GB2233373A

GB2233373A - Method and means for positioning structures on the sea bed

Info

Publication number: GB2233373A
Application number: GB9014426A
Authority: GB
Inventors: Helge Roraas
Original assignee: Norwegian Contractors AS
Current assignee: Norwegian Contractors AS
Priority date: 1989-06-28
Filing date: 1990-06-28
Publication date: 1991-01-09
Anticipated expiration: 2010-06-28
Also published as: GB9014426D0; GB2233373B

Abstract

In a method for accurate positioning of structures (1), particularly large structures, on the sea bed a) a locking dowel (3) is anchored on the sea bed on the mounting site, b) the structure (1) is sunk from a floating position down to a preselected height or level above the sea bed to enable suitable equipment on the structure to come into pivotable contact with the locking dowell, c) the structure (1) is pivoted about the locking dowell, until the structure attains correct orientation in azimuth, d) the pivoting movements are controlled by means of suitable guiding and braking means (7) provided on the under side of the structure, which guiding and braking means contact and in part penetrates the sea bed while the structure is positioned a short distance above the sea bed, and thereby provides the desired friction and control of the pivoting movements, and e) the structure is sunk down to rest on the sea bed in the correct position. The invention also includes means for carrying out the method which comprises a retractible and extensible pile. <IMAGE>

Description

METHOD AND MEANS FOR POSITIONING STRUCTURES ON THE SEA BED The present invention relates to method and means for positioning structures or constructions on the sea bottom, more particularly solutions for exact and safe positioning of large and heavy structures on the sea bottom.

The object of the invention is particularly met in connection with off-shore activities, in which part of the operations involved include positioning, for instance of a bottom structure such as a caisson for a fundament for a platform for drilling or exploration of hydrocarbones, whereby the caisson, which is large and heavy, shall be positioned above or adjacent an already positioned structure on the sea bottom, for instance a well template. The caisson, which shall be positioned, may very easily damage the already positioned structure, and the positioning must therefore be carried out with uttermost care.

It is further. as a rule, a requirement that the caisson shall be positioned relative to an already mounted structure, with great accuracy and with rather small tolerances.

In a known method for such operations, the caisson, subsequent to being towed out to the mounting site, is ballasted carefully, so that it sinks down to the installation position on the sea bed with controlled velocity. The vertical position of the caisson is constantly kept subject to control by means of towing vessels and an anchor line system, and the entire operation is surveyed by measuring equipment which continuously provides information of the position of the caisson and its orientation in azimuth.

This known method is, however, not sufficiently accurate during the last and most critical part of the positioning, since resonances and uncontrolled moments, including rocking movements, due to environmental variations and/or the use of winches are introduced as disturbing elements.

Large structures have large dynamic energy. A positioning system for this purpose will therefore assume large dimensions if the energy in the system can not be withdrawn or dissipated in a different manner. In those cases where the caisson shall be positioned closely adjacent to or in some cases on top of an existing installation on the sea bed, the risk of damage to such equipent will be impending.

It is known to use fixed columns, dowell pins, piles and the like which constitute part of or are mounted on the under side of the caisson in order to provide a braking and guiding of the structure during the final positioning prior to seating the caisson on the sea floor. Such means, such as dowell pins, may penetrate into the sea bed and brake horisontal movements of the caisson. By ballasting/deballasting the caisson, the penetration of such dowell pins into the sea bed can be controlled. The method requires a set of short and long dowell pins, piles or the like in combination and used in a preselected pattern or procedure for positioning.

The present invention relates to a new method for accurate positioning and seating of structure on the sea bed, and the method in accordance with the invention is characterized by the following steps: a) a locking dowell is anchored to the sea bed on the mounting site, b) the structure is sunk from floating position down to a predetermined level above the sea bed,where suitable equipment on the structure enters into pivotable engagement with the locking dowell, c) by use of suitable means the structure is pivoted about the locking dowell until the structure accurately attains correct position in azimuth, d) the pivoting movement is controlled by means of suitable monitoringrand braking means located on the under side of the structure, which braking means contact and partly penetrate the sea bed while the structure is positioned at a short distance above the sea bottom and thereby provides the desirable friction and control of said pivotal movements, e) the structure is sunk to the sea bed in correct position.

The method is suitably carried out by using two or more guiding piles, and that these - one after the other - or all simultaneously, initially are being sunk to contact with the sea bed, and thereafter are gradually withdrawn as required as the structure settles down on the sea bed.

The invention further relates to special means, in order to carry out the method, and these means are in general characterized in that they comprise a locking dowell which initially is being anchored to the sea bed on the mounting site, and a hoistable and lowerable locking socket which is mounted on the under side of the structure and is adapted for cooperation with the locking dowell during the positioning procedure, and finally braking means which also are mounted on the under side of the structure and adapted for contact with the sea bed in order to effect a braking and guiding of the lateral movement of the structure, which can pivot around the locking dowella and further means in order to pivot the structure on and about the locking dowell as a rotational axis, for instance by means of towing vessels which are connected to the structure by cables or the like.

