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Scour holes/Scour protection: Effect on wave loads

Scour holes/Scour protection: Effect on wave loads. EWEC 2007 MILANO. Erik Asp Hansen Erik Damgaard Christensen. 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

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Scour holes/Scour protection: Effect on wave loads

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  1. Scour holes/Scour protection: Effect on wave loads EWEC 2007 MILANO Erik Asp Hansen Erik Damgaard Christensen

  2. 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

  3. 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

  4. 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

  5. Two different tools used: NS3 • Animation:

  6. 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

  7. 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:

  8. 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

  9. Added mass 2D cylinder in an infinite large volume of water Cais equal to one and:

  10. Three types of deflections

  11. Inertia and added mass

  12. The horizontal force Often: is used though not in general correct =>

  13. CFD calculations

  14. Effect of scour protection on wave load: TEST A Calculated horizontal forces and overturning moment for water depth h=10m, wave period T=8s wave height H=5.0m,

  15. Effect of scour protection on wave load: TEST B Calculated horizontal forces and overturning moment for water depth h=10m wave period T=8s wave height H=6.0m

  16. Effect of scour protection on wave load: TEST C Calculated horizontal forces and overturning moment for water depth h=10m wave period T=8s wave height H=5.0m current velocity V=1m/s

  17. 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

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