An innovative step riser configuration and its variations are presented for use in deep or ultra deepwater field developments. The configurations are based on existing proven technologies and free-hanging catenary configurations. The step riser configuration is characterized by the utilization of buoyancy tanks/modules array to generate over hundreds of tonnes net uplift buoyancy, directly exerting that buoyancy force to the riser midline connection rather than the riser body. This technology enables significant improvements in the stress utilization and fatigue performance of a flexible riser. In addition, the reduced tension loads greatly decrease installation vessel requirements. The large buoyancy system not only reduces the top tension but also decouples the motion passing from the upper section to the lower section. As a result, high compression and over-bending issues near the touchdown zone can be mitigated.
This paper also investigates step riser configuration performance and its variations, step wave and tensioned step riser configuration, to address the specific issues associated with different deepwater applications. Parametric studies are performed using commercial software to predict and compare the behavior of the stepped riser system in various dimensions. Also discussed is a scaled step riser model test to validate the performance of the step riser configuration. The step riser model was towed horizontally at varying speeds to investigate the riser's tension, curvature and vortex induced vibration (VIV) responses.
The study indicates that the step riser configuration and its variations are a practical cost effective solution for flexible riser systems in deepwater applications with severe metocean conditions.