Underwater equipment for tensioning anchor lines of an offshore structure and method for installing such equipment

Description

Technical Field

[0001] The present invention relates to the general field of underwater equipment used for tensioning anchor lines of offshore structures, such as in particular oil platforms, floating units for the production, storage and unloading of hydrocarbons, floating supports for offshore wind turbines, etc. It relates more specifically to the launching, deployment and recovery of such underwater equipment. Prior art

[0002] Offshore structures such as oil platforms, floating production, storage and offloading units (FPSOs) and offshore wind turbines are floating structures that do not move very often and generally remain in the same location for several years.

[0003] To moor them to the seabed and ensure their strength and stability throughout their lifetime, it is known to use underwater anchoring systems requiring surface equipment capable of pre-deploying the anchor lines of the offshore structure and carrying out their tensioning and strength tests by surface and / or underwater equipment before it reaches its final destination position. In this way, when the offshore structure is in position, the anchor lines are simply retrieved to be raised onto the offshore structure and then tensioned. The use of such equipment thus eliminates the risk of connecting the offshore structure to anchor lines that have not been previously installed and tested and which could cause damage to the offshore structure in the event of loss of strength or slippage of one of the anchors.

[0004] Numerous embodiments of underwater equipment for tensioning anchor lines of an offshore structure are known from the prior art.

[0005] Generally, this equipment is based on a system for tensioning anchor lines consisting of jacks and a chain locking mechanism. For example, reference may be made to publications WO 2020 / 046127, US 3,845,935 and US 5,934,216 which describe examples of the implementation of such equipment.

[0006] However, this equipment has many drawbacks, particularly during deployment. Launching such equipment from the deck of the installation vessel is typically carried out using a crane connected to the equipment. However, this equipment can be bulky and very heavy, which requires high-capacity lifting equipment suitable for offshore installations. In addition, this lifting is generally carried out from the top of the equipment, so it is necessary, depending on the method of tensioning the anchor lines, after its placement on the seabed to disconnect the lifting points in order to then connect the anchor lines. This operation requires the intervention of divers or a remotely operated underwater vehicle (ROV) from the installation vessel in accordance with the working water depth.Similarly, once the equipment has been placed on the seabed, divers or an ROV intervene to prepare the tensioning of the anchor lines. However, these diving operations are delicate and dangerous, particularly in cases of poor visibility, or require expensive and highly technical means in the case of using a remotely operated underwater vehicle.

[0007] Furthermore, prior art subsea equipment is most often powered during deployment by a power umbilical that is permanently connected to the surface installation vessel. However, this operation is also delicate since there is a risk of damaging the umbilical during deployment. Statement of the invention

[0008] The present invention aims to provide underwater equipment for tensioning anchor lines and the associated deployment, recovery and implementation methods which does not have such drawbacks.

[0009] According to the invention, this aim is achieved by means of underwater equipment for tensioning anchor lines of an offshore structure, comprising: a platform intended to rest on a seabed and to ensure stability of the equipment and its self-alignment by sliding on the seabed when the anchor lines are tensioned, the platform comprising a passive end equipped with a passive ring intended to be connected to a passive anchor line terminating in a retaining anchor, an active end opposite the passive end defining a pulling axis, and two organs aligned along the pulling axis, located in the lower part of the platform and set back from the ends, and capable of allowing deployment from the surface to the seabed and recovery of the equipment by a hauling winch of a surface installation vessel; a V-shaped chute mounted on the platform extending between the passive end and the active end thereof and intended to recover an active anchor line;a system for tensioning the active and passive anchor lines comprising two locks spaced from each other and cooperating with the active anchor line recovered by the chute; means for measuring in real time during the deployment and recovery of the equipment the angles of trim, heel and azimuth of the equipment, as well as its immersion depth; and acoustic communication means for transmitting to the installation vessel the measurements of the angles and the immersion depth of the equipment during its deployment and recovery.

