WO2012104309A2

WO2012104309A2 - Production unit for use with dry christmas trees

Info

Publication number: WO2012104309A2
Application number: PCT/EP2012/051580
Authority: WO
Inventors: Jan Vidar Aarsnes; Kåre SYVERTSEN; Otto SKJÅSTAD
Original assignee: Sevan Marine Asa
Priority date: 2011-02-01
Filing date: 2012-01-31
Publication date: 2012-08-09
Also published as: NO20110173A1; WO2012104309A3

Abstract

The invention concerns a floating platform for drilling, production, storage of oil, comprising: a hull / body (1), at least one rigid vertical riser (4), at least one connecting part (6), wherein at least one of the at least one connecting part (6) is connected to one of the at least one riser (4), a moonpool (3) allowing suspension of the at least one riser (4) close to the center of the hull (1) where the influence of rolling and ramming movements is at a minimum, and where the at least one riser (4) and its corresponding connecting part (6) are particularly protected against external forces such as waves, a first deck (11) positioned on top of hull (1), process tools (10) on or above the first deck of the hull (1), at least one flexible tube or line connection (7) positioned between the process tools (10) and the connecting part (6) for the at least one riser (4), and at least on lip ("bilge box") (2) arrange d at or close to the lowest part of the hull (1) to reduce movements that may be triggered by external forces such as waves. The platform is further characterized in that the at least one connection tool (6) arranged between the process tools (10) and at least one of the at least one riser (4) comprises at least one dry Christmas tree (6) allowing for the installation of said riser (4) to be performed above a waterline of the platform, and that the at least one dry Christmas tree (6) is connected to the process tools (10) via the at least one flexible tube or line connection (7).

Description

Production unit for use with dry Christmas trees.

Technical Field:

The present invention concerns floating platforms forming production units for drilling, production and storage of hydrocarbons offshore. The production unit of the invention is suitable for use with vertical production risers connected to dry Christmas trees, i.e. Christmas trees rendering both well intervention and production from the platform possible. The production unit of the invention may, in addition to processing equipment for treatment of the well stream, also contain drilling tools and/or well intervention equipments. Background of the invention

It is common for a floating production unit to complete the production wells using Christmas trees of the type "wet Christmas trees" on the seabed. Furthermore, the wells are connected to the production unit via a pipeline on the seabed and flexible risers, or alternatively risers of for example steel arranged in a chain configuration or other flexible configuration from the seabed and up to the production unit.

To obtain access to the wells for maintenance and intervention, dedicated vessels positioned above the Christmas trees have earlier been employed, and by the use of special tools connection to the wells have been achieved. The fact that the Christmas trees are located on the seabed causes further difficulties during maintenance.

For this type of field development the drilling of the wells must normally be completed prior the initiating the production, and it is complicated and costly to continue drilling and maintaining the wells at a later stage since dedicated vessels must be hired.

As a consequence, if one could position the Christmas trees on the platform in the same way as the case is for steady platforms significant expenditure cuts and increased production could be attained. For a floating platform this is made possible by is possible, for example tension-leg platforms (TLP), SPAR platforms and special SEMI platforms. These units have different limitations as disclosed below.

From the known publically available art, reference is primarily made to a Norwegian patent NO 319971 which discloses an offshore platform of the cylindrical type, mainly flat bottomed, in which the cylinder diameter is significantly larger than the depth of the platform. The centre of buoyancy of the submersed part of the platform is positioned underneath the centre of gravity of the platform. Such a construction has shown to be advantageous in many ways, for example by providing a large capacity both for the loading on deck and storage of oil. The form of the platform results in turning acts depending on weather and wind conditions redundant, and its movement and strains are small compared to other types of floating installations.

A further development of this platform is disclosed in patent application WO2009136799A1 which is particularly suitable for use in water which is exposed to ice. The hull / body of the platform is divided into sections of which the hull diameter in the downward direction decreases, remains constant and increases, respectively. In addition, the platform may have a coupling part having its own buoyancy for connecting / guiding lines, risers and similar parts.

To be able to operate using "dry" Christmas trees from a floating platform, there exist today different solutions. Tension-leg platforms (TLP) are one alternative solution. However, TLPs often have limited storage capacity and water depths would normally be a limitation due to the increasing weights on the tension-legs locking the platforms to the seabed.

Other types of platform which today are operated using dry Christmas trees are SPAR platforms. Their limitations are often related to the size (deck area and limited storage capacity). Furthermore, the SPAR platforms must be mounted and completed in the field, involving costly and complicated offshore operations.

Semi- submersible platforms (SEMI platforms) having increased depth drafts also use dry Christmas trees. A SEMI platform of this type has a well limited dead weight for tools and small storage capacity.

To develop a field having a floating unit suitable for dry Christmas trees, the production and injection wells may either be drill- and well-completed by a mobile rig prior to installing the unit, or the drilling may be performed from the unit that is to be permanently installed.

