EP3251942B1

EP3251942B1 - Mooring pulley tensioning system

Info

Publication number: EP3251942B1
Application number: EP17174596.1A
Authority: EP
Inventors: Torkjell Lisland
Original assignee: Scana Offshore AS
Current assignee: Seasystems AS
Priority date: 2016-06-03
Filing date: 2017-06-06
Publication date: 2019-01-02
Anticipated expiration: 2037-06-06
Also published as: EP3251942A1; EP3251943A1; EP3251943B1

Description

Field of invention

The present invention relates to mooring and tensioning of mooring lines for floating structures or vessels, such as Floating Production, Storage and Offloading vessels (FPSO's), semi-submersible platforms, turret moored vessels, floating drilling rigs, drilling ships and other floating structures which are moored to the seabed.
The invention is more specifically directed to a method for tensioning a mooring arrangement of a floating structure or vessel.

Background of the invention

Structures and vessels that are supposed to stay moored at the same location for a long time, such as months or years, have to be moored with a prescribed tension. The traditional method is to use chain tensioning equipment on deck for pull-in and tensioning. Normally this equipment is used during the installation period and left on the vessel for a number of years without being used or maintained. When the mooring lines slacken sufficiently over time, due to wear of chains, shifting of anchors, creep in fiber mooring lines etc, the mooring lines have to be re-tensioned, repositioned (typically move the chain one or two chain links in order to engage the chain stopper on a new chain link) or replaced. In this period between installation and next operation, the equipment has normally seized or corroded and requires refurbishment.
Another issue is the weight and deck space. Deck space is often limited and weight on deck is an important factor for the stability of the vessel.
The aim of this patent application is to provide arrangements and methods where there is little or no requirement for equipment on the deck of the vessel and still be able to perform the installation, tensioning, re-tensioning, repositioning and replacement operations.
US 2014/0216323 describes a mooring arrangement and a method of installing the mooring arrangement. An anchor chain is attached to the seabed by an anchor. A submerged chain stopper is attached to an upper end of the anchor chain. A pull chain or installation chain extends from the structure to be anchored through the chain stopper. The lower end of the installation chain is pulled upwards by the use of a winch on the support vessel until the installation chain passes through the chain stopper, a prescribed tension is achieved and the chain stopper engages on the permanent part of the mooring line, here described as the work chain.
The main difference between the abovementioned patents are that the US 2014/0216323 A1 may use a wire from a winch on the anchor handler towards the platform. By doing this it is possible to tension the mooring line with a force based on the winch capacity. The actual tension force will depend on factors such as angle of the mooring chain with respect to the platform and the support vessel. If the mooring line is close to vertical, the effect of this method is close to zero.
In these references, the arrangement and method are relied on pulling the installation chain of the mooring chain vertically or close to vertically upwards to an installation vessel in order to tension the chain. The benefit of both these systems is that there is no chain and chain handling equipment on the deck of the FPSO. However, the main disadvantages are that all mooring line tensioning requires an installation vessel and fine tuning of each mooring line may be difficult. Additionally, the installation requires an additional length of installation chain.
US 2002/0189522 describes a similar arrangement and method as above. WO 03/013950 also describes a similar mooring arrangement and method.
US 5390618 discloses a system and method for mooring a vessel offshore. The vessel mooring line is connected to the vessel at both ends through separate fairlead and winch arrangements. The mooring line is thus tensioned by pulling the winch in one of the ends arranged on the vessel. The fairleads are arranged in a lateral distance from each other on the vessel.
US5566636 discloses a mooring system for mooring a turret of the vessel having a mooring line is extending from a first fairlead towards a block and back towards a second fairlead at the bottom of the vessel turret.
DE102008029982 discloses a stabilisation and maintenance device for rope tensioned carrier device for a wind energy plant and US3151594 discloses an apparatus for resiliently anchoring a floating structure in a substantially fixed position against lateral movement while permitting vertical movement.
It is expensive to use an installation or support vessel every time a re-tensioning of the mooring is required.
The vertical, or close to vertical, pulling of the chain will also pull the submerged chain tensioner upwards, so that an obtuse angle is created between the chain below the chain tensioner and the chain above the chain tensioner extending between the anchor at the seabed and the moored structure, as can readily be seen in the figures of the two references. The size of this angle will depend on several factors, such as pulling tension, weight of the chain, friction in reversing pulley of the chain tensioner, sea currents etc. Due to the angle, the length of chain between the anchor and the floating structure is somewhat greater than the linear distance between the anchor and the floating structure.
Especially if the anchor is at a great distance from the floating structure or the water depth is small, the mooring line can extend at a very shallow angle from the seabed. In such cases the pull to tighten the mooring can be almost at right angle to the mooring line. This will result in the angle between the two parts of the mooring line, below and above the chain tensioner, becoming smaller and thus the length of the mooring line during tensioning will become far greater than the direct distance between the anchor and the floating structure.
When the mooring has been tightened to the prescribed tension, the upper end of the installation chain will be released. Hence, the chain tensioner will sink until it finds itself approximately on the straight line between the anchor and the floating structure. This inevitably leads to a slackening of the mooring. Hence, the mooring must be tightened somewhat beyond the required tension to account for this slackening.
However, it is difficult to predict how much overtightening is required to achieve the correct tension.

