How do oil rigs drill so deep?

Vessel fitted for offshore drilling

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Drillship Deep Value Driller docked in Norway Comparison of deepwater semi-submersible and drillship. In the foreground, offshore support vessel Toisa Perseus with the fifth-generation deepwater drillship Discoverer Enterprise.

A drillship is a merchant vessel designed for use in exploratory offshore drilling of new oil and gas wells or for scientific drilling purposes. In recent years the vessels have been used in deepwater and ultra-deepwater applications, equipped with the latest and most advanced dynamic positioning systems.

History

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Drillship CUSS I.

The first drillship was the CUSS I, designed by Robert F. Bauer of Global Marine in 1955. The CUSS I had drilled in 400-foot-deep waters by 1957.[1] Robert F. Bauer became the first president of Global Marine in 1958.[1]

In 1961 Global Marine started a new drillship era. They ordered several self-propelled drillships each with a rated centerline drilling of 20,000 foot-wells in water depths of 600 feet. The first was named CUSS (Glomar) II, a 5,500-deadweight-ton vessel, costing around $4.5 million. Built by a Gulf Coast shipyard, the vessel was almost twice the size of the CUSS I, and became the world's first drillship built as a new construction which set sail in 1962.[1]

In 1962, The Offshore Company elected to build a new type of drillship, larger than that of the Glomar class. This new drillship would feature a first-ever anchor mooring array based on a unique turret system. The vessel was named Discoverer I. The Discoverer I had no main propulsion engines,[1] meaning it needed to be towed out to the drill site.

Application

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A drillship can be used as a platform to carry out well maintenance or completion work such as casing and tubing installation, subsea tree installations, and well capping. Drillships are often built to the design specifications set by the oil production company and/or investors.[2]

From the first drillship CUSS I to the Deepwater Asgard, the fleet size has been growing ever since. In 2013 the worldwide fleet of drillships topped 80 ships, more than double its size in 2009.[3] Drillships are not only growing in size but also in capability, with new technology assisting operations from academic research to ice-breaker class drilling vessels. U.S. President Barack Obama's decision in late March 2010 to expand U.S. domestic exploratory drilling seemed likely to increase further developments of drillship technology.[4]

Design

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Tower of drillship Deep Value Driller

Drillships are just one way to perform various types of drilling. This function can also be performed by semi-submersibles, jackups, barges, or platform rigs.[5]

Drillships have the functional ability of semi-submersible drilling rigs and also have a few unique features that separate them from all others, first being the ship-shaped design.[6] A drillship has greater mobility and can move quickly under its own propulsion from drill site to drill site in contrast to semi-submersibles and jackup barges and platforms. Drillships have the ability to save time sailing between oilfields worldwide. A drillship takes 20 days to move from the Gulf of Mexico to offshore Angola, whereas a semi-submersible drilling unit must be towed and takes 70 days.[7] Drillship construction costs are much higher than that of a semi-submersible. Although mobility comes at a high price, the drillship owners can charge higher day rates and get the benefit of lower idle times between assignments.[7]

The table below depicts the industry's way of classifying drill sites into different vintages, depending on their age and water depth.[7]

Drillship Launch Date Water Depth (ft.) CUSS I 1961 350 Discoverer 534 1975 7,000 Enterprise 1999 10,000 Inspiration 2009 12,000

The drilling operations are very detailed and in-depth. A simple way to understand what a drillship does to drill, is that a marine riser is lowered from the drillship to the seabed with a blowout preventer (BOP) at the bottom that connects to the wellhead. The BOP is used to quickly disconnect the riser from the wellhead in times of emergency or in any needed situation. Underneath the derrick is a moonpool, an opening through the hull covered by the rig floor. Some of the modern drillships have larger derricks that allow dual activity operations, for example, simultaneous drilling and casing handling.[8]

Types

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There are different types of offshore drilling units such as the oil platform, jackup rig, submersible drilling rig, semi-submersible platform and of course drillships.[9] All drillships have what is called a ”moon pool”. The moon pool is an opening on the base of the hull and depending on the mission the vessel is on, drilling equipment, small submersible crafts and divers may pass through the moon pool. Since the drillship is also a vessel, it can easily relocate to any desired location. Due to their mobility, drillships are not as stable compared to semi-submersible platforms. To maintain its position, drillships may utilize their anchors or use the ship's computer-controlled system on board to run off its dynamic positioning.[10]

