EP0979923B1 - Installation for the exploitation of an offshore oil deposit and method for mounting a riser - Google Patents

Description

The present invention relates to an operating installation of a deposit at sea, of the type comprising a semi-submersible platform, at least one riser connecting the platform to the seabed F, and means for energizing said riser.

Semi-submersible platforms are intended for exploitation oil in deep seas or oceans (see US 4,473,323). They include a hull supported by legs whose lower part is connected to a hollow base. The legs are equipped with buoyancy chambers. The base and the caissons ensure the floatation and stability of the platform. The hull, attached to the legs, is kept above the surface of the sea during the operation of the installation.

One or more risers, generally designated by the English term "riser" connects the platform to the seabed. These columns risers consist of metal tubes.

Their length, which essentially corresponds to the depth of the operating location is commonly 1200 m and their weight is around 100 tons.

In order to prevent the risers from breaking, under the action cross currents, it is known to provide means of energizing them. These energizing means exert a force corresponding to about one to two times the weight of the column uplink.

Due to the floatation of the platform, it is subject to on the one hand, changes in the level of the water due to the tide, and on the other hand to the movements of the swell. As a result, the means to power up the risers must be able to compensate the vertical movement of the platform over time. The Maximum vertical deflection is commonly 4 to 12 m.

In current installations, the powering means risers have hydropneumatically controlled cylinders arranged between the riser head and the platform. These cylinders must have a sufficient stroke to compensate for the displacement relative between the head of the riser and the platform. Of more, these must be powerful enough to withstand traction tensioning the riser.

Thus, it is understood that the cylinders currently used are very bulky and complex technology.

The object of the invention is to propose an operating installation wherein, the power up of the or each riser does not does not require the implementation of complex and bulky means on the hull of the platform.

For this purpose, the subject of the invention is an operating installation of a deposit at sea, of the aforementioned type, characterized in that the means for powering up, for the or each riser, at least one submerged float, connected at a point of the current part of the riser for its traction towards the surface, and a mechanism of traction of the riser, which mechanism is installed on the platform and applied at the top of the riser.

• le ou chaque flotteur est dimensionné pour appliquer sur la colonne montante, une force de traction supérieure à la force de traction appliquée par le mécanisme de traction de tête ;
• le flotteur est dimensionné pour appliquer sur la colonne montante une force de traction comprise entre 1 et 3 fois le poids de la colonne montante ;
• la plate-forme comporte une embase immergée et une coque émergée reliée par des jambes, le ou chaque flotteur étant disposé à la profondeur de ladite embase, laquelle embase comporte des moyens de guidage vertical du ou de chaque flotteur ;
• l'embase comporte, pour chaque flotteur, un passage vertical au travers duquel le flotteur est mobile axialement ;
• elle comporte des moyens de mise en butée du flotteur contre la plate-forme, suivant le sens ascendant ;
• le ou chaque flotteur présente un conduit traversant dans lequel s'étend la colonne montante associée ;
• les moyens assurant la liaison entre le ou chaque flotteur et la colonne montante associée comportent une rotule;
• la rotule comporte un siège annulaire concave solidarisé audit flotteur dans le conduit axial et une collerette à surface convexe portée par la colonne montante, la collerette étant appliquée contre le siège concave pour l'application de la traction sur la colonne montante ;
• le conduit traversant a un diamètre supérieur à trois fois le diamètre de la colonne montante ; et
• le mécanisme de traction de tête comporte au moins un vérin hydropneumatique comportant à chaque extrémité un ensemble de poulies mouflées sur lesquelles est engagé au moins un brin de traction appliqué à ladite colonne montante.

According to particular embodiments, the installation comprises one or more of the following features:

• the or each float is sized to apply on the riser, a tensile force greater than the pulling force applied by the head traction mechanism;
• the float is sized to apply on the riser a tensile force of between 1 and 3 times the weight of the riser;
• the platform comprises a submerged base and an emergent hull connected by legs, the or each float being disposed at the depth of said base, which base comprises means for vertical guidance of the or each float;
• the base comprises, for each float, a vertical passage through which the float is axially movable;
• it comprises means for abutting the float against the platform, in the ascending direction;
• the or each float has a through duct in which extends the associated riser;
• the means ensuring the connection between the or each float and the associated riser comprises a ball joint;
• the ball comprises a concave annular seat secured to said float in the axial duct and a convex surface flange carried by the riser, the flange being applied against the concave seat for the application of traction on the riser;
• the through conduit has a diameter greater than three times the diameter of the riser; and
• the head traction mechanism comprises at least one hydropneumatic jack comprising at each end a set of pulleys on which is engaged at least one traction strand applied to said riser.

