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 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

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

Two different tools used: NS3 l Navier-Stokes solver in 2D and 3D l Finite volume approach l Multi-block domain l Structured grid in each block l Free surface, Volume of Fluid (VOF) l Forces on hydraulic structures

Two different tools used: NS3 Animation:

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

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:

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

Added mass 2D cylinder in an infinite large volume of water C a is equal to one and:

Three types of deflections

Inertia and added mass

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

CFD calculations

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,

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

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 heightH=5.0m current velocity V=1m/s

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 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