Dynamically positioned, concentric riser, drilling method and apparatus

A dynamically positioned, concentric riser, deep water, drilling system comprising:

• a drillship operable to float upon the surface of a body of water and being dynamically positioned on station above a subsea well to be drilled into the seabed, said drillship having a moon pool extending through the hull to permit drilling from the drillship through the moon pool and into the seabed;
• a first marine riser extending from the drillship, through the moon pool, and to the bed of the body of water for supporting drilling operations into the seabed;
• a tensioning system extending between structure mounted on said drillship and an upper end of said first marine riser for supporting said first marine riser through the moon pool and into the body of water down to a well hole;
• a second marine riser having a higher pressure rating than said first marine riser and concentrically extending within said first marine riser from said drillship to the seabed for supporting drilling operations from said dynamically positioned drillship into the seabed including formations where pore pressure and the formation fracture gradient are similar;
• a surface blowout preventor mounted upon the upper end of said second marine riser for facilitating drilling and gas pressure control within said second marine riser;
• means for connecting the upper end of said second marine riser to the upper end of said first marine riser such that said first marine riser operably supports said second marine riser from said drillship;
• a lower marine riser package positioned at a lower end of said first marine riser, said lower marine riser package includes a connector connecting the second marine riser to said lower marine riser package;
• a subsea blowout preventor positioned at a subsea wellhead;
• and
• a connector positioned at the base of said lower marine riser package and above said subsea blowout preventor and being operable for release of the risers from the subsea wellhead below the lower marine riser package, and means positioned beneath the connector to close the well in the event of loss of station of the drillship and wherein drilling in deep water may be safely performed from a dynamically positioned drillship.

A dynamically positioned, concentric riser, deep water, drilling system comprising:

• a drillship operable to float upon the surface of a body of water and being dynamically positioned on station above a subsea well to be drilled into the seabed, said drillship having a moon pool extending through the hull to permit drilling from the drillship through the moon pool and into the seabed;
• a first marine riser extending from the drillship, through the moon pool, and to the bed of the body of water for supporting drilling operations into the seabed;
• a smaller diameter, second marine riser having a higher pressure rating than said first marine riser and concentrically extending within said first marine riser from said drillship to the seabed for supporting underbalanced drilling operations from said dynamically positioned drillship into the seabed including formations where pore pressure and the formation fracture gradient are similar;
• a tensioning system extending between structure mounted on said drillship and an upper end of said first marine riser for supporting said first marine riser through the moon pool and into the body of water down to the well hole including:
• a first plurality of hydraulic ram assemblies spaced equally about an upper end of said first marine risers and extending from a slip joint about said first marine riser to radial locations upon a superstructure of the drillship positioned above the moon pool wherein dynamic tension is applied from the drillship to the first marine riser by said plurality hydraulic ram assemblies;
• a cylindrical load shim coaxially extending about an upper end of said second marine riser between a load ring mounted at the upper end of the second marine riser and a load ring at an upper end of said first marine riser such that tension applied to said first marine riser by said plurality of hydraulic ram assemblies will be used to carry said second marine riser through said load rings and said load shim between the upper end of the first riser and the upper end of said second riser;
• a second plurality of hydraulic ram assemblies extending between superstructure of the drillship positioned above the moon pool and the upper end of said second marine riser to dynamically apply tension to said second marine riser independently of said first marine riser;
• a surface blowout preventor and a rotating head to accommodate underbalanced drilling operations through said second marine riser, mounted upon the upper end of said second marine riser for facilitating underbalanced drilling and gas pressure control within said second marine riser;
• a lower marine riser package positioned at the seabed, said lower marine riser package including:
• riser connectors operable to connect said first and second marine risers to said lower marine riser package;
• a subsea blowout preventor positioned below said lower marine riser package;
• and
• a connector operable to connect said lower marine risers to said subsea blowout preventor, said subsea blowout preventor being operable to close the well in the event of loss of station of the drillship and release of the risers from the well head at the lower marine riser package, wherein underbalanced drilling in deep water may be safely performed from a dynamically positioned drillship.

A method for drilling with a dynamically positioned, concentric riser system in deep water, said method comprising the steps of:

• dynamically positioning a drillship above a wellhole to be drilled;
• extending a first, low-pressure, marine riser through a moon pool in the drillship to a subsea wellhead;
• connecting the submerged end of the first, low-pressure, marine riser to a lower marine riser package;
• concentrically positioning a second, higher pressure, marine riser within the first, low-pressure, marine riser from the drillship to the lower marine riser package;
• connecting the lower marine riser package to a subsea blowout preventor positioned above the subsea wellhead;
• tensioning the first, low-pressure, marine riser from the drillship to provide operative support for the first, low-pressure, marine riser above the wellhole;
• supporting an upper end of the concentric, higher pressure, marine riser upon an upper end of the first, low pressure, marine riser;
• and
• providing a rotatable blowout preventor system at an upper end of the concentric, higher pressure, inner riser for conducting underbalanced drilling operations through the second, higher pressure, marine riser.