Beñat Rodríguez

Julen García

¿What is Aqwa? Aqwa is a software included in Ansys

Is an engineering toolset designed to simulate the effects of wave,

wind and current on both floating and fixed offshore structures

The Hydrodynamic Diffraction tool enables the developing of the

primary hydrodynamic parameters required for the analysis of

motions and responses

It is also possible to generate pressure and inertial loading

The purpose of this work is to guide new users throughout their

first steps of the learning of the program

Modules Aqwa Line

Regular waves

Aqwa Graphical Supervisor

Graphical Supervisor

AqwaWave

Structural FEM analysis

AqwaWorkbench

Used to create an element based model from geometry defined in

Ansys Design Modeler, apply AQWA specific input and view results

It is the one analyzed in this guide

1. Creating the geometry AqwaWorkbench (AQWAWB) requires a geometry fully

compatible with the program

For that purpose, some operations will be carried out in the AnsysWorkbench (do not confuse with Aqwawb)

Open AnsysWorkbench

In AnsysWorkbench, select “Geometry” in

“Component Systems” of the “Toolbox”

That will open a new box with an empty

geometry. Double-clicking on the question

mark will open the Ansys Design Modeler (DM)

1. Creating the geometry Once in the DM, and after selecting the units to work with,

there are two options:

A) Import a geometry already created with a CAD program

B) Create a geometry with the DM

Once that is done, some operations are needed in the DM in

order to make the geometry compatible with AQWAWB

Just for basic tutorial purposes, the creation of a simple cylinder

in the DM is briefly explained here

1. Creating the geometry DM CAD tool is simple, but with a little extra work, simple-

medium geometries can be created

For learning purposes, a generic cylinder’s creation will be

explained. Later, with experience, some tricks can be used to

make it quicker the fulfillment of the subsequent compatibility

conditions

The cylinder is pretended to be a vertical offshore energy system

1. Creating the geometry Will be created a cylinder with the X axis being the longitudinal

one NOTE: Directly creating a cylinder with the Z axis being the longitudinal one would make the

work easier, but for learning purposes here is designed in another way

The characteristics are [MKS]:

Length: 15

Diameter: 4

Cylinder density: 600

Water density: 1028

So, with basic fluid's theory (not explained here):

Draft: 8.75 [Draft=Length*(ρcyl/ρwat)]

1. Creating the geometry DM has three main windows:

Graphics

Tree Outline

Details view

In the tree outline, is important

to remark that there are two tabs:

Sketching

Modeling

In the upper part there are some tool bars which include file management, selection, viewing and operations icons

graphics

tree outline

detail’s view

tool bars

1. Creating the geometry NOTE: DM’s use is not the aim of this tutorial so just a briefly guide of steps is carried out here.

Anyway, no problems should arise since the geometry is very simple. For further explanation check DM tutorials

Select YZPlane and click in the blue icon of the tool’s bar name “New sketch”

It is important to understand that every plane, body… always has a blue arrow pointing its normal direction

Sketch1 will appear in YZPlane. Select it and click in the “Sketching” tab of the “Tree outline”

Select “Circle” and click on the coordinate origin

Move the mouse and click to select its diameter

(select a random one)

1. Creating the geometry In the “Sketching” tab, below all the drawings (line, circle,

ellipse…), click on dimensions

Select “Diameter” and click on the created circle

In “Details View” window set D1 to 4

In the tool’s bar click on the “Generate” icon (a yellow thunder) in order to integrate the new sketch in the geometry

Click on the “Extrude” icon of the tool’s bar

Extrude1 should appear in the “Tree Outline”

In “Detail’s view” window click on the box which is to the right of “Base Object”

1. Creating the geometry Select “Sketch1” either in the “Tree Outline” or directly in the

“Graphics” window, and click “Apply”

In “FD1, Depth (>0)” input 15

Click on “Generate”

With this, the creation of the cylinder is completed

We could have also reached this point importing a CAD geometry: File/Import External Geometry File…

Now, the compatibility requirement’s fulfillment will be explained

NOTE: It would have been better to change, in “Direction”, from “Normal” to “Both – Asymmetric” and set the values that make directly the XYPlane coincident with Global Water Axis

2. General modeling requirements Requirements:

Only Lines and Surfaces are processed (not Bodies)

Surfaces must have normals pointing outwards

The model is oriented Z axis vertical up

The model has to be split at the water line

Each structure should be a part

2. General modeling requirements The structure is a Body. It can be looked in “Tree Outline”, that below ”1

Part, 1 Body” there is one “Solid”

