B-777 First Officer reported a fuel imbalance during cruise flight in oceanic airspace; which lead to a determination of a possible fuel leak. The aircraft diverted to an alternate airport and landed.

Narrative

I was one of two relief pilots who was on duty on the flight deck at the time of the incident. About 5 and 1/2 hours into the flight we arrived at the flight deck to relieve the flying pilots. About 30 minutes after assuming duties we received a FUEL IMBALANCE EICAS. We ran the FUEL IMBALANCE checklist and noted an initial imbalance of 53.2 Left tank and 48.8 right tank. Per the checklist we noted the fuel totalizer about 700 pounds less than calculated fuel. The center fuel tank was empty; both cross feeds were closed and the center fuel tank pumps were both off. We initially thought we did not have a fuel leak and elected to balance the fuel via the xfeed procedure. As we were cross feeding fuel we noticed the right fuel quantity dropping about 100 pounds every 2 to 3 minutes. When we did get the fuel quantity balanced; the gauges read 48.3 left/ 48.3 right. We completed the FUEL IMBALANCE checklist and continued to monitor the fuel quantity and fuel totalizer/ calculated fuel comparison. After about 25 minutes we noted a fuel imbalance of 1000 lbs low in the right tank. We reviewed the FUEL IMBALANCE checklist and determined we had a suspected fuel leak an began the fuel leak unannunciated checklist. At this point we decided to alert the Captain who was on break. both flying pilots returned to the cockpit and we briefed them on the situation to that point and then contacted Dispatch and Maintenance Control via SATCOM. We concurred with Maintenance Control and Dispatch of the possibility of a fuel leak and elected to divert to ZZZZ. We secured the right engine per the Fuel Leak checklist; requested priority handling; executed the drift down procedures; and received a new clearance to ZZZZ. At the time of engine shutdown I noted that the fuel imbalance was 2000 lbs (about 45 minutes after the initial fuel rebalancing procedure) and totalizer fuel was 1300 lbs lower than calculated fuel on Prog 2 page. We notified the flight attendants per the briefing in the FOM and notified the passengers of the diversion to ZZZZ. We calculated single engine landing distance per the Flight Manual (FM); landed flaps 20 on Runway XX without incident; and taxied to the ramp. After the flight we debriefed where I learned that the flying pilots completed the FUEL LOW CENTER procedure about 10 minutes before we had arrived to the cockpit from our rest break. I then reviewed the supplemental section of the FM during the layover regarding the fuel anomalies on the C" model check valves and briefed the other pilots the next day."