WO2006006852A1

WO2006006852A1 - Method and device for connecting a riser to a target structure

Info

Publication number: WO2006006852A1
Application number: PCT/NL2005/000493
Authority: WO
Inventors: Peter Salome; Robert Reinder Narold; Robert Paul Hovinga; Ton Coppens
Original assignee: Heerema Marine Contractors Nederland B.V.
Priority date: 2004-07-12
Filing date: 2005-07-08
Publication date: 2006-01-19
Also published as: US20080089745A1

Abstract

The present invention relates to a method for connecting a free end of an at least partially submerged riser to a target structure, the method comprising: (a) connecting the riser by the free end thereof to a riser support assembly; (b) positioning the target structure near the riser support assembly; (c) connecting a lifting assembly, with the free end (24) of the riser; (d) moving the riser by the free end thereof from the riser support assembly toward the target structure by the lifting assembly; and (e) connecting the riser with the target structure. The present invention also relates to a riser support assembly for connecting a riser by a free end thereof with a target structure.

Description

Method and device for connecting a riser to a target structure

FIELD OF THE INVENTION

The present invention relates to a method and device for connecting a riser to a target structure. Risers are widely applied in the offshore industry in oil and/or gas fields at sea. An oil and/or gas well is located at a seabed level. The product, e.g. oil and/or gas, is produced by the well and is transported to a structure at the water level. From the structure, the product is transported further on, for instance by means of another pipeline to a shore. The product may also be temporarily stored in a storage facility close to, or on the structure. In order to transport the product to the structure, the well is connected to the structure by a pipeline for conveying the product to the structure.

During the installation of oil and/or gas field, a pipeline is laid connecting the well with the structure located at the water surface. Such a pipeline usually has a first part which rests on the seabed, and a second part which rises from the seabed towards the structure at the water level. The second part of the pipeline is known in the field of the art as a "riser".

Typically, the riser follows a curved trajectory from the seabed towards the riser support assembly. At the seabed level, the riser has a substantially horizontal orientation. Near the riser support assembly, the riser may have a substantially vertical orientation.

DESCRIPTION OF THE PRIOR ART

In a known method of installation of a riser, a transfer of a product riser and a transfer of a number of anchor lines from a first, temporary buoy to a second, permanent buoy are combined. The product riser is produced and installed by a pipe-laying vessel, located at the water surface. The first buoy is anchored to the seabed by at least a first and a second anchor line. The product riser connected by a free end thereof to the first buoy. The riser may stay in this position for a considerable period of time, until a second, permanent mooring buoy is put in place. When the second buoy is ready, it is positioned in a desired position, close to the first buoy. The first and the second buoy are connected to one another via a member which can take up tension forces. A tug boat is provided for pulling the second buoy away from the first buoy. Then, the anchor lines are detached one by one from the first buoy and connected to the second buoy. During the time period in which at least one of the anchor lines is detached, the combination of the first and second buoy is no longer anchored in a stable and secure way, and is in risk of drifting away from the desired location. The tug boat is provided for keeping the buoys in place.

Subsequently, the second anchor line is transferred from the first buoy to the second buoy. During this operation, the tug boat also keeps the first and second buoy in place en prevents drifting away of the first and second buoy. Then, the product riser is transferred from the first buoy to the second buoy. This is performed by using a cable which is connected to a winch on the second buoy. The free end of the cable is connected to a free end of the product riser. When the product riser is detached from the first buoy, the cable is pulled by the winch, transferring the product riser from the first buoy to the second buoy.

A drawback of the above mentioned method is that it is a complicated procedure, requiring a tug boat

Further, the combination of transferring the anchor lines and the product riser at the same time provides the drawback that the first buoy is no longer anchored to the seabed at the end of the operation and is to be moved away by the tug boat. A reverse operation of transferring the product riser back to the first buoy cannot be performed without re-anchoring the first buoy back to the seabed.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and device for connecting a riser to a target structure which is substantially free of the above mentioned drawbacks. It is a further object of the invention to provide a method and device for connecting a riser to a target structure, which is relatively fast, reliable and simple to perform.

