I have downloaded the latest version of FAST, FAST_v8.08.00c-bjj.
In the last period I am dedicating to simulations with the semisubmersible platform of the OC4 task, and I have a question about its added linear stiffness matrix.
I suppose that the only non-zero entries, elements (4,4) and (5,5), are used in order to match experimental data for the system.
I have two concerns about that:
(1) If I take off this additional stiffness value, the simulations encounter a fatal error. I tried to change several options (flag for aerodynamic loads, flag for moorings, moorings algorithm convergence options…) but I never succeded to bring a simulation to the end.
(2) The value of the additional stiffness, 1.45E+09 N-m/rad, is very big compared to the corresponding hydrostatic stiffness element (8.81E+08 N-m/rad if gravitation is taken into account, -3.75E+08 N-m/rad if not - approx.). I expected a lower value…I am sure that here I am missing something.
Thanks for your patience.
I have the same question as Michele (above), and one more:
In the OC4 DeepCWindSemi files that are included in the package, in the ‘ElastoDyn’ file, the Platform Mass, inertia and center of mass have been modified. They are much less than that mentioned in the report ‘Definition of the Semisubmersible Floating System for Phase II of OC4’ by you and Amy Robertson and others. In the ‘ElastoDyn’ file, there is a comment after each of these lines:
!FW HUIMIN, subtraction of H^2 self inertia term only
Could you give an idea what this means?
Dear Michele and Mohit,
The HydroDyn model of OC4-DeepCwind semi-submersible that we’ve released is a hybrid model containing a potential-flow solution and a strip-theory solution for viscous-drag loads and water ballasting of the members. It is the water ballasting (filled fluid) modeled in HydroDyn that (1) impacts the mass, center of mass, and inertia of the platform in the ElastoDyn file and (2) requires the addition of an additional linear stiffness matrix (AddCLin) in HydroDyn. See section 6.6.3 of the draft “HydroDyn User’s Guide and Theory Manual” for an explanation on why the (4,4) and (5,5) entries of AddCLin should be nonzero when modeling the water ballast in HydroDyn. Eliminating these terms will effectively eliminate the contribution the water ballast has on the roll and pitch restoring of the semi-submersible, which I can certainly understand would lead to capsizing of the system (and a fatal simulation error).
Regarding the statement about “H^2”, section 4.3.14 of the draft “HydroDyn User’s Guide and Theory Manual” mentions that “rotational inertia of the fluid in the member is ignored”. By this, we mean the following. The rotational inertias of a solid cylinder about its center are:
I_Axial = 1/2mr^2
I_Transverse = 1/12m(3*r^2 + h^2)
m = mass of cylinder
r = radius of cylinder
h = height of cylinder
However, HydroDyn treats the filled fluid as a point mass (not a thin circular disk) at each cross section. This means that a fluid-filled member is treated as a thin rod instead of a solid cylinder, leaving only:
I_Axial = 0
I_Transverse = 1/12m(h^2)
(Only the h^2 term is kept.)
I hope that helps.