Arrangement and Method for Retrieving Broken Moorings

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Brazilian Application No. BR1020240227492 filed on October 31, 2024, the disclosure of which is expressly incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention is applied in the field of ​​subsea inspection and maintenance and, more particularly, relates to a method and arrangement for retrieving broken anchor moorings, for fishing sections of anchor lines that break and are disposed on the seabed.

The present invention can be applied in congested seabeds (with many nearby subsea structures) or non-congested seabeds (with few or no nearby subsea structures). The invention can also be applied in hard soils, which generate little suspension of soil particles in the water, or in soft soils, which generate a lot of suspension of soil particles in the water, hindering the visibility.

BACKGROUND OF THE INVENTION

In maritime operations, the retrieval of broken anchor lines is essential for the maintenance of Maritime Units.

The traditional method of retrieval involves the use of an AHTS (Anchor Handling Tug Supply) - an ocean-going tugboat - to drag a treble hook across the seabed in an attempt to hook the broken line. However, this method is often unfeasible due to the congested layout of the seabed and the proximity of other anchor lines or other subsea structures. The treble hook is a traditional piece of equipment in the industry that is roughly shaped like a four-point hook, in which each point (also called a “throat”) is a metal piece with a shape suitable for engaging and locking the mooring.

As a result, the retrieval of the moorings in congested locations has been carried out mainly by means of steel cable arrangements connected to the AHTS working rope. These arrangements need to be light and flexible in order to be installed with the aid of an ROV (Remotely Operated Vehicle) and, for this reason, they often break, resulting in the anchor line falling back to the seabed.

In view of these challenges, a fishing method and arrangement were developed that eliminate the need for steel cables, providing a more reliable retrieval, avoiding accidents and reducing operating costs.

Document PI 0204708-0 B1 presents a fast anchoring apparatus, which allows the subsea connection and disconnection of anchor lines to anchor devices already attached to the seabed. The fast anchoring apparatus comprises moorings, cables, hooks and floats particularly ordered and arranged, which, attached to the submerged end of the anchor line, allow a single support vessel, together with a remotely operated vehicle, to perform the entire anchor line anchoring operation on a floating platform. This document also refers to a method for installing and uninstalling the fast anchoring apparatus. However, the system proposed in document PI 0204708-0 B1 requires pre-installation and, further, must be perfectly stretched under the seabed, making it unfeasible for removing broken moorings on congested seabeds.

Document CN102219047A presents a device and a method for retrieving a subsea mooring system that sinks in muddy seabeds. The device comprises a small vessel, a winch, a steel cable, and a rescue hook, in which the winch is mounted on the aft deck of the vessel. One end of the steel cable is connected to the rescue hook and the other end is wound onto the winch; the steel cable connected to the rescue hook is released by the winch, and the rescue hook is pulled by the vessel to pass transversely through an anchor chain of the mooring system on the seabed. The anchor chain is hooked and secured by the rescue hook, and the steel cable is pulled by the winch to straighten the anchor chain. However, the method proposed in document CN102219047A cannot handle congested subsea environments, resembling conventional methods of retrieving broken moorings, in which it is necessary to displace the vessel horizontally to couple the broken mooring to the treble hook.

Document US3927636A, belonging to the General State of the Art, discloses a method for retrieving an anchor buried in the seabed and attached by a long mooring line to an anchored floating structure. A submerged remote release hook, having a yieldable catch, is towed with a service line in a direction intersecting the mooring line at a point between the floating structure and the anchor until the hook catches the mooring line. This method is not applicable to broken moorings.

Document CN102632976B discloses a retrieval device for a maritime mooring system comprising: a retrieval rope with one end provided with an anchor and the other end connected to a vessel and a plurality of rescue hooks that are hung from the rope and are located between the anchor and the vessel, wherein the rescue hooks comprise a main shank. The document also provides a method for retrieving maritime mooring systems. CN102632976B is mentioned because it deals with a system and a method for retrieving anchor lines on the seabed. However, the techniques used by the same resemble traditional mooring retrieval methods, in which mechanisms are provided for hooking into the submerged sections from displacements of the vessel. In this way, the method of CN102632976B is also based on the probability of success or failure in an attempt to hook onto the mooring section, unlike the present invention.