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Chikyū

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One of the world's best-known drillships is Japan's ocean-going drilling vessel Chikyū, which actually is a research vessel. The Chikyū has the remarkable ability to drill to a depth of 7.0 kilometres (23,000 ft) below the seabed, bringing that to two to four times that of any other drillship.[11]

Dhirubhai Deepwater KG1

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In 2011 the Transocean drillship Dhirubhai Deepwater KG1 set the world water-depth record at 10,194 feet of water (3,107 meters) while working for Reliance – LWD and directional drilling done by Sperry Drilling in India.[12]

Drillship operators

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References

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Offshore exploration and development wells are often drilled from mobile offshore drilling units (MODUs, pronounced "moe-dooz"). Depending on the water depth and remoteness of the location, these "rigs" may be jack-ups (up to 400 feet of water), or semisubmersibles, or drillships (up to 12,000 feet of water). Jack-ups are bottom-supported units; semisubmersibles and drillships are floating units ("floaters").

In terms of numbers, jack-up rigs drill most offshore wells. Semisubmersibles run a distant second, and drillships come in third, though most of the major new discoveries today are being made by the floaters in deep and ultra-deep water. Oil companies ("operators") select rigs that are specifically suited for a particular job, because each rig and each well has its own specifications and the rig must be matched to the well.  

Jack-up rigs are so named because they are self-elevating-with three or four movable legs that can be extended ("jacked") above or below the drilling deck, or hull. Jack-ups are towed to the drill site with the hull, which is actually a water-tight barge that floats on the water's surface, lowered to the water level, and the legs extended above the hull. When the rig reaches the drill site, the crew jacks the legs downward through the water and into the sea floor (or onto the sea floor with mat supported jack-ups). This anchors the rig and holds the drilling deck well above the waves. 

Jack-up rigs drill through slots in their hull, or have the ability to extend ("cantilever") the drilling package out over the side of the hull. A cantilevered rig is very useful for drilling a series of wells over an established production platform, because the rig can move beside the platform and then extend the drilling package over the top of the platform to drill a sequence of development wells.  

Usually, jack-ups are towed by several towboats to their drilling location. When distances are long, they are placed on heavy-lift vessels for transport. 

The jack-up's deck provides space for drilling equipment, supplies, and living quarters. Helicopters and supply boats transport workers, equipment, and supplies from shore to rig. 

Semisubmersibles do not rest on the sea floor like jack-up rigs. Instead, the working deck sits atop giant pontoons and hollow columns. These float high in the water when the rig is moved. At the drill site, the crew pumps seawater into the pontoons and columns to partially submerge the rig, hence the name semisubmersible. With much of its bulk below the water's surface, the semisubmersible becomes a stable platform for drilling, moving only slightly with wind and currents (this is termed wave transparency).  Like jack-ups, most semisubmersibles are towed to the drill site. 

Because of their exceptional stability, "semis" are well suited for drilling in rough waters. Semisubmersibles can drill in water as deep as 10,000 feet. Because semis lack the legs of a jack-up, they must have a means of maintaining their position over the well during drilling. This is accomplished using either an eight or 12 point anchoring (mooring) systems. In deeper waters, some semis employ dynamic positioning (DP) systems to replace or supplement the mooring system. DP systems employ computer-controlled motor-driven propellers, called "thrusters," to adjust for the action of winds and waves. They respond automatically to satellite GPS signals coordinated with acoustic beacons placed on the sea floor.  

Drillships are specially built seagoing vessels that also drill in waters as deep as 12,000 feet. Drilling equipment is installed on the deck, with the derrick normally placed in the middle of the ship. The well is drilled through an opening (called a "moon pool") that extends to the water's surface below the derrick. 

Because of their cargo-carrying capacity and exceptional mobility, drillships are especially useful for drilling exploratory wells. Although they are not as stable as semisubmersibles in rough water, they can be moved from location to location much faster. They maintain their position through mooring or dynamic positioning systems.

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