THIS PAPER IS FOCUSED ON THE ISSUES LIKE Structural damage detection, damage LOCALIZATION AND SEVERITY ESTIMATION, BASED ON COMPUTATION OF MODAL STRAIN ENERGY. damage OFTEN CAUSES A LOSS OF STIFFNESS IN SOME ELEMENTS OF THE STRUCTURE, SO, MODAL PARAMETERS, MODE SHAPES AND NATURAL FREQUENCIES, IN IMAGED STRUCTURE, ARE DIFFERENT FROM THE UNdamageD STATE. IN THIS PAPER, LOCATION OF damage IS DETECTED BY COMPUTING MODAL STRAIN ENERGY CHANGE RATIO (MSECR) FOR EACH Structural ELEMENT, WHICH ELEMENTS WITH HIGHER MSECR ARE SUSPECTED TO BE damageD. BY COMPUTING CROSS-MODAL STRAIN ENERGY FOR EACH SUSPECTED damage ELEMENT, SEVERITY OF damage AS THE STIFFNESS REDUCTION FACTOR IS ESTIMATED. NUMERICAL STUDIES ARE DEMONSTRATED FOR AN XISTING JACKET PLATFORM. IT IS OBSERVED THAT THIS METHOD CAN BE USED FOR damage detection OF THESE KINDS OF STRUCTURE.