Riser tensioner for an offshore platform

Abstract

An upgraded riser tensioner (10) includes a cylinder end sheave body (12), a hydraulic cylinder (22), a rod (24) extending upward from the hydraulic cylinder, and upper sheave assembly (20) supported on the rod. A piston (26) is movable within the hydraulic cylinder. A blind end cover (22) is spaced at the lower end of the hydraulic cylinder. An adaptor plate (30) is spaced between the blind end cover and the cylinder and sheave body, and includes circumferentially spaced interior bolt holes and circumferentially spaced external bolt holes.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. patent application Ser. No. 60/899,238, filed Feb. 2, 2007 for a RISER TENSIONER FOR AN OFFSHORE PLATFORM, which is incorporated herein in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to tensioner cylinders for use in an offshore platform, such as an oil or gas drilling or production rig. More particularly, the present invention relates to a riser tensioner which may be upgraded for handling an increased load, with the upgrade package being quickly and reliably installed.

BACKGROUND OF THE INVENTION

Various types of offshore vessels or platforms utilize riser tensioners for accommodating the wave action. The term “platform” as used herein includes an offshore vessel. Riser tensioners are commonly used in a drilling platform. The riser system conventionally extends from the floating platform to a subsea blowout preventer or production tree, which in turn is connected to a wellhead at the sea floor. The riser tensioners or motion compensators are thus incorporated into the riser string to compensate for vessel motion induced by wave action and heave. Risers may be constructed of various metal alloys, such as titanium, steel, or aluminum. Risers may also be fabricated from various composite materials, as disclosed in U.S. Pat. Nos. 7,090,006 and 7,144,048. Riser tensioners normally include a lower cylinder sheave body secured to the structural frame of the offshore platform, with a rod supporting an upper sheave body. A cable is wound between the sheave bodies and is connected to a load in a known manner.

At various times, it may be desirable for the platform to handle a larger load and/or a more dense fluid, e.g., to allow the platform to be used in slightly deeper waters than originally intended and/or to accommodate a heavier drilling fluid. In order to accomplish this goal, the load carrying capacity of the riser tensioner systems must be increased.

Proposals for increasing riser tension load include adding additional riser tensioners to the platform, or the increasing pressure to the tensioners increase the cylinder working load. In other cases, it has been proposed to replace each of the riser tensioners with a larger riser tensioner.

Various types of riser tensioning systems have been devised, as disclosed in the following U.S. Patents and Publications: US 2005/0263288, U.S. Pat. Nos. 4,004,532, 4,215,950, 4,272,059, 4,379,657, 4,432,420, 4,449,854, 4,616,708, 4,655,433, 4,787,778, 4,808,035, 5,101,905, 5,252,004, 5,310,007, 5,366,324, 5,628,586, 5,658,095, 5,758,990, 5,846,028, 6,457,527, 6,470,969, 6,688,814, 6,691,784, and 6,692,193.

The following patents provide further details with respect to the specific construction of riser tensioners: U.S. Pat. Nos. 4,615,542, 5,363,920, 6,257,337, 6,296,232, 6,530,430, 6,739,395, 6,869,254, and 7,007,340. U.S. Pat. No. 7,131,922 discloses a riser tensioner having an integral accumulator.

The disadvantages of the prior art are overcome by the present invention, and an improved riser tensioner for an offshore platform is hereinafter disclosed.

SUMMARY OF THE INVENTION

In one embodiment, an upgraded riser tensioner for an offshore platform includes a cylinder end sheave body secured to the offshore platform and housing a lower sheave assembly. A hydraulic cylinder extends upward from the lower sheave body, and a rod extends upward from the hydraulic cylinder. An upper sheave assembly is supported on the rod, and a piston is movable within the hydraulic cylinder and is connected to the rod. A blind end cover is spaced at the lower end of the hydraulic cylinder, and an adaptor plate is spaced between the blind end cover and the cylinder end sheave body. The adaptor plate includes inward bolt holes for bolting to the cylinder end sheave body, and external bolt holes for bolting the adaptor plate to the blind end cover.

