## 8 Conclusions  
In this study, a comparative analysis on the structural reliability of an OWT support structure between a commercial package and a purpose-developed, non-intrusive stochastic formulation was presented. Both methods start by defining the limit states taking account of the various design constraints. The design constraints considered in this study were as follows: stress (ULS), fatigue (FLS), deformation (SLS), buckling (ULS), and vibration (SLS). A 3D (three-dimensional) parametric FEA (finite element analysis) model of a typical OWT jacket-type support structure was developed, taking account of soil-structure interaction and stochastic variables (i.e., soil properties, wind, wave, and current loads). The series of FEA simulation results were post-processed using two methods. In the first method, the design of experiment and response surface module in the DesignXplorer(r) were used in order to map the response domain and then derive an appropriate response surface model. The probabilistic assessment was performed using the six sigma analysis function employing the Latin hypercube sampling method in computing the probability of failure, which was then translated to a reliability index. In the second method, multivariate quadratic regression was employed to derive the performance function expressed in terms of global stochastic variables using results from the FEA simulations. Next, FORM was employed to calculate the reliability index and then the probability of failure.  
The following conclusions can be drawn from the present study:  
* The two methods, i.e., the probabilistic tool package in ANSYS and the developed non-intrusive formulation, match well on the calculated reliability indices calculated for all limit states, cross-verifying applicability of both methods for the structural reliability assessment of OWT support structures.
* The proposed non-intrusive formulation for SRA allows faster calculations, assessment of time-variant fatigue reliability assessment, and calculation of low values of probability of failure in complex engineering systems such as offshore and marine structures.  
Figure 12: Global sensitivity chart: (**a**) stress design constraint (ULS); (**b**) deformation design constraint (SLS); (**c**) fatigue design constraint; (**d**) buckling constraint (ULS); (**e**) vibration constraint (SLS).  
* This study also allowed for verification of the non-intrusive formulation, which can allow multiple problems of complex nature to be solved probabilistically, alleviating the need for an integrated, application-specific tool to be developed, instead of with the proposed modular formulation.  
Writing--original draft preparation, methodology, software, validation, and formal analysis, A.A.S.; methodology, software, validation, formal analysis, and writing--review and editing, L.W.; methodology and writing--review and editing, A.M. and K.S.; supervision, methodology, software, validation, formal analysis, funding acquisition, conceptualization, and writing--review and editing, A.K. All authors have read and agreed to the published version of the manuscript.  
This work was supported by a grant from the Supergen Wind Hub EP/L014106/1, from the UK Engineering and Physical Sciences Research Council (EPSRC), under the Flexible Funding Scheme for the University of Strathclyde.  
Author [PERSON] would like to acknowledge the Petroleum Technology Development Fund (PTDF), Nigeria, for financial support.  
The authors declare no conflicts of interest.