Taut-moored Platform Analysis of the Sustainable Marine Energy PLAT-O

Validation of a ProteusDS tidal energy platform simulation

Overview

Predicting the motion of tidal support systems is critical for device development and deployment. Considerable research and industry study is conducted investigating the performance of the tidal turbines themselves, but there has been less investigation into the support structures. SME tested the first full-scale PLAT-O taut-moored platform at the European Marine Energy Centre (EMEC) in 2016, and the platform is ready for full deployment in early 2017. Development of the second-generation device PLAT-O#2 has been conducted since 2015 and uses the same concept as PLAT-On 1 but will mount four individual turbines instead of two as seen on the first-generation device.

The PLAT-O 2 taut-moored platform at 1/17 scale model was tested in tank tests at the FloWave Tank in Edinburgh in April 2016. The model, tested in axial flow up to full-scale speeds of 4.5m/s. The platform was tested under both normal operating conditions and in line-loss conditions. In the project considered here, the scaled results were compared to simulations created in ProteusDS software of the full-scale platform. This was done to determine the accuracy of modelling the platform behaviour and mooring line tensions in ProteusDS.

Background

Predicting the performance of full-scale tidal turbines and support systems is critical for efficient device development and deployment. Considerable research and industry study is conducted investigating the performance of the tidal turbines themselves, but there has been less investigation into the support structures.

SME tested the first full-scale PLAT-O at the European Marine Energy Centre (EMEC) in 2016, and the platform is ready for full deployment in early 2017. Development of the second-generation device PLATO-O#2 has been conducted since 2015 and uses the same concept as PLAT-O#1 but will mount four individual turbines instead of two as seen on the first-generation device.

The PLAT-O#2 1/17 scale model was tested in tank tests at the FloWave Tank in Edinburgh in April 2016. The model was tested in axial flow up to full-scale speeds of 4.5m/s. The platform was tested under both normal operating conditions and in line-loss conditions.

The scaled results have been directly compared to simulations created in ProteusDS software of full-scale platforms. The comparison was made to determine the accuracy of modelling the platform behaviour and mooring line tensions in ProteusDS.

Challenges

Designed as a taut-moored platform, the PLAT-O operates by balancing the buoyancy and drag forces on the system, using anchors and mooring lines. It is vital to understand the performance of the platform, i.e., how the system pitches, rolls, and yaws. The platform motion directly affects the loads on the mooring lines and anchors, which in turn affects the cost of the system.

Floating tidal platforms like PLAT-O are subjected to significantly complex loading, due to the dynamic wave interaction and turbulent tidal flow. The combination of multiple types of loading results in motions that can be complicated to predict.
Outcomes

The ProteusDS simulations were carried out at full scale, with drag coefficients comparable to tank scale using Reynolds scaling, and then compared to scale model results. The mooring line loads predicted by ProteusDS were very accurate at lower speeds, while slightly overpredicted (<5%) at higher speeds. The platform pitch and roll was also overpredicted somewhat in ProteusDS at higher speeds. The over predictions are attributed to: drag scaling effects that may not be accounted for in the Reynolds scaling of the drag coefficients; surface anomalies that may contribute to drag in the tank model; and the lack of interaction between components in the numerical model. The analysis of the PLAT-O#1 system is intended to determine whether these effects reduce at larger scales.

Also, simulations of line failure accurately predict the loads exerted on the anchor and the extreme orientation that the platform may adopt. Both the simulations and tank tests show that even with a severe attitude, the system is stable and does not move enough for damage to be a consideration.
Conclusion

ProteusDS was successfully used to model the Plat-O taut-moored tidal platform. The balance of buoyancy, thrust and mooring loads were successfully simulated, and the results validated against tank tests. For more information see: P. Jeffcoate, F. Fiore, E. O’Farrell, D. Steinke, A. Baron, R. Starzmann, S. Bischof, “Comparison of Simulations of Taut-Moored Platform PLAT-O using ProteusDS with Experiments,” Proceeding of the 3rd Asian Wave and Tidal Energy Conference (AWTEC), Singapore, Oct 24-28, 2016