Method for installing and connecting a sub-sea riser

Abstract

A method and a device for installing a sub-sea riser for an offshore operation, between a base installation disposed on the sea bed and a sea installation protruding above the base installation. The sub-sea riser is provided with a first end and a second end. The method comprises the following stages in the following order: a) the first end of the riser is joined to the sea installation; b) the sub-sea riser is deployed; c) the second end of the riser is guided towards the base installation; d) the second end is coupled to the base installation by releasing the second end and the efforts of the recoil force exerted on the base installation are transferred; e) the second end and the base installation are connected to each other.

Description

Other features and advantages of the invention will become apparent on reading the description given below of particular embodiments of the invention, given by way of indication but implying no limitation, with reference to the appended drawings in which:

FIG. 1 is a schematic view in vertical section illustrating a first step in implementing the installation method according to the invention;

FIG. 2 is a schematic view in vertical section showing a second step after the first;

FIG. 3 is a schematic view in vertical section showing a third step after the second;

FIG. 4 is a schematic view in vertical section showing a fourth step after the third;

FIG. 5 is a schematic detail view in perspective of one embodiment of the invention illustrating the installation method between the second and third step;

FIG. 6 is a schematic detail view of the embodiment illustrated in FIG. 5 in the installation method in accordance with the third step;

FIG. 7 is a schematic detail view of the embodiment illustrated in the previous figure in accordance with the fourth step; and

FIG. 8 is a schematic detail view of the embodiment illustrated in the previous figure using the installation method in accordance with a fifth step, following the fourth.

FIG. 1 shows, in section, the ocean or sea between the seabed 10 and the surface 12. An offshore installation 14, here consisting of an oil platform, floats on the surface 12 above a subsea or seabed installation 16, consisting of the free end of a flowline 18, which lies on the seabed 10.

In addition, a surface vessel 20 equipped with a reel on which a subsea riser 22 was initially wound and on which a traction cable 24 is partly wound, said surface vessel being located near the platform 14. The subsea riser 22 has a first end 26, which is firstly fastened to the platform, and a second end 28 that is connected to the cable 24 by connection means that will be explained in detail later in the description and that extends it as far as the surface vessel 20. The subsea riser 22 and the traction cable can be wound up onto the same reel. However, as soon as the subsea riser 22 is fully unwound and only its second end 28 is still attached to the surface vessel 20, said connection means and the traction cable 24 can be connected thereto.

Moreover, on the one hand, the second end 28 includes a first frame 30 and parallel arms joined together at one end in order to form a yoke 32, which is mounted so as to pivot on the first frame 30 and which will be described in greater detail with reference to FIGS. 5 to 8, the yoke 32 extending here along the extension of said second end 28, and, on the other hand, the free end 16 of the flowline 18 includes a second guiding frame 34 having at least one hook 36 designed to receive said yoke 32, as will also be explained in detail later in the description. The yoke 32 forms mobile first connection means and the hooks 36 form second connection means. The first and second connection means and the frames constitute fastening means.

In another step, as illustrated in FIG. 2, in which all the elements shown in FIG. 1 can be seen, the second end 28 is guided toward the free end of the seabed installation 16 by means of the cable 24 and of the surface vessel 20, which drives it along the direction of the arrow F approximately along the axis of the flowline 18, and the subsea riser 22 is extended, said riser being heavy and therefore exerting a restoring force R in the opposite direction to that in which the second end 28 is being driven.

In yet another step, following from the previous one and illustrated in FIG. 3, the subsea riser 22 is extended further again by means of the cable 24 towed by the surface vessel so that the first frame 30 and the yoke 32 are made to approach opposite the second guiding frame 34 in order to engage the yoke 32 which pivots in the hook 36. As soon as the yoke 32 is engaged, by using the restoring force R, which until then had been compensated for by the pulling force on the cable 24, and therefore by progressively slackening the cable 24, the yoke 32 catches in the hook 36 and the loads from the restoring force R are progressively transferred to the second guiding frame 34.

By slackening the cable 24 even further, the equilibrium situation illustrated in FIG. 4 is reached, in which the first frame 30 is positioned so as to bear on the seabed 10 after having pivoted about the free end 16 by means of the yoke 32, which itself has pivoted about the hook 36 and the first frame 30. The restoring force R exerted by the subsea riser is then taken up entirely by the frictional forces between the flowline 18 and the first frame 30 on the seabed floor.

