NO20170862A1

NO20170862A1 - A mooring system

Info

Publication number: NO20170862A1
Application number: NO20170862A
Authority: NO
Inventors: Arild Bech
Original assignee: Can Systems As
Priority date: 2017-05-24
Filing date: 2017-05-24
Publication date: 2017-05-24

Description

A mooring system

Field of the invention

The invention concerns the field mooring systems for floating vessels. More specifically, the invention concerns a mooring line control assembly as set out by the preamble of claim 1, a mooring system as set out by the preamble of claim 6, and methods of installing the mooring system as set out by the preambles of claim 10 and claim 11.

Background of the invention

Offshore exploration and production of hydrocarbon resources has for many years been performed from floating marine vessels, such as so-called SPARs, semi-submersible platforms, and purpose-built ships (e.g. Floating Production, Storage and Offloading - FPSO ships). Traditionally, such vessels have been moored by steel chains and/or steel ropes, extending between the vessel and seabed anchors for safe station-keeping. As operations over the years have been shifted into deeper waters and thus requiring longer mooring chains, the weight of these mooring chains has become a major design parameter that the operators seek to reduce. The use of synthetic fibre ropes has therefore become more prevalent, in order to reduce overall weight, reduce fatigue characteristies and eliminate corrosion typically associated with metal-based mooring lines.

Synthetic fibre ropes for offshore mooring purposes normally comprise a core (which typically is made up of a number of polyester strands and bundles) enclosed by a braided jacket. Materials commonly used in making synthetic fibre rope are Polyester, Aramid, ultra high-molecular weight polyethylene (UHMWPE), etc. Most synthetic fibre ropes used as mooring lines range from slightly positive to slightly negative buoyancy.

In addition to the mooring lines, mooring systems for offshore vessels involve large sets of heavy equipment on the offshore vessel; e.g. windlasses or winches, fairleads for guiding the mooring lines, line swivels, and chain stoppers.

The prior art includes WO 2011/102730 Al, which describes an anchor spread for mooring a marine vessel, håving a seabed anchor chain connected to one end of a fibre rope, and an anchor winch chain connected to the other end. The fibre rope is prepared by being packed to a bundle or coil arranged in a protective container on the seabed. The fibre rope is pull ed out of the container at a predetermined pulling force, and picked up by the anchor winch chain which is subsequently connected to the anchor winch on the vessel.

The prior art also includes US 4 446 807 A, which discloses mooring apparatus for a semi-submersible oil exploration and drilling rig. The mooring apparatus has eight drum anchor hoists and winches on which are carried eight wire rope mooring lines. Each of these lines extends up over one of eight head sheaves and vertically down the outside face of the oil rig through a fairlead rotatably mounted with respect to the rig at a bottom submerged portion thereof. A linear line pull machine is mounted to the rig in encompassing relation to each of the vertical runs of mooring line between the head sheave and the fairlead. These eight mooring line assemblies are distributed around the periphery of the rig. Two mooring lines extend at 90° from each other at each corner of the rig. At least one hydraulic power unit is provided for each two of these mooring line assemblies, and air and hydraulic control means are provided utilizing power from this power unit selectively to (1) reel out and reel in on the mooring lines when low line forces are involved using the anchor hoist; (2) to exert extremely high forces at slow speeds when necessary to set anchors attached to outer ends of the mooring lines and to positively hold the rig against the upward buoyant effect of the rig's pontoons; or (3) to operatively disassociate the linear pull machines from the mooring lines so that the mooring lines can be reeled outwardly at very high speeds.

The prior art also includes WO 2010/106134 A2, which discloses a vessel håving a hull with a turret, a cavity in the turret and a mooring buoy releasably attached in the cavity. The buoy comprises a buoyant body and carries a number of risers, and a number of anchor lines (chains) connected to the seabed. The connection of the buoy to the cavity is accomplished by a pulling member connected to a winch on the vessel for lifting of the buoy. Each anchor line is at its upper end connected to a chain stopper on the buoy. The prior art mooring systems are complex, heavy and bulky, and impose weight and layout restrictions on the vessel. It is therefore an object of the invention to provide a mooring system which reduces the vessel weight and occupies less space on the vessel.

