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Can Wadam do long term tuning of roll damping?

No. Wadam can only automatically do short term tuning.

Long term calculations has to be done manually, by giving a maximum roll angle in HydroD, run Wadam and take the results into Postresp and calculate the long term max (using scatter diagram etc:), then run Wadam again with a new max roll angle.

Repeat this until the long term max angle from Postresp converges with the input max roll angle.

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Characteristic Length

When running Wadam, there is a limitation on the definition of the Characteristic Length, as compared to the volume of the model.

This is currently L=100*( VOL**(1/3) )

For older Wadam versions, 8.1-08 and older, the general limit is at L=10*( VOL**(1/3) ).

L defined larger than this limit will typically end in the following message from Wadam, stopping the analysis:

"Error in panel data: volume too small"

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Coordinate systems (HydroD/Wadam)

The origin of the input system is defined as the origin of the model system after a possible translation in HydroD, this because HydroD will perform the translation by actually changing the various coordinates in the T#.FEM file before it is handed over to Wadam. The global coordinate system of HydroD and Wadam has its origin in the water line, directly above the origin of the input system - normal to the water plane.

When a structural model is translated in HydroD, and used in a load transfer analysis, it is very important to use the translated T#.FEM file in any subsequent structural analysis, in Sestra. The translated T#.FEM file is found in the relevant WadamRun folder within the HydroD workspace.

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Coordinate systems for responses vs. load files (HydroD/Wadam)

The responses from Wadam is given in the Motion reference point (Results reference point). This point is by default in the origin of the global coordinate system in HydroD/Wadam. This origin is located in the still water line, directly above the origin of the Input (model) coordinate system.

The Motion reference point may be defined by the user in HydroD: WadamRun setup - Execution DIr. - Result files.

The load file (L#.FEM) is created in the superelement coordinate system of the structural model. Ln.FEM corresponds to Tn.FEM.

When the structural model is defined by one single superelement (one T#.FEM only), this is identical to the Input coordinate system.

If a superelement assembly is created, in Presel, and the superelements are translated/rotated/mirrored in Presel, the top level Input coordinate system may be different from the systems of the individual superelements. The load file will still use the Input coordinate system of the original superelement (T#.FEM).

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Definition of a bilge keel (HydroD/Wadam)

Take care to define the bilge keel, in the roll damping model, within the boundaries of the strip model. Read the Wadam user manual carefully, also page 5-49 etc.

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Dummy hydro pressure on free surface (HydroD/Wadam)

If you want to do a second order QTF calculation in Wadam, then a free surface model is needed. This free surface model can be imported in LoadingCondition>New second order surface model.

Dummy hydro pressure must be defined on the free surface model but can be in both positive and negative Z-directions.

More information is found in Wadam_UM, section 2.1.10.

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Error messages (HydroD/Wadam)

When a Wadam run finishes with error, inside the Wadam.lis file an error message can be found. But sometimes the error messages can be quite generic and not very specific about the real cause of the problem. When such messages are found, please check the Poten.log and Force.log files, as they may contain more detailed information on the root of the error.

If you still have problems to understand the error message you’re getting, please contact software support through the Customer Portal or at software.support@dnv.com so we can make further investigations.

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For a sub-model, do I need to re-map the hydrodynamic loads to the sub-model (local model)? (HydroD/Wadam)

Yes, in normal cases you have to map the loads also to the local model. This is done by defining a new Wadam analysis in HydroD, where the global structural model is replaced by the local model. If you are not using the feature of internal compartments, you can still use the global structural model as a mass model (if this was used in the first place).

If compartments are included, special attention must be made to the definition of the total mass: As the local model does normally not have all compartments included, the total mass may be found from the global analysis and defined as a 'user specified' mass model.

If parts of compartments (i.e. not complete compartments) are included in the local model, the filling fraction cannot be handled properly by HydroD (total volume is needed). In this case the Compartment Points need to be defined in HydroD, giving the acceleration point at the centre of gravity for the compartment, and the zero pressure point at the free surface. Please see the attached document, HydroD_CompartmentPoints_Wadam_QS.pdf.

Please notice that it is required to rerun exactly the same wave periods and directions as in the global analysis. Everything should be identical except the structural model. A good way of doing this is to copy & paste the HydroModel inside HydroD, and replace the local model in the copy. Similar for the Wadam run.

If the panel model is big, using the restart option in the Advanced tab of the Wadam run setup will speed up the calculations (but this has to be done already when running the global analysis).

If the local model is very small, the local loads may in some cases be of less importance, as the bending action from the prescribed displacements on the edges will be governing for the results.

Please see the attached document HydroD_CompartmentPoints_QS.

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How are not-flat elements handled in a panel model

All panels in a Wadam analysis are adapted to be flat panels. For both lower and higher order shell and solid elements, Wadam will adjust the nodes into a flat , 3 or 4 noded, panel. This also when there is an out-of-plane issue with the lower order elements. This means that the normal vector of the panel may be different from the original normal vector of the element, which again can be an issue for example in the use of tolerances in a Wadam load transfer analysis.

Wadam will use the definition of the wet surface, or dummy hydro pressure, in load case 1, for both shell and solid elements, to select the panels.

This is described in more detail in the Wadam user manual, section 2.1.2.

After a Wadam analysis, there will be a file called panelT1.fem in the Wadam run folder. This is the actual panel model used in the calculations, after Wadam has handled the original panel model, including any cutting of the model at the water line etc. This model can be displayed in for example Xtract, as verification.

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How to add a point load to a structural analysis with wave loads from HydroD/Wadam

If you add a load manually in GeniE, the important issue is to define this load in a load case not conflicting with the wave load cases created by HydroD/Wadam.

If you have a Morison model included, and want to use the wave loads on the Morison model in the structural analysis, the only recommended option is to define the additional load in a load case following after all the resulting wave load cases. If you have for example 20 wave periods and 10 directions, and include the static load case from HydroD/Wadam, there will be 201 hydro load cases. The additional load case in GeniE should then be put into FEM load case no. 202, or higher. And, for sure, the load case name is not important; it is the FEM load case number that counts. This number may be adjusted by going into the properties of the load case (right-click). A picture from GeniE is attached

If you do not have a Morison model, and you do not need to run fatigue (ok for ULS), you can specify in HydroD that the wave load cases shall start at e.g. no. 5 (WadamRun - Exe. Dir - Result Files - Load Transfer). That means that the first (static?) load case from Wadam will be no. 5, and you can define the additional load in load case 2-4. The wet surface definition in load case 1 is a dummy load, but should not be used for the additional load. Load cases for compartments, if any, should be available for the additional load.

