EP1799539A1

EP1799539A1 - Apparatus and method for mooring a floating vessel

Info

Publication number: EP1799539A1
Application number: EP05789345A
Authority: EP
Inventors: Luca Canepa
Original assignee: Saipem SpA
Current assignee: Saipem SpA
Priority date: 2004-08-24
Filing date: 2005-08-24
Publication date: 2007-06-27
Anticipated expiration: 2025-08-24
Also published as: GB0418898D0; BRPI0514583A; WO2006021433A1; MX2007002166A; AU2005276561A1; CN101014492A; AU2005276561B2; CN101014492B; EP1799539B1

Abstract

A turret (401) and method for turret mooring a floating vessel. The turret (401) being rotatable with respect to the vessel about a turret axis (405). The turret (401) has a chaintable (413), at a first end of the turret. The chaintable (413) has a plurality of holes arranged at a first distance from the turret axis. The turret also has a hauling device (403) in the turret (401) for exerting a force on a mooring line or riser. The turret also has a beam (423) having a pulley (419; 421) fixed upon it. The beam (423) is rotatable about the turret axis (405). The chaintable (413), hauling device (403) and beam (423) are arranged such that when, in use, a pilot line, connected at a first end to the hauling device (403) and at a second end to the mooring line or riser, passes around the pulley (419; 421), the force exerted on the mooring line or riser by the hauling device (403) via the pilot line is, at the hauling device (403), substantially along the turret axis (405) and substantially towards the chaintable (413).

Description

Apparatus and Method for Mooring a Floating Vessel

The present invention relates to an apparatus and method for mooring a floating vessel. More particularly, but not exclusively, the invention relates to a mooring apparatus and method of the turret mooring type for use with floating production units ("FPUs") such as floating storage and offloading vessels ("FSOs") and floating production storage and offloading vessels ("FPSOs") .

Floating units are utilised in the field of offshore production of oil and gas in deep water. An FPSO is connected to an underwater well or wells by means of risers. The risers are flexible or rigid pipes transporting hydrocarbons from the underwater well or wells to the FPSO. The FPSO is able to store the oil and gas received, to separate the liquid and gas phases and to offload the liquid and gas to oil and gas tankers or pipelines routed to onshore storage sites.

Any floating unit must be secured to the sea bottom by mooring lines in order to keep the unit in position. Several mooring configurations are possible. One mooring configuration is known as spread mooring and is illustrated schematically in Figure 1. Mooring lines 11 are connected to the vessel 13 at several points on the stern and bow of the vessel 13 and each mooring line 11 has its own winch for abandonment/recovery procedures. The risers (not shown) are connected to the vessel, which is stationary relative to the sea bottom. The disadvantage of spread mooring is that, since the vessel is fixed relative to the sea bottom, the mooring lines may be subject to extreme forces in strong winds and currents. In addition, offloading operations can be difficult since the oil/gas tanker must be maintained stationary relative to the vessel throughout. An alternative option for offloading is to route the oil/gas to a separate buoy via export lines. The oil/gas tanker can then offload from the buoy, weathervaining around the buoy as it does so. However, this option involves considerably more equipment and expense.

Another mooring configuration is known as turret mooring and is illustrated schematically in Figure 2. The vessel 21 is formed with a vertical shaft (turret 23) which is fixed relative to the sea bottom. The vessel is allowed to rotate around the vertical axis formed by the turret 23 thereby adopting the most favourable position in light of external forces due to winds and currents. The mooring lines 25 and risers (not shown) are attached to the turret 23. During offloading operations, the oil/gas tanker is subject to the same external forces as the floating vessel so the two vessels remain substantially stationary with respect to one another, thereby facilitating the offloading operations.

In a typical turret mooring arrangement, the turret shaft is made of two elements, the external part and the internal part. The external part is part of the vessel and the internal part is fixed relative to the sea bottom (geostationary) . The two parts are rotatable, relative to one another, around the turret's vertical axis. The lowest portion of the geostationary part is called the chaintable. The chaintable is typically constructed with a number of vertical holes which are used for installation of the mooring lines and risers. Typically, the risers pass through the chaintable, into the turret and are connected to a swivel stack located in the turret. The swivel stack is part of the geostationary part of the turret but contains fluid swivel joints to allow flow continuity between the internal and external parts of the turret and hence the transfer of fluid from the risers to the vessel pipes and tanks. All the mooring chains and risers need to be connectable to a winch for installation and also for abandonment/recovery. In some known arrangements, the winch is located on the main deck of the vessel, in which case the wire path is somewhat restricted due to the relative rotation of the vessel and the turret. In improved known arrangements, the winch is installed in the upper part of the turret with the forces transmitted along a steel wire running down the vertical axis of the turret. In this case, it is well known to use a system of movable pulleys to align the wire (which runs down the turret axis) with the mooring chains and risers (which are located towards the outside of the turret chaintable) . Such an arrangement is shown in Figure 3.

