EP0065695B1 - Leg of an offshore platform and method of placing it - Google Patents

Description

1. The invention applies to any platform intended to be placed at sea on a fixed structure pre-installed and which is therefore subject, at the time of its installation, to the action of the swell.

The platform, transported from its place of construction on a transport barge, comprises legs which, at the end of the operation, must rest on determined locations on the structure (generally piles which protrude from the surface of the water ) and be secured to this structure, most generally by welding.

This operation is easily carried out in the river or in a site protected from swell by simple ballasting of the transport barge.

At sea, one of the main difficulties lies, at the time of contact between the platform and the substructure, in the fact that the transport barge, and consequently the platform which rests on it, being subjected to swell , are driven by an alternating horizontal movement of period equal to that of the swell.

To make the connection between platform and structure, it is necessary that the piles of the structure and the legs of the platform are in coincidence.

Because the swell is not regular, but has waves trains interspersed with periods of relative calm, it is imperative to have a system to ensure the connection in the horizontal plane between platform and structure in a time less than the duration of the periods of calm, that is to say, in the majority of cases, less than one minute.

A classic solution is to suspend the platform at the end of a crane carried by a barge and drop it suddenly, at the time of the coincidence between the piles of the structure and the legs of the platform. Crane operators are skilled enough to succeed in this maneuver.

This type of solution leads to the implementation of very expensive naval means as soon as the platforms reach a weight of the order of a thousand tonnes.

An object of the invention is therefore to define a device making it possible to achieve a beautiful manner. more economical blow an almost instantaneous connection between a platform equipped with legs and the heads of the piles of a fixed structure intended to receive the platform.

The subject of the invention is a leg of a marine platform engaging on a pile belonging to a fixed structure and the head of which has a receiving cone, said platform having legs equal in number to the piles, each leg being intended to be fixed to a stack, said leg being hollow and enclosing an insert which can slide in the leg and intended to secure the leg and the corresponding stack by its partial introduction into the receiving cone, characterized in that the insert is associated to a means which makes it possible to secure it to the leg and to detach it suddenly, causing the insert to fall by gravity.

In a particular embodiment the means comprises at least one finger passing through the leg and cooperating with a bore of the insert.

• a) on accroche la barge de la structure de manière à mettre les jambes au-dessus des têtes de piles,
• b) on désolidarise brusquement au moins deux inserts de leur jambe respective par action simultanée des moyens de solidarisation correspondants,
• c) on ballaste la barge jusqu'à permettre son éloignement de la structure,
• d) on effectue la liaison définitive entre chaque jambe et pile correspondante.

The subject of the invention is also a method of implementing a device for the establishment of an underwater platform carried by a barge on a fixed structure by means of a device according to claim 1, characterized in that it comprises the following phases:

• a) the barge is hung from the structure so as to put the legs above the pile heads,
• b) at least two inserts of their respective legs are suddenly separated by simultaneous action of the corresponding securing means,
• c) the barge is ballasted until it is removed from the structure,
• d) the final connection is made between each leg and corresponding pile.

Advantageously, between phases c and d, a wedge crossing the leg above the insert is inserted into each of the legs concerned by holes facing two by two.

• - les fig. 1A et 1 E représentent schématiquement le dispositif de l'invention et illustrent un procédé pour sa mise en oeuvre,
• - les fig. 2A à 2E illustrent une variante du dispositif du procédé pour sa mise en oeuvre.

The invention will be better understood from the description given below of two preferred embodiments of the invention, with reference to the appended drawing in which:

• - figs. 1A and 1E schematically represent the device of the invention and illustrate a method for its implementation,
• - figs. 2A to 2E illustrate a variant of the device of the method for its implementation.

Fig. 1 A schematically represents a platform 1 carried by a transport barge 2.

On the bottom of the sea 3 is anchored a structure intended to receive the platform. This structure includes piles whose heads can emerge from the water; one of these battery heads, referenced 4, is shown in the figures. It has a receiving cone 5, female.

Each stack 4 corresponds to a leg 6 secured to the platform 1. A platform may for example include four legs which will be secured to 4 stacks.

At least two of the legs consist of a hollow cylinder closed at the upper end by a stop 7 generally called a lifter. It is legs of this type which are shown in the figures.

• - Il peut coulisser dans la jambe dans laquelle il est guidé par des moyens 9 tels que des bossages (et/ou des plaques telles que 9A).
• - Il est maintenu à l'intérieur de la jambe, en butée contre le palonnier, au moyen d'un dispositif mécanique, qui peut par exemple être un doigt 10, traversant la jambe par un orifice approprié, et s'engageant dans un alésage correspondant de l'insert. Le doigt est escamotable au moyen d'un vérin 11. En variante, il peut y avoir plusieurs doigts déplaçables simultanément.
• - Il est terminé par un cône 12 destiné à faciliter son centrage dans le cône récepteur 5 de la tête de pile 4.
• - Il dépasse de la jambe à son extrémité inférieure.
• - Il est constitué de deux tubes 8A et 8B coaxiaux, fixés chacun par l'une de leur extrémité à un dispositif amortisseur 14, de préférence constitué d'un bloc en matériau élastomère.

