Modeling and analysis of autonomous and nonautonomous horizontal-plane slow-motion mooring dynamics

Mooring systems are used for the station-keeping of vessels in oil and gas offshore drilling and production operations. The dynamic behavior of such systems is largely dictated by its slow, horizontal-plane motions. This dissertation presents a mathematical model used to study such motions, and further develops the University of Michigan mooring design methodology for autonomous dynamics. Two original studies based on that methodology are presented in this dissertation. The first is a comparative assessment of mathematical models representing hydrodynamic forces acting on the vessel hull due to steady current and the relative motion of the vessel with respect to water. Four such models, representative of a variety of published models, are classified based on their global dynamic properties and physics, and their advantages and limitations are assessed. The second study is an examination on the effect of hybrid lines on mooring systems dynamics. Hybrid lines are composed of segments of different materials, usually light synthetic fiber ropes and easy-to-operate chains, and are being increasingly adopted for deepwater applications. Another study presented in this dissertation focuses on horizontal-plane, large-amplitude slow motions exhibited by mooring systems. These are caused by both autonomous and nonautonomous excitation, a typical example of the latter being slowly-varying wave drift. It has been published in the literature that resonance with slowly-varying wave drift causes these large amplitude motions. To study the nonautonomous dynamics of mooring systems due to slowly-varying wave drift, two distinct and original approaches were developed. They are both introduced in this dissertation, one based on the analysis of time-averaged parameters from systematic simulations, and the other on harmonic balance. These approaches are used in the design of mooring systems to avoid undesirable large amplitude motions. One of the main conclusions from these approaches is that mooring system motions due to the interaction between Hopf bifurcations and slowly-varying wave drift can be two to three orders of magnitude larger than those induced by resonance.