In model testing of offshore platforms in wave basins it is customary to apply the wave excitation gradually increasing its amplitude over a given period of time. This is often referred to as a loading ramp. The presence of a ramp is practically important when dealing with very flexible structures with very small values of damping since in these cases the free vibrations associated with the initial conditions can lead to responses during the transient much higher than those expected under stationary conditions. Without the ramp it might not be possible to reach the stationary response within the duration of the test, which is limited by wave reflections. This paper presents results of studies conducted at the Offshore Technology Research Center intended to investigate the effect of the ramp duration on the measured responses particularly after the end of the ramp over the latter part of the excitation when a stationary condition should have been reached. The results presented include time histories of response of a Spar platform subjected to irregular waves as well as the Fourier amplitude spectra of these motions. Of particular importance in these spectra are the peaks at the natural frequencies of the platform associated with nonlinear (difference frequency) effects but also due to the free vibration terms. The analyses were conducted using slender body theory with a number of modifications intended to account for various diffraction effects.

INTRODUCTION

The concept of spars is not recent. It has existed for decades. However, using spars as production platforms is relatively new. The interest of the oil industry in spars was sparked by the discovery of large amounts of oil in deep water. Since then, offshore oil designers started looking for innovative ways to extract oil from deep water where using fixed jacket structures is no longer feasible.

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