Rational assessment of wave loads for ultimate strength of ship structures requires appropriate numerical tools capable of dealing with physical phenomena related to severe environmental conditions. In addition to low frequency wave loads, vibration caused by wave action adds to the total wave loading of the primary hull structure. The thesis presents a numerical method for determining ship hull structural loads due to the action of waves, including the contributions from vibration of the fundamental vertical vibration modes of the hull. The proposed numerical method is based on a coupled numerical solution of the dynamics of fluid and structure. The fluid dynamics method is based on solution of the Reynolds-Averaged Navier-Stokes (RANS) equations combined with a multiphase fluid formulation, whereas the structural dynamics method relies on a simplified representation of the hull girder with a finite element beam. The fluid dynamics method is in principle well suited to capture nonlinear flow features relevant for time domain simulation of ships in severe or extreme environments due to the capability of capturing complex flows including highly disturbed free surfaces. Wave-wave interaction, wave breaking and wave impacts are implicitly accounted for. The coupled method presented here implicitly and consistently accounts for complex resonant excitation of hull girder vibrations in waves as well as impulsive vibratory excitation due to slamming. Fundamental tasks of the work documented herein are the improvement and validation of the coupled numerical method, including investigations of required computational accuracy and estimations of numerical uncertainties. Available computational procedures for obtaining statistical properties of ship loads in irregular waves are adopted to the needs of the computationally expensive numerical method, and applied in case studies.