## 1 Introduction
In the 1D beam model, the support structure is discretized into a sequence of elastic Euler or Timoshenko beam elements. It is computationally efficient and capable of accurately modeling global structural behavior, such as deflections and modal frequencies [21]. However, it is incapable of accurately representing the structural responses at a local scale such as stress concentration effects [22]. In the 3D FEA model, the jacket foundations are usually defined by applying brick or shell elements. In comparison to the one-dimensional beam model, the three-dimensional finite element model has the capability of examining detailed stress distributions and accurately capturing structural responses within the structure. The three-dimensional finite element analysis model was extensively applied tostructural modeling of wind turbine structures due to its high fidelity [1; 2; 23; 24; 25; 26; 27]. Thus, the 3D FEA model is adopted in this study to determine the structural responses of OWT jacket foundations. Due to the fact that OWT jacket support structures are embedded via piles into the soil, it is important to consider soil-structure interaction in order to adequately capture their structural response. A simple method to model the soil is the p-y method, in which the soil is modeled using equivalent springs with stiffness based on soil properties. However, this method underestimates the deflection and the modal frequency. An alternative way of modeling the soil is 3D FEA with brick elements, which is capable of providing accurate and reliable results [28; 29]. Considering the accuracy, 3D FEA with brick elements is adopted to model the soil later on in this study. Several studies [30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43] showed that structural reliability assessment is commonly applied to assess the safety level of OWT structures. Most reliability methods are established as time-invariant; however, in numerous instances, owing to the continuous degradation of the resistance of the structure through corrosion, fatigue, etc., or due to fluctuating loads in time, reliability models should be accounted for as time-variant [45; 46]. At first, the structural reliability assessment focuses on the evaluation of the reliability of each component, assuming a series system formulation where a single point failure illustrates a global failure of the structure [40; 47; 48]. In a second level, however, the embedded redundancy in the system is evaluated through methods such as push-over analysis, which is again based on the calculation of component reliability. Level I methods are deterministic reliability methods that only use one characteristic value to describe each uncertain variable. Common design standard formats, such as load resistance and allowable stress, belong to this category. These methods correspond to standard deterministic design methods. They can be combined with more advanced, higher-order methods in the case of partial safety factors calibration, which can optimize their application [49]. The conventional structural reliability methods include level II (approximate) reliability methods or analytical techniques which consist of the first-order reliability method (FORM) [50] and the second-order reliability method (SORM) [51], as well as level III reliability methods which include Monte Carlo Simulations (MCS). In FORM, the limit state function is approached through Taylor's expansion, and the problem of evaluating the reliability of a complex system is translated into a problem of mathematical optimization. SORM generally performs better in cases of highly nonlinear systems than FORM, while, in other cases, the two methods give similar results. MCS involves direct simulations and, hence, reduces the uncertainty in the results due to approximations in the solution algorithm. However, its application is constrained in the calculation of high probabilities for relatively non-complex engineering systems.