Chapter 4

Types of Rigs

Submersible

This is a drilling structure which is used in relatively shallow water, usually 80 feet or less. It is towed to its location where it is submerged until it sits on the bottom. This submerging serves as its mooring system, although anchors may also be used.

Drillship

A drillship can be one of two types: 1) It can be a ship which was designed and built to be a drilling vessel; or 2) A drillship can be an older vessel which has been refitted with drilling equipment. Drillships are self-propelled, carrying a complete ship's crew while underway, as well as a crew of drilling personnel. Drillships are moored either by the standard anchoring system or by dynamic positioning of the vessel. Dynamic positioning is the use of a computer-operated inboard thruster system which keeps the vessel on location without the use of anchors. 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.

Semi-Submersible

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.

Jack-Up

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.

Derrick/Pipe-Lay Barge

This is used to make extremely heavy lifts (The record to-date is 4,400 tons!) or to lay underwater pipelines. When these lifts are being made, there are usually a lot of support personnel on board (up to 200) including welders, electricians, riggers, operators, etc. Derrick barges can be either self-propelled or towed.

Production Platforms

After exploratory drilling has proved the presence of oil or gas deposits, a platform structure is built. These structures are then erected on the site. They are of a permanent nature.