Dynamic response of steel lazy wave riser considering the excitation of internal solitary wave and ocean currents

As the most adequate configuration, steel lazy wave risers (SLWRs) are developed for deep-sea oil and gas exploitation projects. However, SLWRs are very sensitive to ocean currents and internal solitary wave (ISW). Based on a 3D modified slender rod theory, a nonlinear dynamic model for the SLWR suffering from vessel motions (VMs), surface waves (SWs), ocean currents and ISW is established. According to the equivalence principle, equivalent parameters for the buoyancy section are introduced in the dynamic model. Besides, the applicability and formulas for different ISW theories are described. Finally, the nonlinear SLWR dynamic responses under four cases are compared to investigate the influence of the ISW and ocean currents. The results show that the ISW can significantly induce the oscillation displacements of the SLWR, especially along the horizontal direction near the ISW interface. The dynamic bending moments between the hang-off point and interface significantly intensify due to the ISW. The ISW affects the dynamic effective tension of the SLWR in the lower layer fluid more markedly than that in the upper layer fluid. In addition to the interface zone, the impact of ocean currents on the SLWR dynamic response seems to be more significant than that of the ISW.