It is in particular an object of the present invention to provide a method and device of connecting a riser to a target structure, wherein a free end of the riser can temporarily be positioned in a predetermined way during a time period in which the target structure is not yet present and connected to the target structure when the target structure is present at its desired position. Thereto the method according to the invention comprises the steps of:

(a) connecting the riser by the free end thereof to a riser support assembly;

(b) positioning the target structure near the riser support assembly;

(c) connecting a lifting assembly with the free end of the riser;

(d) moving the riser by the free end thereof from the riser support assembly toward the target structure by the lifting assembly; and

(e) connecting the riser with the target structure. With the method according to the invention, risers can be* installed when the target structure is not yet present at the time of installation of the riser itself. The word "riser" may refer to a production, injection, communication and/or control riser for conveying oil and/or gas or another product. A production or injection riser is constructed to convey a product, such as oil or gas or water or any fluid, from the seabed to the target structure or vice versa from the target structure to the well. The word riser may also refer to a communication and/or control riser. A communication and/or control riser is constructed to transmit signals from the target structure to devices on the seabed, such as for instance the wells, or controllable valves. Also, signals may be transmitted by the riser from the oil well and/or other devices on the seabed to the target structure, for controlling the devices on the seabed. Other types of risers also exist, for instance risers that are used to convey water from the target structure toward the seabed, for injection of the water in the well. Other types of riser are well control risers.

The words "target structure" are to be interpreted broadly and can indicate a wide range of structures, floating or resting on the seabed. A number of non-limiting examples of target structures are provided hereinafter.

The words "lifting assembly" are to be understood as to indicate a wide range of lifting assemblies. The lifting assembly may be a crane or a winch or a drilling tower, or another device.

The lifting assembly may be positioned on the target structure or on a separate vessel or structure.

The riser support assembly indicates any construction configured for supporting a riser by a free end thereof. The riser support assembly preferably comprises a buoyancy device for providing a required buoyancy for supporting the riser.

Preferably, the target structure and the riser support assembly are maintained in their respective positions independently from one another. In this way the riser support assembly and the target structure may be accurately positioned with respect to one another.

Preferably, the target structure comprises the lifting assembly. This provides the advantage of obviating the need for providing a separate vessel with a lifting assembly. The moving of the riser can be conducted from the target structure itself, which is simple and reliable.

Preferably, the riser support assembly is anchored to the seabed by a first anchoring device.

The anchoring device advantageously keeps the riser support assembly in a fixed position.

Preferably, the target structure is anchored to the seabed by a second anchoring device which is an independent device from the first anchoring device. In this way, advantageously the step of transferring the anchoring device of the riser support assembly to the target structure can be left out of the procedure.

Preferably, the riser support assembly is directly connected to the first anchoring device throughout steps (a) , (b) , (c) , (d) and (e) . This provides the advantage of having a simple and reliable positioning of the riser support assembly throughout steps (a) , (b) , (C) , (d) and (e) .

Preferably, during step (b) the target structure and the riser support assembly are connected by a connecting means. With the connecting means, a horizontal distance between the riser support assembly and the target structure can be reduced, providing easier transfer of the riser. Preferably, the target structure and the riser support assembly are not connected to one another.

Preferably, during step (b) the target structure and the riser support assembly are moved towards one another by reducing the length of the connection means. Also, the length of the mooring lines of the target structure can be adjusted, thereby moving the target structure toward the riser support assembly. This may be a substantial horizontal movement. Advantageously, the target structure and the riser support assembly can be positioned very close to one another, facilitating the transferal of the riser from the target support assembly to the target structure. Preferably, the riser support assembly is positioned at a predetermined depth under the water surface when the riser is suspended from the riser support assembly. This facilitates an eventual positioning of the riser support assembly close to the target structure. Preferably, the riser support assembly is positioned at a depth at which the riser support assembly is substantially free from influences from wind and waves at the water surface. This provides the advantage of a stable support for the riser with relative low dynamic forces exerted on the riser support assembly and the riser. Preferably, in step (b) the target structure is at least temporarily positioned substantially above the riser support assembly. This provides the advantage that the riser can be moved from the riser support assembly to the target structure in a substantially vertical direction. This enables a simple lift operation by the lifting assembly.

Preferably, the depth at which the riser support assembly is positioned is greater than the draught of the target structure. This enables the positioning of the riser support assembly under the target structure.

Preferably, the method according to the invention comprises a further step (f) of removing the riser support assembly. Advantageously, the riser support assembly may be reused at another location.

Preferably, prior to step (a) the riser is suspended by the free end thereof from a pipe-laying vessel and in step (a) the riser is transferred from the pipe-laying vessel to the riser support assembly. This enables the pipe-laying vessel to start another operation elsewhere, and obviates the need for the pipe-laying vessel to wait for a substantial amount of time for late arrival of the target structure.

Preferably, in step (a) the riser support assembly is moved towards the pipe-laying vessel, for transferring the riser from the pipe-laying vessel to the riser support assembly.

