GB2295174A

GB2295174A - A sleeve for an offshore platform

Info

Publication number: GB2295174A
Application number: GB9523794A
Authority: GB
Inventors: Per N H Jensen; Bjorn Deildok; Leiv Petter Hesstvedt; Tor Aage Goranson
Original assignee: Kvaerner Concrete Construction AS
Current assignee: Kvaerner Concrete Construction AS
Priority date: 1994-11-21
Filing date: 1995-11-21
Publication date: 1996-05-22
Anticipated expiration: 2015-11-21
Also published as: GB2295174B; NO944460D0; AU3797995A; AU698285B2; NO944460L; GB9523794D0

Abstract

In the bottom of a shaft 1 on a movable offshore platform resting on the seabed, a sleeve 2 for penetration of conductor casings or risers has an upper seal 8 and a lower seal 3. For first time installation of the platform, the lower seal is composed of a shear plate or a cover with one or more weakened sections which permits the shear plate or cover to be knocked out of the sleeve by means of a conductor casing or riser during the installation of the conductor casing or riser. <IMAGE>

Description

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 inundation 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 o 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.

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 2,e 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 the well will 5e 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.

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 xhe new holes as quickly as possible.

The conductor casings have to be ail-wcided, 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 platforms 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 cquipment 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 cor storing conductor casings inside the shaft. In addition, as far as the pre-drilled holes are concerned, the 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.

Present day oil and gas platforms are ntln-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 used 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.

The object of the present invention is tc provide an economically advantageous seal in the bottom of the shafts, designed in such a manner that it can be removed quickly and simply after the platform has been installed in the field, without using the drilling equpinent which will occupy storage space for the conductor casings wh;n ir use. It is a further object to provide a seal wherein the problem is avoided of soiling the couplings on the template due to debris from the drilling out of the concrete plugs. At the same time it is an object that the seal should satisfy the requirement that it either should be double, or have a design which enabies it to be considered as equal to the wall of the shaft.A further requirerlent is to provide a design in which the seal can be simply re-installed, which enables the shafts to be deballasted in order to provide buoyancy to the p7atforrn when it has to be moved to a new field.

The said objects of the invention are acnicecd by employing, instead of conductor sleeves, a continual sleeve of the type mentioned in the introduction which is characterized by tne features indicated in the patent claims. The invention consists of kcown per se elements, but combined in a manner which is not previously known, and which entails great economic benefits.

By employing a sleeve with double sealing 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 down into the seabed.The shaft will normally have a concave or raised underside with the penetrations 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 interstention 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 Nonvegian 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 susnmer 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 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 inexpens.se 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 brind plate. Alternatively the upper seal can be made redundant, and consequently remcvcd, 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 sleeves 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 anC1: the risers will be disconnected and hoisted out of the sleeve. In those holes which are drilled from the platform the conductor casing with its casings will be closed and cut immediately above the seabed, whereupon it is hoisted out of the sleeve. The sleeve will 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. rile 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 conductor casings should not oc 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 ntervention 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: Fig. 1 is a vertical section of a platform shaft of concrete, Fig. 2 is a vertical section of a sleeve with double sealing for penetration of conductor casings or risers. where the lower seal is composed of a shear plate, and Fig. 3 is a vertical section of a sleeve with double seal for penetration of conductor casings or risers, where the lower seal is composed of a removable intervention plug.

Figure 1 illustrates a typical platform shaft 1 of concrete according to the invention, containing sleeves 2, two 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 i, clearly showing the pipe flange 7, the blind flange 8 and an air valve l There is further illustrated by dotted lines a conical insertion ring 12, which can be mounted to the flange 7 instead of the blind flange 8.

Figure 3 illustrates the sleeve 2 witlil a hydraulically operated intervention plug 13 with its clamping rings and seating 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 plur 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 by opening an air valves 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 sleeve 2 and knock off the shear plate 3, which will break at its weakened 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 witch 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.

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 15 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 i8 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 pulls, 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 expedicnt 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, e.g., 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 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 pre installation, and give this a height which projects above the sleeve. In this case the seal can be obtained by eqiyping 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, wlll all. lie within the scope of the invention.

Claims

PATENT CLAIMS

1. A sleeve device (2) for use in penetration of conductor casings or risers, for vertical installation in the bottom of a platform shaft (1) on an offshore platform resting on the seabed, wherein the sleeve can be provided with an upper and a lower sealing against the sea, characterized in that at its lower end the sleeve has a watertight shear plate (3) or a cover with one or more weakened sections (3a), the shear plate or cover thus constituting the lower seal, since the shear plate or cover can be knocked out of the sleeve.

2. A device according to claim 1, characterized in that the shear plate (3) or the cover is connected to the underside of the shaft by means of a chain (13) or other connecting device with a similar function, the shear plate or cover thus remaining suspended below the shaft, beside the hole for the penetration, after it has been knocked out of the sleeve.

3. A device according to claim 1 or 2.

characterized in that the upper end of the sleeve projects above the shaft's water level (9), and that the upper 'eal us 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, characterized in that the sleeve is 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 ts a seat.

5. 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 of the conductor casing vr riser, and wherein the sleeve is equipped with an upper and a lower seal for sealing the sleeve before penetration of the conductor casing or tEe riser, characterized by mounting on the sleeve a shear plate or a cover with one or more weakened sections (3a) 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 or the riser.

6. A method according to claim 5, characterized 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, characterized 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 chain 5, characterized 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, characterized in that the shear plate or the cover is knocked out of the sleeve by a special tool.

10. A method according to one of the claims 5-9, characterized 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 again achieved.

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

12. A method according to claim '0, characterized in that the sleeve is sealed by filling a part of it with concrete, thus forming a cast pipe plug