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ARC - Cable Repair Ship

Submarine cables are important for both modern military and private enterprises. The US military, for example, uses a submarine cable network to transfer data from conflict zones to CONUS command centers. Interruption of the cable network during intensive operations can have direct consequences for the military on the ground. Submarine cables clearly play a critical role in military communications. Submarine cables provide the primary means of connectivity between widely scattered undersea acoustic monitoring networks and shoreside processing stations. Navies work with commercial and Government cable owners, planners, surveyors and installation contractors in order to minimize possible damage to military cable systems and other seafloor assets.

Cable ships are mainly used for laying and repairing underwater cables, and can be divided into cable laying ship and cable repair ship. The cable-laying ship can also be used as a cable repair ship. The underwater cable can be divided into a communication cable for transmitting audiovisual information and a power cable for transmitting electric energy. The main object of the cable-laying ship is the underwater communication cable, but it can also be used for laying and repairing electricity. cable. Communication cables mainly include coaxial cables, amplifiers, equalizers and other equipment. On the one hand, the cable-laying vessel accurately lays out the cables in the water according to the designed circuit, and on the other hand, the cables are connected and tested. The ship is equipped with large-capacity cable cabins, amplifier storage yards, cable laying machines and cabins for connection and testing. A cable sliding shaft with a diameter of 30 times the diameter of the cable is provided at the upper part of the bow post, so that the cable can enter the water and board the ship smoothly. The stern deck is made into a square shape to enlarge the working area, and is equipped with cable guide grooves and stern slides for cables when boarding or entering the water. There are two types of cable laying machines, drum type and crawler type, as a tool for pulling and controlling the speed of cable retracting. The position of the cable pulley, the guide groove and the cable laying machine are arranged in a straight line. Generally, new cables are laid at the stern and the cables that need maintenance are collected at the bow. If cables are laid in shallow water areas, in order to prevent the cables from being damaged by fishing nets, anchor chains, etc., a cable burying machine must be used to bury the cables in the bottom mud about one meter. In order to ensure the accuracy of cable routing, the ship is equipped with systems for precise measurement of ships position and automatic steering, and is equipped with dual propellers. Most of them use adjustable pitch propellers and lateral thrusters. There are ballast tanks on the ship for adjusting trim and For the center of gravity. The U.S. Navy uses underwater cables in a wide variety of systems, not only for subsea communication and power transmission, but also for precise placement and orientation of acoustic sensors suspended high above the seafloor. Every situation has different design parameters determined by project purpose, location and materials. Each of these unique projects presents its own challenge.

The first successful trans-Atlantic submarine cable a simple copper wire became operational in 1866, delivering about 12 telegraphed words per minute. As technology and laying techniques improved, the submarine network expanded greatly. In September 1956, the era of submarine coaxial telephone communication began when two coaxial cables capable of carrying multiple voice channels came into service linking London and North America. Finally, in 1988, the first transoceanic fiber-optic cable was installed linking the U.S., the U.K and France. Thereafter, the number of submarine fiber-option cables proliferated as they rapidly outperformed satellites in terms of the volume, speed and economics of data and voice communication.

It is a common misconception that most global communication is accomplished via satellite. In fact, over 95-97 percent of international data and voice transfers are currently routed through the many fiber optic cables that crisscross the worlds seafloors. Commercial undersea cable communications carry most intercontinental electronic communications, facilitating the reach and speed of internet and phone access critical to international trade, official government communications, and daily end user requirements. Overall, the overseas communication industry has built-in resiliency for regular, standard, operational single point cable failures.

Subsea cable breaks or other malfunctions are typically repaired by a cable repair ship working as close as feasible to the break. Typically, an ROV or grapnel is used to find the cable fault or break in a fiber optic cable, cut the cable near the cable fault, and retrieve the two ends for splicing with a new cable on the repair ship. Once aboard, the two cut ends are repaired and spliced back together, and the repaired cable is installed on the seabed or reburied.