It is of course important (as always) to check the sum of loads in sestra.lis after the structural analysis has been run. The additional load case should be seen separated from the wave load cases.

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How to adjust the control surface used for second order (QTF) and mean drift force calculation

Wadam supports use of control surface for mean drift calculation from version 10.1, and full QTF calculation from version 10.2. The control surface is generated automatically by Wadam, which should be suitable to normal structures. However, it is always recommended to check the control surface by importing the cs##.GDF file into HydroD 4, especially when doing manual changes.

If the control surface generated by Wadam is not good enough, especially for multibody analysis when the bodies are very close and the control surfaces intersect with each other, it is possible to adjust it by changing the HYDCSDFT card in Wadam1.FEM file.

| HYDCSDFT | NFIELD | IOVRWRI | IRECT | GAPFAC
|   | SIZEFAC | IBTM | BTMFAC | BTMOT
|   | SIZEZ | NZ | ICSFS | ISORPOT

NFIELD – Number of data fields (so far it is 12 fields utilized for this card)

IOVRWRI – Whether to take the existing csxx.gdf files (The rest of the input are relevant only when IOVRWRI=1) 1 – create control surface gdf file automatically. If the file exists, it will be overwritten. 0 – use the gdf as it exists. The files are to be used in FORCE. If they do not exist, Wadam will bypass the body for drift force calculation from control surface integral.

IRECT – Vertical sided control surface is generated with the shape on free surface and bottom as 0 – Circle 1 – Rectangle 2 – Rectangle (for midship) +half circle (for bow and stern) Each option may fit a specific geometry better than the other, also influences slightly the results.

GAPFAC – Determine the gap size between the wet hull and the control surface. Gap = GAPFAC*maximum panel size along the waterline. 1.0 shall be the default value. A larger GAPFAC leads to more panels on the control surface. A value smaller than 1.0 shall be used with care.

SIZEFAC – Determine the mesh size (in horizontal direction along the waterline) on the control surface. Size = SIZEFAC*maximum panel size along the waterline. 1.0 shall be the default value. A larger SIZEFAC leads to coarse mesh on the control surface. Given good convergence performance, a value somewhat larger than 1.0 can be used to reduce the computational time in FORCE. A value smaller than 1.0 is normally not necessary.

IBTM – Determine whether to have mesh on the bottom. 1: have. Other value: not have. Mesh on the bottom only affects the results in longer waves, whereas mesh quality along the waterline area is crucial to the convergence performance for shorter waves.

BTMFAC – Determine the mesh size on the bottom for the rectangle and inner quad part of the circular shape. The factor is given wrt. the mesh size along the waterline. Default is recommended as 2.0. The integral on the bottom contributes little to the results, so that having a relatively coarse mesh on the bottom can reduce the total CPU time.

BTMOT – The circular bottom area is subdivided into inner quad and outer narrow area along the curve. This factor determines the relative edge length of the outer area wrt. the radius. 0.9 is the default. This option is not relevant when IRECT=1.

SIZEZ – The dimension in Z direction relative to draft. Default is 0, meaning that GAPFAC applies. Otherwise, it should be a value larger than 1.0.

NZ – Number of elements in z direction. Default is 0, meaning that the largest element size in z direction determine the NZ automatically. For some special cases, this number should be increased to have a finer water line mesh to get a better converged result.

Besides, when control surface is very close to the hull (less than one panel size in the gap), source strength option (ISORPOT =1) should be used.

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How to avoid breaking the limit of number of panels in a defined Correspondence (HydroD/Wadam)

When defining a Dual model for Wadam, a Correspondence between panels and Morison elements must be defined. There is a limitation of connecting max. 99 panels to one Morison element. To avoid breaking this limit, apart from changing the models, two main options exist:

  • Try to manually move some panels from one Morison element to another neighbouring element,
  • Split the Morison element into 'subelements', which may have a number of panels for each, max 99. The number of subelements for a Morison element is defined in the Morison Section property.

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How to convert T#.FEM file to Wamit *.GDF

Wadam will create a temporary *.gdf file while running - using the T#.FEM file. This .gdf file may be copied before Wadam is finished, or Wadam can be told to not delete the temporary Wamit files. This may be specified in HydroD (or by editing Wadam1.fem if running Prewad).

See FAQ: Wadam: Save temporary Wamit files

https://emea.salesforce.com/50120000000E2V5

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How to define off body points for Postresp in a HydroD and Wadam analysis? (Postresp)

To use off body point results in Postresp, specific off body points must be defined in HydroD, using coordinates given in the Global coordinate system, i.e. having the origin in the still water line.

Further calculations can be done in Postresp on the results in these off body points.

The off body point grid is used for visualization & animation in Xtract only.

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How to find the connection between external and internal panel numbers?

The external/internal panel numbers are found in section 2.11 PANEL DATA FOR BASIC PART in wadam.lis.

    PI          = PANEL INDEX
    PANO    = EXTERNAL PANEL NUMBER AS READ FROM INPUT INTERFACE FILES

where PI = internal panel number.

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How to include the damping force in sectional loads or load transfer? (HydroD/Wadam)

The only way to include damping forces in sectional loads or load transfer is to use Morison elements.

The drag coefficients should be tuned so that the resulting damping is similar to the case when using an additional (critical) damping matrix or the roll damping model.

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How to input non-linear damping coefficient in HydroD based on quadratic roll damping from model test (HydroD/Wadam)

From Wadam 9.3-06: If you have the quadratic roll damping B2 from tank test with SI unit Nms^2/rad^2, then the input quadratic roll damping coefficient b2 in HydroD under LoadingCondition>AdditionalMatrics>Define Non-linear Damping>Roll damping should be:

b2=B2/(2*M44*(180/pi))

In case B2 is following the unit Nms^2/deg^2, then input to HydroD will be:

b2=B2/(2*M44)

Where M44 is the dimensional mass inertia in roll which is measured in Wadam's global coordinate system. The non-dimensional value M44/(?VL*L) can be found in “MASS INERTIA COEFFICIENT MATRIX” in the Wadam.lis file.