Preferably, the target structure is chosen from a group of floating structures, comprising: a TLP, a Spar, a Semi-submersible, an FPSO, an FPDSO, an FPWSO, a storage barge, an FSO, an FSU. For these structures and other structures, the method according to the invention provides a reliable way of connecting the riser to the target .structure.

Different riser configurations exist to provide a reliable connection between the pipeline on the seabed and the target structure. Preferably, the riser comprises rigid sections.

Alternatively the riser comprises only flexible pipe sections. The riser can also be of a hybrid type comprising rigid sections and flexible sections .

Preferably, in step (a) at least two risers are connected to a single riser support assembly. This provides the advantage of needing only a limited number of riser support assemblies, in some cases only one. Preferably, a first riser support assembly and a second riser support assembly are provided, wherein the first riser support assembly and second riser support assembly are connected by a mooring connecting means for maintaining a predetermined distance between the first and second riser support assembly.

This embodiment advantageously prevents damage to the riser support assemblies. The mooring connecting means may be a wire or a line, a cable or a rigid bar, or another connecting means known in the art. Alternatively, the riser support assemblies may be positioned at a sufficient large distance from one another. The riser support assemblies may comprise fenders to prevent damage at contact between the riser support assemblies.

Alternatively, two or more riser support assemblies are integrated into to a large riser support assembly. A large riser support assembly can support a number of risers, thereby further simplifying the construction.

Preferably, the lifting assembly is chosen from a group of lifting assemblies, comprising: a pull-in device with chain jacks, a winch, a crane and a drill tower. These lifting assemblies have a lifting power that is sufficient to move the riser from the riser support assembly to the target structure.

Preferably, the target structure is anchored to the seabed using a second anchoring device. This provides the ^'advantage of a secure anchoring of the target structure, while obviating the need for detaching the first anchoring device from the riser support assembly.

Preferably, the lifting assembly is connected to the riser by a remotely operated vehicle (ROV) The lifting assembly comprises a lifting means such as a cable or a wire or another lifting means known in the art. The ROV will assist to connect underwater the lifting means and the riser with the advantage of diverless operations.

The invention also relates to a riser support assembly for connecting a riser by a free end thereof with a target structure , the riser support assembly comprising: a buoyancy device for providing a predetermined buoyancy to the riser support assembly; a suspending device configured for suspending the riser by the free end thereof from the riser support assembly; - a first anchoring device for anchoring the riser support assembly to the seabed.

With the riser support assembly, a riser can be connected to a target structure in a reliable and simple way.

Preferably, the riser support assembly is constructed for allowing an upwards movement of the riser relative to the riser support assembly. In this way the lifting assembly can lift the riser from the riser support assembly without hindrance, for instance when the riser support assembly is positioned substantially underneath the target structure. Preferably the riser support assembly comprises a recess configured for receiving the riser therein. Advantageously, the riser is positioned in the recess, thereby exerting a downwards - pulling force on the riser support assembly substantially on the center of the riser support assembly, when viewed from above. The riser support assembly will subsequently stay in a.stable orientation.

Preferably,- the recess is a channel which, in use, extends substantially through the buoyancy device, preferably in a substantially vertical direction. This provides the advantage that the free end of the riser will extend on the upper side of the rise] support assembly, allowing easy access to the riser from above, for instance for connecting a connecting means thereto.

Preferably, the riser support assembly has a substantially elongated form. Preferably, the substantially elongated form extends in a substantially horizontal direction. In this way, space is created for suspending more than one riser from the riser support assembly.

Preferably, the buoyancy device comprises a first buoyancy means and a second buoyancy means, wherein the first and second buoyancy means are connected with one another by means of a bridging body. This provides the further advantage of enabling a configuration of a large riser support assembly. Preferably, the bridging body comprises the suspending device. An integration of the bridging body and the suspending device provides the advantage of a simple design and ease of construction. Preferably, the suspending device is configured for suspending a number of risers therefrom. This provides the advantage that less riser support assemblies will need to be installed when a relatively large number of risers is present.

Often, a number of oil and/or gas wells are located in a same oil and/or gas field at relatively close distances with respect to one another. In that case, a number of riser may rise from the wells and be suspended from a single riser support assembly.

Preferably, the first anchoring device comprises a mooring line and an anchoring means, the anchoring means being chosen from a group of anchoring means, comprising: a piled foundation, a suction pile and a gravity foundation. These anchoring means have shown to be reliable for these purposes.

Preferably, the anchoring means are configured for bearing a substantially upwardly directed force which is exerted on the anchoring means by the mooring line. On the first anchoring means an upwards force may be exerted by the buoyancy device. Therefore, the anchoring means preferably allows for an upwardly directed force.

