WO2002068791A1

WO2002068791A1 - Arrangement for connecting a riser to a floating production assembly

Info

Publication number: WO2002068791A1
Application number: PCT/NO2002/000081
Authority: WO
Inventors: Ole Engen; Johnny Salen; Odd Mohus
Original assignee: Fred.Olsen Production As
Priority date: 2001-02-28
Filing date: 2002-02-27
Publication date: 2002-09-06
Also published as: NO20011042D0; NO20011042L; WO2002068791A8

Abstract

An arrangement (1) in a floating production assembly (10), comprising at least one to the production assembly (10) associated Xmas tree (6) that is interconnected via a riser (5) to a wellhead on the sea bed, is disclosed. The production assembly (10) is exposed for relative motions with respect to the riser (5) and the Xmas tree (6). The riser (5) is connected to the floating production assembly (10) via a number of load supporting and compensating working cylinders (2) that continuously transfer the load of the riser (5). The load supporting and compensating working cylinders (2) are inverse mounted, and each cylinder housing (2b) is connected to the riser (5) and each cylinder rod (2a) is connected to the floating production assembly.

Description

Arrangement for connecting a ri ser to a floating production assembly

The present invention relates to an arrangement in a floating production assembly, comprising at least one to the production assembly associated Xmas tree which is interconnected via a riser to a wellhead on the sea bed. The production assembly is exposed for relative motions with respect to the riser and the Xmas tree, and the riser is connected to the floating production assembly via a number of load supporting and compensating working cylinders that continuously transfer the load of the riser.

The present invention is developed in connection with a new design for arranging a "dry" Xmas tree on a floating rig that is used during production of oil and gas. In this context the term "dry" Xmas tree means a Christmas valve tree located above the sea level, i.e. on the rig. The concept is in particular designed for floating production assemblies, like vessels and rigs, that operate on deep waters, often 500 meters or more. During normal conditions, the relative motion between the rig and the Xmas tree oscillates about two meters and can during severe conditions oscillate more than ten meters.

In addition to the motions of the rig due to waves and tides, the riser will be exposed for thermal expansion, contraction and the influence of pressure. This is source to substantial motion between the rig and the riser, both vertically and horizontally.

It is nearly a premise with existing arrangements of this type that they shall be accessible above the sea level for easier maintenance and replacement operations. However, they inherently have the disadvantage that the load supporting and compensating working cylinders, having a substantial longitudinal extension in order to be able to compensate over a range of at least ten meters, are such arranged that they add a substantial construction height between the main deck and the drill floor. This, in turn, results in a high extending derrick.

The presently proposed arrangement will reduce the construction height of the rig, in particular provide for a smaller distance between the main deck and the drill floor of the rig and thus obtain a derrick extending less high even if the height of the derrick remains. This, in turn, contributes to lower the centre of gravity of the rig, which improves the stability of the rig and provides increased variable load capacity for the main deck.

This is, according to the present invention, obtained by an arrangement of the introductorily described type, which is distinguished in that the load supporting and compensating working cylinders are inverse mounted, and that each cylinder housing is connected to the riser and each cylinder rod is connected to the floating production assembly.

Each cylinder housing is preferably connected to the riser a distance well apart from the lower end of the cylinder housing. In order to provide optimum effect, each cylinder housing can be connected to the riser close to the point where the cylinder rod extends out of the cylinder housing.

In order to be able to handle motions, each cylinder rod is preferably articulated connected to the floating production assembly. Likewise, each cylinder housing can be articulated connected to the riser.

In a suitable embodiment the connection between the cylinder housing and the riser can be in form of a tension ring that is rigidly fixed to the riser.

Further, it is common that the risers are guided in level with the main deck by means of embracing devices that attend to the centring of each individual riser, yet with clearance therebetween. Thus this will be a point about which the riser is swinging and may result in a substantial sway of the upper part of the riser including the Xmas tree proper. When the riser/rig move, the horizontal motions of the Xmas tree will consequently increase with the height thereof above the swinging point. When several rows of Xmas trees are arranged side by side, this may easily create severe collisions, or they need to be placed well apart from each other.

