Multibore hybrid risers
Multibore hybrid risers provide multiple flowpaths from the seabed to an FPS by a combination of a buoyant free-standing rigid-pipe riser (also commonly known as a riser tower) from a subsea riser base to a shallow water depth, plus flexible pipes in a double free-hanging catenary shape connecting from the top of the rigidpipe riser to the FPS. These types of system typically also incorporate all of the small-bore service lines (e.g. gaslift, chemical injection, etc.) in the riser towers, while the control system functions (hydraulic, electrical and/or fibre optic) are usually part of a separate free-hanging umbilical suspended from the FPS, thus avoiding additional connections in these critical lines. The riser tower may also be insulated to address flow-assurance issues associated with temperature losses, such as hydrate and wax formation.
The rigid portion of the riser is typically of construction similar to a multibore top tensioned rigid-pipe riser, as described in the following subclause.
Source: API RP 17A, Design and Operation of Subsea Production Systems—General Requirements and Recommendations, Fourth Edition, Reaffirmed 2011. Global Standards