Numerical Simulation of Free Standing Hybrid Risers

Free Standing Hybrid Riser (FSHR) is an innovative concept for the riser to transport oil and gas from the seabed to a floating production platform deployed in deep water. A FSHR mainly consists of a buoyancy can, a vertical steel riser, a flexible jumper and an upper riser assembly (URA). This study focuses on the development of the numerical scheme for the dynamic simulation of a FSHR whose flexible jumper is connected to a moored floating production vessel, say floating production, storage and offloading (FPSO). The numerical scheme is mainly based on an existing in-house numerical code, known as COUPLE. In using COUPLE, URA is modeled as a rigid body. The URA connects a vertical steel riser and a flexible jumper, both of which are modeled by beam elements (experienced both tensions and bending moments) in COUPLE. The buoyancy can is connected to the URA through a tether chain, which provides the vertical upright force on the URA and in turn the tension to the vertical riser. The tether chain is modeled by bar elements (experienced only tensions) while the buoyancy can is modeled as a rigid cylinder (beam element). The motions and forces of the two rigid bodies, namely URA and the buoyancy can, are coupled in the simulation. Because the connection between the URA and steel riser is considered as ?rigid?, an accurate numerical scheme modeling a rotational spring connecting the URA and steel riser is developed for calculating the bending moment applied at the upper end of the riser. Given the motion at the upper end of the flexible jumper (connecting to a FPSO), numerical simulations are made for two different FSHRs. The results simulated using COUPLE are in satisfactory agreement with those obtained using OrcaFlex, a commercial code widely used by the offshore industry. Furthermore, the vortex induced motion (VIM) of the buoyancy can is considered in the simulation and its effects on the motion of the FSHR are explored, which is compared with the corresponding model tests. The ability to simulate the dynamics of FSHRs developed in this study may be helpful for the future design of FSHRs.