Development of a Multi-Level Approach for the Evaluation of Parametric Roll and its Application to Modern Commercial Ships

Abstract

A multi-level approach for the quantitative evaluation of parametric roll is developed and its application results to modern commercial ships are presented and reviewed in this thesis. Despite many previous studies, it is still a challenging issue to quantify the vulnerability of a ship to parametric roll occurrence on sea-going vessels. Therefore, occurrence mechanism and physical and stochastic characteristics of parametric roll are investigated in depth and this thesis presents a new numerical form for quantitatively analyzing the susceptibility of a ship to parametric roll in random sea ways based on the study. Due to the non-ergodic characteristics of parametric roll motion, numerous direct time-domain simulations are needed to obtain a stable long-term distribution of parametric rolls. To avoid such heavy computational demand and to accelerate numerical simulations, a 1.5-DOF computational model for parametric roll prediction is developed for both regular- and irregular-wave excitations, adopting the approximated GZ curve. In this model, the concept of transfer functions is introduced for the mean and the first harmonic component of GM, which is introduced to approximate GM fluctuation. The simulation results obtained by using this model are compared to those of a three-dimensional weakly nonlinear simulation program. The sensitivities of the simulation results to the initial value, time window and number of simulations are investigated by applying Monte-Carlo simulation, and their proper values are proposed. This thesis also introduces several numerical approaches to analyze parametric roll from simple Mathieu equation to advanced numerical tools based on IRF method and Rankine panel method. Using advantages of various approaches including 1.5-DOF GM-GZ approximation method, a multi-level approach is developed to evaluate the vulnerability of a ship to parametric roll quantitatively. It consists of 1st level check by Mathieu equation, 2nd level check for regular wave, 3rd level check for irregular wave and operational guidance by IRF method. Most advanced tool based on Rankine panel method is used to verify each numerical tool. This multi-level approach is applied to several modern commercial ships including 4 post-Panamax container ships, 3 PCTCs (Pure Car and Truck Carrier), 3 Passenger ships, VLCC and S175 based on North Atlantic wave data. Based on the simulation results, the vulnerability of each ship to parametric roll is evaluated, and the influences of still water GM, roll damping coefficient and ship speed are reviewed and discussed. It is confirmed that each level provides consistent results and the results give very useful information of the vulnerability to parametric roll for each ship type and ship length. As a powerful and effective countermeasure against parametric roll, operational guidance is very important to support crews decision making in harsh environmental condition. In this thesis, the procedure for operational guidance development based on IRF method is proposed and it is applied to 8,000 TEU container ship. All the application results are provided and summarized with easy instruction to help ship crews efficient decision making and to avoid severe parametric roll. The contribution of this study on ship design, dynamic stability criteria are discussed and future works to reinforce the current approach and numerical scheme are proposed