Summary of the invention

The invention is set forth andcharacterized inthe main claim, while the dependent claims describe other characteristics of the invention.

It is thus provided a mooring line control assembly, comprising a mooring line control unit configured for controlling a mooring line,characterized in thatthe mooring line control assembly further comprises a seabed anchor and connection means for interconnecting the control unit and the anchor.

In one embodiment, the connection means comprises an intermediate portion, connected between the control unit and a lug member on the anchor.

The connection means may comprise a pin-and-receptacle connection, and the control unit may comprise a pin member and the anchor may comprise a receptacle. The pin member and receptacle are configured for mating, interlocking engagement.

In one embodiment, the connection means comprises a permanent connection, for example a welded connection.

It is also provided a mooring system,characterized byone or more of the invented mooring line control assembly and one or more a mooring member. Each mooring member has a lower portion configured for connection to a respective mooring line control assembly and an upper portion håving a connection member configured for connection to a floating vessel. The lower portion may comprise an anchor chain.

In one embodiment, at least an intermediate portion of the mooring line comprises a fibre rope. In one embodiment, the fibre rope has inherent buoyancy and/or comprises one or more buoyancy elements.

It is also provided a method of installing the invented mooring system,characterized bythe steps of

a) connecting a lower mooring member portion to a mooring line control unit; b) suspending the mooring line control unit via the lower mooring member portion and associated mooring line, and lowering mooring line control unit to the seabed; c) connecting the control unit to a seabed anchor via connection means; d) applying a tension force to a free end of the lower mooring member portion after

håving connected a connection member to a floating vessel.

Step a) above may be performed at a topsides location, for example on a support vessel. Step d) above may be performed by an ROV. In this embodiment, the seabed anchor may be any suitable mooring anchor, such as a suction anchor or a piled anchor.

It is also provided a method of installing the invented mooring system,characterized inthat the control unit and the seabed anchor are interconnected prior to deployment into the sea, and further comprising the steps of

a) connecting a lower mooring member portion to a mooring line control unit; b) suspending the interconnected control unit and anchor via the lower mooring member portion and associated mooring line, and lowering the interconnected control unit and anchor to the seabed; c) installing the anchor in the seabed; d) applying a tension force to a free end of the lower mooring member portion after

håving connected a connection member to a floating vessel.

Step a) above may be performed at a topsides location, for example on a support vessel. Step d) above may be performed by an ROV. In this embodiment, the anchor may be a suction anchor.

The invention contributes to reducing overall vessel weight and to free up valuable space on the vessel. Heavy and bulky mooring chains, winches, idlers, chain stoppers and fairleads need no longer be accommodated by the floating vessel, but are placed on the seabed, on - or in close proximity to - the seabed anchor. This relocation of equipment has a favourable effect on the vessel's weight, and hence its dynamic characteristics. The combination of a fibre (e.g. polyester) mooring line and the invented subsea mooring control assembly (i.a. tensioning equipment) is also favourable from an operational point of view. With the invention, design tension is reduced, and static and dynamic vertical mooring loads are reduced.

With the invented seabed mooring control assembly, all components are inspectable and replaceable. The mooring line kept above the seabed, and soil contamination or excavation is avoided. All operations (e.g. re-tensioning) are performed on the anchor.

Any surplus anchor chain may be stored on the anchor or on the seabed. The invention removes the need for any top chain, both for buoy turret systems and spread moored systems. Hence, there is no need for fairleads, upper chain stoppers, heavy winches and large HPUs on the vessel. This saves space and weight on the floating vessel, which then may be used for more useful equipment.