Cable faults that necessitate repairs rarely occur on modern buried fiber optic cables. The primary cause of cable disturbance is external damage from human activities, as opposed to damage occurring from natural events, such as earthquakes. The number of length-normalized faults in submarine cables installed in shallow (less than 1,000 meters feet deep) water is less than 0.35 faults per year per thousand kilometers of cable installed.

If a repair is needed, divers, ROV, or grapnel, depending on depth, would be used to retrieve the cable from the seafloor at the point of the fault. A grapnel is long cable with a specialized hook on the end that is dragged either along the seabed or through the subsea floor (if the cable is buried) at a right angle to the cable until the hook grabs one of the cut cable ends.

A damaged submarine cable must be repaired onboard a cable ship. But a cable (whether tensioned or not) that is resting on, or buried in, the seabed will lack sufficient slack to reach the surface for repair. Unless a cable is already severed, therefore, it must first be cut in order to be brought to the surface. This retrieval operation takes at least three passes with the grapnel one to cut the cable, a second to bring up and buoy one end of the cable, and a third to bring up and bring onboard the second end.

A new section of cable would be spliced in by a cable repair ship at the surface. The repaired cable would then be replaced and re-buried. Even when there is a situation affecting traffic on a majority of the fiber pairs in a system, submarine cable operators are often able to adjust and rebalance power feeding equipment to restore traffic within a relatively short period of time without necessitating a large-scale repair operation.

A conventional method of repairing a damaged underwater optical fiber cable is appplied to a cable previously been installed on the seabed. The cable may be lying on top of the seabed but in practice the cable may be buried. If the cable has developed a fault, the approximate location of the fault may be determined by a technique (for instance optical or electrical reflectrometry) before a cable repair ship is sent to repair the damaged cable.

When the ship has reached the approximate location of the fault, a cable severing grapnel is deployed via overboard sheave to sever the cable. This separates the cable into a first length (containing the fault) and a second length. A retrieval grapnel is deployed to retrieve the end of one of the lengths of severed cable onto the deck of the ship. The retrieved length of the cable is tested to determine whether it contains the fault. If this length does not contain the fault, it is buoyed off using a buoy. The retrieval grapnel is used to retrieve the other length of the cable.

The cable repair ship pulls in the first length of the cable until the fault is on the deck of the ship. The exact location of the fault may be determined using reflectrometry or may be clearly visible (for instance the outer casing of the cable may have been visibly damaged by a ship's anchor or fishing gear). Once the fault is on the deck of the ship, the section of cable containing the fault is cut out by cutting the first length of the cable at a location on the sea side of the fault. The damaged section of cable is replaced by a third length of cable which is joined to the first length of the cable using a conventional inline cable joint.

The first length and third length of the cable are then paid out until the cable retrieval ship has returned to the buoy. The buoy is retrieved and the second length of the cable is brought onto the deck of the ship. If at this stage, the third length of the cable is cut at a suitable position and the free end of the third length of cable is joined to the end of the second length of the cable via a conventional inline joint. The inline joint is known as a "final splice joint".

It is not possible to lower the cable on a single deployment rope since this would cause the cable at the point of attachment to bend beyond a minimum bend radius and damage the cable. Therefore two deployment ropes are used to create a loop having a crown of sufficiently low curvature. The deployment ropes are attached to the cable via a pair of suitably spaced stoppers. The cable is lowered overboard using the deployment ropes and at a suitable point the deployment ropes are released, either by cutting the ropes at the ship or by releasing the ropes at the stoppers by actuating a release hook (e.g. an acoustic release hook) Depending on the depth of the water the deployment ropes may be released before the cable has reached the seabed, and may even be released before the stoppers have reached the water.

A similar method of lowering a jointed cable may be employed in a conventional method of installing a cable on an underwater bed between two land masses. In one example, separate lengths of the cable are first installed in the shallow water near the two respective land masses. The two shallow water lengths of cable are then buoyed. An installation ship joins a third length of cable to one of the shallow water lengths of cable, installs the third length of cable in the deep water between the two land masses, joins the third length of cable to the other shallow water length of cable, and lowers the joined lengths.

