Abstract
Tidal and river energy converters can be placed in regions with marine traffic and also floating debris. This introduces significant challenges for the structural and mooring design of these systems to handle both tug and barge towline snag and dynamic impact loading from logs, ice, or other debris.
Dynamics Systems Analysis Ltd (DSA) was tasked with assessing debris impact loads and towline snag effects on floating turbine systems and related technologies, such as floating debris diverters. To better understand these systems, the work presented was carried out in three phases of numerical dynamic analysis:
- Tug and barge navigation through tidal channel in proximity to a hypothetical tidal farm site to assess likelihood of collision
- Floating submerged turbine platform towline snag to qualitatively refine a snag resistant design
- Floating platform debris impact for verification from field test data
The results of the work completed in these phases is intended to inform marine renewable energy standard development by providing indications of loads in debris impact and snag scenarios as well as guidance on how to make floating tidal systems snag resistant. The analysis of tug and barge traffic also provides some reference for assessing potential operating depth and clearance requirements from marine traffic zones to reduce snag risk. All time domain simulation analysis were completed using ProteusDS.
The first phase of analysis was designed to model the barge traffic through Discovery Passage on the east coast of Vancouver Island. Data on a range of tug, barge, and towline configurations were provided by SRM Projects and was used as the basis for establishing a representative tug and barge system to assess in more detail. This system is characteristic of a container barge traveling to Alaska from Seattle and back through the passage. Additional sensitivity studies were completed to examine the effect of lower towing capacity tugs and varied towline lengths. The depth of the towline throughout the transit was monitored to gain an understanding of the risk involved with deploying a submerged turbine in this high traffic area. In order to accurately model this system, hydrodynamically modelled local tidal currents and bathymetry data were supplied by SRM Projects and Cascadia Coast Research Ltd. In addition, a piloting control scheme was developed to ensure the tug and barge follow desired navigation waypoints through the passage. The maximum towline depth observed was 62.4m when the tug and barge system travelled south. The associated maximum lateral tug standard deviation from the waypoints, or intended path through the channel, was 93m. These are extreme results that were produced by the shortest towlines and also smallest tugs investigated. Small tugs spent more time navigating laterally across the channel to maintain the desired track, allowing the barge to drift and catch up to the tug. Short towlines allowed the less time for the barge to catch up to the tug and either pull it off course or increase catenary depth, particularly with small tugs. Increasing towline length also showed some correlation with increased catenary sag. In contrast to these more extreme results, a more reasonable scenario with medium capacity tug with moderate towline length produced a catenary depth of 16.6m and maximum lateral deviation of 12.2m. The lateral deviation from path is of the tug and not the barge. These results give an indication of the dynamic model capabilities in evaluating potential lateral deviation from a controlled channel for this type of tug and barge system when gauging marine traffic proximity to a hypothetical tidal farm.
The second phase of analysis utilised a submerged floating turbine platform designed by MAVI Innovations Inc (MAVI). Using the supplied geometry and mass values, the system was reconstructed in simulation and steady mooring response verified against expected analytical values. The platform was subjected to a snag load scenario by a passing catenary line to mimic a worst-case scenario of a tug and barge interaction with a submerged floating turbine. Several frame designs were tested to provide some insight on design features that would alleviate snagging. Key results are that convex slopes should be used on the top portion of the frame to prevent the line from catching on the hull. In addition, careful assessment of the center of gravity and mooring connection point is needed to ensure the platform can easily tip to allow the line to pass.
The third and final phase of analysis focused on debris impact of floating platforms used for river and tidal turbine systems. In partnership with Alaska Hydrokinetic Energy Research Center (AHERC) via the University of Alaska, field data of impact loads from debris on a floating platform was provided and used for verification of similar impacts in ProteusDS. Field tests showed mooring tension peaked at approximately 30kN during debris impact and DSA used this measured data to compare against a similar floating platform configuration in simulation. Due limited information available, only a qualitative comparison to the field data was conducted. In addition to this, a numerical sensitivity study was completed using various floating debris configurations, masses, and flow speeds to assess the change in impact loads. Generally, the larger the mass and the larger the flow speed, the larger the impact forces. However, the geometry and buoyancy of the debris has a strong effect as well: marginally buoyant but large and heavy structures were easily submerged and rolled off the platform with significantly reduced impact loads.