# Feasibility for Damage Identification in Offshore Wind Jacket Structures through Monitoring of Global Structural Dynamics

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## Abstract

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## 1. Introduction

## 2. Damage Identification Approach

#### 2.1. Metrics and the Modal Assurance Criterion Equation

#### 2.2. Algorithms, MAC vs. Frequency

#### 2.3. Modal Flexibility Based Damage Metric

## 3. Case Study Structure and Model

## 4. Structural Model and Damage Modelling

#### 4.1. General Approach

#### 4.2. Joint Damage

#### 4.3. Pile Interaction

#### 4.4. Bolted Connection

#### 4.5. Corrosion and Marine Growth

## 5. Detectability Considerations

#### 5.1. Dependence on Nacelle Direction of Modal Properties in Intact Condition

#### 5.2. Choice of Damage Metric

^{4}, but not any of the other similar sharp reductions. While the MAC returned to a relatively high value, the damage index continued to increase as more damage was implemented, therefore, the damage index is a better choice for quantifying the level of damage.

## 6. Results

#### 6.1. Joint Damage

#### 6.1.1. Joint Loss

#### 6.1.2. Damage at Varied Nacelle Direction

#### 6.2. Soil

#### 6.2.1. Global Scour

#### 6.2.2. Displacement Factor

#### 6.2.3. Characteristic Strength

#### 6.3. Bolted Connection

#### 6.4. Corrosion and Marine Growth

#### 6.4.1. Corrosion

#### 6.4.2. Marine Growth

## 7. Discussion

## 8. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Diagram of damage process used to map damages implemented in a structural model with structural response caused by a change in the structure.

**Figure 2.**Example case showing the eigenfrequencies of the first 10 modes as the stiffness of a leg element is reduced from design stiffness to complete loss of stiffness.

**Figure 3.**Shapes of first five modes along with their respective frequencies. Colored by total displacement. (

**a**) Tower modes 1 and 2, (

**b**) torsional mode 3, (

**c**) swaying modes 4 and 5.

**Figure 4.**Representation of the structure in ROSA, including appurtenance, sea and soil. Labels of joints presented.

**Figure 5.**Modal assurance criterion (MAC) values for the undamaged case as nacelle direction is rotated in comparison to 0 degrees. Comparison is made to the original direction at 0 degrees. Mode shapes are transformed to the original reference.

**Figure 7.**‘Damage index’ as a result of changing nacelle direction, with no damage implamented. This comparative value is calculated in comparison to the nacelle at 0 degree case.

**Figure 8.**MAC values for the first five modes as simulated damage to a leg element is implemented. (

**a**) Mode frequency value; (

**b**) MAC value, both with comparison based on mode frequency; (

**c**) mode frequency value; (

**d**) MAC value, both with comparison based on maximized MAC values.

**Figure 11.**MAC values for the leg damaged case at varying nacelle directions compared to undamaged case at 0 degrees. Mode shapes have been transformed back to original reference.

**Figure 12.**Effect of gradually increased depth of global scour, compared to 0 scour depth. Modes compared based on maximized MAC (

**a**) Difference in natural frequency, (

**b**) MAC value. Scour depth in meters for the 23 m × 23 m jacket.

**Figure 13.**Effect of displacement factor compared to a factor of 1. (

**a**) Difference in frequency, (

**b**) MAC value.

**Figure 16.**Effect of corrosion relative to the uncorroded structure. (

**a**) Change in frequency, (

**b**) MAC values.

**Figure 17.**Effect of marine growth relative to the unfouled structure. (

**a**) Change in frequency, (

**b**) MAC values.

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**MDPI and ACS Style**

Richmond, M.; Smolka, U.; Kolios, A. Feasibility for Damage Identification in Offshore Wind Jacket Structures through Monitoring of Global Structural Dynamics. *Energies* **2020**, *13*, 5791.
https://doi.org/10.3390/en13215791

**AMA Style**

Richmond M, Smolka U, Kolios A. Feasibility for Damage Identification in Offshore Wind Jacket Structures through Monitoring of Global Structural Dynamics. *Energies*. 2020; 13(21):5791.
https://doi.org/10.3390/en13215791

**Chicago/Turabian Style**

Richmond, Mark, Ursula Smolka, and Athanasios Kolios. 2020. "Feasibility for Damage Identification in Offshore Wind Jacket Structures through Monitoring of Global Structural Dynamics" *Energies* 13, no. 21: 5791.
https://doi.org/10.3390/en13215791