GB2177146A

GB2177146A - Riser system

Info

Publication number: GB2177146A
Application number: GB08615653A
Authority: GB
Inventors: Christian Dumazy; Francois Baduel; Jean-Paul Sarrailh
Original assignee: Societe National Elf Aquitaine
Current assignee: Societe National Elf Aquitaine
Priority date: 1985-06-28
Filing date: 1986-06-26
Publication date: 1987-01-14
Also published as: OA08352A; NO862597L; GB8615653D0; FR2584133A1; NO862597D0; FR2584133B1

Abstract

An accessory device for heads of upright columns (risers) 3, 3' attached to an articulated structure emplaced in deep water is comprised of a working platform 4 which is translationally movable parallelly to the axis of the articulated structure, which platform is rigidly attached to one of the risers 3 of a group of risers, and which platform has orifices 9 in it for the other risers 3' of the said group to pass through said platform. This device facilitates access to the heads of the risers, enables simplification of the connections to the evacuation manifold 11, and, if the risers are not self-supporting, enables improvement of the tensioning system for said risers. <IMAGE>

Description

SPECIFICATION Working platform for an upright column (riser) attached to an articulated structure The invention relates to an improvement applied to flexible risers attached to a pliant structure emplaced in deep water for carrying out an undersea activity, particularly for petroleum exploitation. The types of pliant structures referred to include, e.g., an articulated column, an oil derrick (with or without stays), and a platform with a taut anchorage.

A riser which is translationally movable in a well guide rigidly attached to the pliant marine structure may be additionally furnished with centering means, particularly for its lower region, which eliminate the risk of buckling.

Thus it (the riser) may be considered self-supporting.

On the other hand, in all cases where the lower region of the riser is freely deformable in following the swinging movements [etc.] of the [pliant] structure, it is necessary to provide means of applying tension at the head (said means to be referred to hereinafter as "tensioning means").

In all cases where the swinging structure 1 pivots around its pivot point 2, one observes relative displacements A, ' between a plane aa' fixed on the said swinging structure and a point on the riser 3 attached to the swinging structure 1, which point is associated with said plane (Fig. 1).

The displacement is a function of: The length L of the flexing region of the riser; The inclination Q of the swinging structure; The position I of the articulation; and The distance of offset d of the riser with respect to the axis of the swinging structure.

The relative displacements (A, d') pose problems for the accessibility of the heads of the risers, in view of the [level of] maintenance required with such installations. There are problems posed as well for connection to the system of pipes connected to the structure, particularly at the platform at the top of the structure (collection manifold).

These disadvantages are eliminated or relieved according to the invention, wherein the risers are divided into groups of neighboring risers, and it is proposed according to the invention to provide a working platform for each such group, wherewith the displacements relative to said platform are small. In this way, the problem of accessibility and that of conduit connections are solved simultaneously.

A device according to the invention comprises, for a group of neighboring risers, a working platform rigidly attached to one of the risers, which platform is provided with means of rendering it movable translationally, parallelly to the axis of the pliant marine structure, and which platform has orifices for the other risers of the given group to pass through.

In general, for a pliant marine structure having a cross-member provided with a collection manifold (which manifold is called the "principal manifold") the working platform associated with a group of mutually neighboring risers is provided with a collection manifold (called the "satellite manifold"), wherewith each of the risers of the given group is connected to the satellite manifold by a short flexible connector, and each satellite manifold is connected to the principal manifold by a long flexible connector.

In a number of embodiments, the means for rendering the working platform movable translationally, parallelly to the axis of the pliant marine structure, are comprised of guide devices on rails which are parallel to the axis of the articulated structure and which [rails] are rigidly attached to the articulated structure.

In other embodiments, the rails parallel to the axis of the articulated structure and rigidly attached to the articulated structure link the upper cross-members of the structure to a lower cross-member.

In a number of embodiments, the risers are self-supporting, in which case it is generally assumed that the means employed for this self-support do not include means of tensioning the riser which is rigidly attached to the working platform, which means would take into account the weight of the said platform and the materials attached thereto.

If the risers are not self-supporting, the riser rigidly attached to the working platform is provided with tensioning means which take into account the weight of said riser, said platform, and the accessories of said platform, and each of the other risers in the given group is provided with differential tensioning means applied to the respective (said other) riser and to the working platform.