Therefore, it is apparent that the documents of the State of the Art do not describe the retrieval of any type of mooring (including those broken in operation) without requiring the horizontal displacement of the retrieval device. This aspect makes its application in congested subsea layouts unfeasible. By eliminating the need for horizontal displacement, the present invention minimizes the risk of interference with other mooring lines, as well as other nearby subsea structures, something crucial in environments where the space is limited and other solutions are not effective.

In addition, the adaptation of the treble hook and the use of robust components, such as specific slings and hooks, ensure a safer and more efficient operation, reducing the frequency of breakages and ensuring a successful retrieval of broken moorings. Therefore, the present invention not only simplifies the process of retrieving moorings, but also significantly expands its applicability in complex and congested scenarios, providing a practical solution to the limitations of the traditional methods.

SUMMARY OF THE INVENTION

The present invention proposes an arrangement for retrieving broken moorings comprising: at least three shackles; at least two slings; at least one mooring; at least one hook; at least one ROV needle rope; and a treble hook.

The upper end of the mooring is connected to a working rope with a swivel link, and the lower end of the mooring is connected to the treble hook through its main eye. Typically, treble hooks have an upper main eye and a lower auxiliary eye. However, to apply the present technique, it is necessary to create a hole in the end of one of the treble hook’s throats to serve as a third eye. Optionally, treble hooks with up to three damaged throats can be reused and removed, which is advantageous because it reduces the bending moment on the treble hook when going up the stern of the vessel. The third eye of the treble hook receives a sling, in which the two legs of this sling are joined by means of the shackle.

Additionally, the ROV needle rope is joined to the end of the sling and is further connected to the shackle. The hook, in turn, must be connected to the mooring by using at least one shackle in an intermediate link of the mooring (an arbitrary number of shackles can be added to make the dimensional transition from a large mooring to a small hook). More specifically, the hook must be connected to the intermediate link of the mooring in a position such that the distance between the hook and the treble hook is greater than the combined length of the slings.

In addition, the invention also describes a method for retrieving broken moorings, using the arrangement for retrieving moorings, comprising the following steps: positioning a treble hook close to a mooring to be retrieved; passing a sling around the mooring; connecting a sling to the hook; applying traction; and accommodating the mooring on the treble hook.

Positioning the treble hook involves laying it on the seabed, if the seabed has little suspension; and lowering it close to the bottom, but without touching the ground, if there is a lot of suspension. Passing a sling, in turn, comprises passing the needle rope, with the aid of an ROV, under the broken mooring and pulling the sling from the other side. Next, the connection of a sling to the hook comprises displacing the sling, with the help of the ROV, to the hook and attaching the sling to the hook.

The application of traction comprises: monitoring the ROV, and pulling, by means of the AHTS, the working rope until the broken mooring accommodates itself in the treble hook pocket. Finally, the mooring accommodation procedure comprises: slackening the sling and releasing the connection with the hook.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described below with reference to the typical embodiments thereof and with reference to the appended drawings, in which:

FIG. 1 is a representation of an example of a fishing arrangement from the State of the Art.

FIG. 2 is a representation of a broken mooring treble hooking operation, according to the State of the Art.

FIG. 3 illustrates the mooring retrieval arrangement, according to an embodiment of the present invention.

FIG. 4 illustrates a treble hook adapted with a hole in the end of two throats, according to an embodiment of the present invention.

FIG. 5 illustrates a treble hook adapted in the most recommended manner for application of the present Technique, with removal of three of the four throats and a hole in the end of the remaining throat.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned, fishing for broken anchor lines is traditionally done by means of steel cable arrangements connected to the moorings by ROV in the case of congested subsea layouts. Since such arrangements need to be flexible enough to be manipulated by the ROV, it is quite common to observe the rupture of the arrangements followed by the fall of the broken line back to the seabed.