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How to interpret the printed balance of mass and buoyancy in section 2.7 in Wadam.lis?

First, the vertical tolerance is given in HydroD, relative to the characteristic length. Wadam will stop if the tolerances are exceeded.

Any imbalance between mass and buoyancy will be printed as a vertical resultant force (F3), and as a distance/translation in length unit, at the bottom of section 2.7 in Wadam.lis. The translation is for information only, no translation will be performed by Wadam. A distance of for example 0.5 means that the vessel should be translated up by 0.5 length units to achieve perfect balance.

The individual values for mass and buoyancy are printed in the same section.

Notice that the resulting force in a following structural analysis in Sestra may differ from the results in Wadam. This is mainly because of potential differences in the normal vectors of the models, due to different mesh of the panel model and the structural model. Differences in the mass distribution between the mass used in HydroD/Wadam and the mass of the structural model are also important.

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How to make use of the Haskind relations?

The use of Haskind relations vs. pressure integration is automatic in Wadam. If there is no need for detailed pressure information on the panel model (no pressure integration), the Haskind relations will be used. This happens typically if there is only a panel model in the calculations.

That means that if there are no Morison calculations, no offbody points, no load transfer, no drift forces from pressure integration (6 DOF), only normal print option etc., Wadam will use the Haskind relations.

This will be seen like this in the Wadam results, section 4.3 in Wadam.lis:

     +----------------------------------------------+
     :                                              :
     :  W A V E  L E N G T H        =   4.7920E+01  :
     :  W A V E  P E R I O D        =   5.5401E+00  :
     :  H E A D I N G   A N G L E   =   0.00        :
     :  (IN DEGREES)                                :
     :                                              :
     +----------------------------------------------+

    EXCITING FORCES AND MOMENTS FROM THE HASKIN RELATIONS
    -----------------------------------------------------

              REAL PART       IMAGINARY PART      ABSOLUTE VALUE  PHASE (DEGREES)
    -F1-        3.3511E-01      2.7050E-01          4.3067E-01       38.91
    -F2-        0.0000E+00      0.0000E+00          0.0000E+00        0.00
    -F3-        8.2537E-02     -6.8280E-02          1.0712E-01      -39.60
    -F4-        0.0000E+00      0.0000E+00          0.0000E+00        0.00
    -F5-        5.4511E-02     -4.0366E-02          6.7830E-02      -36.52
    -F6-        0.0000E+00      0.0000E+00          0.0000E+00        0.00

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How to make Wamit .out files with all needed results? (HydroD/Wadam)

Version: Up to 9.1-02

In Wadam version 9.2-03 and later versions, the Wamit.out file contains the complete results. The following work-around is thus not needed.

If you need results in Wamit format, e.g. for further mooring analysis in Simo etc., the option Save Wamit files has to be selected in the Advanced part of the Wadam setup in HydroD.

Some programs use the file .out, not the numerical results files. The .out file is created by the Wamit part of Wadam called Force, which is run twice in a normal Wadam calculation. The .out file is created by the first Force execution, then overwritten by the second Force execution. That means that you need to run one standard Wadam analysis and one where Wadam is stopped before second Force. This can be set up in HydroD - Wadam run - Advanced tab.

The first Force (i.e. stopped before second Force) will give:

  • Added mass and damping coefficients
  • Exciting forces

The second Force (i.e. without stopping) will give:

  • Global mass, damping, stiffness matrices - frequency independent
  • Motion RAOs
  • Offbody point results (pressure, velocities)
  • Drift forces

Please notice that the option Save Wamit files cannot be used together with the Wadam Restart option.

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How to read eigen value table in Wadam.lis

For example you have the following printed eigen value table in your Wadam.lis file.

    EIGEN SOLUTIONS TO THE RIGID BODY MOTION
    ----------------------------------------

   EIGEN VALUES:                   EIGEN VECTORS:

NO   PERIOD [T]  ANG. FREQ.      1           2           3           4           5           6

1      1.3830E+02 4.5433E-02      1.0000 0.0000 0.0000 0.0000 0.0019 0.0000

|
2      1.3791E+02 4.5559E-02      0.0000 1.0000 0.0000 -0.0017 0.0000 0.0000

|
3      5.0252E+01 1.2503E-01      1.0000 0.0000 0.0000 0.0000 0.1460 0.0000

|
4      4.9042E+01 1.2812E-01      0.0000 0.0000 0.0000 0.0000 0.0000 1.0000

|
5      4.7396E+01 1.3257E-01      0.0000 1.0000 0.0000 -0.1361 0.0000 0.0000

|
6      2.2102E+01 2.8429E-01      0.0000 0.0000 1.0000 0.0000 0.0000 0.0000

The column for the periods is sorted on values, not on modes. To find the modes, the eigen vectors must be inspected.

For row 4, eigen vector 1 showing in purple means that eigen period in this wave condition in yaw is being 4.9042E+01 (s) and with angular frequency to be 1.2812E-01 (rad/s).

For row 1 and row 3, 1 in red is showing in both lines, which means that surge and pitch motion are coupled. Under this condition, we should look at other modes, the green one and decide which mode to use.  Plotting the RAOs in Postresp will also help in such an evalustion.

As the eigen values will change for each wave period, because of change in added mass, the wave period with the best match of the eigen period should be selected. Please look up the chapter on the Equation of motion in the Wadam user manual for more information (2.6.7 in the Wadam 10.4 manual).

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How to solve error from drift force calculation with pressure integration?

When drift forces from all degrees of freedom is wanted, the pressure integration method is used. This puts more strict requirements to the mesh of the panel model, especially at the waterline. (The alternative is to calculate only the horizontal drift forces.)

