GB2156283A - Offshore structure for deepsea production - Google Patents

Abstract

An offshore structure for deepsea production is designed for achieving economy both in installation and in operation. It consists mainly of a base 11 to be founded on a sea-bed, a floating body 13 having a containment means 32 for storing and separating the produced fluids and a tension leg assembly 12 for connecting the base and the floating body and transporting the produced fluids from the well 16 through the base to the containment means near the surface of the sea. The containment means has a compartmentalized outer wall for controllably variable ballasting and deballasting. <IMAGE>

Description

SPECIFICATION Offshore structure for deepsea production This invention relates generally to a floating offshore structure for deepsea production and more particularly to an economically installable offshore structure suited for single well production as well as for early production and extended reservoir testing of deepwater fields.

The risk-taking element in deepsea drilling and production of oil remains even after the exploration phase. This is particularly the case where a multiwell system is installed as frequently done for one reason or another before accurately determining the full physical extent of the reservoir and its long term producing characteristics. According to a typical method using a semisubmersible platform, a subsea manifold is disposed on the sea-bed and is connected via subsea pipelines to several subsea production trees at remote wells. Capital expenditures for establishing such an offshore system are generally very large. Should it become necessary for major workover purposes, for example, to position a separate floating system over one of the remote wells, the total expense becomes even higher.

Additional expenses are incurred, furthermore, when land facilities must be upgraded to handle deep draft bulk cargo vessels such as large, very large and ultralarge crude carriers. By limiting the producing system to one well, the volume of production can be carried by shallower draft vessels which can use navigable waterways maintained by agencies such as the U.S. Corps of Engineers. Very few deepwater facilities exist world wide and this severely limits early production and extended reservoir testing production because these reservoirs are generally remote from deepwater ports and cargo terminals. Often, expensive pipelines and/or special deepwater facilities are specially constructed to handle deep draft vessels. Additional expenses can also occur with deep draft vessels which by definition have large widths.Restricted width waterways, such as the Panama Canal, deny passage for such vessels. Shallower, narrower vessels can traverse restricted width waterways, thereby circumventing such problems.

It is therefore an object of this invention to provide an economically advantageous deepsea production system suited for single well production.

It is another object of the prcsent invention to provide a deepsea production system is which major components are adaptable and intended for mass production.

It is a further object of the present invention to provide an offshore structure with process and storage facilities integrally incorporated into a tensioned single central leg anchored system.

It is still a further object of the present invention to provide an offshore structure for production occupying a minimum plan view area so that the site may be restored easily upon abandonment of the system.

The above and other objects of the present invention are achieved by providing a system of which the main components are a base assembly founded on the sea-bed, a central leg assembly allowing vertical alignment for direct access to well bore tubing and a buoy with facilities for storage and separation of the produced fluids.

Figure 1 shows schematically an offshore structure embodying the present invention.

Figure 2 shows schematically the details of a section of the floating body according to the present invention.

There is shown schematically in Figures 1 and 2 a deepwater production system of the present invention which, briefly stated, consists of the following three main components: a base assembly 11, a central tension anchor leg assembly 12 and a floating body 13.

The base assembly 11 is a unit engaged with the sea-bed typically by being secured to piles 15 around a well 16. It may be of a conventional design with a central opening sufficiently large crosssectionally to house therein a wet production tree 20 as well as maintenance space for divers. The base assembly 11 is preferably sufficiently tall to protect the wet tree 20 from damage by ensuing installation operations. The wet 20 is one of the well control means of the system and may be of a conventional type having minimum process functions. Primarily, it is a safety device at the sea floor which fails safe under loss of hydraulic valve pressure. Well control is further achieved by a subsurface safety valve (or downhole safety valve) 17 in the well 16.

At the top, the base assembly 11 is connected via a universal joint means 22 to the bottom end of the central tension anchor leg assembly 12. The leg assembly 12 is essentially an elongate member connecting the base assembly 11 founded on the sea-bed and the floating body 13 which is essentially a semisubmersible tension leg buoy. At the center, there is an independently tensioned production riser assembly 25 for transporting fluids upwards from the well 16 to the floating body 13.

The leg assembly 12 itself may be buoyant or nonbuoyant, depending upon specific site conditions.

A riser tensioning buoy 27 may be provided near the top end of the leg assembly 12 in order to facilitate the vertical positioning of the leg assembly 12 when it is installed vertically onto the base assembly 11.

The top end of the leg assembly 12 is connected to the bottom of the floating body 13 by another universal joint means 30. The floating body 13 has a storage and separation containment means 32 through which the top part of the production riser assembly 25 connects vertically to transport the produced fluids to a dry tree 35 positioned above the containment 32 and above the sea level. The dry tree 35 may be of a conventionally available type where flow control is exercised by an automatic choke with or without a manual override. It is preferably of a type with two wing valves for production, one active and the other used as a stand-by. This will permit switch-over when the active choke must be replaced or maintained. Pro duced fluids are piped directly from the dry tree 35 through one of its discharge pipes 37 into the containment 32 which serves as a vertical buoy separator.Components from the base of the production tubing to the top of the separator inlet flanges 42 are pressure rated. The pressure inside the containment 32 is controlled by a pressure reducing valve 44 at the outlet flange for a gas outlet pipe 45 through which the gas separated inside the tank 32 is transported upwards to the base of a turntable (rotatable turret) 50 through a swivel joint 52 of Chiksan type. Produced fluids discharged into the tank 32 are transported upwards by a submersible pump 55 through the joint 52. Both the produced gas and fluids pass through the same central opening of the turntable 50 and the gas then goes to a scrubber 57 located at the base of a workover rig mast 70 founded on the turntable 50. The workover rig mast 70 also functions as the flare tower.

