WO1999014462A1

WO1999014462A1 - Riser installation method and riser system

Info

Publication number: WO1999014462A1
Application number: PCT/GB1998/002750
Authority: WO
Inventors: Anthony Sweetsur
Original assignee: Kvaerner International Ltd.
Priority date: 1997-09-12
Filing date: 1998-09-11
Publication date: 1999-03-25
Also published as: GB9719410D0; GB9819892D0; GB2329205B; GB2329205A; AU9085998A

Abstract

A method of installing a well production riser bundle (16) for a deepwater oil and/or gas development, which method comprises the steps of: fabricating the riser bundle in a horizontal attitude, such that a path for a production riser for oil and/or gas is surrounded by and stabilized within an outer barrier casing and the outer barrier casing is itself surrounded by and stabilised within a floatation tubular in generally concentric relationship, and the generally annular space between the outer barrier casing and the floatation tubular is divided into discrete lengths by radial bulkheads, so to form a multiplicity of watertight/gastight compartments which are adapted to be pressurised individually; moving the riser bundle (16) to the location of the development by towing it lengthways along the seabed with a tow line attached to one end of the riser bundle, and maintaining the pressures within the compartments so that the external pressure of water does not collapse the riser bundle; attaching the one end of the riser bundle to a line (26) extending down from a buoyant platform (10); arranging (if necessary by pumping or flooding) for the buoyancy of the riser bundle (16) to be slightly less than its weight; raising the one end of the riser bundle to above the sea surface by pulling on the line (26); connecting the one end of the riser bundle to facilities on the buoyant platform (10); and connecting the other end of the riser bundle to a subsea wellhead (14a) or manifold, whereby oil and/or gas can flow from the subsea wellhead or manifold up within the production riser to the facilities on the buoyant platform (10).

Description

RISER INSTALLATION METHOD AND RISER SYSTEM

Technical Field of the Invention

The invention relates to a method of installing a production riser for an offshore platform in deep water, and to a well production riser bundle for use in that method.

In particular, the invention relates to a method of installing a production riser on a buoyant offshore platform having restricted heave characteristics.

Background of the Invention

The exploitation of subsea oil and/or gas deposits is taking place in progressively increasing water depths. For deep water (e.g. water depths in excess of 2,000ft), fixed platforms based on the seabed have become impractical. Instead, buoyant platforms have been used, with sophisticated station keeping systems to keep them permanently in position.

In order to transfer oil and/or gas from subsea wells to processing facilities on the buoyant platforms, a specialised assembly known as a 'production riser^* is used. This riser extends upwards from a wellhead on the seabed to above the sea surface where it completes at an "Xmas Tree". In the past, such risers have been installed by lowering individual lengths of riser from a drilling rig through a well slot on the platform. The lengths of riser are connected together as successive lengths are lowered.

As water depths increase, the length and hence the weight of the riser increases. As the weight of the riser (in practice more probably a group of risers) increases, the requirement for platform buoyancy to support the riser(s) increases. One example of a riser system for a floating platform moored permanently over a deepwater hydrocarbons field has been described in OTC Paper 8386 (published at Houston Texas in May 1997). This system had three separate buoyancy tanks disposed at predetermined depths down each of the production risers. The three buoyancy tanks were sized to support the riser and its associated equipment. The use of buoyancy tanks reduced the effective weight of each riser, and so reduced the required buoyancy of the floating platform (in this case a Spar) needed to support the risers.

This system has been found to work well, in that it reduced the need for platform buoyancy, and hence the overall size and cost of the platform.

However, the buoyancy tanks would attract lateral loads if exposed to wave and current forces. Accordingly the risers were contained within a central shaft below the wellhead area on this platform. Moreover, very substantial heaving/lowering capacity was required on the platform to install the production risers through the platform's moonpool. In practice this capacity was obtained from the casing installation equipment of the platform's workover drilling rig. As a result, the operation of installing a permanent production riser required the stopping of drilling activity. This enabled the drilling rig to install the production riser.

However, use of the drilling rig to install the production riser delayed the drilling programme.

Thus there is a requirement for a production riser installation method that avoids any use of the drilling rig.