Mooring chain 301 is installed by guide wire 303 transmitting force from winch system 305 in the upper part of the turret. The guide wire 303 runs through at least two movable pulleys 307 which align the guide wire connected to the mooring chain to the working position of the winch (i.e. along the turret's vertical axis) . The pulleys are rotatable around the turret's vertical axis to install mooring chains/risers at different angular positions around the turret and are translatable in the radial direction to install mooring chains/risers at different radial positions on the turret. At least two pulleys are required to convert the force at the winch system to a force in the same ^"direction at the mooring chain/riser during installation/recovery or abandonment.

It can be seen from Figure 3 that, in known arrangements, the pulley mechanism which is used to align the guide wires with the vertical axis of the turret is very big and heavy and considerably increases the size and weight of the whole turret. What is more, the large space occupied by the pulley mechanism and winch system cannot be utilised for other purposes.

It is an object of the invention to provide an apparatus for mooring a floating vessel which overcomes or substantially mitigates the problems of known arrangements described above. It is a further object of the invention to provide a method for mooring a floating vessel which overcomes or substantially mitigates the problems of known methods described above.

According to the invention, there is provided a turret for turret mooring a floating vessel, the turret being rotatable with respect to the vessel about a turret axis, the turret comprising: a chaintable at a first end of the turret, the chaintable comprising a plurality of holes arranged at a first distance from the turret axis; a hauling device in the turret for exerting a force on a mooring line or riser; and a beam having a pulley fixed thereon, the beam being rotatable about the turret axis, the chaintable, hauling device and beam being arranged such that when, in use, a pilot line, connected at a first end to the hauling device and at a second end to the mooring line or riser, passes around the pulley, the force exerted on the mooring line or riser by the hauling device via the pilot line is, at the hauling device,- substantially along the turret axis and substantially towards the chaintable.

"Chaintable" is a well known term in the art of turret mooring and need not be described in any detail here. By way of summary, however, the chaintable is typically in the form of a cylinder having a cross section which is substantially the same as that of the turret. The chaintable is customarily positioned at the lower end of the turret which may be located either above or below the water line, but is usually above the water line but within the splash zone. The chaintable is provided with a number of holes for the attachment of mooring lines and/or risers.

By providing an arrangement with the hauling device in the turret and with the force exerted by the hauling device on the mooring line or riser being along the turret axis and being directed towards the chaintable (rather than away from the chaintable as in prior art arrangements), the whole arrangement of the turret may be simplified. For example, only one pulley is required on the beam, rather than two as in many prior art arrangements. Also, the mooring apparatus takes up less space than prior art arrangements, leaving, for example, room in the upper part of the turret for other applications.

A further advantage of the arrangement is that the pulley can be fixed on the beam, rather than being moveable along the beam as in prior art arrangements; this further simplifies the turret arrangement .

It will be appreciated that, where the pilot line is described as being "connected" at a first end to the hauling device, the term "connected" is not intended to mean that the pilot line is permanently linked to the hauling device. Rather, it is intended that the pilot line is sufficiently coupled to the hauling device for the hauling device to be able to exert a force on the mooring line or riser via the pilot line. Similarly, where the pilot line is described as being "connected" at a second end to the mooring line or riser, the term "connected" is not intended to mean that the pilot line is permanently linked to the mooring line or riser.

Preferably, the pulley is fixed on the beam at the first distance from the turret axis such that the pulley is alignable with each one of the plurality of holes in the chaintable. In that case, in use, the beam may be rotated to a particular angular position such that the pulley is aligned with a selected one of the holes in the chaintable. Because the distance of the pulley from the turret axis is the same as the distance of the holes from the turret axis, the pilot line aligns directly with the selected hole once it has passed around the pulley. Thus, the pulley does not need to be moveable along the beam in the radial direction. In that case, preferably, the pilot line passes around only one pulley.

The beam need not necessarily be oblong in shape, but may be any shape capable of being rotated about the turret axis. The beam is most conveniently elongate and substantially rectangular in plan view and preferably extends between the centre of the turret and its outer portion.

