# Calculation of floater Inertias

Dear Jason,

I have a doubt regarding the calculation of PtfmRIner, PtfmPIner, PtfmYIner for a floater with arbitrary dimensions. if we wish to do preliminary analysis of a floater with arbitrary dimensions, how shall we determine this? In my understanding from earlier studies, this depends on the arrangements of structural elements and the shape of floater. For a beginner like me, who wish to try different floater shapes with different dimensions, how can I appropriately calculate and input them in FAST?

Thanks
Vijay

Dear Vijay,

I would need a general understanding of the geometry and make-up of the floater to estimate the platform inertias and center of mass. For a steel structure, for example, you could estimate the center of mass and inertias by assuming a given thickness and use analytical values for the inertia based on the external shape (e.g. single or multiple cylinders and boxes).

Best regards,

Dear Jason,

As i am trying for an arbitrary floater, except the external dimensions I don’t know about the make up arrangement inside the floater. In such cases, is it acceptable to model the floater in some modelling software (say solidworks) assuming it is made up of steel and choosing thickness satisfying the weight conditions in air and no compartmentalization of the floater?

Thanks
Vijay

Dear Vijay,

Without knowing more about your project, I can’t say whether that approach would make sense for your project, but it is a probably a good way to start.

Best regards,

Dear Jason,

Apologies for not putting up clearly in previous interaction.

Th meshed geometry of chosen floater is shown. To get inertia properties for this, I was thinking to model in Solidworks with thickness that satisfies the weight criteria in air, as you also earlier pointed out the same. So how do you basically approach a new floater (preliminary analysis) in choosing its inertial properties? Is it the right approach? How much change does it generally brings in when the internal arrangement is made (say compartmentalization and adding other accessories etc)?

Please give your suggestions as I am pretty much confused in it.

Thanks
Vijay

Dear Vijay,

I’m not sure I can answer your question as I’ve never had to do such a calculation myself. Perhaps someone else on this forum can comment. Another thing to consider is the ballast to be added to the floater, which may have a large effect on the inertia.

It may help to do a quick sensitivity analysis on the impact of the floater inertia on the system response to gauge how important it is to nail these details down.

Best regards,

Dear Jason,