Damping of jacket structures are discussed on the basis of full-scale measurements. During a submarine impact with a jacket structure, unique transient response time series, damping properties are evaluated in terms of the nature and magnitude of the total damping. It is shown that the damping is highly nonlinear and fits a Coulomb-type model reasonably well. The damping magnitude expressed as the relative damping ratio is estimated to be in the range from 0.5 to 3.0 percent, dependent on the motion amplitude. Within the range of motion amplitudes experienced during the impact, the damping decreases with increasing motion amplitudes. The resonant motion amplitudes recorded during the impact are shown to be far greater than the expected dynamic motion amplitudes during a storm period.