Analysis Of Load And Responses Of Ocean Flexible Riser Under The Combined Action Of Internal Solitary Wave And Sheared Flow

With the increasing human need to oil, offshore resources no longer satisfy the demand,and the scope of the exploration gradually extended to the deep ocean. In deep sea, the marine structures will suffer more complex and harsher environments. The internal solitary wave is one of these environments, which has large amplitude and long action time.Therefore, it will bring a great threat to the structures. Meanwhile, these marine structures such as deep-sea risers also suffer vortex-induced vibrations caused by ocean currents. So,under the combined action of internal solitary wave and sheared flow, deep-sea risers will face a huge threat to its life.Since we are exploring in deep sea, the length to radius ratio of riser is large, making the riser more flexible. As a result, the interaction of deep-sea risers with internal solitary waves cannot be ignored, because the interaction is nonlinear. In this article, numerical simulation is conducted to study the interaction between deep-sea risers and the combined action of internal solitary wave and sheared flow.Firstly, a comprehensive review of the state of the art of the research area is taken.Then the analysis of load and responses of deep-sea flexible riser under the combined action of internal solitary wave and sheared flow is performed. The main research work and achievements are as follows:1) Based on m Kdv internal solitary wave theory and the volume of fluid method, the two and three dimensional numerical internal solitary wave flume are established by controlling the velocity of velocity-inlet and absorbing wave by combination of numerical dissipation and open-channel. The numerical results are compared with theoretical ones to verify the feasibility of this wave-generating scheme.2) A code of mathematical model of sheared flow is programmed through UDF. The interaction between ISW and sheared flow is simulated and the characteristics of ISW disturbed by sheared flow. The wave profile and velocity are presented.3) With the feasible ISW and sheared flow numerical flume, the interaction between ISW or ISW with sheared flow and the rigid riser is numerically simulated. The analysis is performed for the lift and drag force loaded on the riser and the velocity distribution and vortex shedding in the wake field.4) Defining riser's material as aluminum, based on the fluid-solid interaction moduleSystem Coupling, a top tensioned flexible riser is put in the numerical flume. Then the two-way interaction between flexible riser and ISW or ISW with sheared flow is simulated numerically, and the dynamic response is analyzed.The numerical results show that the combined action of ISW and sheared flow leads to vortex-shedding in the wake field, which results in VIV of the riser. Meanwhile, the disturbance of sheared flow in ISW makes the displacement in in-line and cross-line direction of deep-sea flexible riser increase by 10 times and maximum stress increase by 38%.