Objectives of the present invention

The present invention has as a main objective to avoid the problem of having to account for a certain amount of overtightening when installing a mooring or re-tensioning a mooring.
A further objective of the present invention is to provide an arrangement and method for installation, tensioning and replacement of mooring lines where there is no requirement for chain handling on the deck of the floating structure.
Tensioning can be performed from an installation vessel and in particular smaller vessels due to multiplication of the tensioning force from the pulley arrangement. The multiplication factor is close to 3 depending on actual angle and friction.
Additionally, a slightly longer work chain can be used instead of an installation chain. An installation wire may act as the main part of the installation chain.
Yet another advantage of the invention is that the chain tensioning may be operated from the FPSO by a wire, rope or hydraulic cylinder.
Another advantage of the present invention is that the weight of the excess top chain is carried mainly by the hull bracket.
A further advantage of the invention is that half of the capacity of the mooring line winch or installation vessel pull is required.
It is yet another advantage of the invention to provide an easy tensioning arrangement for a floating structure with no requirement for chain handling on deck of the vessel and which also requires less/minimum of the tensioning force.
It is yet another advantage of the invention that the tensioning can be performed from an installation vessel or the moored unit, whichever most convenient.

Summary of the invention

The invention relates to a method for tensioning a mooring arrangement of a floating structure or vessel, said mooring arrangement comprising an anchor, a mooring line, a mooring tensioner and a working line, said mooring tensioner having a tensioning pulley and a chain stopper for tensioning said mooring line or said working line and prevent the same from movin relative to said mooring tensioner, said mooring line being attached at a first end to said anchor and at a second end to said mooring tensioner, said working line being coupled at a first end to a fairlead on said floating structure or vessel and at a second end to an installation wire, a line formed by said working line and said installation wire extending over said tensioning pulley on the mooring tensioner and around the fairlead so that said working line having a first flight extending between said fairlead in said floating structure or vessel and said tensioning pulley and that a second flight extending between said tensioning pulley and said fairlead on the floating structure said first and second flight being parallel t.
The method comprising the step of coupling the installation wire (8) to a pulling unit (9, 10) that is remotely arranged from the floating structure (11), tensioning said line (5, 8) by said pulling unit (9, 10) thereby imposing tensioning force on said mooring line directed towards said floating structure or vessel and which does not pull said mooring line out of a straight line between said anchor and said floating structure or vessel.
The method preferably comprises additional features as defined in the dependent claims.

Brief description of the drawings

The foregoing features of the invention will be more readily understood by reference to the following detailed description taken with reference to the accompanying drawings.