[0010] The equipment according to the invention is remarkable in particular in that its launching and its raising from the installation vessel with a hauling winch is made possible by the specific positioning of the two high-capacity connection members centered on the pulling axis. Furthermore, these hauling points that constitute the members are positioned in the lower part of the equipment, and not in the upper part, which minimizes the traction forces necessary for raising the equipment onto the deck of the installation vessel and which avoids having to block access to the recovery chute of the active anchor line with all the disadvantages that this blocking brings. In addition, the members are set back from the passive and active ends of the platform so as to allow passage over the launching rollers of the installation vessel on the surface.In addition, this type of haul-out installation avoids underwater disconnection operations by divers or ROVs, whether for lifting slings or haul-out cables following their installation and reconnection of the latter before recovery of the equipment. Finally, the stability of the equipment during its launching and deployment is guaranteed by the positioning of its center of gravity, which is located below the pulling axis. Any risk of the equipment overturning during launching and recovery on the deck of the installation vessel can thus be avoided.

[0011] The equipment according to the invention is also remarkable in that the integration of measurement and communication means within the equipment makes it possible, during deployment and recovery phases, to ensure its monitoring in real time and continuously from the installation vessel on the surface, to ensure its stability in open water, and to guarantee its deposit on the seabed and its recovery on the deck of the vessel. In particular, it is not necessary to deploy a power and control umbilical between the installation vessel and the equipment during these risky phases, the equipment being autonomous in its movements.

[0012] The equipment according to the invention is further remarkable due to its V-shaped chute mounted on the platform which forms a guide for the active anchor line when it is put in place. Once engaged in the chute, the active anchor line is engaged in the tensioning system which, by a back-and-forth movement formed in particular by the two locks actuated by hydraulic cylinders, makes it possible to "pull" on the active anchor line and thus tension the active and passive anchor lines.

[0013] In this way, the invention proposes equipment whose installation from the surface to the seabed is carried out without divers or ROVs and in a completely energy-autonomous manner with real-time and continuous monitoring from the installation vessel on the surface.

[0014] Preferably, the equipment further comprises a system for pre-tensioning the active anchor line once it is recovered by the chute. This pre-tensioning system makes it possible to precisely position the active anchor line for the tensioning system. This system also makes it possible to recover excess lengths of the active anchor line and tension it to several tens of tons, which saves considerable time during the actual tensioning step.

[0015] Preferably also, the equipment further comprises a system for tracking the active anchor line as it approaches the chute. For example, acoustic sensors are positioned on the chute to visualize the active anchor line and transmit the data in real time to the installation vessel by acoustic communication means. The position and approach data of the active anchor line allow real-time correction of the position, tension and chain length of the active anchor line.

[0016] More preferably, the equipment further comprises a power and control umbilical intended to power and control the system for tensioning the active and passive anchor lines, the power and control umbilical being housed in a basket and capable of being deployed autonomously from the equipment and connected to the installation vessel once the equipment is placed on the seabed. The power and control umbilical is thus stored within the equipment and is therefore protected in open water from mechanical risks, in particular from the risks of tearing.

[0017] The invention also relates to a method of installing underwater equipment as defined above, comprising the successive steps of: placing the passive anchor line retaining anchor on the seabed; connecting the passive anchor line to the passive ring of the equipment on the installation vessel; connecting the organeau located on the active end of the equipment to an abandonment chain connected to the hauling winch of the installation vessel; lowering the equipment to the seabed by hauling the winch and advancing the installation vessel in the opposite direction to the passive anchor line retaining anchor; placing the equipment on the seabed; stopping the installation vessel and disconnecting the hauling winch from the abandonment chain; deploying the active anchor line in the chute of the equipment; deploying a power and control umbilical from the equipment to be retrieved and connected to the installation vessel in order to power and control the tensioning system;and activation of the active and passive anchor line tensioning system.;

[0018] The active anchor line may initially be on standby on the installation vessel. Alternatively, it may initially be on standby on the seabed near the equipment drop-off location.

