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

RELATED APPLICATIONS

This application claims and references the benefit of Provisional Application Ser. No. 60/798,091 filed on May 4, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is generally related to risers used on offshore structures and more particularly to means for controlling tension in the risers.

2. General Background

In the offshore production of oil and gas (fluids), offshore host facilities can be one of many different types, such as TLP's (tension leg platforms), Mini-TLP's, Spars, Semi-submersibles, etc. These host facilities most often bring production fluid onboard and produce the fluid for export through a pipeline or floating transport vessel. The fluid comes from subsea wells either directly to the host facility or via manifolds that commingle the production fluid from several different wells, prior to the fluids being brought onboard a host facility.

The means typically used to bring the production fluids to the host facility include steel catenary risers (SCR's), flexible flowlines, top tensioned risers (TTR's), and free standing risers.

Steel catenary risers (SCR's) are essentially a pipeline that hangs off the side of the host facility by way of a flex joint or a pull tube. Installation involves a lay barge, which drives up installation costs, as field welding is required to manufacture the SCR. The weight of the SCR imparts a high hang off load onto the host facility. There are also fatigue issues associated with host facility motions being transmitted into the SCR.

Flexible flowlines are a multi-layer flexible hose that hang off the host facility via a collar. The hoses hang in a catenary shape similar to an SCR but with a more dramatic sag. The flexible flowline offers the benefit of faster installation times relative to the SCR, as there are no in-field welds that must be made. Typically, the flexible flowline is reeled out during installation. The flexible flowline can also reduce the payload imparted onto the host facility, as the departure angles for flexible flowlines are smaller than SCR's, which yield a shorter free hanging catenary length, reducing the weight, which is another benefit. However, flexible flowlines are expensive to manufacture relative to pipe. Not only is the manufacturing cost high, but the flexible flowlines have temperature and pressure limitations relative to steel pipe.

Another alternative is a top tensioned riser (TTR). These risers are made of steel pipe with various specialty stress joints and connectors located at the sea floor and the keel of the host facility. These specialty joints help reduce localized high bending loads generated in these areas. The weight of the TTR is either supported by the host facility via tensioners or air cans that provide buoyancy independent of the host facility. Installation of the top tensioned riser is accomplished via a rig located on top of the host facility. The subsea wellhead that a TTR is tying back to the host facility must be located within a relatively small horizontal offset distance from the well slot where the TTR enters the host facility such that the TTR remain almost vertical during all operations. This is one of the main restrictions of a TTR. The main advantage of the TTR is that it allows direct access into the subsea well from the host facility. This access allows the operators to stimulate the well and increase the amount of production fluid that can be recovered from a single well. If such intervention is required on wells with the other riser types, a separate vessel known as a Mobile Offshore Drilling Unit (MODU) has to be mobilized at a high cost to carry out the work. Sometimes this cost does not justify the intervention and the well is abandoned with otherwise recoverable product left behind.

A free standing riser (FSR) is a combination of a TTR and a flexible riser. The FSR is a buoyancy can supported TTR that is located outside of the host facility. Another difference is that the top of the FSR's air can is located well below the mean water level, approximately five hundred feet. A flexible flowline is then attached from the top of the FSR to the host facility. This riser concept has a couple of benefits. Host facility motions are decoupled from the riser via the flexible flowline. Another benefit is that the payload imparted on the host facility is small because only approximately one thousand feet of flexible flowline is hanging from the host facility. The flexible flowline just needs to be separated from the free standing riser and hung off once the host facility is in place.

It can be seen that TTR's have some disadvantages. For example, if the wells that the TTR's are producing from have their production deplete before the design life of the riser is up, the TTR cannot readily be moved to accommodate another wellhead, as described above. Because TTR's have not been designed to be tied in to productions from wellheads beyond the normal reach of a TTR, the useful life of TTR's can be limited. This results in the need to have other types of risers, as described above, which adds to the complexity and cost of an offshore host facility. The TTR used to provide direct access to a well must be supported in the interior of the host facility because the intervention requires heavy equipment that is moved around on the facility's working platform. The tensioning device used to support the riser is either a buoyancy can attached to the top of the riser and floating inside an open well in the host facility or a mechanical tensioner hanging directly from the host facility. The tensions in the individual risers must be controlled because of varying weights of contents in the riser as it is subjected to different modes of operation. As water depth increases the length of the riser and thus its weight also increases. Even in shallower depths, where wells produce high pressure fluids at high temperatures the risers require very thick walls and thus become heavier than normal TTR's. In these cases, the traditional buoyancy can and tensioners alone are not adequate to support and control the riser tension as required. There is need to be able to carry out this function using equipment that is currently proven.

SUMMARY OF THE INVENTION

The invention addresses the above need. What is provided is a means for applying a controlled tension to a top tension riser. 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.

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; and

c. 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; and

c. 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.