This rig has the hull design of a catamaran and is either towed or self-propelled. A semi-submersible can also be dynamically positioned or it can use anchors. When the rig is on location, it is ballasted down, in about the same way a submarine submerges, fifty feet or so to give it stability. Semis are heavy-duty rigs and are designed for adverse weather conditions.
A Louisiana shipyard puts the final touches on a monster semi-submersible. This photo shows the two large catamaran-looking hulls, called pontoons, that will take on water when the rig is in place. The huge mass of the flooded platform below the water level greatly reduces the affect of wave action.
Photos by Ron Edwards