[0019] The invention also relates to another method of installing underwater equipment as defined above, comprising the successive steps of: dropping an active anchor line retaining anchor on the seabed; locking and securing the active anchor line in the tensioning system and the chain support chute of the equipment from the surface on the deck of the installation vessel; connecting on the installation vessel the passive anchor line and its retaining anchor to the passive ring of the equipment; connecting the organeau located on the passive end side of the equipment to an abandonment chain connected to the hauling winch of the installation vessel; launching and lowering the equipment by hauling out the abandonment chain and advancing the installation vessel to the intended position for dropping the passive anchor line retaining anchor; stopping the deployment of the equipment following its immersion, the passive anchor line taking up the tension while the abandonment chain is relieved and secured on the passive anchor line;resuming the descent of the equipment to the seabed by hauling out the passive anchor line and resuming the advance of the installation vessel towards the planned position for dropping the passive anchor line retaining anchor; dropping the equipment on the seabed; dropping the passive anchor line towards the planned position of the passive anchor line anchor; dropping the passive anchor line retaining anchor; deploying the power and control umbilical from the equipment to be retrieved and connected to the installation vessel in order to power and control the tensioning system; and activating the active and passive anchor line tensioning system.

[0020] Regardless of the method of installation, the progress of the equipment towards the seabed is advantageously continuously monitored by the installation vessel thanks to measurements of the trim, heel and azimuth angles of the equipment, and immersion depth.

[0021] Similarly, after being disconnected from the haulage winch, the abandonment chain is preferably stored on the seabed and connected to a surface buoy.

[0022] Regardless of the method of implementing the installation, the method may further comprise the recovery of the equipment comprising the successive steps of: deactivating the active and passive anchor line tensioning system to slacken them; extracting the active anchor line from the equipment and placing it on the seabed; recovering and connecting the abandonment chain to the hauling winch of the installation vessel; moving the installation vessel towards the passive anchor line retaining anchor to allow the equipment to be raised; and recovering the equipment on the installation vessel by pulling the hauling winch.

[0023] Whatever the method of carrying out the installation, and in response to tensioning requirements at higher capacities of the anchor lines of the offshore system, the method may further comprise the reduction of the tensioning force of the active and passive anchor lines comprising the successive steps of: connecting a first end of a gearing cable to the passive end of the equipment; routing the gearing cable through a first pulley positioned on the passive end side of the equipment; routing the gearing cable to the active end of the equipment by passing through a second pulley; connecting a second end of the gearing cable to a chain extension connected to the tensioning system of the anchor lines of the equipment; connecting each pulley to one of the active and passive anchor lines; and pulling on the chain extension by the tensioning system of the equipment. Brief description of the drawings

[0024] [ Fig. 1 ] There Figure 1 is a side view of underwater equipment for tensioning anchor lines of an offshore structure according to the invention. Fig. 2 ] There Figure 2is a view of the active side of the equipment of the Figure 1 . [ Fig. 3A-3F ] THE Figures 3A to 3F represent different steps of a method of installing the equipment according to the invention. Fig. 4A-4D ] THE Figures 4A to 4D represent different stages of a method of recovering the equipment according to the invention. Fig. 5A-5B ] THE Figures 5A and 5B represent two stages of a multiplication of the tensioning force of the active and passive anchor lines according to the invention. Fig. 6A-6D ] THE Figures 6A to 6D are magnifying glasses of the pulley and cable device for multiplying the Figures 5A and 5B . Description of the embodiments

[0025] The invention relates to the launching, deployment and recovery of subsea equipment for tensioning anchor lines for retaining an offshore structure such as an oil platform, an FPSO, a buoy, a floating support for an offshore wind turbine, etc. For this type of offshore structure, the anchor tensions can reach more than 1000 tonnes.

