AU698285B2

AU698285B2 - Sleeve with double sealing for penetration of conductor casings or risers

Info

Publication number: AU698285B2
Application number: AU37979/95A
Authority: AU
Inventors: Bjorn Deildok; Tor Aage Goranson; Leiv Petter Hesstvedt; Per N H Jensen
Original assignee: Kvaerner Concrete Construction AS
Current assignee: Kvaerner Concrete Construction AS
Priority date: 1994-11-21
Filing date: 1995-11-21
Publication date: 1998-10-29
Anticipated expiration: 2015-11-21
Also published as: NO944460L; GB2295174A; NO944460D0; AU3797995A; GB2295174B; GB9523794D0

Description

Our Ref: 578504 P/00/011 Regulation 3:2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT r Applicant(s): Address for Service: Invention Title: Kvaerner Concrete Construction as Postboks 18 1322 Hovik

NORWAY

DAVIES COLLISON CAVE Patent Trade Mark Attorneys Level 10, 10 Barrack Street SYDNEY NSW 2000 Sleeve with double sealing for penetration of conductor casings or risers The following statement is a full description of this invention, including the best method of performing it known to me:- 5020 ~Is Sleeve with double sealing for penetration of conductor casings or risers The invention concerns a sleeve with double sealing for penetrating conductor casings or risers, for use in the bottom of the shafts on a movable offshore platform resting on the seabed, which is employed during oil and, or gas production at sea, wherein the platform consists of a concrete foundation with a deck structure, and the concrete foundation consists of one or more shafts.

The use of concrete platforms resting on the seabed is known in the production of oil and gas at sea, wherein the concrete platform's foundation consists of one or more shafts which, apart from their function of supporting the platform, housing equipment and acting as stores for oil, are also used for conducting the flow from the well to the platform's deck for processing.

Before the drilling can begin a conductor casing has to be driven down into the seabed, generally to a depth of 100-200 m. The conductor casing is held in position horizontally by supports, the lowest of which, a conductor sleeve, is located at the bottom of the shaft, and the remaining conductor supports are placed in the shaft on several decks above one another, :ii During the first part of the drilling a drill with large dimensions is employed, "When a certain depth has been reached a casing is fitted inside the conductor casing, and cemented securely thereto. The drill is then changed to one with smaller dimensions for the ensuing drilling, and the drilling continues in this manner until the required depth is reached. Thus the complete pipe from the seabed will consist of a conductor casing on the outside, followed by several layers of alternate casings and concrete.

It is also known to instal a platform over pre-drilled holes, where a template with valves and couplings is provided over the holes before the platform is installed. In this case tihe well will be connected to risers with corresponding couplings. Like the conductor casings, these risers will also be located inside the shafts.

In practice a platform will partially employ pre-drilled holes with a template, and partially drill its own holes without the use of a template.

I- When a concrete platform, which is to be installed on the seabed, is being towed out to sea, the water level inside the shafts will be lower than that outside in order to obtain sufficient buoyancy. This gives rise to an external overpressure, which means that the conductor sleeves in the bottom of the shaft have to be sealed, and the use of a concrete plug is a known procedure for this purpose.

When installing a platform in the field the shafts are filled with water, thus avoiding the necessity of sealing the conductor sleeves in the bottom of the shaft. For economic reasons it is desirable to instal the risers for the predrilled holes and to start the drilling of the new holes as quickly as possible.

The conductor casings have to be all-welded, and the most rational procedure is so-called pre-installation of conductor casings, i.e. the conductor casings are welded together on shore in lengths which are as long as practically possible, and stored inside the platform's shaft during tow-out.

When employing a concrete plug according to the prior art it has to be drilled out by means of the platform's drilling equipment before the work of driving down the conductor casings or connecting the risers can begin. This leads to a 'loss of time and limits the possibilities for storing conductor casings inside the shaft. In addition, as far as the pre-drilled holes are concerned, the S drilling of the concrete can present problems since debris from the drilling is spread over the couplings in the template, making it difficult to connect the risers. The removal of this debris will normally involve the use of a remoteoperated subsea vehicle (ROV), which again leads to loss of time and increased costs.

25 Present day oil and gas platforms are non-replaceable installations, and for several reasons are not suitable for re-use. To-day's low oil prices, together :.:with the fact that the large, easily extractable oil fields have been developed in many places, necessitate a development in the direction of platforms which can be used many times, with only a few years' use in each field.

Consequently it is of general interest within the oil business that technical solutions should be developed which permit the re-use of platforms.