Figure 1 shows a tensioning system of the invention in a first aspect and a first embodiment,
Figure 2 shows a tensioning system of the invention in a first aspect and a second embodiment,
Figure 3 shows a mooring system that has been tensioned according to the invention after the tensioning has been completed in a first configuration,
Figure 3a shows a mooring system that has been tensioned according to the invention after the tensioning has completed in a second configuration,
Figure 4 shows a mooring tensioner to be used in the present invention with a chain passing through,
Figure 4a shows a mooring tensioner to be used in the present invention with a wire or rope passing through,
Figure 5 shows a fairlead of the present invention in a first embodiment,
Figure 6 shows a fairlead of the present invention in a second embodiment,
Figure 7 shows a fairlead of the present invention in a third embodiment,
Figure 8 shows a fairlead of the present invention in a fourth embodiment,
Figure 9 shows a first step in a first embodiment of a method of tensioning according to the present invention,
Figure 10 shows a second step in a first embodiment of a method of tensioning according to the present invention, where a mooring tensioner has been attached to the mooring line,
Figure 11 shows a third step in a first embodiment of a method of tensioning according to the present invention, where a pendant is about to be coupled to a working line,
Figure 12 shows a fourth step in a first embodiment of a method of tensioning according to the present invention, where the working line and pull-in line has been deployed into the sea,
Figure 13 shows a fifth step in a first embodiment of a method of tensioning according to the present invention, where a pull-in line is about to be coupled to a winch on board the installation vessel,
Figure 14 shows a sixth step in a first embodiment of a method of tensioning according to the present invention, where the pull-in line has been coupled to the winch on board the installation vessel,
Figure 15 shows a seventh step in a first embodiment of a method of tensioning according to the present invention, where the pull-in line is tensioned and thereby tensioning the mooring,
Figure 16 shows a first step in a second embodiment of a method of tensioning according to the present invention,
Figure 17 shows a second step in the second embodiment of a method of tensioning according to the present invention, where a mooring line has been brought up to the installation vessel and a working line has been coupled to the floating installation,
Figure 18 shows a third step in the second embodiment of a method of tensioning according to the present invention, where a pull-in line has been connected between a winch on the floating installation and the installation vessel,
Figure 19 shows a fourth step in the second embodiment of a method of tensioning according to the present invention, where a mooring tensioner has been connected to the mooring line,
Figure 20 shows a fifth step in the second embodiment of a method of tensioning according to the present invention, where the mooring tensioner, working line and pull-in line has been deployed to the sea,
Figure 21 shows a sixth step in the second embodiment of a method of tensioning according to the present invention, where the pull-in line is about to be tensioned,
Figure 22 shows a seventh step in the second embodiment of a method of tensioning according to the present invention, where the pull-in line and the working line is tensioning the mooring line, using a winch on the floating structure,
Figure 23 shows the mooring after completing the tensioning using the second embodiment of a method of tensioning according to the present invention, where
Figure 24 shows a cluster of moorings, with a first mooring completed, a second mooring about to be tensioned and a third mooring yet not installed,
Figure 25 shows a close-up of the upper parts of the moorings in figure 24 and a tensioning winch,
Figure 26 shows an alternative mooring tensioner,
Figure 27 shows a tensioning arrangement according to a second aspect of the present invention,
Figure 28 shows a first step in a first installation method for installing a tensioning arrangement according to the second aspect of the present invention,
Figure 29 shows a second step in the first installation method for installing a tensioning arrangement according to the second aspect, where a pull-in line is about to be coupled to the mooring line and a winch,
Figure 30 shows a third step in the first installation method for installing a tensioning arrangement according to the second aspect, where the pull-in line is about to be deployed to the sea,
Figure 31 shows a fourth step in the first installation method for installing a tensioning arrangement according to the second aspect, where the pull-in line is being tensioned,
Figure 32 shows a first step in a second installation method for installing a tensioning arrangement according to the second aspect,
Figure 33 shows a second step in the second installation method for installing the tensioning arrangement according to the second aspect, where a pull-in line has been coupled between two winches on the installation vessel,
Figure 34 shows a third step in the second installation method for installing the tensioning arrangement according to the second aspect, where a mooring line has been coupled to one of the winches,
Figure 35 shows a forth step in the second installation method for installing the tensioning arrangement according to the second aspect, where the pull-in line and the mooring line has been connected, and
Figure 36 shows a fifth step in the second installation method for installing the tensioning arrangement according to the second aspect, where the pull-in line is being tensioned.

