WO2010070586A1

WO2010070586A1 - Cardan tensioning and locking device for anchoring chains of fpso units

Info

Publication number: WO2010070586A1
Application number: PCT/IB2009/055766
Authority: WO
Inventors: Gianfranco Gamba
Original assignee: Rema Engineering S.R.L.
Priority date: 2008-12-18
Filing date: 2009-12-15
Publication date: 2010-06-24
Also published as: IT1392480B1; ITMI20082257A1

Abstract

Tensioning and chain stopper device for vessel anchoring chains, in particular for FPSO units, of the type comprising a hydraulic jack (10) for the intermittent tensioning of the chain (1) and a jaw device (3) for chain locking mounted on a first support (5) oscillating according to a horizontal axis. The jack (10) is mounted on a support (8) oscillating about the same horizontal axis of the oscillating support (5) of the chain stopper device (3) and the angular position of the support (8) is controlled by one or more linear actuators (9), hinged at one end to said support (8) and at the other end to the vessel deck (2). The chain stopper device (3) is furthermore mounted inside said first support (5), through a second support (6) oscillating about a vertical axis to said first support (5), to accomplish a cardan chain stopper device.

Description

CARDAN TENSIONING AND LOCKING DEVICE FOR ANCHORING CHAINS OF FPSO UNITS

DESCRIPTION

The present invention refers to a combined tensioning and locking cardan device for anchoring chains of FPSO units. In particular the invention refers to a device of the above- described type which allows to optimise anchoring operations, and which allows long-term anchoring also in severe marine- weather conditions without stresses being imparted which are critical for the chain and the bearing structures of the FPSO unit . FIELD OF THE INVENTION

The vessel units known under the English acronym FPSO (Floating Production Storage Offloading) are floating units used in the extraction of crude oil from the seabed at great depths . It is a technique introduced in the last few years for the extraction of crude oil from oilfields which - due to their distance from the coast, of adverse environmental conditions, or of the relatively small size of the oilfield - do not make the in- stallation of traditional platforms connected to the coast by submarine pipelines economic .

FPSO units today are normally built by reconverting tankers, by installing chemical plants (for the separation of crude oil from the gas and water possibly present and its pre- refinement with the removal of the high-boiling components) , of systems of energy production for the operation of such plants and for the other requirements of the unit, providing at least one landing platform for helicopters for exchanging labour-force and materials with the mainland and, finally, providing a suit- able system for the permanent anchoring of the vessel. As a matter of fact, FPSO units are fixedly moored at the site of use, where they normally remain until completion of the oilfield exploitation, and hence for periods up to several years. The gas and the partially refined crude oil are stored inside the tanks of the FPSO unit and then periodically transferred into tankers which shuttle between the FPSO unit and the coast terminal of a refinery.

The use of FPSO units is awakening considerable interest in the field, due to the swiftness and flexibility with which such units can be installed and made operational - and possibly repositioned in a different extraction location - compared to traditional platform systems, so that a large number of installations of this type is currently under construction in various areas of the world.

As is evident also from the brief description reported above, a critical aspect of the operation of FPSO units is the anchoring system thereof. As a matter of fact, such system must be capable of ensuring a fixed positioning of the vessel above the extraction site, for periods of time up to various years and with the frequent possibility of having to face particularly se- vere marine weather conditions. In the light of the size of the FPSO units (between 250 and 350 metres and beyond of length) , as a matter of fact the stresses which the anchoring system is subject to in case of storm are very high, especially in correspondence of the critical points of such system, i.e. those connect- ing the anchoring chains and the bearing structures of the FPSO unit . BACKGROUND ART

According to the prior art, during the anchoring operations of an FPSO unit, the anchoring chains - frequently com- prising one or more portions of the median part thereof consisting of ropes of synthetic plastic materials - are fastened, by means of support ships and submarine vessels, on the one hand to non-conventional anchor logs and on the other hand to the FPSO unit. The anchor logs can consist for example of large struc- tures shaped as upturned bells resting on the bottom, then gradually taking away water from the inside so that the lateral walls of the bell progressively sink in the seabed due to the weight of the above-lying water column.

