Simulation of a Dual-Rotor Ocean Current Turbine with Variable Buoyancy and Lifting Surfaces for Motion Control

Open ocean currents provide energy dense resources along the western boundaries of the word’s ocean basins. These
resources contain average energy densities that can exceed 3 kkkk/mm2 in select areas with total extractable energy
levels of several GW. Nearly all areas where ocean currents exceed 0.5 kkkk/mm2 in energy density are located within
the top 100 m of the water column, where the total water depth exceeds 250 m. For this reason, moored ocean current
turbine (OCT) solutions are being pursued to enable the extraction of this resource with variable buoyancy, lifting
surfaces, or a combination of the two being considered from altitude control. Dual-rotor designs enable net torque
cancelation through counter-rotation, minimizing roll motions and misalignment with the current. In this paper, the
numerical modeling and simulation-based performance evaluation for a dual-rotor OCT design that uses coupled
variable-buoyancy and lifting surface control (i.e., dual-control) is presented. A rigid body approach is utilized where
the OCT is modeled as having 8-degrees of freedom (6 for the main body plus the relative rotation speed of each
rotor), in addition to the 3-degrees of freedom assigned to each node within the finite-element, lumped-mass cable
model.