Validation Methodology for Time-Domain Simulation of Maneuvering and Seakeeping

A time-domain ship simulation program is intended to model nonlinear motions, including combined maneuvering and seakeeping. The program is a reduced-order model capable of carrying out thousands of simulations so that it can be used to evaluate the probability of extreme events. The results of the thousands of calculations are used to quantify the capsize risk, for example. The challenge to validate the capsize risk is that an occurrence of capsizing is the result of a chain of (hydrodynamic) events, each of which needs validation. To validate prediction of rare events, we claim validation on many relevant subevents up to a satisfactory level. The program FREDYN is used in this paper, though the approach is valid for similar tools. Initially, FREDYN was meant to be used for naval monohulls: frigates and destroyers. Recently there has been a desire to extend the tool for other ship types as well. Extending the applicability of the tool also extends the more formal validation process, in addition to the verification. The distinction is that verification results in an objective comparison where pass/fail is possible. Validation evaluates the quality of the prediction for a given set of reference cases, usually model tests or full-scale measurements. The validation of the code therefore requires some subjective assessment of the results, which can obfuscate the process. The objective of this paper is to discuss some of these details. After various iterations in this validation methodology, this process is now automated. The validation framework can be repeated efficiently, allowing frequent use when developing or updating the underlying code or adding new validation data, such that the overall performance can be quantified.