Transoceanic - Undersea Cable

- a description of the cable and how it is plowed into the ocean floor -

*** also see Coning's Undersea cable company's site -

Unlike above-ground cable, undersea cable must be capable of withstanding currents, and must have enough slack for repairs.  Many people think that undersea cables lie on the floor of the ocean.  After all, there is little or no current at those depths.  There are cables that lie on the ocean floor - but surprisingly, most undersea cables are buried !!!  This is referred to as "plowing".

The cable must be extremely rugged, and obviously water-proof in design.  The following shows a popular "steel-tuned" brand called "MiniSub" - note the extra layers of steel support bands and the outer rubber sheath:

Laying (Plowing) Undersea Fiber

It is a tremendous effort, with a ship that connects a tow-line to an undersea drag plow (click Here for a detailed explanation).  As the following blurry diagram shows, the ship pulls the plow with a steel tow cable, supplies electrical power to the plow via an umbilical cable, and at the same time lays the cable.

Modern plows are "jet-assisted" and use a powerful jet stream to help dig the trench. 

For repairs - when a cable is damaged, it is simply pulled up, repaired, and lowered back down.  Slack is accomplished by laying the cable along the bottom using a continuous series of "S-curves".

In 1988 the first transoceanic fiber cable, TAT-8, was laid. Economically, this made a tremendous difference, first to industry, and ultimately to consumers. In comparison, the first trans-Atlantic copper cable cost $1 million per circuit to install in 1956; TAT-8 cost less than $10,000 per circuit.

Transoceanic fiber has always been a major undertaking, and a risky one at that. When the telecom boom went bust in the late 90's, companies that invested millions in laying fiber-optic cable on land and at sea went bankrupt as the cable sat unused.  The two largest companies in this area are Tyco International (TyCom Ltd. - TCM),  and Global Crossing. 

Large, specialized ships or submarines are required.  For example, the "Morrison Bridge's work is done far offshore -- laying and repairing fiber-optic cable along the ocean floor. The projects should improve Internet and phone connections throughout the Pacific Rim.  The 460-foot steel vessel has a nautical range of 25,000 miles, meaning it can circle the globe without refueling, and can carry up to 4,600 kilometers of cable on board at a time. The ship is equipped with five diesel engines, a remote-controlled underwater bulldozer and a robotic vehicle that retrieves damaged cable thousands of feet beneath the surface of the ocean.

The Cable

Underwater fiber-optic cable, of course, must be more durable, and more water-and-salt resistant than standard cable.  It is commonly available in lightweight (LW), single-armored (SA) and double-armored (DA) versions. Typical underwater cable requirements include:

The two most common cables are E-glass with a hytrel jacket, and a steel-tubed fiber with FRP and a hytrel jacket. Specifications of the cables purchased are as follows:

Cable Specification E-glass Steel-tubed
break strength 200 lbs. 250 lbs.
working strength 70 lbs. 75 lbs.
specific gravity 1.54 2.2
weight in water 6.2 lbs./km 8.7 lbs./km
diameter 0.067" 0.075"

"TrueWave XL" fiber - in a breakthrough in telecommunications, Bell Labs (owned by Lucent) scientists have designed a novel optical fiber that can dramatically increase the capacity and transmission of transoceanic networks. The new fiber, called TrueWave® XL, is the first of its kind in the industry.  It allows for higher power levels and higher throughput by a factor of 4.  The speed is up to 10 Gbps per wavelength (DWDM offers multiple wavelengths - therefore for n separate wavelengths, the cable can supply n x 10 Gbps).

Ships vs Submarines - intially, undersea cable was laid and repaired by ships, and therefore a large amount of slack was required.  Today, submarines are used, and this allows repairs to be made with a minimal of "lift" of the cable.  Therefore, less slack is required, the S-curve can be less extreme, much less cable is required, and this saves a lot of money.

Corning NSW Underwater Fiber Cable Types 

Here are some common types of undersea cables, made by Corning's undersea cable division, NSW.  There are PDF brochures with cross-sectional diagrams

Repeaterless Undersea Cable Systems

Today, repeaterless fiber-optic submarine cable networks are the preferred solution for offshore telecommunications. Spans between platforms of over 400 km are possible !! Since signals transmitted via repeaterless systems can be amplified onshore, on islands or on platforms, it is possible to avoid the use of electrically powered subsea repeaters which are costly, severely limit the number of fibers a cable can contain and increase the complexity and cost of maintaining and repairing the subsea network. In addition, repeaterless systems make it possible to avoid the risks associated with the introduction of a high-voltage application on the offshore platform.

The Process of Planning and Laying Undersea Fiber