Hybrid risers rely on axial tension, generated by a combination of aircan and distributed buoyancy, to control their static deflection and dynamic response. In deep water, the base tension required to achieve an acceptable response can be significant (70–1000Te). This tension must be reliably reacted into the seabed over the life of the development. Depending on the configuration of the selected foundation there may be, in addition to axial tension, bending moments, shear loads and loads arising from flexure of rigid base jumper spools. The result is a complex set of load conditions, which, when combined with installation issues, can lead to a foundation system that is a significant cost element in the overall riser system.
A number of foundation designs have been proposed and used for free standing risers however, the selection and design process is often not clear. This paper presents a review of the potential design solutions depending on water depth and design loads and outlines approaches adopted on existing projects. The key design and installation drivers are discussed, allowing improved understanding of the issues and the impact on the global riser design and installation strategy. The paper also discusses issues and implications of extending free standing risers to ultra deep water, which often have soft seabed conditions, further compounding the foundation design process.
Free standing hybrid risers are a proven solution for deepwater floating production systems and have been utilized in West Africa and Gulf of Mexico with numerous current proposals and studies for their use in other parts of the world.
The free standing riser uses distributed and local buoyancy to support and tension a vertical riser, extending from the seabed and terminating near the water surface but typically below the influence of high currents and wave action. Aircan buoyancy has been used at or near the top of the riser to cost effectively generates the majority of the tension required to support and control the deflection of the riser under service conditions.
The riser is connected to the floating production system via a flexible jumper(s) configured in a catenary configuration. This arrangement is highly compliant and allows the riser to accommodate large vessel motions and excursions whilst maintaining acceptable storm and fatigue performance.
The ability to free stand without tension applied by a surface vessel offers free standing risers a strong benefit over other riser types in that it allows pre-installation to be considered. This can be an important selection factor for this riser type over catenary systems in that this ability can greatly reduce the risk and cost of the installation phase. This is particularly important in deep and ultra deep locations and also remote locations where the cost of mobilising installation vessels is high.
Freestanding hybrid risers can be either single pipe SLOR (Single Line Offset Riser), pipe in pipe COR (Concentric Offset Riser) or a number of pipes configured in an internal or external bundle, Figure 1.