Once the numerous chains required for the anchoring of an FPSO unit, for example 12, have been positioned radially around said unit in the above-mentioned way, from onboard the FPSO unit the tensioning process of the anchoring chains and the locking of the same begins .

The tensioning operation of the anchoring chains can be performed continuously by special windlasses for rotary-movement chains, or discontinuously by alternate-rectilinear-movement hydraulic jacks with intermittent tensioning. Both these apparatuses are positioned and locked on the deck of the FPSO unit, but due to the greater bulk and complexity thereof, the rotary windlass is normally an irremovable, fixed structure, and neces- sarily found both at the bow and abaft of the FPSO unit, while the hydraulic jacks consist of relatively smaller structures which are hence relatively simply movable by conventional means between the fastening points of the different anchoring chains, where they are fastened to the deck for the sole time necessary for the chain tensioning and locking operation.

The hydraulic-jack tensioning system hence has the advantage - unlike the windlass system - of having a completely free deck of the FPSO unit available after the end of the anchoring operations and then once the hydraulic jacks are stowed or in any case in such a position as not to obstruct the deck. The present invention is applicable to this second anchoring system, to which exclusive reference will hence be made in the following.

When a single anchoring chain has been tensioned to the desired extent, it is fastened by a locking device, essentially consisting of a support structure within which the chain passes, wherein a pair of jaws (so-called "chain stopper") is provided which can fit upon command in a link of the chain to prevent the same from slipping. The chain stopper device is preferably os- dilating about a horizontal axis, so as to allow free variations of the vertical angle of the chain with respect to the deck plane of the vessel, depending on the oscillations experienced by the ship during sea storms or due to the action of winds . When all the mooring chains have been tensioned and locked in the correct final position, the exceeding part of chain chain is cut off and the FPSO unit is ready to begin its long- term operative phase in the extraction, treatment and storage of crude oil .

However, the FPSO units anchored in the above-described way have not proven - in the use made thereof up until today - to be free from drawbacks. As a matter of fact, firstly taking into account the fact that the hydraulic jacks are fastened to the deck and hence operate according to a substantially horizontal axis, it is necessary to bring the end part of the anchoring chain from the sub-vertical position nearly adjacent to the flank of the unit - which it takes up at the beginning of the tensioning operation - precisely to a horizontal position. This operation is carried out by means of large transmission idle wheels fastened in the proximity of the broadside of the FPSO unit, which allow precisely to obtain the necessary deviation by about 90° of the chain towards the hydraulic jack at the beginning of the tensioning operation. However, since in the proximity of the broadside the chain stopper device must also be found, it is necessary that said idle wheels are mounted over- hanging with respect to the broadside, creating remarkable bulk problems - especially during navigation - and serious structural difficulties during construction to obtain a correct load on the bearing structures of the FPSO unit of the remarkable stresses acting thereon both during the tensioning operations and, espe- cially, once the anchoring has occurred in case of severe marine weather conditions .

Another considerable drawback of known anchoring systems, underestimated up until today but which has recently created serious safety problems to FPSO units already in use, is that cur- rent anchoring chain stopper devices allow the chain no degree of freedom in an oscillation direction according to a horizontal angle. In fact in current designs it has been believed that such oscillations are relatively limited and could be absorbed by the mutual oscillations of the end links of the chain. However, practical experience has shown that the extent of such oscillations has instead been sufficient to determine a relatively fast wear of the end links of the chain, in the proximity of the chain stopper device, up until determining the sudden yield thereof in situations of storm overload, with the serious consequences which one can easily imagine. PROBLEM AND SOLUTION

