Submarine cable

Submarine cable is a cable wrapped with insulating material and laid on the seabed for telecommunication transmission. Submarine cables include submarine communication cables and submarine power cables. Modern submarine cables use optical fibers as materials to transmit telephone and internet signals. The world's first submarine cable was laid between Britain and France in 1850. China's first submarine cable was completed in 1988.    Submarine cables are divided into submarine communication cables and submarine power cables. Submarine communication cables are mainly used for communication services, which are expensive but highly confidential. Submarine power cables are mainly used for underwater transmission of high-power electric energy, and have the same function as underground power cables, but the application occasions and laying methods are different. Because the submarine cable project is recognized as a complex and difficult large-scale project by all countries in the world, and complex technology is applied to environmental exploration, marine physical survey, and cable design, manufacturing and installation, the manufacturers of submarine cable are few in the world, mainly Norway, Denmark, Japan, Canada, the United States, the United Kingdom, France, Italy and other countries, which also provide laying technology in addition to manufacturing.

Fabrication materials

The core of submarine optical cable is made of hair-like high-purity optical fiber, which guides light along the path of optical fiber through internal reflection. The submarine cable should be able to withstand the huge pressure at 8 kilometers underwater, which is equivalent to the weight of an elephant placed on a person's thumb. The deep-sea cable manufactured by NEC is made of lightweight polyethylene, and the whole cable is only 17 mm thick.

Use of submarine cable

Submarine communication cables are mainly used in long-distance communication networks, usually between long-distance islands, cross-sea military facilities and other important occasions. The laying distance of submarine power cable is much shorter than that of communication cable, and it is mainly used for connecting between land islands, crossing rivers or harbors, and connecting drilling platforms or drilling platforms from land. In general, the application of submarine cable to transmit electric energy is undoubtedly more expensive than the same length of overhead cable, but it is often more economical than the use of small and isolated power stations for regional power generation, and has more benefits in offshore areas. In countries with many islands and rivers, such cables are widely used.

Submarine cable capacity

The submarine cable can carry up to 80Tbps of data, equivalent to 4.7GB of capacity in one second.

Type classification of submarine cable

1. In terms of structure, it is mainly divided into three-core submarine cables and single-core submarine cables. The majority of medium-low voltage lines use three-core submarine cables, and the majority of high-voltage lines use single-core submarine cables.    2. In terms of function, half a century ago, the submarine cable only had a simple power transmission function. Now the submarine cable integrates two functions, effectively realizing the transmission of power and signal on the same cable line.    3. From the perspective of insulation composition, it can be divided into oil-filled insulated submarine cables and extruded plastic insulated submarine cables.    4. From the perspective of load type, DC submarine cable is divided into DC submarine cable and AC submarine cable. DC submarine cable is characterized by low loss and easy to realize long-distance transmission, but the application experience of DC submarine cable is not rich. The construction cost of DC converter station and other supporting facilities is high, and the loss of AC submarine cable is large, but the operation and maintenance technology is mature, and the supporting construction cost is small. Therefore, the designers of submarine cable lines usually need to make technical and economic trade-offs to maximize benefits.    Impregnated paper-clad cable: applicable to the line with no more than 45kV AC and no more than 400kV DC. At present, it is only installed in waters with water depth less than 500m.    Self-contained oil-filled cable: applicable to DC or AC lines up to 750kV. Because the cable is oil-filled, it can be laid in the sea with water depth of up to 500m without difficulty.    Extruded insulation (cross-linked polyethylene insulation, ethylene-propylene rubber insulation) cable: suitable for up to 200kV AC voltage. Ethylene-propylene rubber is more effective than polyethylene in preventing branch phenomenon and partial discharge, so that the submarine cable can play its role more effectively.    "Oil pressure" pipe cable: it is only applicable to several kilometers of cable system, because it is subject to great mechanical restrictions to pull extremely long cables into the pipeline. Inflatable (pressure assisted) cable: The inflatable cable wrapped with impregnated paper is more suitable for the long submarine cable network than the oil-filled cable. However, due to the need to use high pressure operation in deep water, it increases the difficulty of designing the cable and its accessories, which is generally limited to the water depth of 300m.

How to repair the broken submarine cable?

The repair of submarine optical cable is very complex. In the vast sea, it is necessary to find the optical cable less than 10 cm deep thousands of meters, and also to determine the fault point, like looking for a needle in a haystack.    First, we need to use OTDR instrument to find the fault point, judge the specific location of the break point, and then send an underwater robot (ROV) to cut the optical cable at the break point, and then pull the two ends of the optical cable to the ship for welding. The welding process is quite complicated, because it is necessary to weld the fiber with thin hair in the optical cable one by one.    After the completion of optical cable fusion, the repaired optical cable will be thrown into the sea and "buried", that is, the mud on the seabed will be washed out of a ditch with a high-pressure water gun, and then the repaired optical cable will be "placed" in. This process is also completed by the robot.