A large diameter duct structure is adapted for vertical disposition in an ocean. An elongate tensile core assembly is adapted to be supported at one end, as from a floating platform, and to extend downwardly a desired distance. A plurality of duct wall sections, each of selected length, are alignable in serial order to define a tubular assembly having a diameter substantially greater than that of the core. Support means are engageable with the core at selected locations along the core and with the wall sections. The support means support the wall section substantially concentrically about the core and secure the wall sections from movement relatively along the core. Seals are engageable between the ends of adjacent wall sections disposed about and along the core, and define substantially liquid-tight connections between the wall sections. When the several wall sections are serially disposed along the core and are supported on the core with the seals engaged between adjacent wall sections, the result is a fluid flow duct of large diameter in which the weight of the duct is supported by the core which is loaded in tension below its upper end.