If Wadam stops, you may see the following message in the file poten.log:

Error in evaluating Rankine matrix

If this is the case, please inspect the panel model closely, with respect to the waterline. Avoid defining the waterline just above the element borders of the model, as Wadam will cut the panel model at the waterline. This may result in tiny and badly shaped elements.

It is most useful to check the file panelT1.FEM, created by Wadam (found in the Wadam run folder). This is the panel model actually used in the calculations, after being cut at the waterline (and at the sea bed).

It may be useful to put on the element labels, to more clearly view any small elements at the waterline. An example from Xtract is shown in the attachment, where some additional element labels are shown above the main ones.

Please see the attached document panelT1fem.

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How to use file names for models in a Multibody analysis? (HydroD/Wadam)

Regarding the T#.FEM files in a multibody analysis, the panel models for different vessels must have a unique'superelement' number (e.g. T1.FEM and T2.FEM). They cannot be distinguished by a prefix in the file name, or by a different path, only.

The reason for this is that HydroD will strip off the path\prefix when copying these files into the Wadam run folder, thus making Wadam see only one of the files, if they both have names like <path\prefix>T1.FEM.

Please also remember that this superelement number in the file name needs to be defined in the model, like in GeniE (Edit - Rules - Meshing). Renaming the file from for example T1.FEM to T2.FEM will not work because this number is also given inside the file (in the first line).

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HydroD version 4.8 (and later) requires Wadam version no lower than 9.2

HydroD version 4.8 (and later) requires Wadam version no lower than 9.2

Type: limitation

HydroD version 4.8 (and later) always writes WDDCUR card to Wadam1.fem file, which was started to be supported by Wadam from version 9.2. So HydroD version 4.8 must work together with Wadam version no lower than 9.2.

Always use latest versions of both HydroD 4.x and Wadam.

From version: 4.8-01 To version: 4.ALL

(from the HydroD status list)

(A possible, but not recommended, workaround is to delete the line starting with WDDCUR from the file Wadam1.FEM before starting the Wadam analysis itself.)

Typical error message in Wadam1.lis:

Reference is zero or negative. IDGR:WDDCUR NREF: 1 IREF: 0

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Imported restoring matrix from Wadam G1.SIF file into SIMA (SIMA/Wadam)

Imported restoring matrix from Wadam G1.SIF file into SIMA

Importing a Wadam G1.SIF file into SIMA, the imported restoring matrix contains only the geometry contribution of gravity and buoyancy as described in the user manual of Wadam(see below).

Whatever additional restoring components are added in HydroD, e.g. additional restoring matrix, partially filled compartments, anchor elements or TLP elements, the additional restoring components are NOT imported into SIMA.

SIMA users should manually add additional restoring matrix in SIMA if they had additional restoring matrix or partially filled compartments in HydroD. Mooring lines should be added in SIMA modelling in case that anchor elements or TLP elements were included in HydroD.

It is recommended to compare the restoring matrix and the actual restoring coefficients printed in WADAM1.LIS file(ch.2.7)

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Load transfer analysis with NO MATCH written in Wadam.lis.

Problems with NO MATCH' stated in Wadam.lis for a load transfer analysis should in general be inspected. This means that the listed elements (in the structure model) will have no wave pressure in the structural analysis, and subsequently problems with lower accuracy and increased reaction forces may occur. Local results at these elements will also be affected.

A good way of checking this issue is to plot the pressures in Xtract, from reading the T#.FEM file for the structural model (the L#.FEM file will be read automatically). Elements without pressure will show up as grey elements, without colour contours, when the pressures are displayed.

A work around may be to increase the load transfer tolerances defined in HydroD. If this is not sufficient, the panel model and structural model should be inspected. If there are only a few of these elements, the message can normally be neglected.

Notice that this may also indicate that the definition of the wet surfaces (dummy hydro pressure) in the structure model is not correct. Wadam will find all such elements defined in load case 1, both on the outside and inside of the model.

Example:

      * 2023                   NO MATCH   (24, 1, 0)

     ** A TOTAL OF   29 LEGAL ELEMENTS IN ACTUAL OCCURENCE NO.       2
       OF SUPERELEMENT TYPE      23 DID NOT MATCH!

The numbers in the brackets, (24, 1, 0), indicate that Wadam will search up to 25 panels to find one that can match with the structural element. The load transfer tolerances are used, out of plane/distance and normal vector angle, (dist, ang, 0).

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Load transfer runs but no S#.FEM file created

Must specify in HydroD that load case numbering shall start at no. 1.

WadamRun - Execution directives - Result files - Load transfer - Load case numbering (bottom of dialogue).

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Mass information has to exist in structure model for load transfer and later strength analysis (Wadam/Sestra)

If you want to do load transfer in HydroD and strength analysis in Sestra, mass information has to exist in your structure model.

For the mass related issues, Wadam/HydroD will calculate inertial loads from the mass information. These loads will contribute to calculation of the global forces and motions.

Also the accelerations will be transferred into Sestra. When you do the structure analysis, Sestra will only use the acceleration load from Wadam and the mass from the structure model to add gravity effect. So, your structure model should have mass information, similar to the mass input in HydroD/Wadam.

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Morison model and beam element type (HydroD/Wadam)

Before Wadam version 9.7: lease note that the Morison model used in Wadam has to be created by using the standard 2 node beam element.

Beam elements meshed as second/higher order, 3 node, beam elements will be neglected by Wadam.

A typical message in the print file from Wadam, Wadam1.lis is, in section 2.2:

-  nn - ELEMENTS OF ILLEGAL TYPE ARE DETECTED AND DROPPED.

When creating the Morison model in GeniE, this element type is ensured by NOT selecting the option for use of second order elements in the Edit - Rules - Meshing dialogue.

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Natural frequency explained (HydroD/Wadam)

The natural undamped eigen frequencies are reported in the Wadam1.lis file in section 4.3, together with the dynamic results of motions etc. The equation system is described in section 2.6.5 in the Wadam user manual.

