The Many Models of Offshore Platforms
One of the beauties of my job is the fact that I get to learn so much about the different industries that we work in. We are always doing our best to learn everything about the industries that use our ROVs so that we can continue to develop our products to suit the needs of those who are using them. This is fantastic for us because not only are we experts in ROVs but we strive to be experts in the industries such as oil and energy, aquaculture, commercial diving, municipal infrastructure and more.
In this post I am going to do an overview of the various kinds of oil rigs and platforms that exist and where the use of mini-ROVs fit into each. Even though mini ROVs are not used as often as work class ROVs in the offshore energy industry, mini ROVs still play a vital role in on-going inspections and maintenance. While there are some larger structures that have fewer uses for micro-ROVs, the majority of oil rigs and platforms have many potential areas for mini-ROV inspection. After doing my own research and speaking to various industry professionals I have learned that depending on the type of energy structure, the uses for a mini-ROV can range from inspecting the entire underwater structure to doing on board system inspections.
First off, I need to differentiate between the two main types of offshore oil drilling structures. There are moveable structures and there are fixed structures. As their names indicate, moveable structures are portable and fixed structures are fixed in a specific area.
Moveable Structures
Drilling Barges
These Structures are used mostly for inland and shallow water drilling. They are used in lakes, swamps, rivers and canals. They consist of large, floating platforms that are towed from location to location with the use of a tug boat. They are unable to withstand the water movements that other structures do in large open water areas. Since these structures are used mainly in shallower water, the mini-ROVs can be used to inspect all aspects of these structures. Using a mini-ROV like the Deep Trekker DTG2 or DTX2 offers an inspection solution so that operators can check out their risers (the piping component that is used for exporting oil and gas from the seabed), the hull of the platform and topside components such as water tanks (used for storing water) that are located on the deck of the platform.Jackup Platforms/Rigs
These structures are similar to drilling barges in that they can be moved from location to location. What differentiates these structures are that once they are towed to the correct drilling site, three or four “legs” are lowered until they rest on the bottom floor. This actually allows the working platform to sit above the water as opposed to a floating barge. These systems are also suitable for shallower waters since extending the legs too deep is not practical. Since their working platform is elevated above the water, these structures are generally safer to operate than drilling barges. Mini-ROVs can be used to inspect the underwater infrastructure, such as the jackup legs and risers, of these structures.Submersible Platforms/Rigs
These structures are also suitable for shallow water. They consist of two hulls stacked on top of one another. The upper hull contains the living and working areas for the crew while the lower hull works to make the structure buoyant. When the platform is being moved from location to location, the lower hull is filled with air making it buoyant. Then, when the rig is positioned over the drill site, the air is let out of the hull and it sinks to touch the sea floor. Again, mini-ROVs are perfect for inspecting the underwater infrastructure of these structures as well as any onboard water and cooling tanks.Semi-Submersible Platforms/Rigs
These are the most common type of offshore drilling rigs. It has a floating drill unit that contains columns and pontoons that when flooded with water will sink the pontoons to a pre-determined depth. They work on the same principle as submersible rigs: through the “inflating” and “deflating” of its lower hull. The pontoons sink below the surface of the water so the rig is partially submerged but still floats above the drill site. The lower hull, when filled with water, provides stability when drilling. In most cases these structures are held in place by huge anchors. They are able to drill in much deeper water than the structures listed above, yet there are still a number of application for mini-ROVs on these structures. First off, the ROVs can be used to inspect the inside and outer areas of the submerged hull. Regular inspection of these hulls is a very important task when operating a semi-submersible rig due to the possibility of fatigue and cracking of braces. Of course, the mini-ROVs can also be used for any onboard water and/or cooling tanks.Drillships
These are ships designed to perform drilling operations. They are specially designed to carry out drilling equipment to deep sea locations. In addition to all of the normal ship components, a drillship will typically have a drilling platform and derrick (the tall structure used as part of the rig hoisting system) placed in the middle of its deck. It also has a “moon pool” in the center of the structure that is used to allow the drill string the extent through the hull of the ship and down to the drill site. Drillships use “dynamic positioning systems” to ensure that the ship is directly above the drill sites at all times. They are equipped with underside electric motors that are capable of propelling the ship in any direction. With the use of the ship’s computer system (which uses satellite) technology, the propellers are constantly working to hold the ship directly above the drill site at all times. These structures can drill in very deep waters but based on its structure build, it still has various applications for a mini-ROV. Not only can the ROV be used to perform hull inspections of the structure, but a mini-ROV can be used to inspect the moon pool that larger work class ROVs are unable to reach. Lastly, and possibly the most important use of a mini-ROV is the ability to inspect the underside motors that are integral to holding the ship in place.
Fixed Platforms
Template (jacket) Platform
These structures are used in waters of moderate depths (up to 400m). They are the structures that you would typically see in the Persian Gulf, Gulf of Mexico, Nigeria and California shorelines. On a jacket platform, the working platform is generally sitting above the water line and the risers are lowered into the ground. The “Jacket” is a structure built around the risers to provide a protective layer around the pipes. On these structures ROVs such as the DTX2 can be used to inspect the underwater infrastructure when the structure is not too deep. Units such as the DTG2 can be used to inspect onboard water tanks.Floating Production Systems
These structures are essentially semi-submersible drilling rigs as discussed above but in addition they contain petroleum production equipment along with platform drilling equipment. Ships can also be used as floating production systems. They are kept in place with large, heavy anchors or with similar technology to that of drillships. With these structures, the wellhead is actually attached to the seafloor instead of on deck and is transported via risers to the production facilities on-board. Mini ROVs can be used to inspect the bottom of the hulls and again on-board equipment.Tension Leg Platforms
It consists of a floating rig, much like the semi-submersible platforms but also has long, flexible legs that are attached to the seafloor and run up to the platform itself. The legs allow for significant side-to-side movement to take the pressure of waves off of the structure, but offers very little vertical movement. These structures can be used in waters as deep as 7000 feet. Mini-ROVs can be used efficiently by inspecting the inner hulls of the structures.Seastar Platforms
These are essentially miniature tension leg platforms. With floating hulls and tension legs used to hold them in place, they are generally used in smaller deep-water reservoirs when it is not economically efficient to build a larger structure. Mini-ROVs can be used to inspect the inner and outer areas of the hulls that are filled with water.
In addition to the 9 rig and platform structures discussed above there are also structures like Subsea systems, Spar platforms and Compliant Tower Platforms that are used in much deeper water and typically do not have many underwater uses for mini-ROVs. That being said most rigs do have water tank and cooling tank components that can be effectively inspected with a mini-ROV.
As you can see, there are a number of applications and types of structures that could benefit from having a min-ROV on board. In some of the shallower structures, a mini-ROV can actually be an efficient alternative to purchasing an expensive and larger work class ROV.
Resources used:
http://large.stanford.edu/courses/2014/ph240/liegl2/docs/sadeghi.pdf
http://www.slideshare.net/rahulranakoti/offshore-platformdesign