ABSTRACT

It has long been recognized that ocean waves, and therefore their induced motions and forces on structures, are random phenomena whose exact time cannot be predicted. Today, however, a spectral analysis technique is available that can be used to predict accurately important statistical properties, such as the significant and maximum values of waves, motions, and forces. For these predictions the wave energy distribution over the frequency and the motion, or force responses to regular waves over a wide range of frequencies, should be mown.

In design procedures for offshore structures very often the study of motions and forces is made on the basis of one single regular wave whose properties are believed to be equivalent to those of an irregular sea irregular wave concept).

To investigate the merits of the regular wave concept, the motions of a semi submersible and two supply vessels (one conventional and one twin-hull type), as predicted by the statistical methods and the regular wave concept, are compared. The comparison shows that in some cases the regular wave concept underestimates, while in other cases it overestimates, the motions that may be expected. - It may be concluded that the regular wave concept does not always lead to a conservative motion prediction. In considering the effect of the wave direction on the motions of a semi submersible, both methods give the same tendency in a Beaufort 8 North Sea condition. However, the differences between the motions in various wave directions are considerably smaller when predicted by the statistical methods.

Finally, a motion optimization study reveals that the regular wave concept yields erroneous results.

INTRODUCTION

Ocean waves and their related phenomena are randomly varying in time. However in 1905, R. E. Froude remarked at the end of his paper1 that "irregular wave systems are only a compound of a number of regular wave systems (individually of comparatively small amplitude) of various periods And the effect of such a compound wave series would be more- or less a compound of the effects proper to the individual units composing of it."

In fact, these assumptions were the same as those made by St. Denis and Pierson.2 In 1953, they introduced a statistical method by means of which the response of a ship in a seaway can be predicted when the wave (energy) spectrum and the response to regular waves are mown. The wave spectrum describes the distribution of wave energy over the frequency. Their approach, usually called the spectral analysis technique oath, linear superposition principle, has been used frequently to predict ship motions and performance in irregular seas from model test results in regular waves. The applicability of the superposition principle has been verified many times by comparing the predicted values with those directly measured in irregular seas, as was done by Lewis, 3 Ochi, 4 and Gerri?s. 5 For catamarans and other floating offshore structures, the applicability of the superposition principle has been shown by Van Sluijs6 and Vugts.7

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