In order to resolve the above described problem, i.e. avoid that the Xmas tree is swaying in addition to be able to arrange the Xmas trees pretty close together, the Xmas tree according to the invention comprises a number of guides that cooperate with a corresponding number of guiding elements that extend substantially vertically between the main deck and the drill floor of the production assembly. Suitably a working platform can be arranged around the Xmas tree, preferably with a handrail.

It is not unusual that intervention operations into producing wells need to be performed. This can be necessary to stimulate the production from the well, carry out further drilling of branching wells or to perform downhole logging operations. For such intervening operations the derrick is taken in use and tools are run on a string down through the riser. The derrick is normally transferable back and forth in one main direction along a rail track on the drill floor. Movements laterally to this main direction are normally not possible.

In order to be able to intervene a well by use of the derrick, the upper part of the riser, i.e. the Xmas tree, needs to be brought into the drilling centre of the derrick. This will be performed in a direction extending transversally of the main transfer direction of the derrick. This is obtained, according to the present invention, in that an opening is provided in the main deck through which the riser extends, and this opening is defined by a displaceable frame.

In this embodiment each cylinder rod is suitably fixed to the displaceable frame.

Other and further objects, features and advantages will appear from the following description of one for the time being preferred embodiment of the invention, which is given for the purpose of description, without thereby being limiting, and given in context with the appended drawings where:

Fig.l shows an elevation view of a floating production assembly with an arrangement according to the invention installed,

Fig.2 shows from below and in perspective view an arrangement according to the invention, isolated from the floating production assembly, Fig.3 shows from above and in perspective view an arrangement according to the invention, isolated from the floating production assembly,

Fig.4 shows schematically a principle drawing of the arrangement according to the invention, and

Fig.5 shows schematically a principle drawing of an arrangement according to the prior art.

Firstly reference is given to fig.1 that shows an example of a floating production assembly 10 having pontoons 11, a number of vertical columns 12 and an upwardly extending derrick 15. The floating production assembly 10 includes a main deck 13 and one with respect to the main deck 13 elevated drill floor 14. The derrick 15 is transferable back and forth along a main direction on the drill floor 14, in the shown embodiment transversally of the production assembly 10.

The figure shows two risers 5 that are in communication with a wellhead on the seabed (not shown). It is to be understood that a series of risers 5, e.g. 16, are deployed after each other inwards of the paper plane. Each riser 5 terminates in the upper end thereof in a Xmas tree 6, which is in communication with piping onboard the production assembly 10 via a hose 7. It is to be understood that the respective risers 5 including the Xmas tree 6 remain substantially stationary in the vertical direction, while the floating production assembly 10 follows the motions of the sea.

Fig.2 and 3 show in closer detail how the arrangement 1 according to the invention is installed to the floating production assembly 10. Openings 16 through which the risers 5 pass are provided in the main deck 13. A number of guiding elements 7, here four per riser 5, are tensed between the main deck 13 and the drill floor 14. A corresponding number of guides 8, with which respective guiding elements 7 interact, are associated with each Xmas tree 6. The guiding element 7 can be in form of a wire, stay, rail, etc. The guide 8 can be in form of a sleeve, pulley, sheave, etc. In the illustrated embodiment a platform 9 having handrails, is provided around the Xmas tree 6 for easy access thereto. The respective guides 8 are arranged at the periphery of the platform 9. Thus it is to be understood that during relative motion between the floating production assembly 10 and the Xmas tree 6, the Xmas tree 6 is continuously guided during movements thereof and is not exposed to swaying motions. This because the Xmas tree 6 at any time will coincide with the swaying point of the riser 5 during horizontal motions of the production assembly 10.

A displaceable frame 4 is arranged around each riser 5. Each frame 4 can be displaced back and forth along respective opening 16 in a direction transversally to the transfer direction of the derrick 15. The purpose of this transfer ability is to be able to bring the riser 5 within the drilling centre C in order to enable intervention of the well on the seabed via the riser 5. This is performed by means of the derrick 15, which is transferred into the drilling centre C, and a pipe string (not shown) that is lowered from the derrick 15. After the termination of an intervening operation, the frame 4 jointly with the riser 5 and the Xmas tree 6 are transferred back to the position shown in the figure. It is to be noted that the above described guiding wires 7 normally will be fixed to the frame 4 and need to be disconnected before displacement of the frame 4 can take place.

