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Drilling vessels are special purpose vessels which are used for drilling oil and gas wells on the ocean beds at deep seas. Drillships have always been at the forefront of the developments in the offshore industry. All drillships have 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.  Drillships are equipped with a special drilling derrick and have extensive mooring or positioning equipment, as well as a helipad to receive supplies and transport staff.

Typically employed in deep and ultra-deep waters, drillships work in water depths ranging from 2,000 to more than 10,000 feet (610 to 3,048 meters). Drilling equipment is passed through the vessel's moon pool and connected to the well equipment below via riser pipe, a somewhat flexible pipe that extends from the top of the subsea well to the bottom of the drillship.

Drillship Maersk Viking

Drillships are differentiated from other offshore drilling units by their easy mobility. While semisubmersible rigs can also drill in deep waters, drillships are able to propel themselves from well to well and location to location, unlike semisubs, which must rely on an outside transport vessel to transfer them from place to place.

A proper mooring system on a drillship is integral to drilling successful wells. In shallower waters, drillships are moored to the seafloor with anywhere from six to twelve anchors. Once the water depth becomes too deep, drillships depend on dynamic positioning systems (DPS) to keep the vessel in place while drilling. DPS relies on several thrusters located on the fore, aft and mid sections of the ship, which are activated by an onboard computer that constantly monitors winds and waves to adjust the thrusters to compensate for these changes. Sometimes, both positioning systems are used.

The drillships 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 specification to meet the requirements set by the oil production company and/or investors.

Used sources: RigZone