1. Field of the Invention
The present invention relates generally to offshore or subsea drilling systems or rigs. More particularly, the present invention relates to submersible systems for deepwater drilling for hydrocarbons or other natural resources.
2. Summary of the Prior Art
Drilling for hydrocarbons (oil and natural gas) offshore, in some instances hundreds of miles away from the nearest landmass, poses a number of different challenges over drilling onshore. The actual drilling mechanism used to delve into the seafloor often is much the same as can be found on an onshore rig. However, with drilling at sea, the seafloor can sometimes be thousands of feet below water level. Thus, while with onshore drilling the ground provides a very large load carrying capacity platform from which to drill, at sea, an artificial drilling platform with the same load carrying capacity must be constructed to support the offshore drilling equipment.
Drilling offshore dates back as early as 1869, when a T. F. Rowland developed an offshore drilling rig design. The rig was designed to operate in very shallow water, but the seafloor-anchored, four-legged tower bears much resemblance to a land drilling rig and modern offshore rigs, with the drilling platform at the water level instead of resting on the ground. It wasn't until after World War II that the first truly offshore well was drilled in the Gulf of Mexico. Since then, offshore production, particularly in the Gulf of Mexico, has been very successful, with the discovery and development of a great number of large oil and gas deposits.
There are two basic types of offshore drilling rigs: those that can be moved from place to place, allowing for drilling in multiple locations, and those that are permanently placed. Movable rigs are often used for exploratory purposes because they are much less expensive to use than permanent platforms. Once large deposits of hydrocarbons have been found, a permanent platform is built to allow their extraction. There are a number of different types of movable offshore rigs. These include drilling barges and ships, jack-up rigs, submersible rigs, submersible rigs, and semi-submersible rigs.
Drilling barge rigs are used mostly for inland, shallow water drilling. This typically takes place in lakes, swamps, rivers, and canals. Drilling barges are large, floating platforms, which must be towed by tugboat from location to location. Suitable for still, shallow waters, drilling barges are not able to withstand the water movement experienced in large open water environments.
Drillships are exactly as they sound: ships designed to carry out drilling operations. These drillships are specially designed to carry drilling platforms out to deep-sea locations. A typical drillship will have, in addition to all of the equipment normally found on a large ocean ship, a drilling platform and derrick located on the middle of its deck. In addition, drillships contain a hole (or “moonpool”), extending right through the ship down through the hull, which allows for the drillstring to extend through the boat, down into the water. Drillships are often used to drill in very deep water, which can often be quite turbulent. Drillships also use dynamic positioning systems. Drillships are equipped with electric motors with propellers on the underside of the ship's hull, capable of propelling the ship in any direction. These motors are integrated into the ship's computer system, which uses satellite positioning technology, in conjunction with sensors located on the drilling template at the sea bottom, to ensure that the ship is directly above the drill site at all times.
Jack-up rigs are similar to drilling barges, with one difference. Once a jack-up rig is towed to the drilling site, three or four “legs” are lowered until they rest on the sea bottom. This allows the working platform to rest above the surface of the water, as opposed to simply floating. However, jack-up rigs are suitable only for shallow waters (approximately 450 feet), as extending the legs down too deeply would be impractical. These rigs are typically safer to operate than drilling barges, as their working platform is elevated above the sea water level.
Submersible rigs, also suitable for shallow water, are like jack-up rigs in that they come in contact with the ocean or lake floor. These rigs typically consist of platforms with two hulls positioned on top of one another. The upper hull contains the living quarters for the crew, as well as the actual drilling platform. The lower hull works much like the outer hull in a submarine—when the platform is being moved from one place to another, the lower hull is filled with air—making the entire rig buoyant. When the rig is positioned over the drill site, the air is let out of the lower hull, and the rig's lower hull submerses to the sea or lake floor. This type of rig has the advantage of mobility in water, however, once again, its use is limited to shallow water areas.
Semi-submersible rigs are the most common type of deep water offshore drilling rigs, combining the advantages of submersible rigs with the ability to drill in deep water. Semi-submersible rigs work on the same principle as submersible rigs; through the “inflating” and “deflating” of its lower hull. The main difference with a semi-submersible rig, however, is that when the air is let out of the lower hull, the rig's lower hull does not submerge to the seafloor. Instead, the rig is partially submerged, but still floats above the drill site. When drilling, the lower hull, filled with water, provides stability to the rig. Semi-submersible rigs are held in place by huge anchors, each weighing upwards of ten tons. These anchors, combined with the submerged portion of the rig, ensure that the platform is stable and safe enough to be used in turbulent offshore waters. Semi-submersible rigs can be used to drill in much deeper water than the rigs mentioned above. Semi-submersible rigs can be used for drilling in very deep water by replacing the anchors with multi-computer controlled propellers which are commonly known as a “dynamic positioning system.”
The oil and gas industry in its search for additional hydrocarbon reserves in deep water currently utilizes drillships or semi-submersible drilling units. These are generally called “floating” drilling units and they have a riser (a large-diameter pipe) connecting the floating drilling unit to the BOP stack and template at the sea bottom or floor. The water depth limitation of the current deep water drilling method is approximately 10,000 feet for several reasons:                1. The effect of the long column of drilling fluid (mud) in the riser on the subsea shallow unconsolidated formation integrity.        2. The maximum riser weight the floating drilling unit can support.        3. The ability to maintain the floating drilling unit within a very limited radius above the subsea well location, especially under high current and waves.The latest generations of the floating drilling units are several thousand tons variable load capacity and cost up to $500,000 per day rental and these floating drilling units are currently in a very short supply.        
Therefore, a need exists for a submersible drilling rig or system adapted for deepwater drilling that eliminates the need for a long riser and the associated difficulties presented in position-keeping, as well as avoiding the skyrocketing cost and limited availability of deep water floating drilling units.