AU5885199A

AU5885199A - Device related to risers

Info

Publication number: AU5885199A
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: 2000-01-05
Anticipated expiration: 2019-05-31
Also published as: CA2333730A1; CN1312881A; GB0100777D0; NO306826B2; AU749721B2; GB2356001B; GB2356001A; CN1119498C; WO1999066169A3; NO982742D0; NO982742L; NO306826B1; WO1999066169A2

Description

WO 99/66169 PCT/N099/00170 DEVICE RELATED TO RISERS This invention relates to a device related to risers, especially for transferring fluids between a first construction by the sea bottom and second construction by 5 the sea surface, comprising: an upper, essentially vertically extending part connected in its upper end to the construction by the surface, an intermediate part of the riser coupled to one or more buoyancy elements along at least a part of its length, and a lower part the lower end 10 of which being connected to the construction by the sea bottom. 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", 15 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. 20 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 25 between the anchoring and the construction on the sea floor. A disadvantage related to the "Lazy S" configuration is that it requires relatively much space, so that there is a danger for the risers to come close to each others and be damaged. This makes it difficult to use in larger 30 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 35 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 solution being between these two alternatives is WO 99/66169 PCT/N099/00170 2 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 to provide the riser with a 5 equally well defined and compact progress as the "Steep S" solution. The anchoring is connected to the riser in such a way as to provide 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 10 bottom construction, or be coupled to a pipe on the sea floor in a conventional manner. Because of the anchoring this solution 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 15 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 solution described in the US patent is its low flexibility to large movements in the 20 riser. Also it requires a relatively complex operation for placing of the anchoring connected to the riser at the sea floor. The present invention thus relates to a simple solution for obtaining an approximate "Steep S" configuration of a 25 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. 30 Connecting the weight elements to a possibly preinstalled anchoring will also be easier than in the known art as represented by the abovementioned US patent, as the anhoring line may be mounted in a loose condition, which is simplifies the connecting operation. 35 This is obtained using a device related to a riser as described above, and being characterized as given in claim 1. This way a solution is obtained having the same flexibility as the "Lazy S" configuration and at the same 40 time being almost as compact as the "Steep S" configuration.

WO 99/66169 PCT/NO99/00170 3 The present invention will be described with more detail below with reference to the accompanying drawings, showing possible embodiments of the invention. Figure la shows the complete riser configuration 5 Figure lb shows another configuration according to the invention. Figures 2a and 2b show a detail of figure lb as seen from above. Figure 3 show a detail of the lower part of the riser with 10 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 15 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. 20 In the drawing the upper part 2a of the riser hangs essentially vertically. An intermediate part 2b comprises in a known way a number of buoyancy elements 8 providing the riser with a curved progress 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 are placed 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 the riser floating in the water, thus pulling the riser down 40 again.

WO 99/66169 PCT/NO99/00170 4 In a special embodiment of the invention shown in figure lb 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 5 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 the possible changes in the radius of the curvature 10 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 15 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 20 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 advantag eous when the construction at the surface is positioned 25 directly above the bottom construction, as shown in figure 1b, 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 30 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 35 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 40 buoyancy element 12 with corresponding chains etc. 11,13,14 WO 99/66169 PCT/N099/00170 5 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, such as in movement of 5 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 10 fails. The buoyant body provides, in the same way as corresponding, well known anchoring systems for surface vessels, a softer anchoring than a direct anchoring to the sea floor, thus allowing more drift in the surface 15 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 20 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 30 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 35 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 40 are fastened to each other, so as to allow for a relatively WO 99/66169 PCT/NO99/00170 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. To avoid large curvatures locally on the riser caused 5 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 10 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 15 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 20 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 the invention they will also 25 provide the riser with a certain protection against wear when it is subject to movements relative to the sea floor. Figure 4 show 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 30 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 filled with liquid they 35 may also be made relatively flexible, and thus provide a dampened increase in the bending stiffness when bending the riser, as the weight elements are gradually pressed toward each other. The liquid may be filled in after the weight elements have been mounted, thus simplifying the handling of 40 them.

WO 99/66169 PCT/N099/00170 7 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 5 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 10 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.

Claims

1. Device related to risers (2), especially for transferring fluids between a construction (7) by the sea floor and a second construction at or close to the surface, comprising: an upper essentially vertically extending part (2a) connected in its upper end to the construction at the sea surface, an intermediate part (2b) of the riser (2) connected to one or more buoyancy elements along at least a part of its length, a lower part (2c) having a lower end connected to the construction (1) at the sea floor, c h a r a c t e r i z e d in that one or more weight elements (3) are provided being mounted on the lower part (2c) of the riser (2).

2. Device according to claim 1, c h a r a c t e r i z e d in that each weight element (3) comprises one part made from a heavy material, e.g. lead.

3. Device according to claim 1, c h a r a c t e r i z e d in that each weight elements (3) comprises a liquid with a high specific weight of any known type.

4. Device according to any one of the preceding claims, c h a r a c t e r i z e d in that the weight element(s) (3) comprise annular rings fastened around the riser (2).

5. Device according to claim 4, c h a r a c t e r i z e d in that each weight element (3) comprises two semi-circular ring parts adapted to be screwed together around the circumference of the riser. WO 99/66169 PCT/N099/00170 9

6. Device according to claim 4 or 5, c h a r a c t e r i z e d in that each weight element (3) comprises an inner ring (4), preferably made from a flexible material, fastened to an outer ring (5) made from a heavy material, e.g. lead.

7. Device according to claim 6, c h a r a c t e r i z e d in that each weight element (3) comprises a collar shaped outer ring (6) which essentially covers the other parts of the weight element (4,5).

8. Device according to any of the preceding claims, c h a r a c t e r i z e d in that the lower part (2c) of the riser (2) comprises two or more weight elements (3) with a mutual axial distance between them.

9. Device according to claim 8, c h a r a c t e r i z e d in that it comprises two or more weight elements (3) separated by spacer parts (10).

10. Device according to any one of the preceding claims, c h a r a c t e r i z e d in that the weight elements are distributed along the lower part (2c) of the riser (2), and that the lower part of the riser thus have a curved progress from the essentially vertical part to an essentially horizontally part.

12. Device according to claim 10, c h a r a c t e r i z e d in that at least one of the weight elements (3) lie on the sea floor.

13. Device according to one of the claims 8-12, c h a r a c t e r i z e d in that the lower part (2c) 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. WO 99/66169 PCT/NO99/00170 10

14. Device according to any of the preceding claims, c h a r a c t e r i z e d in that it comprises a number of interconnected weight elements having a continuous bending stiffness in the longitudinal direction of the riser.

15. Device according to claim 14, c h a r a c t e r i z e d in that the bending stiffness decreases toward the ends of the interconnected weight elements.

16. Device according to any one of the preceding claims, c h a r a c t e r i z e d in that at least one of the weight elements are connected to an anchoring system comprising at least one first chain or similar (11), connected to a buoyant body (12) which through a second chain or similar (14) is anchored at the sea floor, the anchoring system being positioned at the convex side of the lower part of the riser, the relationship between the buoyancy of the buoyant body (12) ant the weight and lengths of the chains are adapted to provide an essentially horizontal force at the lower part of the riser.

17. Device according to claim 16, c h a r a c t e r i z e d in that it comprises two buoyant bodies anchored at a chosen angle relative to each other.