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A new approach that models lift and drag hydrodynamic force signals operating over cylindrical structures was developed and validated.This approach is based on stochastic auto regressive moving average with exogenous (ARMAX) input and its time-varying form,TARMAX.Model structure selection and parameter estimation were discussed while considering the validation stage.In this paper,the cylindrical structure was considered as a dynamic system with an incoming water wave and resulting forces as the input and outputs,respectively.The experimental data,used in this study,were collected from a full-scale rough vertical cylinder at the Delft Hydraulics Laboratory.The practicality of the proposed method and also its efficiency in structural modeling were demonstrated through applying two hydrodynamic force components.For this purpose,an ARMAX model is first used to capture the dynamics of the process,relating in-line forces provided by water waves; secondly,the TARMAX model was applied to modeling and analysis of the lift forces on the cylinder.The evaluation of the lift force by the TARMAX model shows the model is successful in modeling the force from the surface elevation.