Because of their large mass and flexible or soft moorings, large floating structures that are moored at sea tend to have natural periods of oscillation in the horizontal degrees of freedom that are of the order of minutes. At these periods there is no first order spectral energy, so they are not appreciably excited by first order forces in these degrees of freedom. However, the difference frequency drift forces vary with very low frequencies, implying large periods that may coincide with the natural period of oscillation of a large floating structure. Excitation at periods close to resonance results in large amplification factors in the motions of the structure. These motions are the drift frequency motions.

From Equation 13–30, the drift frequency equation of motion is:

(13–31)

where is the drift frequency added mass, is the difference frequency force component of the second order force given in Equation 13–29, and and are the is the hydrostatic force and radiation force components given by the impulse function integration, respectively.

It is assumed that the values of drift added mass and damping are constant.

When only drift wave forces are present, the structure will experience drift oscillations. This is termed the slow motion, with a corresponding slow position.