WO2021160457A1

WO2021160457A1 - Spread moored vessel comprising a riser moonpool

Info

Publication number: WO2021160457A1
Application number: PCT/EP2021/052314
Authority: WO
Inventors: Thomas LORIN; Francois Marchais
Original assignee: Single Buoy Moorings Inc.
Priority date: 2020-02-10
Filing date: 2021-02-01
Publication date: 2021-08-19
Also published as: BR112022015624A2; CN115087591A; EP4103464A1; KR20220137987A

Abstract

The invention relates to an offshore vessel (1, 150) comprising a hull (2, 155) having a length of at least 200m, and having a riser moonpool (17, 170) situated on a longitudinal center line (14) of the vessel extending in a vertical direction from deck level to keel level, the moonpool comprising a riser connector member for attaching risers (27, 76, 76', 77, 77'), to the hull (2, 155). The riser connector member has a riser support structure (26) extending across the moonpool (17, 170) and placed in the moonpool near the bottom (18), for attaching the risers (27, 76, 76', 77, 77') at spaced apart positions distributed over the cross section of the moonpool. A manifold module (68, 69) may be situated alongside the moonpool (17, 170). A pulling system (70, 121, 172) is situated near deck level in an area at or near the moonpool, and comprises a movable lifting member (103, 105, 111) that is displaceable across the moonpool (17, 170) for attaching to the risers (27, 76, 76', 77, 77') and exerting an upward pulling force on the risers. The construction is compact and can accommodate a large number of risers with reduced sagging and fatigue weakening.

Description

Spread moored vessel comprising a riser moonpool

Field of the invention

The invention relates to an offshore vessel comprising a hull, a bottom, longitudinal sides, a bow and a stern, spread mooring connectors situated at each side of the hull near the bow and near the stern and a riser moonpool situated on a longitudinal center line of the vessel extending in a vertical direction from deck level to keel level, the moonpool comprising riser connector members for attaching risers to the hull in a fixed position.

Background of the invention

From Sung-Ryng Jung, Jin-Tae Kim, Man-Soo Kim, “Structural Analysis of Riser l-Tube Connection Details of FPSO”, The Seventeenth International Offshore and Polar Engineering Conference, 1-6 July 2007, Lisbon, Portugal, a Floating Production, Storage and Offloading vessel (FPSO) for operation in the Abgami field is known that is attached to the sea bed via spread mooring at the bow and stern side. The FPSO hull structure comprises a continuous single upper deck, a single bottom, double sides, transverse bulkheads and two central longitudinal bulkheads. All riser I tubes are arranged within the water ballast tanks that are situated in the side regions of the hull along the longitudinal sides, and extend from the bottom shell to the upper deck plating.

In WO 2015/041526, a spread moored FPSO with a twin-hull structure is known in which the risers are supported from riser balconies along the sides of the vessel. A number of risers may be supported from the central deck area that interconnects the two hulls.

The large longitudinal dimensions of the deck area along which the risers in the known systems are accommodated and the resulting bulky manifold construction connecting the top ends of the risers to the process equipment on the vessel, take up a large amount of space on the vessel. The length of the piping that extends form the risers to the process equipment on the vessel is relatively long, which significantly contributes to an increase in costs.

Furthermore, the risers that are suspended from riser balconies along the sides of the vessel at a relatively large distance from the center line, are subject to sagging caused by roll motions of the vessel, which may result in fatigue weakening of the risers.

The azimuth orientations at which the risers approach the riser balconies are relatively limited, and risers may need to be routed around the vessel from one side to the other to prevent them from passing underneath the vessel.

Finally, at the position of the risers along the vessel’s sides, no access is possible by supply boats or work-over vessels. The risers that are suspended from the balconies are relatively exposed to vessel collision, waves, slamming, green water and even third party intrusion.

