This data record allows the inclusion of the interaction effects when structures are in close proximity. Up to 20 hydrodynamically interacting structures can be modeled.

The interaction effects modelled include changes to both diffraction and radiation forces. All Aqwa post processing programs recognize these additional forces, including the additional radiation forces which couple the interacting structures.

The input format is as follows:

        5  7   11    16    21
  - --- -- ---- ----- ----- --------
  X|XXX|02|HYDI| LOCK|    1|XXXXX
  - --- -- ---- ----- ----- --------
        |  |        |     |
        |  |        |     |
        |  |        |     |
        |  |        |     |_(3)Structure Number.
        |  |        |
        |  |        |_(4)Optional motion lock keyword.
        |  |
        |  |
        |  |_(2)Mandatory data record keyword.
        |
        |_(1)Optional user data category identifier

(3) All structures between the one specified on the HYDI data record and the current one will be considered to be interacting. For example, if an HYDI data record specifying structure 2 is input for structure 5, interaction effects will be calculated for structures 2,3,4 and 5. The structure number must correspond to a structure which is already defined in the element topology data category.

(4) The motions of hydrodynamically interacting structures can be locked by specifying the LOCK keyword with the HYDI data record. The RAOs calculated for the structures in the interacting group are then consistent with the RAOs of an equivalent single structure. This is particularly useful for the estimation of splitting forces and moments on a structure in a time domain (Aqwa-Drift or Aqwa-Naut) calculation. This is achieved in the time domain analysis by including a joint between each of the bodies in the interacting group to output such forces/moments.