Abstract An offshore jacket platform in the South China Sea was impacted by a large derrick and lay barge during installation. This paper presents a non-linear dynamical analysis procedure for firstly determining the impact action based on the forensic evidence from the damaged components, and then evaluating the overall damage effects on the platform structure. The impact action of the barge is simulated with a triangle impulse load with different collision contact times. The curves relating the indentation deformations of the damaged member with different collision contact times are simulated using an estimated velocity of the impacting ship. On the basis of these curves and the actual detected dent damages, the contact time and the maximum impact load on the platform are determined. Taking into account the force–deflection relationship of the local indentation of the damaged cross-diagonal brace, the transmission of the impact load to the platform structure is simulated by a non-linear spring. The added mass coefficient with hydrodynamic effects and the pile–soil-structure interaction are considered in the computational model of the non-linear dynamic response of the platform structure. Subsequently, the dynamic response of the offshore jacket structure is computed and the critical stress and deformation of the tubular joints are obtained as indicators of the damage effects. The results are useful for choosing a feasible and reasonable repairing and strengthening scheme for the damaged platform. The procedure presented in this paper is generally applicable for the evaluation of typical offshore platform structures in the case of impact or collision.