Preferably, the first anchoring device comprises a mooring line, the mooring line being chosen from a group of mooring lines, comprising: a tendon, a polyester line, a steel wire and a chain. These mooring lines have shown to be reliable.

The word tendon indicates a rigid and stiff pipe, used for vertical mooring and high loads. Preferably the tendon comprises pipe section which are screwed to one another. These tendons technology is well known and has been used to moor Tension Leg Platforms (TLP) to the seabed.

The claims and advantages will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS Hereinafter the present invention will be illustrated in more detail by a drawing. Herein shows:

Fig. 1 a schematic perspective view of a typical configuration of a target structure;

Fig. 2 a schematic top view of a typical configuration of the target structure;

Fig. 3 shows a schematic top view of the target structure in a production field; Fig. 4 shows a schematic top view of an existing method to pre- lay the mooring lines and risers of the target structure to the seabed;

Fig. 5 shows a schematic side view of a pipe-laying vessel approaching a riser support assembly; Fig. 6 shows a schematic side view of the riser support assembly in a vertical position;

Fig. 7 shows a schematic side view of the riser connected to the riser support assembly;

Fig. 8 shows a schematic side view of a part of the riser support assembly;

Fig. 9 shows a schematic top view of a part of the riser support assembly;

Figs. 10a and 10b show an alternative embodiment of the riser support assembly; Fig. 11 shows another alternative embodiment of the riser support assembly;

Figs. 12 and 13 show a side view of an installation of the riser onto a riser support assembly;

Fig. 14 shows a side view of the riser 6 suspended from the riser support assembly;

Figs. 15 and 16 show a side view of the riser 6 with buoyancy elements;

Fig 17. shows a top view of a number of pre-installed risers;

Fig. 18 shows a top view of the target structure in the vicinity of the buoyancy devices;

Fig. 19 shows a side view of the target structure which is moved toward a buoyancy device; Fig. 20 shows a side view of an alternative embodiment of moving the buoyancy device and the target structure towards one another;

Fig. 21 shows a side view of a movement of the riser support assembly after the riser is lifted off the riser support assembly;

Fig. 22 shows a side view of a pull-in device for pulling the riser towards a target structure;

Figs. 23a, 23b, 23c show a chain jacking device for pulling the riser towards the target structure; and Figs. 24, 25 show schematic views of the pull-in device, movably positioned onboard a target structure.

Fig. 1 a schematic perspective view of a typical configuration of a target structure 2 in use. A number of risers 6a, 6b, 6c, 6d, 6e, 6f, 6g, 6h, 6i, 6j and 6k are suspended from the target structure 2. For ease of reference, a riser is referred to as riser 6. The target structure 2 is anchored to the seabed by an anchoring device 8, comprising a number of mooring lines 8a, 8b, 8c , 8d , 8e ,8f, 8g, 8h, 8i, 8j, 8k, 81. For ease of reference, a mooring line is referred to as mooring line 8. The target structure 2 may typically be a floating production facility, such as a Tension leg Platform (TLP) , a Spar, a Semi-submersible, a Floating Production Storage and Offloading Facility (FPSO) , a Floating Production Drilling Storage and Offloading Facility (FPDSO) , a Floating Production well Workover Storage and Offloading Facility (FPWSO) , a storage barge, a Floating Storage and Offloading Facility (FSO) , or a Floating Storage Unit (FSU) .

The riser 6 may be manufactured from well-known construction materials such as steel or titanium. Preferably, the riser 6 is a Steel Catenary Riser (SCR) . The riser 6 may also be manufactured from a synthetic material, or a composite material. It is .also - possible that the riser is a flexible riser. The riser 6 may also be a hybrid curved riser comprising rigid sections and flexible sections (not shown) .

Figure 2 shows a schematic top view of the target structure 2. The risers 6a, 6b, 6c, ... 6q are connected to the target structure 2 on different sides of the target structure 2. Figure 3 shows a schematic top view of the target structure 2 in a production field. Pipelines 9 extend between the wells 10a, 10b, 10c, 1Od, 1Oe, 1Of, 1Og, 1Oh and the target structure 2. The wells produce a product such as oil and/or gas. The product from the wells 10a ... 1Oh is conveyed through the pipeline in the direction of the target structure 2. Some pipelines 9 are not directly connected to the target structure, but are connected to another pipeline 9 at a junction 11, from which junction 11 the product is further conveyed by another pipeline 9. The part of the pipeline which rises from the seabed to the target structure is the riser 6, which is not separately indicated in this figure. Other lines extend on the seabed, such as water injection lines and umbilicals. These are not indicated by a reference sign.

