What was found
AFTER BEING AT CRUISE PROBABLY 30 MINUTES THE ENGINE VIBRATION R CAS ILLUMINATED AND WENT BACK OUT. HAD THE EMER/ABNORMAL CHECKLIST IN HAND WHEN THE CAS REAPPEARED AND STAYED ON. RAN CHECKLIST. REDUCED POWER, DECLARED AN EMERGENCY & DIVERTED. THE ENTIRE TIME DURING THE DESCENT WITH POWER ALMOST AT IDLE THE ENG VIBRATION R CAS WAS STILL CYCLYING. DUE TO BEING 200 NM OFFSHORE, DECIDED NOT TO SHUT THE ENGINE DOWN UNLESS A VIBRATION WORSENED IAW THE CHECKLIST. DURING APPROACH/LANDING/TAXI-IN THE ENGINE VIBRATION CAS WAS NOT ILLUMINATED. R & R RT ACCELEROMETER WITH A NEW UNIT. OPS CHECKED SATISFACTORY IAW AMM 77-20-00.