Detailed description of embodiments of the invention

Figure 1 shows a mooring arrangement of the present invention installed between an anchor 2 that has been attached to the seabed 1 and a floating structure, such as an FPSO 11. How the installation has been achieved will be explained in detail later.
The anchor 2 is conveniently a suction anchor, but may alternatively be any type of anchor known in the field.
The mooring arrangement further comprises a mooring chain 3, a chain tensioner 4 with pulley 20, a working chain 5, a pendant line 6, a hull bracket 33, a fairlead 7 and a pull-in wire 8.
The mooring chain 3 may also be a steel wire, polyester rope or a combination of these and the working chain 5 and pendant 6 may also be a mooring chain, steel wire, polyester rope or a combination of these. The working chain 5 may also extend all the way to the fairlead 7. The installation wire 8 may be steel wire, polyester rope or a combination. It may comprise several segments of these.
Conveniently, the fairlead 7 is attached to the hull bracket 33 and the fairlead 7 may rotate about a vertical axis.
As seen in figure 1, the mooring chain extends between the anchor 2 and the chain tensioner 4. The pendant wire 6 is attached to the fairlead 7 at an upper end. The lower end of the pendant wire 6 is attached to the working chain 5. The working chain 5 extends around the pulley 20 of the chain tensioner 4 and is at the opposite end of the pendant wire 6 attached to the pull-in wire 8.
The pull-in wire, also called installation wire 8 extends around a pulley 22 in the fairlead 7 and further to a winch 10 on a support or installation vessel 9.
Figure 2 shows an alternative mooring arrangement. It is similar to the arrangement of figure 1 except that the installation wire 8 extends to a winch 13 on the floating structure 11 instead of to a winch on a support vessel. A pulley 12 mounted at the edge of the deck of the floating structure 11 ensures proper guiding of the installation wire 8. Alternatively, the winch 13 may be located on the edge of the deck with the pull-in wire 8 extending directly downwards. In this case the pulley 7 is not required, as shown and explained in connection with figure 22.
The arrangements of figures 1 and 2 may be used as alternatives, but they may also be used in stages, where the first part of the tensioning is done with the arrangement of figure 1 and the installation wire 8 is then transferred to the winch 13 on the floating structure 11 so that the final tensioning is done with the arrangement of figure 2. This is especially convenient when the final tensioning has to be done after all the moorings have been installed.
Figure 3 shows the mooring arrangement after the tensioning has been completed. The installation wire 8 will then typically be removed and figure 3 shows that the installation wire 8 has been detached from the working chain 5 and removed.
Figure 3a shows an alternative to removing the installation wire 8. Here a part 8a of the installation wire has been left as a non-tensioned length.
Figure 4 shows the chain tensioner 4 in detail. Although it acts as the main component during the tensioning of the mooring, it also includes a chain stopper 16, which has the function locking the two mooring line parts together to maintain a specific length.
The chain tensioner 4 comprises a steel structure 14 that solid enough to act as a member of the mooring line and as such can withstand at least the same Minimum Breaking Load (MBL) as the mooring chain 3. The structure 14 has a channel 14a through which the working chain 5 can pass. At one end the structure 14 is connected to the mooring line 3 with a connecting bolt 15 or alternatively shackle. Any kind of connecting links, shackles or other connecting elements may be used.
At the other end where the working chain 5 enters the channel 14a of the structure 14 there is a chain stopper 16. The chain stopper 16 is shown with a single latch 16a but may consist of two latches, as is known per se. The latch 16a is connected to a spring 19 that biases the latch 16a towards a closed position, to ensure safe closing of the latch 16a. Additionally, there may be a temporary installed hydraulic cylinder 18, which is capable of opening the latch 16a if the chain has to be paid out. The hydraulic cylinder 18 may be operated from the installation vessel 9 or an ROV (not shown). After the tensioning is finished, the hydraulic cylinder 18 is preferably removed to avoid fouling and corrosion due to prolonged exposure to sea water.
At the upper end, i.e. towards the floating structure or FPSO 11 there is a guide 17 to guide the working chain 5 into the channel 14a.
The chain tensioner 4 also has a chain tensioner pulley 20 that is rotatable about a shaft 21 that is mounted in the structure 14. The purpose of the chain tensioner pulley 20 is to guide the working chain 5 out of the chain tensioner 4 and back towards the fairlead 7.
In Figure 4A, the same chain tensioner 4 is shown as in figure 4 but with the installation wire 8 running through the channel 14a, as will be the case during the installation phase. The latch 16a is open at this stage.