In the case of direct active tensioning, the tensioning means applied to the riser rigidly attached to the working platform comprise a variable hydraulic cylinder.

In the case of direct passive tensioning, the tensioning means applied to the riser rigidly attached to the working platform comprise a weight of predetermined magnitude, acting via a cable and two idler pulleys.

In the case of indirect tensioning, the tensioning means applied to the riser rigidly attached to the working platform may comprise motorized guides operating along rails which run parallel to the axis of the structure. (Alternatively,) said means may also comprise motorized rails running parallel to the axis of the structure, which rails act on the guides.

The invention will be better understood with the aid of the following description of various embodiments with reference to the drawings.

This description is not limitative.

Fig. 1 illustrates the mechanical problem posed; Fig. 2 illustrates the elements of the inven tive solution to said problem; Fig. 3 shows a device for self-supporting risers; Fig. 4 shows a case of direct active tensioning; Fig. 5 shows a case of direct passive tensioning; Fig. 6 shows a case of indirect tensioning; and Fig. 7 shows a second case of indirect tensioning.

It is seen from Fig. 1 that the total displacement of the top end of the riser between two extreme positions A and B is equal to (Q + Al.

Fig. 2 shows that for two neighboring risers (3, 3') having a separation a between the respective points where they are mounted to the base, which separation a is small compared to the offset distance d, the relative displacement between the top ends of the two risers is ( + b'), which is very small compared to (A + ').

Instead of using a reference plane aa' rigidly joined to the articulated structure, we will take a plane rigidly joined to one of the risers (which riser will be referred to as the "reference riser", and the said plane will be referred to as the "reference plane") of the group of neighboring risers. Then the displacement of the top end of a riser in said group, which riser is not the reference riser, will be (d + d') -- which provides a solution to the problem posed.

Fig. 3 shows a device for a self-supporting riser, i.e. a riser not having tension applied at its head.

In Fig. 3 there are seen the principal riser 3 and a riser 3' which is one of a group of neighboring risers. A working platform 4 is affixed to riser 3 perpendicularly thereto. Platform 4 is furnished with guide means 5 along rails 6, which rails are rigidly attached to a lower cross-member 7 and an upper crossmember 8 of the articulated structure (or swinging column) 1.

Riser 3' passes through the working platform 4 at an opening 9 furnished with guide means 10.

The upper cross-member 8 supports a manifold 11, to be referred to as the "principal manifold". The working platform supports a manifold 12, to be referred to as the "satellite manifold".

The top ends of the risers (3, 3') are connected to the satellite manifold 12 by short flexible connectors 13.

The satellite manifold 12 is connected to the principal manifold 11 by a long flexible connector 14.

In this embodiment, the risers (3, 3') are self-supporting, and the riser 3 is dimensioned and equipped to support not only its own weight but also the additional weight represented by the working platform 4, the satellite manifold 12, the short flexible connectors 13, and various accessories. If the riser 3 has not been set up to accept all this additional load, said riser must then be equipped with tensioning means (limited to the effects of these added loads), [since] such means (known in the art) were not planned-for previously [for the self-supporting risers illustrated].

Such a modification by adding auxiliary equipment is not limited to an installation in which a plurality of self-supporting risers is assembled. It may be applied for adding suitable hydraulic, pneumatic, or mechanical tensioning systems to an installation in which a plurality of risers [inherently] requiring tensioning is assembled.

Figs. 4-7 show applications wherein tensioning means (14) are applied to the top end of the riser 3 or directly to the working plafform 4 to which the riser is rigidly attached. In this case a differential system of tensioning 15 connects the working platform 4 to risers other than 3.

In Fig. 4, the direct active tensioning means are comprised of a hydraulic cylinder 14.

Fig. 5 shows direct passive tensioning means, in the form of a counterweight 14 connected to the riser 3 by means of idler pulleys 16 and 17.

The indirect tensioning means shown in Fig.

6 are comprised of motorized guides 5 rigidly attached to the working plafform 4, which guides glide [(travel)] along the guide rails or guide columns 6 which are rigidly attached to the lower cross-member 7 and at the same time to the upper cross-member 8. The motorized guides are powered by a unit 18 which may disposed on the lower plate 7.

Unit 18 is comprised of [i.e, includes] a control device 19 for controlling the relative displacement of the plate 7 and the riser 3.

