WO2002076818A1

WO2002076818A1 - Riser system for use for production of hydrocarbons with a vessel of the epso-type with a dynamic positioning system (dp)

Info

Publication number: WO2002076818A1
Application number: PCT/NO2002/000107
Authority: WO
Inventors: Kåre G. Breivik; Ernst Meyer
Original assignee: Statoil Asa
Priority date: 2001-03-20
Filing date: 2002-03-15
Publication date: 2002-10-03
Also published as: NO20011408D0; NO20011408L; NO313920B1

Abstract

Riser system for use in the production of hydrocarbons with a vessel (1) of the FPSO type having a dynamic positioning system (DP), wherein the vessel is provided with a hoisting means (8) capable of handling the riser system, which riser system during production extends between the vessel and a junction point (13) on the sea bed above a well structure. The riser system is characterized in that it comprises a buoyancy tank (3) which is coupled to the hoisting means in order to be able to be lifted from an immersed operating position to a storage position in the vessel, and vice versa, an upper riser segment (2) comprising a number of flexible risers which during production hang as a catenary between the buoyancy tank and a connecting area in the vessel, the risers being coupled to a process system on the vessel, and a lower rise segment (4) which during production extends between the buoyancy tank and the junction point on the sea bed and having at its upper end fluid communication with the upper rise segment via the buoyancy tank and being at its lower end provided with a coupling head (11) adapted for coupling to the junction point on the sea bed.

Description

Riser system for use for production of hydrocarbons with a wessel of the PPSO-type with a dynamic positioning system (DP) .

The invention relates to a riser system between a floating vessel and a well system with junction points at the sea bed. More particularly, the invention concerns a riser system for use in the production of hydrocarbons with a vessel of the FPSO-type having a dynamic positioning system (DP), where the vessel is provided with a hoist means capable of handling the riser system, which riser system during production extends between the vessel and a junction point at the sea bed above a well structure.

A dynamically positioned (DP) vessel of the type FPSO (Floating Production Storage and Offloading) is particularly useful in connection with the recovery of hydrocarbons from minor fields offshore (for example in test production (EWT), early production (EPS) and tail production), and the main reason being the significant cost and time savings by avoiding investment and installation of an anchoring system secured to the sea bed. Further, a DP FPSO system is especially attractive in deep waters because the costs associated with anchoring systems secured to the sea bed increase with the water depth and because increasing water depth provides a higher degree of freedom of movement to a FPSO.

However, the use of a DP FPSO is dependent upon an appropriate riser system that meets the requirements set by the DP operation. The possibility of rapid and safe disconnecting and subsequent coupling is a requirement connected with DP operations.

As of today, only one larger FBSO unit is available in the market which has been adapted for pure DP operation, viz. the vessel Seillean which currently is operating outside Brazil for the oil company Petrobras. With this unit use is made of only one rigid production riser (drill pipe-riser) consisting of several short sections coupled together. Because of the rigid riser it requires a heave compensating system which as to size and complexity to a great extent corresponds to the equipment required for a drilling ship operating at corresponding water depths. Accordingly, the associated costs are significant.

It is an object of the invention to provide a system of the introductorily mentioned type which is cost efficient, simple and rapid to install, operate and demobilize, which puts relatively low demands to lift compensation and which is particularly suited for smaller hydrocarbon-containing fields offshore, at ocean depths from about 100 m and deeper, which system may be fully handled by means of the equipment on the vessel: To obtain the above-mentioned object a riser system of the stated type has been provided which according to the invention is distinguished in that it comprises a buoyancy tank which also during production is coupled to the hoist means in order to be able to be lifted from a submerged operating position to a storage position in or on the vessel, and vice versa, an upper riser segment comprising a number of flexible risers which during production hang like a catenary between the buoyancy tank and a connecting area in the vessel, the risers being connected to a process system on the vessel, and a lower riser segment which during production extends between the buoyancy tank and the junction point on the sea bed and having at its upper end fluid communication with the upper riser segment via the buoyancy tank and at its lower end being provided with a coupling head adapted for coupling to the junction point on the sea bed.

