There are three main types of offshore drilling rigs used today: platform rigs, jackup rigs, and drillships. Each type is suited for different depths of water and terrains on the seafloor.

Platform Rigs

Platform rigs are large steel structures with legs that rest on the seafloor of relatively shallow water, usually up to about 500 feet deep. The steel deck area of the platform provides work space and housing for crew. Rigs are moved into place and secured by legs lowered into pre-drilled sockets on the seabed. Platform rigs are commonly used in places like the Gulf of Mexico where the continental shelf waters are shallow. They are stable structures well-suited to performing exploratory and development drilling from a fixed location.

Jackup Rigs

Jackup rigs are floating drilling rigs with movable legs that can be lowered to raise the drilling platform out of the water and onto the seafloor. The legs can extend down allowing jackup rigs to operate in waters up to about 400 feet deep. When moving between drilling locations, the legs are lifted up out of the water and the hull is floated to the new site. Once on location, the legs are jacked down allowing work to begin. Jackups provide mobility and are versatile across differing sea conditions.

Drillships

Drillships are marine vessels with dual derrick cranes used for exploratory well drilling in remote ultra-deepwater locations beyond the reach of platform and jackup rigs, typically in over 5,000 feet of water. Rather than having legs that rest on the seafloor, offshore drilling rigs  remain floating at the drill site kept in position using onboard propulsion and dynamic positioning systems. Drillships offer the advantages of mobility and ability to operate independently in offshore areas away from ports or pipelines.

Operating Conditions and Challenges

Drilling offshore presents unique challenges compared to land-based rigs due to harsh ocean environments, remote locations, and greater risks. Rigs are subject to ocean waves, currents, wind, and storms that cause continual motion and vibration. Maintaining position and stability is important to precision drilling operations. Supply boats must battle rough seas to deliver crew and cargo.

Rig crews live and work for weeks at a time aboard the floating infrastructures, far from shore in cramped industrial conditions. Emergency response and evacuation in the event of accidents like fires or explosions is much more difficult offshore. Protecting equipment from corrosion by ocean water and salt air takes careful maintenance planning. Spills of drilling muds, fuels, or hydrocarbons pose severe environmental risks.

Drilling Processes and Equipment

Offshore rigs utilize similar equipment and drilling processes as on land, however everything must be designed to withstand marine conditions and operations moved in larger components suitable for lifting by heavy-lift offshore cranes.

A key piece of equipment is the blowout preventer (BOP), a large specialized valve assembly placed on top of the wellhead to seal, control and monitor well pressures during drilling. BOPs are critical safety devices able to shut wellbore fluid flow in an emergency such as a blowout.

Long strings of drill pipe connected by tool joints are spindled into the hole by top drives or rotary tables powered at the surface. Drilling fluids are pumped down through the pipe and annulus to lubricate and cool the drill bit, then flow back to surface mud tanks carrying rock cuttings from the well.

Geologists and engineers monitor downhole conditions using wireline tools, logging equipment, seismic profiling and testing of formation fluids throughout the process. Casing strings are run and cemented to line the borehole for zonal isolation and well control.

Future Advances

As drilling operations push into ever deeper waters with multi-million dollar rig dayrates, technologies are evolving for safer, more efficient offshore drilling. Automation reduces human exposure to hazards. Robotic systems take over repetitive or dangerous tasks. New BOP designs aim for faster response and redundancies.

advancements in digital sensing, simulators, and condition monitoring improve maintenance and support real-time decision making. Still the challenges of drilling in remote marine environments will always demand constant innovation from the offshore oilfield industry. With discoveries of vast untapped oil and gas reserves beneath the oceans, offshore drilling rig operations will surely continue their progression into even deeper frontier regions.