UNEXPECTED SHORTFALL OF FUEL REMAINING AT OCEANIC CHECKPOINTS CAUSES B777-200 FLT CREW TO DIVERT TO AN INTERMEDIATE STATION FOR FUEL AND MAINT INSPECTION.

Narrative

I WAS PNF ASSIGNED TO THE LAST BREAK. I ARRIVED BACK TO THE COCKPIT IN PREPARATION FOR A LNDG. ONCE I GOT TO THE COCKPIT; THE CAPT ADVISED ME THAT WE WERE DIVERTING TO ZZZ2 FOR FUEL PROB. THE PROB WAS DESCRIBED AS FOLLOWS: AFTER OBSERVING SMALL; BUT NOT INSIGNIFICANT (FROM -.2 AND WITH A FAIRLY CONSISTENTLY INCREASING NUMBER THAT WENT TO -1.7) CONTINUALLY DECREASING FUEL SCORES AT OCEANIC CHKPOINTS FOR MOST OF THE FLT; AND NOTING THAT THE ACTUAL ZERO FUEL WT WAS 5700 LBS LOWER THAN THE FLT PLAN WAS BASED ON (AND THUS THE FUEL SHOULD HAVE BEEN FAR BETTER THAN THE FLT PLAN FUEL); THE CREW BEGAN HAVING SOME CONCERNS ABOUT THE AMOUNT OF FUEL ON BOARD. TWO ACARS MESSAGES WERE SENT TO DISPATCH REGARDING THE LOWER THAN EXPECTED FUEL SCORE AND THE PROJECTED LNDG FUEL AT ZZZZ1. DISPATCH ACKNOWLEDGED THE MESSAGES. FUEL QUANTITY ON BOARD CONTINUED TO DECLINE SOMEWHAT; THEN WHEN JUST ABOUT ABEAM ZZZ2; FUEL QUANTITY INDICATIONS AND PROJECTED LNDG FUEL QUANTITY DECLINED SIGNIFICANTLY. CAPT DECIDED TO MAKE A PRECAUTIONARY LNDG IN ZZZ2 (DECLARED EMER ON ACCOUNT OF POSSIBLE LOW FUEL/FUEL LEAK). LNDG WAS UNEVENTFUL. NO FUEL LEAK WAS SEEN BY FIRE TRUCKS OR MAINT. THE EMER WAS TERMINATED BY THE CAPT AFTER CLRING THE RWY. MAINT DEFERRED 3 PROBES IN THE R MAIN FUEL TANK AND ACFT DEPARTED AND LANDED UNEVENTFULLY IN ZZZZ1.