These and further features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a suitable riser tensioner according to the present invention.

FIG. 2 is a half sectional view of the riser tensioner cylinder shown in FIG. 1.

FIG. 3 is an end view of the adaptor plate;

FIG. 4 is a pictorial view of an upgraded riser tensioner with accumulators.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The complexity of an offshore vessel or platform and the role of the riser tensioners in the successful operation of that platform have resulted in a number of distinct proposals for upgrading the riser tensioner to accommodate a larger load and/or a heavier drilling fluid, thereby allowing the platform to be used in deeper waters. One proposal is to change out and increase the size of each riser tensioner. While this proposal is sound, it practically requires that the cylinder and sheave body which is secured to the offshore platform be changed out. The sheave body is normally rigidly secured to the structure of the offshore vessel or platform, and accordingly the replacement of each riser tensioner in the platform practically requires that platform be shipped to a repair yard. It frequently takes months to upgrade a platform as desired, and the time and expense, and the loss of revenue, associated with the shipment of a platform to and from a field to the repair yard and the time for repairs at the yard make this an expensive solution to the problem. This solution, while theoretically feasible, is impractical since it requires that the entire platform be shipped to a repair yard for lengthy and expensive changes.

Another proposal similarly requires that the vessel or platform be in a repair yard for a considerable time to modify the structure of the vessel or platform to can accommodate additional riser tensioners, which may be of the same size as the original tensioners. Since the length of the riser tensioner determines the stroke, the stroke of the riser tensioners presumably would be the same. An increase in riser tensioners, e.g., from six riser tensioners to 10 riser tensioners, increases the load capacity of the riser tensioning system by over 50%, but again this solution is expensive. This proposal also requires a great deal of time and expense since the platform itself normally has to be shipped to a yard and changes made in the deck and structure of the platform so that additional cylinder end sheave bodies can be positioned within the deck. In view of the high cost for leasing offshore platforms, this approach is not feasible for most applications.

Yet another proposal is to increase the pressure of fluid to the existing riser tensioners. A 15% increase in fluid pressure to the riser tensioner increases the load absorbed by the riser tensioner by 15%. A significant problem with this solution is that the fluid delivery system to the riser tensioner is initially rated for a specific pressure. Even though certain components in the system, such as the pressurizing pump, may be able to generate a higher pressure, other components in the system, such as valves, gauges, accumulators, etc., are rated at this initial pressure, and a significant increase in fluid pressure to these components cannot be made without incurring safety risks. Accordingly, replacement of the entire fluid delivery system to the tensioners may be necessary, and this again is both expensive and time consuming.

The present invention provides a relatively low cost and highly reliable solution which does not require that the offshore vessel or platform be shipped to a yard for repair. The riser tensioner according to the present invention utilizes the existing cylinder end sheave body which houses the lower sheave assembly, and replaces the hydraulic cylinder with a larger diameter cylinder to increase the capacity of the riser tensioner. The existing sheave at the upper end of the cylinder may be used in the new and upgraded riser tensioner, or a new upper sheave assembly may be supported on the rod end of the cylinder.

Referring to FIG. 1, an upgraded riser tensioner 10 is disclosed for use on an offshore vessel or platform. A cylinder end sheave body 12 is secured to structural beams 14 of the offshore vessel or platform, with the sheave body 12 housing a lower sheave assembly 16 which receives cables 18 extending from the lower sheave assembly to the upper sheave assembly 20. A hydraulic cylinder 22 extends upward from the cylinder end sheave body, and rod 24 extends upwardly from the hydraulic cylinder. The upper sheave assembly 20 is thus supported on the rod 24, and moves vertically relative to the structure of the offshore platform during stroking of the cylinder.

