Nonlinear roll motions of a frigate-type trimaran and susceptibility to parametric roll resonance.

摘要

Two scale models (1:80 and 1:125) of a U.S. Navy concept for a frigate-type trimaran have been tested in the Davidson Laboratory towing tank as part of an extensive seakeeping investigation. Numerical models using a 6 DOF potential flow Rankine-Panel Method have been developed for the prototype vessel to study and validate the influence of side-hull longitudinal stagger and transverse spacing on the hydrodynamic coefficients and dynamic stability. The research was motivated by the currently limited understanding of nonlinear roll motions of multihull ships, particularly by the need for a side-hull design method, which allows identification of outrigger locations that can expose the vessel to potentially dangerous nonlinear roll motions. The hydrodynamics part of the research was completed by investigating the effects the predominantly viscous forces have on the free and forced roll motion of a frigate-type trimaran at zero and forward speed. The dynamic stability portion was carried out by isolating the unstable regions on the Ince-Strutt diagram specific for this trimaran, and studying the influence of nonlinear roll-pitch coupling on the inception of parametric roll resonance instability. Since earlier investigations of trimaran roll damping and parametric resonance have been limited to free-roll decay tests and numerical integration of the uncoupled roll equation, this work attempted to address the combined nonlinear roll damping and dynamic stability problem by including numerical and experimental forced oscillation tests in the analysis. A detailed side hull multi-configuration analysis approach is used for determination of roll damping coefficients from free roll decay and forced roll tests. Variation of outrigger location reveals the side hull-main hull wave interaction, which has been seldom investigated on trimarans. First, the analysis of the effect of side hull longitudinal stagger, transverse spacing and speed on roll added mass and nonlinear roll damping was investigated. Second, simulation of forced roll experiments isolating wave trapping mode frequencies with catamaran-like behavior as well as nonlinear ship motions leading to parametric roll resonance were carried out. The latter was done by numerically solving the 1 DOF roll motion equation using the experimentally derived coefficients and then expanded it as a 6 DOF nonlinear boundary-value problem with Det Norske Veritas' potential flow Rankine-panel method ship motion program WASIM. Two nonlinear roll damping models using a roll decrement and an energy method were found suitable in the analysis of trimaran roll damping at zero and forward speed. These models were in good agreement with experimental results and were able to show reasonable agreement when incorporated in analysis in the frequency domain roll response as well as time domain parametric roll resonance simulations carried out in WASIM.