In a second example a first length of the cable is installed from one land mass to a point mid-way between the land masses, and then buoyed off. A second length of cable is then installed from the other land mass until the buoyed-off end of the first length of cable has been located. The first and second lengths are then joined and lowered. Alternatively the first and second lengths may be joined before the second length of cable is installed. In this case the cable joint is simply paid off via the overboard sheave as the second length of cable is installed.

The method of deploying the cable has a number of problems.

Firstly the cable tends to develop loops as it returns to the seabed. As the cable is lowered, the ship steams away from the line of the cable. The aim is to ensure that the additional length of cable (which is required due to the depth of the water) forms a single untwisted loop which can be relatively easily reburied. However in practice the cable often tends to develop loops and twists as it is lowered on the deployment ropes (due to twisting of the deployment ropes and looping of the crown which is not under tension). In addition the cable develops further loops and twists as the cable falls if the deployment ropes are released before the stoppers have reached the seabed. As a result, instead of lying as a single untwisted loop, the cable develops a number of untwisted loops and twisted loops which are difficult or impossible to rebury.

Secondly, if the deployment ropes are released by cutting them onboard the ship then the ropes are left on the seabed attached to the cable via the stoppers. If the deployment ropes are later caught by a ship's anchor or fishing gear then the cable can be damaged. To avoid this problem it is common practice to send a Remotely Operated Vehicle (ROV) down to the seabed after the cable has been lowered to cut the deployment ropes from the cable. This adds cost and complexity to the deployment operation.

Thirdly the requirement of two deployment ropes increases the operational complexity of the deployment operation. In some cases, additional deployment ropes may be attached to the cable between the stoppers to increase the tension on the crown and reduce looping. This increases the complexity of the lowering operation further and increases the number of deployment ropes which must be released and/or cut by an ROV.

Fourthly it may be difficult to achieve sufficient spread at the ship between the deployment ropes. This is a particular problem where there is limited width available on the deck of the ship (such as on bow-working ships).

Conventional deep-sea telecommunication cables are designed to interconnect ground user networks within high-speed paying intercontinental links. These electro-optical cables are highly reliable and have a lifetime of many decades. They are deployed conventionally from the surface by cable ships. They must be particularly reinforced as they must be resistant to deployment efforts due to their linear weight and surface accelerations. Links of a few hundred kilometers are produced in one piece in a factory by assembling cable segments by using repeaters (about every fifty kilometers) as well as bypasses for communicating with other ground user networks (an island for example).

This technology perfectly meets the needs of ground users but is not necessarily adapted to users requiring to be connected on the sea floor. However, the deep-sea sensor networks which are currently under study are either adopting or planning to adopt this technology, mainly for availability and reliability purposes.

According to this technology, in a first phase, the network is deployed continuously from the shore by a cable ship, from point-of-use to point-of-use. The network is formed by transport segments connected in a factory by repeaters or bypasses in order to serve the various points-of-use. In a second phase, nodes (or junction boxes) are set-up and connected at the ends of these bypasses on the sea floor by an underwater vehicle. In a third phase, these sensors are set-up and connected on the sea-floor by an underwater vehicle.

The advantage of this concept is to use for the network, cables and deployment methods that are proven in the telecommunications field. Another advantage is that these electro-optical cables designed to supply repeaters in line can also electrically supply the sensors. Such cables can transmit at high voltage, power levels of around hundreds of kilowatts.

The disadvantage is that these cables are heavy (a ton per kilometer) and are costly. Moreover, their deployment requires specialized and costly cable ships.

However, despite the existence of other communication concepts, there is no way to avoid the (optical) cable for sea floor communications over a great distance. The acoustic transmission concept is not adapted for great distances and requires the use of relay stations every ten kilometers. Moreover, the bandwidth is very limited and the energy per transmitted bit is high. The Hertzian transmission concept by means of surface buoys requires maintenance and the seafloor-surface link is fragile. In addition, these buoys are dangerous with regard to navigation.



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Page last modified: 01-08-2021 14:08:50 ZULU