Fig. 4 shows 'a schematic top view of an existing method to pre- lay the mooring lines 8 and risers 6 of the target structure 2 to the seabed. In this phase, the target structure 2 is not present. The mooring lines 8a, 8b, 8c, ... 81 are connected on one end thereof to a respective anchoring point 12a, 12b, 12c, ... 121. The mooring lines are then laid on the seabed to rest for a predetermined period of time.

In the same way, the risers 6a,..., 6f are laid on the sea bed, possibly connected on one end thereof with a respective well.

Then, the target structure 2 is positioned in its desired position. The mooring lines 8a, ..., 81 are picked up from the seabed by an installation vessel (not shown). The mooring lines 8a, ..., 81 are then connected at their free end to the target structure 2. In the same way, the risers 6a,...6f are picked up from the seabed and connected on a respective free end 24a,..., 24f thereof to the target structure 2. Turning to Figure 5, a side view of a pipe-laying vessel 16 approaching a riser support assembly 20 is shown. The pipeline or riser 6 is suspended from the pipe-laying vessel 16. In this configuration, the pipeline or riser 6 is laid in J-lay mode, wherein the free end 24 of the pipeline or riser 6 extends in a substantially vertical direction near the pipe-laying vessel 16.

Instead of a J-lay vessel, other type of lay vessels can be used to install the risers, such as S-lay or reel-lay vessels. The pipe- laying vessel 16 moves in the direction indicated by arrow 26. The pipeline 6 is formed on the pipe-laying vessel 16 by joining abutting pipe sections (not shown) with the pipeline 6. The pipeline 6 comes to rest on the seabed 18. The riser support assembly 20 comprises a buoyancy device 28 and an anchoring device 29. The anchoring device 29 comprises a mooring line 14 and an anchoring means 30, the mooring line 14 being connected with a first end to the buoyancy device 28 and with a second end to the anchoring means 30, anchored in the seabed at an anchoring point 12. An indicating buoy 32 is connected with a line to the riser support assembly 20 and floats at the water surface 34, for indicating the position of the riser support assembly 20.

The anchoring means 30 may be a piled foundation, a suction pile, a gravity foundation. Preferably, the anchoring means 30 is configured to resist a substantially upwardly directed vertical force exerted on it by the mooring line 14.

The mooring line 14 can be a tendon, a light weight line such as a polyester line, a steel wire or a chain.

The buoyancy device 28 may be completely under the water surface 34 or partially above the water surface 34 when there is no riser 6 suspended from the riser support assembly 20. When the riser 6 is suspended from the riser support assembly 20, the buoyancy device 28 will be under the water surface 34 at a depth 41 where there is no influence from wind and/or waves. When the riser 6 is connected to the riser support assembly ^"20, the mooring line 14 may extend along a substantially straight line or may extend along a catenary line.

Figure 6 shows a riser support assembly 20 having a buoyancy device 28 which is positioned partially submerged and partially above the water surface 34.

Fig. 7 shows the riser 6 connected to the riser support assembly 20. The buoyancy device 28 is positioned at a predetermined depth under the water surface 34.

Fig. 8 and 9 show schematic views from the side and from the top of a part of the riser support assembly 20. The buoyancy device 28 has a substantially cylindrical form. Other forms of the buoyancy device 28, such as box-like, are also possible. A mooring line 14 is attached to the buoyancy device 28 at a bottom side thereof. A recess 36 extends through the buoyancy device 28 to form a channel 37 in a substantial vertical direction through the buoyancy device 28. A person skilled in the art will readily appreciate that if the riser 6 is not positioned substantially vertically but in an inclined orientation, the buoyancy device 28 can adopt the orientation of the riser 6 and the channel 37 will have an inclined orientation.

The riser 6 has at the free end 24 thereof a lifting connector 40 for connecting the riser 6 to a lifting assembly (not shown) , in order to lift the riser 6 from the buoyancy device 28. Further, at the free end 24 of the riser 6 a first collar 42 is provided for suspending the riser 6 from the buoyancy device 28. A second collar 44 is provided to hang the riser 6 from the target structure (not shown) .

