a floating vessel is equipped with perforated plates which exhibit both an added - mass effect and a damping effect . the addition of porosity to an added mass plate phase - shifts the added mass force so that it becomes at least partially a damping force which does not depend on large velocities to develop a large damping force . preferred porosity is in the range of about 5 % to about 15 % of total plate area . a semi - submersible drilling rig may have damper plates fitted between its surface - piercing columns and / or extending from the sides of its pontoons . a truss spar offshore platform may have damper plates installed within its truss structure intermediate its hull and ballast tank . drill ships and similar vessels may be equipped with damper plates extending from the sides of their hulls to reduce both heave and roll . in certain embodiments , the damper plates are retractable so as not to interfere with docking and to reduce drag while the vessel is underway .