A deep-water riserless dual gradient casing while drilling concept allows slender wells to be drilled and completed with a minimum number of casing-strings. Unweighted or weighted drilling fluid is supplied and returned through separate flow conduits. A sealing head located at the lower end of the casing string allows a heavy barrier fluid to be placed between the borehole and the casing to a predetermined level. A choke valve in the return conduit is used to control the borehole pressure. Application of the two different fluids with optimised characteristics eliminates the need for using a subsea located mud lift pump. Simulations show that deep-water wells may be drilled and completed using only three or four casing strings. Due to reduced handling weights, reduced hole volume, etc., a low cost 2 rd or 3 rd generation drilling rig may be used. The total well-cost reduction may be in the order of 40–60% compared to conventional drilling systems.
In general, the main challenges in deep water drilling are as follows:
Handling of the large and heavy 21" marine drilling risers requires large 4 th or 5 th generation drilling vessels to be used, i.e., time consuming and heavy duty operations.
Low margin between the formation pore pressure gradient and the formation fracture pressure gradient requires high number of casing strings, i.e., many trips and time consuming operations.
A number of projects have been focused on reducing the above mentioned limitations:
Casing-while-drilling (CWD) is a method that integrates casing running and drilling.
The CWD process allows the casing strings to be installed in a well during the hole-making process where all, some parts of, or none of the required drilling tools can be recovered after the casing reaches the required depth (Brian and Sukup, 1999).