From WO 2009/141351 a FPSO is known comprising a rotatable turret that is supported in a vertical shaft or moonpool that extends from deck to keel level. The risers are connected to a releasable riser supporting buoy that is anchored to the sea bed and that is received in a conical lower side of the turret. When the riser supporting buoy is latched to the turret, the vessel can weathervane to be aligned with the prevailing wind and current conditions. In case of adverse weather conditions, the riser supporting buoy is detached and is lowered below the wave active zone, while the vessel can be sailed to quieter waters.

WO 2019/025936 discloses a combination of a spread moored FPSO with a riser supporting buoy that is anchored to the sea bed. The riser supporting buoy is kept in position by pre-installed anchor lines and is floated upward to be docked to the FPSO. For use in calm waters, the riser supporting buoy may not be provided with anchor lines, so that the only anchoring is achieved by the spread mooring of the FPSO having anchor lines that are attached to the bottom. The mooring buoy is sailed into the cavity that extends along a larger part of the length of the vessel, via a side of the FPSO that may be closed by a hatch.

WO2017/186284 shows a floating processing vessel of a length-width ratio of less than 2, having a skirt for stability, tanks and a longitudinal moonpool from which risers are hung-off. A movable winch is shown for installation of the risers. The floating structure is moored in a spread moored pattern in three points.

The known turret mooring systems are of relatively complex and bulky construction. The rotating vessels may for some wind directions orient themselves over the risers. If in such orientation, supply boats that dock alongside the FPSO, may accidentally lose objects that could fall overboard and hence pose a risk of damaging the risers.

It is therefore an object of the invention to provide an offshore vessel in which a relatively large number of risers may be accommodated in a compact configuration and the manifold dimensions and piping length connecting the risers to the processing equipment are reduced.

It is a further object of the invention to provide a vessel in which there is a relatively large range of azimuth orientations at which the risers can approach the vessel.

It is another object of the invention to provide an offshore vessel in which fatigue loading of the risers is reduced.

It is again an object of the invention to provide an offshore vessel in which the risers are protected from collisions with supply boats and from environmental impact.

Summary of the invention

Hereto an offshore vessel according to the invention is characterised in that: the moonpool is situated closer to the bow than to a mid-ship position, wherein the moonpool comprises a vertical wall forming a closed casing of circular or square cross section and is adapted to accommodate risers, an internal turret or hydrocarbons or water, the vessel not comprising mooring means at or near the bottom in the area of the moonpool, and at least one landing zone being provided for mooring a work-over or supply vessel along one or along each longitudinal side near the rear and/or front spread mooring connectors with a lifting device on deck near the at least one landing zone, wherein the riser connector member comprises a riser support structure extending across the moonpool and placed in a fixed position and orientation in the moonpool near the bottom, for attaching the risers at spaced-apart positions distributed over the cross section of the moonpool, at least one manifold module is situated in a manifold area that is defined by the longitudinal sides of the vessel and a front and rear transverse region that is situated within 20 m, preferably within 10 m an more preferably within 1 m from a respective front and rear position of the moonpool, and a pulling system is situated near deck level in an area at or near the moonpool, and comprises a movable lifting member that is displaceable across the moonpool for attaching to the risers and exerting an upward pulling force on the risers.

The risers, which may be steel catenary risers, flexible risers or a combination thereof, enter the hull through a central opening of the moonpool in the bottom of the vessel and are mounted in dedicated positions on the riser support structure, which may for instance comprise a frame that carries an array of 48 inclined I tubes.

The riser support structure may extend across the whole cross-sectional area of the moonpool, or may extend across only a part of this area and may have a rectangular, circular or horse-shoe shape.

When extending across the whole cross-sectional area of the moonpool, the riser support structure will be water tight and will allow personnel access for inspection of the riser hang-off collar and flexible riser carcass bleed below the vessel water line. The moonpool will be equipped with bilge pumps and bilge eductors in order to remove the sea water from the moonpool space after riser installation or in case of leakage. The spare I tubes will be closed with blind flanges and gaskets so no water could leak into the moonpool.

The riser support structure will be categorized as a void space and provides a double bottom protection to the riser hang-off structure. This arrangement allows the installation of more risers with various departing angles.