When a concrete platform is re-used the conductor sleeves in the bottom of the shafts have to be sealed, thus enabling the shafts to be deballasted and provide buoyancy to the platform. The strength of the concrete plug which is

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11 W111)(HIS D)S D1 I'SPH:(?RIA78 J144 -3used in present day technology is considered to be equal to the wall of the shaft, When the concrete plug is removed, and the conductor sleeve has to be resealed, the conductor sleeve will be regarded as a pipe penetration, in which case it is normal procedure within the industry to require double sealing against the sea. The obvious solution will be to seal the conductor sleeve with a removable intervention plug, but this is only regarded as a single seal, and consequently cannot be employed alone.

In accordance with the invention, there is provided a sleeve device for penetration by conductor casings or risers, for vertical installation in the bottom of a platform shaft on an offshore platform resting on the seabed, wherein the sleeve can be provided with an upper and a lower sealing against the sea, characterised in that at its lower end the sleeve has a watertight shear plate or a cover with one or more weakened sections, the shear plate or cover thus constituting the lower seal, being adapted to be knocked out of the sleeve.

I' In another aspect, there is provided a method for installing a conductor casing or riser in a platform shaft during an installation of an offshore platform resting on the seabed, 15 wherein after installation the platform is put into operation and may subsequently undergo a move, wherein the platform shaft is equipped with a vertical sleeve for penetration by the conductor casing or riser, and wherein the sleeve is equipped with an tipper and a lower seal for sealing the sleeve before penetration by the conductor casing or the riser, characterised by mounting on the sleeve a shear plate or a cover with one or more weakened sections before the platform is put into operation, in such a manner that the shear plate or the cover constitutes the lower seal, and by knocking the shear plate or the cover out of the sleeve before the penetration of the conductor casing oer tle riser.

By employing a sleeve with double .sit.ng, wherein the lower seal is composed of a shear plate or a cover with a weakened section, a seal is provided which is simple and inexpensive to produce, and reliable when the platform is towed out to sea. The shear plate will be removed by means of the conductor casing, which is dropped or driven down by means of suitable equipment, striking the shear plate in its weakened section. The equipment for driving down the conductor casing will be already available on the platform, since it will be used to drive the conductor casing dovn into the seabed. The shaft will normally have a -7Io concave or raised underside with the penetrations

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for the sleeves under the middle. The shear plate is connected to the sleeve by a chain or other connecting device with a similar function, in such a manner that after it has been knocked aside by the conductor casing it remains suspended below the shaft, beside the sleeve, and does not obstruct or impair the lowering of the conductor casing or the connection of the riser.

As mentioned above, when pre-drilled holes are used a conductor casing will not be employed, but instead a riser which differs from the conductor casing in that it has smaller dimensions and has a coupling mounted at the bottom.

Consequently this riser will not be suitable for knocking off the shear plate, and in this case the shear plate will first have to be knocked off by a riser, or a removable intervention plug will have to be used as a lower seal.

The height of the sleeve is determined by the platform's operating conditions.

The natural design criterion (based on practice on the Norwegian continental shelf with double sealing against the sea) will be to allow the sleeve's upper end to project above the minimum water level required in order for the shaft to maintain stability during a 10 year summer storm.

The upper seal can be operated manually, and in this case it is essential that it should be located above the water level inside the shaft, in addition to which a deck must be provided inside the shaft at a suitable working height 20 under the seal, normally one metre, The upper seal can also be submerged.

and in this case has to be operated by means of subsea vehicles or remote control. The simplest and most inexpensive seal will be to use a standard pipe flange with a blind flange, but if it is justified by weight and space requirements, another seal can be used, for example a clamping ring coupling of the Graylock type with a blind plate. Alternatively the upper seal can be made redundant, and consequently removed, by making the sleeve so high that the upper end will always be located above water. This means that the natural design criterion will be that the sleeve's upper end should always project above the maximum water level which can occur during an accident.

Before moving the platform, as far as the pre-drilled holes are concerned, the valves on the bottom will be closed and the risers will be disconnected and hoisted out of the sleeve. In those holes which are drilled from the platform, the cohductor casing with its casings will be closed and cut immediately above the seabed, whereupon it is hoisted out of the sleeve. The sleeve will

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then be closed in the bottom with a removable intervention plug, several of which are available on the market. This will normally be suspended on a wire, and be remotely controlled via an umbilical. The wire can either pass through a penetration in the upper seal, or be attached inside the sleeve in a wire fastening. The umbilical will pass through a penetration in the upper seal, thus enabling the plug to be controlled after the upper seal is closed.

When the platform is installed in the new field, the seals will be operated in the same way as before moving, but in reverse order.

From the technical point of view there is no reason why the sleeves for the to conductor casings should not be sealed with removable intervention plugs instead of shear plates even during first time tow-out. However, the economic aspect is involved here, since a removable intervention plug is much more expensive than a shear plate, and it is therefore a great economic advantage to avoid or to postpone the purchase of intervention plugs.