Brief description of the drawings

These and other characteristics of the invention will become clear from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached schematic drawings, wherein: Figure 1 is a plan view of a floating vessel and mooring spread, illustrating a plurality of mooring lines extending between the vessel and respective seabed anchors, comprising a seabed mooring control assembly according to the invention; Figure 2 is a schematic side view of a mooring line extending between a floating vessel and a seabed anchor håving a seabed mooring control assembly; Figure 3 is a side view of a first link, between a main mooring line segment and an upper mooring line segment; Figure 4 is a side view of a second link, between a main mooring line segment and a lower mooring line segment in the form of a chain; Figure 5 is a principle side view of a seabed anchor, with a seabed mooring control assembly according to the invention and a portion of the mooring line; Figure 6 is a principle side view of an embodiment of the seabed mooring control assembly on top of the seabed anchor, seabed mooring control assembly in the illustrated embodiment comprising a fairlead and a chain stopper; Figures 7a to 7e illustrate an embodiment of the invented seabed mooring control assembly, and an associated operation sequence; Figure 8 illustrates an installation configuration of the seabed mooring control assembly and seabed anchor; Figures 9a to 9d illustrate another embodiment of the invented seabed mooring control assembly, and an associated operation sequence; Figures 10a to 10d illustrate yet another embodiment of the invented seabed mooring control assembly, and an associated operation sequence; Figure 11 is a principle side view of an embodiment of the mooring line, comprising a buoyant segment; and Figure 12 is a principle side view of an embodiment of the mooring line, comprising discrete buoyancy elements.

Detailed description of a preferential embodiment

The following description may use terms such as "horizontal", "vertical", "lateral", "back and forth", "up and down", "upper", "lower", "inner", "outer", "forward", "rear", etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader's convenience only and shall not be limiting. Figure 1 illustrates a floating vessel (e.g. an FPSO) 1 moored via a plurality of mooring lines 3 extending between the vessel 1 and respective seabed mooring control assemblies 7. The mooring lines 3 are arranged in four groups, wherein each group comprises four mooring lines 3, with a separation a of approximately 4° between the anchors. The separation between the groups, indicated by /? in figure 1, is hence approximately 78°. The invention shall not, however, be limited to this configuration. Figure 2 corresponds to figure 1, but shows only one mooring line 3, for clarity of illustration. The mooring line 3 is connected between a seabed mooring control assembly 7 and a turret mooring buoy 2. The seabed mooring control assembly 7 is arranged on a seabed anchor 4. Mooring buoys 2, and their connection to the vessel turret, allow the vessel to weather-vane, in a manner which is well known in the art. Certain buoys are releasable from the vessel, and are designed to descend to a predetermined depth when released. The invention shall not, however, be limited to this type of connection to the vessel.

In the illustrated embodiment, the mooring line 3 has an upper segment 3 a comprising a wire rope, a central segment 3b comprising a polyester rope, and a lower segment 3c comprising an anchor chain. The wire rope may be a sheathed spiral strand wire. The polyester rope may be a buoyant polyester rope.

In practical applications in an offshore environment, the dimensions and distances may be considerable. For example, referring to figure 2, the water depth (i.e. distance between seabed B and water surface S) may be approximately 375 meters. The horizontal distance between the vessel 1 and seabed mooring control assembly 7 (and seabed anchor 4) may be on the order of 800 meters, while the total mooring line 3 length may be approximately 850 meters. Out of this total mooring line length, the upper segment 3 a may be approximately 100 meters, the central (fibre) mooring segment 3b may be about 700 meters, and the lower (anchor chain) segment 3c approximately 50 meters. These dimensions are only examples, however, and shall not be limiting for the invention.

In the illustrated embodiment, the steel wire 3 a and fibre rope 3b are connected via an upper (first) link 5, and the fibre rope 3b and the anchor chain 3c are connected via a lower (second) link 6. Referring to figure 3 and figure 4, the upper and lower links both comprise an H-link 10 and a thimble 11, arranged and connected in a manner which per se is known in the art. The invention shall thus not be limited to this type of linkage.