a - assurer la mise en butée verticale du flotteur contre la plate-forme;

b - immerger la colonne montante avec son extrémité inférieure maintenue à distance du fond marin ;

c- ballaster la plate-forme ;

d- descendre la colonne montante et relier celle-ci au fond marin F;

e - dégager la butée entre le flotteur et la plate-forme ; et

f- déballaster la plate-forme.

The invention also relates to methods for installing a riser of an installation of the aforementioned type, characterized in that it comprises the successive steps of:

a - ensure the vertical abutment of the float against the platform;

b - immerse the riser with its lower end kept at a distance from the seabed;

c- ballaster the platform;

descend the riser and connect it to the seabed F;

e - clear the stop between the float and the platform; and

f- deballaster the platform.

a - assurer la mise en butée du flotteur contre la plate-forme ;

b - immerger la colonne montante avec son extrémité inférieure maintenue à distance du fond marin ;

c - enfoncer vers le bas le flotteur par mise en place d'un lest sur le flotteur ;

d - descendre la colonne montante et relier celle-ci au fond marin ,

e - dégager la butée entre le flotteur et la plate-forme ; et

f - retirer le lest pesant sur le flotteur.

According to a particular mode of implementation, the method comprises the steps of:

a - ensure the abutment of the float against the platform;

b - immerse the riser with its lower end kept at a distance from the seabed;

c - push down the float by placing a ballast on the float;

d - lower the riser and connect it to the seabed,

e - clear the stop between the float and the platform; and

f - remove the ballast weighing on the float.

• la figure 1 est une vue en élévation d'une installation d'exploitation pétrolière selon l'invention;
• les figures 2 et 3 sont des vues en coupe respectivement longitudinales et transversales d'un flotteur de traction de la colonne montante de l'installation de la figure 1;
• la figure 4 est une vue en perspective de moyens de traction de tête d'une colonne montante;
• les figures 5A, 5B, 5C, 5D et 5E sont des vues schématiques montrant l'installation pétrolière de la figure 1 à des stades successifs d'implantation d'une colonne montante; et
• les figures 6A, 6B, 6C, 6D sont des vues analogues aux figures 5A à 5E, illustrant un second procédé de mise en place d'une colonne montante.

The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the drawings in which:

• Figure 1 is an elevational view of an oil exploitation installation according to the invention;
• Figures 2 and 3 are sectional views respectively longitudinal and transverse of a traction float of the riser of the installation of Figure 1;
• Figure 4 is a perspective view of traction means of a riser head;
• FIGS. 5A, 5B, 5C, 5D and 5E are schematic views showing the oil installation of FIG. 1 at successive stages of implantation of a riser; and
• FIGS. 6A, 6B, 6C, 6D are views similar to FIGS. 5A to 5E, illustrating a second method of placing an riser.

In Figure 1, is schematically represented a platform semi-submersible oil tanker 10. She is willing in a region of great depth, this being for example equal to 1300 m.

The platform essentially comprises an upper hull 12 extending above the surface M of the sea, when the platform is in operation phase. The shell 12 is connected by four legs 14 provided of buoyancy chambers 15, to a lower submerged base 16. The upper shell comprises non-residential buildings represented as well as a derrick 18. The hull 12 and the base 16 are all two of square shape and have in their center through ducts 20, 22 for the passage of a riser 24. The column 24 is connected at its lower end to an operating well.

In Figure 1, only one riser 24 is shown. In practical, several risers are arranged between the platform 10 and the seabed. Vertical ducts similar to ducts 20 and 22 are provided for each riser.

The total weight of each riser 24 is, for example, 100 tons. Its diameter is 10 inches or about 25 cm.

Mooring lines 26, held under tension, are installed between the immersed base 16 and the seabed to immobilize the platform above the deposit.

Each riser 24 is associated with setting means under pressure. According to the invention, these power-up means comprise, for each riser, at least one submerged float 28, connected to a point of the running part of the riser for its traction to the surface, and a pull mechanism of the column rising, which mechanism is installed on the platform 10 and is applied at the top of the riser 24.

