Means For Applying Tension To A Top Tension Riser

Abstract

A means for applying a controlled tension to a top tension riser on a floating offshore structure. The invention combines a mechanical tensioner device and buoyancy can to apply controlled tension to an individual riser or group of risers supported by a floating offshore structure. The buoyancy can applies static tension force on the riser(s). The mechanical tensioner applies additional tension force to assist the can in limiting the stroke of the riser as the supporting structure is displaced from its nominal position

Description

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the present invention reference should be made to the following description, taken in conjunction with the accompanying drawings in which like parts are given like reference numerals, and wherein:

FIG. 1 is a side sectional view of the invention.

FIG. 2 is a top view of the invention.

FIG. 3 is a more detailed side sectional view of the invention.

FIG. 4 is a side sectional view of an alternate embodiment of the invention.

FIG. 5 is a more detailed side sectional view of the alternate embodiment of FIG. 4 that eliminates some of the surrounding structure for ease of understanding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, it is seen in FIGS. 1 and 2 that the means 10 for applying tension to a top tension riser is generally comprised of a mechanical tensioning device 12 mounted on a floating offshore structure 14 such as a spar type structure in combination with a buoyancy can 16 to support a riser 18 for a dry tree 20.

The mechanical tensioning device 12 is rigidly mounted on the floating offshore structure 14 adjacent the center well 15. The mechanical tensioning device 12 includes a hydraulic cylinder 22 and hydraulic ram (stroke tensioner) 24 movably received in the cylinder 22. A line 26 is connected to the ram 24 and buoyancy can 16. The line is received on a sheave 28 mounted on the offshore structure 14. The stroke tensioner may be pressure compensated to restrict the stroke length to a predetermined stroke length. As seen in FIGS. 1 and 2, more than one mechanical tensioning device 12 may be mounted around the center well 15 as required.

FIG. 3 illustrates an example where the hydraulic ram 24 is shown in its normal operating position, indicated by arrow 30, and in a second tensioning position, indicated by arrow 32, in response to movement by the offshore structure 14.

The buoyancy can 16 operates in combination with the mechanical tensioning device 12 via line 26 which is connected between the two.

When the offshore structure 14 is installed at the offshore site and all equipment is operational, the invention operates as follows. When the offshore structure 14 is in the ideal position above the sea floor, the hydraulic ram 24 is in its normal operating position as indicated by arrow 30. As the offshore structure 14 moves horizontally in response to environmental forces, the riser 18 offsets, tends to pull downward, and forms a catenary curve in the riser 18. The top end of the riser 18 pulls down because of the weight of the catenary curve and thus pulls the buoyancy can 16 down until equilibrium is established. If the buoyancy can 16 provides the only top tension on the riser 18, it is possible that the riser will pull down below the limit that will result in damage to the riser 18. The mechanical tensioning device 12, via the line connected to the buoyancy can 16, acts to limit the pull down of the riser 18 before it reaches the damage point by providing tension on the line 26 as the hydraulic ram moves toward the predetermined limit of its second tensioned operating position indicated by arrow 32.

The stroke of the mechanical tensioning device 12 may be adjusted as necessary. As an example, FIG. 3 shows a hydraulic ram 24 with a normal full fifteen foot stroke that has been pressure compensated to a ten foot stroke to meet the requirements of the particular offshore structure, riser, and buoyancy can arrangement.

While only a single buoyancy can is shown, it should be understood that the invention is applicable where there are multiple buoyancy cans. The cables from the tensioning devices are attached to the buoyancy can.

FIGS. 4 and 5 illustrate an alternate embodiment of the invention where the mechanical tensioning devices 12 are mounted on a tensioner support structure 34 instead of directly to the offshore structure. It is seen that the tensioner support structure 34 is attached to the offshore structure 14 and spans the center well 15. The operational principle is the same.

The invention provides several advantages.

It enables the use of risers that are normally too heavy or experience too much stroke for mechanical tensioners or buoyancy cans alone because of the riser's function or water depth of deployment.

It eliminates the requirement for redundancy capacity in buoyancy cans and tensioners.

It reduces stroke ranges as compared to buoyancy cans alone.

It reduces the tension as compared to the use of tensioners alone.

Because many varying and differing embodiments may be made within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.

Claims

1. In an offshore floating structure having one or more top tensioned risers, a means for applying tension to the risers, comprising: a. a mechanical tensioning device supported by the offshore floating structure;b. a buoyancy can supporting the riser; andc. a line connecting said mechanical tensioning device to said buoyancy can.

2. The tension applying means of claim 1, wherein said mechanical tensioning device is attached directly to the offshore floating structure.

3. The tension applying means of claim 1, wherein said mechanical tensioning device is attached to a tensioner support structure mounted on the offshore floating structure.

4. The tension applying means of claim 1, wherein said mechanical tensioning device comprises a hydraulic cylinder and ram.

5. A method for applying tension to top tension risers on a floating offshore structure, comprising: a. providing a buoyancy can supporting the riser;b. providing a mechanical tensioning device supported by the offshore floating structure; andc. attaching a line between said buoyancy can and said mechanical tensioning device such that said buoyancy can and mechanical tensioning device work in combination to support the riser.