Hello,
It is not possible to fix individual freedoms, but you can constrain the structure motions in surge, sway and yaw by defining an Additional Hydrostatic Stiffness matrix for the structure, and setting the X-X, Y-Y and RZ-RZ stiffness terms to be very large. Once you have defined your additional stiffness and run the diffraction calculation, you should check that the natural frequencies in X, Y and RZ are at least an order of magnitude greater than the natural frequencies in the unconstrained freedoms; you can find these values in the diffraction analysis LIS file, by searching for the string 'N A T U R A L F R E Q U E N C I E S / P E R I O D S'.
I hope this helps.
Mike
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