The submerged float 28 is disposed at the depth of the base 16. It is thus mounted movable vertically in the passage 22.

In Figures 2 and 3 is shown in section on a larger scale the float 28 crossing the passage 22.

As shown in these figures, the float 28 has the shape of a sleeve. The float height is for example 13 m and its diameter outside is for example 4.5 meters. A passage 32 is defined along the axis of the float. The riser 24 is engaged through of it.

The diameter of the passage 32 is for example 1.7 m. It is advantageous greater than three times the current diameter of the column rising 24.

The float 28 consists of a toroidal box 34 delimited by metal walls. The interior of the box is filled with a synthetic foam 36 low density. The box 34 is separated into three compartments distinct by radial partitions 38 extending over the entire height of the float. These partitions originate along the wall delimiting the passage 32 and protrude radially out of the casing 34.

Means 40 for guiding the float in the vertical direction are provided between the float 28 and the base 16 of the platform. These guide means 40 comprise, for example, sliding pads 42 carried by the end of the radial partitions 38 projecting out of the caisson. These sliding pads are free to slide in slides guide 44 arranged longitudinally along the passage 22. The guide rails 44 are for example defined by angles U-shaped extending along the entire thickness of the base 16 is about 10 m.

The pads 42 are continuous and extend over a length equal to that of the guide rails 44. As a variant, these consist of distinct elements distributed over the height of the radial partitions 38.

According to another embodiment variant, not shown, the positions slides and pads are reversed. The skates, which are then carried by the base, are integral with a guide sleeve reported and fixed in the through conduit 22. When the pads are worn, the guide sleeve is removed and replaced by a shirt with new skates.

Moreover, means 46 for axially connecting the float 28 and the riser 24 are provided in the passage 32. These means of link are formed by a ball-and-socket arrangement, allowing free movement angular of the riser 24 with respect to the float 28.

This ball-and-socket arrangement advantageously comprises an annular seat concave 48 integral with the float 28 and a collar with a convex surface 50 carried by the riser 24.

The annular seat 48 is advantageously arranged in half bottom of the passage 32. It defines a frustoconical concave surface 52 facing up. This is intended to form a bowl surface intended to support the collar 50. The seat 48 is traversed by a conduit 54 adapted for the passage of the riser 24. The conduit 54 for example a diameter of 1 m.

The flange 50 presents, facing the bearing surface 52 a convex surface 56, formed for example by a spherical crown.

The largest diameter of the flange 50 is smaller than the diameter from passage 32.

In its zone of connection to the flange 50, the riser 24 has an extra thickness, to strengthen its structure.

Since the flange 50, the thickness of the riser is progressively decreasing following two sections marked 57, 58 oriented respectively up and down.

These sections have for example each a length of 3 m. They constitute sections of variable inertia ensuring a regular distribution constraints along their entire length.

Moreover, at the periphery of the passage 22 are provided on the face upper base 16 three latches 60 constituting retractable stops adapted to selectively retain the float 28 and avoid the ascent of it.

The latches 60, each comprise for example a hydraulic actuator controllable from the hull 12 or from an intervention vehicle remote-controlled underwater. They allow the setting up of a penne 64 at the upper end of the slides 44.

The pins 64 are movable between a retracted position in which they allow a free sliding of the skids 42 in the slideways 44 and an active stop position as shown in FIGS. Figures 2 and 3, in which they prohibit the upward movement of skates 42.

The float is sized to apply on the riser a pulling force of between 1 and 3 times the weight of the column. For a riser 24 with a weight of 100 tonnes, the force of traction exerted by the float is for example between 1000 kN and 2000 kN. Advantageously, this traction force is substantially equal at 1500 kN. In these circumstances, the force applied by the mechanism of head tension 30 is substantially equal to 500 kN.

In general, the float 28 is sized to apply on the riser a tensile force greater than the force of traction applied by the head traction mechanism 30.

Advantageously, the traction force of the float is between 1 and 10 times the tensile force applied by the traction mechanism of head.

In practice, the float applies on the riser a tensile force substantially equal to 3 times the applied traction force by the head traction mechanism 30.

The sizing of the float is carried out so that the capacity the head traction mechanism is not more than 500 kN.