Cylinder 22 generally shown in FIG. 1 is more specifically shown in FIG. 2, and includes a piston 26 movable along the axis 27 of the cylinder and connected to the rod 24. The blind end cover 28 is spaced at a lower end of the cylinder 22, and seals with of the cylinder to fluidly close a lower end of the cylinder. FIG. 2 also depicts an adaptor plate 30 which is spaced between the blind end cover and the cylinder end housing 12. Still referring to FIG. 2, the cylinder 22 includes a rod retract port 32 for returning fluid to the top of the piston when the load pulls the upper sheave assembly downward, and a rod extend port 34 for raising the upper sheave assembly with respect to the lower sheave assembly 16. FIG. 2 also depicts a packing gland 36 for sealing between the upper end of the cylinder 22 and the rod 24 during stroking of the riser tensioner.

FIG. 3 is an end view of the adaptor plate 30 generally shown in FIG. 1. The adaptor plate includes a plurality of circumferentially spaced radially inner holes 40 which receive bolts 44 as shown in FIG. 2 for attaching the cylinder to the cylinder end sheave body 12. Adaptor plate 30 also includes a plurality of circumferentially spaced radially outer holes 42 for bolting the adaptor plate 30 to the blind end cover 28. The bolts 44 may have heads each recessed in an upper pocket in the adaptor plate, such that the heads of these bolts are covered by the blind end cover 28. Bolts 46 similarly fit within holes 42 to bolt the adaptor plate 30 to the blind end cover 28. The heads of bolts 46 may press downward against the upper surface of adaptor plate 30.

FIG. 4 shows a riser tensioner according to the present invention which has been upgraded to include the blind end cover 28 and the adaptor plate 30. The riser tensioner as shown in FIG. 2 also includes conventional high and low pressure accumulators 70 and 72 for supplying fluid to and receiving fluid from a cylinder during stroking of the riser tensioner.

In a typical system, the diameter of the cylinder may be increased from 14 inches to 16 inches. This difference in cylinder diameter may be used to upgrade the existing riser tensioners, and results in an approximate 25% increase in the load capability of the riser tensioner. The concept of the invention applies to various size cylinders, and an increase in the load of more or less than 25% is obtainable. The present invention may be used on dual acting riser tensioner, as disclosed herein, but may also be used on single acting riser tensioner.

The method of upgrading the riser tensioner which includes a cylinder and sheave body, hydraulic cylinder, an upper sheave body, a piston, and a blind end cover as disclosed herein is suggested from the foregoing description. The method includes providing and subsequently spacing an adaptor plate between the blind end cover and the cylinder and sheave assembly, with the adaptor plate including internal bolt holes and external bolt holes. The method includes bolting the cylinder and sheave body to the adaptor plate using the internal bolt holes, and bolting the adaptor plate to the blind end cover using the external bolt holes. In a preferred embodiment, the blind end cover fluidly closes off a lower end of the hydraulic cylinder. The adaptor plate preferably may include circumferentially spaced radially inner holes for attaching the hydraulic cylinder to the cylinder and sheave body, and providing circumferentially spaced radially outer holes for attaching the adaptor plate to the blind end cover.

Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.

Claims

1. An upgraded riser tensioner for an offshore vessel or platform, comprising:

a cylinder end sheave body secured to the offshore platform, the cylinder end sheave body housing a lower sheave assembly;

a hydraulic cylinder extending upward from the cylinder end sheave body;

a rod extending upward from the hydraulic cylinder;

an upper sheave assembly supported on the rod;

a piston movable within the hydraulic cylinder and connected to the rod;

a blind end cover spaced at a lower end of the hydraulic cylinder; and

an adaptor plate spaced between the blind end cover and the cylinder end sheave body, the adaptor plate including internal bolt holes for bolting to the cylinder end sheave body, and external bolt holes for bolting the adaptor plate to the blind end cover.

2. An upgraded riser tensioner as defined in claim 1, wherein the cylinder includes a rod extend port for receiving hydraulic fluid.

3. An upgraded riser tensioner as defined in claim 1, further comprising:

a packing gland for sealing between the cylinder and the rod.

