Scour holesScour protection: Effect on wave loads

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Scour holes/Scour protection Effect on wave loads

• EWEC 2007 MILANO

Erik Asp Hansen Erik Damgaard Christensen
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Outline of the presentation

• Added mass for a vibrating cylinder
• (Related to dynamical analyses of the
• entire wind turbine)
• Description of inertia and added mass
• Three deflections of the cylinder
• Effect on damping
• Scour hole effect on loads
• scour protection effect on loads
• (Better estimation of wave loads)
• Method based on CFD

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Two different tools used WAMIT

• WAMIT (Wave analyses MIT, it was developed at
  MIT)
• A first order panel method
• It solves diffraction and radiation problems for
  offshore structures
• It can be used to analyse floating body motions
• In this study the added mass is determined by
  WAMIT

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Two different tools used NS3

• Navier-Stokes solver in 2D and 3D
• Finite volume approach
• Multi-block domain
• Structured grid in each block
• Free surface, Volume of Fluid (VOF)
• Forces on hydraulic structures

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Two different tools used NS3

• Animation

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A comparison A non-cylindrical structure
Regular waves Wave period 9.4 s Water depth
25 m Large diameter 15 m Small diameter
5 m
Panels for WAMIT calculations
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Inertia and added mass

• Definition of inertia coefficeint
• Can be said to be a combination of Froude-Krylow
  force and added mass.
• The Inertia coefficient for a large vertical
  cylinder

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Phase difference
Why do we get a phase difference?
The phase difference is partly due to effect
of diffraction phase difference and size of
added mass
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Added mass

2D cylinder in an infinite large volume of
water Ca is equal to one and
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Three types of deflections
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Inertia and added mass
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The horizontal force
Often
is used though not in general correct gt
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CFD calculations
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Effect of scour protection on wave load TEST A
Calculated horizontal forces and overturning
moment for water depth h10m, wave period T8s
wave height H5.0m,
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Effect of scour protection on wave load TEST B
Calculated horizontal forces and overturning
moment for water depth h10m wave period T8s
wave height H6.0m
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Effect of scour protection on wave load TEST C
Calculated horizontal forces and overturning
moment for water depth h10m wave period T8s
wave height H5.0m current velocity V1m/s
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Conclusions

• Based on the calculation presented, it can be
  concluded that for offshore wind turbines the
  added mass is not related to by the equation.
• This will influence the dynamics of the structure
  and might give wrong estimates of the frequencies
• The wave damping could have a major influence as
  well
• The horizontal forces are affected by the
  presence of a scour hole or a scour protection.
• scour protection will reduce the horizontal
  forces
• scour protection will reduce the horizontal
  forces
• but reduce the overturning moment compared to the
  plane bed situation
• A scour hole will increase the horizontal forces
• but reduce the overturning moment compared to the
  plane bed situation