Referring to figure 5, the anchor 4 comprises in the illustrated embodiment a suction anchor, penetrating a distance into the seabed B. The invention is, however, equally applicable to other types of suitable anchors, for example a pil ed anchor. The seabed mooring control assembly 7 is arranged on (and connected to) the anchor 4. Various embodiments for connecting the control assembly to the anchor are described below.

Referring to figure 6, the seabed mooring control assembly 7 comprises in the illustrated embodiment a fairlead 8 and a chain stopper 9. The anchor chain 3c is controlled and locked by the chain stopper and fairlead in a manner which per se is known in the art. The anchor chain free end 3c' extends out from the fairlead 8. In the following, the combined fairlead 8 and a chain stopper 9 is also occasionally referred to as a mooring line control unit 18. All connections, tensioning and re-tensioning may be performed on the anchor, for example by means of an ROV (remotely operated vehicle), by a local tensioning/re-tensioning unit, or from a surface vessel; or a combination of these. Inspection and repair is also performed without håving any need for seabed excavation.

Figures 7a to 7e illustrate an embodiment of the seabed mooring control assembly, and an associated installation sequence. In figure 7a, the anchor chain 3c is connected to the chain stopper 9 and fairlead 8, i.e. the mooring line control unit 18, in a manner which per se is known in the art. The anchor chain 3c is connected to the other mooring line segments (described above, not shown in figure 7a) which are suspended by, and lowered from, a surface vessel (for example an anchor handling vessel; not shown). The seabed anchor 4, which may be a suction anchor or a pil ed anchor, has been installed in the seabed B. The mooring line control unit 18 (here: the fairlead 8) comprises a connection pin 14 and the seabed anchor 4 comprises a connection pin receptacle 15. In figure 7b, the connection pin has entered, and been locked to, the connection pin receptacle, and the fairlead and chain stopper are thus fixed to the seabed anchor.

Interlocking devices, which are commonly known in the art, are not shown. The anchor chain 3 c and anchor chain's loose end 3 c' are lying on the seabed. Figure 7c illustrates a state where the mooring line has been connected to the floating vessel (e.g. the FPSO as discussed above), for example via a connection member 12 (illustrated in figures 11 and 12). Figure 7d illustrates a pretensioning step, wherein the anchor chain's loose end 3 c' is pulled by a surface vessel (using a suitable line or cable; not shown), indicated by the upward-pointing arrow T. Although not illustrated, it should be understood that the pretensioning alternatively may be performed by a jacking system on the seabed, on or in the vicinity of the subsea anchor, operated by a subsea ROV. Figure 7e illustrates the completed installation, following pretensioning. The anchor chain's loose end 3c'

(surplus anchor chain) is stored on the seabed, from where it may be picked up should re-tensioning or adjustments be necessary. Such operations may be necessary following severe weather or/and may be performed regularly as a part of scheduled inspections.

Figure 8 illustrates an alternative anchor installation method to that described above with reference to figures 7a-e: Here, the chain stopper 9 and fairlead 8 have been connected to the anchor before the installation operation commences, in this embodiment a suction anchor 4a, via for example the pin-and-receptacle connection described above. This connection is preferable done on the surface installation (e.g. anchor handling) vessel (not shown). Following the connection procedure, the assembly is lowered from the surface vessel as shown in figure 8, via the anchor chain 3c and the other mooring line segments (described above, not shown in figure 8), and the suction anchor 4a is installed in the seabed in a manner which is known in the art. The subsequent pretensioning and re-tensioning procedures may be performed as described above, with reference to figures 7b to 7e.

