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Vessel fitted for offshore drilling

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.

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. Robert F. Bauer became the first president of Global Marine in 1958. 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. 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.

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. 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. 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. 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.

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. 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.

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 23,000 feet (7010 m) below the seabed, bringing that to two to four times that of any other drillship.

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.