The pilot reported that when the aircraft was in the cruise at 3500 feet he heard a noise followed by severe vertical and less severe lateral vibration. The aircraft was descended and checks carried out in an attempt to ascertain the cause of the vibration. The cause of the vibration was not determined and the pilot decided to hover taxi the aircraft back to the platform. The aircraft was subsequently landed on a barge without further incident. An inspection of the aircraft found that one of the two lugs on a main rotor blade flapping hinge had failed. The lug fractured as a result of fatigue which originated adjacent to an area of fretting where the blade retaining pin passes through the lug. These lugs had been known to suffer from a defect discovered in service, and the manufacturer introduced a requirement to fit protective washers to the spindle during overhaul in order to prevent the initiating of cracking. The head from this aircraft had been overhauled by the manufacturer some months before the accident and the protective washers were not fitted. The operator had not carried out a recent Maintenance Manual amendment to inspect the lugs of spindles without protective washers. This inspection was not mandatory in Australia at the time of the accident.