Windenergie 3 - Offshore 2

18 June 2026, Po Wen Cheng

Substructure Concepts

• Monopile is used in most cases
• If the seabed conditions dont allow it, or the water depth is too great, Jacket can be used (rarely)
• Tripods are not used any more, it has no advantages over the monopile concept
• Gravity based structure can only be used in shallow water and with small wind tubrines
• Suction bucket is more environmentally friendly, because it can be removed again without much lasting disturbance
• Future designs could utilize the floating platform concept

Hydrodynamic Loads

• Bottom-fixed substructure
  • Morison's equation
  • Hydrodynamically transparent structure (i.e. size of the structure << wave length)
  • The wave does not notice the structure
• Floating substructure
  • Mosison's equation to capture the viscous forces
  • No longer hydrodynamically transparent (structure too large)
  • Panel codes using potential flow theory to calculate the hydrodynamic and hydrostatic matrices
  • Floating structure assumed to be rigid

Morrison Equation

See Slide 13

• Hydrodynamic damping due to viscous drag force is only calculated for very flexible structures
• Added mass can be formulated as an additional term in the mass matrrix, which is considered in both rigid and flexible structures
• Application in different cases on Slide 16

Hydrodynamic Loads on Larger Structures

Methods of achieving stability

• Hydrostatic forces
  • Semi-Sub (Floating Jacket)
• Ballast (mass)
  • Spar
• Tension moorings
  • Tension Leg Platform TLP - has the least motion (very good)

Typically, a combination of these three will be used with one being most dominant

Hydrodynamic loads on larger substructures can not be ignored (Diffraction starts playing a role)

• Wave excitation forces: the platform is restrained from oscillating and there are waves: Froude-Kriloff force + diffraction force
• Radiation forces: assume still water, platform is forced to oscillate with wave excitation rfequencies: added mass + radiation damping

Dynamics of Floating Offshore Wind Turbines

• Rotational degrees of freedom are
  • Pitch (tower fore aft movement) (critical)
  • Roll
  • Yaw (critical)
• Translational degrees of freedom are
  • Surge (in direction of waves) (critical)
  • Sway (along the wavefronts)
  • Heave (upwards along the structure)

Non-linear viscous forces are important for slim structures / elements (e.g. tripod, jacket) and for extreme waves especially in shallow waters. For larger structures / elements (e.g. monopiles) or small waves, linear inertia forces dominate