Operational Characteristics of Electromechanical Cables

During the past decade, there has been a proliferation of operational systems which include electromechanical (or electro-optical-mechanical) cables as vital system components. Examples of such systems include: 1) unmanned remote operated vehicles (ROVs) used for subsea exploration, inspection, recovery, or repair; 2) towed arrays or towed electronic packages used for subsea maping, exploration, or surveillance; 3) moored subsea systems used for surveillance or acquisition of environmental data; 4) tethered aerostats used for communication and surveillance; 5) data acquisition systems used in oil, gas, and geothermal wells. For each of these systems, the success of the mission is contingent upon the reliable operation of the electromechanical (EM) cable which provides the strength, power, and communications link. As the applications for EM cables have become more varied and as the operational conditions for these cables have become more demanding, there has been a corresponding increase in the sophistication of cable design and manufacturing methods. Most contemporary EM cables are highly complex machines which are designed specifically for the intended application. Unfortunately, the complexities and the operational idiosyncrasies of these cables are often misunderstood by the cable user with the result being less than optimum cable performance. As cable technology continues to advance, it becomes increasingly important that cable users devote ample engineering and monetary resources to the development of cables and cable handling systems which are required for critical applications. This paper discusses many of the operational characteristics of typical EM cables and highlights the important points which must be considered during cable design. Examples are given of how cable designs may be tailored to achieve specific performance goals in terms of cable strength, elasticity, torque and rotation characteristics, bending fatigue life, and twist tolerance. The paper also compares and contrasts the operational characteristics which are achievable with cables having metallic strength members versus cables having nonmetallic (typically Kevlar) strength members. The advantages and short-comings of these two basic classes of cables are described for various strength member configurations. A list of references is provided to assist the reader in further investigations into cable response to tension, bending, and twisting.