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Gravity effect on the hydrostatic matrix
Hi David,
I noticed that the hydrostatic stiffness matrix given in the QBlade theory manual is shown in Figure 1, which is different from Figure 2 which takes into account the gravity effect. Does this mean that QBlade automatically corrects it based on the input float weight and center of gravity position, as it is considered in Openfast?
BR,
AsIlll
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Hi AsIlll,
for a given hydrodynamic stiffness matrix, the resulting response in QBlade and OpenFAST is identical. Based on the values within the stiffness matrix, along with the floater’s displacement and rotation across all degrees of freedom (DOF’s), restoring forces and moments are applied to the floater at the REF_HYDRO location. Typically, the SUB_DISPLACEDVOLUME parameter is used alongside the stiffness matrix to account for the constant restoring force caused by the initially submerged portions of the floater.
I cannot provide detailed comments on the differences you mentioned; however, those sections of the documentation are related to the derivation of the hydrodynamic stiffness matrix, rather than its implementation in the code. The implementation is consistent between QBlade and OpenFAST. For example, a stiffness matrix defined in OpenFAST can be used directly in QBlade, and vice versa.
BR,
David
