NUMERICAL ANALYSIS OF OPERATION LIMIT OF UNBONDED FLEXIBLE RISER IN DEEP-SEA MINING

The unbonded flexible riser plays the role of connecting the mining vehicle and the buffer station, and of transporting polymetallic nodules in a deep-sea mining system. The unbonded flexible riser is composed of multi-layer structures, and the material properties and constitutive relations of each layer are different, resulting in complex mechanical behaviors. Considering the nonlinear bending moment-curvature effect of the flexible riser caused by interlayer slips, the contact pressure on both sides of the helical layer is obtained by solving the axis-symmetrical loading mathematical model, and the nonlinear bending moment-curvature model is established. The nonlinear bending model is coupled into the global dynamic model of the flexible riser based on the vector form intrinsic finite element (VFIFE) method, and the dynamic response of the riser in the deep-sea mining process is numerically analyzed. The results show that: before reaching the critical curvature, the bending moment increases linearly with the curvature, and the bending stiffness is mainly provided by the helical layer. When the curvature exceeds the critical curvature, the bending moment increases nonlinearly with the curvature, and the bending stiffness is mainly provided by the polymer layers. With the increase of internal/external pressure and tension, the critical curvature tends to increase. Taking bending failure as the criterion, the curvature of the connector between the unbonded flexible riser and the mining vehicle changes the most, which is the main controlling factor restricting the safe operation limit. For the operation range, there is an optimal value for the elevation of the buffer station. The bending stiffener can alleviate the curvature change of the connector and significantly extend the safe operation range. The study results have reference value for the optimization design of the deep-sea mining riser.