The riser support buoy 28 has a suspending device 31 for suspending the riser 6 therefrom. The suspending device comprises. a contact area 33, wherein the collar 42 engages the buoyancy device 28. Figs. 10a and 10b show an alternative embodiment of the riser support assembly 20. The buoyancy device 28 has an elongated form and extends in a substantially horizontal orientation. Two mooring lines 14a, 14b are connected on a first end thereof to respective ends of the riser support assembly 20. The mooring lines 14a, 14b are connected on a second end thereof to anchoring means 30a, 30b. Other forms of the buoyancy device are also possible.

A number of risers 6a, 6b, 6c, 6d, 6e are suspended from the riser support assembly 20. Each riser has at the free end 24 thereof a suspending device 46 in the form of a hook 46a, 46b ,46c ,46e for suspending the respective risers 6 from the buoyancy device 28. A person skilled in the art will appreciate that many other embodiments such a suspending device 46 are possible, without departing from the scope of the invention.

Fig. 11 shows another alternative embodiment of the riser support assembly 20. In this embodiment, two buoyancy means 28a, 28b are provided which are connected by a bridging body 48. The bridging body 48 is configured to suspend the risers 6a , ..6g therefrom. The bridging body 48 may be a rigid bar.

Turning to figs. 12 and 13, the installation of the riser 6 onto a riser support assembly 20 is shown. When the riser 6 has reached the required length, it is to be transferred to the riser support assembly 20. In fig. 12, the riser support assembly 20 is pulled toward the pipe-laying vessel 16 by a line 50. The buoyancy device 28 is moved in close proximity with the pipe-laying vessel 16. This may be performed xising a winch (not shown) on board the pipe-laying vessel 16 or by using an assisting tug boat (not shown) . Then, the riser 6 is transferred from the pipe-laying vessel 16 to the riser support assembly 20 and is suspended from the buoyancy device 28.

Fig. 14 shows the riser 6 suspended from the riser support assembly 20. In this position, the riser 6 may rest in a suspended position for a period of time. In some cases, this period of time may last for a number of months. The riser support assembly 20 should preferably be in a submerged position at a depth well below the water surface 34, at which depth the riser support assembly 20 experiences no wind and smaller wave induced forces thab it would experience when in the water surface 34.

Figs. 15 and 16 show a riser 6 with a number of buoyancy elements 52a ,52b ,52c , 52d , 52e attached thereto, for providing buoyancy to the riser 6. In Fig. 15, the riser 6 has a lazy wave configuration, to reduce fatigue of the riser at the touch down point and to reduce riser weight.

In Fig. 16, the buoyancy elements 52a ,52b ,52c , 52d , 52e are attached to the part of the riser 6 which extends from the seabed 18 towards the buoyancy device 28 to reduce riser weight and the size of the buoy 28.

Fig 17. shows a top view of a number of pre-installed risers 6a, 6b, 6c, and 6d, suspended from riser support assemblies 20a, 20b, 20^"c, 2Od. A connecting element 54 is provided between the buoyancy device 28b and the buoyancy device 28c, for keeping the buoyancy device 28b and the buoyancy device 28c at a desired distance from one another. In this way, contact between the buoyancy devices 28b, 28c in a current flow, can be avoided, which could result in damage. Connecting elements 54 may also be provided between the other buoyancy devices 28.

Alternatively, in order to prevent contact between the buoyancy devices 28, a sufficient large distance between the buoyancy devices may also be provided. Also fenders on the buoyancy devices 28 may be provided in order to avoid damage during contact. Another possibility of avoiding damage to the buoyancy devices 28 is to integrate a number of buoyancy devices into a single large buoyancy device 28. Fig. 18 shows the target structure 2 in the vicinity of the buoyancy devices. The target structure 2 is moored to the seabed after arrival in the field. In order to reduce installation time of the target structure, use can be made of pre-installed mooring lines 8. These mooring lines 8 can for instance be pre-installed and laid on the seabed, as is shown in fig. 4. The mooring lines 8 can also be connected to buoyancy devices 28 (not shown) allowing pre- installation of the mooring lines prior to arrival of the target structure 2.

The anchoring means 30a, 30b, 30c ... 3Oh are positioned on the other side of the target structure as the respective corresponding buoyancy devices 28a,... 28h.

The risers 6a, 6b, 6c, ... 6h extend in different directions, and are connected to the target structure 2 on different sides thereof. Mooring connecting means 54 connect buoyancy devices 28a, with 28e, 28b with 28c, 28d with 28h, and 28g with 28f. Other ways of connecting the buoyancy devices 28a... 28h with one another are also possible.

