Abstract The studies related to the Structural Health Monitoring have been important to evaluate the operation condition of structures, and let reduce the maintenance costs. The modal strain energy method has been used for this goal. This method requires to compare the information of mechanical vibrations acquired from the structure vs the same structure in initial conditions, called structure in health condition. This work is focused on developing a methodology that differs from the traditional modal strain energy. The methodology allows us to identify and evaluate the damage severity using a severity indicator.The methodology has been implemented in an offshore structure. The results have been satisfactory for the damage quantification for single and multiple damages.