Development of a practical tool to determine the hull damping of modern ship hull forms

Abstract

Predicting the roll damping is a crucial step prior to dimensioning the stabilization systems. Therefore, FLUME® Stabilization systems is using the results provided by seakeeping basin model tests in regular seas to determine the behavior of the ship in roll before the stab is designed. Throughout the decades, FLUME® collected a considerable database that includes general cargo ships, containers, and ships with and without hard chine and ships with various bilge keel aspect ratios etc…That data base was restructured and summarized in a graphical layout Fhull=f(EWS: effective wave slope) that could be used to interpolate the roll damping in a preliminary design stage. Nevertheless, some particular and modern hull forms are not part of the database which makes any attempt of estimation inaccurate. The aim of this study is, to find a reliable alternative to the basin model test through several validation steps. First of all, a state of art of the empirical and numerical tools has been established, analyzed and compared. It’s obvious that computer programs based on strip theory are widely used in seakeeping analysis and particularly roll motion prediction, while the viscous effect on damping is considered by means of empirical formulas. Therefore, PDstrip, an open source Fortran code has been used to perform a frequency domain strip theory analysis. In order to evaluate the accuracy of this program, two particular ships with detailed roll decay and forced roll results, has been selected from FLUME® database. Another reference ship is the DTMB 5415 (5512) surface combatant model which has been the subject of several CFD and roll model test analysis carried out by the US Naval surface warfare center and by the IIHR— Hydroscience & Engineering lab at the University of IOWA towing tank. The output of PDstrip was post processed and presented in form of roll RAOs in beam seas. Afterwards, 2Droll, a FLUME software extracts the roll damping coefficient from the obtained roll RAO based on a regression between similar hull forms and spring mass system equation with a chosen roll damping coefficient to draw a similar RAO. Once the similarity is established this chosen damping coefficient is adopted. IKEDA computer program is an up-to-date tool that can give reasonable results and that has taken into account some advanced hull forms. The same ships tested with PDstrip were also tested with IKEDA computer program and Ikeda simple prediction formula. The results were analyzed and compared to experimental results. Besides, Miller formula with and without forward speed is a preliminary design tool, that have been used in this study in order to analyse its limitations and check the accuracy of the above listed methods. Finally, a comparison regarding the accuracy and the reliability of all the used methods has been presented and interpreted.