Dynamics of a high speed autonomous semisubmersible vehicle

The Remote Minehunting Vehicle (RMV) is an autonomous semisubmersible vehicle used to provide prime motive power for the Remote Minehunting System (RMS). As a semisubmersible, it provides the advantages of having a towing vehicle which is isolated by depth from the large surface wave forces, while having the long endurance available from an air breathing diesel engine. Thus the vehicle has the ability to provide a very stable towpoint for towing a Variable Depth Sensor array at a wide variety of depths. Tow tank model testing of the prototype RMV showed the vehicle to have very good dynamic stability in both the vertical and lateral planes of motion. At sea testing, however, revealed vertical plane instability at high speeds. By increasing the gains of the PID controller in both pitch and heave directions, the vehicle was brought under control at all speeds. Analysis of the system dynamics revealed that near surface suction forces were significant factors in the instability.. This paper develops the required augmented equations of motion for a high-speed semisubmersible, and the linear analysis that shows how near-surface suction effects act to destabilize the vehicle. Control analysis demonstrating that typical PID control action can be used to stabilize the vehicle is also presented.