4. An upgraded riser tensioner as defined in claim 1, further comprising:

at least one accumulator supported on the cylinder for transferring fluid to and receiving fluid from the cylinder.

5. An upgraded riser tensioner as defined in claim 1, wherein the blind end cover fluidly closes a lower end of the hydraulic cylinder.

6. An upgraded riser tensioner as defined in claim 1, wherein the adaptor plate includes circumferentially spaced radially inner holes for attaching the hydraulic cylinder to the cylinder end sheave body, and circumferentially spaced radially outer holes for attaching the adaptor plate to the blind end cover.

7. An upgraded riser tensioner as defined in claim 1, wherein the upgraded hydraulic cylinder has a cross-sectional area of at least 25% greater than the circumferential area of a hydraulic cylinder prior to the upgrade.

8. An upgraded riser tensioner as defined in claim 1, wherein a cable extends between the lower sheave assembly and the upper sheave assembly.

9. An upgraded riser tensioner for an offshore vessel or platform, comprising:

a cylinder end sheave body secured to the offshore platform, the cylinder end sheave body housing a lower sheave assembly;

a hydraulic cylinder extending upward from the cylinder end sheave body;

a rod extending upward from the hydraulic cylinder;

an upper sheave assembly supported on the rod;

a piston movable within the hydraulic cylinder and connected to the rod;

a blind end cover fluidly closing off a lower end of the hydraulic cylinder; and

an adaptor plate spaced between the blind end cover and the cylinder end sheave body, the adaptor plate including circumferentially spaced radially inner holes for attaching the hydraulic cylinder to the cylinder and sheave body, and circumferentially spaced radially outer holes for attaching the adaptor plate to the blind end cover.

10. An upgraded riser tensioner as defined in claim 9, further comprising:

at least one accumulator supported on the cylinder for transferring fluid to and receiving fluid from the cylinder.

11. An upgraded riser tensioner as defined in claim 9, wherein the upgraded hydraulic cylinder has a cross-sectional area of at least 25% greater than the circumferential area of a hydraulic cylinder prior to the upgrade.

12. An upgraded riser tensioner as defined in claim 9, wherein the cylinder includes a rod extend port for receiving hydraulic fluid.

13. An upgraded riser tensioner as defined in claim 9, further comprising:

a packing gland for sealing between the cylinder and the rod.

14. A method of upgrading a riser tensioner for an offshore vessel or platform, the riser tensioner including a cylinder end sheave body secured to the offshore platform, the cylinder end sheave body housing a lower sheave assembly, a hydraulic cylinder extending upward from the cylinder end sheave body, a blind end cover spaced at a lower end of the hydraulic cylinder, a rod extending upward from the hydraulic cylinder, an upper sheave assembly supported on the rod, and a piston movable within the hydraulic cylinder and connected to the rod, the method comprising:

spacing an adaptor plate between the blind end cover and the cylinder end sheave body, the adaptor plate including internal bolt holes and external bolt holes;

bolting the adaptor plate to the cylinder end sheave body using the internal bolt holes; and

bolting the adaptor plate to the blind end cover using the external bolt holes.

15. A method as defined in claim 14, further comprising:

providing a rod extend port in the cylinder for receiving hydraulic fluid.

16. A method as defined in claim 14, further comprising:

providing a packing gland for sealing between the cylinder and the rod.

17. A method as defined in claim 14, further comprising:

supporting at least one accumulator on the cylinder for transferring fluid to and receiving fluid from the cylinder.

18. A method as defined in claim 14, wherein the blind end cover fluidly closes a lower end of the hydraulic cylinder.

19. A method as defined in claim 14, further comprising:

providing the adaptor plate to include circumferentially spaced radially inner holes for attaching the hydraulic cylinder to the cylinder and sheave body, and circumferentially spaced radially outer holes for attaching the adaptor plate to the blind end cover.

20. A method as defined in claim 14, further comprising:

extending a cable between the lower sheave assembly and the upper sheave assembly.