This section will describe in detail how to model different types of tanks and compartments.

Besides a general explanation on how to model tanks using the compartment definition table, this section contains a number of important sections that the user should be aware off when modelling tanks:

§   Number of Sections in Tanks on page 54

§   Tank and Compartment Permeability on page 49

Ø  Select the Compartment Definition table by clicking on the Compartment Definition tab at the bottom of the Input window.

Ø  Select New Compartment Definition from the File menu

, this will give you a new set of compartment definitions with one default tank.

Before you can start adding compartments, make sure you have created a Compartment definition file, see above.

Compartments may be added or deleted by

Ø  Select Add or Delete Compartment from the Edit menu.

Add will add a tank after the currently selected compartment and Delete will delete the currently selected compartment(s). The accelerator keys Ctrl+A and the Delete key may also be used to add and delete entries respectively.

Simple tanks and compartments are created by specifying six values that define a box-shaped boundary for the tank. This box will be called the Boundary Box. The boundary box is made up of the fore and aft extremities of the tank, the top and bottom, and the port and starboard limits of the tank. Each value defines one of the six planes of the tank.

The column headings in the Compartment Definition table include terms such as 'F Bottom, 'A Top', 'F Port' and 'A Starboard'. The 'F' and 'A' abbreviations stand for Forward and Aft, in other words the two ends of the compartment. You will notice that aft columns contain the word "ditto". This means that the value is identical at the aft end of the tank to the forward end, resulting in a parallel tank.

When the “Update Loadcase” command from the Analysis menu is used, or an analysis started, Hydromax will form the sections that define the tanks and compartments. This is done by finding the intersection of the tank bounding box and the hull. Thus it is not necessary to make the tanks fit the hull manually – this is done automatically by Hydromax.

Box shaped compartments can be formed from the numerical values in the compartment definition table.

See Longitudinal Extents of Boundary Box on page 54 for some recommendations regarding setting the boundary box.

The default is for compartments to have parallel sides. If you wish to define tapered compartments, it is possible to enter different transverse and vertical values for the points defining the compartment ends.

If a different value is entered in one of the “ditto” columns, a tapered tank will result. Tanks can be tapered or sloped in Plan or Profile views. Hydromax does not have a mechanism for creating a sloped tank boundary in the Body Plan view.

By changing the “ditto”-input fields, tapered tanks can be formed

Note:

Tapering can be done in Plan and in Profile view. Tapered tanks in Body Plan view have to be created using a boundary surface. See Modelling Tanks Using Boundary Surfaces on page 43.

To define more complex tank geometries, tanks, compartments and non-buoyant volumes may be linked. This means that although they are defined as separate tanks, they act as a single tank with a common free surface. To link tanks, compartments or non-buoyant volumes, first make them the same type as the parent and give them the same name. The easiest way to do this is to copy and paste the name from the Name column of the parent row into the Name column of the linked tank row. They may then be linked to the parent by typing l or linked in the Type column. Linked tanks and compartments do not have to be physically linked in space. However, the fluid in a linked tank or damaged compartment is always assumed to be able to flow freely between the linked volumes.

Tanks, compartments and non-buoyant volumes may have their boundaries defined by surfaces as well as being constrained to particular dimensions. This allows for the modelling of arbitrarily shaped tanks.

Forming tanks using boundary surfaces

The surfaces to be used to define the tank boundaries are selected by clicking in the Boundary Surfaces column in the middle of the Compartments Definition table. A dialog will appear that allows you to select which surfaces form the boundary of the tank. If a tank uses boundary surfaces, the cell in the Boundary Surfaces column is coloured blue.

If you wish to use a Maxsurf surface to define a tank or compartment, tick next to the surface name in the Boundary Surface list. Note that symmetrical surfaces appear twice as there will be a starboard and a port side copy of the surface. The Starboard surface is first in the list and the Port surface second. The port surface is also identified with the suffix (P) after the name.

Note:

Only internal structure surfaces appear in the boundary surfaces list.

