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Data-driven prediction of vortex-induced vibration response of marine risers subjected to three-dimensional current
Slender marine structures such as deep-water marine risers are subjected to
currents and will normally experience Vortex Induced Vibrations (VIV), which
can cause fast accumulation of fatigue damage. The ocean current is often
three-dimensional (3D), i.e., the direction and magnitude of the current vary
throughout the water column. Today, semi-empirical tools are used by the
industry to predict VIV induced fatigue on risers. The load model and
hydrodynamic parameters in present VIV prediction tools are developed based on
two-dimensional (2D) flow conditions, as it is challenging to consider the
effect of 3D flow along the risers. Accordingly, the current profiles must be
purposely made 2D during the design process, which leads to significant
uncertainty in the prediction results. Further, due to the limitations in the
laboratory, VIV model tests are mostly carried out under 2D flow conditions and
thus little experimental data exist to document VIV response of riser subjected
to varying directions of the current. However, a few experiments have been
conducted with 3D current. We have used results from one of these experiments
to investigate how well 1) traditional and 2) an alternative method based on a
data driven prediction can describe VIV in 3D currents. Data driven modelling
is particularly suited for complicated problems with many parameters and
non-linear relationships. We have applied a data clustering algorithm to the
experimental 3D flow data in order to identify measurable parameters that can
influence responses. The riser responses are grouped based on their statistical
characteristics, which relate to the direction of the flow. Furthermore we fit
a random forest regression model to the measured VIV response and compare its
performance with the predictions of existing VIV prediction tools (VIVANA-FD).Comment: 12 pages, presented at Norwegian AI Society Symposium 2019, accepted
for publication in Springer Conference Proceeding
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μ μ μνμλ€. μ§λ λΆμκ³Ό VIV κ²μΆ μλνλ₯Ό μν΄ μ΄λ―Έμ§ κΈ°λ°μ Object detectionμ μν΄ λ리 μ΄μ©λκ³ μλ CNN(Convolution Neural Network) μκ³ λ¦¬μ¦μ μ΄μ©νμλ€. λ³Έ μ°κ΅¬μμλ Object detectionμ μννλ Classificationμ μννμ§ μμλ λλ νΉμ§μ΄ μμ΄ μ΄μ νΉνλ CNN λͺ¨λΈ κ°λ°μ μν΄ Hyper parameterλ₯Ό μ‘°μ νμ¬ Hidden Layerλ₯Ό μ¦κ°νλ λ°©λ²μΌλ‘ 30κ°μ CNNλͺ¨λΈμ κ²ν νμκ³ μ΅μ’
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μΌλ‘ VIVμ κ²μΆμ΄ κ°λ₯ν¨μ 보μλ€. λ§μ§λ§μΌλ‘ VIVλ¬Έμ κ° λ°μνλ μμ μ΄λ°μ μ μμ΄μ μ€ κΈ°κ΄μ€ λ΄μμ κ³μΈ‘λ μ 체 ꡬ쑰 μ§λκ°μ μ΄μ©νμ¬ κ°λ°λ νμ§ μμ€ν
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μΌλ‘ νμ©μ΄ κ°λ₯ν κ²μΌλ‘ 보μ΄λ©° ν₯ν μ€μ λ°μ΄ν°κ° ν보λ κ²½μ° μ μ©μ±μ΄ μ¦κ°ν κ²μΌλ‘ κΈ°λλλ€Due to the International Maritime Organizationβs (IMO) regulations on carbon emission reduction, the shipbuilding and shipping industry increases the size of ships and adopts energy-saving devices (ESD) on ships. Accordingly, design changes of underwater structures such as propellers, rudders, and ESD of ships are required in line with these trends. The lock-in phenomenon caused by vortex-induced vibration (VIV) is a potential cause of vibration fatigue and singing of the propellers of large merchant ships. The VIV occurs when the vibration frequency of a structure immersed in a fluid is locked in its resonance frequencies within a flow speed range. Here, a deep learning-based algorithm is proposed for early detection of the VIV phenomenon. A salient feature in this approach is that the vibrations of a hull structure are used instead of the vibrations of its propeller, implying that indirect hull structure data relatively easy to acquire are utilized. The RPM-frequency representations of the measured vibration signals, which stack the vibration frequency spectrum respective to the propeller RPMs, are used in the algorithm. The resulting waterfall charts, which look like two-dimensional image data, are fed into the proposed convolutional neural network architecture. To generate a large data set needed for the network training, we propose to synthetically produce vibration data using the modal superposition method without computationally-expensive fluid-structure interaction analysis. This way, we generated 100,000 data sets for training, 1,000 sets for hyper-parameter tuning, and 1,000 data sets for the test. The trained network was found to have a success rate of 82% for the test set. We collected vibration data in our laboratory's small-scale ship propulsion system to test the proposed VIV detection algorithm in a more realistic environment. The system was so designed that the vortex shedding frequency and the underwater natural frequency match each other. The proposed VIV detection algorithm was applied to the vibration data collected from the small-scale system. The system was operated in the air and found to be sufficiently reliable. Finally, the proposed algorithm applied to the collected vibration data from the hull structure of a commercial full-scale crude oil carrier in her sea trial operation detected the propeller singing phenomenon correctly.CHAPTER 1. INTRODUCTION 1
1.1 Motivation 1
1.2 Research objectives 8
1.3 Outline of thesis 9
CHAPTER 2. PROPELLER VORTEX-INDUCED VIBRATION MEASUREMNT METHOD 24
2.1 Structural vibration measurement methods 24
2.2 Direct measuremt method for propeller vibration 26
2.3 Indirect measuremt method for propeller vibration 28
CHAPTER 3. DEEP LEARNING NETWORK FOR VIV IDENTIFICATION 39
3.1 Convolution Neural Network 39
3.2 Data generation using mode superposition 46
3.3 Structure of the proposed CNN model 50
3.4 Deep neural networks 53
3.5 Training and diagnosis steps 55
3.6 Performance of the diagnositc model 56
CHAPTER 4. EXPERIMETS AND RESULTS 76
4.1 Experimental apparatus and data collection 76
4.2 Results and discussion 78
CHAPTER 5. ENHANCEMENT OF DETECTION PERFORMANCE USING MULTI-CHANNEL APPROACH 98
CHAPTER 6. VORTEX-INDUCED VIBRATIOIN IDENTIFICATION IN THE PROPELLER OF A CRUDE OIL CARRIER 105
CHAPTER 7. CONCLUSION 114
REFERENCES 118
ABSTRACT(KOREAN) 127λ°
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