A number of working cylinders 2, hydraulic or pneumatic, are arranged to the frame 4 and are projecting downwards therefrom. They are interconnected via a tension ring 3, which is rigidly fixed to the riser 5. The tension ring 3 supports the load of the riser 5 and the strains are in turn transferred to the main deck 13 via the working cylinders 2. For the further description reference is given to the principal drawing according to fig.4 compared with the prior art of fig.5, as we recognise it.

As illustrated in fig.4, the working cylinders 2 are inverse mounted (compared to the prior art), i.e. the free end of the cylinder rod 2a is mounted to the main deck 13 and the cylinder housing 2b is in connection with the riser 5, preferably via a tension ring 3. As indicated on the figure, the cylinder rod 2a is mounted to the main deck 13 by an articulation 2c. The cylinder housing 2b is mounted to the tension ring 3 via an articulation 2d. Fig.5 illustrates how the working cylinders are mounted today. When one knows that the working cylinders 2 must be able to compensate for a range spanning more than ten meters, as indicated with the arrow B, this provides large construction height on existing rigs. Should the cylinder housings 2b, when inverted, project into the sea, it would not really matter.

With regard to the prior art, as illustrated on fig.5, it is to be noted that the riser itself here is guided vertically and supported laterally. Thus a swinging point for the entire riser is formed at this location, on fig.5 marked with P. This may easily result in substantial swaying of the Xmas tree when the horizontal motions of the rig are significant.

A normal range of motions for waves and tides can vary between one and five meters and is indicated with the arrow E on fig.4. What is saved in construction height for the drill floor 14 is indicated with the arrow A. The height from the mean sea level to the tension ring 3 is indicated with the arrow D. The corresponding distance with the solution according to the prior art is indicated with the arrow F on fig.5. The height D could be subject to further decrease, but a certain minimum height should always be present between the sea level and the main deck, depending on the waters in which the floating production assembly 10 is to operate.

Claims

P a t e n t c l a i m s

1.

An arrangement (1) in a floating production assembly (10), comprising at least one to the production assembly (10) associated Xmas tree (6) that is interconnected via a riser (5) to a wellhead on the sea bed, the production assembly (10) is exposed for relative motions with respect to the riser (5) and the Xmas tree (6), and the riser (5) is connected to the floating production assembly (10) via a number of load supporting and compensating working cylinders (2) that continuously transfer the load of the riser (5), characterised in that the load supporting and compensating working cylinders (2) are inverse mounted, wherein each cylinder housing (2b) is connected to the riser (5) and each cylinder rod (2a) is connected to the floating production assembly (10).

2. The arrangement according to claim 1, characterised in that each cylinder housing (2b) is connected to the riser (5) in distance from the lower end of the cylinder housing (2b).

3. The arrangement according to claim 2, characterised in that each cylinder housing (2b) is connected to the riser (5) in proximity of the location where the cylinder rod (2a) extends out of the cylinder housing (2b).

4. The arrangement according to claim 1,2 or 3, characterised in that each cylinder rod (2a) is articulated connected to the floating production assembly (10).

5.

The arrangement according to any of the claims 1-4, characterised in that each cylinder housing (2b) is articulated connected to the riser (5).

6.

The arrangement according to any of the claims 1-5, characterised in that the connection between the cylinder housing (2b) and the riser (5) is in form of a tension ring (3) that is rigidly fixed to the riser (5).

7.

The arrangement according to any of the claims 1-6, characterised in that the Xmas tree (6) comprises a number of guides (8) that interact with a corresponding number of guiding elements (7) that extend substantially vertically from the main deck (13) to the drill floor (14) of the production assembly.

8.

The arrangement according to any of the claims 1-7, characterised in that the Xmas tree (6) is surrounded by a working platform (9), preferably with a handrail.

9.

The arrangement according to any of the claims 1-8, characterised in that an opening (16) in the main deck (13) through which the riser (5) passes is defined by a displaceable frame (4).

10.

The arrangement according to claim 9, characterised in that each cylinder rod (2a) is fixed to the displaceable frame (4).