It has to be a surface. For this purpose, select “Thin/Surface” of the tool’s bar

“Thin1” should appear in “Tree Outline”

In “Details View”, in “Selection Type” select “Bodies Only”

Click on the box to the right of “Geometry” and then on the Body. Then click “Apply”

In “FD1, Thickness (>=0)” input 0

and click on “Generate”

Now, in ”1 Part, 1 Body” there is one

“Surface Body” NOTE: If you create an operation, i.e. “Thin1”, and then you delete it, when creating a new one, its

name would be “Thin2” even if now there is no “Thin1”

2. General modeling requirements Structure’s longitudinal axis must be on the Z axis

For that purpose a rotation and a translation must be carried out. This is done via “Body Operation”

Click on Create/Body Operation

BodyOp1 should appear in the “Tree Outline”

In “Details View”, in type select “Rotate”

In “Bodies” select our body like done previously

In “Axis Definition” select “Components”

The rotation axis is Y, and the required angle is 90º, so in the components put 0, 1 and 0. And the angle 90º

2. General modeling requirements Now, the translation is required

AQWAWB requires that the draft line is at the fixed reference

axis. XYPlane will always be the draft line

Since our structure has a length of 15 and a draft of 8.75, the

structure has to be translated 6.25 units in the Z+ axis (=15-

8.75)

So, a new plane is required in order to make the operation

Click on Create/New Plane

Plane4 should appear in the “Tree Outline”

2. General modeling requirements In “Detail’s View”, select “XYPlane” as Base Plane, in “Transform1

(RBD)” Select “Offset Z” and input 6.25 as a value NOTE: The international version of the program uses comma [,] and not period [.] as decimal

separator, so input “6,25”

Click on “Generate” to proceed

Now, perform a new “Body Operation”, “BodyOp5”, of type

“Move”, where the source plane is “XYPlane” and destination

plane is “Plane4”

2. General modeling requirements When clicking in “XYPlane” and rotating the view, will be easy to

see the how the draft line goes through the right place

Now, the surface must be split at the draft line

For that purpose, it is required to freeze and then split the

structure. This is used with “Freeze” and “Slice”

First, click on Tools/Freeze

Then, click on Create/Slice

Slice1 should appear in “Tree Outline”

2. General modeling requirements Select “XYPlane” as base plane and generate

After this, “2 Parts/ 2 Bodies” appear in “Tree Outline” with 2 “Surface Body”-s below it

One of the requirements is that

the structure has to be a single part.

In order to achieve this, hold the control

key while clicking on both “Surface Body”-s

Free control key, right-click on

the selection and select “Form New Part”

This completes the procedure. The structure is ready

2. General modeling requirements There are two concepts to understand:

One thing is saving the work of the interface. It is done in the DM:

File/Save Project, or directly in the AnsysWorkbench: File/Save

(.wbpj)

But the file imported by AQWAWB has another extension (.agdb).

It is exported from the DM by clicking on File/Export…

The .agdb file is the one to import from AQWAWB

Set its name: “aqwawbguide1”

3. Importing the geometry Open AQWAWB

AQWAWB has three main windows:

Graphics

Outline

Details

Output

In the upper part there are some tool bars which include file

management, selection, viewing and operations icons

graphics

output

outline

details

tool’s bars

3. Importing the geometry For importing the geometry right-click on “Geometry” and select

“Insert Geometry” and “From File…”

Import “aqwawbguide.agdb” from where you saved it

After some time the bar will reach the 100% and if there is no error the geometry should appear very small in the “Graphics” window

There are three elements in the “Graphics” window: A big dark-grey square (the bottom of the sea)

A very small cylinder (the structure)

A big light-grey square dividing the cylinder (the water line)

For viewing purposes, click on “Part” and select “Zoom to Fit” on the tool’s bar (the magnifier with a blue box inside) to fit the zoom to the cylinder

3. Importing the geometry

4. Pre-processing I: The geometry Click on “Geometry” in “Outline” window. In “Details” window, there

are some options needed to know

The sea level must coincide with where the surface is split

The depth of the sea can be defined. For the analysis it will be fixed to 500. Is an important variable since the results will depend on it and, as it will be explained later, frequency range will be determined by this

The density of the water is a function of the temperature. In Europe is 1028 is the most common, being 1026 the standard for USA. In this analysis will be set to 1028

NOTE: Remember that the units are already set. Of course Kg/m^3

The size of the water will affect the display and

will be 1000 for either X and Y

4. Pre-processing I: The geometry Click on “Part” in “Outline” window.

In this section it is possible to show or hide the geometry using

“Part Visibility” and decide if the geometry will take part in the

analysis or not via “Part Activity”. In the analysis, the part will be

active and visible

Also, it is possible to fix the structure or let it free to move

(what is done in this analysis). No internal lid is required here,

but could be useful, for instance, in a moon pool

No linear damping is required

4. Pre-processing I: The geometry Now, click on the first “Surface Body” (check that only the part

below the water is now green) NOTE: if the below water part is the second one, select that second “Surface Body”

It’s required to have the below water surface as a diffracting

surface, and the one above the water as a non-diffracting one.