As an alternative (not shown), the connection between the mooring line control unit 18 and the seabed anchor 4 may be a permanent connection, such as a bolted or welded connection, effectively rendering the mooring line control unit 18 and the seabed anchor 4 as one unit. Figures 9a to 9d illustrate another embodiment of the seabed mooring control assembly, and an associated installation sequence. In this embodiment, an intermediate anchor chain 16 is connected between mooring line control unit 18 (here: the fairlead 8) and an anchor lug 17 on the side of the suction anchor 4a (see figure 9a). The mooring line control unit 18 has been connected to the suction anchor 4a before the installation operation commences, and then lowered from the surface vessel, as explained above with reference to figure 8. The suction anchor 4a is then installed in the seabed in a manner which is known in the art. Figure 9b illustrates the installed anchor, and the mooring line has been connected to the floating vessel (e.g. the FPSO as discussed above), for example via a connection member 12 (illustrated in figures 11 and 12). Figure 9c illustrates a pretensioning step, wherein the anchor chain's loose end 3c' is pull ed by a surface vessel (using a suitable line or cable; not shown), indicated by the upward-pointing arrow T. This causes a shearing device (not shown) in the pin-and-receptacle 14, 15 connection (not illustrated in figure 9c) to break, whereby the mooring line control unit 18 is detached from the pin-and-receptacle connector, and connected to the anchor only via the intermediate anchor chain 16. The intermediate anchor chain is connected to the anchor lug 17 which is connected to the suction anchor at a distance below the anchor top. The anchor lug is not illustrated in figure 9c, but the figure illustrates how the intermediate anchor chain 16 extends at a slanted angle through the soil beneath the seabed B and emerges from the seabed a horizontal distance from the anchor 4a. Although not illustrated, it should be understood that the pretensioning alternatively may be performed by a jacking system on the seabed, on or in the vicinity of the subsea anchor, operated by an ROV. Figure 9d illustrates the completed installation, following pretensioning. The anchor chain's loose end 3 c' (surplus anchor chain) is stored on the seabed, from where it may be picked up should re-tensioning or adjustments be necessary. Such operations may be necessary following severe weather or/and may be performed regularly as a part of scheduled inspections.

Figures 10a to 10d illustrate yet another embodiment of the seabed mooring control assembly, and an associated installation sequence. Figure 10a shows a seabed anchor 4 installed in a seabed B. The seabed anchor 4 may be any suitable anchor known in the art, for example a suction anchor or a piled anchor. The anchor chain 3c is connected to the mooring line control unit 18. The anchor chain 3c is connected to the other mooring line segments (described above, not shown in figure 9a) which are suspended by, and

lowered from, a surface vessel (for example an anchor handling vessel; not shown). The mooring line control unit 18 (here: the fairlead 8) comprises a connection pin 14 and the seabed anchor 4 comprises an offset connection pin receptacle 15a. The offset receptacle 15a is connect to the side of the seabed anchor 4 by means of a shearable connection (not shown), and an intermediate anchor chain 16a is connected between the offset receptacle 15a and an anchor lug (similar to the anchor lug 17 described above with reference to figure 9a; not shown in figure 10a).

In figure 10b, the connection pin has entered, and been locked to, the offset pin receptacle 15a (by locking means known in the art; not shown), and the mooring line control unit 18 is thus locked to the seabed anchor 4. The anchor chain 3c and anchor chain's loose end 3c' are lying on the seabed. Figure 10c illustrates a state where the mooring line has been retrieved and connected to the floating vessel (e.g. the FPSO, as discussed above), for example via a connection member 12 (illustrated in figures 11 and 12). Figure 10d illustrates a pretensioning step, in which the anchor chain's loose end 3 c' is pulled by a surface vessel (using a suitable line or cable; not shown), indicated by the upward-pointing arrow T. This causes the offset receptacle to shear from the seabed anchor whereby the fairlead and chain stopper are connected to the anchor 4 only via the intermediate anchor chain 16a. As explained above, the intermediate anchor chain is connected to the anchor lug, which is connected to the anchor at a distance below the anchor top. The anchor lug is not illustrated in figure 10d, but the figure illustrates how the intermediate anchor chain 16a extends at a slanted angle through the soil beneath the seabed B and emerges from the seabed a horizontal distance from the anchor 4. Although not illustrated, it should be understood that the pretensioning alternatively may be performed by a jacking system on the seabed, on or in the vicinity of the subsea anchor, operated by an ROV. The completed installation, following pretensioning, will in this embodiment be similar to that described above with reference to figure 9d.

