Semi-submersible vessels

Apparatus for damping vertical movement of a vessel, suitable for attachment to a semi-submersible drilling rig or the like having a platform element for support supplies, equipment and the like at an above-water location, at least one support column extending beneath said platform element and having a waterplane area large enough to ensure stability when the apparatus is in a semi-submerged condition, said rig also having at least one buoyant, ballastable support structure secured to said at least one support column, the combination of said platform element and said at least one support column and said at least one support structure being buoyantly supported in said semi-submerged condition primarily due to the buoyancy of said at least one support structure;

• said rig further including means for ballasting and deballasting said at least one support structure to produce semi-submersed and floating conditions of said combination which has a maximum heave amplitude in response to open sea waves whose period is essentially equal to the natural heave period of said combination, said vertical damping apparatus comprising:
• at least one essentially rigid plate including means for permitting a greater flow of water upwardly than downwardly therethrough;
• and
• a plurality of support members attached to said at least one plate, said support members being of sufficient length to locate said at least one plate at a depth beneath the surface of said water in said semi-submersed condition, at which the amplitude of subsurface wave motion is less than said maximum heave amplitude.