WO1997030265A1

WO1997030265A1 - Offshore production piping and method for laying same

Info

Publication number: WO1997030265A1
Application number: PCT/NO1997/000039
Authority: WO
Inventors: Knut Von Trepka; Per Olav Halle
Original assignee: Kværner Oilfield Products A.S
Priority date: 1996-02-14
Filing date: 1997-02-11
Publication date: 1997-08-21
Also published as: NO319277B1; AU1814497A; NO960581D0; NO983668L; NO960581L; BR9707536A; NO983668D0

Abstract

An offshore production tubing system wherein the production tubing extends between at least one Christmas tree (5) on the seabed (11) and a vessel (8) floating on the surface (12). The vessel (8) is capable of limited movement in all directions in the horizontal plane within a defined area, the production tubing is divided into two sections: a section (1) lying on the seabed and emerging from a Christmas tree (5) and a section (2) which is 'suspended' in the water and terminates in the vessel (8). The bottom section (1) and the suspended section (2) are connected at one point (4) and at some distance from the seabed. Either the bottom section (1) or the suspended section (2) close to the connection point (4) describes an elbow bend (3) and the suspended section (2) is compliant anchored to the seabed (11) in the vicinity of the connection point (4), the pipes thereby being held under tensile load.

Description

OFFSHORE PRODUCTION PIPING AND METHOD FOR LAYING SAME

The present invention relates to an offshore production tubing system where the production tubing runs between at least one wellhead Christmas tree on the seabed and a vessel floating on the surface, which vessel is capable of limited movement in all directions in the horizontal plane within a defined area, the production tubing is divided into two sections: one section lying on the seabed and emerging from a Christmas tree, and one section which is "suspended" in the water and terminates in the vessel.

The invention also relates to a method for laying production tubing between a Christmas tree on the seabed and a surface vessel which, after the tubing has been laid, is capable of limited movement in all directions in the horizontal plane, which production tubing is divided into two sections: a bottom section and a suspended section.

The present production tubing system has been developed for floating production units and forms a connection between a Christmas tree on the seabed and a floating vessel. The use of floating production units is ever-increasing because of their cost efficiency, especially in deeper waters. It is increasingly common for the floating production units to have capacity both for drilling and production. This means that as soon as a well has been drilled and completed for production, production can take place through a first riser whilst well no. 2 is drilled and so forth. Thus, an early production start is achieved, and the capacity increases as new wells are drilled.

In the case of floating production systems, special riser constructions are used so as to allow the floating vessel to move to all the well sites for normal operations and to move to the side during an emergency situation, should one arise. The riser systems commonly used are flexible risers made of steel-reinforced thermoplastic pipes. These risers have a limited useful life and are extremely expensive. It is difficult to use flexible risers, in particular in the case of oilfields with high temperature and high pressures. The use of dynamic metal risers has therefore been considered for the next generation of offshore oilfields.

In the case of solutions which make use of flexible risers running down to Christmas trees, a lazy wave or steep S -configuration will usually be used. The steep S has a riser base with a horizontal connection to the static flowline and a vertical connection to the riser. The lazy wave configuration can be installed in one length by using flanges. The riser base in this solution is a block weight which is anchored to a 90° bend. The disadvantages of this steep S-configuration are extra installation time in order to carry out two pipeline pull-ins, extra equipment on the riser base and finally the fact that the vertical riser has a fixed support. This last-mentioned point means that high bending moments must be taken close to the vertical connection point.

The use of rigid metal risers laid in wave configuration instead of flexible risers raises several new problems. These problems are:

(1) Installation of risers. The installation can be carried out by a vessel using reels or a carousel, where the riser can be welded on site, or it can be towed out in a continuous length. The 90° bend causes problems for these solutions which result in a need for a subsea connection.

(2) Attainment of necessary flexibility for the movement of the vessel .

It becomes much more important to have a good riser design because the bend radius is much more critical for metal risers than for flexible risers. Risers having a lazy wave configuration are best suited for metal risers, since the 90° bend is a "hinge point" and bending moments are taken up by a large part of the horizontal flowline during movement of the rig.

