I. Abstract

The paper describes a method to calculate motions and loads for a catamaran at zero speed in regular waves of arbitrary frequency and direction. The hydro dynamical interaction between the two hulls has been taken into consideration. It has been found that this interaction has a significant influence on the coefficients for added mass and damping. Comparisons with model tests have given good agreement. Short and long term response in irregular waves have been predicted using wave conditions for the North Sea. Forces, moments, accelerations and motions have been calculated. The methods developed are considered to be useful tools for predicting motions and loads for catamarans or similar vessels.

II. Introduction

One of the greatest problems in connection with the design of offshore structures, is the lack of adequate knowledge about the loads imposed by the sea. As a result the frequency of serious damages to offshore structures is very high. This applies particularly to the unconventional types of structures like floating drilling platforms for which the experience is limited, while for conventional ships the trial and error procedure gradually has developed an adequate strength standard over a long period of time.

The merits of the trial and error procedure are obvious - successful service experience is the best test of a design. However, the limitations of the trial and error procedure are equally obvious - it does not work for unconventional designs and for this reason it becomes difficult to solve new problems in connection with new demands like big ships, fast transport or offshore oil drilling. The demands are changing fast nowadays and consequently the attention has lately been more and more focused on theoretical predictions of wave induced motions and loads for offshore structures.

One may ask why naval architects who have been working on these problems for centuries, did not arrive at successful solutions a long time ago. The answer is simply that methods to describe the confused sea were not available until Rice /1/ in 1945 developed his mathematical theories for random noise. The introduction of statistical methods was the beginning of a revolution which has already completely changed the situation.

In 1953 St. Denis and Pierson /2/ applied the new statistical theories to ship problems and introduced the linear superposition principle for calculation of response spectra from wave spectra and transfer functions. Another important step forward was the strip-theory development by Korvin-Kroukovsky and Jacobs /3/ for calculation of transfer functions of heave, pitch and vertical bending moments and shear forces. Lately Salvesen, Tuck and Faltinsen /4/ have developed methods to calculate transfer functions also for lateral motions (sway, yaw and roll) as well as the lateral bending moments and shear forces and the torsional moments.

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