ACR FLT CREW HAS TXWY INCURSION AT ORD WHILE ASSESSING RWY LENGTH REQUIRED FOR ACFT WEIGHT.

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IN SHORT; I ENTERED A TXWY WITHOUT CLRNC. WE WERE DEPARTING THE GATE AT ORD. IT IS A PWR OFF THE GATE WITH A HARD R TURN WITH VERY LITTLE DISTANCE TO THE INNER TXWY. I BEGAN TO TAXI OFF THE GATE ANTICIPATING A HDOF FROM METERING TO GND WITH A QUICK CLRNC TO TAXI OUT TO OUR DEP RWY. HOWEVER; METERING KEPT QUESTIONING US ABOUT WHICH RWY WE WANTED FOR TKOF. WE WERE PLANNING A VERY HVY TKOF WT WITH TAILWINDS SO WE WERE SIMULTANEOUSLY TRYING TO DETERMINE A LEGAL AND SAFE RWY FOR HIM. THIS DISTR ME ENOUGH THAT I ENTERED THE INNER TXWY BEFORE RECEIVING CLRNC FROM GND. GND WAS TALKING TO OTHER ACFT SO WE WERE NOT ABLE TO CONTACT HIM RIGHT AWAY. AT THIS POINT I SAW NO ACFT COMING AROUND THE INNER SO I THOUGHT I COULD MAKE A QUICK R TURN ONTO THE RAMP TO CLR THE INNER; BUT AS I BEGAN THIS MOVEMENT A COUPLE OF REGIONAL JETS STARTED INTO AND OUT OF THE RAMP AHEAD OF ME. AT THIS POINT AN ACFT CAME UP BEHIND US ON THE INNER AND GND CTL THEN ASKED WHO WE WERE. WE IDENTED OURSELVES AND HE GAVE US TAXI CLRNC TO OUR RWY. I HAVE THOUGHT BACK ABOUT HOW I COULD HAVE ALLOWED MYSELF TO GET CAUGHT IN THIS SIT. WE WERE RUNNING LATE SO I ALLOWED MYSELF TO BE RUSHED AND I ALLOWED MYSELF TO BE DISTR WITH RWY CALCULATIONS. ALSO; WHEN I REALIZED I WAS ON THE INNER I WAS CONCERNED ABOUT STOPPING BECAUSE I WAS WORRIED ABOUT JET BLAST TO PERSONNEL AND EQUIP JUST BEHIND US. I ALSO KNOW THAT I SHOULD NOT HAVE PWRED OFF THE GATE BEFORE RECEIVING CLRNC FROM GND. THE ONLY EXPLANATION I CAN THINK OF IS THAT WE WERE BOTH FATIGUED. WE JUST HAD A VERY LONG DAY FOLLOWED BY A VERY SHORT NIGHT FOLLOWED BY THE CURRENT LONG DAY. I DON'T THINK WE WERE FUNCTIONING NORMAL. SUPPLEMENTAL INFO FROM ACN 796266: WE WERE PARKED AT THE GATE AT ORD. I HAD REVIEWED THE PROCS FOR GATE DEP FROM THE GATE IN MY CHICAGO COMPANY PAGES; ABOUT COORD WITH RAMP; METERING; AND GND CTL. I CALLED RAMP; AND WAS TOLD WE WERE CLRED TO PUSH WITH A SHORT PUSH. WE DID THAT. THE ACFT WAS HVY; ATIS WAS ADVERTISING A TAILWIND FOR RWY 32L. WE GOT THE CLOSE-OUT DURING THE PUSHBACK; AND WHILE IT WAS BETTER THAN EXECUTED; IT WAS STILL A HVY WT TKOF. WE SAT FOR A BIT DISCUSSING OUR OPTIONS ON WHICH RWY TO ASK FOR FROM METERING. I ASKED METERING FOR RWY 32R; AND WAS TOLD IT WAS ONLY 8700 FT IN LENGTH. I WAS LOOKING UP THE INTXN DATA FOR RWY 32L FROM TXWY M TO ENTER THAT INTO THE PERFORMANCE COMPUTER. SOMEWHERE IN THERE; METERING TOLD US TO GO TO GND; AND HE'D PASS ALONG OUR REQUEST TO GND. I FLIPPED THE SWITCH TO GND; STARTED TYPING AND THE NEXT THING I KNEW; GND CTL IS ASKING US OUR POS. I LOOKED UP; AND SAW WE WERE ON TXWY A. GND WAS NOT HAPPY. CONTRIBUTING FACTORS. FATIGUE. I SHOULD HAVE NOT GONE HEADS DOWN UNTIL AFTER WE GOT SQUARED AWAY ON THE TXWY. I NEEDED HOWEVER TO KNOW WHICH RWY WE WERE HDG TO BEFORE TALKING TO GND CTL.