Riserless dual gradient technology enables drilling tophole with weighted and engineered mud without discharge to sea. A Suction Module is run and installed subsea. This has a collection chamber where well returns are collected and diverted to a subsea mud pump. A Mud Return Line allows pumping or lifting the mud and cuttings back to surface. In this way a "closed loop system" is established; mud and cuttings are collected at seafloor and returned to the shakers as with post-BOP drilling techniques. The technique eliminates discharges of mud and cuttings at seafloor and thereby a weighted and engineered mud system may be used to solve difficult drilling challenges, improve drilling efficiency and safety.
Selection of the technology is typically driven by its ability to impact drilling efficiency and safety, but in arctic and other environmental sensitive areas also by the fact that discharges to sea can be reduced of totally eliminated. Riserless dual gradient technology has been used to drill wells in artic locations. If a 0-discharge requirement also apply for the surface conductor, a purpose designed SpudBase may be used. This will allow spudding the well / setting and cementing of conductor pipe without discharges at seafloor. Thereafter the 26" hole can be drilled with weighted mud, the surface casing run and cemented. The entire tophole drilling may hence be drilled riserless with return to surface. Cuttings may be collected at the shakers and shipped to shore for disposal. The mud is recirculated and reused throughout the entire drilling operation.