Submergence Depth Control of a Free-Fall Submersible Escape Capsule for an Offshore Oil Rig

This paper describes some exploratory work on a new evacuation system for an offshore oil rig. The proposed system would free-fall to the ocean surface and then sink down to a level where water motions are insignificant. So, in severe storms, it would avoid the violent air and water motions that exist near the surface. The paper concentrates on the control features needed to make the escape capsule hover at a certain depth below the surface. The describing function concept of classical automatic control theory is used to study the performance of a number of control strategies. Comparisons with data from a simple experiment show reasonable qualitative agreement. For the experiment, a spherical capsule was constructed and a digital computer was used to sense depth error and take corrective action by adjusting an air flow into or out of a central buoyancy chamber.