B777 FLT CREW HAS ENG COMPRESSOR STALL AT ALT; DECLARES EMER AND DIVERTS FOR LNDG.

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THE FO WAS FLYING AND I WAS RUNNING THE RADIOS. BOTH RELIEF PLTS LEFT THE COCKPIT AS WE CLBED OUT OF 18300 FT. WE GOT A CALL FROM ONE OF THE RELIEF PLTS STATING THAT A FLT ATTENDANT WITNESSED SPARKS COMING OUT OF THE R ENG DURING THE CLB. HOWEVER THE RELIEF PLT DIDN'T SEE ANYTHING UNUSUAL AT THE TIME. IN THE COCKPIT WE DIDN'T HAVE ANY INDICATION OF ANY ABNORMAL ENG OP. THE ACFT SEEMED TO BE OPERATING NORMALLY WITHOUT ANY VIBRATION. WE WERE LEVEL AT FL250 AND IN CONTACT WITH ATC. PERHAPS 5-10 MINS PASSED WHEN WE HAD 'BANGING' AND VIBRATION COMING FROM THE R ENG. ENG INST INDICATIONS CONFIRMED THE R ENG WAS SURGING. I INFORMED ATC THAT WE HAD A PROB AND THAT WE WOULD LET THEM KNOW MORE INFO AS WE DEALT WITH THE SITUATION. ATC COOPERATED FULLY. I TOLD THE FO TO CONTINUE FLYING AND I WOULD RUN THE CHKLISTS. DURING THE R ENG COMPRESSOR STALL CHKLIST; THE RELIEF PLTS RETURNED TO THE COCKPIT. ONE OF THEM STATED THAT SHE SAW A 10 FT FLAME EMITTED FROM THE R ENG. SINCE WE HAD A FULL COMPLEMENT OF CREW MEMBERS PRESENT; ONE OF THE RELIEF PLTS TOOK THE RADIO CALLS AND BACKED UP THE PF WHILE THE OTHER RELIEF PLT BACKED ME UP WHILE I RAN THE CHKLIST. THE RELIEF PLT BACKING ME MADE A SATCOM CALL TO DISPATCH. AFTER GETTING DISPATCH ON THE PHONE HE REQUESTED MAINT TO ALSO BE IN ON THE CALL. WHEN MAINT GOT ON THE PHONE I EXPLAINED THE PROB AS COMPRESSOR STALLING AND VIBRATION. MAINT DIDN'T SEEM TO HAVE ANY INDICATION OF ANYTHING ABNORMAL. I EXPLAINED THAT AFTER FOLLOWING THE CHKLIST THAT WE WOULD GET COMPRESSOR STALLING AFTER THE ENG WAS ACCELERATED SLOWLY FROM IDLE TO JUST OVER 50% N1 AND THE COMPRESSOR STALLING CONTINUED UNTIL THE ENG WAS RETURNED TO IDLE AT WHICH POINT THE ENG WOULD OPERATE NORMALLY WITHOUT COMPRESSOR STALLING. MAINT AGREED THAT WE SHOULD OPERATE THE ENG AT IDLE AND NOT SHUT IT DOWN. DISPATCH GAVE US ZZZZ1 OR ZZZZ2 AS ALTERNATES AND I ELECTED TO DIVERT TO ZZZZ2. DURING THIS TIME THE FLYING FO REQUESTED FL190 DUE TO ACFT PERFORMANCE. DISPATCH CHKED PERFORMANCE NUMBERS REGARDING OUR WT AND CONFIRMED THAT WE MET PERFORMANCE CRITERIA EVEN AT OUR WT OF APPROX 566000 LBS. OBVIOUSLY STRUCTURAL WT RESTRS WOULD BE EXCEEDED BUT THE DISPATCHER AGREED WITH ME THAT DUMPING OF FUEL WOULD NOT BE REQUIRED. AFTER FINISHING THE SATCOM CALL; I INFORMED ATC THAT WE WERE DECLARING AN EMER AND REQUESTED CLRNC TO ZZZZ2. CLRNC WAS GIVEN WITHOUT DELAY. I INFORMED THE CREW THAT I WOULD MAKE THE LNDG. A RELIEF PLT GOT THE ATIS. WE WERE SURPRISED THAT THE WINDS WERE 30+ KTS GUSTING TO 41 KTS. WE WERE COMMITTED NOW AND AFTER CALCULATING THE XWIND LNDG WIND COMPONENT OF 20 KTS; WE FELT THAT IT WAS SATISFACTORY. DURING THIS PERIOD ONE OF THE RELIEF PLTS INFORMED THE FLT ATTENDANTS AND THE PAX OF OUR DECISION. THE FLT ATTENDANTS MADE A CABIN ADVISORY. I BRIEFED THE APCH AND TOOK THE CTLS BECOMING THE PF. WE WERE CLRED DIRECT TO THE IAF AND FLEW THE APCH ON AUTOPLT AND AUTOTHROTTLE TO APPROX 1000 FT WHEN I DISENGAGED THE AUTOPLT. BY THIS TIME THE WIND SPDS HAD DROPPED DOWN INTO THE 20 KT RANGE AND THE RIDE WAS SMOOTH. THE ACFT TOUCHED DOWN AT ABOUT A 320 FPM RATE AND AT AN INDICATED SPD OF 172 KTS; FLAPS 20 DEGS. A NORMAL ROLLOUT FOLLOWED WITH AUTOBRAKES AT LEVEL 3. WE EXITED THE RWY WITHOUT ANY COMPLICATION AND TAXIED TO THE GATE.