Disclosure of the Invention

The invention provides a method of installing a well production riser bundle for a deepwater oil and/or gas development, which method comprises the steps of:- fabricating the riser bundle in a horizontal attitude, such that a path for a production riser for oil and/or gas is surrounded by and stabilised within an outer barrier casing and the outer barrier casing is itself surrounded by and stabilised within a floatation tubular in generally concentric relationship, and the generally annular space between the outer barrier casing and the floatation tubular is divided into discrete lengths by radial bulkheads, so to form a multiplicity of watertight/gastight compartments which are adapted to be pressurised individually; moving the riser bundle to the location of the development by towing it lengthways along the seabed with a tow line attached to one end of the riser bundle, and maintaining the pressures within the compartments so that the external pressure of water does not collapse the riser bundle; attaching the one end of the riser bundle to a line extending down from a buoyant platform; arranging (if necessary by pumping or flooding) for the buoyancy of the riser bundle to be slightly less than its weight; raising the one end of the riser bundle to above the sea surface by pulling on the line; connecting the one end of the riser bundle to facilities on the buoyant platform; and connecting the other end of the riser bundle to a subsea wellhead or manifold, whereby oil and/or gas can flow from the subsea wellhead or manifold up within the production riser to the facilities on the buoyant platform.

There may be a plurality of production risers for oil and/or gas within the outer barrier casing of the riser bundle, and these are connected to a plurality of corresponding subsea wellheads.

It is preferred that there is an inner barrier casing between the production riser(s) for oil and/or gas and the outer barrier casing. ln this form it is further preferred that there is provision for electrohydraulic controls (e.g. for SSSVs or wellhead equipment) or measuring devices (e.g. for temperatures or pressures) within the inner barrier casing, and these controls or measuring devices are connected to the subsea wellheads after installation. The one end of the riser bundle may be raised to above the sea surface within a moonpool of the buoyant platform.

It is preferred that the pressures within at least some of the compartments are maintained to react only the local external pressure of water at the particular depth of the compartments in their in-place position when they have been installed. It is also preferred that the line attached to the one end of the riser bundle is pulled by means on the platform other than the drilling rig used to drill the subsea wells (e.g. by a deck mounted winch).

In one form the other end of the riser bundle is manoeuvred over the subsea wellhead by moving the position of the buoyant platform laterally on the sea surface. In this form it is preferred that the method is used for installing a plurality of riser bundles related to an existing array of subsea wellheads by moving the buoyant platform laterally on the sea surface over the seabed locations of the wellheads in the array.

The invention also provides a well production riser bundle for installation by, or when installed according to, any one of the methods described above. More specifically the invention provides a well production riser bundle comprising a path for a production riser surrounded by and stabilised within an outer barrier casing, which outer barrier casing is itself surrounded by and stabilised within a floatation tubular in generally concentric relationship, and in which the generally annular space between the outer barrier casing and the floatation tubular is divided into discrete lengths by radial bulkheads, so forming a multiplicity of watertight/gastight compartments which are adapted to be pressurised individually.

Brief Description of the Drawings

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 is a diagrammatic section (not to scale) from sea surface to seabed through the water overlying an offshore oil/gas field development;

Figure 2 is a cross section through a well production riser bundle used in that development; Figure 3 is a vertical cross section through the riser bundle, showing key levels up the length of the riser;

Figure 4 is a diagrammatic section from sea surface to seabed showing a group of riser bundles leading up from an array of subsea wellheads, and Figure 5 is a plan on those riser bundles (to an enlarged scale).

Description of the Specific Embodiment

Figure 1 shows a buoyant platform 10 floating in a water depth of 6,000 ft. The buoyant platform is in this case formed of a substructure as described in our co-pending UK Patent Application 98,18142.3. However, the buoyant platform could be a conventional Spar, or a Tension Leg Platform (TLP), or even a guyed tower with a buoyant upper portion. If the platform was a guyed tower, an upper part of the tower would need a recess or slot to accept a well production riser bundle. In any case, the substructure for the platform would have very restricted heave characteristics.

The buoyant platform 10 has a deck mounted winch 12. The platform may have other equipment such as a drilling rig and processing facilities. However, this other equipment is not relevant to the installation method, and is not shown.

The buoyant platform 10 is moored permanently in position over a subsea wellhead 14, which is located on top of a predrilled subsea oil well 14A. To enable the production of oil/gas from the subsea wellhead 14, a production riser must be installed between the subsea wellhead and an "Xmas Tree" located on the well deck of the platform 10.

In order to install the production riser, a well production riser bundle 16 is fabricated in a horizontal attitude on a flat onshore site. The riser bundle 16 is shown in cross section in Figure 2. The riser bundle 16 comprises three concentric steel tubes, viz. a production riser

18, an inner barrier casing 20, and an outer barrier casing 22. The outer barrier casing 22 is surrounded by a floatation tubular 24.