[0026] THE Figures 1 and 2 represent underwater equipment 2 according to the invention used for tensioning the anchor lines of such offshore structures.

[0027] As shown in these figures, the equipment 2 includes in particular a rectangular platform 4 which is intended to rest on the seabed and to ensure the stability of the equipment when the anchor lines are tensioned and to ensure the self-alignment of the equipment.

[0028] This platform 4 comprises two end flanks 4a, 4b connected to each other by two lateral flanks 4c, 4d, these flanks being inclined relative to the horizontal.

[0029] At one of the end flanks 4a, called the “passive end” of the equipment, the platform 4 is equipped with a passive ring 6 which is intended to be connected to a passive anchor line.

[0030] The opposite end flank 4b, called the “active end” of the equipment, defines a pulling axis 8 for the active anchor line 10.

[0031] Furthermore, the platform is equipped with two high-capacity organs 12 (or rings on which a chain, a cable, etc. is moored), these organs being aligned on the pulling axis 8 and intended to be connected to a hauling winch of a surface installation vessel.

[0032] More specifically, the organs 12 are centered on the end flanks 4a, 4b, located in the lower part of the equipment and set back from the ends of the equipment. They thus define two hauling points for the deployment and recovery of the equipment from the surface installation vessel.

[0033] The equipment according to the invention also comprises a V-shaped chute 14 which is mounted on the platform 4 extending between the passive end 4a and the active end 4b thereof.

[0034] This chute 14 consists of two inclined plates forming a V open towards the surface when the equipment rests on the seabed. The function of this chute is to recover the active anchor line 10 to guide it towards the tensioning system.

[0035] The tensioning system 16 of the active and passive anchor lines is positioned on the platform and under the chute 14. It comprises in particular two locks - a mobile lock 18 and a fixed lock 20 - which are spaced from each other and which cooperate with the active anchor line 10 recovered by the chute according to an operating mode which will be described later.

[0036] The movable lock 18 is coupled to hydraulic cylinders 21 for creating tension in the active anchor line. As for the hydraulic cylinders 21, they are powered and controlled from the surface installation vessel via umbilicals described later.

[0037] The passive end 4a of the equipment serves in particular to provide the reaction force to the anchor line tensioning system 16. A chain support chute 23 allows the active anchor line 10 to pass through the equipment lengthwise (from the active end 4b to the passive end 4a) by passing through the two locks 18, 20.

[0038] The equipment according to the invention also comprises measuring instruments grouped within a housing 25 and known from the prior art for measuring in real time during the immersion, deployment and recovery of the equipment the trim, heel and azimuth angles of the equipment, as well as its immersion depth.

[0039] These various measurements are transmitted in real time to the surface to the installation vessel using acoustic communication means (not shown in the figures).

[0040] Preferably, the equipment according to the invention further comprises a system 22 for pre-tensioning the active anchor line once the latter is recovered by the chain support chute 23.

[0041] This pre-tensioning system 22 is positioned under the V-shaped chute 14 downstream of the tensioning system 16. In particular, it makes it possible to apply tension in the passive and active anchor lines in order to minimize the duration of tensioning by the two locks 18, 20 of the tensioning system.

[0042] Also preferably, the equipment according to the invention further comprises a system for locating the active anchor line (for example housed within the housing 25 grouping the measuring instruments) as it approaches the V-shaped chute 14 of the equipment.

[0043] Typically, this tracking system may be in the form of acoustic sensors (for example SONAR type or acoustic cameras) arranged in opposition on each plate of the V-shaped chute 14. The signals from these sensors are transmitted in real time to the surface installation vessel in order to enable it to know the relative position of the active anchor line with respect to said chute. This data then makes it possible to correct in real time from the surface installation vessel the position, tension and length of the active anchor line as it approaches the equipment.