Depending on the field configuration and in particular on the potential movements of the surface installation, it is generally recommended that a portion of the riser rest on the seafloor before the connection.

When installing the riser, this portion remaining on the seafloor makes it possible to reduce the force exerted by the catenary at the second end 28, which is extended by the cable 24, this force being reduced to an acceptable value R.

The way in which the subsea riser is connected to the flowline will be explained in greater detail with reference to FIGS. 7 and 8.

The device suitable for implementing the invention can now be explained in detail with reference to FIG. 5 in a step of the installation method between the step illustrated in FIG. 2 and the step shown in FIG. 3.

In this FIG. 5, there are again the flowline 18, the free end 16 of which is accompanied by the second guiding frame 34 that is connected to it and has hooks 36, and the subsea riser 22, the second end 28 of which has the first frame 30 on which the yoke 32 is pivotally mounted, said yoke consisting of two parallel arms 40, 42 joined at the end by an arch 44.

The second frame 34, which is entirely attached to the free end 16, is designed to rest on the seabed 10 and the free end 16 is extended in the form of a swan-neck and terminates in a connection end-fitting 46 lying above the mean plane defined by the second guiding frame 34.

Moreover, it includes parallel spaced-apart guiding and protection arches 48 that extend transversely across the second frame and go around the free end 16 so that they are perpendicular to the axis of the flowline 18 and also conical guiding housings 50 oriented longitudinally and designed to house guiding rods 52 that extend longitudinally at the front of the first frame 30.

The hooks 36 are mounted toward the rear 53 of the second frame, upstream of the free end 16, and are bent over toward the rear 53. Furthermore, in one advantageous embodiment, they can move translationally.

Moreover, the second end 28, which is attached to the first frame 30, is also extended in the form of a swan-neck that terminates in a second connection end-fitting 54, lying above the mean plane of the first frame 30 and between the two arms 40, 42 of the yoke 32.

The second end 28 as shown in FIG. 5 does not include the cable, which is connected to it and makes it possible both to pull the yoke 32 and hold it in place in its extension during this step, so as to simplify this figure. For similar reasons, the swan-neck ends are not shown in FIGS. 1 to 4.

FIG. 6 shows the relative positions of the free end 16 and the second end 28 of the subsea riser 22 during the step illustrated in FIG. 3.

Thus, the yoke 32 has pivoted with respect to the first frame 30 and, on the one hand, the ends of the arms joined together by the arch 44 are engaged in the hooks 36 by pressing on the second frame 34 and, on the other hand, by pivoting, the arms 40, 42, which are spaced apart by a distance approximately equal to the width of the guiding and protection arches 48, have been guided by these guiding arches 48. In this way not only is the yoke 32 capable of being fastened in the hooks 36 but the second end 28 of the subsea riser is in alignment with the free end 16 of the flowline 18.

FIG. 7 illustrates the device according to the invention in the step illustrated in FIG. 4. The first frame 30 rests here on the seabed 10, the yoke 32 is held fast in the hooks 36 which transmit, to the flowline 18, the restoring force exerted by the subsea riser 22, and the arms 40, 42 of the yoke 32 are held in place laterally by the guiding and protection arches 48. In this position, the connection end-fittings 46, 54 which terminate the swan-necks lie opposite each other and the guiding rods 52 lie respectively along the axis of and facing the conical guiding housings 50.

Thus, the restoring force from the subsea riser is transmitted to the seabed installation and the connection between the second end and the seabed installation may be made without said restoring force being exerted on the actual connection.

In this situation, the presence of a surface vessel is no longer absolutely necessary, since the remotely operated subsea robots (not shown) are designed to make the actual connection.

Thus, thanks to the hooks 36 that can move translationally and are designed to be driven toward the rear 53 of the second frame 34 by driving means (not shown), for example worms, jacks, or the like, the yoke 32 is designed to drive the first frame 30 toward the second frame 34 so that the guiding rods 52 can be inserted into the conical guiding housings 50 so as to guide the first frame 30 relative to the second frame 34. Said worms are, for example, designed to be rotated by said subsea robots. Consequently, the connection end-fittings 46, 54, which are already facing each other, are joined together to allow them to be connected when the hooks 36 are at the end of travel of the second frame 34 toward the rear 53. Said robots are also designed to carry out the various steps of the connection, such as the cleaning and sealing steps, or other operations.

