Dual gradient drilling systems - PetroWiki

Petrowiki 

Perhaps one of the most important ventures in the area of high-cost technologies for deepwater challenges is the development of dual gradient drilling systems (DGDSs). DGDS is often referred to as riserless drilling. It is generally accepted that DGDS is required in water depths of 5,000 ft. There have been a number of unpublished examples, however, in which application of the technology was needed in water depths as shallow as 3,000 ft. The need for DGDS is caused by the reduced fracture gradient of formations below the mudline, resulting from the reduced weight, or gradient (0.5 vs. 1.0 psi/ft), which, itself, is a result of water above the mudline as viewed from a drillship operating at sea level.