[0044] More preferably, the equipment according to the invention further comprises at least one power and control umbilical which is intended to supply and control the tensioning system 16, the jacks 21 and the pre-tensioning system 22 as well as all the equipment for controlling the active and passive anchor lines.

[0045] This power and control umbilical is advantageously housed in an umbilical basket 27 equipped with its release system and carried either by the port side or by the starboard side of the equipment. This basket provides structural protection during immersion and deployment of the equipment. This umbilical is capable of being deployed autonomously from the equipment and of being connected to the installation vessel once the equipment is placed on the seabed.

[0046] In connection with the Figures 3A to 3F , we will now describe an embodiment of a method of installing such underwater equipment according to the invention.

[0047] There Figure 3Aschematizes the initial conditions prior to starting the installation process. Among these initial conditions, the passive anchor line 24 and its retaining anchor 26 are placed on the seabed F. The end of the passive anchor line opposite the anchor is connected to the passive ring 6 of the equipment 2. As for the active anchor line 10, it is either also placed on the seabed or is waiting on the deck of the installation vessel N.

[0048] Finally, a length of abandonment chain is connected, on the one hand to the organs 12 of the equipment to allow the abandonment of the latter, and on the other hand to a hauling winch (not shown in the figures) of the installation vessel.

[0049] During the first stage ( Figure 3B), the installation vessel N begins to advance in the opposite direction to the retaining anchor 26 of the passive anchor line 24. If necessary, port and starboard hooking lines can be installed on the equipment and be relieved or taken up to adjust the position of the equipment relative to the launching device of the installation vessel.

[0050] The installation vessel N continues its course towards the equipment deployment position, gradually hauling out the abandonment chain in order to delicately launch the equipment ( Figure 3C ). In particular, the equipment is overflowed from the installation vessel and its overflow is controlled by the hauling winch.

[0051] The descent of equipment 2 then continues by hauling the hauling winch and advancing the installation vessel N ( 3D figure). During this descent, the equipment is suspended on the chain catenary between the installation vessel and the seabed.

[0052] The installation vessel N continues on its azimuth away from the holding anchor 26 of the passive anchor line. The equipment gradually descends ( Figure 3E ). The progress of the equipment towards the seabed F is monitored in real time thanks to the acoustic transmission of measurement data (trim, heel and azimuth angles of the equipment, and immersion depth of the equipment) to the installation vessel.

[0053] Once the equipment 2 reaches the seabed F and is stabilized thereon, the installation vessel N stops and the operation of the hauling winch is stopped. The position of the equipment is checked against a predetermined position. The installation vessel N then completes its trajectory by abandoning the abandonment chain 28 which is connected to a surface buoy 30 ( Figure 3F ).

[0054] The active anchor line is installed by the surface installation vessel in the V-shaped chute of the equipment to engage with the pre-tensioning and tensioning systems. This deployment of the active anchor line within the chute is carried out either directly from the installation vessel following the installation of the active anchor line or following its recovery by the installation vessel from the seabed where the active anchor line was lying awaiting deployment of the equipment. This recovery is controlled using the means described above.

[0055] Prior to tensioning the active and passive anchor lines, the power and control umbilical is deployed from the equipment to be retrieved and connected to the installation vessel to power and control the tensioning system.

[0056] The tensioning of the active and passive anchor lines by the equipment tensioning system is known per se. It is based on a back-and-forth movement formed by the two locks 18, 20 and the hydraulic cylinders 21.

[0057] A pulling cycle comprises the following successive steps: the chain of the active anchor line is engaged in the equipment and rests in the chain support chute 23, the fixed lock 20 of the tensioning system 16 being open (i.e. in the low position) and the movable lock 18 being in the closed position (i.e. in the high position). At this stage, the hydraulic cylinders 21 are retracted (i.e. in the start-of-stroke position). The hydraulic cylinders 21 are then deployed to their end-of-stroke position, which causes a movement of the movable lock 18 which drives the active anchor line over a length corresponding to the stroke length of the cylinders 21. The fixed lock 20 is then tilted into the closed position (high position) so as to block the active anchor line. As for the mobile lock 18, it is tilted into the open position (lower position) and thus releases the active anchor line.The hydraulic cylinders 21 can then be retracted to their start-of-stroke position so as to start a new pulling cycle.

