Hydrodynamics

This section enables you to select the different models and algorithms that are applied during the simulation. In particular, the section Default coefficients enables you to change the hydrodynamic coefficients applied to the model. 

The hydrodynamic coefficients defined here are only applied on parameterized support sections. If you import your own support section, the hydrodynamic coefficients will be defined in the text file defining the support section.

Hydrodynamic loads

Algorithm for perpendicular loading

There are two alternative algorithms available to calculate perpendicular hydrodynamic loading.


Options:

Morison (default):

Loading is calculated as two components - drag and inertia - based on water particle velocity and acceleration. The movement of the structure can also be included in the equation.

MacCamy-Fuchs:

For large volume structures (large dimensions compared to wave length) the inertia term of Morison's equation becomes increasingly inaccurate. The MacCamy-Fuchs algorithm is an exact solution for a vertical, circular, bottom-fixed, surface piercing cylinder of any size. When implementing MacCamy-Fuchs, the inertia coefficient is set to 2. Thus, the inertia coefficents can not be set below. MacCamy-Fuchs should only be used with circular cross-sections (not rectangular) and for vertical surface piercing structures (typically monopiles).

Potential flow:

Loading is calculated using the Cummins equation. Frequency-dependent hydrodynamic coefficients, typically computed using potential flow theory, must be specified on the support section(s).

Integration time

The integration time when computing the potential flow radiation memory effect load.


  • Default value: 30
  • Unit: $\text{s}$
  • Range: 0 — 1e+07

Reduce load due to partly submergence

Reduce loading in the surface zone for elements that are partly submerged. If not checked, then the loading on a surface element is 100% or 0%.

  • Default value: 1
  • Unit: —

Moving structure for perpendicular drag?

Perpendicular hydrodynamic loads (drag and inertia) can be implemented assuming either a fixed or a moving structure. Whether movement (response) of the structure is taken into account, can be set independently for drag and inertia loading. This parameter only affects drag loading. Default: Partly

Options:

Fixed:

Perpendicular drag loading is calculated only based on water particle velocity at the initial (start) position of the structure. This is more numerically stable and is usually OK for bottom-fixed wind turbines as the movement is typically relatively small.

Partly (default):

Perpendicular drag loading is calculated based on water particle velocity and structural velocity at the current position the beginning of each time step. However, it is NOT updated during nonlinear iterations. The reason to use this option instead of 'Moving' is that it is much faster for irregular waves. It is especially relevant for floaters where movements can be large. For bottom-fixed wind turbines it is usually OK to use 'Fixed'.

Moving:

Loading effects are calculated based on both wave particle velocity at the current position and time and the structural position and velocity at the current time. This can cause numerical problems, but typically gives more accurate results if successful (every real structure moves, even if it is so little that it is not observed). This option will increase the simulation time, especially for jacket (truss) substructures (with many elements). Moving should be used for floating wind turbines as relatively large movements are expected.

Moving structure for perpendicular inertia?

Perpendicular hydrodynamic loads (drag and inertia) can be implemented assuming either a fixed or a moving structure. Whether movement (response) of the structure is taken into account, can be set independently for drag and inertia loading. This parameter only affects inertia loading. Default: Partly

Options:

Fixed:

Perpendicular inertia loading is calculated only based on water particle acceleration at the initial (start) position of the structure. This is more numerically stable and is usually OK for bottom-fixed wind turbines as the movement is typically relatively small.

Partly (default):

Perpendicular inertia loading is calculated based on water particle acceleration and structural acceleration at the beginning of each time step. However, it is NOT updated during nonlionear iterations. The reason to use this option instead of 'Moving' is that it is much faster for irregular waves. It is especially relevant for floaters where movements can be large. For bottom-fixed wind turbines it is usually OK to use 'Fixed'.

Moving:

Loading effects are calculated based on both wave particle acceleration at the current position and time and the structural position and acceleration at the current time. This can cause numerical problems, but typically gives more accurate results if successful (every real structure moves, even if it is so little that it is not observed). This option will increase the simulation time, especially for jacket (truss) substructures (with many elements). Moving should be used for floating wind turbines as relatively large movements are expected.

Froude-Krylov force on heave plates

Enables the Froude-Krylov force on the heave plates. This force is computed from a shallow-water approximation of the dynamic pressure acting on the bottom and top surfaces of the heave plate.

  • Default value: 0
  • Unit: —

Hydro drag loads on mooring lines below seabed

If enabled, segments of mooring lines that are in contact with or fully submerged in the seabed will still have hydrodynamic drag. This option will give a damping effect on the mooring lines when below the seabed.

The wave particle (fluid) velocity will be zero in the seabed, so the drag loads are only from the velocity of the line. Note that this option does NOT give any hydro inertia loads.

  • Default value: False
  • Unit: —

Seabed ramp-down distance

This parameter can ramp-down perpendicular hydro loads (Morison) to zero at the seabed. If the parameter is greater than 0, then both drag and inertia loads are linearly ramped-down to 0. However, if the Hydro drag loads on mooring lines below seabed parameter is checked, then hydro drag loads are NOT ramped-down even if the ramp-down distance is nonero.

This parameter is most relevant for mooring lines because they can go in and out of the seabed during a simulation and that can cause numerical convergence issues. Using this parameter in combination with Hydro drag loads on mooring lines below seabed can improve the convergence (smoothness) of the siumulation.


However, the parameter does work also for bottom-fixed WTs.

  • Default value: 0.1
  • Unit: $\text{m}$
  • Range: 0 — 100

Added mass tuning factor

This factor tunes all added mass effects. It is used to easily change all added mass coefficients. Added mass effects can be turned off by setting it to 0.


  • Default value: 1
  • Unit: —
  • Range: 0 — 10

Default coefficients

Hydro drag coefficient of circular cross section

The default drag coefficient for circular cross sections in (sea)water. Used in the drag part of the Morison equation. If this coefficient is 0 then the corresponding drag loads will not be made.


  • Default value: 1.2
  • Unit: —
  • Range: 0 — 100

Hydro inertia coefficient of circular cross section

Inertiacoefficient, CM (sometimes called mass coefficient) for a circular cross section in (sea) water. This coefficient is used to calculate the inertia (mass) force both for fixed and moving structure. For the moving structure case, an inertia term is also applied. For the moving structure case the applied coefficient is ( CM - 1 ). It is typically unphysical to use CM ≤ 1, but it is possible to run simulations with this input. However, if CM ≤ 1 then the inertia term is not applied. Thus, if CM = 0, then no mass (nor inertia) load is applied, neither for the moving nor the fixed structure.

Note: This parameter is not used for MacCamy-Fuchs loading.


  • Default value: 1.63
  • Unit: —
  • Range: 0 — 10

Heave plate drag coefficient

Coefficient for calculating the drag force for the heave plates in the longitudinal direction.

  • Default value: 4.8
  • Unit: —
  • Range: 0 — 10

Heave plate inertia coefficient

Inertia (mass) coefficient for calculating the force in the longitudinal direction of the heave plates.


  • Default value: 1.67
  • Unit: —
  • Range: 0 — 10