FIG. 1 shows an example of a fishing arrangement from the State of the Art, using steel cables. As can be seen, the typical fishing arrangements consist of a main steel cable (also called working rope) (2), shackles (4A, 4B, 4C), mooring tails (6A, 6B), detachable reduction links (8A, 8B), a swivel link (also called a swivel or untwist) (10), intermediate steel cables (12), a final steel cable (14), and a barbed hook (16).

The main steel cable (2) is connected to a shackle (4A), where mooring tails (6A, 6B) are attached, connected to a swivel link (10) by reduction links (8A, 8B). At the other end of the final portion of the mooring tails, another shackle (4B) is attached, which is connected to intermediate steel cables (12), which also contain a shackle (4C) at their other end.

A final cable (14) and a barbed hook (16) are attached to this shackle, in which typically the two ends of the final cable are connected to the shackle, and the two parallel halves of the final cable are joined by rope ties. In this arrangement, the main steel cable (2) is the high-capacity winch cable of the AHTS vessel, with diameters typically greater than 80 mm. Considering that the main steel cable (2) is too rigid for ROV operation, the arrangement has one or more intermediate steel cables (12), which are thinner, lighter and more flexible, with diameters between 25 mm and 51 mm, carrying at their lower end the final cable (14), with a diameter between 12 mm and 25 mm, and the hook (16), joined by the shackle (4C).

The function of the intermediate cables is to allow sufficient flexibility for the ROV to bring the hook (16) close to the point where it is desired to connect to the broken mooring. The ROV then passes the midpoint of the final cable (14) through a link of the broken mooring and connects the same to the hook (16).

Typically, it is more efficient to pass the midpoint rather than the end of the cable through the link, with the additional advantage that the traction in the final cable becomes approximately one-quarter of the load.

Within the variations of this type of arrangement, it is possible to replace the main steel cable with a high-capacity synthetic fiber cable, typically of torsion-neutral construction, where the swivel link (10), together with the elements (6A, 8A and 8B) become dispensable. It is also possible to use synthetic fiber cables with appropriate dimensions to replace one or more intermediate cables (12). However, in this case, the final cable (14) must be made of steel, since the synthetic fiber cable can be damaged by the ROV’s hydraulic manipulators. Even with all possible optimizations, this type of arrangement still represents a weak point in the retrieval system, causing several ruptures with the broken mooring falling back to the seabed.

In this sense, FIG. 2 represents a broken mooring treble hooking operation, according to the State of the Art. As can be seen, an AHTS vessel (21) drags a treble hook (23) over a broken mooring hook (24). It is worth noting that the AHTS vessel (21) is connected to the treble hook (23) by a steel cable (22), with specifications similar to the main steel cable (2) in FIG. 1. Between the steel cable (22) and the treble hook (23), there is a similar set of elements that appear in FIG. 1 between the main cable and the secondary cable, these being: shackles (4A, 4B), mooring tails (6A, 6B), reduction links (8A, 8B), and swivel link (10). For the treble hooking operation to be successful, the catenary formed by the suspended steel cable (22) must never reach the treble hook (23); that is, there must be enough cable lying on the ground in front of the treble hook to compensate for the vertical movements of the vessel under the action of the sea. To achieve this, the total length of the cable must be at least 1.4 times the operating depth.

It is possible to slightly reduce the length of the cable required by adding a section of mooring of great dimension between the steel cable (22) and the treble hook (23) to provide ballast, and a small section of steel cable with a specification similar to the main cable.

The heavy mooring will tend to remain on the bottom, preventing the cable between the same and the treble hook (23) from being suspended. In both cases, the vessel will only be able to apply force to cause the treble hook to move after releasing the entire length of cable necessary for the operation into the water, so that, at the moment immediately prior to the application of force, several dozen meters of cable will be lying on the seabed.

Given the limitations of the fishing arrangements exemplified by FIG. 1 and the treble hooking method illustrated by FIG. 2, the present invention presents an arrangement for retrieving moorings, as shown in FIG. 3, comprising:

at least three shackles (30A, 30B, 30C);

at least two slings (32A, 32B);

at least one mooring (34);

at least one hook (36);

at least one ROV needle rope (38); and

a treble hook (40).

