X-MED: EXtreme Loading of Marine Energy Devices due to Waves, Current, Flotsam and Mammal Impact

Marine energy should make a substantial contribution to the UK renewable energy target of 30% electricity by 2020. Tidal stream turbines are a more mature technology than wave energy devices while the potential of wave energy is considerable. There is a growing capability and confidence in the loading and performance of marine energy devices in operating conditions as designs rapidly develop. However knowledge of extreme loading is less mature and indeed there is some uncertainty about their origin. Tidal conditions are relatively well defined in terms of water levels and mean flows but large scale turbine deployment will have an uncertain effect (not considered here). Tidal flows particularly in areas of high velocity attractive for energy extraction are however bathymetry dependent. For example headlands and islands cause large-scale unsteady eddy structures affecting extreme loads. To complicate matters further tidal turbulence in the horizontal plane has length scales about six times those in the vertical giving a horizontal length scale of about half the water depth, similar to a typical turbine diameter. This will affect extreme loading to an uncertain degree and is not understood. In addition waves superimposed on currents cause unsteadiness which penetrates below the water surface; this may be due to long swell waves or breaking waves where concentrated, generally oblique, vortex structures propagate downwards. The effect of breaking waves is an important component of this project. Breaking waves also have a major impact on extreme loads on wave energy devices and it is appropriate to apply physical knowledge obtained from experiments and modelling to both tidal stream and generic wave devices. We consider only a moored, floating wave energy device as fixed structures have high costs likely to inhibit at least large scale deployment. Floating structures may also be used for tidal turbine deployment. Extreme loading will also be strongly influenced by impacts due to flotsam, debris and marine mammals or sharks. Such occurrence is highly uncertain but the impact will be high if it occurs. Risk is normally defined as the product of probability and cost of damage and so this is of particular concern for tidal turbine blades which are vulnerable since they must be slender. In this project we will not investigate the likelihood of occurrence of impact at large scale but will identify the possibility and magnitude of impact when there is flotsam or marine life in the flow. Flotsam is generally slightly buoyant, floating at the water surface, and in normal conditions of little danger to turbines. However in breaking conditions downwards jet-like flow is generated and entrained flotsam is likely to impact turbines. This has not been researched to our knowledge. This will be investigated experimentally and using a numerical modelling method known as smoothed particle hydrodynamics (SPH) which is well suited to handling debris (represented as small bodies in the flow).

The Supergen Marine Challenge is concerned with Accelerating the Deployment of Marine Energy. There is no question about the need for renewable energy and security of supply. Here we consider extreme loads on both tidal turbines and wave energy converters. Understanding and quantification of these loads are essential to develop efficient designs and to reduce investor risk. Various questions need to be answered.
There is a general question for extreme loading on tidal turbines and wave energy devices:
What is a design sea state with currents and waves in shallow to intermediate depths, less than about 60m? A secondary question is whether breaking waves due to storms or extreme swell waves generate a design sea state for extreme loads.
For tidal turbines there are two main questions:
1. What are extreme loads due to superimposed waves, tidal turbulence, wake impact in arrays or larger scale unsteady flow due to headlands or other bathymetry ? Has the worst case load a single origin, or more likely, a combination?
2. What are the impact loads due to flotsam in breaking waves and marine mammals or sharks?
For wave energy devices the main question concerns the loads and response due to extreme, probably breaking, waves.
An important general question is to what extent may extreme loads and response be modelled using computational modelling, and how can these results be incorporated in design.
The identification and quantification of extreme loads is a most important element in reducing investor risk enabling pathways to impact of this research. It should be said that there is remarkably little research history on extreme loading of marine energy devices.