The group structure of records, containing abnormal waves, is studied by the non-stationary HHT method.
The envelope, phase and frequency, define the local wave instantaneous characteristics.
A refined definition of wave group is proposed considering that waves are amplitude and phase modulated.
The Hilbert spectrum is used as a detector of wave groups and group length and energy are determined.
It was found that the symmetrical abnormal waves appear in records with pronounced groupiness.