Angle of GZmax and Load Cases

Hi!

I am naval architecture student and I am designing a tanker ship hull. I have a several questions about it.

Firstly, Loadcases. I have created loadcases in maxsurf. My first load case is full loaded. Are ballast tanks %100 full on full load case? I examined the capacity plans of some ships. According to the plans, when all the tanks were full, it was more than the ship's displacement. For example; in one tanker ship total cargo tanks capacity is 120.000 m3 and total ballast tanks capacity is 40.000 t. However, ship's displacement is approximately 122.000 t at scantling draught.

Secondly, Angle of GZmax. How can I increase angle of GZmax? You know, angle of GZmax must be greater than 25 degrees but it is 23,6 degree at full-load case. I can decrease KG but if I do it , GM value will increase. My GM value is 3,9 m at full load case. GM is 1.0 - 6.0 m on tanker ship at fully-loaded according to Preliminary design book.

Thanks in advance for your helps

 

Welcome to the forum dundrfikret

In a way you have already answered your own question:

The full load cannot be greater than the scantling draft...another way to think of it..this scantling draft is generally coincident with the Load Line mark too.

So, you have think..what is the vessel...well...you have stated it is a tanker.
This implies it is carrying "goods" of some kind...it could be sand, Oil, Gas, grain...most likely it is fluids of some type.

So, when the vessel is full, with all the holds/voids/tanks full...this would be at the scantling draft....the full laod.
When the vessel bunkers or offloads some of the fluids in the hold/tank.voids...what happens...??

well 2 things... i) she will now be floating on a lesser draft...and ii) if the removal of said fluids creates a trim, and could be excessive, thus it needs correcting.
So ballast tanks will be used to correct the trim to ensure she floats level.
The amount of ballast used, will be to correct the trim and to not exceed the scantling draft.

That's for you, the student, to investigate. You know that changes in the VCG can have an influence on the stability as well as the hull form.
So, your job is to investigate what changes, of which parameter and see what occurs.

I recommend you read Basic Ship Theory - by Rawson & Tupper, and in chapter 4 Stability - there is a section - Adjustment of Transverse Metacentric Height by Small Changes in Dimensions.
This shall give you the clues you need to investigate.

No point me giving you the answer... that's is not learning!

 

The first consideration that must be taken into account is that a ship that at full load needs to carry ballast is a poorly designed ship. We would be planning a ballast carrier instead of a crude carrier and even if that were necessary it should be fixed ballast and therefore included in the lightship. The issue of the trim is solved by the captain with a correct distribution of the load, for which they use the loading manual (computer program). It is not possible for the scantling draft to correspond to a displacement of 122,000 tons when the tank capacity is 120,000 m3. One of those two figures, or both, is not correct. When the scantling calculations are carried out, the weight of the structure is not yet known (although it can be estimated). Therefore, the displacement at full load is estimated, as possible (knowing that it may be wrong), and a certain margin is taken to obtain the displacement for scantling. This prevents the final displacement from being greater than expected and the scantlings, on a finished ship, it is seen that they are not enough. For this reason, in most of the scantling drawings there are some MAIN PARTICULARS with the adjective "for scantlings".
To increase the maximum GZ you have to lower the CoG position. That will also increase the GM, since the water plane should not change, which seems to be necessary on your ship. The other solution would be to increase the beam, but that might not be feasible at this point in the project.
Before correcting anything, check the calculations because, although it is difficult for your CAD/CAM software to make a mistake, it is very possible that the user enters incorrect data.
If you have no way of obtaining correct GZ values, it is best to redefine the shapes, especially the beam, of your ship. Do not worry about it since it is relatively normal, when you do a totally new project, to have to go back to the beginning if the provisional results are not satisfactory or if, for whatever reason, it is necessary to improve them..
If we knew your boat, we could surely give you more specific answers.

 

Hi!

As far as I understand, I don't have to fill the ballasts when calculating in fully loaded condition. I thought like this; 120,000 m3 * (density of oil: approximately 0,85) = 102 t and according to plans the lightship weight 17.000 t. So, total weight is 119.000 t. But when I added the weight of the ballast tanks its like impossible. I was confused here. Now I will calculate with empty ballast tanks and full cargo tanks. Thanks for your helping.

Actually, I don't want to increase maximum GZ value, I want to increase angle of GZ value. I did a quick calculation by arranging the rate of tanks. In Full load condition (Cargo Tanks full, ballast empty), my GM value is 6,5 m (approximately). I want lower GM and higher angle of GZ. So, I think, I should change form of the ship. Now, as you say, I carefully check my calculations. If there is no solution, I will look at the form of the ship.

Also, I can send folders. (Model, Stability and Loadcases folders).

Thank you both very much for your help.

 

Sorry for the mistake I made when computing the weight of the liquid in the cargo tanks. I thought that, among other things, a payload of 120 tons, for a ship that weighs 120 000 tons fully loaded, was not possible (I'm still not quite sure what figures we're talking about. In any case, the ship, fully loaded, shouldn't be carrying ballast).
The only way I see to increase the angle of the maximum GZ is to increase the beam of the ship. In order not to change the total displacement you will have to decrease the draft.