[0058] In order not to create any "jamming" of the active anchor line at the equipment outlet on the passive end side 4a when it is tensioned, the active anchor line is kept under tension from the surface.

[0059] Another embodiment of a method of installing such underwater equipment according to the invention will now be described.

[0060] The initial conditions for starting the installation process according to this embodiment variant are as follows: the retaining anchor of the active anchor line is placed on the seabed. The end of the active anchor line opposite the retaining anchor is kept on the surface on the deck of the installation vessel and installed and secured in the locks of the equipment tensioning system. As for the passive anchor line with its retaining anchor, they are waiting on the deck of the installation vessel.

[0061] Finally, a length of abandonment chain is connected, on the one hand to the equipment organs on the passive end to allow it to be abandoned, and on the other hand to a hauling winch on the installation vessel.

[0062] During the first stage, the installation vessel begins to move towards the intended position for dropping the passive anchor line retaining anchor. If necessary, port and starboard hooking lines can be installed on the equipment and can be eased or taken up to adjust the position of the equipment relative to the installation vessel's launching device.

[0063] The installation vessel continues its course towards the deployment position of the passive anchor line retaining anchor, gradually unhooking the abandonment chain in order to gently launch the equipment.

[0064] The descent of the equipment is stopped after it has been submerged. The passive anchor line takes up the tension while the abandonment chain is relieved. The latter is secured to the passive anchor line.

[0065] The descent of the equipment is then resumed by hauling the passive anchor line hauling winch and moving the installation vessel forward.

[0066] The installation vessel continues its advance towards the position for dropping the passive anchor line retaining anchor while hauling the hauling winch. The progress of the equipment towards the seabed is monitored in real time thanks to the acoustic transmission of measurement data (trim, heel and azimuth angles of the equipment, and immersion depth of the equipment) to the installation vessel.

[0067] Once the equipment reaches the seabed and is stabilized on it, the installation vessel stops and the hauling winch is stopped. The position of the equipment is checked against a predetermined position. The installation vessel then completes its trajectory by abandoning the passive anchor line and the passive anchor, which is connected to a surface buoy. The active anchor line is then ready to be tensioned.

[0068] Prior to tensioning the active and passive anchor lines, the power and control umbilicals are deployed from the equipment to be retrieved and connected to the installation vessel to power and control the tensioning system.

[0069] The tensioning of the active and passive anchor lines by the equipment tensioning system can then begin according to the pulling cycle previously described with reference to the other embodiment of the installation process.

[0070] Whatever the embodiment, it will be noted that this step of tensioning the active and passive anchor lines can advantageously be preceded by pre-tensioning using the pre-tensioning system 22 of the active anchor line.

[0071] In connection with the Figures 4A to 4D , we will now describe an embodiment of a method for recovering underwater equipment according to the invention.

[0072] Prior to starting this recovery process, the active 10 and passive 24 anchor lines must be “de-tensioned” from the tensioning system of the equipment 2. The active anchor line 10 is extracted from the equipment 2 by hauling from the installation vessel N on the surface and deposited on the seabed F. The abandonment chain 28 is recovered and connected to the hauling winch of the installation vessel N, the latter being in the equipment recovery position ( Figure 4A ).

[0073] The hauling winch of the installation vessel begins to regain tension on the abandonment chain 28. The installation vessel then advances towards the retaining anchor 26 of the passive anchor line 24 to allow the equipment 2 to be raised ( Figure 4B ).

[0074] The installation vessel continues to move towards holding anchor 26, the hauling winch regaining tension to support the raising of the equipment ( Figure 4C ).

