How should I treat the water in the moonpool and in the so-called “soft tanks” commonly used in the design of caisson-type spars?
It is usually best (and simplest) to include all the water in the moonpool, “soft tanks” and “hard tanks” in the vessel hydrostatic and mass distribution quantities, including KB, KM, Displacement (hydrostatics) and KG, Gyradii (mass distribution). Note, however, that only water which will clearly move with the vessel during its oscillations should be included; water inside a very open truss structure should therefore *not* be so included. The lone exception to this rule is the waterplane area, which should reflect the “tons per inch”-type hydrostatic resistance due to incremental vessel immersion (i.e., it should exclude any contribution from the moonpool, which is clearly not a part of the waterplane area). This will result in the correct estimation of heave periods and offset-versus-pulldown characteristics. (Note that it is perfectly reasonable to treat the KM’s in a manner similar to the waterplane area; that is, to exclude the moonpool contribution to the righting moment variable