The invention will be described in more detail in the following by means of working examples with reference to the drawings, wherein

Fig. 1 schematically shows a site elevation of a riser system according to the invention,

Fig. 2 shows the most preferred embodiment of the riser system according to the invention during production mode,

Fig. 3 shows the preferred embodiment of the buoyancy tank according to the invention,

Fig. 4 shows a preferred embodiment of the coupling head according to the invention,

Fig. 5 shows how operation of the system is taking place with the vessel within certain shut off sectors, for the most preferred embodiment according to the invention,

Fig. 6 shows the disconnecting mode of the riser system according to the most preferred embodiment of the invention,

Fig. 7 shows the connecting mode of the riser system according to the most preferred embodiment of the invention,

Fig. 8 shows transit with the riser system according to the most preferred embodiment of the invention.

Reference is made to Fig. 1 which generally illustrates the invention, the riser system clearly appearing with its particular features, and the vessel is illustrated having minimum equipment for being able to handle the riser system and also for obtaining the technical effect of the invention. More particularly, in Fig. 1 a riser system is shown for use in the production of hydrocarbons with a vessel (1) of the FPSO type with a dynamic positioning system (DP), the vessel being provided with a hoist means (8) capable of handling the riser system, the latter during production is extending between the vessel and a junction point on the sea bed above a well structure, and the riser system is characterized in that it comprises a buoyancy tank (3) coupled to the hoist means in order to be able to be lifted from a submerged operating position to a storage position in or on the vessel, and vice versa, an upper riser segment (2) comprising a number of flexible risers which during production hang like a catenary between the buoyancy tank and a junction area in the vessel, the risers being coupled to a process system on the vessel, and a lower riser segment (4) which during production extends between the buoyancy tank and the junction point on the sea bed and which at its upper end has fluid communication with the upper riser segment via the buoyancy tank and which at its lower end is provided with a coupling head (11) adapted for coupling to the junction point on the sea bed.

The riser system is not specifically tied to any certain type of vessel, except that the vessel must have dynamic positioning and be a vessel of the type FPSO with hoist means and other devices in order to be able to handle the riser system. The riser system is based on the above-mentioned use of flexible pipes, possibly with rigid pipes in parts of or in the entire lower riser segment, and a buoyancy tank, according to what has been mentioned above. The possibly rigid pipes will preferably be made of composites due to weight, particularly at larger depths of water. Accordingly, the simplest use of the riser system according to the invention may be by hanging the riser system over the side of the vessel by use of an appropriately dimensioned crane system, as illustrated in Fig. 1, however, a such system is not among the most preferred ones and, accordingly, will here not be further described.

The riser system is particularly useful for a FPSO having 2 moonpooles. The vessel which is most preferred for use together with the riser system is a FPSO of the type "Multi Purpose Shuttle Tanker" (MST) (reference is made to international patent application WO 95/21091) which vessel is a ship which may be supplied with two moonpools, in the middle of the ship and ahead of the tank area respectively, as appears from Figure 2 and discussed below. The decided position of the moonpooles in the axis of rotation and along the longitudinal axis of the ship respectively has been chosen in order to minimize the strain on the riser system both upon rotation of the vessel and by the wave induced movements. The above-mentioned positioning of moonpools and the riser system is considered most advantageous because the strains of the riser system due to weather and movements of the vessel are minimized and the positioning of the ship and riser system is made simpler. It has been found that flexible pipes tend to fail particularly as a result of repeated mechanical stretching and bending, and the above-mentioned positioning of the moonpooles confers great advantage with respect to the working life of the riser system in addition thereto that the handling is most simple in the areas of the ship where the movements are smallest. The forward moonpool has a cylindrical shape with a diameter of about 15 m. The midship moonpool is rectangular having as dimensions a length of about 20 m and a width of about 12 m. Both moonpools are preferably inclined in the lower part in order to minimize conflicts during movement of the risers.

According to the most preferred embodiment of the invention use is made of the ship which is considered best suited for operation with the riser system according to the invention, and in the riser system further means are present which increase the usefulness and the technical effect.