The upper end of the mooring (34) must be connected to the working rope with a swivel link, or polyester working rope from the AHTS. The treble hook (40) must be connected to the lower end of the mooring (34) using the shackle (30C). The sling (32A) must be passed through the third eye of the treble hook (40) (located at the end of the throat), as shown in FIG. 3. The two legs of the sling (32A) must be joined using a shackle (30C).

The ROV needle rope (38) must be joined to the sling (32B) using the common eye splice technique. The free end of the sling (32B) must be connected to the same shackle (30C) where the legs of the sling (32A) were joined.

In turn, the hook (36) must be connected to the mooring (34) using at least the shackle (30A) (more shackles of suitable dimensions may be used, if the dimensional difference between the hook and the mooring is too large) to connect the hook (36) to an intermediate link of the mooring (34). The hook (36) must be connected in a position such that the distance between the hook (36) and the treble hook (40) is greater than the combined length of the slings (32A, 32B). For this reason, the mooring (34) and the slings (32A, 32B) must be chosen such that the length of the mooring (34) is greater than the sum of the lengths of the slings (32A, 32B).

FIG. 4 presents a traditional treble hook adapted with a hole at the end of two throats. It is possible to observe the main eye (42), the throats (44A, 44B, 44C, 44D), the pockets (geometry for locking the links) (46A, 46B, 46C, 46D), the auxiliary eyes (48A, 48B), and the lower eye (50). The treble hook presented is a reinforced treble hook design, developed for loads of up to 400 ton.

FIG. 5 presents the treble hook adapted in the most recommended way for application of the present Technique, in which three of the four throats are removed. It is possible to observe the main eye (60), the body (62), the lower eye (64), the throat (66), the pocket (68), and the third eye (70).

Considering the arrangement proposed in this invention, the method of fishing for broken moorings comprises the following steps:

I. positioning of a treble hook;

II. passing of a sling;

III. connecting a sling to the hook;

IV. applying traction; and

V. accommodating the mooring.

In step I, the treble hook must be positioned close to the section of mooring that is to be retrieved. If the seabed has little suspension, the treble hook can be placed on the seabed. If there is a lot of suspension, the treble hook must be positioned close to the bottom, but without touching the ground.

Next, in step II, the ROV must pass the needle rope under the broken mooring and pull the sling from the other side. In this step, it is necessary to make sure not to pull the sling excessively to avoid jolting the ROV due to the movement of the vessel.

Step III comprises connecting the sling’s leg to the hook. The ROV, in possession of the sling, moves to the hook, where it attaches the sling.

In turn, in step IV, with ROV monitoring, the AHTS must slowly apply traction to the working rope. The sling will stretch first, guiding the broken mooring into the throat of the treble hook. The ROV has the function of providing visual feedback to the winch operator of the AHTS.

Finally, in step V, after the broken mooring is accommodated in the treble hook’s pocket, the AHTS must relieve the traction on the working rope, moving the treble hook towards the seabed. Next, the ROV must slack the sling so that it is disconnected from the hook.

It is worth noting that other types of commercial hooks suitable for handling moorings can be applied to the present invention without prejudice to the same, as long as they have an eyelet at the end by design or adaptation, similar to the adaptations shown in treble hooks.

This approach allows the retrieval of broken moorings in congested subsea areas; that is, with subsea structures around the broken mooring, such as pipelines, production equipment and other anchor lines, by eliminating the need to drag the treble hook over the mooring, and mitigates the risk associated with the low load capacity of the retrieval arrangements consisting of exclusively of steel cables. In addition, the method combines the flexibility and adaptability necessary to operate in different suspension conditions on the seabed, ensuring the efficient retrieval of the broken moorings, both in environments with seabeds with high suspension and in seabeds with low suspension. By simplifying the procedures, the practicality and efficiency of the operation are increased, minimizing the risk of interferences and breakages during the mooring retrieval.