Final report for WP4.3: Enhancement of design methods ... - Upwind

• A supplementary load vector for the master nodes Fm is calculated with the reaction force matrix R
from step 3 and the slave load vector Fs that was defined in step 1. This vector Fm is written to the
nodal load file. With this approach, all wave loads on the slaves are distributed to the neighboring
masters by means of the reaction force matrix as a load distribution key.
• Steps 1 and 4 are repeated for each time step. This results in a modified nodal load file for the
dynamic time domain simulation.
With this approach, the global values of the wave loads remain the same as for an unmodified model.
Only a distribution to other nodes has been performed. The modifications described herein are verified as
shown in [64].
Verification of super-element implementation
The correct implementation of the super-element in ADCoS-Offshore is verified via code-to-code comparison
against the general purpose FE tool ANSYS using component models and static load cases. The
stiffness matrix, the mass matrix and the static load vector of the super-element in ADCoS are compared
to their ANSYS counterparts. A realistic model of a tripod central node is used. The stiffness matrix is
checked via displacement comparisons along all six degrees of freedom, and for the mass matrix verification
the first ten natural frequencies are compared. The static load vector (or the dead weight) of the
model is verified comparing vertical force and two moment components for an asymmetrically clamped
structure loaded only by gravity. The wave load input is verified by means of a comparison of the wave
loads on the basic beam structure and the wave loads modified as described above. Here as well, the resulting
differences are negligible. The results show that the implementation of the super-element and the
wave load input are realized correctly as only very minor deviations are found in general. This is described
in more detail in the respective UpWind report [63] and published in [64].
4.2 Results of using super-element technique
The impact on simulation results of using super-elements to model joints was studied using the NREL
5MW baseline turbine [53] on the tripod support structure in 45m of water that was defined in the course
of the Offshore Code Comparison Collaboration project which operated under IEA Wind Task XXIII [16].
In the aeroelastic code ADCoS-Offshore, the tripod was modelled
• as a basic beam model (beam model) and
• in terms of a model including beam- and super-elements (super-element model)
The tripod standard beam model for the use in ADCoS-Offshore is defined with FE Euler-Bernoulli beam
elements. The conical parts of the structure are set up as stepped members in the model. The modeling is
mainly based on the findings from the third phase of the OC3 project as described in [16], [18] and [19] but
some further modifications were realized. Firstly, the overlapping parts of the tripod members at the joints
are excluded from the wave load and buoyancy calculation using small supplementary elements. Doubling
of masses is avoided because the mass of the small supplementary is set close to zero. This is a simplified
approach as the real "doubled" volume (buoyancy), surface (wave load calculation) and steel mass
(gravity and dynamics) is still roughly estimated, however, the model is significantly improved by using the
supplementary members. Furthermore, the tapered main column of the tripod is finely discretized because
this was found to have a significant influence on wave load calculation in the OC3 project. To conclude,
this model is as realistic as achievable with standard beam elements and reasonable effort. Therefore, it is
suitable to investigate the differences between beam models and the newly developed and more sophisticated
structural model.
The super-element model is based on the beam model, whereas the beam elements representing all
joints are replaced by super-elements as described above. Master nodes are defined to connect the detailed
sub-models to the residual structure. Being located in the centerlines of the tubular chord and brace
members those nodes are rigidly connected to the outgoing beam element and rigidly connected to the
detailed model via radially arranged rigid link connections. Figure 4.3 shows the tripod model with shell
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