The object of the invention

The object of the present invention is to provide a robust and cost effective solution for using dry Christmas trees on a floating production platform, which in addition provides a high storage capacity for produced oil (Floating Production, Storage and Offloading, FPSO) compared to the aforementioned prior art.

Summary of the invention

The above mentioned object is achieved with the present invention by utilizing a floating production unit or platform for drilling, production, storage or other applications in accordance with claim 1. Preferred embodiments are defined in the associated dependent claims.

In particular, the invention concerns a floating platform for drilling, production, storage of oil, comprising a hull / body, a moonpool allowing suspension of at least one rigid vertical riser positioned close to the center of the hull where the influence of the rolling and the ramming is at a minimum, and where the at least one riser and its corresponding connection tools are particularly protected against external forces such as waves, a first deck positioned on top of hull, process tools on or above the first deck of the hull, at least one flexible tube or line connection positioned between the process tools and the connection tools for the risers, and at least on lip ("bilge box") arranged at the lowest part of the hull to reduce movements that may be triggered by external forces such as waves.

The platform is further distinguished in that the connection tool arranged between the process tools and at least one of the risers comprises one or more dry Christmas trees allowing for the installation of the risers to be performed above the waterline, and that the dry Christmas tree(s) is/are connected to the process tools via the flexible tube(s) or line connection(s). The arrangement of the riser(s) in a central moonpool results in reduced wave induced movements on the riser(s), which again provides higher protection against external forces such as collisions against ice and vessels.

A preferred embodiment of the present invention concerns designing the hull in a substantially circular cylindrical form. A cylindrical floating platform has good carrying capacity for tools including drilling tools and intervention tools, and may give significant storage possibilities for the production of oil and condensate.

Another preferred embodiment of the present invention concerns the platform further comprising a second deck positioned above the first deck.

Another preferred embodiment of the present invention concerns the moonpool being extended upwards past the first deck, and alternatively the second deck, positioned on top of the hull, within an extension having reduced diameter compared to the lower cylindrical part of the moonpool for liberating space on the first deck and/or the optional second deck.

Another preferred embodiment of the present invention concerns the extension of the moonpool being continued further up above the first deck and/or the optional second deck, in the form of an enclosure to prevent occurrence of an undesired incident in the moonpool area which may influence the tools and the activity on the first and/or the optional second deck. This undesired incident may be undesired accidents such as a gaseous emission, a fire and/or an explosion.

Another preferred embodiment of the present invention concerns the moonpool in its lower end having a conical form with extended diameter directed downwards to allow static and dynamic angular deflections between the direction of the at least one riser and the center axis of the moonpool.

Another preferred embodiment of the present invention concerns the moonpool having an inwards directed structure below the water line when the platform is submerged in water for damping water movements inside the moonpool.

Another preferred embodiment of the present invention concerns the platform further comprising equipments for geostationary positioning depending on weather and wind conditions. Such positioning allows the platform to receive several risers, and allows for the riser(s) to be extended downwards from at least one heave compensating mechanism without the use of revolving segments or swivels.

Another preferred embodiment of the present invention concerns the platform comprising at least one integrated ballast arrangement for ballasting so that the water line of the platform may be set to a certain desired water line sector by filling or emptying one or more ballast tanks.

As aforementioned, the lower part of the hull has at least one protruding "lip" which advantageously constitutes a very marked protrusion from the hull, and hence significantly increases the bottom area of the hull. This again results in an increased damping which contributes to a reduction of the platform movements when, for example, the platform is exposed to waves.

The at least one riser can advantageously be kept under tension. This may be achieved by various methods such as:

employing heave compensation for keeping individual risers under tension, and suspending risers in a common deck that is heave compensated.

Heave compensation may be achieved by inter alia using wire tensioners or tensioners based on direct use of hydraulic cylinders. Both tensioner parts are prior art for drilling riser arrangements. Alternatively, buoyancy elements can be arranged to provide tension in the riser(s). These elements may be installed either on the individual riser, or on a group of several risers. If buoyancy elements are used, a guiding system for the risers must be included. Combinations using tensioners and buoyancy elements are also a possible solution.

The production unit / platform may be completed at the yard or installed in the field ready for initiating the production.

Short summary of the drawings

FIG. 1 is an illustration of a schematic side view of a floating platform having internal risers connected to dry Christmas trees at their top end, wherein the platform has a lower stabilizing platform disc in the form of a lip ("bilge box").

