Hydrostatics and hydrodynamics

Articles on this page:

• Hydrostatic calculation in Bladed
• Foundation reaction loads vs member loads
• How to get Bladed to read BEM hydrodynamic data generated using WADAM in Bladed
• SifToWdnhConverter tool error: "SIF file contains no Zero Frequency data"
• How to import AQWA .LIS files with newer versions into Bladed
• How to include second order hydrodynamic excitation loads in Bladed if using AQWA QTF data
• How to generate the QTF files from Wadam analysis for second order hydrodynamic loading calculation

Problem

How are the hydrostatic loads calculated in Bladed?

Solution

The hydrostatic forces are calculated individually for each member, and are made up as a combination of point forces and distributed forces. The hydrostatic forces are split down into three parts, which together account for buoyancy:

1. The hydrostatic pressure force on each end of the member (area x density x g x water depth), applied as point loads on the end of the element.
2. The pressure force acting perpendicular to the member, applied as a linearly varying distributed load. This is a pressure integral in a circle around the member axis. This will be zero if the member is vertical.
3. The pressure force acting parallel to the member, applied as a linearly varying distributed load. This will be zero if the member has no tapering.

If a member crosses through the sea surface then the distance along the member that the surface passes through the member is calculated. The surface is assumed to be perpendicular to the member axis and then the distributed forces are split up between the underwater section and above water section.

For underground members, the seabed is assumed to be saturated, which means that the same hydrostatic loading is applied as underwater.

For more details of the hydrostatic calculation:

• Bladed 4.17 and later: view online documentation
• Bladed 4.16 and earlier: view PDF documentation

Keywords

Hydrostatic; Buoyancy

Problem

Why are vertical foundation reaction and member loads different for offshore turbines?

Solution

See PDF document.

Keywords

Hydrostatics; Foundation; Loads

Problem

Does Bladed have a hydrodynamic pre-processor for WADAM?

Solution

Yes. Download the SIF to Wdnh Converter tool from the Downloads page. Documentation and example data are available within the installation package.

Keywords

Boundary Element Method; BEM Hydrodynamics; WADAM; WAMIT; Hydrodynamic data; Flow solver

Problem

When using SifToWdnhConverter tool for converting SIF files containing first order hydrodynamic coefficients computed from a Wadam analysis to a Wdnh file that can be imported into a Bladed project, the following error occurs:

Solution

It is required to include the added mass at zero/infinite frequency in the Wadam calculation. Users should re-run the Wadam analysis to get this output data in order to use the Wadam parser developed by the Bladed team.

Users with HydroD v4.10:

Users can refer to this link to find how to include the added mass at zero/infinite frequencies in the Wadam analysis.

1. Find the .FEM file by running the first two steps in HydroD monitor
   
   
   
   
2. Modify the card HYDPOTT in the .FEM file
   
   
   
   
3. Run step 3 without selecting the first two steps to avoid overwriting the FEM file.
   
   
   
   

Users with HydroD V6:

1. Select the following option directly in the Wadam Analysis Properties.
   
   
   
   

Keywords

Hydrodynamics; Wadam; SifToWdnhConverter

Problem

Bladed (prior to 4.15) supports the reading of *.LIS files outputted by AQWA versions 13 to 16 inclusive.

Is there a workaround to import .LIS files of the latest AQWA versions?

Solution

With the latest versions, the output files have changed in format. Users need to manually edit the output .LIS file of later versions to make it work with Bladed versions (up to 4.14).

Steps:

1. Change
   Aqwa-Line - 2019 R2 64BIT VERSION 2019 R2
   to
   AQWA-LINE - RELEASE Release 16.
2. Change
   STRUCTURE C.O.G.
   to
   C.O.G
3. Insert the text below between the different direction sectors for the excitation force data. The code is looking for: FROUDE KRYLOV + DIFFRACTION FORCES-VARIATION WITH WAVE PERIOD/FREQUENCY, which needs to be present 6 lines from the previous row of data (note that there are two empty lines that are required to make the row count correct).

1
              * * * * H Y D R O D Y N A M I C   P A R A M E T E R S   F O R   S T R U C T U R E   1 * * * *
                      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 


                        FROUDE KRYLOV + DIFFRACTION FORCES-VARIATION WITH WAVE PERIOD/FREQUENCY

Example:

Keywords

AQWA; BEM Hydrodynamics

Problem

From version 4.14 Bladed supports the Second Order Hydrodynamics calculation. The feature supports QTF data compatible with Wadam and WAMIT formats. For AQWA data users the 2nd order file " .qtf" is not read in by Bladed directly.

Solution

A temporary solution is to use an open-source tool to convert the 2nd Order QTF data to Wadam/WAMIT format. The proposed application is: BEMRosetta: Hydrodynamic coefficients viewer and converter for Boundary Element Method solver formats

BEMRosetta allows the user to load the AQWA 2nd Order Hydrodynamic data and convert it to Wadam/WAMIT format.

A few notes:

• The QTF AQWA data is non-dimensionalised in BEMRosetta to match the Wadam/WAMIT file format specification. The "Physical Constants" defined in the simulation parameters in Bladed are used to dimensionalise the data read by Bladed (water density and gravity).
• As for the first order data, the user still needs to make sure the BEM Hydrodynamic body Node in Bladed matches the AQWA simulation outputs location.

We continue to advise the users to use the Bladed pre-processor for import of the AQWA 1st order linear data.

Note: We have performed testing to evaluate the BEMRosetta tool to check suitability and are satisfied with the results. However the BEMRosetta project is not a supported DNV product and therefore DNV will not provide support or fixes relating to the use of this application.

Keywords

AQWA; Second order hydrodynamic excitation load; Floating; BEM; Hydrodynamics; QTF data

Problem

Second order hydrodynamic loading calculation is available in Bladed 4.14 onwards. To compute these loads the user is required to provide a Quadratic Transfer Function (QTF) by a frequency domain potential flow solver.

How can the user generate the QTF from Wadam analysis?

Solution

Prerequisites: WADAM Advanced license

In WADAM analysis, a file with extension .11d/12d and .11s/12s contains the difference and sum frequency QTF respectively. Files with extension 11 and 12 are generated using two different methods (indirect and direct method), however content of 11s and 12s, as well as 11d and 12d, are the same.

By default, the file with extension .11d/11s will be generated.

Users with HydroD4

Right click “Wadam analysis” -> “Edit”, then select the option “Save temp Wamit files” in the following figure. Run the simulation, the .11d/11s output files will be generated.

Users with HydroD6 onwards

Click “Wadam analysis”, then select the option “Save temp Wamit files” in the following figure.

Run the simulation, the .11d/11s output files will be generated.

Keywords

Wadam; Quadratic Transfer Function (QTF) data; Second order hydrodynamics