Figure 5 shows the hull bracket 33 and the fairlead 7. The fairlead that comprises a steel structure 26 and a pulley 22. The hull bracket 33 comprises a lower hull support 27 and an upper hull support 28. A vertical shaft 29 connects the steel structure with the supports 27, 28, so that the structure 26 can rotate about a vertical axis.
At the outer end of the structure 26 the pendant wire 6 is attached by means of a connection bolt 24 that attaches an end termination 23 of the pendant wire to the structure 26. The end termination 23 can rotate about the bolt 24 in a vertical plane.
The fairlead pullet 22 is rotatably supported in the structure 26 by a shaft 25. The structure 26 has a channel 26a that receives the installation wire 8, which extends about the fairlead pulley 22.
The hull bracket 33, structure 26 and shaft 24 and 29, all these elements are part of the mooring load line and has to be strong enough to carry the mooring load.
An alternative fairlead construction can be seen in Figure 6. Here the steel structure 26 is welded to the hull and is not capable of rotating in a horizontal plane as in Figure 5. Instead, a connecting link 30 with a vertical shaft 31 has been added between the end termination 23 and the bolt 24, to endure free movement in two planes for the pendant 6.
A further another alternative solution for the fairlead construction is shown in Figure 7. As for the previous embodiments, the pendant 6 with end termination 23 is connected to the steel structure 26 via a shaft 24. However, the steel structure 26 is connected to the hull via a bracket 33 and a connecting link 30. Two shafts 31, 32 ensures freedom of movement in two planes.
In the alternative embodiment of Figure 8, the connecting link 30 and shaft 32 has been replaced by a connector comprising a male part 34 and a female part 35, the female part 35 being capable of retaining the male part 34 within a hole (not shown) in the female part 35. The female part 35 is rotatable about a horizontal axis with respect to the bracket 33.
A rope or wire 36 is initially connected at the end of the male part 34. The rope or wire 36 is fed through the hole in the female part, and by pulling the rope or wire 36, the male part 34 can be brought to enter the hole of the female part 35.
The male connector 34 and female connector 35 will engage automatically or with the intervention of ROV, a diver or by other mechanic means, depending on the environment.
The pendant 6 may in some cases be replaced by extending the working chain 5.
In some cases, the pendant 6 or working chain 5 may be fixed directly to the hull of the floating structure 11 by a bolt through a bracket. In such a case the pulley 22 may be arranged separately of the attachment of the pendant 6 or working chain to the hull.
The arrangement of the present invention has the benefit that the feeding of the pull-in line 8 over the pulleys 20 and 22 ensures both that the mooring line 3 and working chain 5 are in the same line and extends directly between the anchor 2 and the floating structure 11, and due to the fact that the system has the configuration of a block and tackle, the tensioning of the mooring arrangement can be done with double tension force by the winch 10 or 13 as compared to the prior art configurations.
In the case where the installation wire 8 is connected to the pull-in winch 10 on the installation vessel the tensioning can be done both by rotating the winch and by moving the installation vessel 9 relative to the floating structure 11. This will provide the possibility of achieving a much higher tension force than the winch can achieve alone. Instead of using the winch 10 as the point of attachment of the pull-in line 8 on the support vessel 9, the installation line 8 can also be attached to the vessel 9 itself. This way a vessel 9 without a powerful winch can also be used to tension the mooring.
An installation sequence according to the invention will now be described, referring to figures 9-15.
During the installation, a number of additional ropes, wires and winches than described above are typically used. These items will be described below.
Figure 9 shows the FPSO 11 at the start of the installation of the mooring arrangement. The pendant line 6 is connected to the fairlead 7a forerunner 8a of the installation wire 8 has been passed through the fairlead pulley 22. The installation vessel 9 or specialized anchoring vessel 2 has installed the anchor at the seabed. Typically, the mooring chain 3 has been left on the seabed attached to a rope 37 with a buoy (not shown) at the free end.
In figure 9 the buoy and rope 37 has been picked up and the winch 10 of the vessel 9 is pulling in the mooring line 3.
In Figure 10, the chain tensioner 4 has been attached to the mooring line and the pull-in line 8 has been fed through the chain tensioner 4. The installation line 8 has the working chain 5 attached to the training end. The free end of both the pull-in line 8 and the working chain 5 are on board the vessel 9, with the free end of the installation line connected to the winch 10, and the free end of the working chain 5 is fixed to the installation vessel 9, typically in shark jaws (not shown).
The installation winch 10 on the installation vessel 9 now pays out and lowers the chain tensioner 4 to reduce the tension in the mooring line 3. This operation conveniently takes place at a safe distance from the FPSO 11.