The invention will now be explained in more detail by means of the drawings which illustrated a preferred embodiment of the invention, in which: i oO Fig. 1 is a vertical section of a platform shaft of concrete, I Fig. 2 is a vertical section of a sleeve with double sealing for penetration of conductor casings or risers, where the lower 3eal is composed of S 20 a shear plate, and Fig. 3 is a vertical section of a sleeve with double seal for penetration of S•conductor casings or risers, where the lower seal is composed of a S• removable intervention plug.

Figure 1 illustrates a typical platform shaft 1 of concrete according to the invention, containing sleeves 2, twvo of which are illustrated, cast in the shaft's bottom section 4. In the lower edge of the shaft's underside 5 are located the sleeve's lower seals in the form of shear plates 3. The figure shows that the central section of the shaft's bottom section 4 is raised above the seabed 6, the distance between the shaft's underside 5 and the seabed 6 in this embodiment being 7.5 metres. Figure 1 further illustrates the sleeve's upper seal consisting of a standard pipe flange 7 with a blind flange 8. Also illustrated are the shaft's water level 9 and access deck 10 for operation of the upper seal.

Figure 2 illustrates the details of the sleeve 2, with the shear plate 3 with its weakened section 3a, in the form of an annular groove, and the welded-on lug 3b for attaching the chain 13, the other end of which is attached to the lug 2a, which in turn is welded to the conductor casing 2. The upper seal is illustrated in more detail than in figure 1, clearly showing the pipe flange 7, the blind flange 8 and an air valve 11. There is further illustrated by dotted lines a conical insertion ring 12, which can be mounted to the flange 7 tO instead of the blind flange 8, Figure 3 illustrates the sleeve 2 with a hydraulically operated intervention plug 13 with its clamping rings and sealing rings 14. The intervention plug 13 is suspended on a wire 15, which is attached to the wire fastening 17. The umbilical 16 connects the intervention plug 13 to its control unit (not shown), being passed from the intervention plug 13, along the wire 15 and through the penetration 18 in the blind flange 8, After first time installation of the platform, the platform shaft 1 will be ballasted by admitting water via valves (not shown) in the known manner, to the indicated water level 9. Any pressure inside the sleeve 2 will be equalized 20 by opening an air valve 11, whereupon the blind flange 8 will be opened and replaced by the insertion ring 12. Before drilling can start, the conductor casing (not shown) will then be inserted into the sleeve 2 by means of the insertion ring 12. The conductor casing will be dropped down through the S 25 section 3a. The shear plate will remain suspended in the chain 13 below the shaft's underside 5, beside the sleeve 2, in such a manner that it does not obstruct the conductor casing. The conductor casing will then be driven down into the seabed in the known manner, and form the first part of the well. The further drilling of the well will be conducted according to the prior art, by drilling down to a certain depth, inserting casings, cementing these to the conductor casing, continuing drilling with a smaller pipe dimension, inserting a new casing, etc.

When the platform has to be moved to a new field the conductor casing with its casings will be cut above the seabed and shut off in the known manner.

It~aUsl~~ll~dl~~Y11~WlZ~+tl nW~a 7 The sleeve 2 will then be closed at the bottom by means of a removable intervention plug 13 which will be lowered down by means of the wire and connected to its control unit via the umbilical 16, The wire 15 will be attached to the wire fastening 17 inside the sleeve 2, while the umbilical 16 will be passed through the penetration 18 in the blind flange 8. This is done in order to relieve the pressure on the hoisting device which holds the intervention plug, for example a pulley, while at the same time maintaining control over the intervention plug 13 via the umbilical 16. After the lower seal has been installed, the blind flange 8 will be mounted on the flange 7, and the penetration for the umbilical 18 will be tightened, thus closing the upper seal.

When the platform is installed for the second time the opening of the upper and lower seals will be performed in a similar manner to closing before moving, but in reverse order.

In the method described above a conductor casing was used to knock the shear plate out of the sleeve, It is, of course, also possible to use other objects for this purpose, since any object can be employed which has sufficient weight, and which at the same time can be accommodated inside the sleeve. In particular it can be expedient to use a part of a drill string, since this will be easily accessible on the platform. It is also possible to *".employ a special tool, which in its simplest form, can consist of a pipe section suspended on a wire. This pipe section may be filled with concrete in order to increase the weight.

Similarly, when moving the platform it is possible to do this by other methods than by the use of a removable intervention plug, An example of an alternative method for sealing the sleeve is to fill a part of it with concrete, thus forming a cast intervention plug.