As the calculated natural periods will change slightly for each wave period, the best match of wave period and natural period should be inspected. For example, the following table may be printed for a certain wave period (the table is unfortunately wrapped):

EIGEN SOLUTIONS TO THE RIGID BODY MOTION
----------------------------------------

EIGEN VALUES: EIGEN VECTORS:

NO PERIOD [T] ANG. FREQ.    1     2     3      4      5      6

1 INFINITE

2 INFINITE

3 INFINITE

4 5.3937E+01  1.1649E-01  0.0293  0.0001   0.0032  -0.0018   1.0000   0.0009

5 4.6670E+01  1.3463E-01  0.0001 -0.0514   0.0000   1.0000   0.0021  -0.0064

6 2.0611E+01  3.0485E-01  0.0007  0.0000   1.0000  -0.0003  -0.2010   0.0000

The periods at the left of the table are sorted by value; the actual mode is found by looking at the eigen vectors, where the column 3 is heave etc. (surge, sway, heave, roll, pitch, yaw). This means that the natural period for heave is 20.6 s, pitch is approximately 53.9 s and roll is 46.7 s. The horizontal modes (surge, sway, yaw) are infinite when there is no restoring from mooring elements etc.

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Option to include current

Including current in Wadam is limited to a fixed structure, valid for the Morison model only, and using time domain (deterministic) output.

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Prefix of T#.FEM files (HydroD/Wadam)

When T#.FEM file are read into HydroD/Wadam, a prefix of the T file will automatically be neglected and only the characters after ‘T’ will be read by Wadam.

For example, ‘VesselT1.FEM’ and ‘ShipT1.FEM’ will have the same name in a Wadam run in HydroD, which is T1.FEM. This will cause trouble if more than one HydroModel exists in one workspace and different T files are read in different places, for example in a multibody analysis.

So T#.FEM files are needed to be assigned with different recognized numbers, like ‘VesselT1.FEM’ and ‘ShipT2.FEM’.

Please also remember that this superelement number in the file name needs to be defined in the model, like in GeniE (Edit - Rules - Meshing). Renaming the file from for example T1.FEM to T2.FEM will not work because this number is also given inside the file.

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By default from HydroD, the natural undamped eigen frequencies are reported in the Wadam1.lis file in section 4.3, together with the dynamic results of motions etc. The equation system is described in section 2.6.8 in the Wadam user manual.

The periods are sorted on values of eigen periods. To find the corresponding modes, you should look at the normalized eigen vectors, the right-hand side table. These are given in the sequence of the modes: surge, sway, heave, roll, pitch, yaw.

As the eigen values will change for each wave period, because of change in added mass, the wave period with the best match of the eigen period should be selected.

In the example below, it is easy to find the modes because the value of 1.00 is found in the different modes. Period no. 4 is roll (4), period no. 5 is pitch (5) and period no 6 is heave (3). Because there is no horizontal stiffness (like mooring stiffness) in this case, the horizontal modes (surge, sway, yaw) are in this case, with no horizontal restoring/stiffness defined, given as INFINITE.

A wave period around 92 s should be used for the best simulation of the roll mode, and a wave period of around 18 s for heave.

    EIGEN SOLUTIONS TO THE RIGID BODY MOTION
    ----------------------------------------

   EIGEN VALUES:                   EIGEN VECTORS:
   NO PERIOD [T] ANG. FREQ.  1      2    3     4    5     6

   1  INFINITE
   2  INFINITE
   3  INFINITE
   4  9.2236E+01 6.8121E-02 -0.00  0.01 0.00  1.00 0.02 -0.0180
   5  8.3899E+01 7.4890E-02 -0.10 -0.00 0.00 -0.02 1.00  0.0007
   6  1.8081E+01 3.4750E-01  0.00  0.00 1.00  0.00 0.00  0.0000

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Roll damping model bilge keel (HydroD 4.x/Wadam)

There is unfortunately a defect in HydroD 4 connected to the definition of the bilge keel angle within the roll damping strip model. This is listed in a separate entry in the HydroD status list.

It is important to know that the bilge angle actually being used in Wadam is listed in the Wadam1.lis print file, in section 3.1. As stated below the table of bilge keel data in this section, the angle PHI, initially given from HydroD, is the relative angle of the bilge keel from the line between (Yi,Zi) and the intersection of the centerline/waterline, seen from bow to stern.

PHI is given in HydroD relative to the input coordinate system. HydroD transforms this angle to the global coordinate systemand writes this to the Wadam input file, Wadam1.FEM.

The display defect in HydroD means that the correct angle to be used in Wadam may be displayed incorrectly in HydroD.

This issue is only relevant when the Integration option for the ITTC method is used (or the now old Tanaka/Kato method). When the Simplified option is used, the bilge keel angle is not relevant.

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Save temporary Wamit files

Wamit files are retained from Wadam by the following setting in HydroD:

- Select Save Wamit files in the Advanced tab of WadamRun Execution directives

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Save/Restart option vs Save/Bypass of Wamit parts (HydroD/Wadam)

Two different options for saving and reusing Wadam results are available in HydroD.

  1. The Save/Restart option is the "Wadam way" of saving the potential solution, to avoid recalculating this when not needed. This is the recommended method for such time saving operations. The option for Save/Bypass/Stop of Wamit parts is included to help users knowing Wamit input/output files to obtain special results and features not available in the standard Wadam analysis. If the Wamit files are saved in the first run, the "bypass Poten" option will be similar to the Wadam Save/Restore option.
  2. To be able to reuse the potential solution, from Save/restart or Bypass, the wet part of the structure cannot be changed. This means that the panel model and the draught (displaced volume) cannot be changed. Because equilibrium of mass & buoyancy is needed, the mass cannot be changed either. The method of defining the mass may possibly be changed, though, as long as its resulting mass properties are not changed. The same wave periods must be used as well. Typical changes are variations of damping, and variations of output results like adding load transfer to a previous analysis.
  3. The Save/Bypass/Stop option makes it possible to edit intermediate Wamit files. There can be many reasons for wanting to change the Wamit files, like the need of special features not accounted for in a standard Wadam analysis. Knowledge in Wamit is of course required for this kind of editing. When Wadam is told to "Stop before Force", the input files will be made by Wadam. These may be edited, Force can be run independently of Wadam, before running Wadam again with "Bypass Force" selected. Some more information is found in the status list for Wadam (an example for a multi-body analysis is attached).
  4. To put it simply, the first Force execution will compute added mass, damping, excitation forces etc., while the last Force execution will compute e.g. off-body kinematics, drift forces etc. Handling of Morison effects (drag coefficients, added mass etc.) will be done in Wadam after the first Force.