In an embodiment of the invention, the chaintable comprises a second plurality of holes arranged at a second distance from the turret axis. In that case, the beam preferably has a second pulley fixed thereon at the second distance from the turret axis such that the second pulley is alignable with each one of the second plurality of holes in the chaintable.

In that case, in a similar way to the first plurality of holes in the chaintable, in use, the beam may be rotated to a particular angular position such that the second pulley is aligned with a selected one of the second plurality of holes in the chaintabie. Because the distance of the second pulley from the turret axis is the same as the distance of the second plurality of holes from the turret axis, the pilot line aligns directly with the selected hole once it has passed around the second pulley. Thus, at least two radial positions of holes in the chaintabie are operable simply by providing two pulleys at different radial positions, each of which are fixed on the rotatable beam. Neither pulley need be moveable along the beam in the radial direction.

In one arrangement, the second distance is greater than the first distance such that the second pulley is positioned towards the outside of the turret relative to the first pulley. In that arrangement, when the pilot line is required to pass through one of the holes in the first plurality of holes, it passes around only the first pulley but when the pilot line is required to pass through one of the holes in the second plurality of holes (outside the first plurality of holes) , it passes around both the first pulley and the second pulley.

It will be appreciated that the first plurality of holes may be spaced out around the turret axis. In that case, they may form a ring centred on the turret axis. In addition, the second plurality of holes may be spaced out around the turret axis and may form a second ring also centred on the turret axis, i.e. concentric with the first ring of holes. The holes in the two rings of holes may or may not be angularly aligned. The holes in each ring of holes may or may not equi-angularly spaced.

In one embodiment, the hauling device is a jack arranged to selectively draw in or feed out the pilot line. The jack may comprise two clamps for clamping the pilot line, at least one of the clamps being moveable, such that the jack draws in or feeds out the pilot line step wise.

Alternatively, the jack may comprise two moveable clamps for clamping the pilot line, such that the jack draws in or feeds out the pilot line substantially continuously.

In an alternative embodiment, the hauling device is a winch.

Preferably, the hauling device is located in the turret, substantially adjacent the chaintable. Locating the hauling device close to the chaintable allows other parts of the turret to be used for other applications.

Advantageously, the pilot line includes a connection device at its second end, for connecting to a mooring line or riser head. Such a connection device may be of a type that is well known in the art.

The turret may further comprise a joint for connection of a mooring line. The joint may be located on the chaintable itself or on the outer periphery of the chaintable. The joint is preferably angularly aligned with one of the holes in the chaintable. The turret may further comprise a riser guide tube for connection of a riser. The riser guide tube may be mounted on the chaintable.

According to the invention, there is also provided a vessel comprising a turret as described above.

According to the invention, there is also provided a vessel comprising a turret for turret mooring the vessel, the turret being rotatable with respect to the vessel about a substantially vertical axis, the turret comprising: a chaintable at a lower end of the turret, the chaintable having a plurality of holes arranged at a first distance from the turret axis; a hauling device in the turret for exerting a force on a mooring line or riser; and a beam having a pulley mounted thereon, the beam being rotatable about the turret axis, the chaintable, hauling device and beam being arranged such that when, in use, a pilot line, connected at a first end to the hauling device and at a second end to the mooring line or riser, passes over the pulley, the force exerted on the mooring line or riser by the hauling device via the pilot line is, at the hauling device, substantially along the turret axis and substantially in a downwards direction.

Once again, by providing an arrangement with the hauling device in the turret and with the force exerted by the hauling device on the mooring line or riser being along the turret axis and being directed downwards (rather than upwards as in prior art arrangements), the whole arrangement of the turret may be simplified.

Once again, it will be appreciated that, where the pilot line is described as being "connected" at a first end to the hauling device, the term "connected" is not intended to mean that the pilot line is permanently linked to the hauling device. Rather, it is intended that the pilot line is sufficiently coupled to the hauling device for the hauling device to be able to exert a force on the mooring line or riser via the pilot line. Similarly, where the pilot line is described as being "connected" at a second end to the mooring line or riser, the term "connected" is not intended to mean that the pilot line is permanently linked to the mooring line or riser.

The pulley is preferably fixed on the beam at the first distance from the turret axis such that the pulley is alignable above each one of the plurality of holes in the chaintable. In that case, in use, the beam may be rotated in a substantially horizontal plane to a particular angular position such that the pulley is aligned above a selected one of the holes in the chaintable. Because the distance of the pulley from the turret axis is the same as the distance of the holes from the turret axis, the pilot line aligns directly above the selected hole once it has passed over the pulley. Thus, the pulley does not need to be moveable along the beam in the radial direction. In that case, the pilot line preferably passes over only one pulley.