This allows a less expensive floating tender rig to be used for major workover activities such as packer and tubing replacements rather than mobilizing a more expensive rig such as a drill ship.

Liquids deposited in the scrubber are discharged back into the containment 32.

The containment 32 is further provided with a fixed ballast 59 at the bottom for keeping the center of gravity of the floating body 13 safely low and to minimize tension on the leg assembly 12 even when the containment 32 is empty. The interior of the containment 32 is segregated from the sea by surrounding ballast compartments 60. In other words, the external walls of the containment 32 are of a compartmentalized structure divided into a number of vertically elongate parallel chambers so that the containment 32 can be deballasted suitably as product accumulates inside and deballasting further controls the tension to the base assembly 11 and the central tension anchor leg assembly 12 to approximately uniform levels.Liquid level in the containment 32 is constantly measured by level indicators (not shown) which feed data into a microcprocessor which in turn controls ballasting and deballasting. Measured liquid level is telemetered to shore at all times. In addition, low and high level telemetered alarms may be provided to trigger shutdown.

The floating body 13 may further be provided with loading devices of standard types such as loading booms, tanker loading hoses, tanker mooring, pipe ramps and/or tender rig mooring bridles.

Conventional floating hoses can be used in lieu of the loading boom if site conditions warrant their use. Since this system of the present invention is intended for use in an open seaway, the loading boom method will reduce hose damage and maintenance costs.

Hydraulic power furnished by the tanker will operate the shipping pumps (not shown). The loss of weight during product unloading is compensated by taking on seawater ballast. Depending on the severity of sand production, a desanding device can be placed aboard the shuttle and jet nozzles installed within containment 32.

An offshore structure of the present invention provides various advantages. The primary characteristic which differentiates it from the conventional designs is that process and storage facilities are integrally incorporated into a tensioned single central leg anchored system. Since the structure and other components of the present invention are intended for mass production, economic benefits associated with volume fabrication is readily gained. Installation economies are gained by using the exploration rig to place the facility and revenues occur immediately following installation and well completion. If components are manufactured in advance, furthermore, the installation period is extremely short and early cash flow can be achieved.

The system of the present invention can also be used economically because production and ballasting can be monitored by shore-based telemetering which can also be used to control the equipment system. Workover capability is provided with the assistance of a tender rig, thus eliminating the need for a major exploration drilling vessel during the producing phase. Since production is via a single well and through vertically aligned components, direct access is allowed to well bore tubing, casing and other equipment which require periodic maintenance and workover. Conventional downhole tools are sufficient for workovers.

A further advantage of the present invention is found in its use of exploration and delineation wells. Exploration and reservoir delineation wells are often plugged and abandoned while a multiplicity of new wells are drilled from one or more central facilities during the producing phase. By the present invention, the expense of these wells can be recovered and expensive new well drilling can be avoided.

The present invention has been described above in terms of only one embodiment but the description above should be regarded as illustrative rather than as limiting, and should therefore be construed broadly. For example, the accompanying figures are intended to be schematic and not to represent any preferred dimensional relationships or shapes of the various components. Although a design with two joints have been shown, the number of universal joint means in the system is by no means to two. Although a hydraulic jack (not shown) may typically be placed below the dry tree 35 for applying tension to the production riser assembly 25, any other type of pressure applying means can be included in the system. The base assembly 11 need not be firmly affixed to a see floor by being secured to piles; it may be a simple gravity base.

The ballastable compartments may be horizontally divided instead of vertically. At a higher investment cost, an offshore structure of the present invention can even be made adaptable for a multiwell subsea system. The scope of the present invention is therefore to be considered limited only by the following claims.

Claims (8)

1. An offshore structure for deepsea produc tion, comprising a base founded on a sea-bed over a well, a floating body with a controllably ballastable and deballastable storage and separation containment means, and an elongate tensioned leg vertically aligned to said well and connected to said base and said floating body.

2. The structure of Claim 1 wherein said base contains a wet production tree.

3. The structure of Claim 1 wherein said floating body contains a dry tree.

4. The structure of Claim 1, Claim 2 or Claim 3 wherein said leg contains a product riser assembly for transporting fluids from said well upwards into said containment means.

5. The structure of any preceding claim wherein said leg is connected to said base and to said floating body respectively by a universal joint means.

6. The structure of any preceding claim wherein said containment means has an outer wall which is divided into compartments.

7. The structure of any preceding claim further comprising a means for controllably ballasting and deballasting said containment means.

8. The offshore structure substantially as hereinbefore described with reference to and as shown in the accompanying drawings.