Mechanical behavior of a drilling riser with buoyant blocks when it enters into deep water

Abstract

A novel three-dimensional dynamic model is developed to investigate the vortex-induced vibrations (VIVs) in flexible risers with buoyant blocks conveying the internal fluid flow. The unsteady hydrodynamic forces associated with wake dynamics are modeled using two distributed van der Pol wake oscillators that consider the coupling between the structure and the fluid. Furthermore, our model considers the loads of buoyancy weight, tension, pressure, temperature, and seawater current owing to wind, tide, and waves, as well as the nonlinear effect of the large deflection of the riser on the loads. The riser is discretized into slender-beam elements, the distributed van der Pol wake oscillators are also discretized on the element nodes, and each node has six degrees of freedom, allowing it to describe the vibrations of the risers in lateral, axis, and torsion simultaneously. The boundary conditions include the displacements and rotations at the up end and free bottom end in the presence of additional weight. The computational program is prepared using Matlab (R2014a). The simulation results exhibit good agreement with the existing experimental data, verifying the validity of the model and the parameters. Finally, the VIV simulations are conducted for more realistic risers when they are entering 500–3000 m deep water, denoting the effects of buoyant blocks, entry speed, and water depth upon the drop-point offset of the bottom end of the risers, the tension stresses as well as the values and positions of the maximum bending stresses with respect to the risers. These data can be used to select a good operational condition for ensuring the entry of a riser. This study provides a computational framework for conducting subsequent research, and the obtained results can be applied to construct ocean-floating platforms more scientifically, safely, and economically.