A B777 DEVELOPED AN ACRID SMELL AFTER TKOF. THE CREW INVESTIGATED BUT THE SMELL DISSIPATED. AS THE FLT CONTINUED CLB; SMELL RETURNED. AN EMER WAS DECLARED WITH A RTN TO LNDG.

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AFTER TKOF; CLBING THROUGH APPROX 7000 FT RECEIVED EICAS SMOKE FORWARD LOWER CREW REST ACCOMPANIED BY VERY ACRID ODOR THROUGHOUT ACFT. SMOKE ALARMS WERE GOING OFF IN CABIN. COCKPIT CREW DONNED OXYGEN MASKS AND LEVELED ACFT AT 10000 FT. WHILE SORTING THROUGH THE CAUTION; THE EICAS MESSAGE WENT AWAY. ODOR STARTED TO DISSIPATE. COM ESTABLISHED WITH DISPATCH AND MAINT VIA SATPHONE/ACARS; AFTER A BIT OF CONVERSATION; ODOR DISSIPATED. FLT ATTENDANTS RPTED THEY WERE OK. FO WENT BACK THROUGH THE CABIN TO THE FLT ATTENDANT CREW BUNK AND RPTED ALL WAS NORMAL. DECISION WAS MADE TO CONTINUE TO ZZZ. AFTER RECEIVING CLRNC TO CLB AND ADDING CLB PWR; ODOR RETURNED VERY STRONG. LEVELED AT 14000 FT. FLT ATTENDANTS RPTED NAUSEA; HEADACHES AND DIZZINESS. DECISION WAS MADE TO DIVERT BACK TO ZZZ. VECTORS RECEIVED FROM ZZZ DEP AND APCH TO HOLD AND DUMP FUEL AT 14000 FT. DUMP TOOK APPROX 35 MINS. DUMP TO MAX LNDG WT DONE DUE TO XWINDS AT ZZZ AND WINDSHEAR ADVISORIES AND RPTS ON THE FIELD. EMER WAS DECLARED. CABIN ADVISORY FOR THE FLT ATTENDANTS. UNEVENTFUL LNDG. ODOR WAS VERY ACRID; BUT NOT EASILY RECOGNIZABLE. MECHS BACK AT ZZZ DESCRIBED IT AS AN OIL SMELL PROBABLY IN THE PACKS. SUPPLEMENTAL INFO FROM ACN 766869: THERE WAS A NOTICEABLE MECHANICAL ODOR IN THE CABIN WHEN WE BOARDED THE ACFT AT THE GATE. SOME COMPARISON WAS MADE TO EQUINE EXCREMENT (I'M NOT BEING SARCASTIC; IT REALLY DID HAVE THE ODOR OF HORSE FECES). THE SMELL COULD ALSO BE DESCRIBED AS SLIGHTLY MECHANICAL AND OILY; SIMILAR TO HYD FLUID BUT NOT PRECISELY THE SAME. ALL CREW MEMBERS I SAW COMMENTED ON THE SMELL AND PURSER ASKED CAPT IF HE WOULD COME BACK AND INVESTIGATE. HE DID BUT IT WASN'T APPARENT WHERE SMELL ORIGINATED; THERE WAS SOME DISCREPANCY WHETHER THE SMELL WAS COMING FROM ACFT OR JETWAY. IT IS NOT AT ALL UNUSUAL TO HAVE A STRANGE SMELL ON AN AIRPLANE. THIS SMELL WAS STRONGER THAN SOME BUT NOT STRONG ENOUGH TO CAUSE ANY UNDUE CONCERN ON MY PART. WITHIN A MIN OR SO AFTER TKOF I NOTICED A SIGNIFICANT INCREASE IN THE STRENGTH OF THE SMELL AND SEVERAL MINS AFTER TKOF WE RECEIVED AN EICAS REGARDING SMOKE IN THE LOWER CREW REST. WE ALL DONNED OXYGEN