Wasim

On this page

How to include compartments when using multiple instances of the superelement in Presel? (HydroD/wadam/Wasim)

It is not possible to reuse superelements with compartment definitions in HydroD. The only exception is if a compartment is actually split in two or more superelements, e.g. at the centre line of a vessel.

The reason for this is that the load case definitions for compartments must be unique in the 1st  level models. Reused superelements will have the same load case numbers as the original, modelled, superelements. Load combinations in Presel are not used for these dummy load cases. HydroD, Wadam and Wasim  will read the definitions as created in the 1st level model only.

The way to do this with superelements is to create more superelement T-files in GeniE, with specific load case numbers for the compartments in each; one superelement for each location in the complete model. Or, alternatively, not use the superelement technique this way.

Back to top

How to input prescribed wave elevations?

It is possible to define prescribed wave elevations for a Wasim analysis.

It is important to define constant time steps for the wave elevations. Wasim is using the last time step together with all the data as input to calculate the wave components, especially the wave frequency. Therefore a precise input on the time step will be important. Inside the program we use double precision to get correct results. When we get a small difference in the expected wave time step, a small error will be introduced in the wave components, which will shift the values by some percentage and more and more with time growing.

This issue may typically be seen as differences in the wave phase angles when plotting the input wave elevations vs. the calculated ones.

More information will be found in the Wasim user manual, e.g. section 2.16 Prescribed waves.

Back to top

How to make sure Morison loads are included in the Sestra structural analysis? (HydroD/wadam/Wasim)

How to use a composite model (Morison model plus shell model) in a HydroD/Wadam/Wasim load transfer analysis?

In order to use the anchor loads and any other loads from the Morison model in the structural analysis in Sestra, the Morison model has to be part of the structural model.

The recommended method is to use a “Single superelement composite model”. This means that the complete model can be created in one model, i.e. one superelement (one T#.FEM file), and this file can be read into HydroD as both a Morison model and the Structural model. In this method, all loads, both accelerations, surface pressures and Morison loads, will be part of the same load file (L#.FEM). More information is found in the Wadam user manual. The HydroD user manual and release notes (for HydroD 6 versions) can also be useful.

If the Morison model is a separate model (T#.FEM, e.g. T2.FEM), Wadam/Wasim will create a load file, e.g. L2.FEM, for the Morison loads, including anchor loads etc. Unless T2.FEM is also a part of the structural model, Sestra will not read the L2.FEM file. Only the L files matching the T files in the structural model will be used. In this case a superelement model assembled in Presel is required. This means that supernodes (special boundary conditions) need to be defined in the Morison model, e.g. T2.FEM. These need to match the supernodes for the connecting superelements (shell model). Then read T2.FEM into Presel together with the other superelements etc.

Back to top

How to select filling fractions for sub model? (HydroD/wadam/Wasim)

If the whole compartment in the sub model is fully wet or dry, then the filling fraction for that compartment can be defined to be 1 or 0.

If a compartment is not completely modelled in the sub model, and it is partly filled only, then some special technic is needed for filling fraction defined.

Go to GeniE first, using the global model and look at the interesting compartment. Carefully select the wetted/submerged internal compartment surface in the sub model, according to the filling fraction, assign them ‘wet surface’ property. Then apply the correct load case number.

Then run submod (the same as before).

Import into HydroD the modified sub model with only partly defined wetted internal surface.

Assign filling fraction for this compartment to be 1.

So the correct internal wet surface will be generated from HydroD and read into Wadam/Wasim.

Also remember to define the correct Compartment points, typically taken from the global analysis, from Wadam.lis or from HydroD information.

Back to top

How to transfer Sima outputs to Wasim

How to transfer Sima outputs to Wasim?

If the users would like to read the Sima outputs into Wasim for a time domain hydrodynamic analysis, they need to select storing the results in Dynamic Calculation – Storage – WASIM export.

The new version Sima v4.2-00 changes the formats of WASIM export compared with previous versions. This change is to align with the updates in Wasim v6.3-10. Now Sima could output the following files in Wasim required formats and store the files in the Dyanmic results folder.

Incoming wave:

  • sima.wave: The incoming wave elevation at global origin.
  • sima.wavecomponents: Export as wave components.

Body motion:

  • sima.motion: Motions (position, velocity and acceleration), 6 DoF.

External force and moments on body:

  • sima.force: The time series of forces and moments, 6 DoF.

There are two kinds of wave files, one is wave elevation time series, the other is described as wave components. Both can be read by Wasim, the user should choose one of them. Details about the file formats can be seen in the Wasim user manual, the commands for Wasim extra parameters can also be found.

