Drillships: Option for DeepwaterTue, 01/22/2013 - 17:42
Drillships have been used in offshore drilling operations since the 1940s, when the US Navy adapted one of its numerous ships for a drilling operation in California. Currently used all over the world, drillships are modified marine vessels that are equipped with a drilling derrick and moon pool, and have extensive mooring capacity.
Drillships are a viable alternative to semisubmergibles in deep and ultra-deepwater drilling operations. While semisubmersibles depend on various other mechanisms to either be installed, constructed, or transported, drillships have the advantage of being able to move from well to well independently at a speed of 15-16 knots, compared to a semisubmersible’s speed of 5-6 knots. Also, drillships are better suited for dual activity derrick, the blowout preventer (BOP) can be carried closer to the waterline, and they usually have more tank space available for liquids. Besides these advantages, drillships have a moon pool at the center of the ship, where drilling equipment is passed through and connected to the well equipment via a riser pipe, a flexible pipe that extends from the top of the well to the bottom of the drillship.
Nevertheless, drillships have various disadvantages, the most important is instability. While they are still able to drill in deep and ultra-deepwater, they can be easily affected by winds, currents, and waves, which can be dangerous during drilling, since the vessel is attached to pipes that are in turn attached to an oil or gas well thousands of meters below. Therefore, drillships are more suitable for locations with benign waters. However, companies like Transocean, Pride, Seadrill, Frontier Drilling, and Noble have created mooring systems that allow for the successful drilling of wells with drillships in most locations. Drillships depend on GPS and dynamic positioning systems (DPS) to keep the ships in place through various thrusters located at strategic locations around the vessel in order to move it and keep it in place, despite winds, waves, and currents. “It is not that one is better than the other; semisubmergibles or drillships each do different things,” explains Bob Mankin, Country Manager for Seadrill Mexico. “There is an overlap where both semisubmergibles and vessels could function perfectly, but there are many places where drillships simply cannot work.”