a marine buoyancy unit which fits at least partly around a marine object such as a riser with auxiliary conduits is composed of syntactic foam with an external layer or skin of harder polymeric composite . to reinforce the unit slots or channels are cut in the exterior of the unit or borings are created which extend within the unit and reinforcement in the form of webs or strips are placed into the slots or channels or borings . the reinforcement is embedded into the slots , channels or borings by introducing a curable resins .