When extending across only part of this area and with a rectangular, circular or horse-shoe shape, the riser support structure will need to be accessible by divers or ROVs for inspection and will be protected by anodes or ICCP system. The riser heng-off collar and flexible riser carcass bleed will in this case be situated above the maximum vessel water line. This arrangement allows the installation of steel rigid risers with open receptacles. Alongside and/or on top of the moonpool, the valve modules are situated, comprising the Emergency Shut Down Valves (ESDVs). The risers are pulled upward to the riser support structure or to a connect position at an intermediate deck, for connecting with the ESDV’s, via the lifting member that can be vertically aligned with the risers to exert a pulling force that is directed along the riser’s natural inclination. The pulling system may be situated alongside the moonpool at the moonpool edge at deck level, or over the moonpool at a vertical height within a range of about 20 m from deck level. The top ends of the risers are connected to a respective ESDV on the manifold module so that they are in fluid connection with the processing equipment on the vessel.

The spread moored vessel with the riser moonpool comprising the lower riser support structure and the upper tensioning device according to the present invention, forms a relatively low cost and compact solution compared to the known complex systems having starboard and port riser porch systems in cantilever. The manifold modules according to the invention are compact and are situated close to the moonpool, which results in the piping between the process equipment on deck of the vessel and the risers being of reduced length. Since this piping is of relatively high wall thickness, a large weight and cost saving can be achieved in this manner.

Since the position of the risers according to the invention is close to the neutral axis of the vessel, the bending moments caused by roll motions are reduced and the risers will be less subject to sagging and fatigue weakening.

The combination of the spread moored configuration and the riser moonpool allows risers and umbilical to approach the hull from a wide range of azimuths such that the subsea lay-out can be optimized. The central riser position in the hull of the vessel, allows access of supply boats or installation vessels to the sides of the vessel and mitigates the risk of the risers being damaged by collision or by objects that are accidentally over boarded from the supply boats. The risers within the moonpool are furthermore protected from waves, green water and 3^rd party intrusion.

For a given moonpool diameter, the number of risers that can be hooked up to the moonpool in the spread moored configuration according to the invention is higher than for a corresponding turret that is anchored to the sea bed.

The invention could be applied to a generic new built hull construction with a watertight riser support structure across the whole cross-sectional area of the moonpool. A double bottom, watertight riser support structure could allow the conversion of the moonpool into an oil cargo tank or a ballast tank in case an external turret is selected. A water tight riser support structure ease the tow of a generic new built hull to a conversion yard. The riser support structure and the moonpool could also be converted into an internal turret space or a riser support structure with openings for riser I tubes and/or steel rigid riser open receptacles. This approach allow a reduction in the project schedule or late changes in riser load and azimuth. From WO2002/38438 a drilling rig is known having a central moonpool and transverse skirts to reduce roll motions. At its upper end, the moonpool is provided with a riser tensioning deck that is supported from cables via sheaves and counterweights.

WO2016/066295 discloses a FPSO with spread mooring connectors and an internal turret reinforcement member. Depending on user demands, the vessel can be constructed with an internal turret at the position of the turret reinforcement or can be configured as a spread moored FPSO with riser balconies.

The moonpool according to the present invention can be of rectangular or of circular cross section.

The walls or the moonpool are formed of steel plate.

The pulling system is formed by one or more winches that may be moved over the moon pool via a movable winch platform, or can alternatively be formed by a fixed winch situated near the moonpool in combination with a movable sheave that that can be brought in proximity to the riser upper ends.

In WO 2012/104308, a cylindrical hull is described of a production unit having a riser support frame inside a central moonpool, situated below the water line. The risers are tensioned by adjusting the buoyancy of the production unit.

In an embodiment, a manifold area, carrying at least one manifold module, is situated on each side of the moonpool, between the moonpool and a respective longitudinal side. In this arrangement, the riser pulling system can easily access the moonpool area for installation of the risers.

In another embodiment, the manifold area is situated over the moonpool, on a deck above the riser pulling system.

