Patent ID: 8640778

Claim:
A system for drilling subsea wells from a Mobile Offshore Drilling Unit (MODU), comprising: a marine drilling riser arranged from the MODU to a seabed located Blow Out Preventer (BOP); a drill string arranged from the MODU through the marine drilling riser and BOP and further down a wellbore, an annulus being formed between the drill string and the drilling riser, and between the drill string and the wellbore, said annulus being filled with drilling mud to a low mud level where an interface is formed between the drilling mud and either gas or liquid that extends in the annulus above the drilling mud; at least one closing devise arranged either in the marine drilling riser, integral with the BOP, or in a Lower Marine Riser Package (LMRP), the closing device being able to close the annulus; at least one mud return outlet and mud conduit fluidly connected to the annulus either at a lower part of the marine drilling riser or below the drilling riser, said mud return outlet and mud conduit being connected to the annulus at a level below the low mud level and below said closing device, said mud return outlet and mud conduit being adapted to enable drilling mud to flow from the annulus to a subsea lift pump that via piping or conduits can pump the drilling mud to a location above sea level, said mud conduit including a choke line, a subsea choke valve being provided in said choke line, such that a choked flow of mud can be directed to the subsea lift pump via said gas separator; a well pressure regulator that is able to dynamically maintain a constant pressure in the annulus, said well pressure regulator being fluidly connected to the mud conduit, said well pressure regulator also including a pressure regulating conduit extending upwards from the seabed, or from a level near seabed level, through the sea to a level above sea level, adjustment of a height in the pressure regulating conduit of an interface between drilling mud and a gas or liquid being able to adjust and regulate the pressure in the annulus, said well pressure regulator including a gas separator, said pressure regulating conduit extending upward from said gas separator through the sea; and at least one pipe and one valve are provided that can by-pass said gas separator and connect the choke line directly to the subsea lift pump.