This is done with “Surface Type”

“Structure Type” must be set as a “Physical geometry” in the

analyzed structure

Now, the mass and inertias must be included

Click on “Part” and then, in the upper bar, select Add/Point Mass

4. Pre-processing I: The geometry Physical characteristics of the cylinder are:

Mass: 113097 [Mass=Density*Volume]

Inertias:

For generic cylinder with z axis longitudinal

Ix=(1/2)*M*(R^2)

Iy=Iz=(1/12)*M*{[3*(R^2)]+(L^2)}

Iyz=(1/12)*M*(L^2)

Ixy=Izx=(1/4)*M*(R^2)

For this guide’s cylinder:

Ix=226194

Iy=Iz=2233666

Iyz=2120569

Ixy=Izx=113097

4. Pre-processing I: The geometry Click on “Point Mass”

In “Mass Definition” change it to “Manual” and set it to 113097

The inertia can be input directly or using the radius of gyration as can be seen in “Define inertia values by”.

Using “Direct input of inertia”: Ixx=226194

Ixy=113097

Ixz=113097

Iyy=2233666

Iyz=2120569

Izz=2233666

4. Pre-processing I: The geometry It is important that “X”, “Y” and “Z” are selected so that the

“Point Mass” is applied right into the mass centre of the cylinder.

For this example, Z is required to be -1.25 [=(Length/2)-Draft]

“Point Buoyancy” and “Disc” can also be included

Point Buoyancy requires a position and a volume

Disc can be used to create an area that has drag and added mass in

the direction perpendicular to the disc

With this work, the geometry is done and we are ready to set the

Mesh

5. Pre-processing II: The mesh Click on “Mesh”

In “Meshing Type” the algorythm used for the mesh generation can be controled. It works nice as “Program Controlled” but if any problem arised, change it to adjust it to the required geometry

There are 2 variables which can be changed:

“Defeaturing Tolerance” (how small the details are treated)

“Max Element Size” (maximum size of an ellement)

Note that “Max Allowed Frequency” is inversely proportional to “Max Element Size”

It is required that:

Defeaturing tolerance < 0.6 * Max element size

No. Of Elements < 18000 (of which diffracting < 12000)

5. Pre-processing II: The mesh For our analysis, set:

DefeaturingTolerance: 0.8

Max Element Size: 1.4

If you click in “Mesh” and then in the upper bar select InsertMesh Control/Sizing, a new option is avaible

This option enables the refinement of a mesh on any given partor body, by enabling a smaller element size to be associated tothe geometry. Will not be used in this guide.

NOTE: In case you added it, it is possible to delete it by right-clicking it and selecting “Delete”. Note that “Supress” also deletes it, but by unsupressing it, becomes avaible again

5. Pre-processing II: The mesh Once it is all set, click on “Generate Mesh” (yellow thunder)

After a while (large time if the selected size is too small), the

mesh will be generated

To view it, click on “Part”,then in “ZoomFit” (remember, the

magnifier with a blue box inside), and then on “Mesh”

Once it is done, it is time to set the analysis

6. Pre-processing III: The analysis Click on “aqwawbguide1” and the on Insert analysis/Hydrodynamic

Diffraction

A new icon named “Analysis XX” should have appeared

Click on “Analysis Options”

The “Sea Grid Factor” controls how much larger the area is than the

structure. Set it to 2

In “Output File Options” there are multyple options to manage the files

outputted by AQWAWB. Let all them as “No”

“Common analysis options” control how the analysis is performed. It is useful

to activate “Ignore modelling rule violations”. Activate it and let the others

desactivated

6. Pre-processing III: The analysis “Structure selection” tree object enables the definition of

interacting structures

Is used to exclude structures that you want to exclude for any

particular analysis

It also enables the order of solving to be changed

For this analysis, do not change anything here

“Gravity” enables the definition of gravity

It should not be changed in most cases

6. Pre-processing III: The analysis Click on “Wave Directions”

It is possible to add a forward speed to the structure, but if that is done, only

a single wave direction can be analyzed. We will not use it

Waves are automatically created in -180 and +180 directions

It is possible to set the interval between the required directions, or simple

put how many intermediate directions are. Selecting too many can make the

solving costly. Put 7 intermediate directions

It is also possible to add intermediate directions or ranges of directions

NOTE: 0º is always X+

6. Pre-processing III: The analysis Click on “Wave frequencies”

It is possible to select either one single frequency or a range of them. Select

“Range”