In both embodiments, the length of piping that extends between the valve modules and the process equipment on the vessel is reduced compared to the use of riser balconies. A large distance of the manifolds from the accommodation decks, which are usually situated near the stern, can be maintained.

The risers may be grouped in two rows or more extending along each side of the moonpool, leaving the central moonpool area free for diver access for inspection and/or intervention.

In again an embodiment, a landing zone for mooring a work-over or supply vessel, is provided along each longitudinal side near the rear and/or front spread mooring connectors. The absence of risers in the area of the landing zones allows unrestricted mooring of supply vessels or work boats. Equipment, tools, supplies and people may be offloaded from the supply vessel, by a crane situated in a laydown area of the deck of the vessel near the landing zone, without the risk of objects that would accidentally fall overboard, impacting with the risers.

The moonpool can be situated in the front half of the vessel, preferably closer to the bow than to the midship position. Risers can access the moonpool of the offshore vessel according to this embodiment, along the moonpool’s longitudinal sides and along its front transverse side, so that a large number of risers can be accommodated. Placing the moonpool near the bow also provides a large access area along the length of the sidewalls between the stern and the moonpool for supply vessels orworkboats. Mooring members may be situated along the sidewalls in the access area for receiving mooring hawsers of the supply boats.

The moonpool of the vessel according to the invention is near the bottom bounded by ballast tanks that extend along the bottom of the vessel, and cargo tanks situated above the ballast tanks, the bounding sides of the ballast tanks with the moonpool being of tapering shape up to the height of the cargo tanks.

In an embodiment of a vessel according to the invention, on a deck area between a rear side of the moonpool and the stern, hydrocarbon processing modules, in particular oil processing modules are situated. The close proximity of the hydrocarbon processing equipment to the manifold area results in a compact construction, and further minimizes the length of the piping.

On a deck area bounded by a front side of the moonpool and the bow, gas processing modules are situated, such as compressors, gas treatment modules, injection gas compression module, FhS removal equipment and a flare tower.

A power generation module may be situated in an area between by the rear side of the hydrocarbon processing module area and the stern.

A safety gap area may extend adjacent the rear side of the power generation module area, across a width of the vessel. In this way, personnel accommodation near the stern is isolated in case of accidents in the hydrocarbon processing areas or in the power generating areas.

Brief description of the Drawings

Some embodiments of an offshore vessel according to the present invention will, by way of nonlimiting example, be described in detail with reference to the accompanying drawings. In the drawings:

Fig. 1 is a perspective view of a spread-moored FPSO with a riser balcony, according to the prior art, Fig. 2 is a perspective view of a spread-moored FPSO according to the invention, with a riser moonpool near the bow,

Fig. 3 is a perspective view of an embodiment of a hull according to the invention, without any process modules on the deck,

Fig. 4 is a perspective view of the bottom of the hull according to fig. 3,

Fig. 5 is a longitudinal cross-section of the moonpool area, showing a riser support structure comprising a riser frame,

Fig. 6 shows a plan view of a vessel with the riser moonpool situated between the longitudinal bulkheads, near the bow,

Fig. 7 shows an aft and forward extending riser and umbilical configuration from a circular riser moonpool near the bow,

Fig. 8 shows a transverse view of the piping connecting the riser upper end to the valve module,

Fig. 9 shows a plan view of the riser moonpool with a central access area, and

Fig. 10 shows an embodiment of a lifting member with fixed linear winches and a movable sheave.

Figure 1 shows an embodiment of a known FPSO 150 with a bow 151 , a stern 152 and a hull 155 having longitudinal sides 153, 154. Mooring line connectors 156, 157, 158 are attached to the mooring lines 160, 161 , 162 via a quick release mechanism. The mooring lines 160-162 anchor the FPSO to the sea bed in a spread moored configuration. Along the longitudinal side 153, a riser balcony 165 is provided form which risers 166, 166’ are vertically suspended, for instance 30 risers or more. Manifold modules 167 are placed along the side 153. Process modules 168 are placed on the deck, for processing oil and/or gas produced through the risers. A flare tower 169 is located near the bow 151 .