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Second order analysis cannot run with limiting frequency

Wadam second order analysis cannot run with limiting frequency, and Wadam analysis will stop with message below in force.lis:

PER in PT2 file is less than or equal to zero

WADAM1.LIS file will write the following message:

   *** TRYING TO READ NONEXISTING DATA
       CALL TO PGHYD WITH OPERATION CODE    2
       FOR DATATYPE      32
       OFFSET IN POINTER ARRAY            0
       KEY  1 SPECIFIED =      1  MAX=     19
       KEY  2 SPECIFIED =      1  MAX=      6
       KEY  3 SPECIFIED =      1  MAX=      1
       CALLED FROM SUBROUTINE OUTPUT
       CALLED FROM SUBROUTINE  MN1462

P R O G R A M    E R R O R    D E T E C T E D

In case that both full QTF results and added mass at limiting frequency are needed, for example FOWT modelling in Sima or Bladed, one should split the Wadam analysis into two analyses, and run them separately.

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Sectional loads (HydroD/Wadam/Wasim)

When computing sectional loads for a vessel, we recommend to specify the side of the cross section with the smaller part of the vessel as the integration side. The integration side should be changed at midship to achieve the best results.

The numerical issues from integrating the pressure over all the panels may be fairly large. This will typically grow when the part of the model on the integrated side is large, and is one of the reasons for this recommendation.

The mesh size (discretization) may also affect the results; a finer panel mesh will be more accurate than a coarse mesh.

It must also be remembered that the total sectional load is the difference between the huge values of buoyancy and inertia. Small inaccuracies in these values may have a big consequence for the sectional load.

The end sections should be positioned to include all of the panel and mass model from the applicable side of integration, i.e. a bit on the outside of the model. This to include the complete horizontal pressure in the integration.

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Sectional loads (Wadam)

The cross sectional forces can be obtained in Wadam by defining calculation of cross sectional loads. This calculation contains all six forces and moments, and may be calculated in a number of locations along the axis.

The use of sectional loads is defined in HydroD: Define the cross sections as described in the user manual for HydroD in the HydroStructure folder in HydroModel in HydroD's browser.

For a ship, the sections will typically be defined in the YZ plane, and the definition of multiple cross sections is most efficient.

The cross sections may be calculated for several loading conditions (different Wadam runs) in the same HydroD workspace. The forces and moments are printed in the print file from Wadam, wadam1.lis, including the still water results.

Please note that these results are non-dimensional (as the other results in the print file) and must be multiplied with the correct numbers given in the start of section 4 in the print file.

The dynamic results should normally be postprosessed in Postresp, where you get results with real dimensions. If you have a number of cross sections along the axis, you may also display force and moment diagrams in Postresp.

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Still water bending moment

The still water bending moment is printed in Wadam1.lis, section 4.7 for a panel model. See also section 2.5.18 in the user manual for Wadam. Note that these results are non-dimensional, and have to be multiplied with the factors listed in the beginning of section 4.1.

    SECTIONAL LOADS     NON-DIMENSIONALIZED BY:    ROVOLGWA/L      ROVOLGWA

The values of these factors (RO, VOL, G, etc.) are also given below the table in section 4.1

The dynamic sectional loads are also printed by Wadam, but these are normally displayed and printed in Postresp (with dimensions). The still water loads are not read into Postresp, because Postresp can only use harmonic results.

When performing a hydrostatic stability analysis in HydroD (separate license), the still water loads are found in the Hydrostatic report.

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Tanaka and Kato roll damping strip model limitations

The roll damping method is based on empirical results for certain cross sections of ships, see section 2.5.19 in the user's manual for Wadam. These results were tabulated by Tanaka and Kato back in the sixties.

*** WARNING FROM TANAKA : OUTSIDE TABLE : BDG= 0.1333E+02 FOR STRIP 5

This message means that the cross section for this strip is outside the current table for this parameter. Wadam will use a maximum value for the parameter, in stead of interpolating in this table, to find a damping coefficient for this strip, thus it will in a way be less accurate. BDG is interpreted as B/KG (B=beam of section).

There may be problems like this when the method is used on ships which differ from the ships used when the tabulated data were defined, for example wide ships with small draught.

Sometimes it helps when the user specifies that the strip type is e.g. bow type or stern type instead of "automatic". This is done by right-clicking the actual strip definition in HydroD, see also section 5.3.10.2 in the user's manual for HydroD. It helps also to increase the print switch to print of model data, this will give more information on the strip and bilge keel computations.

This message should not be critical, as it is just a warning, and the analysis is continuing. It is, however, a signal that the model is not within the standard assumption of cross sections for the roll damping method.

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Total damping

Summary of the damping in Wadam:

  1. 'Total damping' as shown in Wadam.lis file, section 4, includes potential damping and user defined damping (critical damping and general damping matrix) and viscous damping from Morison model, when the constant drag velocity is selected.
  2. Other viscous damping in the Wadam.lis file may include linearized viscous damping from stochastic and regular wave linearization of Morison drag (in section 3.3) or from the roll damping model, i.e. skin-friction of the hull, viscous damping from bilge keel and damping from eddy-making of the bilge keel (in section 4). Because this damping is direction dependent, it is not transferred to G1.SIF/Postresp.

The final total damping from Wadam will be 1+2, when other viscous damping is included.

  1. 'Total damping' as shown in G1.SIF/Postresp is potential damping plus viscous damping from Morison model, when the constant drag velocity is selected. Only potential damping and frequency/direction independent viscous damping, from Morison drag, are included in G1.SIF/Postresp.

Please notice that the effect of the damping on the motions is of course included in all cases. The discussion here is on the reporting of the damping matrix itself.

Critical damping is calculated by equation 2*sqrt((M44+A44)*C44). Here you have to use global coordinate system.

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Use of non-Morison elements in the Morison model

How to avoid general beam elements being taken as Morison elements by HydroD/Wadam/Wasim? (HydroD version 6 and later.)

