Mobile Offshore Drilling Units: MODUs
Offshore drilling has grown rapidly over the last 100 years from drilling off the end of a dock to drilling in thousands of feet of water in the middle of the ocean. The focus on offshore drilling increased as more deposits are found farther offshore. Not long ago, it was widely accepted that most of the oil in the world was in the middle east, and in fact the largest oil field in the world is the Ghawar oilfield in Saudi Arabia. As technology has improved, we have found more oil and gas deposits offshore and in deeper waters that were before unreachable. Recent findings predict that areas like the Gulf of Mexico may have more oil and natural gas than we ever thought possible. Since most of the deposits on land have already been found, new deposits are usually offshore, some deep underwater, and we reach them through offshore and deepwater drilling. The vessels used for offshore drilling are called Mobile Drilling Units, or MODUs, and there are three main types: jack-ups, semi-submersibles, and drillships.
“Jack-Up Rig courtesy Maersk”. Image from Subsea Oil and Gas Directory.
Jack-ups are vessels that use legs to push themselves out of the water, creating a stable platform above the waves for drilling. They are large platforms that usually have three or four legs that are stored vertically up above the vessel. Once on location, they lower the legs to the seafloor and preload them in stages, raising the vessel slowly in small increments while filling the ballast tanks with seawater. This puts more pressure on the legs than they would otherwise experience, and make sure they don’t sink further in a soft bottom or unexpectedly punch through a hard crust, which could tip the whole vessel or bend the other legs. Preloading is done in stages for safety and a vessel could go through 7 or more sequences of raising, filling the ballast tanks, emptying the tanks, and raising again, according to PetroWiki. When the legs no longer move, the jack-up is set.
The legs on a jack-up can only be so long, and these vessels usually operate in waters 400 or less feet deep. This means they are relegated to mostly coastal areas. They generate their own power and some have their own propulsion, giving them the ability to travel on their own. For long distances or for those jack-ups without propulsion they may be towed or moved with a heavy lift vessel. Some jack-ups even have dynamic positioning systems like the DEME Neptune, equipped with a DP2 system with four azimuth thrusters. Once on location, drilling is done either over the side of the vessel with a cantilever system or through a slot in the hull. Open truss legs are the strongest type of leg, so most newer jack-ups have legs with a triangular or box shape with a lattice structure inside. Water jets inside the legs ensure the legs get solid footing in the sea bed, as well as help to loosen the legs when it’s time to move again.
“Maersk Developer. Credit: Maersk Drilling.” Image from Offshore Technology.
Semi-submersibles are huge platforms that are column stabilized. This means they have large columns underwater that can be filled with air or water depending on the height requirements of the vessel. At the bottom of the columns, there is typically a continuous pontoon connecting the legs or twin pontoons connected to two legs each, according to Wärtsilä. This design creates a very stable platform resistant to severe weather and waves and is great for extreme environments like the North Sea. They can be spread moored with cables connecting them to the sea bed or use dynamic positioning to keep station. Semi-subs are capable to almost any depth of water and have worked in thousands of feet of depth in waters over 9,000 feet deep, according to PetroWiki. They don’t work as well in shallower waters due to the large columns that could collide with an underwater blow out preventer (BOP). The resonance period of waves is also taken into consideration, meaning the period in between swells. Semi-submersibles must be built so that they are not upset by the waves in the area in which they will be operated.
These vessels usually have drilling equipment mounted somewhere in the middle of the platform with everything else built around it. They are also more expensive to operate than jack-ups, but provide a very stable surface and operate in deep waters and extreme environments. Fun fact: SpaceX is turning two old semi-submersible MODUs into rocket launch platforms, Deimos and Phobos.
Stena Forth drillship. Image from Offshore Technology.
Compared to jack-ups and semi-submersibles, drillships are very different – they have a ship shape instead of a platform, and they have their own power and propulsion. They have some of the deepest drilling capability and their appearance is usually marked by a large drilling derrick in the middle of the ship that operates through a moonpool. These vessels are dynamically positioned with azimuth thrusters and are fast and nimble, making them perfect for drilling multiple exploratory wells in quick succession. They use DP2 or DP3 dynamic positioning with redundancy in case of failure of a component or compartment. Since these ships are much smaller than jack-ups and semi-submersibles, they are not as stable in rough waters. Drillships can carry thousands of feet of drillpipe, and make great research vessels with the ability to have more than 100 people on board.
The Chikyū is one of the most fascinating drillships, built to study faults to learn more about earthquakes by drilling thousands of meters into the sea bed to extract magma directly from the Earth’s mantle. The vessel was built by Japanese Agency for Marine Earth Science and Technology (JAMSTEC), and it contains many onboard laboratories for on-site research of core samples, including living microbes, rocks, fossils, and magnetic forces. It carries about 100 crew members and 50 scientists and is equipped with seven thrusters for precise station keeping while drilling. So far, Chikyū has drilled to a depth of 3,250 meters with the goal of drilling to 5,200 meters to reach the mantle. With the capability to drill to 7,000 meters, they will get there eventually!
Types of drilling rigs. Image from Deep Trekker.
For offshore drilling, you need a MODU! Jack-ups are just right for shallower waters where they can sink their legs into the sea floor, creating a stable drilling platform. When deepwater drilling, semi-submersibles can operate in almost any environment with underwater columns to keep their platform stable in harsh conditions. Either of these vessels can be self-propelled or towed into position. For deepwater drilling done fast or with multiple wells, drillships can relocate and drill wells the fastest, as long as the waters aren’t too rough. All of these vessels play important parts in drilling for oil and gas and research in all areas of the offshore world.
For more information on dynamically positioned vessels, check out 22 types of dynamically positioned vessels.