In this connection, reference is made to Fig. 2 which shows the most preferred riser system according to the invention, in production mode, the vessel of the type DP FPSO being a ship (1) provided with equipment to ensure safe and exact positioning at significant wave height of at least 7 m, and in both ends the ship is provided with bow which can be oriented against the direction of the weather, and, further the ship is provided with a forward moonpool (6) centrally placed in the axis of rotation of the ship, and a moonpool (5) at the aft which is placed in the centre line of the ship, a heave compensated hoist means (8) which via a wire (14) is tightly coupled to a buoyancy tank (3) during production, which buoyancy tank during production is attempted to be held below the forward moonpool and thereby in the axis of rotation of the ship.

The riser system further comprises an upper riser segment (2) which at the upper end, via the aft moonpool and with flange connections, is coupled to a process plant in the ship, and which at its lower end is coupled to the buoyancy tank by means of one or more "in-line" swivels, a lower riser segment (4) with a number of flexible pipes corresponding to the number in the upper riser segment, the buoyancy tank during production being coupled between the upper and the lower riser segment with fluid communication through an adapted number of pipes (16) in the buoyancy tank, wherein the lower riser segment in the lower end is provided with a coupling head (11) and a high pressure swivel (12) with electric or hydraulic driving motor for rotation when exceeding predetermined limiting values for rotation of the ship, for coupling to the junction point on the sea bed.

Further, the riser system comprises bending restricting means in the form of bending stays (7) and framework arranged near the buoyancy tank and the coupling head, at least one umbilical (10), and a swivel (9) located above the buoyancy tank.

The in-line swivels may alternatively be arranged at the side of the vessel in the upper riser segment. The high pressure swivel is particularly advantageous for use in the smallest water depths.

The forward moonpool (6) is provided with support and locking means for storing and maintaining the buoyancy tank in a stored position therein, and, further, it is provided with working platforms for use by personnel in connection with said storing and later release of the buoyancy tank.

It is remarked that the detailed layout of the riser system and the choice of vessel may vary based on economic, operative and technical considerations. For example, at favourable weather and current conditions and when producing from marginal hydrocarbon fields it may be preferred to use a more simple version of the riser system and the vessel, such that the most advantageous embodiment of the invention under certain conditions may be equal to or more close to the embodiment shown in Fig. 1, in preference to the embodiment shown in Fig. 2. It is considered to be within the competence of persons of skill in the art to consider simplified embodiments taking into consideration the variables which have been mentioned in the description, in order to thereby decide which is the most advantageous embodiment under given conditions within the scope of the invention.

Some advantageous features of the riser system according to the invention are as follows, referring to Figures 2, 3 and 4.

The buoyancy tank (3) has cylindrical shape and is divided into sections so that it can be ballasted and buoyancy adjusted according to demand.

Further, the buoyancy tank (3) is provided with a number of vertically extending tunnel pipes (15) in suitable number and dimensioned for the passing through of risers, umbilicals and wires.

During production the buoyancy tank (3) is coupled to the hoist means via a wire (14). As an additional function the angle of the wire (14) in the vertical plane will represent a position reference for the DP-system of the vessel because the wire extends through the forward moonpool.

In the lower end of the buoyancy tank there is preferably arranged a protective construction (17) which surrounds the upper part of the lower riser segment in order to protect this part when the vessel turns and the upper riser segment according to the existing operating conditions possibly arranges itself in spiral form around said part.

The coupling head (11) comprises a separate shut off valve (19) for each riser, and by-pass flow connections (20) with associated isolation valve are arranged between the nearest risers.

A high pressure swivel (12) is arranged between the coupling head and the junction point on the sea bed, the high pressure swivel being provided with a driving motor for rotating the coupling head upon demand. The upper riser segment (2) preferably comprises 2 to 4 flexible pipes which when the system is in operation hang like a catenary between the buoyancy tank (3) and the aft moonpool (5) and are oriented and spaced apart in the vertical plane with a certain least mutual distance or longer in order to avoid conflict during movement.

The lower riser segment (4) preferably comprises 2 to 4 flexible risers and/or rigid pipes of composite material flanged together. Replacement of flexible pipes with rigid pipes of composite material completely or partly in the lower riser segment can prove technically and economically advantageous at very large sea depths, for instance deeper than 1500 m.

