GB2348454A

GB2348454A - Floatation device for lifting a platform and a substructure

Info

Publication number: GB2348454A
Application number: GB0017909A
Authority: GB
Inventors: Robert Paul Hovinga; Gelder Klass Boudewijn Van; Den Dool Arnold Andries Va; Jong Peter Ronald De
Original assignee: Allseas Group SA
Current assignee: Allseas Group SA
Priority date: 1996-04-24
Filing date: 1997-04-22
Publication date: 2000-10-04
Anticipated expiration: 2017-04-22
Also published as: GB2348454B; GB0017909D0

Abstract

A flotation device for use in lifting a sea platform and a substructure comprises a flotation body 7 and fixing means 9 attached to the body for fixing the body to vertical posts (6, Fig 4) of the substructure below sea level Z, the flotation body being of elongate form. The fixing means may be pivotally attached via pivot 10 either directly to the flotation body or by means of a spacer arm 12 between the flotation body and the fixing means. The fixing means may be a clamp or clamps formed by at least two shell-like parts (21 and 22, Fig 9) which are closeable around the substructure post. The flotation body may be provided with volume enlarging means such as flaps (13 and 14, Fig 6) at a top part thereof.

Description

FLOTATION DEVICE FOR LIFTING A SEA PLATFORM FROM A SUBSTRUCTURE The invention relates to a flotation device for lifting a sea platform from a substructure.

In the exploitation of oil wells in relatively shallow areas of water, for instance the continental shelf of Europe, it is usual to place sea platforms or the like and anchor them on the seabed using a jacket. Such constructions have to be removed again after a period of time and the methods existing heretofore are rather time-consuming and costly.

An object of the invention is to provide a device for use in lifting a sea platform or like construction from a substructure so that the sea platform can then be taken to a location where total dismantling is easier to carry out.

Reference is directed to UK patent application 9708161.6 (GB 2312704)"the parent application"which contains subject matter common to this application. The parent application has a claim defining a method of lifting a sea platform or like construction in the following terms: A method of lifting a sea platform or like construction from a substructure comprises the steps of : fixing a flotation body to a part of the substructure; giving the flotation body buoyancy by removing water; making a separation in the substructure below the connection point between flotation body and substructure; increasing the buoyancy of the flotation body by removing more water, such that the flotation body obtains a half-submersible position, so projecting out of the water, and moving the or each flotation body with the substructure fixed thereto to an end location.

Due to the use of a flotation body which is fixed to the substructure, i. e. the jacket, it is possible after increasing the buoyancy thereof and after separating the substructure below the connecting point of the or each flotation body to raise and remove the whole construction including the substructure or a portion thereof.

By first submerging the or each flotation body below the water surface prior to fixing to the substructure, for instance by filling this body at least partially with water, disrupting influences resulting from swell (waves and current) can be largely avoided whereby the fixing entails less risk ofdamage.

In a refinement of such a method each flotation body is rotated through a predetermined angle around a lying pivot axis after fixing to the substructure in order to be able to transect the water level and thus obtain the desired stability. During fixing of the flotation body to the substructure this body remains far below sea level and is thereby virtually unaffected by the swelI.

According to an aspect of the invention there is provided flotation device for use in lifting a sea platform or like construction from a substructure, the device being fixable to the substructure below sea level and the device comprising: a flotation body and fixing means attached to the body for fixing the body to vertical posts of the substructure, wherein the flotation body is of elongate form.

According to another aspect of the invention there is provided a flotation device for use in lifting a sea platform or like construction from a substructure, the device comprising: a flotation body and fixing means attached to the body for fixing the body to vertical posts of the substructure, wherein the fixing means are pivotably arranged to the body so that in use the flotation body is rotatable with respect to the fixing means and substructure to obtain a desired stability.

The flotation means may be of substantially box-like elongate form and the fixing means are preferably pivotably arranged to the body by a pivot or hinge.

The fixing means may be pivotably attached directly on the flotation body or pivotably attached to an arm or spacer disposed between the fixing means and body. The spacer is provided to allow the flotation body to position partially above sea level after rotation. The loads on the jacket are smaller with this construction.

Preferably the fixing means are in the form of at least two fixing means and may be a clamp.

The clamp can consist of two shell-like parts which can be closed around the substructure post.

Above mentioned and other features will be further elucidated hereinbelow in the figure description of a number of embodiments. Figures from the parent application have been included along with the accompanying description to assist in understanding the invention. In the drawings: Figure 1 shows a perspective top view of a drilling platform, on the substructure of which two flotation bodies are fixed, figures 2,3,4,5 show in each case standing views of the drilling platform of figure 1 with differently embodied flotation bodies which are fixable to the jacket and displaceable relative thereto, figure 6 is a perspective view of a construction with jacket substructure provided with flotation bodies of changeable volume, figure 7 shows a standing view corresponding with figure 2 of flotation bodies fixed to the jacket, wherein use is made of tidal movement, figure 8 shows a detail in perspective of the fixing means in the form of a clamp between the flotation body and an upright post of the substructure. figure 9 shows a variant of the fixing means, figure 10 shows a third variant of the fixing means.

