Abstract
Laminar Scientific has designed a unique WEC system which is meant to capture energy through horizontal wave forces and vertical buoyancy forces. The system is composed of a rectangular shaped buoy with concave sides that is tethered by 4 mooring lines. Each mooring line is attached to a reel located inside of the buoy structure. As the buoy moves along with the waves, the mooring lines pay in and out of the reels which drives a series of generator mechanism to capture the wave energy and convert to electrical power.
AMOG was engaged by TEAMER to conduct a series of numerical analysis to assess the performance of Laminar Scientific’s WEC design. Using PacWave South as the primary environment, AMOG performed hydrodynamic analysis on the system and investigated design parameters such as buoy size and geometry to optimize the performance of the system.
To assist Laminar Scientific, AMOG:
- Created an OrcaFlex model of the Laminar Scientific WEC system. This model was built using specifications which were provided by Laminar Scientific.
- Performed regular wave hydrodynamic analysis of the WEC system using PacWave South site conditions to understand the performance of the WEC for these wave conditions. The analyses were performed for the original buoy model and four subsequent iterations of the buoy considering regular wave approximations. Each iteration considered variation in buoy parameters such as overall dimensions, damping, inertia and geometry. Of the Base Model and four iterations, one iteration was chosen for irregular wave testing.
- Performed irregular wave hydrodynamic analysis on the chosen iteration to investigate factors such as buoy stability and mooring line loading. The chosen iteration was assessed over a simulation period of 3 hours for a range of sea states which were determined based on the PacWave south site conditions. These simulations considered first order wave loading as well as drag and radiation damping.
- Assessed the system’s performance for the base model and each of the iterations. The motions and behavior of the models were compared to determine which design parameters had the most influence on the performance of the WEC system.
The following conclusions were made as a result of the work undertaken.
- The power at the mooring line connection to the reels is driven by the surge and heave motions of the buoy, and the system performs best when the buoy follows the elliptical path of the waves.
- The Base Model configuration, which is included the original buoy design parameters, exhibited uncontrolled buoy motion in some regular wave conditions. Typically, these conditions had small wave periods or wave periods near the heave or roll natural response periods of the system.
- Increasing the size of the buoy resulted in an increase in the stability of the system. A larger buoy showed less uncontrolled motions than a smaller buoy for the same wave cases. Analysis of the larger buoy size also indicated that the average power at the mooring line to reel connection point is increased when the size of the buoy is increased.
- Reducing the inertia of the internal rotational mechanisms has a significant effect on the stability of the system. As this inertia becomes smaller, it becomes easier for the system to recover from uncontrolled motion and snatch loading of the mooring lines. The reduction of inertia does not have a noticeable effect on the average power seen at the mooring line to reel connection points.
- When the concavity of the buoy is removed and replaced with flat sides the performance of the system is similar to that of the concave buoy. However, the concave does see slightly more power at the mooring line to reel connection points than the flat sided buoy.
- For the original range of irregular sea states tested, the buoy motions were unstable, and the mooring lines exhibited snatch loading. This resulted in line tension spikes which exceeded the MBL of the mooring lines. Additional simulations were performed for sea states with a reduced significant wave height. The results of these cases indicated that the system is more stable and that the snatch loading is reduced in smaller sea states.
- The current mooring configuration of the system offers little resistance against yaw motions. Slight yaw motions quickly can quickly become uncontrolled, and the mooring system is unable to recover reliably.
- While the system performed best in smaller sea states, further analysis and iteration of the system may be conducted to improve its performance. This may include investigation into reducing the inertia