The most typical number of risers will be from 2 to 4, however, the number may both be lower or higher.

The junction point (13) on the sea bed comprises locking mechanisms (21) and shut off valves (19) which may be controlled by means of controlling signals sent from the control room of the vessel, and, moreover, they are controllable by means of a ROV.

The arrangement of the buoyancy tank (3) relative to the lower riser segment (4) enables the FPSO to rotate typically 180° in each direction without overloading the risers. No other known system allows a corresponding flexibility for several parallel risers.

The heave compensating system may be arranged to function both against the wire (14) and during the introduction of the upper riser segment (2), which may be advantageous in order to minimize the strains on the riser system.

During production with the riser system it may be advantageous to restrict the buoyancy in the buoyancy tank (3) by partly transferring the weight to the lift compensated hoisting means (8) via the wire (14).

The by-pass flow connections (20) arranged in the coupling head are used for emptying/flushing the risers in connection with disconnection, for instance after having disconnected the risers from the junction point on the sea bed. The by-pass flow connections may for example also be arranged for the injection of methanol, or for other purposes.

Referring to the most preferred embodiment of the riser system according to the invention, and the specific FPSO, some conditions in connection with normal operation, disconnection, coupling and installation will be discussed.

In Fig. 2 normal operation is illustrated, whereby the FPSO is dynamically positioned and rotates about the buoyancy tank (3) against the prevailing wind direction. When maximum angular rotation has been reached and it is required to further change the heading, the FPSO may use the DP system when rotating back in order to assume heading from the opposite angle. The risers are mutually oriented in the length direction of the vessel and have been optimally placed in relation to the structure of the moonpool during rolling movements. As concerns the position of the vessel during operation, reference is made to Fig. 5 which illustrates operation of the system within certain shut down sectors. As appears from Fig. 5, the zones have been divided in accordance with positioning on the sea surface in a circle about the nominal position N for the vessel, and when rotating about this point, whereby a secure zone S occurs. The reference I signifies shut down class 1 which is shut down according to the lowest level of danger, whereby the system will be made ready for disconnection should exceeding occur. This involves inter alia that the process on the vessel is shut down and that the isolation valves against the well system are closed. Shut down class I occurs according to Fig. 5 when position of the vessel is outside the circle defined by means of I or when the rotation of the ship is greater than the defined limits indicated by means of I.

Shut down class II may occur if the dangerous situation develops further, in that the FPSO moves outside the circle designated II or outside the rotation limit indicated by II. Upon a shut down class II the FPSO with associated production system will be prepared for a very rapid (about 20-30 seconds) disconnecting of the coupling head (11) from the sea bed and well installation. The locking mechanism (21) is hydraulically released controlled from the control room of the vessel.

The positive buoyancy in the buoyancy tank (3), possibly in combination with lifting from the hoisting means (8), lifts the coupling unit (11) from the construction (13) on the sea bed until the buoyancy balances the weight of the released coupling head and increased proportion by weight which is transferred from the upper riser segment (2) to the buoyancy tank (3) as it rises towards the surface. The hoisting means (8) is then used. In Figure 6 the disconnection is illustrated. An important property of the riser system is that it is unnecessary to empty the risers before disconnection because such operation may be carried out later, for example by circulating diesel from the FPSO via the other risers after the disconnection, via the by-pass flow connections (20). More precisely, the buoyancy tank (3) may be taken up in the forward moonpool and be secured therein, whereupon the risers for example are taken on to drums (23) of sufficient diameter onboard the vessel, whereby the risers slide through the tunnel pipes of the buoyancy tank.

The connecting of the riser system is illustrated in Fig. 7. The vessel is kept in position by means of dynamic positioning so that the coupling head (11) is kept at short distance above the junction point on the sea bed (13). The buoyancy tank (3) is filled with ballast until weak negative buoyancy. This may for example take place in that a ROV (remotely operated vehicle) opens a valve and thereby lets in a controlled amount of water into the tank. With the buoyancy tank (3) hanging via lifting straps and the wire (14) in the heave compensated hoisting means (8) the coupling head (11) may in controlled manner be guided down and secured to the junction point on the sea bed (13). The junction point on the sea bed has guide pins which assure exact positioning and connecting, for example under assistance of a ROV in order to guide the coupling head laterally when carrying out the connecting.

