Deep-sea Riser Research Articles

Nonlinear Transient Motion of Deep Ocean Mining Pipe

For accurate and economic time-domain analysis of deep ocean pipes, a realistic representation of hydrodynamic forces along the pipe and an efficient numerical method are required. A transient analysis procedure is formulated and implemented for the numerical determination of nonlinear transient motion of pipes using, as an initial condition, the nonlinear static configuration. The pipe is modeled by three-dimensional beam finite elements which account for coupled axial, bending and torsional deformations. We used 1) vertical variation of seawater properties and hydrodynamic force in the stratified ocean, 2) relative pipe velocity and response for each state in the force computation, and 3) an implicit time-integration method. Several cases are presented for accelerating, turning, and oscillatory motions of the ship and an 18,000-ft pipe, bottom end free. The subsurface environment and force coefficient selection greatly affects the results. The method can be directly applied to the analysis of deepsea risers and OTEC pipes.