A drillship is a maritime vessel that has been fitted with drilling apparatus. It is most often used for exploratory drilling of new oil or gas wells in deep water or for scientific drilling. The drillship can also be used as a platform to carry out well maintenance or completion work such as casing and tubing installation or subsea tree installations. It is often built to the oil production companies and/or investors design and specifaction but it can also be a modified tanker hull and outfitted with a dynamic positioning system to maintain its position over the well. The greatest advantages these modern drillships have is their ability to drill in water depths of more than 2500 meters and of course the valuable time saved sailing between oilfields worldwide as per contractual agreement, they are completely independant compared with Semi-submersibles and jackup barges.
Deepwater Millennium in Singapore October 2013. Picture taken from Maersk Logger.