GB2192592A - A floating or semi-submerged plant connected to a riser system - Google Patents

Abstract

A floating or semi-submerged plant connected to a riser system, especially in connection with pontoons (21) on a floating production unit, where, in contrast to the conventional arrangement using a hyperbaric compartment in which the riser connection is accomplished, according to the present invention the risers (1) are connected to a substantially atmospheric compartment (22) in the pontoon (21), a fact which entails a far simpler structure and simplified operation and maintenance, because it is no longer necessary for the operating staff to be divers, or for the production plants to be provided with a diving system and life-boats with pressure chambers. The pontoon (21) may be divided into a plurality of watertight compartments (22) provided with a watertight entrance door (18) communicating with an entrance corridor (16). <IMAGE>

Description

SPECIFICATION A foating or semi-submerged plant connected to a riser system The present invention relates to a floating or semi-submerged plant for example for handling crude oil and/or gas, provided with one or more pontoons for adequate buoyancy and connected to at least one riser coming from the sea floor.

From US-patent specification 3,525,388 (R.A. McClintock) there is known a subsea driiling apparatus for drilling and working in the earth beneath a body of water. The apparatus comprises a submersed structure containing a working chamber which is maintained free of water and at a predetermined pressure which is most generally atmospheric. However, the structure according to US-patent specification 3,525,388 does not form an integral part of a floating or semi-submerged plant for handling crude oil and/or gas, said known structure primarily being used for storing and extending drilling pipe strings or similar means from the chamber to a well in the subsea floor.Access means are provided to the structure from the water surface, and means under tension are connected to said structure and to the earth beneath the body of water for preventing said structure from moving upward or horizontally and for maintaining said structure at a predetermined depth.

From NO patent publication 85.3125 (Helmerson) there is known a device for remotely detachable connections or riser conduits in connection with a vessel provided with an operating deck which is carried by columns resting upon pontoons. In such known vessels, the connections of the riser conduits are made in a compartment in the pontoon which is subjected to ambient pressure. This kind of compartment is classified as hyperbaric, and has a high risk of explosions during maintenance operations. Also, the workers who are to do their jobs in the compartment must be competent divers, and the production plant must be equipped with a diver system and a life-boat including a pressure chamber.

A similar concept is also disclosed in the introductory part of GB patent publication 2,175,060A in which a remotely releasable riser pipe connection piece is located in a compartment which is open downwards to the surrounding water, and which is connected to a compressor for providing an air pressure within the compartment corresponding to the external water pressure. In such a manner the level of the water within the compartment can be maintained approximately at the downwardly directed opening. This solution means that the bulkheads surrounding the compartment must be dimensioned to withstand a rather high pressure, and that operating staff can remain within the compartment for only a limited time without having to be subjected to time-consuming decompression afterwards.

According to GB 2,175,060A it is suggested to include a sealing collar connected to the bottom plate of the compartment, so as to simplify inspection and overhaul of the riser connections. However, this solution does not give a connector system which is directly accessible for inspection and maintenance.

According to the present invention there is provided a floating or semi-submerged plant, for example for handling crude oil and/or gas, provided with a pontoon for adequate buoyancy, and at least one riser system comprising at least one riser extending between the sea floor and the plant, the or each riser system being connected to the pontoon by a connection device through a compartment at substantially atmospheric pressure.

The present invention may provide an improved arrangement of a system for example for transporting oil/gas/water to or from the pontoon of a floating production unit, and more specifically an improved arrangement of a riser connection of the type stated in the preamble.

The necessity for the compartment to be classified as hyperbaric, so that the operating staff must include divers, is thereby avoided.

By such an arrangement the connection and disconnection of the risers may be carried out in the compartment by means of an operating staff that can work under normal pressure, and consequently does not have to be subjected to time-consuming decompression after the task is terminated. Each connector will then be directly accessible and can be maintained without providing any additional equipment, e.g. a sealing means.

One development of the present invention may be to provide a riser connection including devices for facilitating the handling thereof.

In a further development of the invention there may be provided a system for lowering a pull-in line.

Another development of the present invention may be to provide an installation of a pipe system in a separately ventilated corridor on the pontoon, the corridor being constructed in such a way that any explosion will not lead to leakage of wqter into the pontoon.

Yet another development of the present invention may be to provide an alternative installation of the pipe system in ballast tanks.

The present invention will now be further explained with reference to the accompanying drawings, which illustrate a preferred embodiment of the invention.

Figure 1 is a schematical section through a pontoon supporting a floating or semi-submerged production plant for processing crude oil and/or gas.

Figure 2 is a section taken along the line B Bin Fig.1.

Figure 3 is a side view, partly in section, of a pontoon including a preferred arrangement of several compartments.

In Figures 1, 2 and 3 there is illustrated a plurality of flexible risers 1 connected to a pontoon 21, which carries one or more columns 31 which in turn support an operating deck or similar of a production plant, which can be of the floating or semi-submerged type. There may be provided more than one pontoon 21, which can be provided with thruster propellers 35 for propelling and positioning the production plant.

Figure 1, which is a section through the pontoon 21, illustrates how a group of risers, or riser system, is connected to the bottom portion of the pontoon. It is to be understood that the risers 1 extend from the sea floor 33 and are connected to the underside of the pontoon 21 for conducting crude oil and/or gas from a well 34 at the sea floor 33, as shown in Figure 3.

The upper end of the riser system is provided with a connection device 19, which is provided with a locking mechanism 2 for locking the connection device 19 to the pontoon 21.

The pontoon 21 is divided into watertight compartments 22 provided with e watertight entrance door 18 from an entrance corridor 16. To each compartment 22 there are connected for example two riser connection devices 19.

Each riser system is connected via its connection device 19 to flexible pipes 13, which in turn are connected to rigid pipes (not shown) through remote-controlled valves 14.