The problem on which the present invention is based is hence that of proposing a tensioning and chain stopper device of anchoring chains of the type described above which is free from the drawbacks mentioned above and which hence allows to perform the tensioning operations of an anchoring chain for FPSO units without the need to use transmission idle wheels and, in chain- locking position, allows the same degrees of freedom both in a vertical and in a horizontal direction, so as to avoid any dangerous continuous friction action between the chain links . This object is achieved by a tensioning and chain stopper device for anchoring chains having the features defined in the claim 1 herewith enclosed. Further features of such device are defined in the dependent claims . BRIEF DESCRIPTION OF THE DRAWINGS Further features and advantages of the device of the invention will in any case be more evident from the following detailed description of a preferred embodiment of the same, given purely by way of a non-limiting example and illustrated in the accompanying drawings, wherein: fig. 1 is a diagrammatic, perspective view of the cardan tensioning and chain stopper device according to the present invention; fig. 2A is a front view of the internal side of the chain stopper device with chain tightening jaws in an open position; fig. 2B is a view similar to fig. 2A, with the chain tightening jaws in a closed position; fig. 3 is a lateral view of the device of fig. 1, at the beginning of the tensioning travel of the hydraulic jack; fig. 4 is a view similar to fig. 3, at the end of the jack tensioning travel; fig. 5 is a lateral view of the device of fig. 1 which shows some of the different positions which the device can take up rotating in a vertical plane, during the tensioning operations of the anchoring chain; and fig. 6 is a diagrammatic plan and partly section view of the device of fig. 1, which shows the possible field of oscillations of the chain according to a horizontal angle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As clearly visible in the drawings, an anchoring chain 1 is tensioned and locked in a stable way on the deck 2 of a vessel, such as an FPSO unit, by means of the device of the invention. The fastening occurs in the proximity of the broadside of the vessel, i.e. of the outer edge of the deck.

The device according to the invention comprises firstly a chain stopper device 3, apt to lock the chain, despite allowing oscillations both according to vertical angles and to horizontal angles, i.e. such as to accompany all possible movements induced on the vessel by marine weather phenomena without non-axial forces acting on the chain links, i.e. forces apt to induce dangerous chafing movements between the chain links . For this purpose the chain stopper device comprises a pair of parallel brackets 4, securely fastened to the deck, whereon an outer frame 5 is mounted oscillating about a horizontal axis, in a way already known per se. According to a first important feature of the invention, a second internal frame 6 is then mounted inside frame 5 and oscillating on the same, according to a rotation axis 6' perpendicular to the one of outer frame 5, so as to accomplish a cardan- type joint. Chain 1 is caused to pass in the inner span of internal frame 6, wherein two jaws 7 are provided, here too in a conventional way, apt to fit upon com- mand in the links of chain 1 (fig. 2B) , to prevent the same from slipping off and hence accomplish chain locking.

According to another essential feature of the invention, on the same axis of outer frame 5, but on the outer side of brackets 2 , there is further mounted an oscillating yoke 8 , kept in the desired angular position by a pair of hydraulic cylinder- piston assemblies 9 hinged on one side to deck 2 and on the other side to the free ends of yoke 8. With yoke 8 a hydraulic jack 10 is finally made integral, apt to accomplish on chain 1, in the alternate and intermittent way known per se, the tensioning action clearly illustrated in figs . 3 and 4. This arrangement allows to obtain very remarkable advantages in the tensioning operations of the anchoring chains, fully removing the need for the large external transmission wheels and avoiding to impart considerable stresses on the outer edge of the ship. This result is obtained by progressively changing the angle of the hydraulic jack during the tensioning operation, so that the longitudinal axis of the jack remains always substantially tangential to the anchoring chain generatrix in the tensioning point. In particular, hydraulic jack 10 can for example rotate between an initial sub-vertical position (in- clined by about 80° to the vertical, shown in fig. 5 by reference Is) which substantially corresponds to the inclination of the chain at the beginning of the tensioning operation, up to an inclination of 30° (shown in fig. 5 by reference Ie) which corresponds to the angular position of the anchoring chain at the end of the tensioning.

During the subsequent tensioning actions, the position of cylinder-piston assemblies 9 is gradually changed, manually or even automatically, so as to maintain the desired alignment between the direction of the end portion of the chain and the lon- gitudinal axis of the jack. As is evident, in this position the load on the cylinder-piston assemblies 9 is theoretically null and actually minimal. With a suitable control of the inclination of hydraulic jack 10, such load can hence be kept smaller than a predetermined value and hence always directed towards the deck 2 of the vessel, so as to guarantee that a positive, capsizing- preventing torque is constantly present during the tensioning steps of hydraulic jack 10. For this purpose it is also useful to provide locking pins 12 for making integral during rotation the hydraulic jack 10 and the outer frame 5, so as to guarantee a constant alignment thereof during the tensioning operations.