A landing zone 175 extends along the longitudinal side 154, for mooring a supply vessel, or work-boat, that is of a relatively small length compared to the FPSO 150, that may be 200 m in length or more. A crane 176 is situated on the deck in a laydown area 177 for offloading of equipment, supplies or personnel from a vessel moored at the landing area 175 onto the laydown area 177 of the deck,

Figure 2 shows a FPSO according to the invention. Near the bow 151 , a riser moonpool 170 is provided, carrying for instance 48 risers or more, with on top of the moonpool area, a manifold module 171 . Placing the manifold module 171 on top of the riser moonpool and above or underneath a riser pulling system, results in a very compact construction.

Along each longitudinal side 153,154, a landing zone 175, 175’ is provided for allowing mooring of relatively small-sized vessels, such as supply vessels or work-over vessels. Cranes 176, 176’ can unload the vessels moored in the landing zones 175, 175’ and place cargo in the laydown areas 177, 177’. As no risers extend underneath the landing zones 175, 175’, the offloading operation can be carried out according to strict safety standards.

In figure 3, a FPSO 1 is shown with a hull 2 having a bow 3, a stern 4 and longitudinal sides 5, 6. Near the bow 3, anchor line connectors 9,10 are provided for attaching to bow mooring lines. Stern anchor line connectors 12,13 project outward from the sides 5,6 and connect the rear anchor lines of a spread moored anchoring configuration that keeps the vessel 1 in a fixed orientation. The anchor lines may be connected to the connectors 9,10, 12, 13 via quick release members for rapidly moving the vessel 1 in case of emergencies. Near the stern, personnel accommodation 15 is provided.

Near the bow 3, a riser moonpool 17 is provided, that in this example is of a circular cross-section and that is situated on the longitudinal center line 14. The riser moonpool 17 extends from deck level to the bottom 18 (see figure 4) of the vessel 1 and hydrocarbon risers and umbilicals extend from subsea wellheads, upwards through the moonpool 17 and are connected via product piping 16 to processing equipment on the deck 19. The landing zones may in this embodiment extend on each side 5,6 of the vessel, along the larger part extending between the rear anchor line connectors 12, 13 and the front anchor line connectors 9,10.

As shown in figure 5, the riser moonpool 17 comprises a vertical wall 21 that extends from the main deck 19 to a box-shaped reinforcement member 22. The reinforcement member 22 defines the lower opening 25 of the moonpool 17 and closes off the ballast tanks 23 that extend along the bottom 18. A riser support structure, which in this example is formed by a frame 26, is connected to the reinforcement member 22 and carries the risers 27 and the umbilicals 27’. The risers 27 are in this example formed by steel catenary risers (SCR’s) that are inclined at an angle of about 12° to the vertical direction. The SCR’s end at a riser deck 28. The umbilicals 27’ are inclined at an angle of about 5° and extend upwards to the main deck 19. No rotating parts are present in the riser moonpool 17 and the spread mooring connectors 9-13 and the vessel 1 , the risers 26 and umbilicals 27 all have a fixed orientation relative to the sea bed.

Figure 6 shows a plan view of the vessel 1 in which the riser moon pool 17 is situated near the bow 3 and is placed between two longitudinal bulkheads 30,31 that extend from the stern 4 to the bow 3 along the length of the vessel 1 . The moon pool 17 is of rectangular cross section and defines manifold areas 33,34 on each side of the moonpool, that are bounded by a front transverse region 35, a rear transverse region 36, the sidewalls 5,6 of the vessel and the longitudinal sidewalls 37, 38 of the moonpool 17. In the manifold areas 33,34, manifold modules are placed on the deck 19, comprising emergency shut down valves (ESDV’s) that connect the risers to the process equipment on the deck. The transverse regions 35, 36 may have a width L1 that is larger than the length L2 of the moonpool 17 by 25m or less, such as 10 m. Preferably the length of the manifold areas 33,34 is about the same as the length L2 of the riser moonpool, for instance 22 m, so that a very compact construction is obtained in which the length of the piping that interconnects the risers 27 and the process equipment on the deck 19 of the vessel is minimized.

