ABSTRACT:

The dynamic characteristics involved in the loading process in the ocean environment are complicated and difficult for exact evaluation of the response of offshore structures From the engineering point of view, it is Important to obtain reasonably reliable results by means of simplified descriptions of the complex problems. This paper investigates the effects of those simplified descriptions on the dynamic response evaluations of offshore structures Parametric studies based on the perturbation method are presented to assess the influence of uncertainties associated with various assumptions and approximations. These uncertainties are represented by the inertha coefficient, the drag coefficient, the mean wave height and the shear wave velocity of the soil it is found that among -all the parameters considered in the present study, variations in the mean wave height provide the most significant contributions to the dynamic response it is suggested that in order to perform reliable design of offshore structures, it is important to clarify the uncertain parameter effects on the response evaluation.

INTRODUCTION

The environmental loads such as waves, winds and earthquakes acting on any offshore structure are dynamic and highly random in nature. On the other hand, the uncertainties in the properties of the material and in the characteristics of the foundation soil may also give use to some statistical variation in the eigen values and eigenvectors of the structure (Boyce, 1968) and consequently the dynamic response may be affected (Kareem and Sun, 1986). The analysis and design of offshore structures is an extremely complicated process and requires several assumptions and approximations. Empirical models such as the Monson equation for the wave force or the expressions for the wave energy such as the Bretschneider" s spectrum involve several coefficients which have to be determined by experiments or field observations and have to be extrapolated to a particular problem.

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