RIASSUNTO
ABSTRACT
Submarine cables and pipelines placed on the ocean floor must operate reliably for periods of up to 30 years. To ensure such reliability, cables and pipelines must be protected against both natural phenomena as well as human activity. As part of the Department of Energy Ocean Thermal Energy Conversion, and Underground Transmission Programs, a"" detailed analysis and characterization of cable hazards and cable protection techniques has been conducted.
Information on approximately one thousand incidents of submarine cable faults have been compiled. These data cover a 101 year period and are reported in probabilistic terms keyed to water depth. Chafing and corrosion were found to be the most common hazard representing just over half of the reported incidents. Fishing activities, specifically trawlers and dredgers, were the second most common hazard accounting for approximately one-quarter of the incidents. Ship anchor drags, submarine landslides, earthquakes, and miscellaneous other hazards accounted for less than 10% each of the reported incidents.
An assessment of cable protection technology has also been performed. Embedment, or placing the cable below the natural sea bottom is the most widely used technique. Alternative techniques include hazard avoidance, cable stabilization and cable immobilization. Capabilities and characteristics of these techniques are described in detail.
INTRODUCTION
A cable hazard is a natural or manmade phenomenon that is potentially a source of damage to a cable system. Usually the time of occurrence and the extent of damage cannot be predicted for an individual case; however, overall probabilities can be assumed. General techniques for precluding hazardous conditions in the design of a cable system are: (1) to avoid the hazard, (2) to accept a calculated risk and (3) to provide protection for the assumed ""worst case"" during the cable design life. This paper discusses various characteristics of common natural and manmade hazards, and ways to protect against them.
TYPES OF HAZARDS
Chafe and Corrosion. Current- and wave-induced motions can increase the chafe and corrosion experienced by a cable. The maximum hazard from currents exists when their speed is greatest and their direction is perpendicular to the cable. The maximum hazard from ocean waves exists when their heights are greatest, their periods shortest and their direction is perpendicular to the cable.
Biologically induced corrosion and fouling by bottom organisms such as bryozoans, shells and sponges to a degree that could impair or interfere with cable operations are limited to water depths of less than 10 fathoms. The more common types of biological fouling found to affect cable installation are kelp and coral.
Submarine Landslides. Submarine landslides are the phenomenon of mass movement of seafloor sediments caused by unbalanced gravity forces. Submarine landslides are of particular importance in the delta areas where there is a rapid accumulation of the very low shear strength unconsolidated deposits.
Fish Trawling. Two types of seafloor fishing gear are commonly used by trawlers:
Otter-Trawls - Otter-trawl equipment consists of the fishing net, otter-boards and the towing wires.