Verifying Dynamic Analysis Techniques for Floating Tidal Platforms

Download Case Study

About the Project

The ecoSPRAY tidal energy test platform was developed as part of the Natural Resources Canada (NRCan) EcoEnergy Innovation Initiative (EcoEII) research project entitled “Reducing the cost of in-stream tidal energy through comprehensive site assessment.” The project led by Dr. Richard Karsten of Acadia University is focused on tidal energy site assessment using numerical models and innovative monitoring methods.

In support of the project, the ecoSPRAY platform was collaboratively designed and built by DSA and local suppliers Clare Machine Works (Meteghan, NS) and Bear River Plastics (Cornwallis, NS). DSA equipped the platform with state of the art instrumentation to measure motion, mooring line loads, and wind speeds along with current and wave conditions. The platform was fitted with a drag plate to simulate the thrust created by an underwater turbine.

Project Motivation

Floating tidal power platforms are developed for their advantages in positioning turbines in strong flows near the surface. However, there are many questions about these systems which need to be addressed, for example: What types of motions and loads will the platforms and moorings be subjected to in turbulent tidal flows? What kind of fatigue life do mooring lines exhibit in these flows? How do you install these platforms?

To better quantify the gaps in knowledge about how wave and current loads affect floating platforms and their moorings, DSA developed a simple catamaran structure which mimics a floating turbine platform. The focus of the project was to gather in-situ data about the performance of the platform in waves and currents and compare these to numerical predictions.

  • Project challenges
    • Develop an economical and safe deployment technique for the platform using local assets.
    • Establish a design basis (wave, wind and current conditions) for a short-term platform deployment in a tidal area.
    • Design an anchor and quantify the bottom-type(seabed) and holding power.
  • Scope of Work
    • Simulate the platform, moorings lines, and anchors in ProteusDS to determine the loads and motions of the system.
    • Evaluate the mechanical design and platform stability.
    • Develop a deployment and recovery plan for the anchors, moorings, and platform.
    • Coordinate anchor deployment and mooring connection.
    • Process mooring line loads, wind speeds, current data and platform motion data to validate numerical models.
  • Outcomes
    • The platform was successfully deployed in March 2016 using the Spray ferry. An excavator was used to deploy the anchors and local boats and divers were used to deploy and connect the mooring to the platform.
    • The full-scale simulation data closely matched the ecoSPRAY instrumentation data for both mean and peak mooring line loads, pitch, and heave motion.
    • The simulations underpredicted mooring line standard deviations, which may be important for fatigue. This may be a result of turbulence loading and vortex shedding that is not accounted for in the simulation. An upcoming project will look into this in more detail and add a synthetic turbulence model to the simulation.
    • Mooring line loads were not distributed as evenly in the measurements as in the simulation. The likely cause of this is the anchor placement. This highlights the importance of having the ability to set the mooring line tension using winches after the anchors are deployed.
    • The analysis highlighted the importance of using site-specific custom wave spectra as opposed to standard wave spectra such as JONSWAP. This is especially important when considering mooring line and platform fatigue load analysis.
  • Conclusion

    Validating the numerical model against physical experimentation was useful in better understanding the limitations of the numerical approach and the possible requirements of future analyses. The methods and assumptions that were applied can be used to inform the development of future numerical models of floating tidal platforms.