A flotation system includes one or more floatation modules, each having a first section and a second section, each having an internal core made of a buoyant material. The first section includes two end faces and a projection extending from one of the end faces. The projection has a contoured profile. The second section includes two end faces and a reciprocal recess in one of the end faces. The end faces of the first section engage the end faces of the second section such that the projection engages the reciprocal recess in a mating engagement. A stop collar may be disposed adjacent each end of the floatation module. The floatation system may be attached to a tubular string that is lowered into a marine riser to reduce a load applied to a rig disposed over the marine riser.