AU749721B2

AU749721B2 - Device related to risers

Info

Publication number: AU749721B2
Application number: AU58851/99A
Authority: AU
Inventors: Knut Harry Fjell
Original assignee: Den Norske Stats Oljeselskap AS
Current assignee: Equinor Energy AS
Priority date: 1998-06-12
Filing date: 1999-05-31
Publication date: 2002-07-04
Anticipated expiration: 2019-05-31
Also published as: CA2333730A1; CN1312881A; GB0100777D0; AU5885199A; NO306826B2; GB2356001B; GB2356001A; CN1119498C; WO1999066169A3; NO982742D0; NO982742L; NO306826B1; WO1999066169A2

Description

P:\OPER\ArW8851-99 pe.doc-07/05/02 -1- A RISER This invention relates to risers, especially a riser for transferring fluids between a first construction by the sea bottom and second construction by the sea surface.

Many different types of risers are known for transferring fluids from the sea bottom to the surface. Usual configurations are for example "Lazy S" and "Steep S", where the riser extends toward the sea bottom via an intermediate area. In the intermediate part the riser is connected to one or more buoyancy elements lifting the corresponding part of the riser and making a curve with its concavity directed downward.

From the intermediate area the riser in a "Lazy S" configuration stretches in a gentle curve toward the sea bottom, while a riser in a "Steep S" configuration stretches in a straighter line toward an anchoring on the sea bottom, which also comprises a coupling for a pipe stretching between the anchoring and the construction on the sea floor.

•A disadvantage related to the "Lazy S" configuration is that it requires relatively i **much more space, so that there is a danger of the risers coming close to each other and be damaged. This makes "Lazy S" risers difficult to use in larger installations comprising a plurality of risers, and at large depths.

The anchored "Steep S" solution provides better control over the riser, both regarding vertical and horizontal movements, but the coupling in the anchoring against the sea floor is relatively complex and it demands specialized equipment for performing the coupling. Also, it is basically unstable as the riser will float up to the surface if the anchoring breaks.

A proposed alternative to these two types of risers is shown in US 4,906,137. In this case the anchoring is fastened to the lower part of the riser to obtain a stretch between the buoyancy elements coupled to the intermediate part and the sea floor, so as .ooooi S"to provide the riser with an equally well defined and compact progression as in the -"Steep S" configuration. The anchoring is connected to the riser in such a way as to P:\OPER\Ari\8851-99 spe.doc-07i05/02 -2provide the riser with a smooth, curved transition from a vertical to a horizontal path, with a predetermined curvature. The riser may extend further along toward the bottom construction, or be coupled to a pipe on the sea floor in a conventional manner.

Because of the anchoring this proposed riser is also vulnerable if the bottom conditions or the anchoring fails. Also, the coupling point between the anchoring on the riser will be subject to strains in the form of stretching and bending, and thus possibly damage, because of large movements introduced in the top of the riser by the vessel or platform.

The disadvantage with the riser described in the US patent is its low flexibility to large movements in the riser. Also it requires a relatively complex operation for placing of the anchoring connected to the riser at the sea floor.

Accordingly, the invention provides a riser for transferring fluids between a construction at the sea floor and a second construction at or close to the surface, comprising: an upper substantially vertically extending part connected at its upper end to the second construction; an an intermediate part of the riser connected to one or more buoyancy elements along at least a part of its length, and a lower part having a lower end connected to the construction at the sea floor, wherein one or more weight elements are mounted on the lower part of the riser.

Preferred embodiments of the present invention thus relate to a simple solution for obtaining an approximate "Steep S" configuration of a riser without requiring complicated operations for connecting of risers to the sea bottom, as the weight elements being fastened to the riser may be mounted over the sea surface in an ***installation vessel during the installation of the riser.

Connecting the weight elements to a possible preinstalled anchoring will also be easier than in the known art or than that described by the abovementioned US patent, as the anchoring line may be mounted in a loose condition, which is simplifies the connecting operation.

This way a solution is obtained having the same flexibility as the "Lazy S" configuration and at the same time being almost as compact as the "Steep S" S configuration.

The present invention will be described, by way of example only, with reference to the accompanying drawings, as set out below.

Figure 1 a shows a complete riser configuration according to a preferred embodiment of the invention.

Figure lb shows another configuration according to another embodiment of the invention.

Figures 2a and 2b show a detail of figure Ib as seen from above.

Figure 3 show a detail of the lower part of the riser with weight elements.

Figure 4 show a cross section of two weight elements mounted on the riser.

In figure la a floating platform 1 is shown. To this platform a riser 2 is connected being connected to a coupling on the installation deck. The platform per se is not important for the invention, and the upper part of the riser may of course be coupled to other constructions, e.g.

ships, especially a submerged turret loading or production vessel.

