weight and sea fastenings.

The hydraulic pull-down system is particularly suited for use with a catamaran formed of two pontoons. The small water plane area of such an arrangement ensures that only relatively small loads need to be applied by the hydraulic system.

Claims

1. A seagoing transportation system for a deck or modular assembly to form part of an offshore platform, and comprising two elongate pontoons fixed together in aligned side by side relationship with a gap between them, in combination with a framework arranged to act as a structural link between the pontoons so that the pontoons act together as a catamaran, and in which the same framework also acts as a support frame for the deck.

2. A system as claimed in claim 1 in which at least a part of the framework forms an integral part of the structure of the offshore plafform when the deck is installed on a substructure for that offshore platform.

3. A system as claimed in claim 1 or claim 2 in which both of the pontoons have elongate lower hull portions and elongate upper portions mounted on top of the lower hull portions and narrower than the lower hull portions.

4. A system as claimed in any of the preceding claims in which at least one of the pontoons has an upstanding caisson at or near at least one of its ends.

5. A system as claimed in any of the preceding claims in which both of the pontoons have at least one controllably ballastable compartment fitted with rip out diaphragms or burstable discs to allow rapid flooding.

6. A system as claimed in any one of the preceding claims in which the framework (or means securely attached thereto) has at least three downwardly extending tendons arranged to engage corresponding latch means on a substructure for the offshore platform, and there is hydraulic means to tension the tendons, and so to pull the framework down onto the substructure.

7. A system as claimed in claim 6 in which the hydraulic means to tension the tendons includes non return valves which open to allow the deck to move down in response to downward wave/swell action, but close to restrict upward movement of the deck in response to upward wave/swell action.

8. A system as claimed in claim 6 or claim 7 in which the hydraulic means to tension the tendons includes pressure relief valves which open under excess pressure.

9. A transportation system substantially as hereinbefore described with reference to and as shown in Figures 1 to 5 of the accompanying drawings.

10. A transportation and installation system substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of transporting a deck or modular assembly for an offshore platform which comprises the steps of arranging two pontoons in aligned side by side relationship with a gap between them, fixing them together with a framework, setting the deck on the framework, and then moving the pontoons, framework and deck to the site of the offshore platform.

12. A method of installing a deck transported according to claim 11 which includes the steps of locating the deck over a substructure, and then drawing the deck down onto the substructure using hydraulic means attached to at least three tendons arranged to engage latch means on the substructure.

13. A method of transporting a deck substantially as hereinbefore described with reference to Figures 1 to 5 of the accompanying drawings.

14. A method of transporting and installing a deck substantially as hereinbefore described with reference to the accompanying drawings.

15. An offshore plafform in which a deck or modular assembly has been set upon a substructure (e.g. jacket structure or lower part of a catenary anchored floater or tension leg plafform or spar buoy) using the system of any one of claims 1 to 10 or the method of any one of claims 11 to 14).

15. An offshore plafform in which a deck or modular assembly has been set upon a substructure (e.g. jacket structure or lower part of a catenary anchored floater or tension leg plafform or spar buoy) using the system of any one of claims 1 to 10 or the method of any one of claims 11 to 14.

Amendments to the claims have been filed as follows CLAIMS 1. A seagoing transportation system for a deck or modular assembly to form part of a free standing offshore platform, and comprising two elongate pontoons connected together in aligned side by side relationship with a gap between them, in which the primary connection between the pontoons is a framework arranged to form a structural link between the pontoons so that the pontoons act together as a catamaran, and in which the same framework also acts as a support frame for the deck.

2. A system as claimed in claim 1 in which at least a part of the framework forms an integral part of the permanent structure of the offshore platform when the deck is installed on a substructure for that offshore platform and the pontoons are removed individually.

6. A system as claimed in any one of the preceding claims in which the framework (or means securely attached thereto) has at least three downwardly extending tendons arranged to engage corresponding latch means on a substructure for the offshore plafform, and there is hydraulic means to tension the tendons, and so to pull the framework down onto the substructure.

11. A method of transporting a deck or modular assembly to form part of a free standing offshore platform, which comprises the steps of arranging two elongate pontoons in aligned side by side relationship with a gap between them, connecting them together with a framework in which the framework is the primary connection between the pontoons and is arranged to form a structural link between the pontoons so that the pontoons act together as a catamaran, setting the deck on the framework, and moving the pontoons, framework and deck together to the site of the offshore platform.