A study on the hydrodynamic modelling of pontoon bridges is presented, and exemplified with the Bergsoysund Bridge. The structural modelling is based on the finite element method using the FE-software ABAQUS; the fluid-structure interaction, as well as the wave action, are modelled applying linearized potential theory represented by the DNV HydroD WADAM software. The main emphasis is put on the stochastic response analysis within the framework of the power spectral density method. The accuracy of the presented modelling is outlined. Convergence of the response obtained by modal technique is dealt with. By means of state-space representation and an iteration algorithm, the quadratic eigenvalue problem is solved. Contribution from the fluid-structure interaction to the overall modal damping is presented and discussed, and is found to be significant. Critical damping ratios close to 11%, corresponding to modes with natural frequency around 1 rad/s, are found. The displacement response is considered near-converged when approximately 30 modes are included.