Dear @Jason.Jonkman ，

I want to try to change the additional stiffness and damping matrix of the platform inside hydrodyn to simulate a linear mooring system.But I don’t know how to change the values of the mass and stiffness matrices, and how these two matrices affect the operation of the platform.Are there any relevant cases or materials that can be referred to?

thanks in advance.

Best regards，

Monge.

Dear @Monge.Low,

In most cases I would expect that you’d be able to zero out the mass matrix for the mooring system because the mass contribution from the moorings is typically small relative to the FOWT mass.

What typically *is* important is the mooring pretension and mooring stiffness. If you do not know these for your mooring system, you can derive them, e.g., using the python driver for MAP++.

Best regards,

Dear @Jason.Jonkman ，

Thank you very much for your reply.I try to build OC4 semi-submersible mooring system in simpack.

There is a default AddCLin matrix in the hydrodyn calculation example of simpack, I want to know how this matrix will affect the platform motion.

--------------------- PLATFORM ADDITIONAL STIFFNESS AND DAMPING --------------

0 0 0 0 0 0 AddF0 - Additional preload (N, N-m)

0 0 0 0 0 0 AddCLin - Additional linear stiffness (N/m, N/rad, N-m/m, N-m/rad)

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 1451298897 0 0

0 0 0 0 1451298897 0

0 0 0 0 0 0

0 0 0 0 0 0 AddBLin - Additional linear damping(N/(m/s), N/(rad/s), N-m/(m/s), N-m/(rad/s))

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0 AddBQuad - Additional quadratic drag(N/(m/s)^2, N/(rad/s)^2, N-m(m/s)^2, N-m/(rad/s)^2)

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

In addition, I try to couple simpack and moordyn, is there any relevant experience or case reference?

Best regard，

Dear @Monge.Low,

The nonzero values in the `AddCLin`

matrix are there to account for the roll and pitch restoring associated with the water ballast modeled in HydroDyn. SIMPACK is currently interfaced to an old version of HydroDyn which did not recompute hydrostatics in the strip-theory solution dependent on displacement, so, the hydrostatic stiffness matrix associated with strip-theory members (including internal water ballast) had to be specified through the `AddCLin`

matrix. (The functionality to recalculate hydrostatics in the strip-theory solution was added to HydroDyn in OpenFAST v2.6 and newer, so, these `AddCLin`

terms are no longer needed in newer versions of HydroDyn).

Regarding the coupling of MoorDyn to SIMPACK, that is a better question for SIMPACK technical support.

Best regards,

Dear @Jason.Jonkman ，

I see, thank you very much for your reply！

Best regards，