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Buoyancy Diameter

The buoyancy diameter (Db) is the effective outer diameter for buoyancy force calculations. For geometric properties in the Rigid Riser format, Db defaults to Do, but you can override this. Db is also used in some circumstances in the calculation of the added mass/inertia components of the Morison’s Equation hydrodynamic force – this is discussed in Element Diameter and Hydrodynamic Forces.

If you specify the Distributed buoyancy option, then specification of a value for Db greater than Do would be typical. Do is used to calculate the buoyancy loads generated by the riser itself, while the difference between the buoyancy loading calculated using Do and the buoyancy loading calculated using Db is then applied as a uniformly distributed load in the element axial direction, as discussed earlier in this section.