What shape and tension does a cable take when it is payed-out or reeled-in from a ship? This paper presents a numerically simple but physically complete method for modeling the structural behavior of an underwater cable being payed-out and reeled-in. This new capability was a natural addition to Multi-Body Dynamic Simulation (MBDSIM), a computer code that can robustly simulate the structural response of cables, seafloor plows, and other highly compliant ocean structures. Example problems at the end of the paper demonstrate the robustness of the code, even for radical pay-out or reel-in maneuvers. This robustness is not found in traditional cable analysis codes.
Cables are often payed-out or reeled-in from ships on the surface to hold underwater objects, to trail fishing gear, or to moor the ship to an anchor. Pay-out means adding a length of cable to its end. Similarly, reel-in means removing a length of cable from its end. The process of paying-out or reeling-in cable is not normally an easy structural problem to model, even for a highly developed nonlinear finite element model. This difficulty arises because in most nonlinear structural analysis or simulation codes, the basic geometry and makeup of the structural problem cannot be changed during the analysis or simulation. This also means that the number of elements also can not normally be changed. Some nonlinear finite element codes can modify certain aspects of the basic geometry and makeup of the structural problem. To change the length of elements that model a cable or otherwise redefine the structure, SEADYN must perform an intricate process of modifying various state variables throughout the numerical matrices that represent the cable problem. If these matrices are not carefully modified in small enough increments, the iterative nonlinear step-by-step solution can easily become unstable.