In Fig. 7 tensioning means are comprised (right side of Figure 7) of a fixed helix device (worm) or gear rack, 6, which cooperates with the motorized guides 5 which are rigidly attached to the working platform 4 and are powered by unit 18; or alternatively (left side of Figure 7) tensioning means are comprised of a motorized worm 6 which cooperates with a guide 5 rigidly attached to the working platform 4, which guide 5 comprises a nut or the like. The worm 6 is moved by a mechanism 20 powered by unit 18 and regulated by the displacement control device 19.

Functioning of the Inventive Device A riser attached to an articulated structure is inherently subjected to continual displacements. The equipment at the head of such risers, such equipment comprising various systems of valves and flexible connectors of the collection manifold rigidly connected to the articulated structure, constitutes a complex assembly which is cumbersome and heavy. Furthermore, the equipment at the head of the riser is subjected (as is the articulated structure) to dynamic stresses of exterior origin.

This equipment can be forced into movements which are undesired because they are incompatible with normal operation and efficient maintenance.

The devices according to the invention offer the following operational advantages: Efficient guiding of the riser by the intermediary of one or more guide columns or guide rails connected to the articulated structure and parallel to the movement of the said risers relative to their guide tubes.

The possibility of locating servicing and maintenance of a group of neighboring risers at the location of a common working platform, at which location the connections of said risers to a satellite manifold are also disposed, and achieving a substantially reduced scale of displacement relative to said platform, in comparison to the displacements relative to the reference structure comprised of the upper and lower cross-members.

The possibility of guiding a plurality of neighboring risers with a single guiding device, wherewith tensioning (if tensioning is necessary) may be provided by an exterior active or passive system.

The possibility of improving the system of flexible connectors by separately providing short flexible connectors between the riser heads and the satellite manifold, and a long flexible connector between the satellite manifold and the principal manifold.

The applicability of the inventive device is not limited to improving the accessibility of heads of petroleum risers attached to pliant undersea structures. Similar devices may be employed in any system with relative displacement, whether terrestrial or marine; e.g., cables, hydraulic lines, fiber optics, etc. Such applications are still within the scope of the invention described supra.

Claims

1. An accessory device for heads of upright columns (risers) which risers are attached to a pliant marine structure; characterized in that, for a group of neighboring risers, said device is comprised of a working platform rigidly attached to one of said risers, [said device being] furnished with means for rendering said platform translationally mobile in movement parallel to the axis of the articulated structure, and [said platform] having orifices for the other risers of the given group to pass through [said platform].

2. A device according to claim 1; wherein, with the articulated structure having a crossmember equipped with a collection manifold (which manifold is called the "principal manifold"), the working platform corresponding to a group of risers is equipped with a [(another)] collection manifold (which manifold is called a "satellite manifold"); and wherein each of the risers in a given group is connected to the satellite manifold by a short flexible connector and each satellite manifold is connected to the principal manifold by a long flexible connector.

3. A device according to claim 1; wherein the means for rendering said platform translationally mobile in movement parallel to the axis of the articulated structure comprise guiding-devices on rails running parallel to the axis of the articulated structure, which rails are rigidly attached to said articulated structure.

4. A device according to claim 3; wherein the rails running parallel to the axis of the articulated structure and being rigidly attached to said articulated structure connect the upper cross-members of the structure to a lower cross-member.

5. A device according to claim 1; wherein the risers are self-supporting.

6. A device according to claim 1; wherein the riser rigidly attached to the working platform is provided with tensioning means taking into account [i.e., "which can accommodate"] the weight of said riser and that of the working platform and of the accessories of said platform, [and] each of the other risers in the given group is provided with differential tensioning means applied to the respective (other) riser and to the working platform.

7. A device according to claim 6; wherein the tensioning means applied to the column which is rigidly attached to the working platform are comprised of a worm or the like having variable action.

8. A device according to claim 6; wherein the tensioning means applied to the column which is rigidly attached to the working platform are comprised of a counterweight of predetermined weight which acts by the intermediary of a cable and two idler pulleys.

9. A device according to claim 6; wherein the tensioning means applied to the column which is rigidly attached to the working platform are comprised of motorization means for the guides along the rails running parallel to the axis of the [articulated] structure.

10. A device according to claim 6; wherein the tensioning means applied to the column which is rigidly attached to the working platform are comprised of motorization means for the rails running parallel to the axis of the [articulated] structure, said motorization-means being with respect to the guides.