Study On Characteristics Of Ship Rolling Motion

Ships sailing in the sea will swing due to the influences of wind, wave, currentand other interferences. Swinging will cause negative impacts on navigational safetyand operational efficiency of ships. Large amplitude swinging may even lead to shipscapsizing. Rolling motion is not only the most common behavior but also the mostdangerous motion mode in ships sailing in the sea. The ships capsizing is mostlycaused by the large rolling angles.Rolling is a very important and complex subject in the field of ships’ stability andseakeeping research. In the early stage, the theoretical basis that experts and scholarsused to study ship swinging was linear theory, but that approach ignored thenon-linear factors in the movement of ships swinging, and could not give asatisfactory explanation to some ships capsizing suddenly phenomenon. In recentyears, along with the development of nonlinear dynamics theory and the wideapplication of computer science and technology, people began bringing nonlineardynamics theory into the study of ship rolling motion and capsizing mechanism.Thus, how to apply the nonlinear dynamics theory to explain, forecast andcontrol ships motion state in real conditions to avoid the ship capsizing accidents, hasbecome urgent work to do in the field of ship motions research. Study on ship rollingmotion performance has a remarkable significance in revealing the capsizingmechanism, predicting motion state, developing anti-rolling device, etc. Based on shipstatics，dynamics，seakeeping and fluid mechanics theories, this paper focused on thefollowing aspects:(1) The characteristics of linear rolling motion of ships in regular beam waveswere analyzed. By using statistical analysis methods, the basic method of the forecastof ships rolling in irregular waves was proposed.(2) Considering the nonlinear damping and nonlinear righting moment, nonlinearequations of ship roll motion in regular beam waves were established. By usingmulti-scale method, the approximate analytical solutions of the nonlinear equation inthe case of primary resonance were obtained, then, they were verified by real casevalidation. Furthermore, the approximate first-order amplitude-frequency responsefunction of the system was analyzed, and the mutation phenomenon mechanism wasexplored.(3) In order to facilitate the forecast of ship roll with statistical analysis methods,the nonlinear equation of ship roll motion was linearized by energy method, and thepracticality of the conversion equation was verified by real case validation.(4) The characteristics of the parametrically excited rolling in regularlongitudinal waves were studied, and the conditions of ships producing substantial rollmotion in longitudinal waves were explored by real case validation.