Other than dynamically-positioned units, vessels engaged in offshore drilling and production operations are usually maintained in position by some kind of mooring arrangement. Ideally, the mooring system is intended to maintain the floating vessel in a constant unvarying mean position at the sea surface but, as a result of dynamic effects, this ideal can only be approximately attained. The mooring lines, which may be chains, wire rope or a combination of the two, have properties of elasticity, mass and hydrodynamic drag that can affect the dynamic response of the vessel to waves and wind. As perhaps the most obvious affect, the mooring provides a spring or restoring term to the equations of horizontal motion of surge, sway and yaw, making these motions subject to resonance in their response to wave excitation. At the same time they can change the resonance characteristics in the other three motions, and the roll especially is affected.