In Figure 11, the installation vessel 9 has moved closer to the FPSO 11. When close enough the free end pendant 6, which has been kept on board the FPSO, is transferred to the deck of the installation vessel 9. This free end is connected to the working chain 5.
In Figure 12, the pendant 6 and the working chain 5 is lowered from the installation vessel 9 using a wire 37 that is connected to the winch 10 at one end and at the connection point between the working chain and the installation line 8 at the other end. The upper end of the installation line 8 has now temporarily been attached to the vessel 9. During the lowering of the connection point between the working chain 5 and the pull-in line 8, the installation vessel 9 moves away from the FPSO 11.
As shown in Figure 13, both ends of the forerunner 8a are transferred from the FPSO 11 to the installation vessel 9. The lower end of the forerunner 8a, i.e. the end that had been fed downwards through the fairlead 7, is connected with the upper end of the installation wire 8, that extends upwards from the chain tensioner 4. The other end of the forerunner, i.e. that extends upwards from the fairlead 7, is connected to the installation winch 10. Figure 14 shows the pull-in wires 8 and the forerunner 8a connected.
As shown in Figure 15, the wire 37 has been detached, and the forerunner 8a has been wound in so that the installation line 8 has reached the winch 10. The connection between the working chain 5 and the installation line 8 has passed through the chain tensioner 4. Tensioning is now in progress by the installation winch 10 on the installation vessel 9 or alternatively by the installation winch 13 on the FPSO 11.
As stated above, after final tensioning has been completed, the installation wire 8 is usually removed, as shown in figure 3. The installation winch 13 on the FPSO 11 may also be removed. Alternatively, a short part of the installation wire 8 may be left for further work on the mooring line, as shown in Figure 3A.
An alternative installation method according to the present invention will now be explained, referring to Figures 16-21.
The main difference between this installation method and the one described above is that in the following all the tensioning of the mooring line 3 is performed from the FPSO 11.
Figure 16 shows the FPSO 11 with the installation winch 13, a hull bracket 33, which in this case is mounted close to the deck of the FPSO, for the work chain 5 or pendant 6 and the installation vessel 9 with the installation winch 10. This is the initial state of the installation operation.
In Figure 17, the mooring line 3 has been pulled up from the seabed and locked on the deck of the installation vessel 9. The installation of the anchor and retrieval of the mooring line 3 is done in the same way as explained above with regard to figure 9. The free end of the pull-in wire 8 has been transferred to the installation vessel 9 and connected to the work chain 5 and is being pulled over towards the FPSO 11 using the winch 13.
In Figure 18, the working chain 5 has been connected to the hull bracket 33 with a bolt, shackle, H-link or any kind of connecting link. The other end of the working chain 5 is fixed to the installation vessel 9.The installation wire 8 is connected to one end of a forerunner 8a that has its opposite end attached to the vessel 9.
In Figure 19, the mooring line 3 is connected to one end of the chain tensioner 4, which is resting on the deck of the installation vessel 9. The position of the mooring chain 3 is arbitrary as it will most likely be coming in from the stern of the vessel 9. The forerunner 8a is pulled over the pulley 20 of the chain tensioner 4, through the chain stopper 16, and is then connected to the working chain 5. The chain tensioner 4 is now ready to be deployed into the sea.
In Figure 20 the chain tensioner 4 together with the mooring line 3, working chain 5, installation line 8 and forerunner 8a are lowered from the installation vessel 9 with the installation winch 10 with a wire 37 that is attached to the chain tensioner 4.
In Figure 21, the installation winch 13 is pulling in the working chain 5 through the chain tensioner 4 to tension the mooring line 3.
In Figure 22, the installation vessel 9 has disconnected from the chain tensioner pulley 4 and final tensioning is performed by the winch 13 on board the FPSO 11. All of the pull-in wire 8 is now on the drum of the winch 13 when final tension is achieved.
Figure 23 shows the finished mooring. The forerunner 8a has been attached at its upper end to the FPSO for further tensioning operations if required. This line may, however, be removed. The installation winch 13 has also been removed.
As an alternative to using a two- part installation wire 8, 8a, only one may be used. This requires disconnection of the pull-in wire 8 from the working chain 5 after the tensioning operation.
Figure 24 shows a cluster for three mooring lines. One has already been installed and tensioned, and the second is under tensioning and a third mooring has not yet been initiated. The installation winch 13 on the FPSO 11 is fixed in one position and can handle all mooring lines from this position.
