lab#4 modelling a ship x-section

6002 Lab#4: Hull Section Model in ANSYS, page 1

Engineering 6002 - Ship Structures I

Lab#4

Modelling a Ship X-section By C. Daley

Overview

In this lab we will model the cross-section of a ship. We will

limit the problem to a relatively simple arrangement and

model everything with shell elements. The initial geometry is

modelled in Rhino, and imported as an IGES file into

SpaceClaim to make some adjustments, and then use ANSYS

to model the structural behavior. The aim is to see how to do

larger problems in ANSYS.

ANSYS Model #4 – Ship X-section

Step 1: describe and sketch the problem:

I’ve drawn a 10m section of a ship in Rhino. Before exporting from Rhino, I made sure that all

surfaces contacted all other surfaces along edges. For example the top deck is a set of surfaces,

rather than being one surface. The same is true for all the frame intersections. This will allow the

meshing to work and represent a fully welded structure (i.e. one solid body made up of many

welded surfaces.) For this lab, I will provide this iges file called MidSh.igs.

We will be applying a hydrostatic water pressure to the outside as well as an overall bending

moment.


Step 2: estimate expected results (analytically):

Just imagine what the resulting deflections will look like.

Step 3: open ANSYS Workbench 19.0

1) First, save the (empty) project as AnsysL4.wbpj 2) The left-hand window shows a set of analysis type options. Select Static Structural and

drag the icon to the right, placing it in the Project Schematic window.

Step4: open Geometry and create the CAD model

1) Right-Click on Geometry and select Import Geometry and Browse… Find the file

called MidSh.igs and select it. The Geometry component will get a green check mark, to

indicate geometry is available for ANSYS. We could open Model, but we need to open

SpaceClaim to fix a few things.

2) Now Click Geometry in the Project window, ANSYS will open SpaceClaim. You will see

a window like this;


3) If you want to change options or units, but I’m going to stay with mm.

4) In a realistic case, there would be a wide range of different thicknesses. In our case, for

simplicity we will assume all the outer shell (deck, sides and bottom) are 25mm, and all

the frame webs and flanges are 15mm. We can set all these in SpaceClaim, relatively

easily. We will make a component of all the outer plate and a second component of all

the framing. Then we will make a third component of the first two components. We need

components to all us to merge (weld) everything together, and we can quickly change the

thickness of all parts of a component.

Start by selecting panels that are part of the out hull. Hold the Ctrl key down to select

more, (or unselect ones you selected in error). They will turn orange. When you have all

or most selected, go over to the left hand list of TrimSrf and right-click and select Move to New Component. You will then see that the selected surfaces are listed below a

Component.


You can un-check the component and the Component parts will disappear, making it

easier to see what more you need to do.

If some part of the outer hull was left out, you can select it and then over on the left, drag

the highlighted selections down into the component. When only the frame parts are left

showing, you can select them all at once (‘elastic band select’) and move them all to a

second component.

5) You can select all outer plate and set the thickness to 25mm, and set the thickness of the

frames to 15mm. The thicknesses will be listed in the list at left;


For each component, select Merge, in the Share Topology part of the properties window

(lower left).

Now with the two Components selected, move them into a 3rd component;

And for the new component, select Merge again in the properties area.

This completes all work in SpaceClaim.

Step5: open Model and create the Finite Element model

1) Return to the ANSYS window, and click on the Model feature in the Project window.

This will start the ANSYS ‘Mechanical’ program.

2) The Mechanical window shows the 1 component, with a checkmark indicating that all the

plates have a thickness.

At first the model is shown with no mesh or loads yet. On the left is a list of the model

features that have to be set. By default, the material to be used will be structural steel.


Note:

A green checkmark means that everything is OK

A yellow lightning bolt means that something hasn’t been done, but its ready to be

done.

A question mark means that there is something missing, or not yet set. ANSYS can’t

solve the model if there are any question marks.

Select the Mesh icon in the Project and right-click Generate Mesh. This will take a little

while.

The mesh on the body is;


3) Now we will set the applied load and support condition on the grillage.

First we change the view to Front. .

Change select to Box Select

Now Right-click on static structural, select Insert

and select Fixed Support;

With lines selected use the box select tool to select all the edges of one cut of the

model. The edges will be highlighted in green.


Now click on Apply

You should see the base highlighted in purple and a green check by fixed support

under Outline.

Right-click on static structural, select Insert

and select Remote Displacement;

Now select the other cross section and click Apply in the panel on the left that lists Details of Remote Displacement.


By default, all 6 degrees of freedom are set to free. Edit the Rotation Y component to 0.1

deg., and the Behavior to Rigid.

This action will impose a 0.1 deg rotation to the whole cross section.

Let’s find the size of the moment reaction that this imposed displacement has created.

Under Solution, Insert a Probe of the Moment Reaction

And then select Remote Displacement as the location of the Moment

You should see the moment as 1.139e9 N-m. You will also see the exaggerated deformed

shape;


Now lets apply water pressure (hydrostatic pressure) to the outer hull.

Apply this pressure to all the outer hull (not deck);

Set the Fluid Density to 1012. Set the Acceleration to 9.81 and have the direction up;


Now set the free surface z=8m.

The Hydrostatic Pressure will look like;

Now solve, insert Normal (X) Stress, Equivalent Stress and Total Deflection as output;

Examine the results. One way is to look at the color contours. The other is to use the probe.

The probe shows typical deck x - stresses of 186 MPa (tension). Typical deck y - stresses of 34 MPa

(tension). And then the typical equivalent stresses are 170 MPa.


This completes the Analysis.


Exercises: Student:______________

For these exercises, prepare and submit a 2 page report.

Exercise #1 – In the above analyses, can you explain why the typical von-Mises

equivalent stresses were less than x-direction stress?

And why are the highest stresses in the corners? Are they real?

Exercise #2 – Re-do the above analysis, but instead of an imposed bending, us the

remote displacement to apply a 10mm vertical shear displacement on the non-fixed cross

section. Add a plot of shear stress and make sure the plot represents shear in the plane of

the side shell. What level of shear is present?

lab#4 modelling a ship x-section

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