The advantage with extendible and retractable guiding piles is that the clearance during the last part of the positioning can be increased, and further, that the guiding dowells may be dimensioned as to length and cross section to conform to the actual mass and dimensions of the structure, and including sufficient safety factors.

The procedure is controlled by adjusting the active part of the equipment, and is not limited to the principle of ballasting and deballasting the element in order to obtain the necessary vertical guiding, a procedure which is the only available when fixed dowells are being used.

The penetration into the sea bed can be attained by means of the weight of the dowells alone, or in combination with vibrating weights, vacuum or high pressure jets at the tips of the dowells.

Through locking of the dowells in fully extended position, the dowell can also be forced down into the sea bed using the weight of the structure.

It shall be understood that the present invention is not limited to caissons, but can also be used in all other connections in which large, heavy structural elements shall be positioned on the sea bed with great accuracy, and particularly in cases where there is a risk of damage on constructional elements which are located on or adjacent the location site. As example of other fields of use for which the present invention is applicable, can be mentioned protectional shields to bemounted around an oil production platform as protection against influence of dangerous waves, or ice formations.

The invention shall in the following be further illustrated by means of a couple of embodiments shown in the attached drawings, wherein: Fig. 1 shows a caisson 1 to be sunk down to the sea bed 2.

A locking dowell 3 is already anchored on the sea bed 2 and serves as a reference point for the caisson. The locking dowell 3 cooperates with a locking sleeve 4 which is hoistably and lowerably connected to the caisson by means of bushing shaped guides 5, 6. On the caisson is furthermore provided guiding piles 7 which also are furnished with hoistable and lowerable brackets 8, 8. The guiding dowells 7 may be sunk into the sea bed for use in connection with the positioning procedure. The degree of penetration into the sea bed is adjusted at any time in accordance with the need for braking force, Only one guiding dowell is shown in the drawing, but it shall be understood that two or more guiding piles my be used in order to secure the required guiding.In fig. 1 the locking sleeve 4 is in position above the locking dowell 3, and the guiding piles 7 are sunk down into the sea bed in order to control the movements of the caisson.

In fig. 2 is shown the next stage wherein locking sleeve 4 has been sunk into engagement with the upper part of the locking dowell 3. The guiding pile 7 is now being hoisted up to inoperable position.

If the orientation i azimuth of the caisson must be adjusted, this is carried out by the previously mentioned towing vessels and anchor line system, whereby the caisson can be pivoted about the locking dowell 3 until correct orientation in azimuth has been obtained. Also under this part of the operation the guiding piles 7 may with advantage be used for control of the movements of the caisson.

Fig. 3 and 4 show, respectively, a lateral view and a plan view of the entire caisson shown in fragmentary views in fugures 1 and 2 where the same references designate same parts as shown in fig. 1 and 2.

In the embodiments shown in fig. 3 and 4, two guiding dowells 7, 7 positioned about 1800 apart, are used.

Claims

1. Method for accurate positioning of structures, particularly large structures on the sea bed, c h a r a c t e r i z e d by the following steps: a) a locking dowell (3) is being anchored on the sea bed on the mounting site, b) the structure (1) is being sunk from a floating position down to a preselected level above the sea bed, so that suitable equipment on the structure come into pivotable engagement with the locking dowell, c) the structure (1) is being pivoted about the locking dowell (3), until the structure attains correct orientation in azimuth, d) the pivoting movement is co?.trlled by means of suitable guiding and braking means provided on the under side of the structure, which guiding and braking means contact and in part penetrate the sea bottom while the structure is positioned at a short distance above the sea bed, and thereby provide the desired friction and control of the pivoting movements, e) the structure is being sunk down on the sea bed in the correct position.

2. Method in accordance with claim 1, c h a r a c t e r i z e d in that two or more guiding piles (7, 7) are being used, and that these, one by one or all simultaneously, initially are being sunk into contact with the sea bed, and thereafter are gradually retracted as the structure is being set down on the sea bed.

3. Method in accordance with claim 1 or 2, c h a r a c t e r i z e d in that the guiding pile (7) is being pressed down into the sea bed by its own weight and if necessary in combination with other means, such as vibrators, high pressure jets or the like,

4. Means in order to carry out the method as defined in any of the preceeding claims, c h a r a c t e r i z e d in that the means comprise: a) a locking dowell (3) which is anchored to the sea bed on the mounting site, b) a hoistable and lowerable locking sleeve (4) mounted on the structure (1) and adapted for cooperation with the locking dowell (3), c) means on the under side of the structure adapted for contact with and penetration into the sea bed while the structure still is at a short distance or height above the sea bed and in rotational engagement with the locking dowell, d) means for pivoting the structure with the locking dowell (3) as a pivot until the structure attains accurate orientation in azimuth.

5. Means in accordance with claim 3, c h a r a c t e r i z e d in that the means for braking and guiding the structure during the pivoting movement consist of at least one hoistable and lowerable guiding pile (7) mounted on the structure (1), which guiding pile can be sunk down to penetrate the sea bed in order to provide the required guiding and braking of the structure.