Designated in figure 1 with the numeral 1 is a work platform which can have a random function and be or random structure. Shown in the drawing is a drilling rig (2), a hoisting frame (3), a landing platform (4) ana further work areas an accommodation areas for personnel.

The platform is arranged on a substructure or jacket (5) which consists here of a plurality of vertically directe posts (6) which are mutually connected by cross struts to form a diagonal bracing.

The posts stand on the seabed (not shown).

It is the objective according to the invention to release the substructure (6) from the seabed and lif. it with flotation bodies (7) such that the whole construction can be taken to a location for further dismancling.

The flotation bodies according to the invention can be of random type, for instance in the form of lighters or in the form of closed pontoons embodied in box shape, see figures 2 and following. Each flotation body is filable with water and can be emptied by means of for instance a pump (8) as shown in figure 2, the suc-ion line of which debouches in the space of the flotation body (7). The flotation body can be provided with tanks (not shown) placed above sea level which can be emptied quickly, whereby the buoyant effect can be increaseå rapidly.

The side of flotation body (7) facing toward jacket (5) is provided with fixing means (9) which are further elucidated hereinbelow.

It is the intention to fill tanks in flotation body (7) with water such that a comparatively small submerge weight is obtained (for instance between 100 and, 000 tons) wherein it comes to lie practically underwater, see figure 2, whereafter the flotation body can be carried using a crane or derrick in the direction of arrow Pi towards the side of jacket (5) and the fixing means (9) can be fastened round the posts (6). The water is removed from flotation body (7) by means of pump (8) so that after cutting through the posts (6) at the position of level A the whole jacket with platform 1 standing thereon can be moved upward, wherein stability is achievec by the flotation bodies (7) which protrude some height above sea level Z. This whole construction can then be removed by means of tugboats.

The construction of figures 2 and 3 in respect of the flotation bodies is suitable for small swell. However, should it be expected that the sea swell will be rougher, then it is recommended to fix the flotation bodies (7) to jacket (5) at a lower level. This is shown in figure 4. Flotation bodies (7) are submerged to a distance below the level Z, whereafter the fixing means (9) can be connected to posts (6) at a lower level.

Because fixing means (9) are connected to flotation body (7) by means of the. horizontal pivot axis, the flotation body (7) can be swung upward in the direction of arrow P2 after fastening of the means (9) to posts (6), whereby the outer side wall (11) comes to lie above the level Z.

By admitting air into a relatively small compartment of the flotation body the water can be urged out of the flotation body (7) and the desired rotation as according to figure 3 can be effected. A buffer 11'bounds the upward rotation in that it strikes against the side of the substructure. It is known that wave effects are damped at greater depth, as are surface currents resulting from wind. Due to the low coupling of flotation bodies (7) to jacket (5), wave forces of 1st and 2nd order, i. e. high respectively low frequency, will hardly affect the flotation bodies (7) even in rough weather or high swell, whereby the fixing takes place more easily.

Although when the flotation bodies rise upward the above mentioned forces will once again affect the bodies, they are now secured to the jacket and the bodies can be controlled more easily with ballasts or buffers and traction wires.

Figure 5 shows an alternative embodiment of a fully submersible flotation body (7), since this is provided with a spacer (12) between the hinge (10) with horizontal pivot axis and the flotation body (7). In this manner the fixing means (9) can be arranged at an even lower level since after swinging as according to arrow P2 the pontoon can still be carried with side wall (11) above the level Z.

Figure 6 shows an embodiment wherein each flotation body (7) is provided along the upper wall with folding or upwardly slidable flaps (13,14) which can be swung upward in the direction of arrow P3 on the hinges (15). While it is thus possible to couple the flotation bodies (7) to the jacket (5) below the level Z, it is also possible, by then moving the flaps (13, 14) upward, to create a water-transecting surface in the level Z. The buoyant effect of flotation bodies (7) is brought about by pumping empty the volume in the flotation body and that between the flaps, so that the platform can be raised in stable situation.

Figure 7 shows an option for the submersible flotation bodies (7) which are embodied with fixing means (9,9') such that with use of tidal movements the flotation bodies (7) can be fixed to jacket (5) at a high sea level Zh such that at low tide Zl the flotation bodies (7) come to protrude above the level Z. After separating the posts (6) of the jacket the flotation body (7) moves automatically upward, carrying with it the platform (1).

Hereinbelow follows a discussion of the fixing means shown in figures 8,9 and 10.

Figure 8 shows a cross section of the flotation body (7) which is here a closed box-like body reinforced by cross struts (15). Mounted on the slanting wall (16) are two ears (17) between which a U-shaped holder (18) is arranged pivotally round the pin (19). Pin (19) is mounted in an eye of the ear (17).

