B747-400 ACFT ON OCEANIC RTE ENCOUNTERED AREA OF SEVERE WX AND HAD TO DIVERT AROUND WX. FLC HAD SOME DIFFICULTY PROGRAMMING THE FMC PROPERLY FOR OFFSET PLUS HIGH WORKLOAD. FLC FORGOT TO PLOT POS AND GIVE POS RPT. BOTH PLTS IN COCKPIT WERE FO'S AND THE CAPT WAS ON BREAK SLEEPING. FLT WAS FANS FLT.

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I WAS A RELIEF PLT AND HAD JUST ASSUMED MY SHIFT AFTER BEING ON BREAK. DISPATCH SENT US A MESSAGE VIA ACARS RECOMMENDING A REROUTE TO THE W OF CLRED CUS VIA 10 DEGS S 160 DEGS W TO AVOID CUMULO NIMBUS ACTIVITY ASSOCIATED WITH TROPICAL CYCLONE 'OSEA.' THE CAPT (JUST BEFORE LEAVING THE COCKPIT FOR HIS BREAK) SAID; THAT 'SHE (THE DISPATCHER) WAS OVERLY HELPFUL AND THAT MOST OF THE WX WAS BEHIND US AND WE SHOULD HAVE SMOOTH SAILING.' HE COULD NOT HAVE BEEN MORE WRONG. GOING BY WHAT THE CAPT SAID; I HAD THE OTHER (NON-FLYING) FO TELL DISPATCH THAT OUR CURRENT FLT PLAN LOOKED OK. WITHIN 15 MINS OF THAT; AN INTENSE; 100 NM WIDE RETURN STARTED TO GROW ON OUR RADAR 200 MI AHEAD. I TRIED TO CONVINCE THE OTHER FO THAT WE SHOULD ASK FOR 10 DEGS S 160 DEGS W REROUTE AS RECOMMENDED. HE THOUGHT IT WAS TOO FAR OFF COURSE AND SUGGESTED THAT IT LOOKED BETTER TO THE E. I AGREED TO THAT AND WE ASKED TO DEVIATE 40 NM L OF CUS. AS SOON AS WE STARTED L (S) THE LINE WE WERE TRYING TO GO AROUND APPEARED TO GROW ON OUR CRT. I HAD HIM ASK FOR 70 NM AND SOON FOR 99 NM (THE MOST THE SOFTWARE WILL ALLOW; THIS WAS A 'FANS' RTE). DURING THIS DEV IT GOT INCREASINGLY TURBULENT TO ALMOST CONTINUOUS MODERATE. THE OTHER FO WAS INVOLVED IN FANS COMS AS I WAS FLYING THE ACFT AND WE FORGOT TO GIVE A POS RPT AT 11 DEGS S AND 155 DEGS W. WE ALSO DIDN'T DO A PLOT WHILE DEVIATING. AS WE CLRED THE END OF THE CUMULO NIMBUS LINE AND ASKED FOR DIRECT TO 14 DEGS S 153 DEGS W; OR TO CONTINUE OUR L OFFSET FOR A WHILE LONGER; TAHITI RESPONDED 'UNABLE.' WE INQUIRED AS TO WHICH WAS UNABLE; THE DIRECT OR THE OFFSET; THE NEXT CTR; AUCKLAND ASKED FOR OUR POS WE WERE ABEAM 13 DEGS 27 MINS S 157 DEGS W (OUR FIR CHK POINT). SO WE GAVE AN ESTIMATE TO THAT POINT; AND AUCKLAND CLRED US TO GO DIRECT TO THE SUCCEEDING POINT; S 27 DEGS W 170 DEGS. DURING THIS DEV THE WORKLOAD SEEMED HIGH TO ME; BECAUSE THE OTHER FO (NON-FLYING) WAS HAVING SOME DIFFICULTY WITH THE FMC AND RESPONDING TO ATC. ON THE FMC WE MADE THE MISTAKE OF NOT 'EXECUTING' THE 99L OFFSET (JUST USING THE DOTTED LINE) AND THAT AFFECTED HIS ABILITY TO USE THE 'POS RPT' SCREEN TO RESPOND TO ATC QUICKLY. I THOUGHT ABOUT AWAKENING THE CAPT WHEN I REALIZED WE HAD MISSED A POS RPT AND A PLOT; BUT IT LOOKED LIKE SINCE WE WERE GIVING THE PRESENT POS TO AKL; THIS SIT WAS SOLVED. IN HINDSIGHT; I SHOULD HAVE ADVOCATED MORE STRONGLY TO FOLLOW DISPATCH'S RECOMMENDATION FOR A REROUTE REQUEST. I ALSO SHOULD HAVE BEEN MADE AWARE OF HOW THE OTHER FO WAS KEEPING UP ON THE POS RPT AND PLOTTING CHART.