Substructure load is sometimes the most concerned wave induced load exerting on jacket structures. In computing these loads, one of the most sensitive parameters is the characteristics of the waves hitting the structure. These waves which are usually modelled as regular waves, are in reality random in nature. The present study aims to quantify the changes on the substructure when nonlinear random waves are employed in the calculation of these forces. A simplified jacket structure located in a uni-directional wave field is considered in the present study. Also, in the computation of substructure loads, Morison's equation is used. The wave loads simulated based on various wave theories are discussed in the present paper. © Published under licence by IOP Publishing Ltd.