The FPSO is supplied with all necessary equipment to carry out mobilization/demobilization of the riser system in connection with new commissions. Thereby an integrated system has been provided for floating production, storage and offloading in connection with the flexible riser system which can be mobilized/demobilized in the cause of 1-2 days, whereby the system is particularly attractive in connection with operations of short duration.

When the FPSO is moved from one field to another the buoyancy tank (3) will be stored and secured in the forward moonpool, with the coupling head (11), as illustrated in Fig. 8, wherein it is also illustrated that the risers are wound up on drums (23).

The riser system can be mobilized/demobilized in many ways, and the system comprises further means which have not been specifically described or mentioned, however, which are considered to be obvious to people of the skill in the art. Here should particular mention be made of wires and hoisting and handling means between the various components of the system and between the front and aft moonpool. It is considered technically obvious to adapt such well known further means to the system as required.

Some important features in connection with suitable installation are as follows:

By installation, with the coupling head and the buoyancy tank placed in the front moonpool, it is purposeful to lower the lower riser segment and the coupling head, with wire, umbilical etc., connected while the buoyancy tank still is present in the front moonpool. Bending stays/framework against the lower riser segment are arranged on the buoyancy tank while it is in the front moonpool. The upper riser segment is suitably installed while the buoyancy tank is still stored in the front moonpool, by means of wire connections between front moonpool and aft moonpool outside the hull of the vessel. The risers in the upper riser segment are drawn into the aft moonpool and connected by means of flange connection against the process plant in the vessel. It is first when all the risers have been lowered down through the buoyancy tank, the riser segments have been connected via the buoyancy tank and the bending restrictors have been mounted on, that the buoyancy tank is lowered. The connection against the junction points on the sea bed may advantageously be undertaken by making use of a ROV of the working type, i.e. with sufficient power to move the coupling head laterally and sufficient manipulator means to serve the connecting means in the coupling head and the ballast adjustment means in the buoyancy tank. It is advantageous if the adjustment of the ballast in the buoyancy tank and the connecting means against the junction point on the sea bed may both be operated remotely from the control room of the vessel and by means of ROV. A summary of significant features of the riser system according to the invention, and in particular the most preferred embodiment thereof, is as follows:

• The only system which as of today is based on flexible risers in combination with pure DP operation and a larger FPSO.

• Several risers in parallel.

• Simple and efficient disconnection possible in case of positioning problems.

• After disconnection it is possible to bleed off the pressure in the riser system so that subsequent coupling may be carried out simple and efficient.

• May be used both in shallow and deep water.

• Allows the FPSO to rotate until ± 180° without the use of swivel.

• The system is adapted to installation of high pressure swivel on the sea bed when there is demand for larger operational flexibility with respect to rotation than ± 180° and/or when operating in less deep waters, down to about 100 m.

• Self aided installation/deinstallation, whereby the system is completely integrated.

• The system renders full flexibility to the field operators with respect to location of wells.

• By means of the most preferred FPSO the mutual location of the risers is optimum with respect to the structure of the moonpools and the risk for touching when the vessel is rolling.

• Use of high pressure swivel on the sea bed, according to the most preferred embodiment, is a technology which has previously not been used.

Claims

81600-EHClaims

1. Riser system for use in the production of hydrocarbons with a vessel (1) of the FPSO type with a dynamic positioning system (DP), wherein the vessel is provided with a hoisting means (8) capable of handling the riser system, which riser system during production extends between the vessel and a junction point (13) on the sea bed above a well structure, characterized in that it comprises a buoyancy tank (3) which also during production is coupled to the hoisting means in order to be able to be lifted from an immersed operating position to a storage position in or on the vessel, and vice versa, an upper riser segment (2) comprising a number of flexible risers which during production hang as a catenary between the buoyancy tank and a connecting area in the vessel, the risers being coupled to a process system on the vessel, and a lower riser segment (4) which during production extends between the buoyancy tank and the junction point on the sea bed and having at its upper end fluid communication with the upper riser segment via the buoyancy tank and at its lower end being provided with a coupling head (11) adapted for coupling to the junction point on the sea bed.