In a particularly advantageous embodiment, the chaintable comprises a second plurality of holes arranged at a second distance from the turret axis. In that embodiment, the beam may have a second pulley fixed thereon at the second distance from the turret axis such that the second pulley is alignable above each one of the second plurality of holes in the chaintable. In that case, in a similar way, in use, the second pulley may be aligned above a selected one of the second plurality of holes in the chaintable. Because the distance of the second pulley from the turret axis is the same as the distance of the second plurality of holes from the turret axis, the pilot line aligns directly above the selected hole once it has passed over the second pulley.

Thus, neither pulley need be moveable along the beam in the radial direction. The beam need not necessarily be oblong in shape, but may be any shape capable of being rotated about the turret axis. The beam is most conveniently elongate and substantially rectangular in plan view and preferably extends between the centre of the turret and its outer portion.

In an embodiment, the hauling device is a jack arranged to selectively draw in or feed out the pilot line. The jack may comprise two clamps for clamping the pilot line, at least one of the clamps being moveable, such that the jack draws in or feeds out the pilot line step wise. Alternatively, the jack may comprise two moveable clamps for clamping the pilot line, such that the jack draws in or feeds out the pilot line substantially continuously.

In an alternative embodiment, the hauling device is a winch.

The hauling device is preferably located in the turret, substantially adjacent and above the chaintable. Locating the hauling device close to the chaintable (i.e. in the lower part of the turret) allows the upper part of the turret to be used for other applications. It also lowers the centre of mass of the turret, thereby providing extra stability.

The pilot line may include a connection device at its second end for connecting to a mooring line or riser head. The turret may further comprise a joint for connection of a mooring line. The joint may be located on the chaintable.

According to the invention, there is also provided apparatus for mounting in a turret for turret mooring a floating vessel, the turret being rotatable with respect to the vessel about a substantially vertical axis, the apparatus comprising: a hauling device for exerting a force on a mooring line or riser via a pilot line; a beam arranged to be rotatable about the turret axis; and a pulley fixed on the beam, wherein, in use, the force exerted on the mooring line or riser by the hauling device via the pilot line is, at the hauling device, substantially along the turret axis and substantially in a downwards direction.

By arranging the apparatus such that the force exerted on the mooring line or riser by the hauling device via the pilot line is, at the hauling device, substantially along the turret axis and substantially in a downwards direction, the mooring apparatus in the turret takes up considerably less space than prior art arrangements.

Preferably, the apparatus further comprises the pilot line. The pilot line may be arranged to be connected at one end to the hauling device, to pass over the pulley and to be connected at the other end to a mooring line or riser.

It will be appreciated that, where the pilot line is described as being "connected" at a first end to the hauling device, the term "connected" is not intended to mean that the pilot line is permanently linked to the hauling device. Rather, it is intended that the pilot line is sufficiently coupled to the hauling device for the hauling device to be able to exert a force on the mooring line or riser via the pilot line. Similarly, where the pilot line is described as being "connected" at a second end to the mooring line or riser, the term "connected" is not intended to mean that the pilot line is permanently linked to the mooring line or riser. In an embodiment of the invention, the apparatus further comprises a second pulley mounted on the beam. The beam need not necessarily be oblong in shape, but may be any shape capable of being rotated about the turret axis. The beam is most conveniently elongate and substantially rectangular in plan view and preferably extends between the centre of the turret and its outer portion.

In an embodiment, the hauling device is a jack arranged to selectively draw in or feed out the pilot line. As already described, the jack may comprise two clamps for clamping the pilot line, at least one of the clamps being moveable, such that the jack draws in or feeds out the pilot line step wise. Alternatively, the jack may comprise two moveable clamps for clamping the pilot line, such that the jack draws in or feeds out the pilot line substantially continuously.

In an alternative embodiment, the hauling device is a winch.

According to the invention, there is also provided a method for mooring a floating vessel comprising the steps of: providing a turret rotatable with respect to the vessel about a substantially vertical axis, the turret comprising a chaintable at its lower end, the chaintable having a plurality of holes arranged at a first distance from the axis; providing a beam in the turret, the beam having a pulley mounted thereon; providing a pilot line, the pilot line being arranged to pass over the pulley and to be connected at a first part to a hauling device mounted in the turret; rotating the beam about the axis such that the pulley is aligned above a selected one of the plurality of holes in the chaintable; feeding out the pilot line through the selected hole in the chaintable; connecting the pilot line at a second part to a selected mooring line; operating the hauling device to draw in the pilot line to lift the mooring line towards the chaintable, force exerted on the mooring line by the hauling device via the pilot line being, at the hauling device, substantially along the turret axis and substantially in a downwards direction; securing the mooring line to the turret; and disconnecting the pilot line from the mooring line.

