Abstract

Cable burial by plowing into the ocean bottom has been the most widely used method of protecting undersea telecommunications cables from ocean hazards, including fishing activity and heavy ships' anchors. For the most part, this is accomplished by using heavy sled-type cable plows towed by large powerful cable ships to overcome the significant plow forces associated with most ocean bottom conditions. This technique has been generally effective and reliable, however the resulting depth of burial is limited by the amount of tow force the ship can impact to the plow. In some soils, achieving even the standard 1.0- meter burial depth requirement of most cable projects within the bollard-pull capabilities of most commercial cable ships is difficult with conventional passive plow technology.

Recognizing the need for deeper burial over a range of soils to counter the increased threat of the fishing industry which is using larger vessels, heavier gear and extending its activities into deeper waters, TyCom Submarine Systems Ltd. in cooperation with Perry Slingsby Systems Inc. have developed a new generation of cable plows with both passive and jet-assisted plowing capability. The newest in this suite, Sea Plow IX, was commissioned in July 2000 and has recently completed burial of over 1000 kilometers of fiber-optic cable in conjunction with the Hibernia Project in the North Atlantic Ocean. This lightweight (less than 17 metric tons in air) jet-assisted cable plow can be deployed from standard shipboard handling equipment and is capable of burying cables and repeaters to 1.5 meters depth at low tension and increased speed. Overall, this new fiber optic cable system installed by TyCom has been continuously buried to an average depth of 1.5 meters. Tow tensions ranged from 15 to 25 metric tons at an average forward speed of 1.4 kilometers per hour in soils ranging from medium clay to abrasive mixtures of sand and gravel. This lightweight ow-tension approach to plowing is the subject matter of this paper.

Introduction

Soon after AT&T-Submarine Systems (now TyCom Submarine Systems Ltd.) installed the first two transatlantic telephone cable systems (TAT-1 in 1956 and TAT-2 in 1959), it became apparent that some reliable means of protecting these cables from ocean hazards was needed. The greatest danger to submarine cables exists on the continental shelves resulting from bottom fishing activities such as trawling and scalloping. Approximately 95 percent of all past submarine telephone cable failures have been caused by human intervention, primarily from fishing activities.¹,Â2 Characteristically, these shelves slope gently from shore to about 100 miles seaward where the water depth typically approaches 100 fathoms. Beyond a bottom depth of 100 fathoms, the bottom slope increases rapidly as the edge of the shelf is reached and the transition to deep water begins. While most cable failures occur in relatively shallow waters (less than 500 meters), it was anticipated that commercial fishing would eventually move into deeper waters.

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