# Numerical Modelling of a Floating Wind Turbine Semi-Submersible Platform
## 1 Introduction  
Reducing carbon emissions is one of the main challenges that the world must face. This is reflected in the announcements that several governments have made to go carbon neutral in the next few decades [1]. One of the most popular ways to achieve this is the electrification [2] of all the energy sectors. This creates a unique scenario for renewable technologies, which are one of the most important protagonists of this transition. Among renewable technologies, wind energy has shown its maturity and continuous evolution, with an increase in the size of turbines [3] and expansion from onshore to offshore. Furthermore, in recent years, this transition has reached deeper areas, paving the way for floating systems to exploit a larger, and better-quality, resource while having fewer social impacts [4]. Despite this expansion, floating wind technology has some challenges that must be addressed to optimize its functioning. Offshore wind turbines are larger and more powerful than their onshore counterparts [5]. However, floating offshore wind systems, composed of the wind turbine, the floating sub-structure and the mooring lines, have a complex behaviour, as they are affected by environmental, aerodynamic, hydrodynamic, and mooring loads. This complexity is reflected in the turbine working at sub-optimal angles and in constant oscillation and movement. In pursuit of a better performance of these systems, several concepts of floatingplatforms have been developed, such as spar buoys [6; 7], semi-submersible platforms [8] and tension leg platforms (TLP) [9], among others. To study the design and behaviour of these systems, several approaches exist, which are reflected in different software packages [10]. Numerical simulations are used to design these floating sub-structures and reduce the total cost of the installation of this type of structures due to their complexity. Software packages that combine strip theory and panel methods are well-known in the industry, e.g., Orca Flex [11] and FAST [12], but they present some limitations due to their neglect of viscous effects [13; 14]. Another approach, which is gaining popularity in recent years due to the increase in computational power, is to apply high-fidelity numerical models such as Computational Fluid Dynamics (CFD), which combined with six degrees of freedom (6 DoFs) present accurate solutions that only require the geometry and the mass properties of the platforms. These numerical models, based on the Reynolds-Averaged Navier-Stokes (RANS) equations, are used to overcome the problem of the viscous effects [15]. Some researchers have compared both approaches [8; 13], achieving good agreement between both methods. It is important to highlight that there are more approaches in CFD, such as Direct Numerical Simulations (DNS), which directly apply the Navier-Stokes equations. However, this approach is very computationally expensive, and it is mainly done in academia. Another approach for turbulent modelling is the Large Eddy Simulation (LES) approach, which resolves the large-scale turbulence while modelling the small isotropic turbulent scales. For this article, the RANS approach has been used due to its maturity and its good computational cost in relation to the accuracy of the method, besides its very strong performance coupled with the maturity of the turbulent models developed around this approach. Along these lines, comparisons of different approaches for the simulation of a numerical wave flume have been done to have a better understanding of how different types of turbulence modelling could affect the behaviour of regular waves and the reflection process due to the existence of an extinction system [16]. The first studies that were carried out around CFD simulations were focused on cylindrical floaters [17]. In recent years, more complex simulations have been carried out to better understand the behaviour of these types of structures. For example, numerous articles have been published simulating different decay tests of different structures.