Nonlinear vibration model and response characteristic of drilling risers in deep-sea under soft suspension evacuation condition.

• A dynamic model for drilling risers under soft suspension evacuation is established. • The nonlinear coupling of longitudinal-CF-IL in three directions is considered. • The correctness and effectiveness of model are verified by experimental results. • The progressiveness of model is verified by comparing with the hard suspension model. • The effect of environmental and operation parameters on response characteristics are investigated. In order to provide theoretical reference for the design and operation of soft suspension riser, firstly, a three-dimensional nonlinear vibration model of risers is established using the Hamilton variation and virtual work principles. In the model, three main nonlinear factors were taken into account: longitudinal-lateral (LG/LT) coupling vibration, vortex induced vibration (VIV) and large deformation. The finite element method (FEM) is used to realize the numerical solution of the nonlinear vibration model. Secondly, the correctness and effectiveness of the nonlinear vibration model is verified by the experimental measured data and ANSYS software. The progressiveness of the model is verified by comparing with the uncoupled model and hard suspension, and it demonstrate that, the proposed model can reflect the high nonlinearity of the riser system and describe the nonlinear vibration of the real riser more accurately. The natural frequency of the nonlinear model was much higher and the modal response more complex. The nonlinear coupling effects increase the vibration amplitude in the cross flow (CF) direction and the in-line (IL) direction. Thirdly, the model was used to analyze the effects of structure parameters, environmental parameters and operation parameters on the response characteristics of soft suspension evacuation riser. Some important phenomena have been found: reasonable allocation rate of buoyancy block can effectively restrain the vibration of riser. The optimal allocation rate of buoyancy block is related to the riser structure and operation conditions, which can be determined using proposed nonlinear model. Meanwhile, it is necessary to evacuate along the flow direction as much as possible, with an evacuation speed of lower than 1 m/s. [ABSTRACT FROM AUTHOR]