For the application of flexible riser in ultra-deepwater beyond 2000m water depth, one of the technical challenges is the existence of ultra-high-tension loads induced from riser self-weight and amplified by its dynamic responses. The high-tension loads impose additional equipment and operational constrains in riser installation and application. For example, the high-tension loads limit the choice of installation vessel and prohibit the use of disconnectable turret moored system technology, which protects the riser system during extreme weather events. The industry has been developing innovative technologies to mitigate the high-tension loads anticipated in ultra-deepwater riser applications, such as using light weight composite material, buoyancy modules, hybrid riser tower structures, etc.
This paper presents an innovative fully flexible riser solution for deep water application: a step riser configuration and it's variation-patented by BHGE. The technology was evolved from traditional free hanging configuration and based on existing proven reliable technologies. The step riser configuration is characterized by utilizing buoyancy tank/modules arrays to maximize uplifting buoyancy efficiency and exerting the buoyancy force directly to riser midline connections instead on the riser body directly. Extensive numerical simulations were performed to better understand the step riser's dynamic response characteristics, with validation through scale-model tank tests. This paper also includes an alternative buoyancy module and buoyancy system design, for potentially significant savings in fabrication cost and improvements in offshore installation efficiency. The step riser configuration permits various buoyancy designs to suit the capacity of installation vessels and to improve the riser extreme and fatigue performance in service. The technology also enables applicability for disconnectable turret moored systems in ultradeepwater application. It shows that the presented technology is a practical and reliable solution for ultradeepwater applications, with providing potential cost saving opportunity.