International Shipbuilding Progress

The paper deals with natural vibration of deepwater tensioned uniform risers related to vortex-induced excitation. Due to weak flexural rigidity cable vibration and beam vibration are considered. The exact and asymptotic solutions of cable differential equation are developed. Beam vibration is determined by modifying the cable solution for each half-wave of natural mode. The analytical procedures are verified by the finite element method. Some additional explanations of the riser dynamic behaviour are given.

Parametric roll resonance is known as one of dangerous modes of ship motions in waves. This resonance occurs as a result of restoring variation in astern or head seas. This paper focuses on the parametric resonance observed in head seas for a model of an Icelandic trawler that has no distinct wave-induced change in her metacentric height, which is hydrostatically calculated. Two mathematical models, a 1 degree-of-freedom (DOF) model with balanced heave and pitch motions and a 6 DOF model were used to explain the parametric roll resonance of the ship in head waves. Both the mathematical models take the change of the righting arm into account; the 1 DOF model assumes static balance in heave and pitch, while the 6 DOF one takes coupling between roll and vertical motions taken into account. Only the 6 DOF model, however, successfully reproduced the occurrence of parametric rolling of the ship. This suggests that coupling effect from heave and pitch to roll restoring is essential for parametric rolling in head seas. Furthermore, numerical simulation based on the 6 DOF model shows parametric rolling does not depend on small change in heading angle very much but is sensitive to the relationship between the natural pitch period and the natural roll period.

The current practical long-term prediction method for ship responses estimates higher values than real values, because it does not consider ship operation conditions and does not assume non-linearity between significant wave height and standard deviation of the ship response. We propose a long-term prediction method with operation criteria for ship responses that considers these influences. We define a critical standard deviation of the ship response and propose the utilization of onboard monitor data of ships. The validity of the proposal method is confirmed by comparative results of the long-term predictions and the onboard monitor data from an actual ship.