Claims

Detailed description of embodiments of the invention In FIG. 1, a production platform or unit has a cylindrical hull 1 with an approximately circular external form defined by sidewall(s) of the hull 1. A notion 'approximately circular' includes in this case also polygonal designs, such as when the side walls are assembled or formed from a large number of joined plane elements or surfaces all around a perimeter of the hull 1. The hull 1 includes underneath or below a heavily outward protruding platform disc 2, often called lip or "bilge box", and this platform disc 2 contributes to a significant reduction in platform movements when the platform is exposed to waves. A vertical opening, hereinafter referred as "moonpool 3", is located in a center of the hull 1. Risers 4 of the platform are guided vertically from a suspension within the moonpool 3 down to a seabed. The risers 4 are suspended in a heave compensating mechanism 5 and are connected to their respective dry Christmas trees 6 positioned at ends of the risers 4. The Christmas trees 6 are further connected to process tools 10 of the platform on deck via flexible tubes 7 capable of absorbing relative movements arising between the Christmas trees 6 and the process tools 10. In the illustrated embodiment in FIG. 1, the moonpool 3 has a cylindrical form, wherein the form in its lower part may be given an outwardly-directed conical form to achieve a better distance between the riser 4 and the hull 1 in case of heeling of the platform. Above a main deck / a first deck 11, the shown extended vertical side(s) of the moonpool 3 is/are narrowed down in order to increase a space available for tools on the main deck 11 and on an above positioned process deck / the second deck 12. At the top of the platform, within the extension of the moonpool 3, there may, as shown in FIG. 1, be mounted explosion panels 13 for weather protection. To reduce water movements inside the moonpool 3, there is in the exemplary embodiment arranged an inwardly directed structure 14 close to the lower edge of the moonpool 3. P a t e n t c l a i m s

1. A floating platform for drilling, production, storage of oil, comprising:

a hull / body (1),

at least one rigid vertical riser (4),

at least one connecting part (6), wherein at least one of the at least one connecting part (6) is connected to one of the at least one riser (4),

a moonpool (3) allowing suspension of the at least one riser (4) close to a center of the hull (1) where an influence of rolling and ramming movements is at a minimum, and where the at least one riser (4) and its corresponding connecting part (6) are particularly protected against external forces such as waves,

a first deck (11) positioned on top of the hull (1),

process tools (10) on or above the first deck of the hull (1),

at least one flexible tube or line connection (7) positioned between the process tools (10) and the connecting part (6) for the at least one riser (4), and

at least on lip ("bilge box") (2) arranged at or close to a lowest part of the hull (1) to reduce movements that may be triggered by external forces such as waves,

characterized in that

the at least one connecting part (6) arranged between the process tools (10) and at least one of the at least one riser (4) comprises at least one dry Christmas tree (6) allowing for installation of said riser (4) to be performed above a waterline of the platform, and that the at least one dry Christmas tree (6) is connected to the process tools (10) via the at least one flexible tube or line connection (7).

2. A platform in accordance with claim 1, characterized in that the hull (1) has a substantially circular form.

3. A platform in accordance with claim 1 or 2, characterized in that the platform further comprises a second deck (12) positioned above the first deck (11).

4. A platform in accordance with claim 1 or 2, characterized in that the moonpool (3) has an extent upwards past the first deck (11) positioned on top of the hull (1), and within an extension (8) having reduced diameter compared to a lower cylindrical part of the moonpool (3), for liberating space on the first deck (11).

5. A platform in accordance with claim 3, characterized in that the moonpool (3) has an extent upwards past the first deck (11) positioned on top of the hull (1), and within an extension (8) having reduced diameter compared to a lower cylindrical part of the moonpool (3) for liberating space on the first deck (11) and/or the second deck (12).

6. A platform in accordance with claim 4, characterized in that the extension (8) of the moonpool (3) has an extent further up above the first deck (11) in a form of an enclosure (13) for preventing an occurrence of an undesired incident such as a gaseous emission, a fire and/or an explosion from influencing the tools and activities on the first deck (11).

7. A platform in accordance with claim 5, characterized in that the extension (8) of the moonpool (3) has an extent further up above the second deck (11) in a form of an enclosure (13) for preventing an undesired incident such as a gaseous emission, a fire and/or an explosion from influencing the tools and activities on the first deck (11) and/or the second deck (12).

8. A platform in accordance with any one of the preceding claims, characterized in that the moonpool (3) in its lower end has a conical form with an extended diameter in a downward directions for allowing static and dynamic angular deflections between a direction of the at least one riser (4) and the center axis of the moonpool (3).

9. A platform in accordance with any one of the preceding claims, characterized in that the moonpool (3) has an inwards directed structure (14) below the water line when the platform is submerged in water for damping water movements inside the moonpool (3).

10. A platform in accordance with any one of the preceding claims, characterized in that the platform further comprises equipments for geostationary positioning, thereby allowing for the at least one riser (4) to be extended down from at least one heave compensating mechanism (5) without a need to use of revolving segments or swivels.

11. A platform in accordance with any one of the preceding claims, characterized in that the platform further comprises at least one integrated ballast arrangement having ballast tanks for ballasting so that the water line of the platform may be set to a certain desired water line sector by filling or emptying one or more ballast tanks.