Symmetrical surfaces are duplicated, with the port-side surface having “(P)” appended to the surface name.

After selecting the internal surfaces, it is necessary to type in the extents of the boundary box. Hydromax will automatically set the “Fore” and “Aft” limits of the boundary box to just within the longitudinal limits of the Boundary Surface. This ensures that at least 12 sections are inserted in the tank.

Also see:

Forming Compartments on page 52

Number of Sections in Tanks on page 54

Longitudinal Extents of Boundary Box on page 54

External tanks may not be modelled in Hydromax. However, it is normally possible to add "Hull" surfaces in the Maxsurf model, which will enclose the external tanks. The tanks can then be modelled in Hydromax.

Additional box-shaped hull surfaces used to define deck tanks

Non-buoyant volumes are effectively permanently flooded compartments. They can normally be modelled using trimmed hull surfaces. However, there are occasions where it is more convenient to use non-buoyant volumes. In some cases, where the volume to be flooded forms sections within the hydrostatic section, this is the only option, e.g. waterjet ducts. The choice whether to use trimmed surfaces or non-buoyant volumes is primarily determined by the length of the non-buoyant volume relative to the length of the vessel.

Using trimmed hull surfaces

When the length of the non-buoyant volume, relative to the length of the model, is large enough; the non-buoyant volume can be calculated accurately from the hull sections. If possible, trimmed surfaces should be used. The picture below is a good example of when to use trimmed surfaces.

Propeller tunnels modelled with trimming surfaces

Using tank type: Non-buoyant volume

In some cases using trimmed surfaces is just not possible. For example, when the sections of the non-buoyant volume are entirely enclosed within the hull sections (as is the case for a water jet duct) the use of a non-buoyant volume is the only way in which these features can be modelled.

Water-jet ducts modelled as non-buoyant volumes

Another occasion when non-buoyant volumes should be used, is when the length of the compartment relative to the length of the hull is too small to calculate its volume from the hull sections. A good example of this is a bow thruster on a long ship. If the vessel is very long, and the thruster duct is of small diameter, there may not be sufficient sections to model it accurately (even if you use the maximum of 200 sections for the Hydromax model). In this case you are better off modelling the thruster duct as internal structure and using these surfaces to define a non-buoyant volume. For example: in the image below the bow thruster volume is only calculated with one section.

For more information, see Number of Sections in Tanks on page 54.

Tip: Besides increasing the number of sections through the bow thruster from 1 to 12, modelling the thruster duct as a non-buoyant volume has the additional advantage of being able to specify a Tank and Compartment Permeability, and hence also account for the thruster.

Bow thruster tube modelled as two non-buoyant volumes

When a tank is defined within a compartment, Hydromax will automatically deduct the volume of the tank from the compartment volume using a “linked neg. (negative) compartment”. This is necessary for damage cases where the compartment is flooded and the volume of the tank should be treated completely separately from the compartment.

Linked negative compartments are deleted and recreated whenever a tank or compartment is added, deleted or modified. Negatively linked compartments are displayed on the bottom of the Compartment Definition table solely for reference purposes and are not under direct user control. This means that linked negative compartments cannot be added, deleted or modified.

Linked negative compartments are named based on both the parent compartment as well as the tank from which the linked negative compartment was derived. For example a linked negative compartment might be named “Compartment3 (Stbd Hydr Oil)” to reflect that it is derived from the intersection of Compartment3 with the Stbd Hydr Oil tank.

As mentioned earlier in this manual, only compartments and non buoyant volumes or tanks can overlap with each other. Tanks or compartments of the same type (eg two tanks) can not overlap. A tank and a non-buoyant volume are also not allowed to overlap.

Hydromax will first try to form tank sections and then check whether these sections overlap tank sections of adjacent tanks. When two conflicting or overlapping tanks or compartments are detected during the forming process, you will receive an error message:

Notice that the compartment definition row number of the tank is given in brackets
i.e. tank #8 intersects tank #3.

