Validating Numerical Predictions of Floating Offshore Wind Turbine StructuralFrequencies in Bladed using Measured Data from Fukushima Hamakaze
Haruki Yoshimoto, Takumi Natsume (Japan Marine United Corporation)
Junichi Sugino, Hiromu Kakuya (Hitachi, Ltd.)
Robert Harries, Armando Alexandre, Douglas McCowen (DNV GL)

1. Fukushima FORWARD
The government of Japan has started the experimental research project of the world’s
first floating offshore wind farm, which is conducted by the consortium made up ofindustry-academic-government organization. This project is sponsored by Ministry ofEconomy, Trade and Industry and named as “Fukushima FORWARD (Fukushima
Floating Offshore Wind Farm Demonstration Project)”.
The wind farm consists of three floating offshore wind turbines (FOWTs) and asubstation floater. The wind farm’s total amount of rating capacity is 14 MW.

3. Method of Validation

2. Fukushima Hamakaze (5MW FOWT)
Fukushima Hamakaze is floating offshore
wind turbine with a 5 MW horizontal axis
wind turbine, has been installed at about
20 km off the coast of Fukushima
Prefecture of Japan since July 2016 and isnow operating.

ItemLengthBreadth
Hub height
DraftValue
58.9 m51.0 m
86.4 m33.0 mFig. 2. Isometric view andprincipal particulars

http://www.fukushima-forward.jp/

Fig. 1. Overview of Fukushima FORWARD

The structure of the floating offshore windturbine is “Advanced Spar Type”.
Advanced spar is the newly developed
structure for FOWT and enables tosuppress the motion of the float.
This floater wasdesigned using
commercial wind turbine modelling
software “Bladed”. The purpose of thispaper is to validate the structural
frequencies using measured data.

To validate the first tower natural frequency estimation model,
we investigated several approaches to the modelling of thefloater.
Model#1 Baseline#2 Flex#3 Flex + DynML

Tab. 1. Investigated models
Structural Flexibility
Dynamic Mooring Lines
××✔
×
✔
✔

The natural frequencies are
extracted through counting
the tower base overturning
moment peaks after anexternal impulsive load isapplied to the tower top.
(like “Hammer test”)
Fig. 3. Hammer test example result

4. Modelling Structural Flexibility

5. Modelling Dynamic Mooring Lines

The submerged structure was divided into rigid and flexible
sections and the added mass was distributed to each part. To
break down the added mass into several parts, the boundary
element method hydrodynamics was post processed using outputsof the individual panel potentials.

To consider the inertia of the chain and hydrodynamic added mass,
dynamic mooring lines were included in the model.

Timoshenko
beam elements

Rigid elements

Fig. 4. Flexible structure modelFig. 5. Hydrodynamic model & sections

6. Result and Recommendation
5.0%

4.5%
Calculation/Measured -1

The FOWT is mooredby six chain catenary.
Nominal diameter ofthe chain is 132mm.
The water depth atwhich the anchor isinstalled is 110 to 120
m. The upper end ofthe chain is connectedto the submerged deck.

4.0%

Fig. 6. Mooring arrangement

Thelineshydrodynamic loadings
aremodelledasMorison model.

Each model in Tab.1 has been simulated in Bladed and the results are shownin Fig.7, the percentage difference between the calculation and measuredvalues.
Effect of Structural Flexibility (#1 - #2)

3.5%

About
. 1.5% improvement in the tower frequency prediction can be seen.

3.0%
2.5%

Effect of Dynamic Mooring Lines (#2 - #3)

2.0%

Reducing the tower natural frequency, however the differences are verysmall (0.4%).

1.5%
1.0%
0.5%
0.0%
#1 Baseline#2 Flex#3 Flex + DynML•

Investigated Models

Fig. 7 Comparison result of tower natural frequencies•

It is recommended to identify where significant flexibilities exist
within the floater and model it appropriately for the estimation of thetower natural frequencies. (This will be platform dependent.)
For this model, dynamic mooring lines could be safely ignored.