## 4 Design Load Analysis  
### Sources of Loads  
OWTs are exposed to several load sources usually imposed by their environment. In design codes, such as DNV-OS-J101 [71] and IEC 61400-3 [72], loads which are relevant to the design of OWT jacket support structures are outlined. Calculations and formulations of each load produced by the environment in this study are based on DNV-RP-C205 [73]. The loads relevant to OWT support structures can be roughly classified into six groups, i.e., (1) aerodynamic loads transferred from the rotor, (2) wind loads on the tower, (3) inertia loads, (4) current loads, (5) wave loads, and (6) hydrostatic loads [24]. These loads are illustrated in Figure 2.  
Gravitational and inertia loads are static and dynamic loads which result from seismic activity, the entire structure's weight and imposed equipment on the structure under operation, and the exerted forces on the structure due to operations [74]. The inertia loads, as a result of the support structure and rotor-nacelle assembly (RNA) weight at the tower top, can have a significant impact on buckling and considerably influence the eigen-frequencies of the OWT support structure [24]. Aerodynamic loads are dynamic and static loads resulting from the airflow interaction with the moving and stationary wind turbine components. The load magnitude varies, and this depends on the air density, the mean wind speed and turbulence across the rotor plane, and the aerodynamic shapes of the components of the wind turbine and their interactive effects. These loads can be computed using aeroelastic models [74]. The aerodynamic loads imposed by the rotor are transmitted to the top of the tower and are usually decomposed into a load matrix defined in the referential axis of the wind turbine. The fatigue loads were determined by employing the damage equivalent load (DEL) method, which was described in Reference [75]. The support structure was subjected to thrust load directed along the diagonal of the jacket base and applied at the assumed RNA center of mass (CM). This loading set-up resembled a yaw error situation and produced an additional torsional moment at the tower top. The presence of a horizontal offset for the RNA CM led to a gravity moment at the tower top [8]. Wave load and rotor thrust vectors were directed along the jacket base diagonal, causing maximum compression and tension on the opposite legs [5]. Environmental loads are those caused by environmental phenomena, i.e., the set produced by wave force, current force, and wind force. Wave loads are dynamic loads produced by waves and their interaction with the OWT support structure. [PERSON]'s graph indicates the validity range of each wave theory and can be employed to determine the most applicable theory for given characteristics of waves, such as wave period (T), wave height (H), and water depth (d) [74]. The regular wave theories are ideal for designs where a single wave method is adopted, and this is a widely used method in offshore structure design. In this case, a regular wave with the appropriate period and height is used to represent an extreme wave. This method provides an easy analysis in estimating the extreme response of offshore structures. On the other hand, the random ocean wave is defined by an energy density spectrum. The ocean wave energy content and its distribution over a random wave frequency range are described by the wave energy spectrum. Therefore, the random wave design method may be ideal for designing floating structures [76]. In this study, the wave profile data presented in Table 1 fall within the area of validity of the Stokes third-order wave theory. Stokes wave theory is one of the most commonly used methods in the analysis of offshore structures because of its accuracy in predicting the kinematic properties of the wave. A typical bottom-fixed jacket offshore structure is considered to be dominated by drag owing to the wavelength, water depth, structural shape, and size; thus, [PERSON]'s equation is adopted.