Figure 7 shows a spread moored vessel having a riser moon pool 17 of cylindrical cross-section, situated near the bow 3. The longitudinal bulkheads 30, 31 are spaced apart further near the bow to accommodate the cylindrical riser moon pool 17. Mooring lines 50,50’-53, 53’ are attached to the respective mooring line connectors 9,10,13 and 14. The moon pool 17 accommodates a first group of risers 54 and umbilicals 55 entering the moon pool 17 from aft in a sector 180 that is defined by the mooring lines 52,53. A second group of risers 56 extends from the moon pool 17, in forward direction in a sector 181 that is defined by the mooring lines 52’, 53’. In this forward position of the riser moonpool 17, a larger number of risers can be accommodated and the risers are subject to reduced bending moments..

Figure 8 shows the riser moon pool 17 being situated between cargo tanks 58, 59. The risers 27 enter into the moon pool through the bottom 18 via an opening 25 in the ballast tanks 61 that are located between the outer wall 62, 63 of the hull 2 and the inner walls 64, 65. An inclined cylindrical wall 60 allows the risers 27 to follow an inclination of about 12 degrees to the vertical. From the riser frame 26 at which the risers are hung off, piping 66 extends upward via an intermediate deck 67 to manifold modules 68, 69 on each side of the moon pool 17. A riser pulling system 70 is situated over the moon pool 17 for pulling in the risers from the seabed to their connect position on the riser frame 26.

As shown in figure 8, the riser 27 is guided through a sleeve 71 of the riser frame 26. The riser 27 is connected to the upper part of the sleeve 71 via a flanged part 73. A detachable spool piece 72 connects the riser 27 to the vertical piping 66. During pulling in of the riser 27 by the pulling system 70, the spool piece 72 is disconnected and the riser is pulled upward with a pulling line 74 that is attached to a winch of the pulling system, along the inclined riser direction. After pulling in and attaching the upper flanged part 73 of the riser 27 to the sleeve 71 , the spool piece 72 is installed so that the riser is placed in fluid connection, via the vertical piping 66, with the ESDV 75 of the manifold module 68. At the manifold module 68, the connection of the riser with piping of the process equipment on deck of the vessel is made.

Figure 9 shows the lay-out of the manifold areas 33, 34 on the deck 19, situated alongside the riser moonpool 17 and bounded in the length direction by the front and rear transverse sides 39,42 of the moonpool. The manifold modules 68,69 are located within the manifold areas 33, 34. The risers 76,

76’ and 77,77’ are placed in two pairs of rows 78,79; 80,81 that extend along the longitudinal sides 37,38 of the riser moonpool 17, leaving a free central area 84,85 for diver access.

In figure 10, a riser pulling system is shown that is suitable for loads larger than 300 tons, in which a linear winch 107 and storage drum 108 are positioned along the rear transverse edge 42 of the moonpool 17. A pulling line 109 extends from the storage drum 108 along a fixed sheave 110, to a movable sheave 11 that can travel along the rows 80-81 of risers in the length direction of the moonpool 17.