It is required to set the starting and ending frenquency

Approximate requirements: Start Frequency > 0.16 / (Water Depth)^0.5

End Frequency < 0.51 / (Max Element Size)^0.5

End Frencuency > 1.1 Start Frequency

Set “Start Frequency” to 0.10 and “End Frequency” to 0.40

Also, add 12 intermediate values

Of course, period is the inverse of the frequency so any change in one of

them would affect the other

7. Processing There are two analysis options for a Hydrodynamic Diffraction analysis:

either to calculate the Hydrostatics only, or to calculate the full Hydrodynamic results

Typically, you will first do the first one if you are not sure of the pre-processing and want to check for errors in a quick solving, or simply if youjust require the hydrostatics

The solving is done by clicking on “Analysis XX” and then on “Solve” or“Solve Hydrostatics”

Click on “Solve”

You will be able to follow the progress of the analysis through the smallwindow that is opened during the analysis

Once it ends, is time to check the results

8. Post-processing Right-click on “Results”. Add “Hydrostatic”, “Hydrodynamic Graph” and “Pressures

and Motions”.

Click on “Part” and then on “ZoomFit” (remember: the magnifier with a small blue

box inside)

Click on “Hydrostatic”

The Centre of Gravity appears in green (is equal to the “Point Mass” in this case)

The Centre of Buoyancy appears in red

The Centre of Floatation appears in blue

In “Graphics” window there is a small tab called “Properties”

Clicking on it, detailed hydrostatics results will appear:

Hydrostatic Displacement Properties, CutWater

Plane Properties, Small Angles Stability Parameters

8. Post-processing

8. Post-processing “Hydrodynamic Graph” enables the plotting of up to 4 comparative graph

results

Can be plotted against either Frequency or Period and can be Amplitude orPhase based

In “Line A” select the structure and view how the results operate

It is possible to plot different types of results as hydrodynamic coefficients orforces

NOTE: Check theory for the meaning of the different results

For this guide application, just select the RAOs (Response AmplitudeOperators) in Z axis in any direction (Amplitude based against Frequency) and check how, starting from 1 (no amplification), the maximum for thisparticular case happens close to 0.15 Hz, and then when the frequency is toohigh the RAOs are negligible

Also, add another line to plot how the Diffraction + Froude-Krylov forces in Z axis is smaller when the frequency of the waves increases

8. Post-processing

8. Post-processing Finally, “Pressures and Motions” enables the visualization and display of a number of

results generated from AQWA once a hydrodynamic solve has been performed

Using “Result Selection”,all the combination of the analyzed wave types can beselected (wave’s frequencies and directions)

“IncidentWave Amplitude” can be selected

In “ResultType” can be selected constat ones like “Maximum”, “Minimum or“Amplitude”, or also “Cyclic” where just a single phase or a “Range” of them can beselected (allowing a video output)

In “Contour Selection” both the Preassure (Interpolated or Panel) can be selected. There are options where it is possible to select if the body above the water has “ZeroPressure” or is simply “Dimmed”. The pressure can be measured as “Head of Water” or as “Force/Area”. There is also the possibility to visualize the “ResultantDisplacement” in “Structure ContourType”. In both cases, it is possible to visualizeor not the waves (“Wave ContourType”)

“Component Selection” allows the disabling of different parts of the visualization

8. Post-processing In this guide, the following case will be analyzed:

Frequency: 0.22

Direction: 45

IncidentWave Amplitude: 1.5

ResultType: Cyclic

Wave Position (Phase): Range

No. Of Steps: 60

Structure ContourType: Interpolated Preassure

AboveWater Body Display: Zero Pressure

Pressure Measurement: Head of Water

Wave ContourType: Wave Height

Click on “Part” and then on “ZoomFit”. Then click on “Pressures and Motions”, and in “Output”window, below the Graphical window, click on thered “Play”. A video file (.avi) can be exported

8. Post-processing

Beñat Rodríguez (undergraduate scholarship)

b.rodriguez.madariaga@gmail.com

Julen García (undergraduate assistant)

jxgarciaibanez@hotmail.com

Faculty of Engineering in Bilbao

Tecnalia Researching Room