Flight Crew Commercial; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Flight Crew Took Evasive Action
Weather
I was the [Relief Pilot]. Captain and First Officer were on break. We received a severe turbulence segment on our outs of flight from FL240 to FL410. Requested deviation below which was denied and then to offset laterally 150 miles; but only given 120 miles. We considered weather deviation without a clearance procedures as well as declaring an [urgent situation]. As Captain was awake in the seat; and had requested we wake him in prior briefing should an event like this be considered; he came up to flight deck. I then sat in Observer's seat. He had [another Relief Pilot] CPDLC to ATC that we would be deviating to the lat/long of outer most point of severe turbulence segment which was a 148 offset. As this was not our clearance; I suggested the weather deviation without a clearance procedure as well as possibility of declaring an [urgent situation]. We had just been cleared from FL400 to FL390 while preceding on the south offset. The Captain did not want to go up or down 300 feet with the decency of the altitude clearance. We turned on all lights and broadcast our position and intentions on both 121.5 and 124.45. We talked to Dispatch on SAT comm. We reached Nadi Radio who said we were only cleared to offset 120 miles and we were at 148 miles. We received the same message over CPDLC. I spoke with Nadi Radio and said we were given no other options as we were flying through an area of forecast severe turbulence. We were then given a clearance through Nadi Radio to deviate up to 148 miles and received the same on CPDLC.
Air carrier First Officer reported needing to deviate around an area of forecast severe turbulence and was unable to get complete clearance to do so.