Abstract: A method and apparatus for forming a hollow column of concrete on a floating offshore platform by applying successive layers of concrete and reinforcing to a single surface slipform which outlines the inner or outer contour of the column. The column is intermittently or continuously lowered by means of gravity under the control of one or more cables. The column may be round, oval, rectangular or of any cross section, or any combination thereof, and may be open or closed at either end or at intermediate levels. The column may be segmented into sections and have relatively thin walls as in an OTEC cold water pipe, an outfall sewer, or an underwater tunnel; and a method is described for safely deploying such columns at great depth on to an uneven seabed. The column may be monolithic with walls thick enough to support a structure above the surface or to maintain the hollow interior at amospheric pressure and provide access to an undersea habitat or to gas and oil well equipment.