The Hybrid Riser Tower (HRT) concept has been established as a viable solution for deepwater developments, but only a handful of systems exist; this is in contrast to the 25 or more Steel Catenary Riser (SCR) based systems that exist, mostly in the Gulf of Mexico and Campos Basin. Recent FPSO developments offshore Nigeria are also being developed with SCR systems.
So what are the drivers for using a HRT system, and will the HRT concept offer a competitive solution for future offshore developments? The key to answering this question is to understand the characteristics of the system.
BP has actively studied the application of Hybrid Riser Tower (HRT) systems in several very different offshore development areas. The key characteristics are discussed in this paper.
The hybrid riser tower concept can be defined as; 'a bundled, free-standing vertical riser with near surface flexible connections', see figure 1. The 'hybrid' description relates to the partial use of steel lines and flexible lines within the configuration. Typically a near surface cylindrical buoyancy tank is used, but other configurations are also feasible.
Several 'tensioned' hybrid bundled riser systems have been operated using drilling rig tensioning systems, e.g. BP Buchan in the North Sea, but it wasn't until the ground breaking Placid Green Canyon 29 project  that the concept came to full fruition as a free-standing version. Subsequently the Girassol project  extended the concept to a deeper water version, constructed onshore, towed to site, and upended. The Rosa-Lirio field subsea tieback to Girassol FPSO will also be developed with a hybrid riser tower design.
The concept has been almost universally configured with the following elements from bottom up: seabed foundation, flexible joint / flowline jumpers, bundle riser, buoyancy tank, and flexible jumpers.
Figure 1 Hybrid Riser Tower Configuration (Available in full paper)
Two variations for the configuration of the buoyancy tank have developed; the integral buoyancy tank with product lines passing through it, versus a separate tethered buoyancy tank, with the product lines emerging at the top of the bundle. Both of these options have been successfully engineered and implemented, and there is no reason to exclude either of these configurations from future design studies. As always, 'the devil is in the detail', and a successful design will have to address fundamental challenges with both approaches; the integral option requires a high integrity stress joint; the independent option requires a high integrity tether.
The use of a hybrid riser tower will typically allow a very compact subsea layout of lines and umbilicals. The various product lines must all converge to the base of the tower, but from that point the subsea layout is not restricted by the riser requirements, and lines can be routed directly to the subsea infrastructure.
This is in contrast to SCR and flexible systems where the lines must take very long, separated, straight paths to the touch down location before routing towards the required subsea destination.