Introduction

In this issue of SmartDO eNews, we will introduce how SmartDO was utilized to optimize the structure of the Wave Energy Converter. In this example, the stress in the initial design was much more than the allowable value, and the weight was about 3000 kg. (The total height of the Wave Energy Converter is about 7 meters.) After being optimized by SmartDO, the stress was reduced to the allowable value, and the weight was reduce by about 19%.

Part of the the content in this article is the courtesy of the Green Energy Department of Industrial Technology Research Institute at Taiwan. Proprietary information has been removed from this article.

Description of the Problem

The Wave Energy Converter is a large scale structure with the height of around 7 meters. Figure 1 shows the basic mechanism of the structure. It contains the buoy and the wave press board, which is then anchored to the sea bed by steel cable. The buoy sliding up and down due to the movement of the wave. The kinetic energy of the buoy is then converted to electric power by the converter. Figure 2 shows the major components of the structure.

Figure 1 The Basic Mechanism of the Wave Energy Converter

Figure 2 Major Components of the Wave Energy Converter

Since the wave energy converter is repeatedly flushed by the wave, the structure reliable is an important issue. FEA/CAE was utilized understand the hydrodynamics and mechanics of the structure ANSYS AQWA was used to calculate the dynamics of the buoy under specific wave condition, and the pressure and interacting force was exported to ANSYS Mechanical under the Workbench platform to perform structural analysis. Figure 3 shows the schema of the analysis process under ANSYS Workbench.

Figure 3 Using ANSYS Workbench to Integrate AQWA and Mechanical

After the integrated AQWA/Mechanical is done, SmartDO was employed to linked with the parameters in the model for further design optimization. The following design requirement needs to be satisfied when performing the structural optimization of the wave energy converter.

• The hydrodynamics property of the converter should not be altered too much. For example, center of gravity should remain in certain location range. The structure should be lighter, since more buoyant force can be contributed to electric power.
• The stress must be lower than the allowable value.
• There should not be two many thickness groups for the steel plate, to reduce the manufacturing and material cost.

Design Optimization Modeling

After the parametric model of the finite element model has been built, it can be easily integrated with SmartDO. In the current project, the design optimization model is defined as

Find : DV1, DV2, DV3, DV4, DV5, DV6, DV7, DV8 (8 different thickness groups of the steel plates)
To Minimize : The total weight of the structure
Subjected To: -10 cm < change of COG location < 10 cm
                              Maximum Stress < 260 MPa

Results of the Design Optimization

After being optimized by SmartDO, the final design has the following properties

• The total weight of the structure is reduced by about 19%.
• The maximum stress is reduced from 507MPa to 260 MPa.
• The COG location is raised by about 7 cm.

Figure 4 shows the changes of the objective function (Weight of the structure) and the normalized maximum constraint value (VMAX, the maximum stress and the location change of COG.) during design history. I can be seen that during iteration 27 to 28, SmartDO has jumped out of the local minimum and later reached a lower minimum. During the design iteration, SmartDO reduce the weight of the structure and also change the design into a feasible design simultaneously. Even after the later iteration where the weight is reduced, the design proposed by SmartDO still satisfies all the constraints. This behavior is important in all practical engineering application.

Figure 4 Design History of the Objective function (Weight of the structure) and the Normalized Maximum Constraint Value (VMAX, the Maximum Stress and the Location Change of COG.)

Conclusion and Remarks

In this issue of the SmartDO eNews, we show you how SmartDO can perform design optimization of the large scale structure of the wave energy converter to reduce its weight as well as keeping its performance. The optimized structure is currently being manufactured and will be put on field service soon. In the future, we will continue to develop more Direct Global Optimization Technology, and will keep you posted in our eNews.