2. Riser system according to claim 1, characterized in that the vessel of the type DP FPSO is a ship (1) having equipment for providing secure and exact positioning at significant wave height of at least 7 m and being at both ends provided with bow which can be oriented against the direction of the weather, and the ship being further provided with a front moonpool (6) centrally placed in the axis of rotation of the ship, an aft moonpool (5) placed in the centre line of the ship, a heave compensated hoisting means (8) which via a wire (14) during production is tightly connected against a buoyancy tank (3), the buoyancy tank during production being sought to be held under the front moon pool and thereby in the axis of rotation of the ship, the riser system further comprises an upper riser segment (2) which in its upper end, through aft moonpool and with flange connections, is coupled against a process plant in the ship and in its lower end, with one or more "in-line" swivels, is coupled against the buoyancy tank, a lower riser segment (4) with a number of flexible pipes corresponding to the number in the upper riser segment, the buoyancy tank during production being connected between the upper and lower riser segment with fluid communication through an adapted number of pipes (16) in the buoyancy tank, wherein the lower riser segment in its lower end is provided with a coupling head (11) and a high pressure swivel (12) with electric or hydraulic driving motor for rotation when exceeding predetermined limiting values for rotation of the ship, for coupling to the junction point on the sea bed, and further the riser system comprises bending restrictors in the form of bending stays (7) and framework arranged near the buoyancy tank and the coupling head, at least one umbilical (10) and a swivel (9) located above the buoyancy tank.

3. Riser system according to claim 1 or 2, characterized in that the buoyancy tank (3) has cylindrical shape and is divided into sections so that it may be ballasted and buoyancy adjusted according to demand.

4. Riser system according to claim 1, 2 or 3, characterized in that the buoyancy tank (3) is provided with a number of vertical extending tunnel pipes (15) in suitable number and dimensioned for the passing through risers, umbilicals and wires.

5. Riser system according to claim 2, characterized in that the front moonpool (6) is provided with support and locking means for storing and keeping the buoyancy tank in a stored position therein, and further is provided with work platforms for use by personnel in connection with said storage and later release of the buoyancy tank.

6. Riser system according to one of claims 1, 2, 3 or 4, characterized in that during production the buoyancy tank (3) is coupled to the hoisting means via a wire (14) which also is a position reference for the DP system of the vessel, the wire extending through the front moonpool.

7. Riser system according to claim 1, 2, 3, 4, or 6, characterized in that at the lower end of the buoyancy tank a protective construction (17) is arranged which surrounds the upper part of the lower riser segment, in order to protect this part when the vessel rotates and the upper riser segment takes on spiral form around said part.

8. Riser system according to claim 1 or 2, characterized in that the coupling head (11) comprises a separate closing valve (19) for each riser, and by-pass flow connections (20) with associated isolation valve being arranged between adjacent risers.

9. Riser system according to claim 1 or 2, characterized in that it comprises a high pressure swivel (12) placed between the coupling head and the junction point on the sea bed, the high pressure swivel being provided with a driving motor for rotation of the coupling head according to demand.

10. Riser system according to claim 1 or 2, characterized in that the upper riser segment (2) comprises 2 to 4 flexible risers which during operation of the system hang as a catenary between the buoyancy tank (3) and the aft moonpool (5) and being oriented and spaced apart in a vertical plane with a certain least mutual distance or longer in order to avoid conflict during movement.

11. Riser system according to claim 1 or 2, characterized in that the lower riser segment (4) comprises 2 to 4 flexible risers and/or rigid pipes of composite material flanged together.

12. Riser system according to claim 1, 2 or 8, characterized in that a junction point (13) on the sea bed comprises locking mechanisms (21) and shutting off valves (19) which are controllable by means of control signals given from the control room of the vessel and, moreover, are controllable by means of a ROV.