Page 246 - Practical Design Ships and Floating Structures

Practical Design of Ships and Other Floating Structures 22 I
You-Sheng Wu, Wei-Cheng Cui and Guo-Jun Zhou (Eds)
0 200 I Elsevier Science Ltd. All rights reserved

EFFECTS OF A DRAFT ON HYDROELASTIC RESPONSES OF A
PONTOON TYPE VERY LARGE FLOATING STRUCTURE

H. Maeda I, T. Ikoma I, C.K. Rheem and M. Arita
' Nihon University, Chiba, Japan
University of Tokyo Tokyo, Japan

ABSTRACT
In this paper, the limitation of the zero draft assumption is examined. Then, the elastic motion and the
wave drifting forces of a calculation model with lOOOm in length and 250m in width are computed due
to two calculation methods. Three-dimensional singularity distribution method is applied to the
analysis of a finite draft body, and the pressure distribution method is applied to that of a zero draft
body.
The authors conclude that the zero draft assumption is effective and practical for designing a VLFS in
waves within wave frequency range. The hydrodynamics on a VLFS is calculated by the pressure
distribution method with higher order boundary element in case of the zero draft assumption, while the
three-dimensional singularity distribution method is applied to the finite draft case.

KEYWORDS

VLFS, Very large floating structure, Draft effect, Shallow draft assumption, Hydroelasticity, Elastic
response, Momentum theory, Far field method, Near field method

1 INTRODUCTION

There are many examples on the elastic response analysis of a VLFS(Very Large Floating Structure)
for which the shallow draft assumption is applied. The shallow draft assumption corresponds to the
zero draft assumption. The effectiveness of the zero draft assumption has been confirmed already in
case of rigid body motions on a ship or small scale offshore structure. In the same context, the shallow
draft assumption is considered to be effective even for a flexible pontoon type VLFS without deeper
investigation. However, there have been no big problems occur empirically yet, while there has been a
few examples which investigated the effectiveness of the shallow draft assumption analytically. Kim et
al. (2000) pointed out that there is not big discrepancy in the wave frequency range between the elastic
deformation due to zero draft assumption and finite draft assumption, however, there are some
discrepancy on elastic deformation in lower frequency or on critical angle of incident waves.
In this paper, the authors investigate the applicability of the zero draft assumption to the hydrodynamic
forces on a VLFS. The 3D singular point distribution is applied to the finite draft assumption, while