[0075] The ascent of equipment 2 continues until it reaches the launching device of the installation vessel to allow its recovery ( figure 4D ).

[0076] It should be noted that this recovery process also applies regardless of the method of carrying out the underwater equipment installation process.

[0077] In connection with the Figures 5A and 5B And 6A to 6D , we will now describe additional steps for multiplying the tensioning force of the active and passive anchor lines.

[0078] These additional steps apply to either of the two equipment installation methods described above. They are based on the use of a pulley and gear cable arrangement that is mounted once the equipment is installed on the seabed.

[0079] As shown in the Figures 5A And 6A , the first step consists of connecting a first end of a multiplication cable 32 to the passive end 4a of the equipment 2, that is to say to the passive ring 6 thereof.

[0080] The gearing cable 32 then passes through a first pulley 34 which is positioned on the passive end side of the equipment. The gearing cable is thus doubled on this side of the equipment (see Figure 6C ).

[0081] The gearing cable 32 continues its path by passing towards the active end 4b of the equipment to pass through a second pulley 36 positioned on the side of the active end of the equipment (see Figure 6B ).

[0082] The second end of the gearing cable 32 (opposite the first end) is then connected to a chain extension 38 which is itself connected to the tensioning system of the anchor lines of the equipment (see Figure 6D ).

[0083] Finally, as shown in the Figure 5B , each pulley is connected to one of the active and passive anchor lines, namely here the first pulley 34 is connected to the passive anchor line 24, while the second pulley 36 is connected to the active anchor line 10.

[0084] With such a pulley and gear cable device, the actuation of the system tensioning the anchor lines of the equipment will exert traction on the chain extension 38, this traction creating a multiplied force in the active and passive anchor lines.

Claims

1. An underwater equipment item (2) for tensioning anchor lines of an offshore structure, comprising: - a platform (4) intended to rest on a seabed (F) and to ensure the stability of the equipment and its self-alignment by sliding on the seabed when tensioning the anchor lines, the platform comprising a passive end (4a) equipped with a passive ring (6) intended to be connected to a passive anchor line (10) terminating in a retaining anchor, an active end (4b) opposite the passive end defining a pulling axis (8), and two haulage rings (12) aligned along the pulling axis, located in the lower part of the platform and set back from the ends, and able to allow deployment from the surface to the seabed and recovery of the equipment by a hauling winch from an installation vessel (N) on the surface; - a V-shaped trough (14) mounted on the platform (4) extending between the passive end and the active end of the latter and intended to recover an active anchor line (10); - a tensioning system (16) of the active and passive anchor lines comprising two bolts (18, 20) spaced apart from one another and engaging with the active anchor line recovered by the trough; - means for measuring, in real time during the deployment and recovery of the equipment item, trim, list and azimuth angles of the equipment item, as well as its depth of immersion; and - means of acoustic communication to transmit to the installation vessel the measurements of the angles and the immersion depth of the equipment item during its deployment and its recovery.

2. The equipment item according to claim 1, further comprising a system for pre-tensioning (22) the active anchor line once the latter is recovered by the trough (14).

3. The equipment item according to one of claims 1 and 2, further comprising a system for locating the active anchor line as it approaches the trough.

4. The equipment item according to any one of claims 1 to 3, further comprising a power and control umbilical intended to supply and control the system for tensioning the active and passive anchor lines, the power and control umbilical being housed in a basket (27) and capable of being deployed autonomously from the equipment and connected to the installation vessel once the equipment item has been placed on the seabed.

