Response of Vertical Cylinder to Random Waves

The design of many offshore drilling platforms has utilized the Morison equation for wave forces on cylindrical members by considering a viscous drag component and an inertia component. Recently, several authors have shown that the drag component may be ignored in some instances, which linearizes the wave force equation. This paper reinforces that information and shows that the inertia component may be ignored for some installations in shallow water. Two methods of measuring a wave spectrum in the laboratory are described: (1) by obtaining a continuous record of the water surface, digitizing the record and performing a numerical spectral analysis; and (2) by using a narrow banded vibrator such as a single vertical cylinder whose natural frequency of vibration can be changed in order to sample various frequency bands of the spectrum. It is shown how total damping between the water surface record and the structural response can be estimated from the cospectrum and the quadrature spectral density functions. The significance of the calculation of coherence is analyzed.