Insofar as the installation of risers is concerned, there are several major factors which must be taken into consideration with a view to choosing the installation method. Firstly, it is important that the vessels are independent of one another during installation. Furthermore, it is important that individual components, such as the Christmas tree, the riser or the flowline, can be replaced or retrieved easily whilst adjacent wells continue to produce. In addition, the layout on the seabed is important and an installation method must be used which allows the Christmas trees and the risers or flowlines to be located close to one another.

Here, the pull-in method that will be used for connection on the seabed must also be taken into consideration.

As can be seen from this description, it is important that the system allows the possibility of replacing individual modules or pipe sections without disturbing the others. To retrieve the Christmas tree it is essential that the riser generates tension backwards in the flowline and assists in the pulling out thereof from the Christmas tree. In accordance with the present invention, a production tubing system of the type mentioned above is provided, which is characterised in that the bottom section and the suspended section are connected at one point and at some distance from the seabed, that either the bottom section or the suspended section close to the connection point describes an elbow, and that the suspended section is compliant anchored to the seabed in the vicinity of the connection point, the pipes thereby being held under tensile load.

It would be of advantage for the point of application of the tubing anchoring on the tubing to be some distance from the said elbow bend, for example 1 to 20 meters.

In order to prevent the flowline from becoming stuck in the seabed mud it is proposed to have a protective sliding sleeve of a plastic material encasing the pipe when the flowline is laid. The sliding sleeve ensures enhanced heat insulation, provides mechanical protection and maintains low friction between the flowline and the seabed.

Furthermore, it will be able to take up any torque along the entire length of the flowline. The pipes may to advantage be non-flexible and be made of titanium, steel or stainless steel (Duplex).

In accordance with the present invention a method of the type mentioned above is also provided which is characterised in that the bottom section is laid on the seabed substantially rectilinearly and is secured at one end thereof to the Christmas tree, that the suspended section is laid in a catenary form and floating with the aid of floats in the water and is secured at one end thereof to the vessel, whilst at its other end it is secured non-rigidly to a seabed anchor, and with the pipe end at some distance from the seabed, and that the pipe ends are pulled towards each other and connected at said distance from the seabed, the compliant of the seabed anchor and the elasticity of the rigid pipes being utilised to allow this without the introduction of spot loads in the pipes, and also to keep these under tension.

Expediently, the pipe ends are pulled towards one another by means of a pull-in tool which is put in place by an ROV.

The pull-in tool can be connected to the end of the suspended section, whereupon a line is positioned and secured with the aid of an ROV to the end of the bottom section before the pull-in tool is put into operation and pulls the pipes towards one another and carries out the connection. As a practical measure, it would be advantageous to lay the suspended section before the bottom section.

The method can be used to replace only the bottom section in a production tubing system, where the pipes are disconnected and the end of the bottom section is allowed to fall in a controlled manner towards the seabed, whereupon the other end of the bottom section is detached from the Christmas tree and removed, and a replacement bottom section is installed according to the previously disclosed method.

A method is also provided for replacing only the suspended section in a production tubing system, where the pipe ends are disconnected and the end of the bottom section is allowed to fall in a controlled manner towards the seabed, whereupon the other end of the suspended section is detached from the vessel and removed, and a replacement suspended section is installed according to the previously disclosed method.

Other and additional objectives, features and advantages are set forth in the following description of, for the present, preferred embodiments of the invention, which are disclosed for the purposes of description, without thereby being limiting, and disclosed in connection with the appended drawings, wherein:

Fig. 1 shows the production tubing system according to the invention during deployment between a Christmas tree on the seabed and a floating vessel;

Fig. 1A shows the actual connection point between the bottom pipe section and suspended pipe section according to Fig. 1 in more detail;

Fig. 2 shows the production tubing system ready connected;

Fig. 2A shows in more detail the elbow portion of the suspended section at the connection point.

