B767 FLT CREW INITIATED WX DEV IN OCEANIC ENVIRONMENT WITHOUT PRIOR ZNY APPROVAL.

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WE WERE IN OCEANIC AIRSPACE. ABOUT 70 MI W OF DIDLE INTXN; WE ADVISED ATC WE WOULD LIKE 15 MI R OF COURSE FOR WX AFTER DIDLE INTXN. THERE WAS A LONG LINE OF TSTMS ON THE AIRWAY BTWN DIDLE AND HUBER. JUST AFTER DIDLE; ATC GAVE US CLRNC TO DIVERT R OF COURSE 15 MI. WE STARTED TO DIVERT. WHEN WE WERE ABOUT 13 MI; I ASKED THE FO TO ASK IF WE COULD HAVE 25 MI. ATC TOLD US TO STAND BY. SEVERAL MINS WENT BY AND NO ANSWER FROM ATC. THERE WAS A LOT OF CONGESTION ON THE RADIO. ATC THEN CALLED US BACK AND SAID THAT WE COULD NOT HAVE THE 25 MI. ABOUT 2 MINS LATER; ATC THEN INFORMED US WE COULD NOT HAVE THE 15 MI HE HAD ALREADY CLRED US TO DO. HE TOLD US WE HAD TO RETURN TO OUR PREVIOUS RTE. WE ADVISED HIM WE WERE TURNING TO THE L TO RETURN TO OUR RTE. HOWEVER; THERE WERE TSTMS IN OUR WAY SO WE COULD NOT RETURN TO COURSE IMMEDIATELY WITHOUT RISKING THE SAFETY OF THE ACFT. WE DID RETURN TO OUR RTE SLOWLY TRYING TO AVOID WX. AFTER WE WERE BACK ON OUR RTE; WE COULD SEE THAT ABOUT 80 MI AHEAD OF US THERE WAS A TSTM RIGHT ON THE AIRWAY. AGAIN; WE TRIED TO REACH ATC. HF COMS WERE POOR. WE FINALLY GOT A HOLD OF THEM AND ASKED FOR A R DEV TO AVOID WX. THEY TOLD US TO STAND BY. AS WE NEARED THE WX; WE TRIED AGAIN;. NO ANSWER. I TURNED THE ACFT TO THE R TO AVOID THE WX. JUST AS WE PASSED THE WX AND STARTED OUR TURN TO THE L; ATC ADVISED US THEY COULD NOT GIVE US A R TURN; BUT WE COULD GO L. IT WAS OF COURSE TOO LATE BECAUSE WE HAD ALREADY PASSED THE WX AND WERE IN A L TURN TO RETURN TO OUR RTE. BEFORE TURNING THE ACFT; WE MADE SURE VISUALLY AND WITH THE USE OF TCASII THAT THERE WERE NO TFC CONFLICTS.