The main difference between onshore and offshore drilling is that onshore drilling has a solid platform. The offshore drilling rig digs thousands of feet below sea level. The offshore rig has to invent platforms. These become artificial platforms that have to be constructed to serve as a solid platform. There are different types of rigs with different types of uses and platforms.
The first type of platforms is movable. Mobile platforms look for oil. They are cheaper than the permanent structure. If large deposits of oil are discovered, a permanet structure is built.
1.) Drilling barges are immense floating platforms used for shallow-water drilling. Tugboats tow the barges from place to place. They can only be used in still water.
2.) Jack-Up Rigs are towed to the drilling site. It then uses tripod legs or four legs to keep the platform above the water. This is another rig for shallow waters but deeper than the drilling barge.
3.) Submersible Rigs are similar to jack-up rigs in the sense that they touch the ocean floor. There are two hulls. The top hull is where the crew lives and the location of the drilling platform. The bottom hull is similar to a submarine. After the bottom hull fills with air, it floats to the drilling site. Upon arrival to the drilling site, the air is let out of the lower hull. This allows the rig to submerge to the sea floor. Even though the rig is mobile, it still needs to stay in shallow water.
4.) Semisubmersible Rigs are the most prevalent offshore rig. It has many advantages. The main one is that it can drill in deep sea water. The difference is that the lower hull does not submerge to the sea floor. The lower hull is filled with water and makes the rig stable. Heavy anchors that weigh more than ten tons are used to make the platform stable in unstable offshore waters.
5.) Drillships are designed to carry out drill operations. The ships carry the drilling platform out to deep waters. The drilling platform is at the middle of the deck with the derrick. The intriguing part is that there is a hole (“moonpool”), which begins on the main deck and goes straight down through a hole in the hull to the sea floor. Drillships are equipped with “dynamic positioning” systems. This means that the ship has electric motors on the underside of the ship. The electric motors are able to change the ship's direction. The motors are an integral part of the ship’s computer, which uses the satellite technology. There are sensors on the drilling line that feed the information into the computer. This, in turn, activates the motors and moves the ship so that it is directly above of the drill site. As you can imagine, the water is quite turbulent out in the middle of the ocean. This added security helps the crew members not to veer off course.
6.) The permanent platforms are built when large qualities of oil are discovered. It is more economical to build a platform for locating oil. The largest offshore platforms are found in the North Sea. The North Sea is known for its adverse weather. The platforms have to be built to withstand wind speeds of over 85 miles per hour. It also must resist waves that are more than 60 feet high. The permanent platforms are the strongest built. The platforms are made in sections and then carried out to the rig for assembly.
Fixed Platforms can be used in shallower water. The fixed platforms can be attached to the sea floor with “legs”. The legs are made of concrete or steel. The main reason that these are not used in deep water is that it would not be worth the cost to build “legs” that long.
7.) Compliant Towers have a narrow tower that is attached to a foundation on the sea floor. The tower is flexible because it does not have rigid legs. It is able to operate in deeper water. It has the added advantage of withstanding hurricanes. They have a drilling capacity of 1500 - 3000 feet.
8.) Seastar Platforms have a floating rig similar to the semisubmersible rig. This fixed platform fills the bottom half of the hull with water and uses tension legs. The legs are kept tense as they become attached to the floating platform. They are flexible and can withstand the impact of the ocean and wind. It is primarily used in the Gulf of Mexico where there is not enough room for a large platform. The larger platform tension leg platforms are able to drill much deeper. This includes the Subsea System and the SPAR platform.
9.) Floating Production Systems are platforms that not only drill, but also has petroleum production on board. The wellhead is not on the platform but is on the sea floor. It uses the same dynamic positioning system as the drillship to keep the ship over the drilling area. It can drill and produce oil to depths of 6,000 feet.
This is an overview of the different platforms and drilling equipment. The actual production process is another story.