Fig. 1 shows a floating drilling and production vessel 8 floating on the water surface 12 above a production Christmas tree 5 which is set down in the seabed 11. Between the Christmas tree 5 and the vessel 8 there runs a production tubing system consisting of bottom pipe section 1, often called a flowline, and a suspended pipe section 2, often called the riser, which pipe sections are connected 4 in an area which lies just above the seabed 1 1. It would be advantageous to lay the bottom section 1 substantially rectilinearly from the Christmas tree 5, in contrast to the previously commonly used S-configuration. The suspended pipe section 2 floats in the water and describes an approximate sine shape, or an inverted S configuration between the seabed 1 1 and the floating vessel 8. The lower end of the suspended section 2 is connected to the bottom section 1 by means of a pull- in tool 6 and an ROV. The ROV is controlled from a surface vessel 10 which is in communication with the ROV via a cable 15.

As can be seen from Fig. 1 A, the pipe ends are drawn together by means of a pull-in tool 6 which is described in detail in Norwegian Patent Application No. NO-920518. A more detailed discussion of this tool will not be given here, apart from mentioning its capacity to pull in a pipe cable towards the tool 6 and connect two adjacent pipe ends. The tool 6 is transported and manoeuvred into place with the aid of a remote-operated vehicle (ROV) which is of a commonly known type, and will not be described in more detail here.

As also can be seen from Fig. 1 A, the suspended section 2 in its lower end describes an elbow 3 which expediently can form about 90°. The end of the suspended section 2 is anchored to the seabed 11 by means of a block weight 7 and a mooring line 13 which has a point of application 9 to the suspended section 2. This type of anchoring gives the end portion 4 of the suspended section 2 freedom of movement in substantially the horizontal direction. Furthermore, through certain positioning it will help to orient the end flange of the suspended section 2 to align with or point towards the pipe end which is pulled in. This entails that initially it points in the direction of the Christmas tree 5. It will thus be understood that the elbow 3, together with the anchoring 7, 9, 13, is both orienting and gives the pipe end 4 freedom of movement.

Figure 2, like Fig. 1, shows a production tubing system between a Christmas tree 5 on the seabed and a production vessel 8 on the sea surface. The figure is a schematic illustration of two different conditions: one where a suspended section 2 is transporting gas and a second one where the suspended section 2 is filled with water or oil. In such a situation, it would be advantageous to equip the suspended section with floats 2' to contribute to increased buoyancy in the suspended section 2, at least along parts thereof.

The connection point 4 is shown in more detail in Fig. 2A. I.e., it is the end of the suspended section 2 that is shown, and its orientation. The figure shows an embodiment of the anchoring that is different from that shown in Fig. 1 A. The anchoring consists, as before, of a block weight T and a mooring line 13" and a point of application 9'. The point of application 9' has, however, been moved some way up along the vertical part of the suspended section 2, and the distance from the elbow bend 3 to the point of application 9' can, for example, vary from 1 to 20 meters. This helps to further increase the flexibility in the end 4 of the pipe section 2. It should also be pointed out that this design of the suspended section and the anchoring helps to hold the bottom section 1 under tension. This tension is maintained even if the production platform 8 moves on the sea surface 12 in any direction whatsoever within a defined, limited area.

As mentioned, the proposed method sequence is based on the use of a pull-in system including a pull-in tool 6 which is put in place using an ROV. The pull-in tool 6 and the ROV are used both to pull in the bottom section 1 towards the Christmas tree 5 and to pull together the opposite end of the bottom section 1 and the lower end of the suspended section 2. The flowline 1 can be equipped with protecting caps to prevent the penetration of seawater. The caps may have ROV-operated filling ports which allow filling with sea water prior to the pull-in. Rotational orientation of the Christmas tree is necessary within an exactitude of about ± 1°. Different solutions are possible, such as a position indicator in relation to the downhaul wire of the flowline 1. Pull-in is carried out by using the ROV-operated pull-in system in the direction of the Christmas tree. It is important that the riser has sufficient flexibility to allow the pull-in at the Christmas tree end to be about 1 meter. The pull-in can be carried out using the ROV-operated pull-in system between the horizontal bottom section and the suspended section. The bottom section is lifted up whilst the end of the suspended section is forced horizontally until the connection members are brought together and connection with a clamp connector is carried out. The emptying of water and inner pressure testing are carried out. Production can then start.