The steps of the method need not necessarily be in the above order.

The arrangement may be such that the pulley is fixed on the beam at the first distance from the turret axis. In that case, once the step of rotating the beam is complete, the pulley is automatically in line directly above a selected one of the holes in the chaintable without any radial movement of the pulley being required.

The step of operating the hauling device may comprise operating a jack for selectively drawing in or feeding out the pilot line. Such an arrangement is advantageous because the jack takes up considerably less space than a standard winch. The jack may comprise two clamps for clamping the pilot line, at least one of the clamps being moveable, such that the jack draws in or feeds out the pilot line step wise. Alternatively, the jack may comprise two moveable clamps for clamping the pilot line, such that the jack draws in or feeds out the pilot line substantially continuously.

Alternatively, the step of operating the hauling device may comprise operating a winch.

Optionally, the step of feeding out pilot line comprises operating the hauling device.

Preferably, the step of securing the mooring line to the turret comprises securing the mooring line to a joint on the turret for connection of a mooring line. The joint may be located on the chaintable.

Advantageously, the step of connecting the pilot line at a second part to a selected mooring line comprises connecting a connection device at the second part of the pilot line to the mooring line.

Other advantageous features have been described above with reference to the other aspects of the invention.

According to the invention, there is further provided a method for releasing a floating vessel from its mooring comprising the steps of: providing a turret rotatable with respect to the vessel about a substantially vertical axis, the turret comprising a chaintable at its lower end, the chaintable having a plurality of holes arranged at a first distance from the axis; providing a beam in the turret, the beam having a pulley mounted thereon; providing a pilot line, the pilot line being arranged to pass over the pulley and to be connected at a first part to a hauling device mounted in the turret; rotating the beam about the axis such that the pulley is aligned above a selected one of the plurality of holes in the chaintable; feeding out the pilot line through the selected hole in the chaintable; connecting the pilot line at a second part to a selected mooring line secured to the turret; de-securing the mooring line from the turret; operating the hauling device to feed out the pilot line to lower the mooring line away from the chaintable, force exerted on the mooring line by the hauling device via the pilot line being, at the hauling device, substantially along the turret axis and substantially in a downwards direction; and disconnecting the pilot line from the mooring line.

The steps of the method need not necessarily be in the above order.

The step of operating the hauling device may comprise operating a jack for selectively drawing in or feeding out the pilot line. The jack may comprise two clamps for clamping the pilot line, at least one of the clamps being moveable, such that the jack draws in or feeds out the pilot line step wise. Alternatively, the jack may comprise two moveable clamps for clamping the pilot line, such that the jack draws in or feeds out the pilot line substantially continuously.

It will be appreciated that any features described with respect to one aspect of the invention may also be applicable to another aspect of the invention.

Known mooring arrangements have already been described with reference to Figures 1 to 3 of the accompanying drawings, of which:

Figure 1 is a schematic plan view of a spread mooring configuration; Figure 2 is a schematic plan view of a turret mooring configuration; and Figure 3 is a sectional view of a turret mooring arrangement according to the prior art. An embodiment of the invention will now be described with reference to Figures 4 to 6 of the accompanying drawings, of which:

Figure 4 is a cross sectional view of a turret mooring apparatus according to the invention; Figure 5 is a sectional view of the chaintable

(along line V-V in Figure 4) ; and Figure 6 is an enlarged view of box VI of Figure 4.

Figure 4 is a cross sectional view of the turret of the present invention and Figure 5 is a section along line V-V in Figure 4. Figure 6 shows the lower portion of Figure 4 in more detail.

