Sometimes we need to make a design of a jacket structure starting on the Deck structure designed in the past and specifically considering the stiffness matrix of this one.

one of the worst scenarios is when we don't have the structural model, but we have the stiffness matrix and mass for the structure, in this case, the user could create a super element as long as they have the Stiffness Matrix and the total mass for the structure.

To create the Deck super element you have to create a new Super Element input file and once the file was created we work only with the "SUBOPT", "STFHEA", and "STFR" cards.

           

In the card SUBOPT you can set the primary function to be executed RET, XPN, MER, MOD, and INP. Also in this card, you can set the units and the direction for the vertical coordinates 

• RET - Retrieve Substructure From Psi File.
• XPN - Expand Substructure.
• MER - Merge Two Substructures.
• MOD - Modify Substructure.
• INP - Direct Stiffness Input.

In the card STFHEA, you should set the 1st and 2nd joints for the stiffness partition.

you can input the Stiffness Matrix Row with the card STFR 

Below is presented the normal Stiffness Matrix for a structure with 2 points that we have to input to create the super element for the Deck

As you can see the stiffness matrix in the image is shown as a triangular matrice so we have to input this as a symmetrical. It means that K(row#, column#) = K(column#, row#) -That’s why you have only the upper part of the stiffness matrix in the Picture. we can't leave the lower part of the matrix blocks blank in the input file. so we need to enter the stiffness matrix components based on the symmetry of the stiffness matrix. For example, in the first block (joint 1 to joint 1), K(1,3) = K(3,1). 

Below is shown the correct way to input the stiffness matrix

finally, the value shown in the screenshot above could be input at our input file.

Also, you need to make sure that your model (the one you will import this stiffness matrix) has the same joint labels as your super element input - for this case are, 1, 2. Otherwise, you can't import the stiffness matrix. 

finally, to include the mass in the analysis, we can use the ‘WTSTR’, ‘SURFWT’, ‘WGTFP’, ‘WGTJT’, ‘WGTMEM’, ‘WGTNS’ or ‘WTCMB’ cards to make this mass participates in the analysis