The Stress Wave Method For Calculating Snap Loads Of Marine Cable In The Deepwater Platforms

Cables are widely used for offshore and subsea engineering. They provide an economic means for various offshore activities. It is only during the second half of this century that cables have been subjected to systematic research as the increasing activities involved in offshore gas and oil exploration requires a good understanding of the statics and dynamics of cables.In applications such as mooring a buoy, towing a ship or tethering a subsea unit, cables can become slack if the tension temporarily falls to a level which is comparable to the distributed force along the cable, and thus operate in alternating taut-slack conditions under periodic environmental excitation. The transition from the slack state to the taut state may cause high tension in the cable which can have detrimental effects and may even cause cable breakage. In this case, it is meaningful to study the dynamics of cables.The snap tension of small-sagged cables operating in alternating taut-slack conditions is considered in this paper. The cable is suspended at the two ends on the same level. One of the two ends is fixed, while the other is subjected to horizontal excitation. Two partial differential non-linear equations of dynamical motion of the cable is derived and solved by the stress wave method. The approach adopted here is capable of tackling the pertinent nonlinearities of cable dynamics including damping, geometric nonlinearity and the curvature.Numerical results and analyses under four sets of parameters show that: There are two types of waves which propagate through the cable, the longitudinal wave travels much faster than the transverse wave, both of their speed may be attenuated as they propagate through the cable. The largest dynamic tension appears at ends and their neighbourhood.As the increase of the excitation amplitude and frequency, the dynamic force is becoming larger and larger. The largest dynamic force is more related to the speed than the displacement. The dynamic force at the excitation side is the largest at beginning, and the force at the other end increases as the time passed.