Arranged in the U-shaped holder (18) is a vertical pin (20) around which two shells (21,22) are pivotable.

Each shell is coupled to a cylinder (23,24) which sup- ports on the side wall (16) of flotation body (7). Each shell (21,22) can be moved away from respectively toward the other by means of cylinder (23,24). The shell is formed such that it can be arranged clampingly round a post (6) of the jacket. With the pivot pin (19) the flotation body 7) can be carried from the lower to the upward folded position as shown in figure 4.

The ears (: 7) can also be arranged on a body portion 17', which body portion is displaceable along the sloping wall via a longitudinal guide (15') therein to enabie varying of the distance between the clamps and adapt it to the distance between the posts of the substructure.

Figure 9 once again shows a fixing means (9) which consists here o two shells (21,22) which are rotatable round a pin (20) relative to each other. In the closed position the shells form a sleeve with a conically tapering inner wall. Wedges (13) are arranged round the post (6) of the jacket such that when the closed shells are moved upward as according to arrow P4 the funnel-shaped inner wall thereof presses against the outside of wedges (31) to clamp fixedly thereor.. In the case of an upward force on sleeve (21,22) this force will be transmitted to post (6) and carry it along in upward direction. It will be apparent that shell (21) is connected to the flotation body via a random connection (32). This may for instance be the spacer, 12) as according to figure 5.

Figure 10 shows an embodiment of a connecting means (9) which is a combination of the embodiment of figure 8 and that of figure 9. The outer shell (22) can be clamped to the inner shell (21) by means of any suitable pressing member. The inner shell is provided with a pivot pin corresponding with pin (19) in figure 8 which is mounted rotatably in ears (17) fixed to the upper part of flota tion body (7), this as according to the embodiment of figure 4. With this pivot pin (19) the flotation body can be swung upward in the direction of arrow P2.

Compared with the embodiment in figure 8 this embodiment has a more favourable load on the post of the substructure. The line of force here runs through the centre of the post whereby it does not come under strain of bending.

The invention is not limited to the above described embodiments. It is noted that a flotation body constructed in accordance with the invention not only can be used to remove the sub-and superstructure in one operation but that it can also be in several parts. This may be particularly important in the case of deep-water jackets, of which the superstructure with a part of the jacket is first removed and then the remaining part or parts of the jacket, for which purpose the flotation body must in any case be fully submersible.

It is finally noted that the clamps as described can be replaced by fixing means formed in other manner. Here can be envisaged for instance: -a hook, -a hoisting sling, -a hook or eye fixed to an intersection point of the jacket by a"plastic matrix"cast round the intersection, -a support point pressing under the deck of the superstructure.

Claims

Claims 1. A flotation device for use in lifting a sea platform or like construction from a substructure, the device being fixable to the substructure below sea level and the device comprising: a flotation body and fixing means attached to the body for fixing the body to vertical posts of the substructure, wherein the flotation body is of elongate form.
2. A flotation device according to claim 1, wherein in use the flotation body rises to transect the water level and obtain a desired stability.
3. A flotation device according to claim 1 or 2, wherein the fixing means are pivotably arranged to the body so that in use the flotation body is rotatable with respect to the fixing means and substructure to obtain a desired stability.
4. A flotation device according to claim 3, wherein the fixing means are pivotably attached directly on the flotation body.
5. A flotation device according to claim 3, further comprising a spacer or arm between the flotation body and fixing means and wherein the fixing means are pivotably attached to the arm.
6. A flotation device according to any one of claims 1 to 5, wherein the fixing means are in the form of at least two fixing means.
7. A flotation device according to any one of the claims 1 to 6, wherein the flotation body is of substantially box-like elongate form.
8. A flotation device according to any one of claims 1 to 7, wherein the or each fixing means is formed by a clamp.
9. A flotation device according to claim 8, wherein the clamp is formed by at least two shelllike parts which are closeable around the substructure post.
10. A flotation device according to claim 9, wherein the inner walls of the shell-like parts together form a conical surface which co-acts with wedge-shaped members to be arranged around the substructure post.
I I. A flotation device according to any one of claims 3 to 10, wherein the fixing means are pivotably arranged to the body by a pivot or hinge.
12. A flotation device according to any one claims I to Il, wherein the fixing means are provided on a wall of the flotation body and the wall runs at an inclination to the vertical in order to serve as an impact or buffer surface against the substructure.
13. A flotation device according to any one of claims 1 to 11, wherein the flotation body is provided on the top part thereof with volume-enlarging means such as flaps.
14. A flotation device substantially as described herein with reference to and as illustrated in any one or more of figures 4,5,6,8,9 and 10 of the accompanying drawings.
15. A flotation device for use in lifting a sea platform or like construction from a substructure, the device comprising: a flotation body and fixing means attached to the body for fixing the body to vertical posts of the substructure, wherein the fixing means are pivotably arranged to the body so that in use the flotation body is rotatable with respect to the fixing means and substructure to obtain a desired stability.