and use the invention and do not limit the scope of the invention referring to fig1 an offshore oil and gas platform is schematically illustrated and generally designated 10 a semi submersible platform 12 is centered over a subterranean oil and gas formation 14 located below sea floor 16 a well 18 extends through the sea 20 penetrating sea floor 16 to form wellbore 22 which traverses various earth strata a wellbore extension is formed by wellbore 24 which extends from wellbore 22 through additional earth strata including formation 14 platform 12 has a hoisting apparatus 26 and a derrick 28 for raising and lowering pipe strings such as drill string 30 including drill bit 32 located in wellbore 24 and casing string 34 including drill bit 36 crossover subassembly 38 and downhole drilling apparatus 40 located in wellbore 22 as used herein the term u201c casing string u201d is used to refer to a tubular string which includes sections of casing or liner as in a typical drilling operation wellbore 22 is formed by rotating drill bit 36 while adding additional sections of pipe to casing string 34 when drill bit 36 reaches total depth however casing string 34 and drill bit 36 are not retrieved from wellbore 22 rather casing string 34 and drill bit 36 are cemented in place by cement 42 which fills the annular area between casing string.