In the drawing the upper part 2a of the riser hangs essentially vertically. An intermediate part 2b comprises a number of buoyancy elements 8 providing the riser with a curved portion over the part on which the buoyancy elements are positioned.

25 The riser also comprises a lower part 2c below the buoyancy elements. The lower part 2c extends-at least down to the sea floor and preferably a distance along the sea floor. A number of weight elements 3 areplaced on the lower part so as to balance the buoyancy of the buoyancy 30 elements, and at the same time contribute in limiting the movements of the riser in the vertical direction.

Preferably at least one of the weight elements lie on the sea floor and thus contributes in limiting horizontal movements perpendicular to the riser, so that the danger of 35 collisions between risers is reduced. This also contributes in stabilizing the vertical position of the riser in the water as a lifting of the riser will lift the weight element from the sea floor and thus raise the weight of the part of ,he riser floating in the water, thus pulling the riser down 0 gain.

4 In a special embodiment of the invention shown in figure Ib one or more of the weight elements 3b, being lifted from the sea floor, are connected through a first chain or similar to a buoyant body 12 being in its turn anchored at the sea floor through a second chain or similar 13 to an anchor 14. This gives the upper part of the weight elements an extra anchoring which works against movements in the riser in the horisontal direction and at the same time reduces possible changes in the radius of the curvature of the lower part 2c of the riser. When the top of the riser is pulled, for example because of drift of the platform 1, in the same direction as the curvature of the lower part 2c, the radius of curvature usually is reduced.

This anchoring will usually dampen this reduction in the radius of curvature as the chain lying on the sea floor is pulled up from the bottom, and the chain being fastened to .the weight element thus contributes with a gradually increasing horizontal force against the movements of the riser. This will contribute together with the rigitity of the weight elements in avoiding a decrease in the radius of curvature of the lower part of the riser under the critical limit.

The extra anchoring 11,12,13,14 is specially advantageous when the construction at the surface is positioned 25 directly above the bottom construction, as shown in figure Ib, in which e.g. a drill pipe 15 extends from the platform 1 to a well head 7. A current from the in or out of the paper plane in the drawing will, without the extra anchoring, bend the riser in a direction into or out of the paper plane. This bending may damage the riser, both by being bent beyond the critical limit of the riser and by resulting in collisions with other risers being located in the area. The abovementioned anchoring system 11,12,13,14 positioned on the convex side of the lower part of the riser 2c may reduce this bending to an acceptable-level.

To keep the balanced configuration of the buoyancy and weight elements 3,8 the buoyancy of the buoyant body 12 is adapted so that the first chain 11 only contributes with a horizontal force to the weight elements 3. Thus the Sbuoyancy element 12 with corresponding chains etc. 11,13,14

B

may be positioned on the sea floor independently and be connected to this without affecting the equilibrium situation, while at the same time improving the properties outside the equilibrium situation, suchas in movement of the surface construction. The buoyant body may be anchored in a chosen position and the first chain may thereafter simply be connected to one of the weight elements using a ROV (Remote operated vehicle). Also, the riser will keep essentially the same vertical position if the anchoring fails.

The buoyant body provides, in the same way as corresponding, anchoring systems for surface vessels, a softer anchoring than a direct anchoring to the sea floor, thus allowing more drift in the surface installations before the riser is damaged and at the same time limitting the weight being lifted by the ROV when the first chain 11 is coupled to the weight elements 3.

Figures 2a and 2b show the anchoring part of figure lb as seen from above, where figure 2a shows the use of one buoyant body 12 connected to the riser 2, while figure 2b shows correspondingly the use of two buoyant bodies with attached chains 11,13 connected to the riser 2. The solution in figure 2 provides an improved stability for movements in horisontal directions.

25 A detail shown in figure 3, illustrating the lower part of the riser provided with weight elements 3, of which one 3b comprises a fastening device for connecting of the anchoring 11,12,13,14. In addition to the anchored weight element 3b the drawing shows a gradually decreasing size of the weight elements 3a providing a gradual transition between the riser and the weight elements. The weight elements may be positioned similarly on the lowest weight elements lying on the sea floor.

Figure 3 shows how the weights are placed on the riser so as to maintain the flexibility, as they are positioned along the riser at a certain distance between them, with spacer parts 10 between the weight elements 3.

Preferably only one of the weight elements, preferably the lowest is fastened to the riser, while the others only are fastened to each other, so as to allow for a relatively o o* WO 99/66169 PCT/N0O99/001707 6 free movement in the longitudinal direction inside the weight elements. Thus the riser is not subject to any extra tension when the curvature changes.