FIG. 8 illustrates such a configuration in which the connection end-fittings 46, 54 are connected together and the second end 28 of the subsea riser 22 is held in a fixed position relative to the free end 16 of the flowline 18 thanks to the hooks 36 and the yoke 32.

A seabed installation consisting of a flowline has been described above, this having the advantage of being able to pick up the entire restoring force of the subsea riser. However, it would not be outside the scope of the invention to provide a seabed installation formed from a collector, which would be anchored to the seabed in order to take up the restoring force.

Claims

1. A method for installing and connecting a subsea riser between a seabed installation placed on the seabed and a second installation that lies above the seabed installation, the riser having a first end and a second end, the method comprising the following steps in order: a) connecting the first end of the riser to the second installation;b) deforming the subsea riser in a catenary between the second installation and the seabed installation;c) guiding the second end of the riser toward the seabed installation by extending the subsea riser wherein the extending exerts a restoring force on the second end;d) fastening the second end of the riser to the seabed installation, while slackening the second end to position the second end to face the seabed installation under the restoring force and to transfer the loads of the restoring force to the seabed installation; ande) connecting the second end of the riser and the seabed installation together.

2. The method for installing a subsea riser as claimed in claim 1, wherein the seabed installation is formed from a free end of a flowline.

3. The method for installing a subsea riser as claimed in claim 1, wherein the second end of the riser includes mobile first connections and the seabed installation includes second connections, and the mobile first connections are engaged in the second connections before the slackening of the second end in order to attach it the second end.

4. The method for installing a subsea riser as claimed in claim 1, further comprising guiding the second end of the riser from the water surface by a cable extending between the second end of the riser and a surface vessel.

5. The method for installing a subsea riser as claimed in claim 4, wherein the seabed installation is formed from a free end of a flowline, and the surface vessel extends the subsea riser by orienting a lower portion of the riser in a direction approximately parallel to the flowline until said second end of the riser is located approximately in line with the seabed installation.

6. The method for installing a subsea riser as claimed in claim 1, further comprising fastening the second end of the riser and the seabed installation together after the second end of the riser and the seabed installation have been connected together.

7. A device for installing and connecting a subsea riser between a seabed installation placed on the seabed and another installation that lies above the seabed installation, the subsea riser having a first end and a second end; the device comprises: connectors operable for connecting the first riser end to the other installation;a deployment device operable for deploying the subsea riser in a catenary between the other installation and the seabed installation;a guiding device operable for guiding the second end of the riser to face toward the seabed installation by extending the subsea riser, and the subsea riser having a characteristic of exerting a restoring force on the second end of the riser;fastenings operable for fastening the second end of the riser to the seabed installation, the guiding device being operable to slacken the second end of the riser to position the riser, so that the second end of the riser faces the seabed installation due to the restoring force and to transfer the loads from the restoring force to so the seabed installation; andconnectors for connecting the second end of the riser and the seabed installation together.

8. The device for installing and connecting a subsea riser as claimed in claim 7, wherein the seabed installation comprises a free end of a flowline.

9. The device for installing and connecting a subsea riser as claimed in claim 7, wherein the second end of the riser comprises a first frame and parallel arms joined together at one end of the arms to form a yoke, the arms being mounted to pivot on the frame between a first position, in which they extend longitudinally in the extension of the second end of the riser, and a second position, which is inclined to the first position.

10. The device for installing and connecting a subsea riser as claimed in claim 9, wherein the seabed installation comprises a free end of a flowline; the free end of the flowline comprises a second guiding frame having an upstream part which extends from the free end of the flowline along the flowline and includes at least one hook in which the yoke engages in the inclined second position.

11. The device for installing and connecting a subsea riser as claimed in claim 10, wherein the second guiding frame includes lateral guides for guiding the second end of the riser along the axis of the free end of the flowline.

12. The device for installing and connecting a subsea riser as claimed in claim 10, wherein the second guiding frame includes a driver for driving the hook for driving the second end of the riser toward the seabed installation.