Fig. 19 shows a target structure 2 which is moved toward a buoyancy device 28. The target structure 2 is anchored to the seabed by mooring lines 8a, 8b, shown and designated in a first position as 8al and 8bl. In order to move the target structure 2 towards the buoyancy device 28, the mooring line 8b is winched to shorten the length thereof. Consequently, the target structure 2 will move in the direction of arrow 56, toward the buoyancy device 28. The mooring lines 8a, 8b will adopt respective second positions 8a2, 8b2. In an alternative embodiment, a tug boat is used to move the target structure toward the buoyancy device 28. When the target structure 2 is in the desired position, the riser 6 can be transferred to the target structure.

If a number of risers 6 are to be connected to the target structure 2, the target structure may be moved from one desired position to the next one, picking up a riser 6 in each position. After the risers 6 are connected to the target structure 2, the target structure 2 is moved to its final position.

Fig. 20 shows an alternative embodiment of moving the buoyancy device 28 and the target structure 2 towards one another. In Fig. 20, the buoyancy device 28 is winched towards the target structure 2 by a winching device 58 located on the target structure 2. A guiding assembly 60 is provided for guiding a connection line 62 between the winching device 58 and the buoyancy device 28, in order to ensure a correct positioning of the buoyancy device 28 relative to the target structure 2.

Fig. 21 shows a movement of the riser support assembly 20 after the riser 6 is lifted off the riser support assembly 20 and transferred to the target structure 2. The buoyancy device 28 will move with respect to the anchoring point 12, until it reaches its equilibrium position vertically above the anchoring point 12.

The buoyancy device 28 can be left in this position, allowing a reverse operation of transferring the riser 6 from the target structure 2 onto the riser support assembly 20. Alternatively the riser support assembly 20 may be removed and reused in another location. The riser support assemblies 20 may be disconnected from the anchoring means 30 by a remote operated vehicle (ROV, not shown), known in the art. Partial flooding of the buoyancy device 28 may be performed prior to disconnecting from the buoyancy device 28. The ROV may also be used to assist in connecting the connection line 62 to the buoyancy device 28.

In an alternative embodiment, the riser 6 is lifted off the riser support assembly 20 by a support vessel (not shown) and moved towards the target structure 2 by the support vessel. The support vessel requires a high capacity winch to lift the heavy riser (possibly weighing in the order of 2000 - 6000 kiloNewton) .

Fig. 22 shows a pull-in device 63 for pulling the riser 6 towards a target structure 2. In this embodiment, the pull-in device 63 is a drilling tower 64. The pull-in device 63 may also be a crane or a jacking device (not shown) .

Fig. 23 shows a chain jacking device 66 for pulling the riser 6 towards the target structure 2. The chain jacking device 66 comprises lift cylinders 68a and 68b, a fixed latch 70, a travelling latch 72 and a traveling cross-arm 74. Chain jacking devices are known in the art.

Figs. 24 and 25 show a configuration of a pull-in device 63, movably positioned onboard a target structure 2, on rails 76a, 76b, such as skidding rails.

A number of risers 6 are positioned on the side 83 of the target structure 2. Hydraulic rams 78a, 78b are provided for moving the pull-in device in the directions of arrows relative to the target structure. A pulling rig skid base 81 is provided for skidding on the skidding rails 78a, 78b. In this way, a single pull- in device 63 can be used to sequentially pull in a number of risers 6a, 6b, 6c towards the target structure 2.

It will be obvious to a person skilled in the art that numerous changes in the details and the arrangement of the parts may be varied over considerable range without departing from the spirit of the invention and the scope of the claims.

Claims

1. A method for connecting a free end of an at least partially submerged riser to a target structure, the method comprising:

(a) connecting the riser by the free end thereof to a riser support assembly;

(b) positioning the target structure near the riser support assembly;

(c) connecting a lifting assembly with the free end (24) of the riser;

(e) connecting the riser with the target structure.

2. The method of claim.1, wherein the target structure and the riser support assembly are maintained in their respective positions independently from one another.

3. The method of claim 1 or 2, wherein the target structure comprises the lifting assembly.

4. The method of any of claims 1-3, wherein the riser support assembly is anchored to the seabed by a first anchoring device.

5. The method of claim 4, wherein the target structure is ^~ anchored to the seabed by a second anchoring device which is an independent device from the first anchoring device.

6. The method of any of claims 1-5, wherein the riser support assembly is directly connected to the first anchoring device throughout steps (a) , (b) , (c) , (d) and (e) .

7. The method of any of claims 1-6, wherein during step (b) the target structure and the riser support assembly are connected by a connecting means.

8. The method of claim 7, wherein during step (b) the target structure and the riser support assembly are moved towards one another by reducing a length of the connection means.