Figure 25 is a close-up of the mooring line interface to the FPSO 11. The installation winch is fixed in one position. An auxiliary winch 38 may be used for pulling in the working chain 5 during the initial stages of the operation. This auxiliary winch 38 can be moved to suitable positions each hull bracket 33. The figure shows that the forerunner 8a is fixed to a hull bracket 33.
Figure 26 shows a modified version of the chain tensioner 4 as compared with Figure 4 and 4A. Here a lifting yoke 39 has been added for lifting or holding the chain tensioner 4 by a wire 37 from the installation vessel 9. Conveniently, the lifting yoke 39 is rotatably attached to the chain tensioner 4 at a common rotation axis with the pulley 21. This ensures that the chain tensioner 4 does not rotate if the pulling force from the wire 37 or the working chain 5 changes.
Another alternative configuration can be seen in Figure 27 where the chain tensioner 4 has been attached to the FPSO 11 via the hull bracket 33 possibly via a link or shackle.
In this embodiment, the vessel 9 installs and tensions the mooring line 3 via a pull-in wire (8) that at one end is connected to the winch 10 or a fixed point on the vessel 9. In the latter case, the mooring is tensioned using the bollard pull of the vessel 9.
A typical installation method for this embodiment will be explained referring to Figures 28-31.
In Figure 28, the chain tensioner 4 has been installed with a forerunner 38 passing through it. The vessel 9 is pulling up the mooring line 3 from the seabed, which has been installed as explained in connection with figure 9 above.
In Figure 29, the vessel 9 has already pulled the mooring line 3 to the deck and attached a hook 39 to the mooring line 3. The hook 39 is attached to a wire 37 running from the winch 10 and the mooring line 3 has been lowered. As the vessel 9 moves closer to the FPSO 11 while the winch 10 pays out the wire 37 to reduce the tension in the mooring line 3. The forerunner 38 is passed from the FPSO 11 and connected to another winch 10a at one end, i.e. the end on the upper side of the chain tensioner 4. The other end, i.e. the end on the lower side of the chain tensioner 4 is connected to the upper end of the pull-in wire 8.
In Figure 30 the vessel 9 has moved away from the FPSO 11, the forerunner 38 has been pulled onto to the winch 10a and the mooring line 3 is being held by the first winch 10 via the wire 37 and hook 39.
In Figure 31, the hook 39 and wire 37 has been disconnected. Tension is now on the pull-in wire 8 and mooring line 3 via the chain tensioner 4. The pull-in line 8 is pulled in until the upper end of the mooring chain 3 has passed through the chain tensioner 4, as shown in figure 27, which represent the final tensioning. After final tensioning, the installation wire 8 is disconnected from the mooring line 3.
A further alternative method to the one described in Figures 28-31 can be seen in Figures 32-36.
In Figure 32, the vessel 9 approaches the FPSO 11. On the FPSO 11 the chain tensioner 4 is attached to the FPSO 11 via a hull bracket 33 via a possible link, shackle or shaft. A forerunner 38 has been installed through the chain tensioner 4.
In Figure 33, the vessel 9 moves closer to the FPSO 11 and both ends of the forerunner 38 are transferred to the vessel 9. One end of the forerunner 38, i.e. the end emerging from the lower side of the chain tensioner 4, is connected to the installation wire 8. The installation wire 8 is spooled onto the winch 10. The other end of the forerunner 38, i.e. the one emerging from the upper side of the chain tensioner 4, is connected to another winch 10a. By paying out the installation wire 8 and pulling in on the other winch 10a, the pull-in line passes through the chain tensioner 4.
In Figure 34, the vessel 9 moves away from the FPSO 11 while paying out the pull-in wire 8. The mooring line 3 is pulled up from the seabed with a wire 37 and the winch 10. The mooring line 3 and anchor 2 has been installed, and the mooring line 3 has been retrieved as explained above in connection with figure 9.
In Figure 35, the vessel 9 has already pulled the mooring line 3 to the deck and attached a hook 39 to the mooring line 3. The hook 39 is attached to a wire 37 running from the winch 10 and the mooring line 3 has been lowered. The installation wire 8 has been attached at one end to the mooring line 3 and the mooring line 3 is in the process of being tensioned by the winch 10 by pulling the installation line 8.
In Figure 36, the hook 39 and wire 37 has been disconnected, tension is now on the installation wire 8 and mooring line 3 via the chain tensioner 4. The tensioning will continue until the upper end of the mooring line 3 has passed through the chain tensioner 4.
Final tensioning can be done as shown and described in connection with Figure 27. After final tensioning, the installation wire 8 is disconnected from the mooring line 3, as explained above.
It is to be understood that the present invention is not to be limited by the embodiments of the invention described herein. Indeed, those skilled in the art will readily understand that various modifications and embodiments of the invention may be made and practiced without departing from the scope of the claims.