The system is dependent upon a number of functional requirements for the riser system. In case of detachment of the Christmas tree, the suspended section must take up necessary kick of about 450 mm. This kick at the Christmas tree should not cause pressure in the bottom pipeline 1. The riser system should create positive tension in the static lines also during detachment. In the event of replacement of a suspended section, the connection between the bottom section and the suspended section should be detachable. This is possible without disengagement between the bottom section 1 and the Christmas tree. In the event that the bottom section is to be replaced, the two connections must be disengaged without retrieving or damaging the suspended section.

Claims

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

1.

An offshore production tubing system wherein the production tubing extends between at least one Christmas tree (5) on the seabed (11) and a vessel (8) floating on the surface (12), which vessel (8) is capable of limited movement in all directions in the horizontal plane within a defined area, the production tubing is divided into two sections: a section (1) lying on the seabed and emerging from a Christmas tree (5) and a section (2) which is "suspended" in the water and terminates in the vessel (8), characterised in that the bottom section (1) and the suspended section (2) are connected at one point (4) and at some distance from the seabed, that either the bottom section (1) or the suspended section (2) close to the connection point (4) describes an elbow bend (3) and that the suspended section (2) is compliant anchored to the seabed (11) in the vicinity of the connection point (4), the pipes thereby being held under tensile load.

2.

The production tubing system according to claim 1 , characterised in that the point of application (9) of the tubing anchoring on the tubing is at some distance, such as 1-20 meters, from said elbow bend (3) (Fig. 2A).

3.

The production tubing system according to claim 1 or 2, characterised in that the bottom section (1) is laid in a sleeve with a view to allowing some axial movement of the bottom section (1) and also to take up any torque along the entire length of the bottom section.

4.

The production tubing system according to claim 1, 2 or 3, characterised in that the pipes are made of titanium, steel or stainless steel (Duplex).

5.

A method of laying production tubing between a Christmas tree (5) on the seabed and a surface-floating vessel (8) which, after the tubing has been laid, is capable of limited movement in all directions in the horizontal plane, which production tubing is divided into two sections: a bottom section (1) and a suspended section (2), characterised in that the bottom section (1) is laid on the seabed substantially rectilinearly and is secured at one of its ends to the Christmas tree (5), that the suspended section (2) is laid in a catenary form and floating with the aid of floats in the water and is secured at its one end to the vessel whilst at its other end it is secured compliant to a seabed anchor (7), and with the pipe end at some distance from the seabed, and that the pipe ends are pulled towards each other and connected at said distance from the seabed, the compliant of the seabed anchoring and the elasticity of the rigid pipes being utilised in order to allow this without the introduction of spot loads in the pipes, and to hold these under tension.

6. The method according to claim 5, characterised in that the pipe ends are pulled towards each other with the aid of a pull-in tool which is brought into place by an ROV (6).

7.

The method according to claim 6, characterised in that the pull-in tool is connected to the end of the suspended section, whereupon a line is brought into place and secured by means of an ROV (6) to the end of the bottom section before the pull-in tool is put into operation and pulls the pipe ends towards each other and carries out the connection.

8. The method according to claim 5, characterised in that the suspended section (2) is laid before the bottom section (1).

9.

A method for replacing the bottom section of a production tubing system according to claims 1-4, characterised in that the pipe ends are disconnected and the end of the bottom section is allowed to fall controllably towards the seabed, whereupon the other end of the bottom section is detached from the Christmas tree and removed, and a replacement bottom section is installed following the method according to one of claims 5-7.

10.

A method for replacing the suspended section of a production tubing system according to claims 1-4, characterised in that the pipe ends are disconnected and the end of the bottom section is allowed to fall controllably towards the seabed, whereupon the other end of the suspended section is detached from the vessel and removed, and a replacement suspended section is installed following the method according to one of claims 5-7.