This arrangement allows vessels to drill in ultra-deep water. The record is held by Sonat Offshore's Discovery Seven Seas, which drilled in 6,448 feet of water in 1984. 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.
The name of this rig implies exactly what it does. Jack-ups are towed to their location where rig personnel use heavy machinery to jack the legs down into the water until they are on the ocean floor. When this is competed, the platform containing the work area rises above the water. After the platform his risen about 50 feet out of the water, the rig is ready to begin drilling. Jack-up rigs are limited to a water depth of about 600 feet or less.