It will be obvious that many modifications are possible within the scope of the invention. The shear plate or cover with its weakened section can be designed in many ways. For example, a curved cover can be used, which will be advantageous from the point of view of strength. The cover can be fastened with screws or pins with weakened sections, thus causing them to break under the impact from the conductor casing. The lower seal can also be WI)~R--Jlsb~PLIISYI*)11C )Z~L1 designed as a removable cover or a blind flange, in which case it can be operated by a subsea vehicle.

As an upper seal it will be possible to replace the blind flange or the clamping ring coupling of the "Graylock" type with other types of seals, for example a ball valve or special valve which permits penetration of the conductor casing when it is open. It will also be possible to place the conductor casing or the riser inside the sleeve before tow-out, so-called preinstallation, and give this a height which projects above the sleeve. In this case the seal can be obtained by equipping the top of the sleeve with a sealing ring between the sleeve and the conductor casing or the riser.

These examples of modifications and similar modifications which will be natural for one skilled in the art, will all lie within the scope of the invention.

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Claims

1. A sleeve device for penetration by conductor casings or risers, for vertical installation in the bottom of a platform shaft on an offshore platform resting on the seabed, wherein the sleeve can be provided with an upper and a lower sealing against the sea, characterised in that at its lower end the sleeve has a watertight shear plate or a cover with one or more weakened sections, the shear plate or cover thus constituting the lower seal, being adapted to be knocked out of the sleeve.

2. A device according to Claim 1, characterised in that the shear plate or the cover is operatively connected to the sleeve by means of a chain or other connecting device with a similar function, the shear plate or cover thus remaining suspended below the platform shaft, after it has been knocked out of the sleeve. 15

3. A device according to Claim 1 or 2, characterised in that the upper end of the sleeve projects above the platform shaft's water level, and that the upper seal is composed of a removable dry seal, including a removable pipe plug, blind flange or clamping ring coupling.

4. A device according to one of the preceding claims, characterised in that the sleeve is S 20 of such a length that it projects above the maximum water level which can occur during an accident, in such a manner that the upper end of the sleeve is located at such a height that it provides a security which is equivalent to a seal.

A method for installing a conductor casing or riser in a platform shaft during an installation of an offshore platform resting on the seabed, wherein after installation the platform is put into operation and may subsequently undergo a move, wherein the platform shaft is equipped with a vertical sleeve for penetration by the conductor casing or riser, and wherein the sleeve is equipped with an upper and a lower seal for sealing the sleeve before penetration by the conductor casing or the riser, characterised by mounting on the sleeve a shear plate or a cover with one or more weakened sections before the platform is put into 2 operation, in such a manner that the shear plate or the cover constitutes the lower seal, and ~I by knocking th-. shear plate or the cover out of the sleeve before the penetration of the conductor casing or the riser.

6. A method according to C-laim 5, characterised in that the shear plate or the cover is knocked out of the sleeve by a conductor casing.

7. A method according to Claim 6, characterised in that the conductor casing is installed in the sleeve during a building or preparation phase prior to the installation of the platform.

8. A method according to Claim 5, characterised in that the shear plate or the cover is knocked out of the sleeve by a part of a drill string.

9. A method according to Claim 5, characterised in that the shear plate or the cover is knocked out of the sleeve by a special tool.

A method accoiuing to one of the Claims 5-9, characterised in that before a possible moving of the platform and after removing the conductor casing or the riser, the sleeve is sealed in such a manner that a double sealing against the sea is achieved. 20

11. A method according to Claim 10, characterised in that the sleeve is sealed by a removable pipe plug, which thus constitutes the lower seal.

12. A method according to Claim 10, characterised in that the sleeve by filling a part of it with concrete, thus forming a cast pipe plug,

13. A sleeve device substantially as hereinbefore described with reference to the drawings. Dated this 15th day of May 1998 KVAERNER CONCRE TE CONSTRUCTION AS 1 By Their! or /Its Patent Attorneys 9 AVIES COLLISON CAVE (2 wi th double sealing for penetration oft conductor caiuS Or rihcr, lor use in the bottom of the shafts (I onl a mwwbale otliore platiorm rest in e onl the seabed, which is employedl during oil or gas prodICti n1 at sea, herein the platform consists of a concrete tbundation with a deck structure, ,lid thle concrete foundation consists 01 one or more ihalts vkl i thle lee% es t2placed in the bottom, and where during the platl'orni's moving, plia:;e thle ,leeve has double scaling againmt the iea, the Jeeve'-, loAer end helm-, remiovably sealed )at thle shiafi's underside For first time installation oit thle Platform, in a preferred embodiment thle sealinu,- at the slIce\ e's lower end is composed of a shear plate 3) or a cover with one or mol(re weakened sections which permiits the shear plate or cover to be knocked out by the sleeve by means of thle conductor easing during the instal lationi of thle conductor casing. 0i .0S 0