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Table of Contents

Environmental Loads

Click either Land or Sea in the Environment panel to define whether the turbine is sited onshore or offshore. The Tower Geometry diagram then shows either the ground level or the seabed and mean water level, as appropriate. For an offshore wind turbine, enter the mean water depth. To change the mean water depth relative to the base of the tower, for the monopile tower model change the Depth of first tower station as appropriate, and for the multi-member model, change the Z-axis origin above mean water level. Note that the mean water depth and mean sea surface elevation can also be adjusted by specifying the tide height.

Based on the environment the tower is placed (onshore or offshore) it will experience different loads.

Tower Aerodynamic Load

For a monopile or multi-member tower a single global aerodynamic drag coefficient can be defined for the computation of tower windage loads on all sections of the tower exposed to air.

Tower Hydrodynamic Load

For an offshore turbine defined using either a monopile or multi-member, the user may define a single global value for the hydrodynamic drag and inertia coefficients. This enables wave and current forces and hydrodynamic drag to be calculated using Morison's equation. This global value will be used for all tower elements that are submerged in water.

Note

The application of Morison's equation for capturing hydrodynamic forces on a structure is suitable in cases where the element diameter is small relative to the wavelength of incident waves. If the element diameter is large relative to the wavelength then a radiation-diffraction method for computing hydrodynamic loads should be used instead. This alternative option is only available when using the multi-member option.

Drag and Inertia Coefficients per Element

For multi-member towers a user can specify drag and inertia coefficients for the computation of wave and current forces and hydrodynamic drag individually for each tower element. The user can further improve the detail of the hydrodynamic calculations in Bladed with the following options.

Heave Plates

The Froude-Krylov force is already accounted when applying forces due to dynamic pressure from wave kinematics. Heave plates specified in Project Info are only included when using one of the following hydro loading models: “Morison’s Equation”, “BEM Hydrodynamics (with Morison drag)” or if the heave plate element is a Morison element in the "Specify different model for each member". Only the contribution due to quadratic drag (HeavePlateCd values) are included when using “BEM Hydrodynamics (with Morison drag)”.

The following code should be used to add Heave Plates

MSTART EXTRA
NHeavePlates  *Number of heave plates
HeavePlates  *A comma separated list of indexes of the members that are 
representing heave plates as they appear in the user interface screen
HeavePlateCa  *A comma separated list of added mass coefficient for each member
HeavePlateCd  *A comma separated list of drag coefficient for each
member*
MEND

The \(C_d\) is based on the plan area of the element (the length multiplied by the diameter), and the \(C_a\) is referenced to the volume of the element.

Anisotropic Drag and Inertia

On support structure elements \(C_d\) and \(C_m\) are usually the same in the member \(Y\) and \(Z\) directions. It is however possible to specify different values of in the Y and Z directions, using the following Project Info inputs.

MSTART EXTRA
N_ANISOTROPIC_CDS * number of members on which to specify y and z drag coefficients*
ANISOTROPIC_CDS * list of member numbers on which y and z drag coefficients are specified
ANISOTROPIC_CDY * drag coefficient in local member y direction at end 1 and end 2 of each member
 {(mem1, end1), (mem1, end2), (mem2, end1)…….} - there should be double the number as in row above*
ANISOTROPIC_CDZ * drag coefficient in local member z direction at end 1 and end 2 of each member*
N_ANISOTROPIC_CMS * number of members on which to specify y and z hydrodynamic inertia coefficients*
ANISOTROPIC_CMS * list of member numbers on which y and z hydrodynamic inertia coefficients are specified*
ANISOTROPIC_CMY * hydrodynamic inertia coefficient in local member y direction at end 1 and end 2 of each member 
{(mem1, end1), (mem1, end2), (mem2, end1)…….} - there should be double the number as in row above
ANISOTROPIC_CMZ * hydrodynamic inertia coefficient in local member z direction at end 1 and end 2 of each member
MEND

Hydrostatics of Isolated Elements

In general, hydrostatic forces are calculated on a tower by considering the pressure forces on structural elements in isolation.

For an offshore multi-member turbine only, support structure members can be defined as [sealed or unsealed]. Sealed elements can then be defined as either flooded or unflooded.

A user may also specify the density of any marine growth. If any members have non-zero marine growth thickness specified, this density is used to calculate the additional mass due to marine growth. The additional thickness will be used in the computation of hydrodynamic and hydrostatic loads.

Note

The Turbine Information window shows the total masses and inertias of all turbine components whose mass characteristics are defined. Click the Mass totals button or use the Windows pull-down menu on the main toolbar to open this window. Marine growth is excluded from these mass totals.

Hydrodynamic Added Mass in Simulations

Structural mass and hydrodynamic added mass are treated separately in Bladed simulations.

For Modal Analysis, the hydrodynamic added mass is not included. For flooded sealed elements, the mass of water enclosed within the structural element is added to the structural mass, so does affect modal analysis.

In a time-domain simulation and linearisation calculations, the hydrodynamic added mass is included as modification to the system total mass matrix (i.e. structural + hydrodynamic mass). The hydrodynamic added mass changes at each time step depending on which parts of the structure are submerged.

Note that the additional mass due to enclosed water, and any hydrodynamic added mass, are not included in the tower mass reported in the Turbine Information window.

Last updated 10-09-2024