The following is more relevant when doing a load transfer analysis to a structural model including Morison beams and loads than for a pure motion analysis. Care must anyway be taken to avoid including all general beam elements as Morison elements, like plate stiffeners and other structural beams.

Why? Mainly because all beam elements in the Morison model will by default be taken as Morison elements. This will be wrong, and it may easily break the limitation of max. number of Morison elements (15000 in Wadam). It is especially important when a big structural model is also used as a Morison model (the general way of doing it, also called a Single Superelement Composite Model).

The method is different from the practice in previous version 4.x of HydroD, where this part was more automatic, using the "Dry Morison element" option. The Morison concept was updated quite a bit in HydroD 6, and the Dry option is no longer available.

In version 6.0 and later versions of HydroD, do both following points:

  • Select "Drag only" for all dry Morison element sections from the Morison 2D tab om the Morison model properties, i.e., all non-Morison structural beam elements
  • Do not select any Morison2D property for these cross sections (obviously)

This will make HydroD ignore volume of the Morison sections and write only the specific Morison sections to the Wadam input (Wadam1.FEM). In the below example, the actual Morison section is also defined to be of type “Drag only”.

See the attached pdf: HydroD_Morison_SSCM.pdf. This and more information is also found in the document called "HydroD x vs. HydroD 4.10", found from the Help menu in HydroD 6.2 and later versions (and as a chapter in the release notes for version 6.0 and 6.1-01, also from the Help menu).

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User specified motion reference point

The default coordinate system for Wadam output files is using global coordinate system, with its origin located on the mean free surface.

The user can input their own motion reference point under ‘Define Run>Result Files>Global Response’ window.  Under this condition, forces and motions printed in Wadam.lis and G1.SIF file will be transformed into user specified coordinate system.

Please notice that all hydrostatic characteristics printed in section 2.7 in Wadam.lis file will still be using the ‘mixed coordinate’ system. Also Wamit files will still using global coordinate system. Drift forces, being 2nd order results, are not transformed correctly.

This FAQ is valid for HydroD 4.6-03, Wadam 9.0-04 and the relevant previous versions of HydroD/Wadam. Please find more information in the Wadam user manual, section 2.5.2.

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Very long wave and relavant water depth set up (HydroD/Wadam)

When very long waves are applied in HydroD/Wadam, the setup for ‘Water depth’ is important.

Different water depth will cause the vessel floating in shallow water, intermediate water or deep water condition for one fixed wave height and wave length.

Ranges of validity for various wave theories in relationship with the non-dimensional wave height and non-dimensional water depth are specified clearly in DNV-RP-C205, Figure 3-2 (look for Rules and standards in www.dnv.com).

User needs to have a proper water depth to make sure the floating structure is actually working within linear wave theory.

For example, if we have a long wave with amplitude =1m; period=180s, water depth needs to be at least around 48000m to make sure deep water condition is applied.

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Wadam (9.1-02): How to set up a coupled damping matrix for a multi body analysis?

OLD VERSION OF WADAM: Due to a defect in Wadam 9.1-02, a coupled damping matrix for a multi-body analysis must be handled partly manually.

It involves not defining the damping matrix in HydroD directly, but adding it later; and stopping Wadam before the 2nd run of Wamit-Force.

A status list entry for Wadam will be updated as well.

Please see the attachments for more details.

  • Multy body with coupled damping.pdf
  • Damping1.txt
  • WAMIT_5S.FRC

Please see the attached document Multi body with coupled damping.

Please see the attached document Damping1.

Please see the attached document WAMIT_5S.

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Wadam crashes sometime when pressure panels are defined, why?

The selection of pressure panels is done on the panel model. Since many loading conditions may be defined, the selection of panels may include panels above the water line for a specific loading condition.

If a panel above the water line is selected, Wadam will stop.

Wadam will print: "No match for the element triplets given on input"

Removing the panels above the water line from the list of selected pressure panels will avoid this.

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Wadam error messages (HydroD/Wadam)

When a Wadam run finishes with error, inside the Wadam.lis file an error message can be found. But sometimes the error messages can be quite generic and not very specific about the real cause of the problem. When such messages are found, please check the Poten.log and Force.log files, as they may contain more detailed information on the root of the error.

If you still have problems to understand the error message you’re getting, please contact software support through the Customer Portal or at software.support@dnv.com so we can make further investigations.

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WARNING FROM TANAKA : OUTSIDE TABLE

The roll damping method in Wadam is based on empirical results for certain cross sections of ships (see section 2.5.19 in the user's manual for Wadam). These results were tabulated by Tanaka and Kato back in the 1960s.

The message "WARNING FROM TANAKA : OUTSIDE TABLE" means that the cross section for this strip is outside the current table for this parameter. This means that Wadam will use a maximum value for the parameter, instead of interpolating within this table.

This should not be critical, as it is just a warning, and the analysis is continuing. It is, however, a signal that the model is not within the standard assumption of cross sections for the roll damping method. It could be useful to try other locations for the strip definitions, i.e. change the distance between the strips somewhat, or use more/fewer strips. Some more info on the strip types is found in the Wadam UM, page 5-51.

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What are the contents of the different Wamit results files?

When running Wadam, parts of our version of Wamit will be used for some of the calculations. There is an option, controlled from HydroD, to save the temporary Wamit files when Wadam is complete. As per default, these files will be deleted.

The file names will typically be of the form "WAMIT_5S.*". The contents of the files are as follows:

WAMIT_5S.out    - print file, feedback and information on input, results etc.