Claims

1 . Offshore vessel (1 ,150) comprising a hull (2,155) with a length of at least 250m and a width of at least 30 m, a bottom (18), longitudinal sides (5,6;153,154), a bow (3,151) and a stern (4,152), spread mooring connectors (9,10,12,13;156,157,158) situated at each side of the hull near the bow and near the stern on the longitudinal sides, near deck level, and a riser moonpool (17,170) situated on a longitudinal center line (14) of the vessel extending in a vertical direction from deck level to keel level, the moonpool comprising a riser connector member for attaching risers (27, 76, 76’, 77, 77’), to the hull (2,155), characterized in that, the moonpool is situated closer to the bow (3,51) than to a mid-ship position, wherein the moonpool comprises a vertical wall forming a closed casing of circular or square cross section and is adapted to accommodate risers, an internal turret or hydrocarbons or water, the vessel not comprising mooring means at or near the bottom in the area of the moonpool, and at least one landing zone being provided for mooring a work-over or supply vessel along one or along each longitudinal side (5,6; 153, 154) near the rear and/or front spread mooring connectors (9,10,12) with a lifting device on deck near the at least one landing zone, wherein the riser connector member comprises a riser support structure (26) extending across the moonpool (17,170) and placed in a fixed position and orientation in the moonpool near the bottom (18), for attaching the risers (27, 76, 76’, 77, 77’) at spaced-apart positions distributed over the cross section of the moonpool, at least one manifold module (68,69) is situated in a manifold area (33,34) that is defined by the longitudinal sides (5,6;153,154) of the vessel and a front and rear transverse region (35,36) that is situated within 20 m, preferably within 10 m an more preferably within 1 m from a respective front and rear position (39,42) of the moonpool (17,170), and a pulling system (70, 121 ,172) is situated near deck level in an area at or near the moonpool, and comprises a movable lifting member (103,105,111) that is displaceable across the moonpool (17,170) for attaching to the risers (27, 76, 76’, 77, 77’) and exerting an upward pulling force on the risers.

2. Offshore vessel (1 ,50) according to claim 1 , wherein the manifold module (68,69) comprises shut-off valves (75,94,95).

3. Offshore vessel (1 ,50) according to claim 1 or 2, wherein shut-off valves (75,94,95) are situated at the upper end of the risers (27, 76, 76’, 77, 77’).

4. Offshore vessel (1 ,50) according to claim 1 , 2 or 3, wherein the moonpool comprises a wall 30 that near the bottom of the vessel is provided with a reinforcement construction.

5. Offshore vessel (1) according to any of claims 1-4, wherein a manifold area (33,34), carrying at least one manifold module (68,69), is situated on each side of the moonpool (17,170), between the moonpool and a respective longitudinal side (5,6).

6. Offshore vessel (150) according to any of claims 1-5, wherein the manifold area is situated over the moonpool (170).

7. Offshore vessel (1 ,150) according to any of claims 1-6, wherein a central moonpool area (84,85) is free of risers for allowing access to the moonpool for inspection and/or intervention.

8. Offshore vessel (1 ,150) according any of the preceding claims, wherein the riser support structure (26) comprises riser connection members along two longitudinal sides (37,38) .

9. Offshore vessel (1 ,150) according to any of the preceding claims wherein in the rear half, on each side of the vessel, landing zones are provided with mooring members for mooring vessels alongside the landing zone and lay down area.

10. Offshore vessel (1 ,150) according to any of the preceding claims, wherein along two opposed sides (37,38) of the moonpool (17), a support rail (100,101) is provided, the opposed support rails (100,101) carrying a winch support member (102,104), supporting a winch (103,105) and extending transversely to the support rails from one side of the moonpool to an opposing side, the support members (102,104) being displaceable along the support rails (100,101).

11. Offshore vessel (1 ,150) according to claim 8 or 10, the risers extending in rows (78,79,80,81) on each side of the centerline (14), the support members (71 ,90,91) of the riser support structure (26) extending along the riser rows (78,79,80,81).

12. Offshore vessel (1 ,150) according to any of the preceding claims wherein the risers are arranged in rows (78,79,80,81), a movable sheave (111) being displaceable along tracks extending above the riser rows, a winch (107) being situated along a side of the moonpool (17), a pulling member (109) extending from the movable sheave (111) to the winch (107) via a fixed sheave (110) near the winch.

13. Offshore vessel (1 ,150) according to any of the preceding claims, wherein, in a moored position of the vessel, mooring lines (52,52,53,53’) extend from the bow anchor line connectors (9,10) to the sea bed, a first group of risers extending from the turret in a forward direction, in a forward sector bounded by the mooring lines (52,52,53,53’) on each side of the vessel, a second group of risers extending from the turret in a rearward direction in a rearward sector bounded by the mooring lines (52,52,53,53’) .