Referring to Figures 4 to 6, the lower portion of the turret 401 includes chain jack 403 which, in this embodiment, replaces the winch or winches of previous known arrangements. Operation of the chain jack will be described in more detail below. The chain jack is located upside down along the vertical axis 405 of the turret. The lower portion of the turret 401 also includes a horizontal beam 423 on which are mounted two pulleys, first pulley 419 and second pulley 421. The horizontal beam 423 is positioned between the turret's vertical axis 405 and the outer circumference of the turret. The chain jack 403 is equipped with a pilot chain 407 which may pass over only first pulley 419 (first position 408a) or over both first 419 and second 421 pulleys (second position 408b) . The pilot chain transmits the force from the chain jack to a selected mooring chain or riser. The two pulleys 419, 421 are fixed on the beam but the whole arrangement (chain jack plus beam with fixed pulleys) is mounted on bearings to be rotatable in a horizontal plane around the turret's vertical axis thereby aligning first pulley 419 with a selected hole on the chaintable 413 or second pulley 421 with a selected hole on the chaintable 413 for installation of a selected mooring chain or riser. The whole arrangement is rotated, in order to align the selected pulley with the selected hole in the chaintable, by independent drive means (not shown) . The beam/turret interface is, in this embodiment, equipped with Teflon pads 605 (see Figure 6) to facilitate rotation.

As can be seen clearly in Figure 5, chaintable 413 includes a number of holes 415 for installation of the risers 411 and a number of holes 416 for installation of the mooring chains 409. The holes 415 for the risers are located in two circles, each having their centres on the vertical axis of the turret, so there are two rings of riser holes, inner holes 415a and outer holes 415b. The holes 416 for the mooring chains 409 are angularly aligned with joints 417 for connection of the mooring chains.

First pulley 419 is positioned towards the vertical axis of the turret in line with inner holes 415a in the chaintable 413. Second pulley 421 is further outwardly than the first pulley 419 in line with outer holes 415b in the chaintable 413. Thus, in position 408a, the pilot chain passes over first pulley 419 and is aligned with an inner hole 415a in the chaintable 413 for installation/abandonment/recovery of a riser at that radial position. In addition, in position 408a, the pilot chain passes over first pulley 419 and can then pass through a hole 416 in the chaintable 413 for installation/abandonment/recovery of a mooring chain. It will be appreciated that the holes 416 are, in this embodiment, not radially aligned with first pulley 419. This means that during installation/abandonment/recovery of a mooring chain, the pilot chain does not pass vertically downwards hole 416 from pulley 419 but, instead, passes downwards from pulley 419 to hole 416 at a small angle from the vertical. In position 408b, the pilot chain passes over first pulley 419 and second pulley 421 and is aligned with an outer hole 415b in the chaintable 413 for installation/abandonment/recovery of a riser at that radial position. The pulley or pulleys convert the force transmitted through the pilot chain 407 to the correct direction for the upside down mounted chain jack.

During installation or recovery, a downward net force on the pilot chain at the chain jack is transmitted to an upward net force on the pilot chain at the mooring chain/riser and during abandonment, a downward net force on the pilot chain at the mooring chain/riser is transmitted to an upward net force on the pilot chain at the chain jack.

In this embodiment, the chain jack is a specially designed pulling device which operates as a linear winch. The chain jack releases or recovers the chain in a non-continuous movement, i.e. :m a step by step type movement via two clamps able to grip one or more chain rings. One of the clamps 601 is moveable to carry out the pulling operation by releasing or recovering sections of the chain at a time. The moveable clamp 601 is shown in its two alternative positions in Figure 6. The other clamp 603 is fixed. The chain jack could equally be constructed with two moveable clamps so that the releasing or recovering of the chain occurs in a continuous movement.

Because the chain jack 403 acts as a linear winch and is mounted upside down and in the lower part of the turret just above the chaintable 413, the whole arrangement takes up less space than prior art arrangements. The upside down mounting of the chain jack 403 means only one pulley 421 is required to transmit the force along the pilot chain. The unused part of the pilot chain 407 is able to hang from the chain jack 403 through an aperture in the chaintable 413 aligned with the turret's vertical axis 405 rather than being stored on the vessel or upper part of the turret. The chain jack arrangement could equally be replaced by an upside-down non¬ linear winch arrangement also just above the chaintable.

The pulley and beam mechanism is much simpler than the pulley mechanisms of the prior art. The two pulley arrangement and the positioning of the holes in the chaintable allow the pilot chain to be aligned with the selected mooring chain or riser without the need for any translating pulley i.e. the two pulleys can be fixed to the beam and do not need to be moveable along the beam. Of course, if required, more than two pulleys could be fixed to the beam for use with more than two sets of rings of holes in the chaintable.

