Session: 02-02-03 Probabilistic Models of Forces and Motions

Paper Number: 123035

123035 - Simplified Estimation for Extreme Value Distribution of Nonlinear Roll Motion of Ship by Rtp Method 

In a previous study, the author proposed a new RTP (RAO-based Translation Process) method for simple estimation of EVD (Extreme Value Distribution) of narrowband nonlinear responses. The RTP method approximates the EVD in arbitrary irregular waves from the nonlinear response in regular waves based on the idea of the transformation process, in which the nonlinear process is regarded as a transformation of a linear process. This method makes it possible to take nonlinearities into account in long-term prediction based on full-spectrum analysis (i.e., taking into account the response in all short-term sea states). Since this method is highly effective in relatively computationally demanding analyses such as DLSA (Direct Load and Structure Analysis) and is positioned as an extension of the conventional method using equivalent design waves, it is expected to become a new basic calculation method for advanced structural strength assessment using DLSA.

 In the previous paper, it was shown that the RTP method can estimate the EVD of wave-induced hull girder sectional forces (including shape nonlinearity) with sufficient accuracy, but linearity was assumed for the ship motion. Among ship motions, roll motion is known to have a strong nonlinear effect, so it is necessary to consider this nonlinear effect in the evaluation of EVD of stresses especially on transverse strength members. Although there are several methods to estimate the nonlinear EVD of roll motion itself, they are mainly based on the equation of roll motion, and thus cannot be applied to the evaluation of the EVD of stresses including the effects of sectional forces and local pressures in addition to the roll motion. On the other hand, the RTP method is a semi-analytical method that is not based on the basic equations. If the EVD of nonlinear roll motion can be reasonably evaluated by the RTP method, it will be possible to obtain the nonlinear EVD of stress including nonlinear effects of wave loads with it.

In this paper, the applicability of the RTP method to roll motion considering nonlinear damping force is confirmed. The EVD of the roll motion obtained by the RTP method is compared with the results of direct simulation of the time series to verify its validity. It was shown that the EVD of roll can be reasonably estimated by the RTP method if the equivalent linearization of damping force coefficients is properly performed, although it is necessary to be careful in treating linear roll motion because the linear damping coefficients of roll motion may be very small.

Presenting Author: Sadaoki Matsui National Maritime Research Institute: Kaijo Gijutsu Anzen Kenkyujo

Presenting Author Biography: I'm developing the DLSA (Direct Load and Structure Analysis) system and studying its application to ship structural design. My areas of expertise are structural analysis, seaworthiness theory, and statistical theory.

Authors:

Sadaoki Matsui National Maritime Research Institute: Kaijo Gijutsu Anzen Kenkyujo