Prediction of ultimate strength reduction caused by corrosion for offshore wind turbines

다국어 초록 (Multilingual Abstract)

The harsh environmental conditions of oceans cause significant pit corrosion on floating offshore wind turbines (FOWTs). Corrosion is a factor that decreases the ultimate strength of a structure, which can lead to structural failure in an aged structure. To overcome this issue, comprehensive ultimate strength assessment of these structures are required. The purpose of this work is to evaluate the ultimate strength performance of steel plates used on the body of FOWTs by considering the effects of pit corrosions. This study proposes a reliable method to predict the ultimate strength reduction (USR) caused by pit corrosion of a structure at various ages. This method can assist structural designers to consider the impacts of pit corrosion on the ultimate strength of the structures; thereby helping them to design more reliable and safer structures. The presented method has 6 steps and was assessed by applying it to the simplified example of a thin-plate panel structure. The ultimate strengths of the panel with different pit depths were calculated using Finite Element (FE) analysis. The effects of
material and geometric nonlinearities, as well as the initial imperfections and buckling were also considered. A linear regression model, representing the relationship between the USR and pit depth, was then developed. Furthermore, using the experimental data and probabilistic approach, statistical analysis was carried out to acquire the weighted mean value of USR for different ages of the panel. Employing the results of this statistical analysis, a plot to elucidate the connection between the USR and age of the structure was then produced. The ultimate strength of a structure may be decreased up to 33% due to pit corrosion after 27 years of service. The introduced stepwise approach contemplates the nonlinearity of pit corrosion with time by considering the experimental data. As corroborated by the successful application of the presented methodology, this method is more accurate, compared to accepted practices available in the literature.

목차 (Table of Contents)

• Abstract 5
• List of Tables 9
• List of Figures 10
• Preface 12
• Notations 13
• Predict Ultimate Strength Reduction caused by Corrosion for Offshore Wind Turbines 16
• 1. Introduction 16
• 1.1. Background and literature review 16
• 1.2. Objective 19
• 2. Proposed method: predict USR of structures with pit corrosion 20
• 3. Application of the proposed method on a panel 23
• Step 1: Loading and boundary conditions 23
• Step 1.1: Loading conditions 23
• Step 1.2: Plate edge boundary conditions 24
• Step 2: Material properties and geometric dimensions 25
• Step 2.1: Material properties 25
• Step 2.2: Geometry Dimensions 26
• Step 3: Pit corrosion experimental data 28
• Step 4: Initial imperfections 31
• Step 5: Ultimate strength analysis 32
• Step 5.1: 3D modeling and meshing 32
• Step 5.2: Nonlinear finite element analysis 33
• Step 5.3: Ultimate Strength Reduction (USR) 36
• Step 6: Relations of USR, pit depth, and structure age 43
• Conclusion 46
• Future Work 48
• References 49
• Appendix A. Nonlinear Buckling Analysis Results 52
• Abstract (Korean) 56