connected to the risers by flexible flow lines 132 etc arranged in a catenary configuration for the transfer of fluids between the fpu and the seabed via riser towers 112 and 114 individual pipelines may be required not only for hydrocarbons produced from the seabed wells but also for various auxiliary fluids which assist in the production and or maintenance of the seabed installation for the sake of convenience a number of pipes carrying either the same or a number of different types of fluid are grouped in u201c bundles u201d and the riser towers 112 and 114 in this embodiment comprise each one a bundle of conduits for production fluids lifting gas water and gas injection oil and gas export and treatment chemicals e g methanol all the component conduits of each bundle are arranged around a central core and are often held in place relative to each other in the two lateral dimensions longitudinal movement not being prevented by guide frames attached to the central core an embodiment of the invention will now be described with reference to the remaining figures fig2 a to 2 i show the buoyancy device according to an embodiment of the invention in different stages of fabrication fig3 a to 3 c also show in closer detail the top and bottom of the buoyancy device according to an embodiment of the invention with one six and all buoyancy tubes attached fig4 shows the buoyancy device in cross section with only one buoyancy tube.