AND ESTABLISHED COMS. CABIN INTERPHONE BEGAN RINGING WITH FLT ATTENDANT ALERTING US TO A SMOKE WARNING THEY RECEIVED ON THEIR CABIN SVC PANELS. CAPT AND FLYING FO RAN A SMOKE; FUMES; ODOR CHKLIST. WE LEVELED AT 10000 FT AND GOT VECTORS FROM DEP SO WE COULD INVESTIGATE THE PROB WITHOUT GETTING TOO FAR FROM AN AIRFIELD. I WAS COORDINATING WITH FLT ATTENDANTS AND HAD DOOR 3R FLT ATTENDANT INVESTIGATE THE LOWER CREW REST BUNK ROOM. FLT ATTENDANT RPTED BACK THAT THERE WAS NO EVIDENCE OF ANY SMOKE OR FIRE. CAPT THEN REQUESTED THAT I EXIT COCKPIT AND INVESTIGATE THE LOWER CREW REST MYSELF. I TOOK THE SMOKE HOOD AND GOT A FIRE EXTINGUISHER FROM DOOR 3. MY INVESTIGATION REVEALED NOTHING OUT OF THE ORDINARY. IN THE MEANTIME THE STRENGTH OF THE SMELL HAD NOTICEABLY DIMINISHED. AFTER I RETURNED TO THE COCKPIT A SHORT DISCUSSION ENSUED AND WE DETERMINED THAT IT WAS SAFE TO CONTINUE. WE RECEIVED A CLB CLRNC AND AS WE BEGAN TO CLB THE STRENGTH OF THE SMELL RETURNED TO THAT OF AFTER TKOF. THE PURSER CALLED AND SAID PAX AND FLT ATTENDANTS WERE GETTING NAUSEATED BY THE SMELL. WE ALL AGREED THAT THE TIME HAD COME TO SOLVE THIS PROB ON THE GND. WE DECLARED AN EMER AND RECEIVED VECTORS FROM CTR FOR A SAFE SPOT TO DUMP FUEL. ENSUING FUEL DUMP AND RETURN TO ARPT WAS UNEVENTFUL. MY ONLY CONCERN IN REVIEWING THE EVENT IS THE DIFFICULTY IN HANDLING COCKPIT SECURITY IN THIS EVENT. I MADE A JUDGEMENT CALL TO QUICKLY EXIT THE COCKPIT WITHOUT A CART IN PLACE. I DID THIS KNOWING THE FORWARD LAVATORY WAS EMPTY AND HAVING SURVEYED THE HALL THROUGH THE PEEPHOLE IN THE COCKPIT DOOR. TIME WAS OF THE ESSENCE AND THE POSSIBILITY OF THIS BEING AN ENGINEERED SCENARIO TO BREACH THE COCKPIT SEEMED SO REMOTE AS TO BE IMPLAUSIBLE. PERHAPS I SHOULD HAVE TAKEN THE TIME TO HAVE THE FLT ATTENDANT PULL A CART AND FOLLOW SOP SECURITY FOR THE COCKPIT. THAT IS EASY TO SAY KNOWING THE EVENT TURNED OUT SATISFACTORILY; BUT THE JUDGEMENT CALL TO LEAVE THE COCKPIT WITHOUT FOLLOWING SOP SECURITY WAS MADE ONLY KNOWING THAT WE HAD A POTENTIAL FIRE ON THE AIRPLANE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT A CAUSE OF THIS SMELL WAS NEVER DETERMINED. IT WAS THOUGHT THAT AN ENG OIL SEAL WAS DEFECTIVE. THE ENG WAS CHANGED AS WELL THE APU. THIS EVENT HAS OCCURRED ON THIS ACFT THREE ADDITIONAL TIMES THAT THIS REPORTER KNOWS OF; EACH OCCURRENCE LEADING TO AN INFLT TURN BACK.