The head traction mechanism 30, shown in FIG. 4 comprises two hydropneumatic cylinders 70 mounted in parallel.

At each end of the cylinders are mounted four pulleys noted 72 and 74. A cable 76 for energizing the column rising 24 is engaged around the pulleys. The cable 76 is passed on a deflection pulley 78 and directed towards the head of the riser at which it is fixed.

The cylinders 70 are supplied with a hydraulic fluid by a set of hydraulic pressure regulations noted 80. The variation of the hydraulic pressure in the cylinders 70 allows the control of their travel.

The engagement of the cable 76 between the pulleys 72 and 74 ensures a reduction in the travel of the cylinders, so that for ensure at the level of the head of the riser 24 a deflection axial 15.2 m, the stroke of the cylinders is only 3.8 m.

The head traction mechanisms 30 are integrated into the thickness of the shell 12 as shown in Figure 1. Thus, they do not encumber not the upper deck of the hull 12.

Alternatively, the head traction means 30 are deported on the side walls of the hull, the cables 76 then flowing from the rail up to the column head through the hull 12.

It is conceivable that with such an installation, the riser 24 is biased upwards both by the float 28 and by the traction means head 30.

Thus, because of the traction exerted by the float 28, the capacity of traction means 30 can be reduced. So, it is not necessary to use bulky cylinders with a corresponding long stroke the maximum displacement encountered between the riser head and the platform.

In addition, the diameter of the duct 32 in which the column passes rising 24 being much greater than the diameter thereof, and the connection between the float and the riser being provided by a ball joint, the column rising is free to move angularly relative to the float, this which reduces the stresses applied on the riser 24.

FIGS. 5A to 5E show a first mode of implementation place of a riser 24.

As shown in FIG. 5A, the riser 24 is first submerged with its lower end kept at a distance from bottom F. The float 28 is held in abutment against the pinnae 64, prohibiting thus the rise of the float. In this position, the flange 50 is substantially at the depth of the seat 48. The bottom of the float 28 outcrops substantially bottom of the base 16.

In the next step of the method, the platform 10 is ballasted, for example by partial filling of the base 16. Thus, the platform 10 sinks to a depth l as shown in Figure 5B. The depth l is for example equal to 1.5m. Thanks to the 18 derrick, the column rising 24 is raised upwards during the descent of the platform, so that the lower end of the riser stays apart of the seabed F of an interval J and is for example one meter from background. In this position, the flange 50 is disposed above the seat 48 and is spaced therefrom by an interval K substantially equal to 1.5 m.

After this step, and as shown in FIG. 5C, the column rising 24 went down to the bottom and is connected to a well previously drilled and barbed. During this descent, the immersion depth of the platform is kept constant.

In this position, the flange 50 is moved away from the seat 48 of a K 'interval substantially equal to 0.5m. The section of the riser between its lower end and the float is relaxed.

The next phase of the process is to first link the mechanism head traction 30 to the riser 24, then gradually déballaster the platform, until the flange 50 comes to bear on the seat 48, as shown in Figure 5D. The platform 10 is thus raised by the interval K '. During deballasting, the derrick 18 is gradually released to allow relative movement between the riser and the platform.

During the subsequent deballasting of the platform, the float emerges stops 60 since it is retained by the riser 24. Thus, as shown in FIG. 5E, the platform continues its back to its operating position while the float 28 remains at a constant depth. This second phase of ascent corresponds at an interval l-K 'of height equal to about 1 m.

In this position, the float 28 exerts a restoring force towards the surface of the lower part of the riser.

After detachment of the float 28 stops 60, the latter are retracted to allow maximum vertical movement of the float relative to the base 16.

Likewise, the head traction means 30 are actuated so to ensure the traction of the upper section of the riser 24 included between the derrick 18 and the float 28.

We understand that because of the height of the float, it is likely to perform large amplitude deflections with respect to the base 16 of the platform, while being properly guided by the lateral guide means 40.

Another method of setting up a riser of a installation according to the invention is illustrated in FIGS. 6A to 6D.

For the implementation of this method, the hull 12 of the platform is equipped with winches 90 to suspend an annular ballast 92 above the float 28. The ring ballast 92 is formed of two half-rings assembled around the riser 24. The length of the winch is sufficient to deposit the ballast 92 on the upper annular surface 28. In addition, the weight of the ballast 92 is adapted to ensure the depression of the float 28 towards the bottom.