The method of installation of a mooring chain is as follows:

a) the chain jack 403 and beam 423 are rotated so that the beam is aligned above the selected hole 416 in the chaintable

413; b) the pilot chain 407 is fed through the selected hole 416 by "release" operation of the chain jack 403; c) the pilot chain 407 is connected to the selected mooring chain 409 head; d) the mooring chain 409 is lifted by "recover" operation of the chain jack 403 until the mooring chain 409 head approaches the chaintable 413; e) the mooring chain 409 is appropriately tensioned; f) the mooring chain 409 is fixed to a joint 417 on the outside of the chaintable 413 via a pre-installed short chain length; and g) the pilot chain 407 is slackened then is disconnected from the mooring chain head and the excess pilot chain is lifted clear of the chaintable 413 by "recover" operation of the chain jack 403.

Recovery of a mooring chain follows essentially the same set of steps. Abandonment of a mooring chain follows essentially the reverse procedure as follows:

413; b) the pilot chain 407 is fed through the selected hole 416 by "release" operation of the chain jack 403; c) the pilot chain 407 is connected to the selected mooring chain 409 head and appropriately tensioned; d) the selected mooring chain 409 is disconnected from the joint 417; e) the pilot chain 407 is fed out via "release" operation of the chain jack 403 until the moor-ing chain 409 is sufficiently discarded; f) the pilot chain is disconnected from the mooring chain 409 head; and g) the excess pilot chain is lifted clear of the chaintable 413 by "recover" operation of the chain jack 403.

The method of installation of a riser is similar to that of a mooring chain, as follows:

a) the chain jack 403 and beam 423 'are rotated so that the first/second pulley is aligned above the selected hole in the inner/outer ring of holes in the chaintable 413; b) the pilot chain 407 is fed through the selected hole 415 by "release" operation of the chain jack 403; c) the pilot chain 407 is connected to the selected riser 411 head; d) the riser 411 is lifted by "recover" operation of the chain jack 403 until the riser 411 head approaches the chaintable 413; e) the riser 411 is lifted through the selected hole 415 in the chaintable 413; f) the riser 411 is appropriately tensioned; g) the riser 411 is secured to the chaintable; and h) the pilot chain 407 is disconnected from the riser 411 head.

Recovery of a riser follows essentially the same set of steps. Abandonment of a riser follows essentially the reverse procedure as follows:

a) the chain jack 403 and beam 423 are rotated so that the first/second pulley is aligned above the selected hole in the inner/outer ring of holes in the chaintable 413; b) the pilot chain is connected, to the selected riser 411 head; c) the selected riser 411 is de-secured from the chaintable 413; d) the pilot chain is fed through the selected hole 415 and further fed out by "release" operation of the chain jack 403 until the riser 411 is sufficiently discarded; e) the pilot chain is disconnected from the riser 411 head; and f) the excess pilot chain is lifted clear of the chaintable 413 by "recover" operation of the chain jack 403.

It should be noted that, in practice, all the mooring lines are installed before the risers.

Claims

CLAIMS :

1. A turret for turret mooring a floating vessel, the turret being rotatable with respect to the vessel about a turret axis, the turret comprising: a chaintable at a first end of the turret, the chaintable comprising a plurality of holes arranged at a first distance from the turret axis; a hauling device in the turret for exerting a force on a mooring line or riser; and a beam having a pulley fixed thereon, the beam being rotatable about the turret axis, the chaintable, hauling device and beam being arranged such that when, in use, a pilot line, connected at a first end to the hauling device and at a second end to the mooring line or riser, passes around the pulley, the force exerted on the mooring line or riser by the hauling device via the pilot line is, at the hauling device, substantially along the turret axis and substantially towards the chaintable.

2. A turret according to claim 1 wherein the pulley is fixed on the beam at the first distance from the turret axis such that the pulley is alignable with each one of the plurality of holes in the chaintable.

3. A turret according to claim 1 or claim 2 wherein the pilot line passes around only one pulley.

4. A turret according to claim 1 or claim 2 wherein the chaintable comprises a second plurality of holes arranged at a second distance from the turret axis.

5. A turret according to claim 4 wherein the beam has a second pulley fixed thereon at the second distance from the turret axis such that the second pulley is alignable with each one of the second plurality of holes in the chaintable.

6. A turret according to any one of the preceding claims wherein the hauling device is a jack arranged to selectively draw in or feed out the pilot line.

7. A turret according to claim 6 wherein the jack comprises two clamps for clamping the pilot line, at least one of the clamps being moveable.

8. A turret according to any one of the preceding claims wherein the hauling device is located in the turret, substantially adjacent the chaintable.