Figure 11 illustrates an embodiment of the invented mooring system (not showing the vessel to which the mooring line is connected). Here, reference number 3b' denotes a buoyant fibre rope, and 12 denotes an upper connection member. This connection member 12 by be connected to a buoy 2 (as described above), but also any other vessels connection means. Figure 12 illustrates another embodiment, where the fibre rope 3b" comprises a plurality of discrete buoyancy elements 13..

Although the invention has been described with reference to connection to a turret buoy, it should be understood that the invention is applicable also to other vessel connection means.

It should be understood that the seabed mooring control system 7 may be modified to accommodate mooring members other than chains. Although the invention has been described with reference to a mooring line comprising an upper wire rope, a central polyester rope, and a lower chain, the invention shall not be limited to this configuration.

Claims

1. A mooring line control assembly (7), comprising a mooring line control unit (18) configured for controlling a mooring line (3; 3 c),characterized in thatthe mooring line control assembly (7) further comprises a seabed anchor (4; 4a) and connection means (14, 15; 16, 17; 15a; 16a) for interconnecting the control unit (18) and the anchor (4; 4a).

2. The control assembly (7) of any one of claims 1 and 2, wherein the connection means comprises an intermediate portion (16; 16a), connected between the control unit (18) and a lug member (17) on the anchor (4; 4a).

3. The control assembly (7) of claim 1 or claim 2, wherein the connection means comprises a pin-and-receptacle connection (14, 15; 15a).

4. The control assembly (7) of claim 3, wherein the control unit (18) comprises a pin member (14) and the anchor (4; 4a) comprises a receptacle (15; 15a), and the pin member (14) and receptacle (15; 15a) are configured for mating, interlocking engagement.

5. The control assembly (7) of claim 1, wherein the connection means comprises a permanent connection, for example a welded connection.

6. A mooring system,characterized byone or more mooring line control assembly (7) according to any one of claims 1-5 and one or more a mooring member (3;

3 a, 3b, 3 c); each mooring member håving a lower portion (3 c) configured for connection to a respective mooring line control assembly (7) and an upper portion (3a) håving a connection member (12) configured for connection to a floating vessel (1).

7. The mooring system of claim 6, wherein the lower portion comprises an anchor chain (3 c).

8. The mooring system of claim 6 or claim 7, wherein at least an intermediate portion (3b) of the mooring line comprises a fibre rope.

9. The mooring system of claim 8, wherein the fibre rope (3b'; 3b") has inherent buoyancy and/or comprises one or more buoyancy elements (13).

10. A method of installing the mooring system according to any one of claims 6-9,characterized bythe steps of a) connecting a lower mooring member portion (3 c) to a mooring line control unit (18); b) suspending the mooring line control unit (18) via the lower mooring member portion (3c) and associated mooring line (3), and lowering mooring line control unit (18) to the seabed (B); c) connecting the control unit (18) to a seabed anchor (4; 4a) via connection means (14, 15); d) applying a tension force (T) to a free end (3c') of the lower mooring member portion (3c) after håving connected a connection member (12) to a floating vessel (1).

11. A method of installing the mooring system according to any one of claims 6-9,characterized in that the control unit (18) and the seabed anchor (4a) are interconnected prior to deployment into the sea, and further comprising the steps of a) connecting a lower mooring member portion (3 c) to a mooring line control unit (18); b) suspending the interconnected control unit (18) and anchor (4a) via the lower mooring member portion (3 c) and associated mooring line (3), and lowering the interconnected control unit and anchor to the seabed (B); c) installing the anchor (4a) in the seabed; d) applying a tension force (T) to a free end (3 c') of the lower mooring member portion (3c) after håving connected a connection member (12) to a floating vessel (1).

12. The method of claim 10 or claim 11, wherein step a) in both claims are performed at a topsides location, for example on a support vessel.

13. The method of claim 10 or claim 11, wherein step d) in both claims are performed by an ROV.

14. The method of claim 11, wherein the anchor is a suction anchor.