Reliability of offshore platforms is an important issue in the prevention of environmental disasters. In this paper, the variation of the horizontal force exerted on an offshore gravity platform is analyzed to achieve a deep comprehension of a storm scenario. Considering the wave motion as a potential motion, the quasi-determinism theory is applied to obtain the kinematic characteristics of a storm. The evolution of force against time is analyzed through the Morison's equation, diffraction theory, and a simplified method. Moreover, two case studies are examined, one in the Ortona region of the Italian coast and the other in the Gulf of Alaska, USA.