9. A turret as hereinbefore described with reference to Figures 4, 5 and 6 of the accompanying drawings.

10. A vessel comprising a turret according to any one of the preceding claims.

11. A vessel comprising a turret for turret mooring the vessel, the turret being rotatable with respect to the vessel about a substantially vertical axis, the turret comprising: a chaintable at a ^'lower end of the turret, the chaintable having a plurality of holes arranged at a first distance from the turret axis; a hauling device in the turret for exerting a force on a mooring line or riser; and a beam having a pulley mounted thereon, the beam being rotatable about the turret axis, the chaintable, hauling device and beam being arranged such that when, in use, a pilot line, connected at a first end to the hauling device and at a second end to the mooring line or riser, passes over the pulley, the force exerted on the mooring line or riser by the hauling device via the pilot line is, at the hauling device, substantially along the turret axis and substantially in a downwards direction.

12. A vessel according to claim 11 wherein the pulley is fixed on the beam at the first distance from the turret axis such that the pulley is alignable above each one of the plurality of holes in the chaintable.

13. A vessel according to claim 11 or claim 12 wherein the pilot line passes over only one pulley.

14. A vessel according to claim 11 or claim 12 wherein the chaintable comprises a second plurality of holes arranged at a second distance from the turret axis.

15. A vessel according to claim 14 wherein the beam has a second pulley fixed thereon at the second distance from the turret axis such that the second pulley is alignable above each one of the second plurality of holes in the chaintable.

16. A vessel according to any one of claims 11 to 15 wherein the hauling device is a jack arranged to selectively draw in or feed out the pilot line.

17. A vessel according to claim 16 wherein the jack comprises two clamps for clamping the pilot line, at least one of the clamps being moveable.

18. A vessel according to any one of claims 11 to 17 wherein the hauling device is located in the turret, substantially adjacent and above the chaintable.

19. Apparatus for mounting in a turret for turret mooring a floating vessel, the turret being rotatable with respect to the vessel about a substantially vertical axis, the apparatus comprising: a hauling device for exerting a force on a mooring line or riser via a pilot line; a beam arranged to be rotatable about the turret axis; and a pulley fixed on the beam, wherein, in use, the force exerted on the mooring line or riser by the hauling device via the pilot line is, at the hauling device, substantially along the turret axis and substantially in a downwards direction.

20. Apparatus according to claim 19 further including the pilot line.

21. Apparatus according to claim 19 or claim 20 further comprising a second pulley fixed on the beam.

22. Apparatus according to any one of claims 19 to 21 wherein the hauling device is a jack arranged to selectively draw in or feed out the pilot line.

23. Apparatus according to claim 22 wherein the jack comprises two clamps for clamping the pilot line, at least one of the clamps being moveable.

24. A method for mooring a floating vessel comprising the steps of: providing a turret rotatable with respect to the vessel about a substantially vertical axis, the turret comprising a chaintable at its lower end, the chaintable having a plurality of holes arranged at a first distance from the axis; providing a beam in the turret, the beam having a pulley mounted thereon; providing a pilot line, the pilot line being arranged to pass over the pulley and to be connected at a first part to a hauling device mounted in the turret; rotating the beam about the axis such that the pulley is aligned above a selected one of the plurality of holes in the chaintable; feeding out the pilot line through the selected hole in the chaintable; connecting the pilot line at a second part to a selected mooring line; operating the hauling device to draw in the pilot line to lift the mooring line towards the chaintable, force exerted on the mooring line by the hauling device via the pilot line being, at the hauling device, substantially along the turret axis and substantially in a downwards direction; securing the mooring line to the turret; and disconnecting the pilot line from the mooring line.

25. A method for releasing a floating vessel from its mooring comprising the steps of: providing a turret rotatable with respect to the vessel about a substantially vertical axis, the turret comprising a chaintable at its lower end, the chaintable having a plurality of holes arranged at a first distance from the axis; providing a beam in the turret, the beam having a pulley mounted thereon; providing a pilot line, the pilot line being arranged to pass over the pulley and to be connected at a first part to a hauling device mounted in the turret; rotating the beam about the axis such that the pulley is aligned above a selected one of the plurality of holes in the chaintable; feeding out the pilot line through the selected hole in the chaintable; connecting the pilot line at a second part to a selected mooring line secured to the turret; de-securing the mooring line from the turret; operating the hauling device to feed out the pilot line to lower the mooring line away from the chaintable, force exerted on the mooring line by the hauling device via the pilot line being, at the hauling device, substantially along the turret axis and substantially in a downwards direction; and disconnecting the pilot line from the mooring line.