A towable pipeline bundle for installation underwater at a subsea oil or gas production site has two or more bundled elongate elements, at least one of which is a flowline for carrying production fluid along the bundle. A rigid buoyancy pipe of polymer-composite material extends along and supports the bundled elongate elements. The buoyancy pipe has an internal buoyancy chamber and at least one port for introducing fluid into the buoyancy chamber. The buoyancy pipe is arranged to have positive buoyancy in seawater when its buoyancy chamber contains a fluid less dense than seawater. In this way, the buoyancy pipe confers substantially neutral buoyancy on a towable unit comprising the bundle. The unit can then be towed in mid-water to the production site and lowered there onto the seabed.