This is a broad term applied to the offshore drilling industry. Basically, any rig that drills over water is an offshore rig. There are four main types – barge rig, jack up rig, semi-submersible rig, and a drillship. The barge rig usually sits on the bottom of the ocean in very shallow water. They come in two types: conventional barge rigs and posted barge rigs. The conventional rig works in 6’ to 12’ of water. The posted barge works in water as deep as 30’. These are usually used in South Louisiana, Lake Maracaibo, Venezuela, and the swamps in Nigeria. Jack-up rigs come in several sizes and shapes. Basically, they consist of a drilling rig that sits on a vessel that has three or four legs that can be “jacked” down to the ocean floor. These rigs operate in 30’ to 300’ of water in many places around the world. A semi-submersible rig is also called a “floater” because is does not rest on the ocean floor. It floats over the desired drilling location and is anchored in place using anchors and anchors lines. There is a version that is “dynamically position” meaning that it is not anchored, but is kept on site by computer controlled jets that push and pull the rig onto the correct site. These are used in water depths from 300’ to 5000’. A drill ship is a large ship that has drilling rig built in the center of it. These are dynamically positioned and are used in water depths from 5000’ to 12,000’.