9. The method of any of claims 1-8, wherein the riser support assembly is positioned at a predetermined depth under the water surface when the riser is suspended from the riser support assembly.

10. The method of claim 9, wherein the riser support assembly is positioned at a depth at which the riser support assembly is substantially free from influences from wind and waves at the water surface.

11. The method of any of claims 1-9, wherein in step (b) the target structure is at least temporarily positioned substantially above the riser support assembly.

12. The method of any of claims 1-11, comprising a further step (f) of removing the riser support assembly.

13. The method of any of claims .1-12, wherein prior to step (a) the riser is suspended by the free end thereof from a pipe-laying vessel and in step (a) , the riser is transferred from the pipe- laying vessel to the riser support assembly.

14. The method of claim 13, wherein in step (a) the riser support assembly is moved towards the pipe-laying vessel, for transferring the riser from the pipe-laying vessel to the riser support assembly.

15. The method of any of claims 1-14, wherein the target structure is chosen from a group of floating structures, comprising: a TLP, a Spar, a Semi-submersible, an FPSO, an FPDSO, an FPWSO, a storage barge, an FSO, an FSU.

16. The method of any of claims 1-15, wherein the riser is chosen from a group of risers, comprising: a steel catenary riser, a lazy wave riser with buoyancy elements, a steep wave riser, and a flexible riser.

17. The method of any of claims 1-16, wherein the riser is manufactured from a material, chosen from a group of materials, comprising: steel, titanium, a synthetic material, and a composite of a metal and a synthetic material.

18. The method of any of claims 1-17, wherein the riser comprises flexible pipe sections.

19. The method of any of claims 1-18, wherein at least two risers are connected to the target structure.

20. The method of claim 19, wherein in step (a) at least two risers are connected to a single riser support assembly.

21. The method of claim 19 or 20, wherein a first riser support assembly and a second riser support assembly are provided, and wherein the first riser support assembly and second riser support assembly are connected by a mooring connecting means for maintaining . a predetermined distance between the first and second riser support assembly.

22. The method of claim 21, wherein the mooring connecting means are selected from a group of mooring connecting means, comprising: a flexible mooring connecting means and a rigid mooring connecting means .

23. The method of any of claims 1-22, wherein the lifting assembly is chosen from a group of lifting assemblies, comprising: a pull-in device with chain jacks, a winch, a crane and a drill tower.

24. The method of any of claims 1-23, wherein the target structure is anchored to the seabed using a second anchoring device

25. The method of any of claims 1-24, wherein the lifting assembly is connected to the riser by a remotely operated device.

26. A riser support assembly for connecting a riser by a free end thereof with a target structure, the riser support assembly comprising: a buoyancy device for providing a predetermined buoyancy to the riser support assembly; a suspending device configured for suspending the riser by the free end thereof from the riser support assembly; a first anchoring device for anchoring the riser support assembly to the seabed.

27. The riser support assembly of claim 26, wherein the suspending device is constructed for allowing an upwards movement of the riser relative to the riser support assembly.

28. The riser support assembly of claim 26 or 27, wherein the riser support assembly comprises a recess configured for receiving the riser therein.

29. The riser support assembly according to claim 28, wherein the recess is a channel which, in use, extends substantially through the buoyancy device, preferably in a substantially vertical direction.

30. The riser support assembly according to any of claims 26- 29, wherein the riser support assembly has a substantially elongated form.

31. The riser support assembly according to claim 30, wherein the substantially elongated form extends in a substantially horizontal direction.

32. The riser support assembly according to any of claims 26-

31, wherein the buoyancy device comprises a first buoyancy means and a second buoyancy means, wherein the first and second buoyancy means are connected with one another by means of a bridging body.

33. The riser support assembly according to any of claims 26-

32, wherein the bridging body comprises the suspending device.

34. The riser support assembly according to any of claims 26-

33, wherein the suspending device is configured for suspending a number of risers therefrom.

35. The riser support assembly according to any of claims 26-

34, wherein the first anchoring device comprises an mooring line and an anchoring means, the anchoring means being chosen from a group of anchoring means, comprising: a piled foundation, a suction pile and a gravity foundation.

36. The riser support assembly according to claim 35, wherein the anchoring means are configured for bearing a substantially upwardly directed force which is exerted on the anchoring means by the mooring line.

37. The riser support assembly according to any of claims 26-

36, wherein the first anchoring device comprises a mooring line, the mooring line being chosen from a group of mooring lines, comprising: a tendon, a polyester line, a steel wire and a chain.

38. The riser support assembly according to any of claims 26-

37, having at least one riser suspended therefrom.