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Design on substructure‘s inertia

Dear David,

To get more accurate results, I need to calculate the inertia of the platform and then add it to the 6×6 SUB_MASS matrix.

From the definition of OC4, I found the geometry of the whole substructure. I got all the locations and mass of every element and tried to calculate the inertia the document presented as Graph 1 shows.

In my attempt, I use the formula ” I_roll=Mass multiply distance^2″ to compute each element’s inertia. In this approach, I assumed the element was a lump point of its centroid, and multiplied the distance from the centroid to CM.

But in this way, the result of my hand calculation can’t be consistent with the given data.

So my problem is: Do they use each subelement as a lump point to calculate inertia in the official document? Or they were using some software to get the value of inertia, and every point of the element distribution is considered instead of simply one concentrate point? Is that the reason I can not reproduce the same value by hand calculation?

Best Regards,


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Hi Easey,

from your description it seems that your approach is correct. However, I cant really help you with the details of the floater model, since we were not involved in its design and also dont know all the details regarding the published reports etc.

One critical aspect to consider, though, is the inclusion of the ballast mass (the mass of water-filled members) in your inertia calculations. Overlooking this could lead to inaccuracies in your results.