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

UPWIND WP4: Offshore Support Structures and Foundations
frequency motions in the finite element representations of the mooring lines. Other results confirmed that the
mooring line tensions were interacting with the floating platform as expected: for instance, in the simulations
with regular wind and waves the upstream fairlead tension was higher than the downstream fairlead tension,
which is what you would expect given that the mooring line tensions are counteracting the thrust from the rotor.
The fairlead tensions were also higher overall in 3Dfloat, which had a higher mean thrust. The results from the
effective RAO calculations showed that the behaviour of the fairlead tension was similar to that of the surge
displacement, which confirms that platform surge is what most influences fairlead tensions.
One of the most significant outcomes of the project is that it has helped to identify deficiencies and areas in
need of improvement in the participating codes, which has led to significant improvements in the accuracy of
modelling and response prediction. This is extremely beneficial both for the developers of the floating design
tools and for the industry in general. More details and full results from the project can be found in [17].
7.3 Case study: testing and validation of GH Bladed v4.0
The GH Bladed code has recently undergone development from a modal representation of structural dynamics
to a multibody representation, renamed as Bladed v4.0 [10] and described in Part I Section 1 of this report. One
of the benefits of re-structuring the code in this way is to better enable the modelling of floating wind turbine
platforms. The floating capabilities of Bladed v4.0 are based on the capabilities which are already present in the
code: the same aerodynamic and hydrodynamic models are used, and the same structural dynamics for the
deflection of blades and support structure. The main difference lies in the turbine and support structure not being
constrained by a direct connection to the ground. The structure is connected to a reference frame by a free
joint and constrained by mooring line forces, which enables all six support structure degrees of freedom to be
modelled with large rotations and displacements. Mooring line forces are applied using the foundation module
which is flexible enough to incorporate non-linear mooring responses.
The testing and validation of the new code structure is important due to the fact that Bladed is used commercially
by a large number of wind turbine manufacturers worldwide for design calculations. The testing and validation
of the new Bladed v4.0 code is described fully in [98].
Several different types of testing and validation have been carried out. In the first level of testing each individual
feature of the multibody code was checked against hand calculations and simulations using the previous version
of Bladed. Cases were chosen which were simple enough for the results to be known, either in terms of
the output values or the expected dynamic behaviour of the turbine. Results confirmed the new code structure
to be performing as expected [98].
In the next level of testing code-to-code comparisons were performed against the general purpose FEM package
ANSYS. The first test case was a simple beam modelled by separate beam elements, mainly in order to
calibrate the methods. The following results were calculated and compared:
• The frequency of the ten (10) lowest natural vibration modes
• The displacement (translation and rotation) of two of the stations in global coordinates
• The section force and moment (stress resultants) at two of the nodes in element coordinates
This test case showed that the results were identical, which verified the method for defining the input parameters
in ANSYS.
The second test case considered a separate blade, assuming the blade is fixed at the root and free at the tip.
The lowest mode shapes were calculated and compared with respect to frequency and shape. The results from
this test case showed the relative differences between the calculated frequencies of the lowest four modes to
be smaller than 0.2%, which indicates a close agreement between the modes calculated by ANSYS and Bladed
v4.0. For mode 7 (the torsional mode) the relative difference between the calculated frequencies was 0.5%,
which still indicates a close agreement between the calculated modes. The relative differences between the
calculated mode shapes were generally small for the most significant displacement components. An important
exception is that a very large relative difference appears for the axial displacement (elongation) and the axial
rotation (torsion). It is believed that this discrepancy is mainly caused by the fact that the axial displacement is
not completely constrained in the ANSYS model.
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