Troubleshooting Overlapping Tanks

Sometimes the reason for the conflict can be quite simple: eg an overlapping boundary box. However, when you are modelling tanks using boundary surfaces, the surface boundaries act as a boundary between two adjacent tanks and the bounding box extents are allowed to overlap. In these cases, it can be quite difficult to see why the tanks overlap, especially if you have a large number of tanks already defined.

By temporarily deleting all tanks except for the one that does not form, it often becomes clear why the tank overlaps. In the case of the image above, the tank’s fwd most section goes all the way to the CL (probably because the fwd boundary box extent is just fwd of the boundary surfaces or exactly on the edge of a boundary surface). This causes this particular tank to “overlap” with surrounding tanks.

Procedure to Fix Overlapping Tanks:

Ø  Save Model

Ø  Go into Comp def window

Ø  Save comp def

Ø  Delete all tanks except for one you wish to investigate

Ø  form tanks, inspect tank sections

Ø  Try to fix tank definition, eg by selecting additional boundary surfaces

Now that you know how to fix it..

Ø  Close comp def file. Do NOT save!!

Ø  Open saved Comp def file

Ø  Fix compartment.

Ø  Save & move on to next compartment.

Tanks may have two permeabilities; one, which is used when the tank is intact, and the other when it is damaged. Compartments and non-buoyant volumes have only one permeability, thought it is listed in both columns.  The compartment permeability is applied when the compartment is flooded in a damage condition and the non-buoyant volume permeability is applied at all times since it is always flooded.

In the case of damaged tanks and compartments, the permeability fraction is also applied to the free-surface-moment contribution of that tank or compartment.

Permeability of Compartments

As opposed to tanks, compartments typically have structure (other than plate stiffeners) and equipment inside. In case of large variations in permeability within a compartment it is recommended to model separate linked compartments with separate permeability to increase accuracy.
For example an engine room with engines and auxiliaries at the tanktop could be divided up in a lower- and an upper engine room compartment. The lower compartment will have a permeability of, for example, 60% and the upper compartment a permeability of 95%. Depending on the level of accuracy required, the engines and equipment could also be modelled individually as empty tanks.

Relative Density (Specific Gravity) values can be typed directly into the Relative Density column of the Compartment Definition table.

Alternatively the fluid type can be entered into the Fluid Type column, either as the name or as one of the single letter codes (when entering the name, auto complete is used, so it is normally only necessary to type the first few letter of the name). If a fluid type is entered, the relative density value is obtained from the value specified in the Density dialog. Whenever values are changed in the Density dialog (see Density of Fluids on page 101), all entries for that fluid in the compartment definition are automatically updated.

If you have specified that Hydromax should include the surface thickness, the tanks, compartments and non-buoyant volumes will correctly account for the surface thickness and its projection direction: the tanks will go to the inside of the hull shell.

Note:

Thickness of boundary surfaces are not taken into account, hence you should design these surfaces to the inside of the tank.

Tanks defined in the Compartment Definition table appear in the loadcase in the same order as they are defined in the Compartment Definition table. To reorder the tanks:

Ø  Copy the tank definition data to Excel

Ø  Sort the rows in to the desired order

Ø  Paste the data from Excel back into the Compartment Definition table.

Take care if you have linked tanks – unlink them first.

When creating complicated tank plans, it is often useful to check individual tanks. Selected tanks may be displayed in the following manner:

Ø  Define a damage case

Ø  Select only damaged tanks and compartments for display, turn off the display of intact tanks and compartments.

Ø  Select whether you want to see the tank outline or the tank sections (tanks sections are preferable when checking that tanks have been formed correctly since it is these sections which are used to determine the tank volume and other properties).

Ø  Choose the damage case from the Analysis toolbar

Ø  Set any of the tanks and compartments you wish to be visible to damaged in the damage case window.

You can make the damage case window quite small and tile it next to the perspective view. Use this to quickly turn tanks on and off by changing their damage status.

Using a damage case to quickly change the tank and compartment visibility