A concrete section of an offshore platform substructure comprises a concrete body with a central opening and at least one guidepost hole extending through a height of the concrete body, wherein a width of the concrete body is greater than the height. An offshore platform substructure comprises a base portion resting on the ocean floor, and a plurality of concrete support sections stacked one on top of another on the base portion. A method of assembling an offshore platform with a concrete substructure comprises locating a guidepost in the ocean floor at a well site, towing a plurality of concrete sections to the well site, sequentially engaging each of the plurality of concrete sections with the guidepost, and sequentially sinking each of the plurality of concrete sections, thereby forming a stack of concrete sections on the ocean floor.