What Are Underwater Cables?

Underwater cables are insulated conductors or optical fiber assemblies laid on or buried in the seabed to transmit electrical power or communications signals across bodies of water. They span distances from short harbor crossings a few hundred meters long to transoceanic routes stretching tens of thousands of kilometers. The technology is foundational to global telecommunications: NOAA estimates that more than 95 percent of international data traffic travels through undersea fiber-optic cables rather than satellite links, owing to the far greater bandwidth and lower latency that fiber provides. Underwater cables also carry bulk electrical power between islands, from offshore wind farms to shore, and across straits where overhead transmission is not possible.

The first practical submarine telegraph cable linked England and France in 1850; the first successful transatlantic cable became operational in 1866, carrying telegraphed text at roughly twelve words per minute. The Engineering and Technology History Wiki milestone for TAT-8 records how the 1988 installation of the TAT-8 cable marked the transition to fiber-optic transmission, replacing coaxial copper cables with glass fibers capable of carrying orders of magnitude more traffic at far lower signal loss.

Fiber-Optic Telecommunications Cables

Modern submarine telecommunications cables are built around one or more fiber bundles housed inside a polyethylene core, surrounded by a steel wire armored layer and an outer polyethylene sheath. Signal amplification is provided by optical repeaters spaced 50 to 100 kilometers apart, powered by a DC current running through a copper conductor in the cable. In deep water, where the seabed is relatively stable and free of fishing gear, cables are often unburied and rely on the armoring for protection. In shallow coastal zones, cables are buried one to three meters into the seabed using a jetting sled or plow, protecting them from ship anchors and trawling. The world's active subsea cable network, comprising more than 500 systems and approximately 1.7 million kilometers of cable, handles international voice, internet, and financial transaction traffic around the clock.

Submarine Power Cables

Submarine power cables transmit electricity in configurations similar to underground power cables but adapted for the marine environment. Extruded XLPE insulation is standard for AC systems, while mass-impregnated non-draining (MIND) insulation and, increasingly, extruded HVDC polymeric insulation are used for high-voltage direct current (HVDC) links. HVDC is preferred for long submarine routes because it eliminates the reactive power losses that make long AC cables inefficient. The cable's steel armor also serves as a mechanical anchor against tidal currents and provides cathodic protection against seawater corrosion. IEC 60840 and IEC 62067 govern testing and design of high-voltage and extra-high-voltage submarine power cable systems. Offshore wind energy has driven rapid growth in short-range inter-array cables (connecting turbines within a farm) and long-range export cables (connecting the farm to the onshore grid).

Cable Protection and Repair

Underwater cables face hazards including ship anchors, trawl gear, submarine landslides, seismic activity, and biofouling. Armoring provides mechanical protection, and burial reduces exposure in coastal zones, but fault rates in shallow water remain higher than in deep water. Repair requires a cable ship equipped with grapnel tools to lift the cable, submersible equipment for inspection, and splicing technology to join fiber or conductor ends after a cut is removed. The IEEE Oceanic Engineering Society publishes research on cable fault location, repair vessel operations, and structural health monitoring methods for subsea systems.

Applications

Underwater cables have applications across a wide range of sectors, including:

• International internet, telephone, and financial data transmission
• Offshore wind and tidal energy export to onshore grids
• Interconnection of island power grids and cross-strait HVDC links
• Scientific observatories on the ocean floor, including seismographic and oceanographic sensor networks
• Military communications and surveillance infrastructure