Wave-frequency and low-frequency motions of a deep-draft spar buoy in irregular waves based on a consistent second-order theory

This study investigates the effect of horizontal low-frequency (LF) displacements and velocities on the responses of floating structures in irregular waves, with a focus on a deep-draft spar buoy. It employs a consistent second-order hydrodynamic model in the time domain and its two variations to assess the influence of LF motions. To accurately estimate LF velocities from total velocities, an improved wavelet low-pass filter is proposed and applied, overcoming challenges posed by one-sided data from simulations. For the examined spar buoy, the incorporation of LF displacements and velocities in the seakeeping analysis is deemed essential, as they are found to contribute to the reduction of slowly varying, and consequently, total surge and pitch responses. The study finds that the standard deviations of LF surge and pitch motions scale with significant wave height Hs as (Hs)b, with b close to 4/3 for surge motion, highlighting viscous damping as the dominating damping mechanism. For the storm seastate, third-order viscous drag loads may also have made a non-negligible contribution. Meanwhile, pitch motion exhibits b values around 1.1 for both low and storm seastates, indicating the significance of both wave-drift and viscous damping in LF pitch motion prediction. The effects of mooring stiffness are also discussed.

Automated summary

This study investigates the effect of horizontal low-frequency (LF) displacements and velocities on the responses of floating structures in irregular waves, focusing on a deep-draft spar buoy. The study finds that incorporating LF displacements and velocities in the seakeeping analysis is essential for reducing surge and pitch responses. The standard deviations of LF surge and pitch motions scale with significant wave height, highlighting viscous damping as the dominating damping mechanism.