Pressure balanced bumper sub

Claim 1

A COMPACT VIRTUALLY BALANCED DRILLING SUB FOR USE IN SUBSEA DRILLING OPERATIONS WITHIN A WELL BORE AND INSTALLATION IN A STRING OF DRILL PIPE ABOVE A DRILLING COLLAR AND TOOLS AND BELOW A FLOATING DRILLING PLATFORM, WHEREBY RISE AND FALL OF SUCH PLATFORM DOES NOT AFFECT THE EFFECTIVE DRILLING LOAD ON THE TOOLS, COMPRISING: AN EXTERNAL, CYLINDRICAL HOLLOW BARREL PROVIDED WITH A SMOOTH OUTER SURFACE HAVING AN OUTER DIAMETER VIRTUALLY EQUAL TO THE OUTER DIAMETER OF DRILL COLLARS OF THE DRILL STRING IN WHICH IT IS TO BE USED, AND MEANS AT ONE END THEREOF FOR CONNECTION TO A DRILL STRING; SAID BARREL HAVING A MAIN SECTION PRESENTING AN INTERNAL SURFACE OF UNIFORM INTERNAL DIAMETER AND CARRYING A PAIR OF LONGITUDINALLY SPACED INWARDLY EXTENDING ANNULAR SEALING MEANS OF SMALLER INTERNAL DIAMETER; A HOLLOW MANDREL IN ROTATIONAL ENGAGEMENT WITH SAID BARREL, SAID MANDREL BEING RETAINED IN SAID BARREL FOR LIMITED RELATIVE AXIAL MOVEMENT, MEANS AT ONE END OF THE MANDREL FOR CONNECTING THE SAME TO A DRILL STRING, SAID MANDREL HAVING AN AXIAL BORE FOR DRILLING FLUID AND INCLUDING A SECTION IN SEALING CONTACT WITH ONE OF SAID INWARDLY EXTENDING SEALING MEANS OF SAID BARREL, ANOTHER SECTION OF SMALLER OUTER DIAMETER IN SEALING CONTACT WITH THE OTHER INWARDLY EXTENDING SEALING MEANS, AND MEANS CONNECTING THE MANDREL SECTIONS AND INCLUDING A SEAL MEANS, SAID SEAL MEANS BEING IN SLIDING SEALING CONTACT WITH THE INTERNAL SURFACE OF UNIFORM DIAMETER OF THE BARREL; SAID SEAL MEANS, SEALING MEANS AND MANDREL SECTIONS COOPERATING WITH THE BARREL TO FORM TWO ANNULAR CHAMBERS OF DIFFERENT EFFECTIVE CROSS-SECTIONAL AREAS; PORT MEANS IN ONE MANDRELL SECTION COMMUNICATING ONE OF SAID ANNULAR CHAMBERS WITH THE DRILLING FLUID BORE OF SAID MANDREL; AND PORT MEANS IN THE MAIN BARREL SECTION COMMUNICATING THE OTHER CHAMBER OF LARGER EFFECTIVE CROSS-SECTIONAL AREA WITH THE EXTERIOR OF THE BARREL.