The rigid pipes are installed in a pipe conduit 15 outside the watertight compartment 22.

In the compartment 22 there is provided a watertight cover 3 which is open when the riser system has been connected as illustrated on the right in Figure 2, and is closed when the riser system is disconnected, as illustrated on the left in Figure 2.

In the bottom area of the watertight compartment 22, there is provided a conically shaped opening 24, serving to guide the riser connection 19 during the connection thereof and for protecting a buoyancy means 17 which is mounted an the connection device 19.

In the watertight compartment 22 there is also provided a winch system 4 serving to haul in the riser system with a hoisting cable 5 during the attachment of the connection device 19 to the pontoon 21 of the production plant.

A brief explanation of the connection procedure will be given in the following.

First of all, the watertight cover 3 is closed as illustrated by the left-hand part of Figure 2, the flexible pipes 13 being moved away from the area of said cover 3.

Thereafter, the winch system 4 is activated, the hoisting cable 5 being threaded through a cable lock 11 and connected to a flush connector 6, which by means of a remotely operated vehicle (ROV) is connected via a collet 7 to a beam or column of the connection device 19 of the riser system extending up through a cap 9 for protecting manually operated closing valves 8. It is to be understood that the cable lock 11 permits the cable 5 to run up and down without substantial amounts of water entering the compartment 22.

After the above connections are made, the riser 1 is hauled up by means of the winch system 4, and through a glass window 25 in the compartment 22, the connection procedure can be supervised by the operating staff.

When the riser system with its connection device 19 has been pulled completely into the conically shaped opening 24, the opening 24 is sealed by means of a further sealing means 12 at the bottom of the compartment 22.

Some water will have collected between the watertight cover 3 and the riser connection device 19, and in order to check that the seal 12 is tight, a test valve 23 in the watertight cover 3 is opened.

After the checking of the tightness of the seal 12, and after the locking mechanism 2 on the connection device 19 has been put in its locking position, the collet 7 is disconnected from the connection device 19 and is pulled up towards the cable lock 11, as illustrated in Figure 1.

After the collet 7 has been put in the upper position closely underneath the watertight cover 3, the hoisting cable 5 is disconnected from it, whereafter the area enclosed by the watertight cover 3 is drained by removing water therefrom by pumping means (not illustrated).

The watertight cover 3 can thereafter be opened, as is illustrated in Figure 1, and the protective cap 9 removed. The flush connector 6 and the collet 7 are lifted off as the cover 3 is opened. Thereafter the flexible hoses or pipes 13 are connected to the risers 1, and the manually operated valves 8 at the top of the riser connection device 19 can be opened. Finally, the remote-controlled valves 14 connecting the flexible hoses 13 to the rigid pipes 29 in the pipe conduit 15 can be opened according to the plant production program.

When the cover 3 is removed from the area of the opening 24, for example by providing the cover 3 with an appropriate hinge mechanism, the operating staff can have direct access to the connectors so as to facilitate the maintenance thereof without the need of introducing any additional equipment, for example further sealing means.

The above connection procedure has been carried out in a watertight compartment 22 which is at substantially atmospheric pressure, a fact which means that the operating staff can work therein without needing to be divers, and without being subjected to timeconsuming decompression after the connection procedure.

It is to be understood that the pontoon 21 can be divided into a plurality of watertight compartments, each of which is provided with a watertight entrance door 18 communicating with an entrance corridor 16.

It is further to be understood that each compartment can have connected thereto one or more riser systems, the above illustrated example including two riser systems being connected to the atmospheric compartment 22.

In Figure 3 it is further illustrated how the various watertight compartments 22 can be located in a production plant having a pontoon 21 as illustrated therein.

Claims (11)

1. A floating or semi-submerged plant, for example for handling crude oil and/or gas, provided with a pontoon for adequate buoyancy, and at least one riser system comprising at least one riser extending between the sea floor and the plant, each riser system being connected to the pontoon by a connection device through a compartment at substantially atmospheric pressure.

2. A plant as claimed in claim 1, wherein the pontoon has a plurality of compartments at substantially atmospheric pressure, each of which is adapted to receive one or more riser systems.

3. A plant as claimed in claim 1 or 2, wherein each connection device is provided at the upper end of the corresponding riser system and includes a locking mechanism for attaching the riser system with its connection device to the bottom portion of the pontoon.

4. A plant as claimed in any preceding claim, wherein the or each riser can be con necked via its connection device to an end of a corresponding flexible pipe arranged in the atmospheric compartment.

5. A plant as claimed in claim 4, wherein the flexible pipe or pipes are connected at their other ends to individual rigid pipes mounted in a pipe conduit outside said atmospheric compartment.

6. A plant as claimed in any preceding claim, wherein the or each compartment is associated with a winch system-used for pulling at least one riser system with its connection device towards the pontoon.

7. A plant as claimed in any preceding claim, wherein the or each compartment contains a watertight cover for the or each riser system, which is closed and sealed to the bottom portion of the pontoon when the riser system is being connected.

8. A plant as claimed in any preceding claim, wherein the bottom of the or each compartment has a conically shaped opening serving to guide the associated riser connection device during the connection procedure, and also serving to protect buoyancy devices mounted on said connection device.

9. A plant as claimed in claims 6 and 7, or claims 6,7 and 8, wherein the or each connection device has a flush connector which is connected via a collet to a column at the top of the connection device, said flush connector being adapted for attachment to a hoisting cable in the winch system and being adapted to be kept, when disconnected, at the underside of the watertight cover.

10. A plant as claimed in claims 8 and 9, wherein the watertight cover is provided with a hinge mechanism so as either to cover or to expose the upper area of said conically shaped opening in the bottom of the or each compartment.

11. A plant as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.