Subsea wellhead system and method for drilling shallow water flow formations

A subsea well assembly comprising:

• a first wellhead housing (26) secured to a first string of pipe (14) which is cemented into earth formations, said wellhead housing (26) having a sidewall with a plurality of flow ports (29), said first wellhead housing (26) having an internal landing profile (46);
• a second wellhead housing (32) secured to a second string of pipe (30) which extends through said first string of pipe (14), said second wellhead housing (32) having an external landing profile (46) by which said second wellhead housing (32) is supported on said internal landing profile (46) within said first wellhead housing (26), said first and second wellhead housings defining an annular space (33) which communicates with said flow ports (29) of said first wellhead housing (26) and with a return path between said first and second stings (14, 30) of pipe;
• a seal assembly (50), including a seal (51), disposed at least partially in said annular space (33), said seal assembly (50) characterized by being selectively capable of being placed in said annular space (33) such that said seal (51) is above said flow ports (29) for allowing cement returns from between said first and second strings of pipe (14, 30) to exit said first wellhead housing (26), and of being placed such that said seal (51) is below said flow ports (29) for sealing said annular space (33) from formation water flow from said return path between said first and second strings of pipe (14, 30);
• a running tool assembly (111) selectively capable of being controlled via a hydraulic path from a surface vessel (200) supply (102) of hydraulic pressure for forcing said seal assembly (50) downward in said annular space (33) until said seal (51) is below said flow ports (29);
• and
• a feedback mechanism coupled to said running tool assembly (111) and to said seal assembly (50) for generating a pressure signal at said surface vessel (200) in said hydraulic path, said pressure signal being representative of the position of said seal (51) in said annular space (33).

A method of casing a subsea well comprising the steps of:

• installing a first wellhead housing (26) secured to a first casing string (14) into earth formations of a first borehole drilled to a depth above a shallow water flow zone (9), where said first casing string (14) is cemented into said borehole and said first wellhead housing (26) has a side wall with a flow port (29),
• drilling a second borehole through said first casing sting (14) through lower earth formations to a depth below said shallow water flow zone (9),
• with a running tool (111) lowered by means of drill pipe (95) from a surface vessel (200), where said running tool (111) carries a second wellhead housing (32) having a second casing string (30) secured thereto and carrying a seal assembly (50), installing, with a single lowering of said running tool with said drill pipe,
• said second wellhead housing (32) with said second casing string (30) attached thereto such that said second casing string (30) is placed in said second borehole and said second wellhead housing (32) is landed in said first wellhead housing (26), said first and second wellhead housings being arranged and designed such that when said second wellhead housing (32) is landed within said first wellhead housing (26), an annular space (33) is defined which communicates with said flow ports (29) of said first wellhead housing (26), and
• a seal assembly (50) including a seal (51) in said annular space (33) at a position such that said seal is above said flow ports (29).