Use of high modulus fiber ropes in large scale towed marine applications

The advent of high modulus fiber (HM fiber) ropes utilizing materials such as Kevlar(R), Vectran(R) and Dyneema(R) present a cost-effective alternative over wire rope for towed in-water seismic equipment. Ease of handling, lightweight and high strength allow for longer towing offsets and the associated increase in the number of sensors which can be deployed at one time. Long-term use has shown that the fiber ropes exhibit a minimal yet marked decline in strength based on exposure to the elements, static loads and cyclical loading. Ropes used to replace traditional wire or wire rope stress members in complex constructions such as towed arrays and electro-mechanical cables occasionally show greater strength reductions due to an uneven load distribution. These load distributions are the result of structure's design and handling method. During destructive testing of cable assemblies using Vectran fiber rope stress members, breaking strengths were found to be 30-40% lower than the summed rated strength of the stress members. Individual stress members of equal age and under the same amount of wear and tear were found to fail at higher strengths. Further investigation showed stress banding and kinking at certain points of the cable where the stress member was either secured or had the possibility of encountering a bend radius smaller than the specified minimum. This difference in breaking strengths can be attributed to the nature of the cable construction and the handling of such which exceeded the individual stress member bend radius. Individual ropes used as replacements for wire tow ropes show wear and tear in line with traditional wire ropes.