Heave response spectra for a semisubmersible in long period swell

Abstract: The resonant heave response of a conventional twin pontoon drilling semisubmersible driven by first order excitation is studied experimentally and numerically. Full-scale data of semisubmersibles excited at resonance have seldom been previously reported in the literature. Here, measurements from an operating semisubmersible and a nearby wave buoy are used to derive the heave response spectra and response amplitude operators (RAOs) around resonance in low to moderate seastates. Numerical computations of the heave response are made in the time domain and used to assess the appropriateness of a Morison drag treatment for viscous damping from the pontoons, without the complication of scaling. The uncertainties associated with the estimation of the spectra are also quantified. The study can facilitate more reliable assessments of the heave response expected operationally in regions where long period swell is prevalent such as Australia. Keywords: Hydrodynamics, Fluid-structure interaction, Viscous flows