Toavoid large curvatures locally on the riser caused by bending, the weight elements, and the intermediate spacer parts, are provided with a smooth stiffness. The stiffness decreases toward the ends to provide a gradual transition to the stiffness of the riser.

To avoid curvatures over a certain limit the weight elements 3 may be formed with a radial extension larger than the distance between the weights 3. The relationship between the radius of the weight elements and the distance between them is chosen to make the weight elements touch each other at a certain curvature, and thus hinder further bending of the riser 2.

Alternatively spacer parts 10 may be provided between the weight elements which may adjust the bending stiffness so that the operational stiffness does not come below the critical bending radius of the riser 2. If the riser with the weight elements is bent sufficiently to make the weight elements touch each other the spacer parts may take up the resulting tension on the outside of the curve.

In addition to the abovementioned advantages in the use of weight elements according to a preferred embodiment of the invention they will also provide the riser with a certain protection against wear when it is subject to movements relative to the sea floor.

Figure 4 shows a cross section of an example of how the weight elements 3 may be made. The weight elements 3 may be shaped as rings, e.g. of lead, being fastened to the riser 2 at certain distances from each other. Thus the flexibility of the riser is maintained.

In an alternative embodiment the weight element may be a liquid with a high specific weight, e.g. the drilling fluid. If the weight elements are filledwith liquid they may also be made relatively flexible, and thus provide a S"dampened increase in the bending stiffness when bending the riser, as the weight elements are gradually pressed toward Seach other. The liquid may be filled in after the weight 0 lements have been mounted, thus simplifying the handling of Preferably the weight elements 3 are half circle shaped parts being screwed together outside the riser. The rings 3 shown in figure 4 are assembled by an inner ring 4 which may be of a flexible polymer of rubber-like material. The inner ring 4 is glued or vulcanized to an outer ring 5 made from a heavy weight material, preferably lead or comprising a heavy liquid. On the outside a collar shaped ring 6 may be provided giving outward protection.

The stiffness in the weight elements and in the coupling between them may be provided in different ways, e~g. by using different rubber mixtures in the production of the weight elements and possible spacer parts, possibly in combination with the use of heavy liquids in the weight elements giving them a certain resilience.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the c gg common general knowledge in Australia.

a.o..

Claims

1. A riser for transferring fluids between a construction at the sea floor and a second construction at or close to the surface, comprising: an upper substantially vertically extending part connected at its upper end to the second construction; an intermediate part of the riser connected to one or more buoyancy elements along at least a part of its length, and a lower part having a lower end connected to the construction at the sea floor, wherein one or more weight elements are mounted on the lower part of the riser.

2. A riser according to claim 1, wherein each weight element comprises one part made from a heavy material.

3. A riser according to claim 2, wherein the heavy material is lead. 000:

4. A riser according to claim 1, wherein each weight element comprises a liquid Sgo with a high specific weight.

5. A riser according to one of the preceding claims, wherein each weight element comprises at least one annular ring fastened around the riser. S.

6. A riser according to claim 5, wherein each weight element comprises two semi- circular ring parts adapted to be screwed together around the circumference of the riser.

7. A riser according to claim 5 or 6, wherein each weight element comprises an inner ring fastened to an outer ring.

8. A riser according to claim 7, wherein the inner ring is made from a flexible material. P:\OPER\AI5885 1.99 sp.do-07/05/02 -9-

9. A riser according to claim 7 or 8, wherein the outer ring is made from heavy material.

A riser according to claim 9, wherein the heavy material is lead.

11. A riser according to any one of claims 7 to 10, wherein each weight element comprises a collar shaped outer ring which substantially covers the other parts of the weight element.

12. A riser according to any of the preceding claims, wherein the lower part of the riser comprises two or more weight elements with a mutual axial distance between them.

13. A riser according to claim 12, wherein two or more weight elements are separated by spacer parts. ooooo

14. A riser according to any one of the preceding claims, wherein the weight elements are distributed along the lower part of the riser, and the lower part of the riser has a curved portion from a substantially vertical portion of the lower part to a substantially horizontally portion of the lower part.

A riser according to claim 14, wherein at least one of the weight elements lie on the sea floor.

16. A riser according to one of the claims 12 to 15, wherein the lower part of the riser has a least radius of curvature defined by the spacing between the weight elements and the radial extension of the weight elements, the radial extension being larger than the mutual distance between the weight elements. the mutual distance between the weight elements. P:NOPER\Ar1\5885.99 spo.doc.14/05/02

17. A riser, substantially as described with reference to the drawings and/or examples. DATED this 14th day of May, 2002 Den norske stats oljeselskap a.s. By DAVIES COLLISON CAVE Patent Attorneys for the Applicant a S *SS~ I. S S.. *.SS