5. A method for installing an underwater equipment item according to any one of claims 1 to 4 for tensioning anchor lines of an offshore structure, comprising the successive steps of: - placing, on the seabed, the retaining anchor (26) of the passive anchor line (24); - connecting, on the installation vessel (N), the passive anchor line (24) to the passive ring (6) of the equipment item; - connecting the haulage ring (12) located on the side of the active end of the equipment item to an auxiliary handling chain (28) connected to the hauling winch of the installation vessel; - lowering the equipment item toward the seabed by shifting the winch and advancing the installation vessel in the direction opposite the retaining anchor of the passive anchor line; - placing the equipment (2) on the seabed (F); - stopping the installation vessel and disconnecting the hauling winch from the auxiliary handling chain; - deploying the active anchor line (10) in the trough of the equipment; - deploying a power and control umbilical from the equipment item to be recovered and connected to the installation vessel in order to supply and drive the tensioning system (16); and - activating the tensioning system (6) of the active and passive anchor lines.

6. The method according to claim 5, wherein the active anchor line is initially held in standby on the installation vessel.

7. The method according to claim 5, wherein the active anchor line is initially held in standby on the seabed proximate to the equipment item drop-off location.

8. A method for installing an underwater equipment item for tensioning anchor lines of an offshore structure according to any one of claims 1 to 4, comprising the successive steps of: - placing, on the seabed (F), an anchor retaining the active anchor line (10); - locking and securing the active anchor line (10) in the tensioning system (6) and the chain support trough (23) of the equipment item from the surface on the deck of the installation vessel; - connecting, on the installation vessel (N), the passive anchor line (24) and its retaining anchor (26) to the passive ring (6) of the equipment item; - connecting the haulage ring (12) located on the side of the passive end of the equipment item to an auxiliary handling chain (28) connected to the hauling winch of the installation vessel; - launching and lowering the equipment item by shifting the auxiliary handling chain and advancing the installation vessel toward the planned position for placing the retaining anchor of the passive anchor line; - stopping the deployment of the equipment item following its immersion, the passive anchor line taking up tension while the auxiliary handling chain is slackened and secured on the passive anchor line; - resuming the descent of the equipment item toward the seabed by shifting the passive anchor line and resuming the advance of the installation vessel in the direction of the planned placement position of the retaining anchor of the passive anchor line; - placing the equipment item on the seabed; - placing the passive anchor line in the direction of the planned position of the anchor of the passive anchor line; - placing the retaining anchor of the passive anchor line; - deploying the power and control umbilical from the equipment item to be recovered and connected to the installation vessel in order to supply and control the tensioning system; and - activating the active and passive anchor line tensioning system.

9. The method according to any one of claims 5 to 8, wherein the progress of the equipment item (2) toward the seabed (F) is continuously monitored by the installation vessel (N) owing to the measurements of the trim, list and azimuth angles of the equipment, and the immersion depth.

10. The method according to any one of claims 5 to 9, wherein, after being disconnected from the hauling winch, the auxiliary handling chain (28) is stored on the seabed and connected to a surface buoy (30).

11. The method according to any one of claims 5 to 10, further comprising recovering the equipment item comprising the successive steps of: - activating the tensioning system of the active and passive anchor lines to relax them; - extracting the active anchor line from the equipment item and placing it on the seabed; - recovering and connecting the auxiliary handling chain to the towing winch of the installation vessel; - moving the installation vessel in the direction of the retaining anchor of the passive anchor line to allow the raising of the equipment; and - recovering the equipment item on the installation vessel by pulling the hauling winch.

12. The method according to any one of claims 5 to 11, further comprising reducing the tensioning force of the active and passive anchor lines, comprising the successive steps of: - connecting a first end of a force-multiplication cable (32) to the passive end (4a) of the equipment item; - routing the force multiplication cable through a first pulley (34) positioned on the side of the passive end of the equipment item; - routing the force multiplication cable toward the active end (4b) of the equipment item by passing through a second pulley (36); - connecting a second end of the force multiplication cable to a chain extension (38) connected to the tensioning system (6) of the anchor lines of the equipment item; - connecting each pulley (34, 36) to one of the active and passive anchor lines (10, 24); and - traction performed on the chain extension (38) by the system for tensioning the equipment item.