## 1 Introduction
Other notable works include the application of the PISA design model to monopiles embedded in layered soils [10]. In contrast, more complex frameworks, such as the one provided by the OpenFAST code, are used for studying aerodynamic, hydrodynamic, elastic, and structural characteristics in wind turbines. OpenFAST has been widely used for the detailed analysis and optimization of monopile designs, as the fact that it is open-source software allows for its continuous evolution. The first study by [PERSON] [11] investigates the coupled dynamic response of OWTs using this comprehensive aero-hydro-servo-elastic simulation tool, highlighting the importance of considering the interaction between aerodynamic, hydrodynamic, and structural dynamics. [PERSON] et al. [12] conducted an optimization study of a tuned mass damper for large monopile wind turbines. These studies demonstrate the capabilities of OpenFAST in capturing complex interactions, such as soil-structure interaction effects. Other significant contributions include a study by [PERSON] [13] on the dynamic responses of offshore wind turbines using FAST, and research by [PERSON] et al. [14] comparing FAST results with previous international code-comparison projects. Additionally, the work by [PERSON] [15] on the validation of FAST for OWT applications is also representative of the noteworthy capabilities of this software. The choice of the structural model depends on the type of problem, its complexity, and the desired level of precision. Different numerical models should not be considered mutually exclusive in their application. In the initial stages of structural design for wind turbines, simplified models are an appropriate tool to begin addressing the problem using minimal computational resources. Their simplified nature also reduces complexity in data processing and requires less time. However, for detailed structural analysis in later stages, time-domain multi-physics tools provide a broader and more detailed approach, allowing for a deeper study of the problem. However, the comparison between a simplified model approach and more rigorous time-domain multi-physics tools (software such as OpenFAST, HAWC2 or Bladed, among others) is crucial to validate the accuracy of these models and evaluate their reliability. In this field, to the authors' knowledge, only [PERSON] et al. [16] have conducted a study comparing OpenFAST with simplified formulas for environmental load combinations described by [PERSON] et al. [6]. They focused on comparing maximum bending moments at the mudline level and shear forces at hub height for extreme environmental conditions. The objective of this paper is helping to elucidate whether or not a simplified static-equivalent model can be used to predict major trends in the dynamic structural response of monopile OWTs. To do so, this study presents a comparison between the results of two numerical models of different levels of complexity used for estimating the structural response of OWTs under dynamic loads: the advanced time-domain multi-physics numerical tool OpenFAST and a simplified static-equivalent model based on beam elements and lumped masses. The results of this simplified model are compared to those of the baseline OpenFAST model in terms of fundamental frequencies, and shear forces, and bending moments along the system. To achieve this, a series of simulations are conducted, including scenarios with only wind loads and with combined wind and wave loads, corresponding to normal operation and extreme conditions. The present paper is structured into five sections. After this introduction, Section 2 presents the methodology, detailing the two numerical models used, including soil-structure interaction phenomena. Section 3 describes the properties of the OWT, the environmental conditions adopted in this study, and the complete set of cases analyzed. Section 4 discusses the results obtained from both numerical models.