Thesis
Fracture mechanics analysis of offshore wind turbine monopile structures for lifetime extension
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2022
- Thesis identifier
- T16538
- Person Identifier (Local)
- 201864107
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- As more early offshore wind farms reach the end of their design life, the wind industry needs to prepare for end-of-life decision making for offshore wind turbine (OWT). One idea gaining traction is lifetime extension, that is, operating an OWT monopile beyond its original lifetime if it can be shown that the monopile possesses structural reserves. Currently, the industry practise is to establish the extent of structural reserves by repeating the initial S-N fatigue calculation with updated actual environmental data, turbine operational data and improved simulation tools. However, this approach does not allow the integration of inspection results as the S-N calculation encapsulates the three stages of a structure’s fatigue life into a single value. Monopiles are large, welded structures. Thus, it is likely that an aged monopile already has a crack initiated. There may actually be multiple cracks on the same plane due to manufacturing defects from the time of fabrication or due to loading. Therefore, later life fatigue calculation must focus on the crack propagation phase. Crack propagation is analysed using fracture mechanics. It requires the calculation of stress intensity factor (SIF) which is a function of structure geometry, crack geometry and the applied load. The SIF can be obtained from handbook solutions for simple cases or by finite element analysis for complex cases. However, SIF by finite element analysis (FEA) is not practical for fatigue load cases because the procedure is computationally expensive. It has been shown that the SIF provided in handbooks is imprecise for large diameter pipes. This thesis investigated fracture mechanics approach to assessing the integrity of OWT monopiles with multiple cracks. The thesis proposes that suitability of cracked OWT monopile be assessed using a failure assessment diagram (FAD). The FAD simultaneously assesses both brittle failure and plastic collapse. The core of this thesis is twofold: 1) the development of an efficient and accurate method to calculate the SIF of an OWT monopile containing a semi-elliptical external surface crack subjected to arbitrary stress loading. The SIF is integral to the assessment of brittle failure. The approach proposed in this thesis is based on the theory of weight functions. 2) the development of a simplified and accurate methodology for calculating the plastic collapse (limit) bending moment load of an OWT monopile with external circumferential flaws. The limit load is integral to the assessment of plastic collapse. The proposed methodology is based on net section collapse (NSC) theory. Both methods allow the assessment of multiple cracks. For SIF, a new equation for the interaction of SIF between co-planar, circumferential, semi-elliptical, external surface flaws located in offshore wind turbine monopile is also derived. The effect of multiple cracks on plastic collapse is explicitly captured in the methodology for obtaining the limit load. As part of this thesis an application was written using the Visual Basic .Net programming language that incorporates the proposed methods. The new application is tested with two case studies to demonstrate its performance. The case studies demonstrate fast and accurate assessment of the integrity of cracked OWT monopiles. The results of this thesis provide a clear demonstration of the fracture mechanics approach to assess OWT monopiles for lifetime extension.
- Advisor / supervisor
- Brennan, Feargal
- Resource Type
- DOI
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PDF of thesis T16538 | 2023-04-12 | Public | Download |