  • 2.18.2 Prescribed wave components: New file Format
  • 2.18.4 Prescribed wave elevation
  • 2.19.2 Prescribed motions from Sima      
  • 2.20.1 Prescribed auxiliary forces and moments

SIMA Export files should be defined in Wasim Activity – Execution Directives – Extra parameters.

Regarding the prescribed waves, the user could use the commands for either wave components or wave elevation time series. Two templates can be found in the attachments:

Back to top

How to understand the Morison results in the file *_loadTransferMorison

  1. Wasim writes the Morison load in L file from a Wasim snapshot load transfer analysis. Before doing this, Wasim keeps the Morison load at each time step, and write the load in *_loadTransferMorison. The format of the file is following, and the forces are given in the FEM coordinate system.

No. of Morison beams,  No. of Morison nodes, Total time steps

1st time step

Beam forces on the 1st Morison beam in x, y, z direction, Immergence fraction of this beam (1 for submerged, 0 for dry, other value for partially wet)

Beam forces on the 2st Morison beam in x, y, z direction, Immergence fraction of this beam

…

Point forces on the 1st Morison node in x,y,z direction

…

  1. Load file should be interpreted with another file named *_loadTransfer_geoMorison. This file gives the relationship between the internal Morison beam/node numbering and the beam/node numbers in FEM file. The format of the file is following

Total number of Morison beam elements, Total number of Morison nodes

(For the 1st Morison element) Which FEM beam element it belongs to, no. of sub element on the FEM element, location of the sub element, length of the element

…

(For the 1st Morison node element) Which FEM node element it attached to.

Back to top

How to understand Wasim output file: ‘***_loadTransferMorison’

File ‘***_loadTransferMorison’ includes morison load after Wasim load transfer analysis.

This file contains four or three columns.

The lines with 4 columns are the loads on the 2D Morison elements. There is one line per sub-element. The first three numbers are the three force components on the element in the global coordinate system. The fourth column is the wet fraction of the sub-element. A value of 1.0 means that it is totally submerged. Thus, to get the actual force on the sub-element the force components should be multiplied with the wet fraction.

The lines with three columns are loads on other types of Morison elements, which represents nodal loads. This can be due to pressure area elements, anchor elements or TLP elements. This is also a part of the Morison model, but the loads are not computed from Morison’s equation, which is only relevant for the line loads.

Back to top

Mooring force time series of soft spring system

Users can obtain time series file (detfrc_mooring.frc) with the below extra parameters. The file contains total mooring forces and moments in body fixed system.

ifout=40*1
detfrc='detfrc'

The parameters will write other .frc files in addition to the mooring force file. Input files:

Output mooring force file:

Back to top

The order of sections and points in a section model

For a section model, we have the following requirements:

  • Sections must be given from fore to aft (x coordinates from large to small) 
  • A section can only have one intersection with the free surface
  • On a partially wet patch, the points on a section must be given from bottom to deck (z coordinates from small to large)

For example, a vertical column must be split into two patches to fulfill the first requirement, and the order of the sections and the points are shown as below.

Back to top

Torsethaugen wave spectrum cannot be used in an irregular sea state (HydroD 4.x/Wasim)

A Torsethaugen wave spectrum cannot be used in an irregular sea state.

The Bretschneider and Jonswap spectra may be used.

Back to top

Use of non-Morison elements in the Morison model (HydroD/wadam/Wasim)

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).

Back to top

What is the difference between RelativeMotionPoints and WaveElevationPoints?

Version: 5.2-07

There are two concepts in Wasim version 5.2-07.

‘Wave elevation points’ is always defined on the free surface and only reports total wave elevation in a file 'XXX .wel'.

‘Relative motion points’ is similar to offbody points in Wadam. This point can be defined anywhere. Velocity and pressure will be available for this point in Postresp.

In order to get the velocity and pressure, a frequency domain analysis in Wasim is needed. Read the G file into Postresp and the velocity and pressure at the Relative motion points will be available.

Back to top

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.

Back to top

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

Back to top

Why are rudder and/or springs needed in Wasim?

Wasim is a time domain and nonlinear program, which means that your structures will drift away from its original position. No natural stiffness is available for horizontal positions (surge, sway and yaw).

In order to hold your structure back, additional mechanism rudder/spring is needed in Wasim.

Controlling equations for rudder/spring are given in Wasim_UM section 2.4. More information is given here.

  1. Rudder is preferred to be placed behind the ship with half-ship-length distance relative to the stern.
  2. Rudder force and/or spring force will participate when motion of equation is solved. But they do not contribute to load transfer.
  3. The eigen periods of the spring should be away from the eigen periods of the structure. The longer spring eigen periods, the softer it is.
  4. If your structure drifts away when running it in calm water with forward speed, then it is a good time to adjust your rudder/spring setting. You must make sure other disturbances like static balancing, mesh quality etc. are excluded.

Back to top