The tensioning and chain stopper device of the present in- vention finally is preferably equipped with a slide 11 accompanying chain 1 out of the operation area of the jack and with a protection skirt (not shown) on the vessel edge, to accompany the movement of chain 1 in the very first tensioning steps, use- ful especially when the chain has been fastened to the device in a particularly loose manner.

At the end of the tensioning operation of anchoring chain 1, jaws 7 are definitively locked, hydraulic jack 10, cylinder- piston assemblies 9 and yoke 8 are taken off the chain stopper device and off deck 2 and the residual chain is removed. In this locking position, shown in fig. 6, anchoring chain 1 is free to oscillate as well as in the vertical plane - as already known - also according to a horizontal angle of ± 30°, as shown by the two limit-positions 11 which the chain can take up, thereby be- ing perfectly suitable to follow any possible movement of the vessel without non-axial stresses rising between the individual chain links which continue to work hence by simple traction, guaranteeing an optimal duration of the chain.

The tensioning device at this point can be mounted in the proximity of a second chain stopper device, to accomplish the tensioning and hence the locking of a second anchoring chain, and so on until it has completed the anchoring of the FPSO unit. As is evident from the above-reported description, the cardan tensioning and chain stopper device of an anchoring chain according to the present invention has fully reached the set objects. As a matter of fact, chain tensioning can be accomplished without employing large external transmission wheels, removing the costs and the drawbacks of these structures . Once fastened in the chain stopper device, the chain is then free to move both according to a vertical angle and to a wide horizontal angle, so that it always operates in optimal conditions of perfectly axial load, regardless of the possible vessel oscillations.

However, it is understood that the invention must not be considered limited to the particular arrangement illustrated a- bove, which represents only an exemplifying embodiment thereof, but that a number of variants are possible, all within the reach of a person skilled in the field, without departing from the scope of protection of the invention, as defined by the following claims .

Claims

1) Tensioning and chain stopper device for vessel anchoring chains, in particular for PPSO units, of the type comprising a hydraulic jack (10) for the intermittent tensioning of the chain (1) and a chain-locking jaw device (3) mounted on a first support (5) oscillating according to a horizontal axis, characterised in that said jack (10) is mounted on a support (8) oscillating about the same horizontal axis of the oscillating support (5) of the chain stopper device (3) . 2) Tensioning and chain stopper device for anchoring chains as claimed in claim 1) , wherein the angular position of the support (8) of the jack (10) is controlled by one or more linear actuators (9), hinged at one end to said support (8) and at the other end to the vessel deck (2) . 3) Tensioning and chain stopper device for anchoring chains as claimed in claim 2) , wherein said hydraulic jack is free to oscillate between a tensioning start position (Is) inclined by about 80° and a tensioning end position (Ie) inclined by about 30° to the horizontal. 4) Tensioning and chain stopper device for anchoring chains as claimed in claim 3 ) , wherein said chain stopper device (3) is mounted within said first support (5), through a second support (6) pivoted along a vertical axis to said first support (5) . 5) Tensioning and chain stopper device for anchoring chains as claimed in claim 4) , wherein the oscillation of said second support (6) ranges in a horizontal angle of ± 30° (1) .

6) Tensioning and chain stopper device for anchoring chains as claimed in any one of the preceding claims , wherein said linear actuators are hydraulic cylinder-piston assemblies.

7) Tensioning and chain stopper device for anchoring chains as claimed in any one of the preceding claims, wherein said hydraulic jack (10) and said first support (5) of the chain stopper device (3), are locked to mutual rotation during the tensioning operations .

8) Tensioning and chain stopper device for anchoring chains as claimed in any one of the preceding claims, wherein the angle of inclination in the vertical plane of said hydraulic jack is controlled so that the load on said actuators is smaller than a predetermined value and furthermore constantly directed towards the inside of the vessel .

9) Tensioning and chain stopper device for anchoring chains as claimed in any one of the preceding claims , further comprising a slide (11) apt to accompany the chain (1) to an area upwards of the tensioning point and outside the operation area of the jack (10) .

10) Tensioning and chain stopper device for anchoring chains as claimed in any one of the preceding claims, further comprising a protection skirt on the vessel edge, to accompany the movement of the chain (1) in the very first tensioning steps.