International Shipbuilding Progress

The paper treats the important problem of prediction of ship's inherent course stability, based on a large amount of model experimental data for a well-known bare hull shape – the Delft catamaran 372, covering a comparatively wide range of Froude numbers and depth to draught ratios. The analysis is based on a linearized maneuvering model.

As a result, dependency of the stability criterion on Froude number and the water depth to draught ratio is established and clearly demonstrated for this particular ship case.

Large amplitude rolling in random beam seas is studied by moment equations and stochastic excitation is represented through a cascading filter driven by pure Gaussian white noise in order to apply Markov approximation. The moment equation is generated from a six state space rolling model using the Itô differential rule. The Gaussian and non-Gaussian cumulant neglect method is applied to close the infinite hierarchy of moment equations. In this paper, an automatic neglect tool is developed to handle the complex and untraceable higher-order cumulant neglect closure method to capture the non-Gaussian effect of nonlinear rolling phenomena. Both transient and stationary responses of statistical moments are obtained through the solution of the closed moment equations.

A numerical investigation of hydrodynamic force and pressure distribution for planing hull has been carried out. The problem is solved by using a pressure patch distribution method. The method is based on potential flow theory and with a linearised free surface boundary condition. In some of the previous studies, it was found that the inversed pressure distribution on the planing hull will be oscillating, i.e., a numerical instability problem occurred. In the present study, a robust numerical approach was adopted to calculate the elementary solution of a pressure patch moving on the free surface. The numerical approach is applied to flat plate, flat plate catamaran and prismatic planing hulls. No numerical instability has been found for the predicted pressure distribution on the planing hulls. The predicted results are compared with those of other published numerical calculations and model tests, a fairly good agreement has been found.

Certain class of offshore structures exhibit highly intense nonlinear behavior called springing and ringing. Dynamic response of compliant structures like Tension Leg Platforms (TLP) under impact and non-impact waves responsible for ringing and springing phenomenon is of large interest to marine engineers. This paper develops a mathematical formulation of impact and non-impact waves and discusses the method of analysis of TLPs of triangular geometry under these wave effect; response of square and equivalent triangular TLPs are compared. Heave response in square TLPs show bursts but there are no rapid build-ups; gradual decays are seen in most cases looking like a beat phenomenon while such results are not predominantly noticed in case of equivalent triangular TLPs. Ringing, caused by impact waves in pitch degree-of-freedom and springing, caused by non-impact waves in heave degree-of-freedom in both the platform geometries are undesirable as they pose serious threat to the platform stability. Analytical studies conducted show that equivalent triangular TLPs positioned at different water depth are less sensitive to these undesirable responses thus making it as a safe alternative for deepwater oil explorations.