The production riser 18 is designed to carry oil/gas from the wellhead 14 up to the platform 10. The barrier casings (20, 22) give fail safe protection to the production riser 18 to prevent the leakage of oil/gas. The annular space between the outer barrier casing 22 and the floatation tubular 24 is divided lengthwise by radially extending bulkheads to form a series of watertight (and gastight) compartments.

The production riser 18 is typically a 4.50" (114.3mm) diameter pipe rated to 10,000 psi (69Mpa) working pressure. It can be installed as part of the riser bundle 16, or post installed from the platform 10. Pre-installing the production riser will reduce the amount of offshore hook-up, whilst post installation will result in a smaller riser bundle diameter. In Figure 2 only one production riser 18 is shown. It will be understood that more than one riser may be contained within the inner barrier casing 20. Additionally, electro-hydraulic controls or leads from measuring devices may be contained within the riser bundle.

When complete, the riser bundle 16 is connected to a tug (not shown) and towed lengthwise along the sea bed to the location of the offshore development. Several riser bundles could be towed out in a riser bundle train. The compartments (between casing 22 and floatation tubular 24) are pressurised with nitrogen (e.g. at 2670 psi at 40°F) to resist the crushing pressure of the water under a head of c6,000ft surrounding the riser bundle. Pressurisation minimises the wall thickness of the floatation tubular 24. When the end of the riser bundle connected to the towline is close to the subsea wellhead 14, that end is attached to the lower end of a line 26. The line 26 extends down through the platform. The upper end of the line is connected to the winch 12 on the deck of the platform. The tug is released when the line is safely connected.

The volumes of the compartments between the outer barrier casing 22 and the floatation tubular 24, and the pressurisation of those compartments, are such that the riser bundle is almost self buoyant. In other words, the buoyancy of the riser bundle is slightly less than its weight. For instance, the resultant negative buoyancy (or effective weight) might be of the order of 5 to 7 lb. per linear ft of the riser bundle.

The diameter of the riser bundle 16 is reduced to the diameter of the outer barrier casing 22 in the wave effected zone to reduce wave and current loading. (See Figure 3.)

Because the riser bundle 16 is almost self buoyant, it can be raised from the seabed using the deck mounted winch 12. As the upper end of the riser bundle is raised, nitrogen gas is vented from the compartments formed by the bulkheads between the outer barrier casing 22 and the floatation tublar 24. This results in the weight of pressurised nitrogen being reduced, thereby giving the bundle a greater buoyancy per unit length at its upper end. Venting of nitrogen ensures that internal pressurisation does not explode the floatation tubular. Some counterballasting may be necessary.

When the riser bundle is suspended vertically beneath the buoyant platform - as shown by a broken line and designated 16A - the production riser 18 is connected to the subsea wellhead 14. This can be achieved by moving the platform around laterally until the lower end of the riser bundle is directly over the wellhead, and then lowering the riser bundle using the winch 12 and/or other control devices such as mechanical or hydraulic jacks.

(If the production riser is designed to be post installed, it can be run at this time.) Figure 3 shows particular lengths of the riser bundle 16 in its installed condition. The production riser 18 extends up from the well 14A, through the special wellhead and BOP connector system 14 to a tapered stress joint 14B. The buoyancy tubular 24 then surrounds the outer barrier casing 22 up to a keel joint 28 at the base of the platform. Within the height of the platform, the buoyancy tubular is absent, so that it does not attract side loads from waves and currents. Within this height, plan bracing joints (e.g. 30) support the outer casing 22. Directly below the deck of the platform a deck joint 32 gives further support. At the joints 28, 30, 32, the riser bundle is made stiffer by increasing the wall thicknesses. This is intended to counteract wear and excessive point loadings. A dry Xmas Tree 34 is located on the deck. Riser tensioning is required to support the weight of the riser bundle 16 above the keel of the platform and to provide a tensile force to reduce lateral deflections in the riser due to environmental loading. Tension can be varied depending on environmental forces applied to the riser bundle and any additional weight to be carried (e.g. mounting of a surface BOP for workover). Equipment is sized to support the maximum load and to allow the required stroke for heave compensation to accommodate relative vertical motions.