Claims

A method for tensioning a mooring arrangement of a floating structure or vessel (11), said mooring arrangement comprising an anchor (2), a mooring line (3), a mooring tensioner (4) and a working line (5), said mooring tensioner (4) having a tensioning pulley (20) and a chain stopper (16) for tensioning said mooring line (3) or said working line (5) and prevent the same from moving relative to said mooring tensioner (4), said mooring line (3) being attached at a first end to said anchor (2) and at a second end to said mooring tensioner (4), said working line (5) being coupled at a first end to a fairlead (7) on said floating structure or vessel (11) and at a second end to an installation wire (8), a line (5, 8) formed by said working line (5) and said installation wire (8) extending over said tensioning pulley (20) on the mooring tensioner (4) and around a pulley (22) on the fairlead (7) so that said working line (5) having a first flight extending between said fairlead (7) on said floating structure or vessel (11) and said tensioning pulley (20) and a second flight extending between said tensioning pulley (20) and said fairlead (7) on the floating structure (11) said first and second flight being parallel, wherein
the method comprising the step of
coupling the installation wire (8) to a pulling unit (9, 10) that is remotely arranged from the floating structure (11),
tensioning said line (5, 8) by said pulling unit (9, 10) thereby imposing a tensioning force on said mooring line (3), directed towards said floating structure or vessel (11), and which does not pull said mooring line (3) out of a straight line between said anchor (2) and said floating structure or vessel (11).
The method for tensioning the mooring arrangement according to claim 1, characterised in that said second flight of said working line (5) extends over a fairlead pulley (22) that is attached to said floating structure or vessel (11) close to, or at the same position as, said fairlead (7) of said first end of said working line (5) to said floating structure or vessel (11), so that said second flight of said working line (5) extends parallel with said first flight between said tensioning pulley (20) and said fairlead pulley (22).
The method for tensioning a mooring arrangement according to any one of the preceding claims, characterised in that said mooring line (3) or said working line (5) is coupled to the installation wire (8), which in turn is coupled to said pulling unit (9, 10).
The method for tensioning a mooring arrangement according to claim 3, characterised in that the method further comprises removal of the installation wire (8) after tensioning of the mooring arrangement.
The method according to claim 4, characterised in that the method further comprising the step of
pulling the mooring line (3) and working line (5) via several pulleys (20, 21) arranged respectively at the mooring tensioner (4) and at the offshore structure (11), so that the tensioning force on the mooring line (3) are multiplied without increasing the pulling force from the pulling unit (9, 10).
The method according to any one of the preceding claims, characterised in that said tensioning force is created by a pulling unit (9, 10), which is a winch (10) on an installation vessel (9) or said vessel (9) and its propulsion system.