WAMIT_5S.GDF  - panel model

WAMIT_5S.1   - Added mass and damping coefficients

WAMIT_5S.2   - Exciting forces from Haskind relations

WAMIT_5S.3   - Exciting forces from diffraction potential

WAMIT_5S.4   - Motions of body (RAO)

WAMIT_5S.5p - Hydrodynamic pressure on body surface

WAMIT_5S.6   - Pressure at offbody points

WAMIT_5S.7   - Fluid velocities at offbody points (7x, 7y, 7z)

WAMIT_5S.8   - Mean drift force and moment from momentum conservation (horizontal far field)

WAMIT_5S.9   - Mean drift force and moment from pressure integrations (6 dof)

WAMIT_5S.10 - Quadratic second-order forces (10s, 10d)

WAMIT_5S.11 - Second-order forces by indirect method (11s, 11d)

WAMIT_5S.12 - Second-order forces by direct method (12s, 12d)

WAMIT_5S.13 - Second-order hydrodynamic pressure on the body (13s, 13d)

WAMIT_5S.14 - Second-order hydrodynamic pressure in the fluid (14s, 14d)

WAMIT_5S.15 - Second-order wave elevation (15s, 15d)

WAMIT_5S.16 - Second-order RAO (16s, 16d)

Some of these results are optional, depending on user input settings. The exciting forces will be given either by the Haskind relations (.2) or from the diffraction potential (.3).

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What does this mean: WARNING FROM TANAKA : OUTSIDE TABLE : BDG= 0.3127E+01 FOR STRIP 9

The roll damping method is based on empirical results for certain cross sections of ships (see section 2.5.19 in the user's manual for Wadam). These results were tabulated by Tanaka and Kato back in the 1960s.

This message means that the cross section for this strip is outside of the Tanaka table which was used for viscous roll damping based on model tests, etc. Wadam will use a maximum value for the parameter, instead of interpolating in this table, to find a damping coefficient for this strip, thus it will in a way be less accurate.

BDG is interpreted as B/KG (B=beam of section).

This is mainly related to whether your model is inside the studied range in which the Tanaka method was valid.

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What is the difference between a composite model and dual model? (HydroD/Wadam)

Version: All

The main difference is that in a load transfer analysis, the composite model will transfer panel pressure loads to the shell part of the structural model and Morison loads to the beam (Morison) part of the model. The use of a dual model will only transfer loads to a beam (Morison) model.

To change from a composite to a dual model, a correspondence between the panels and Morison elements has to be defined.

In a composite model, any Morison elements overlapping the panel model need to have a small diameter, with an increased drag coefficient. This is to ensure a correct calculation of the displaced volume and a correct drag calculation.

More information is available in the HydroD and Wadam user manuals.

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Why and when is free surface mesh needed in Wadam/Wasim?

Wadam first order wave analysis does not need free surface mesh since a complex green function is applied there. Free surface boundary condition is automatically satisfied in Wadam.

A free surface mesh is needed in Wasim, since the Rankine panel method is applied.  Afree surface mesh can be created from the ‘automatic surface meshing’ function in Wasim. Some default factors are suggested in Wasim already. Or a more advanced tool, HydroMesh, can be used. In HydroMesh, you can generate any mesh you like on the free surface. You can control the size and shape of your free surface domain; create different patches on the free surface and define stretching of the mesh as you need.

The free surface mesh from HydroMesh will by default be exported as T7373.FEM file. T7373.FEM file is created in the Global coordinate system. If you have a symmetry model, then T7373.FEM is also included by the symmetry properties. 

When you want to do second order (QTF, sum/difference frequencies) analysis in Wadam, a free surface mesh is also needed in Wadam, at least for sum frequency calculations. Then T7373.FEM file can be imported to be used as a free surface model.

Further on Wadam QTF: It is a common understanding that for difference frequency problems (comparing with the sum frequency problems), the contribution from the free surface integral is small, especially when the two wave components are with the same frequency. As the difference between the two frequency components becomes larger, the contribution from the free surface will increase, especially for higher frequency region.

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Why are the mass data in Wadam files and Wamit files not the same?

By inspecting the Wadam1.lis file and the G1.SIF file, it may be found that the mass matrix is different from that printed in WAMIT_5S.out.

This may happen when a Morison model is included. In order to be able to compute 2nd order results, when asked for, Wadam is including the added mass for the Morison model into the Wamit.FRC file, one of the input files to the Wamit execution (as part of a Wadam run).

If, for instance, the Morison model has only tranverse Morison elements (outside the panel model), like bracings between pontoons, the (2,2) value in Wamit.out is identical to the similar mass value in G1.SIF.

These mass values in the Wamit files are not used in the Wadam analysis, unless this will be a 2nd order analysis. The first order motions are solved in Wadam, not in Wamit.

If the mass data from Wamit.out are to be used elsewhere, you will have to evaluate whether this added mass should be included or not. If not, the Wamit.out file can of course be edited.

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Why are there some artificial restoring terms for fluid dynamics of tanks?

The internal dynamics for a tank is in Wadam computed with the same boundary conditions as for the external wave dynamics.

This means that the mean free surface in this context is considered as fixed to the earth instead of following the tanks vertical movement.

Then the PDF solver for the external problem can be used directly for solving the internal problem.

This approach for solving the problem introduces extra, artificial terms in both the restoring and the radiation. It can be shown from Bernoulli’s equation that the artificial terms in the restoring are cancelled by corresponding extra terms in the radiation solution, so that the total solution for the pressure is correct.

Still, it might be a bit confusing that the restoring coefficients and added mass coefficients presented for the tanks are not the actually valid coefficients.

Instead, they are representatives of the restoring and added mass, containing extra terms that cancels in the computation of the forces.

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Why do wadam.lis report zero mass?

May be due to an error in the panel model, like wrong definition of external wet surfaces / dummy hydro pressure, causing zero displacement / volume. This makes Wadam report the mass as zero as well.

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Workaround for phase angle set in deterministic calculation (HydroD/Wadam)

When defining an array of phases within a phase set, HydroD writes WADAMD1 card into WADAM1.FEM file. In this card, the third value describes the number of phase input (NPHA). And then write out all the phase angles as users' input. If NPHA is larger than zero, Wadam will use the first entry as a starting value and the second entry as a phase step value. It subsequently ignores the others. WADAM thus interpreted phases in a wrong way and these wrong phases can be found in the output from WADAM.LIS.

There are two workarounds depending on users' input phase angles:

  1. If the users input phase angles has a constant phase step, then users can manually modify the second phase entry, i.e. the fifth value in WADAMD1 card as the phase step value.
  2. If the phase angles users input does not have a constant phase step, then users can modify the third value, i.e. NPHA=0 in WADAMD1 card. Thus WADAM will read the next 8 phase values and find out the total number of NPHA by counting non-zeros.

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