Date: 201706
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B777 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Human Factors; Procedure; Aircraft
Primary problem: Ambiguous

Narrative: Shortly after arriving at the aircraft a mechanic came to the cockpit and stated they needed to do some work because the fueler couldn't get the correct amounts in the tanks due to fuel shutoff valves closing. This isn't unheard of so I was not concerned. After stepping back to say hi to the flight attendants I saw the mechanics had opened the equipment access hatch in first class. This I have never seen before to work a fuel transfer issue; but again I wasn't concerned.Shortly afterwards the fuel issue was resolved and the fuel sheet was brought to the cockpit. At this point I noticed the fuel load of Left 63.1 Center 118.0 Right 63.1. I took note of it because it was exact (left and right) this is unusual but good; I thought to myself the mechanics did a perfect job!About 45 minutes later when preparing for the before push checklist I noticed the fuel was now 2000 lbs lower in the left tank and 2000 lbs higher in the center tank. Also after my before push flow the center right fuel pump should have load shed but this time it didn't. Right away I thought something in the fuel system is still set to transfer fuel or was left 'connected' between the left main and the center tank.After having maintenance come back out to the aircraft they looked the plane over made some adjustments (I am not sure what exactly they did) the fuel was again balanced and the center right fuel pump now load shed like it should.At this point we believed the issue was corrected. We then taxied for takeoff. After takeoff when passing 10;000 an IRO (International Relief Officer) asked what was happening with the fuel. We brought the fuel synoptic up and shockingly the left tank was now 3000 lbs higher than it was at the gate.We decided the best course of action was to get up to altitude before an in-depth discussion. After getting close to cruise the Captain called Dispatch and [Maintenance] on the satellite phone and I was in charge of flying and talking to ATC. The Captain; with input from the IROs; Dispatch; and [Maintenance] attempted to understand and remedy the situation. After quite a while it became apparent that none of the crew was comfortable taking the aircraft over the Pacific Ocean like it was.We elected to return to [departure airport] for further maintenance work. The crew was also not comfortable jettisoning fuel which would open more valves and further complicate what we considered an 'unknown' fuel system. By now we had the system burning out of the center tank and decided to use delaying vectors altitude and landing gear to increase our burn to a weight we were comfortable with.After arriving at that weight we conducted the arrival and made an uneventful overweight landing.




Synopsis: B777 First Officer reported returning to departure airport after experiencing fuel system anomalies.