This paper presents details of the development and application of a numerical model, SPECMOD, for modal analysis of offshore lattice platforms, along with details of a structural monitoring programme carried out on such a platform. The model was developed using the finite element method. Three-dimensional beam elements were employed to represent structural members. The paper outlines how added mass, entrained water and marine growth on submerged piles are incorporated into the consistent mass matrix, having first linearized Morison's equation. The paper also outlines how the soil-structure interaction of foundation members is rep resented in the model. Kinsale Head Gas Production Platform Alpha, located off the south coast of ireland, is sited in approximately 92 m of water and is often subjected to harsh environmental loadings. The structural monitoring programme was undertaken to determine some of the dynamic characteristics of the platform. In particular the project team sought to estimate the five fundamental natural frequencies of the platform. The results from this project are presented in the paper. The finite element model, SPECMOD, was used to develop a numerical model of the platform and to predict its natural frequencies. Details of this model application are presented along with a comparison between the estimated and numerically predicted natural frequencies. The model was then used to determine the sensitivity of the natural frequencies to hydrodynamic masses, results from these sensitivity analyses are presented. (C) 1997 Elsevier Science Ltd.