As in the previous embodiment, the riser 24 is submerged with its lower end held at a distance of the bottom F. During this installation of the riser, the float 28 is in abutment against pennes 64.

The ballast 92 is then conveyed to the float. So, the float 28 is made to sink as shown in Figure 6B ,.

After a sufficient depression of the float 28, the riser has come down and its lower end is connected to an operating well of oil as shown in Figure 6C. Due to the depression of the float 28, the flange 50 of the riser is away from the seat 48. In these circumstances, the riser 24 is relaxed, allowing its connection to the operating well.

After connecting the lower end of the riser, the ballast 92 is reassembled, as shown in Figure 6D. The limit stop by the latch 60 having been disengaged, the float 28 tends to rise to the surface, so that it exerts on the riser 24 a force of upward traction applied to the flange 50.

According to this method of implantation of a riser implement a ballast, it is not necessary to ballast the platform or the float, thus avoiding the transfer of seawater.

Claims (12)

1. An installation for the exploitation of an offshore field, of the type comprising a semi-submersible platform (10), at least one riser (24) connecting the platform (10) to the seabed F, and means (28, 30) for tensioning said riser (24), characterised in that the tensioning means comprise, for the or each riser (24), at least one submerged float (28), connected at one point of the run of the riser (24) for pulling it towards the surface, and a mechanism (30) for pulling the riser (24), the mechanism (30) being installed on the platform (10) and applied to the top of the riser (24), and in that it comprises means (60) for bringing the float (28) into abutment with the platform (10), in the upward direction.
2. The installation according to Claim 1, characterised in that the or each float (28) is dimensioned to apply to the riser (24) a pulling force that exceeds the pulling force applied by the top pulling mechanism (30).
3. The installation according to Claim 1 or 2, characterised in that the float (28) is dimensioned to apply to the riser (24) a pulling force of between 1 and 3 times the weight of the riser (24).
4. The installation according to any one of the preceding claims, characterised in that the platform (10) comprises a submerged base (16) and a hull located above water (12) connected by means of legs (14), the or each float (28) being arranged at the depth of said base (16), the base comprising means (40) for vertically guiding the or each float (28).
5. The installation according to Claim 4, characterised in that the base (16) comprises, for each float (28), a vertical passage (22) through which the float (28) can move axially.
6. The installation according to any one of the preceding claims, characterised in that the or each float (28) has a through-conduit (32) through which the associated riser extends (24).
7. The installation according to any one of the preceding claims, characterised in that the means for ensuring the connection of the or each floater (28) and the associated riser (24) comprises a ball-joint (46).
8. The installation according to Claims 6 and 7 taken in conjunction with one another, characterised in that the ball-joint comprises a concave annular seat (48) made integral with said float (28) in the axial conduit (32) and a flange (50) with a convex surface (56) borne by the riser (24), the flange (50) being applied against the concave seat (48) to apply a pulling force to the riser (24).
9. The installation according to Claim 6, characterised in that the through-conduit (32) has a diameter greater than three times the diameter of the riser (24).
10. The installation according to any one of the preceding claims, characterised in that the top pulling mechanism (30) comprises at least one hydropneumatic jack (70) comprising at each end a block-and-tackle pulley assembly (72, 74) on which there is engaged at least one pulling line (76) that is connected to said riser (24).
11. A method for installing a riser (24) of an installation according to Claim 1, characterised in that it comprises the successive steps of:

    a bringing the float (28) into vertical abutment with the platform (10);

    b submerging the riser (24) with its lower end held some distance from the seabed F;

    c ballasting the platform (10);

    d lowering the riser (24) and connecting it to the seabed F;

    e releasing the float (28) from abutment with the platform (10); and

    f unballasting the platform (10).
12. A method for installing a riser (24) of an installation according to Claim 11, characterised in that it comprises the steps of:

    a bringing the float (28) into abutment with the platform (10);

    b submerging the riser (24) with its lower end held some distance from the seabed F;

    c sinking the float (28) towards the seabed by placing a ballast (92) on the float (28);

    d lowering the riser (24) and connecting it to the seabed (F);

    e releasing the float (28) from abutment with the platform (10); and

    f removing the ballast (92) weighing down the float (28).

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