Inside the tube is an insert 8, the characteristics of which are as follows:

• - It can slide in the leg in which it is guided by means 9 such as bosses (and / or plates such as 9A).
• - It is held inside the leg, in abutment against the lifter, by means of a mechanical device, which can for example be a finger 10, passing through the leg through an appropriate orifice, and engaging in a bore correspondent of the insert. The finger is retractable by means of a jack 11. As a variant, there can be several fingers that can be moved simultaneously.
• - It is terminated by a cone 12 intended to facilitate its centering in the receiving cone 5 of the battery head 4.
• - It protrudes from the leg at its lower end.
• - It consists of two coaxial tubes 8A and 8B, each fixed by one of their ends to a damping device 14, preferably consisting of a block of elastomeric material.

• Durant le transport, l'insert 8 est rentré à l'intérieur de la jambe et est maintenu en butée contre le palonnier 7.

The operation of the device is as follows:

• During transport, the insert 8 is inserted inside the leg and is held in abutment against the lifter 7.

The transport barge is positioned using anchor lines, the platform and its legs passing freely above the pile heads.

When the barge is introduced, the distance (or clear) between the lower end of the insert and the pile head depends on the tide, the amplitude of the barge's vertical movements and certain factors of security. This value is designated by AGo.

The transport barge / platform assembly being positioned using the anchor lines oscillates, due to the swell, around an average position of perfect coincidence between stacks and legs.

The transport barge is first ballasted until the clear AGo is reduced to a minimum value compatible with the vertical movements of the assembly and in such a way that there is no contact between pile head and leg of the platform. This is the position shown in fig. 1 A.

During a swell lull, the vertical coincidence being assured for a time of the order of a minute, inserts are released simultaneously and fall into the cones of reception of the battery heads, instantly making the horizontal connection between the plate -form and structure. We operate this release by simultaneous operation of the corresponding cylinders 11. We can be content to drop two inserts, which is sufficient to ensure the horizontal immobilization of the platform. But of course you can drop more than two inserts simultaneously and even all of them. The number of inserts to be dropped simultaneously depends on their capacity and that of the piers of the structure to withstand the horizontal forces generated by the wave trains. Their number is at least two to avoid any rotation of the platform which could occur after connection on a single insert and which would prevent the others from going down into the receiving cones.

Once the inserts have been released (fig. 1 B), the barge / platform assembly remains free to move vertically with the swell, along the insert. As a safety measure, the depth of the reception cone will advantageously be taken at least equal to the value of the maximum possible heaving to prevent any accidental dislodging of an insert after connection by friction of the insert inside the leg, at the moment of the passage of wave trains.

The receiving cone can advantageously be mounted on a damper 14 made of rubber, elastomer or any other suitable elastic material, to absorb the shock created by the fall of the insert and to ensure that the latter does not damage the battery head of the structure, if the centering of the platform is not perfect when the inserts are released.

The removal of the platform on the structure is then carried out by ballasting the barge until the lifter of the leg of the platform comes into abutment on the head of the insert, the relative position leg / insert being again the same as before dropping the insert (fig. 1 C).

During this ballasting, the vertical movements of the barge and the platform cause shocks between the lifter and the insert, which are absorbed by the damper integrated into the insert.

Continuing ballasting of the barge then makes it possible to release the platform from the latter, the platform resting at the end of the operation on the pile head of the structure, by means of the inserts. Fig. 1 D shows the crushing of the shock absorber at the end of ballasting.

The length of the insert and the location of the spreader inside the leg are studied so that the distance, at the end of the operation, between the lower end of the platform leg and the head of the structure's stack is sufficient to allow two metal half-ferrules (16A and 16B) to be welded, which ultimately provide continuity of the structure installed between the stack and the leg of the platform.

The shock absorber can then be destroyed, for example by chemical action (fig. 1 E).

• - la hauteur D correspondant à l'allégement du poids de la plate-forme,
• - le clair initial AGo,
• - la valeur H du demi-pilonnement,
• - la profondeur h du cône de réception (h=2H).

The ballast height, that is to say the height which the barge must be able to sink to allow the above operation, is at least equal to the sum of four terms:

• - the height D corresponding to the reduction in weight of the platform,
• - the initial clear AGo,
• - the H value of the half-heave,
• - the depth h of the reception cone (h = 2H).

