mooring cable modeling with moody - aau
TRANSCRIPT
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Mooring cable modeling with MOODY
04-06-2014
• MOODY – software description
• Choosing suitable cable parameters – Filtering – End-point motion importance – Ground model – Parameter choice
• Results
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Software description
04-06-2014
• MOODY - developed as a Matlab® program package.
• A Finite Element Code with intermediate sinks and floaters, ground
interaction, drag forces and added mass effects.
• Validated against experimental data.
• Coupled to several different solvers for the motion of the device.
• A user’s manual has been produced during this project
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Modular approach
04-06-2014
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Software description
04-06-2014
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Software description
04-06-2014
• However: Bending stiffness is neglected, leading to numerical noise when the cable goes slack (at no tension).
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Choosing settings
04-06-2014
• Several parameters
• Only one target value: Tension force
• Axial stiffness • Added mass coefficients, CMn and CMt • Drag force coefficients, CDn and CDt • Ground stiffness, K • Friction coefficient, μ • Viscous force cut-off speed, vc
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Smoothing end point motion
04-06-2014
• 3-point moving average and cubic interpolation between sampled positions.
• Quasi-static results
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
End-point sensitivity
04-06-2014
• 3-point moving average filter and cubic interpolation between sampled positions.
• MOODY results
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Cable parameters
04-06-2014
• Decreasing cable axial stiffness – Allows larger time-steps – Gives smaller noise from measurement errors – Very small mean force difference
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Cable parameters
04-06-2014
• Decreasing axial stiffness – Allows larger time-steps – Gives smaller noise from measurement errors – Very small mean force difference
• Added mass -> small impact
– Chosen from Orcaflex manual. Standard values
• Drag forces -> small impact – Chosen from previous studies.
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Ground model settings
04-06-2014
• The ground is modeled as a bi-linear spring-damper.
• Vertical stiffness (Winkler module) chosen from 1 mm penetration depth at rest.
• Vertical ground damping is critical (ξ=1)
• Friction coefficient from literature, μ=0.3 between wet concrete and steel.
• Maximum friction force at vc = 0.1 m/s – Trade-off between large damping and high noise – Chosen to match the phase and noise level of experimental results
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Results – Long waves
04-06-2014
• Good phase and amplitude
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Results – Short waves
04-06-2014
• Overestimated force amplitude
Structural Design of Wave Energy Devices – w
ww
.sdwed.civil.aau.dk
Conclusions and outlook
04-06-2014
• Validation of mooring codes on slack mooring systems is problematic in experimental, lab-test scale.
– Sensitive to measurement errors – Ground interaction dominates response.
• Field test data or large-scale experiments are more suitable for validation
of cable codes and for tuning cable parameters.
• Future and ongoing work – Translation of MOODY into C++ – Further development and study of the ground interaction – Improvement of the interpolation-scheme of the API
