Modeling of Ship Roll Dynamics and Its Coupling with Heave and Pitch
In order to study the dynamic behavior of ships navigating in severe environmental conditions it is imperative to develop their governing equations of motion taking into account the inherent nonlinearity of large-amplitude ship motion. The purpose of this paper is to present the coupled nonlinear equations of motion in heave, roll, and pitch based on physical grounds. The ingredients of the formulation are comprised of three main components. These are the inertia forces and moments, restoring forces and moments, and damping forces and moments with an emphasis to the roll damping moment. In the formulation of the restoring forces and moments, the influence of large-amplitude ship motions will be considered together with ocean wave loads. The special cases of coupled roll-pitch and purely roll equations of motion are obtained from the general formulation. The paper includes an assessment of roll stochastic stability and probabilistic approaches used to estimate the probability of capsizing and parameter identification.
ROLL DAMPING OF A SHARP-CORNERED BARGE AND ROLL CONTROL BY A NEW-TYPE STABILIZER
In the first part of the paper, prediction methods of the roll damping of a ship proposed by one of the authors are experimentally examined to confirm their applicability to a barge. Measurements of the roll damping of sharp-cornered barge-type ships are carried out by free decay tests and a forced roll test. The experimental results are compared with results obtained by the prediction methods. The comparisons demonstrate that the prediction method is valid only when the roll amplitude is smaller than five degrees and that at large roll amplitude the methods give overestimated results. In the second part of the paper, the effect of a new anti-rolling device using vertically oscillating fins on roll motion reduction of a barge in waves is investigated experimentally. Since the system is effective even if the barge has no forward speed, it can reduce the ship motion when the barge is carrying out installation work in the ocean at zero forward speed.
A new approach to the overturning stability of floating structures
Abstract The increasingly widespread use of floating offshore production facilities has led to a need for a better understanding and accurate prediction of dynamic behavior. The approach described in this paper uses an approximate criterion for safe, forced, roll oscillations of a typical semi-submersible based on a stability margin. This stability margin is developed from a comparison between the total energy of the steady-state oscillating system and the total energy required to cause instability in the corresponding unforced, undamped system. The accuracy and utility of the new approach is assessed by comparing the loss of stability of numerical simulations with the approximate criterion.