Buoyancy unit for use on a riser for sub sea applications, particularly for great depths, with a clamp 11 to be tensioned around the riser 16, and with a tensioning member 18 to damp the clamp 11. Buoyancy bodies 26, 27 are provided to engage with the clamp 11, said clamp 11 having at least one arched element 12-15 providing an annular structure to enclose the riser and to be pressed thereto with a tensioning strap 18. The buoyancy bodies 26, 27 are attached to the arched segments, The clamp 11 is cast from of a concrete material where at least the outer shell or layer is provided to prevent the intrusion of water, the casting of the clamp preferably containing powdered aplite.