For a barge with a dimension of 120 m x 30 m, supporting a platform of 6000 tonnes, the relief height D is close to 2 m. If the initial clear is 0.6 m and the heaving 0.9 m, the ballasting height is close to 4 m; ballasting then requires a very large quantity of water, of the order of 13,000 tonnes.

A variant of the system makes it possible to reduce this ballasting height; this variant will preferably apply to very large platforms.

The variant is based on the following consideration:

• AGo+h±H, H étant toujours le pilonnement de la barge.

Immediately after the release of the insert 8, the distance between the lifter 7 of the leg 6 and the top of the insert became:

• AGo + h ± H, H being always the heaving of the barge.

If we then interpose, between the lifter and the top of the insert, a block of total thickness: (AGo + h-H), the ballasting height is reduced accordingly.

• (0,60+0,90-0,45)=1,05 m, soit plus de 25% sur la hauteur de ballastage.

In the numerical example considered above, the gain is:

• (0.60 + 0.90-0.45) = 1.05 m, i.e. more than 25% of the ballasting height.

Figs. 2A to 2E represent the variant of the device of the invention using this wedge.

The leg is provided with orifices 20A, 20B, 21 A facing two by two. The lowest pair is located at a maximum distance from the spreader such that the distance between the spreader and the lower part of the wedge is equal to d '= AGo + h-H, where these symbols have the same meanings as before.

As shown in fig. 2C which represents the device at the start of the ballasting operation, a shim 19 is introduced by a pair of orifices (here the orifices 21 A, 21 B lower), so as to serve as a stop for the insert at the end of ballasting (fig. 2D).

Depending on the level of the tide, the state of the sea at the time of the operation, in particular the value of the heaving, we choose the pair of orifices which is best suited.

Fig. 2E shows the final position of the various elements after welding of two half-ferrules 16A and 16B and destruction of the shock absorber.

The wedge is mounted on a slide (not shown) which crosses the leg of the platform so as not to impede the sliding of the insert - passing, for example, on either side of the insert to the inside of the leg - and is extended to the outside of the leg where the wedge is in the waiting position before connection.

The introduction of the wedge inside the leg can be carried out manually or using a mechanical device such as for example a jack (not shown) whose required capacity is limited to the value of the friction due to the dead weight of the hold.

An additional advantage of this variant lies in the fact that, after dropping the inserts, the resumption of the horizontal forces due to the swell induces smaller moments in the inserts, the distance between the bottom of the reception cone and the guide 9 mounted at the lower end of the leg being reduced, relative to the device of FIGS. 1A to 1 E, of a length equal to that of the protrusion of the insert under the leg; this length can reach 1.5 m.

Claims (10)

1. A leg of an offshore platform (1) engaging on a pillar (4) which belongs to a fixed structure and the head of which presents a receiving cone (5), said platform presenting as many legs as there are pillars, each leg being destined to be fixed to a pillar, said leg (6) being hollow and enclosing an insert (8) which can slide inside the leg and which is destined, by its partial introduction into the receiving cone, to fix the leg to the corresponding pillar, characterized in that the insert (8) is associated to a means (10, 11) which is able to fix the insert to the leg and to release it abruptly, thus provoking the fall of the insert by gravity.

2. A leg according to claim 1, characterized in that the means comprises at least one finger (10) passing through the leg (6) and cooperating with an opening of the insert.

3. A leg according to claim 2, characterized in that the finger (10) is associated to a jack (11).

4. A leg according to one of claims 1 to 3, characterized in that it comprises at least one pair of openings (21 A, 21 B) for the introduction of a wedge (19) for limiting the course of the insert.

5. A leg according to one of claims 1 to 4, characterized in that the insert (8) is composed of two tubular coaxial parts (8A, 8B) which are each fixed by one end to a common shock-absorbing block (14).

6. A leg according to claim 5, characterized in that the shock-absorbing block is of the destructible type.

7. A method for depositing, on a fixed structure, a submarine platform carried by a barge and comprising at least two legs according to claim 1, characterized in that it comprises the following steps:

(a) approaching the barge (2) to the structure so as to place the legs (6) above the heads of the pillars (4),

(b) abruptly disengaging at least two inserts (8) from their respective leg by simultaneous action on the corresponding fixing means (10, 11 ),

(c) ballasting the barge (2) until it can be withdrawn from the structure,

(d) carrying out the final connection between each leg and the corresponding pillar.

8. A method according to claim 7, characterized in that between the steps c and d, a wedge (19) is inserted into each one of the concerned legs which passes through the leg above the insert by openings (21 A, 21 B) which are facing each other two by two.

9. A method according to one of claims 7 or 8, characterized in that after the step e, the shock-absorbing block is destructed.

10. A method according to one of claims 7 to 9, characterized in that the step e is a welding phase with possible adjunction of two metal half-rings for each couple composed of leg and pillar.

Images