Drilling rigs are some of the most important pieces of oilfield equipment. They are used during a number of stages throughout oil and gas fields’ lifecycles. See the summary below for descriptions of how offshore rigs are used throughout oil and gas fields lifecycles.
During the exploration stage, rigs are used to drill exploration wells and ‘wildcat’ wells for potential hydrocarbon-bearing geological structures after various geological studies and seismic surveys have identified locations in which these structures could be placed. Most of the time, vertical wells are drilled to ensure safety and well stability and to acquire sufficiently high-quality subsurface data and knowledge.
During the appraisal stage, rigs are used to drill several wells to understand flow rates and reservoir dynamics and to determine the size and limits of the reservoir in order to confirm the assumption that hydrocarbons can be produced economically.
During the development stage, rigs are used to drill wells (at a much higher level of activity) to the depth of a productive zone of the reservoir. At this point, wells can be vertical, horizontal or deviated, and they can be drilled on a grid or on a pad.
During the production stage, rigs are used to drill more wells, also known as repair/work-over wells, to fix existing wells. This practice is also known as infield drilling Depending on the complexity of the job, a smaller work-over rig may be used for a work-over program to repair wells, enhance production or provide other well treatments.
A number of rig types are used offshore. Each rig type serves a different purpose, and each one is best suited to a particular drilling environment.
The key differentiation factors in offshore drilling rigs are as follows:
- Rated water depth
- Drilling depth
- Leg length (for jack-ups)
- Lifting capacity
- Top drive system
- Number of mud pumps and the power and ability to circulate drilling fluid
Types of offshore drilling rigs ( source: Maersk)
Offshore drilling is more challenging than onshore drilling due to the lack of stability (particularly for floaters), the corrosive water environment, space constraints and the need for more complex logistics and support. Offshore drilling rigs are broadly divided into bottom-supported rigs and floaters.
Unlike floaters, bottom-supported rigs are anchored to the bottom of the seafloor. There are two main types of bottom-supported rigs: platform rigs and jack-up rigs.
Platform rigs consist of steel or concrete platforms standing on top of fixed columns that are made of tubular steel and driven into the seabed. Platform rigs are limited to water depths of about 150 meters.
Jack-up rigs are floated out to the drilling location, and they have retractable legs that are lowered down to the seafloor. Jack-up rigs can only work in water depths less than the length of their legs, typically limiting operations to less than 150 meters/500 feet. When drilling is completed, the legs are raised out of the water, and the rig becomes a floating barge that can be towed away (‘wet tow’) or placed on a large transport ship (‘dry tow’).
Jack-up rigs can be segmented by their specifications and water depth ratings. The three most common types of these rigs are standard, high specification and harsh environment.
Standard rigs are generally old, and they usually have low hook-load capacities and mechanically operated drilling equipment with little automation. These rigs operate at water depths less than 300 feet. However, standard rigs can do almost the same job as high-specification rigs at a much lower rate.
High-specification rigs are mostly used in Southeast Asia, but they are growing in popularity in GCC and GOM. The rigs are very robust, and they can drill at water depths of up to 400 feet due to their modern automation systems and drilling equipment.
Harsh-environment rigs are mainly used in the North Sea. These rigs are designed to withstand harsh weather conditions and water depths of up to 490 feet.
Floaters are not limited to the same water depths as jack-up rigs because they do not have to stand on legs. Floaters are ships with drilling equipment that are self-propelled. When the ship arrives at a location, the floating rig anchors with the help of anchor handling vessels and support vessels (AHTSV) in a complex, lengthy process. Unlike jack-ups, floaters move up and down with the tides, but the fixed wellbore does not move. This is achieved through hydraulic wave-motion and heave compensators.
The two different types of floater rigs are semi-submersible rigs and drillships.
- Semi-submersible rigs are supported (floating) by large pontoons, which provide enough buoyancy to keep the rig afloat or to move it from location to location. They are semi-submersible rigs (or ‘semis’) because the floaters operate in a ‘semi-submerged manner. While some semis use mooring lines to connect to the anchors on the seafloor, others have propellers that rotate to hold the rig in the exact location; these rigs are often referred to as dynamically positioned.
- Drillships are used in deepwater and remote fields due to their large load-carrying capabilities, mobility and ease of moving. Drillships use dynamic positioning systems to station at an exact position.