# 4.2.6.1. Usage

The *ExtPtfm* module uses superelement properties of the support structure
provided by the user (e.g., mass, stiffness, damping, and time series of excitation forces)
to compute the reaction of the support-structure at the interface.
The module uses a linear formulation and internally keeps track
of Guyan and Craig-Bampton degrees of freedom.

## 4.2.6.1.1. Typical sequentially coupled workflow with *ExtPtfm*

The overall workflow includes the following steps:

The substructure designer performs a time-domain simulation of the isolated substructure under a given sea state, using an external tool such as a finite-element tool that has the capability to generate a superelement model. The underlying model behind the external tool and the time series of loads are reduced using the CB technique described in Theory, where the leader DOF are selected as the ones at the substructure interface node. Results from the reduction are written to a file containing the reduced system matrices, \(\boldsymbol{M}_r\), \(\boldsymbol{C}_r\), \(\boldsymbol{K}_r\), and the time series of reduced loads, \(\boldsymbol{f}_r\).

The file is imported in

*OpenFAST*by the*ExtPtfm*module, and a time-domain simulation of the full wind turbine is run with the reduced representation of the substructure. At every time step, the*ExtPtfm*module receives as inputs the displacement, velocity, and acceleration of the interface point and returns the loads at this point.*OpenFAST*exports times series of loads and displacements at the interface, which are then returned to the substructure designer. These inputs are used as boundary conditions to the external tool and then another time-domain simulation of the substructure is run. Stress concentrations are computed, and code checks are performed.

## 4.2.6.1.2. Using the module

The `ExtPtfm`

module is activated by setting the flag `CompSub`

to
\(2\) in the main OpenFAST input file. The variable `SubFile`

in
this same file needs to be set to a valid input file for the module (see
Input Files for the input file specifications).

```
--- [...]
2 CompSub - Compute sub-structural dynamics (switch) {0=None; 1=SubDyn; 2=ExtPtfm}
--- [...]
"Turbine_ExtPtfm.dat" SubFile - Sub-structure input file (SubDyn or ExtPtfm)
```

## 4.2.6.1.3. Recommendations

**Time step** superelements may contain high frequencies.
As a rule of thumb, it is recommended to use a time step
satisfying the following criteria: \(\Delta t=\frac{1}{10 f_\text{max}}\), where \(f_\text{max}\) is the maximum frequency present in the superelement or full OpenFAST model.

**Number of Craig-Bampton modes** It is recommended to perform a sensitivity analyses of the results
for an increasing number of Craig-Bampton modes to get an idea of how many modes are needed to reach convergence
and see which modes contributes the most to the sytem response.