To give an impression of typical proportions of the length of the riser bundle 16 incorporating the buoyancy tubular 24 to the length of the riser bundle within the platform (and not incorporating the buoyancy tubular), corresponding dimensions have been marked on Figures 3 and 4. Dimension "a" represents the length of the riser bundle without the buoyancy tubular, and dimension "b" represents the length of the buoyancy tubular. The buoyancy tubular 24 surrounds by far the greater part of the length of the riser bundle 16.

If there is a need to produce oil from an existing array of subsea wellheads 14 (e.g. as shown in Figures 4 and 5), several riser bundles 16 can be installed by the method described above. In this case the buoyant platform can be moved around laterally to connect riser bundles with each of the wellheads. If the wellheads are to be used sequentially rather than concurrently, then a single well production riser bundle would be moved to each of the wellheads in turn.

Advantages of the Invention

The system described above by way of example has several advantages. It eliminates time consuming deployment from a drilling rig on the platform, and does not impose excessive tension loads on the platform. Since the drilling rig is not used to lower the production riser, simultaneous drilling and well completion is possible. Additionally, it reduces production well spacing at deck level, and reduces deck structure loads and weight; and reduces production riser loading from wave and current loads.

Claims

1. A method of installing a well production riser bundle for a deepwater oil and/or gas development, which method comprises the steps of:-

fabricating the riser bundle in a horizontal attitude, such that a path for a production riser for oil and/or gas is surrounded by and stabilised within an outer barrier casing and the outer barrier casing is itself surrounded by and stabilised within a floatation tubular in generally concentric relationship, and the generally annular space between the outer barrier casing and the floatation tubular is divided into discrete lengths by radial bulkheads, so to form a multiplicity of watertight/gastight compartments which are adapted to be pressurised individually,

moving the riser bundle to the location of the development by towing it lengthways along the seabed with a tow line attached to one end of the riser bundle, and maintaining the pressures within the compartments so that the external pressure of water does not collapse the riser bundle,

attaching the one end of the riser bundle to a line extending down from a buoyant platform,

arranging (if necessary by pumping or flooding) for the buoyancy of the riser bundle to be slightly less than its weight,

raising the one end of the riser bundle to above the sea surface by pulling on the line,

connecting the one end of the riser bundle to facilities on the buoyant platform, and

connecting the other end of the riser bundle to a subsea wellhead or manifold, whereby oil and/or gas can flow from the subsea wellhead or manifold up within the production riser to the facilities on the buoyant platform.

2. A method as claimed in claim 1 in which there are a plurality of production risers for oil and/or gas within the outer barrier casing of the riser bundle, and these are connected to a plurality of corresponding subsea wellheads.

3. A method as claimed in claim 1 or claim 2 in which there is an inner barrier casing between the production riser(s) for oil and/or gas and the outer barrier casing.

4. A method as claimed in claim 3 in which there is provision for electrohydraulic controls (e.g. for SSSVs or wellhead equipment) or measuring devices (e.g. for temperatures or pressures) within the inner barrier casing, and these controls or measuring devices are connected to the subsea wellheads after installation.

5. A method as claimed in any one of the preceding claims in which the one end of the riser bundle is raised to above the sea surface within a moonpool of the buoyant platform.

6. A method as claimed in any one of the preceding claims in which the pressures within at least some of the compartments are maintained to react only the local external pressure of water at the particular depth of the compartments in their in-place position when they have been installed.

7. A method as claimed in any one of the preceding claims in which the line attached to the one end of the riser bundle is pulled by means on the platform other than the drilling rig used to drill the subsea wells (e.g. by a deck mounted winch).

8. A method as claimed in any one of the preceding claims in which the other end of the riser bundle is manoeuvred over the subsea wellhead by moving the position of the buoyant platform laterally on the sea surface.

9. A method as claimed in claim 8 for installing a plurality of riser bundles related to an existing array of subsea wellheads by moving the buoyant platform laterally on the sea surface over the seabed locations of the wellheads in the array.

10. A method substantially as hereinbefore described with reference to the accompanying drawings.

11. A well production riser bundle for installation by, or when installed according to, any one of the preceding claims.

12. A well production riser bundle as claimed in claim 11 , and comprising a path for a production riser surrounded by and stabilised within an outer barrier casing, which outer barrier casing is itself surrounded by and stabilised within a floatation tubular in generally concentric relationship, and in which the generally annular space between the outer barrier casing and the floatation tubular is divided into discrete lengths by radial bulkheads, so forming a multiplicity of watertight/gastight compartments which are adapted to be pressurised individually.