Numerical simulation method to study the effect of damping
plates for a new-type spar platform
Sharath SRINIVASAMURTHY*, Wataru ABE*, Shigesuke ISHIDA* and Shigeo YOSHIDA**Institute of Ocean Energy, Saga University
1 Honjo-machi, Saga-city, Saga 840-8502, Japan
E-mail: sharath@ioes.saga-u.ac.jp

Abstract
Floating offshore wind turbines (i.e., FOWTs) are emerging as one of the most promising renewable energy resources
and studies have been carried out regarding the floating platform. In a previous study, a new-type spar with damping
plates was proposed as a solution to reduce the overall motion characteristics of the floating platform and model scale
experiments were carried out. However, there is no numerical simulation method to understand the potential use ofdamping plates and this is a limitation to realize new-type spar platforms. The objective of this study is to develop a
numerical simulation method to obtain the response characteristics of the new-type spar models with damping plates. In
this study, the calculations based on boundary element method is adopted to evaluate the added mass and dampingcoefficients of the new-type spar models. Overall, three spar models (i) Type-A (Conventional spar platform without
damping plates), (ii) Type-B (Spar model with horizontal damping plates) and (iii) Type-C (Spar model with vertical
damping plates) are studied. The calculation results are compared with the findings of the model-scale experiment. It isfound that there is a good correlation between the numerical simulation and experimental findings. The potential use of
the proposed numerical simulation method is confirmed.
Keywords : Spar platform, Damping plates, Wave response, Numerical simulation

1. Introduction
Various platforms have been developed over the years to host floating offshore wind turbines. In a previous study, anew-type spar platform with damping plates was proposed to reduce the pitching motion of the offshore wind turbine.
The experimental study showed the potential use of damping plates replacing lower part of the spar platform. However,there is lack of numerical simulation method to understand the effect of damping plates. This study is dedicated todeveloping a numerical simulation method to incorporate damping plates and study the effect of different configurations.
This paper is divided into four sections. Section 2 will discuss the research methodology including the new-type sparmodels. The results of the study is described in Section 3 along with validation results. Section 4 will conclude thefindings of the study.

2. Research models and methodology
Table 1 shows the principal particulars of the new-type spar platforms considered in the study. The experimental
models were crafted based on 1/120 model scaling to host a 5MW wind turbine. The stability parameter (Mass x GM)
for all the three models is almost same.
Table 1 Principal particulars of the new-type spar models with damping plates.
Model types
Diameter of the main cylindrical part (m)Diameter of the water plane (m)Mass (kg)Moment of inertia (kg m2)
GM (m)KG (m)

Type-A
0.11
0.07
7.50
1.288
0.084
0.313

Type-B
0.11
0.07
5.65
1.288
0.114
0.346

Type-C
0.11
0.07
5.84
1.223
0.112
0.350

Figure 1 shows the CAD models of spar platform. Type-B model has horizontal damping plates and Type-C hasvertical damping plates. Type-A will be used as a base case without any damping plates. Figure 2 explicitly describes theproposed numerical simulation procedure. After modelling the damping plates in a CAD software, the open source BEM(Boundary Element Method) solver NEMOH is utilized for calculating the added mass, damping coefficients and wave
excitation forces. The equation of motion is setup in frequency domain, and solved to obtain the response amplitudeoperators. It is noted that modelling only the floating body below the free water surface is sufficient to obtain the addedmass and damping coefficientsFig. 1 New-type spar models.

Fig. 2 Numerical simulation procedure

3. Results and discussion
The response amplitude operator in six degrees of freedom can be obtained based on the proposed method. In thispaper, the heave RAO and pitch RAO are shown as demonstration. The simulation results are compared with the model
scale experimental results for validation. Figure 3 shows the heave and pitch RAO results for Type-A, Type-B and TypeC spar models. There is a good correlation between the experiment and simulation results.

Fig. 3 Heave and Pitch RAO – Experiment vs Simulation

4. Conclusion
It is concluded from the results that the proposed numerical simulation method can predict the experimental findings.
The simulation method to incorporate damping plates is validated.

References
Ishida, S. and Imai, Y., New spar design for floating offshore wind turbine with damping plates, Proceeding of the ASME
2019 38th International Conference on Ocean, Offshore and Arctic Engineering, Vol.9, 2019.