## 1 Introduction  
The worldwide energy demand is steadily increasing, therefore the supply must follow. Rising greenhouse gas emissions are moving energy suppliers focus heavily towards the renewable energy sector. Wind energy has become a major energy contributor to the renewable energy sector, the global wind capacity in 2018 reached 540 GW [1]. In 2018 a report from the International Energy Agency (IEA) estimated that the wind energy capacity is to increase to between 1700 GW and 2800 GW in 2040 [1], where the offshore wind energy capacity is estimated to be between 160 GW and 350 GW [2]. The wind energy sector is continually moving towards reducing the levelized cost of electricity (LCOE) to make wind energy more competitive with other sources of energy e.g. fossil fuels. Today, offshore wind turbine suppliers are interested in non-conventional offshore wind concepts such as floating foundations. The floating foundation offers the possibility to move the wind turbines out into deeper waters. This opens up for large installation areas where offshore wind turbines was not feasible before. At deep water locations there is often a more steady and continuous wind, which increases the full load hours during the wind turbines lifetime. Furthermore, some concepts of platforms for floating offshore wind turbines (FOWT) offers the possibility to tow out the assembled wind turbine for installation, and also tow it back for maintenance. This could lead to a reduction in installation and maintenance cost. But FOWTs are complex structures with multiple variables, which all contribute to a highly complex dynamical behavior: Simultaneous wind and wave loads, which are irregular by nature, hydrodynamic and aerodynamic couplings, flexible structures, a non-stationary platform and interaction with electrical components, these all make the design and control of the floating wind turbine a difficult challenge. To obtain a better understanding of the dynamical behavior of FOWTs there have been extensive research in the area. However, large-scale testing of FOWT are expensive to acquire and operate. Therefore, there has been an increasing focus to use small-scale models which have been experimented on in wave tanks and wind tunnels to validate their numerical models and the design of the structure. A review of some of the projects which addresses the subject of FOWT model validation by use of scaled model experiments is given. At MARIN, three different 1/50 scaled models were tested by the DeepCWind consortium led by the University of Maine in 2011 and in 2013 [3; 4]. The test was part of a model test campaign, which were to produce experimental data which could be used for validating offshore wind modeling tools. The FAST code has been calibrated and validated with use of these data [5; 6]. The initial tests in 2011 were on a Tension-Leg Platform (OC-4 TLP), semi-submersible (OC4-semi) platform and a spar-buoy (OC4-spar). These platforms used a rotor which was based on Froude scaling of the NREL 5 MW wind turbine. The platforms were tested in simultaneous wind and wave environments. Due to the geometrically scaled rotor, the Reynolds number was not scaled; as a consequence the aerodynamic performance not match the scaled aerodynamic performance. In 2013, a new rotor was developed, this was designed to replicate the aerodynamic loads of the 5 MW NREL wind turbine and a scaled flexible tower was designed. This new rotor and tower were mounted on the previously used OC4-semi platform from 2011 and retested. These tests showed that the scaled aerodynamic properties was better matched. In 2015, a 1/60 scaled model of a 10 MW wind turbine was developed to match the scaled rotor thrust and the OC-4 semi platform was modified to support this [7]. The scaled model was tested in different scaled operational conditions with simulations wind and wave loads and under extreme load conditions. In addition, a feedback controlled ducted fan was used to simulate the thrust force in some of the experiments.