Questions & Answers
FUNCTIONALITY 
    
What makes the mud with rock cuttings go up the inner pipe?   It is the only return flow path for the system. The well annulus is sealed off with a Rotating Control Device (RCD) on top of the BOP.
RDM uses a small inner pipe. Does that make the ECD higher?   No, the pipe-in-pipe system requires a lower drilling fluid circulation volume to remove cuttings - RDM uses approximately 50 % of the volume used by conventional drilling. Typical flow rates for RDM are 600-1200 l/min.
What if the flow X-over (entrance) holes in the DFV get plugged?   The holes can be backflushed by pumping down the inner pipe.
Can the passive well annulus fluid have a different density than the active, circulating fluid?   Yes, the active fluid can be an underbalanced fluid. The passive fluid in the well annulus could have a different density than the active fluid, e.g. a kill fluid.
How long does it take to clean out the well when you stop drilling - bottoms up?   This depends on the well depth but a rule of thumb is that cuttings transport takes 5-7 min per 1000 m.
SAFETY
    
How is well control handled?   Well control is handled conventionally by circulating the kick up the annulus and having the ability to pump down both channels in the dual drill string.
Can the Dual Drill String be sheared by the BOP shear ram?   Yes, the dual drill string can be cut off by the BOP shear ram and this has been verified by tests.
EQUIPMENT
    
Are there any differences in the handling of Dual Drill String compared to conventional drill strings?   No. The Dual Drill String is a conventional 6 5/8” drill pipe where each joint is equipped with 3 ½” inner pipe (ID=3”). The DDS is handled according to normal procedure on the drill floor.
Is the Bottom Hole Assembly different from conventional solutions?   No. The BHA is made up of standard conventional components, but motor and bit need to be optimized for the lower flow rates.
Is it possible to use tri-cone and PDC bits?   Yes, both tri-cone and PDC bits can be used. However, it is important to select bits and nozzle configurations that suit the actual flow rate.
Can the system use either RSS or mud motors for directional drilling?   Yes, the system can use either a RSS or mud motor optimized for lower flowrates.
Can standard MWD tools be used?   Yes, the system has been used with different brands of MWD tools.