Ultimate force
A case study highlighting the importance and optimisation of blade design
Fibre reinforced epoxy [FRP] composites are already recognised as being the material of choice for many parts of typical ocean energy generators whether they are driven by tidal stream or wave power. The material’s properties which contribute to this choice are already well proven and have been widely accepted by many marine based industries such as oil & gas, working and leisure craft, submarine, offshore wind energy and military marine. Thus the take-up of FRP composites within the ocean energy sector is rapidly expanding.
For a tidal turbine developer assessing the feasibility and cost of their device there are many variables. One of these variables is the structural design of composite blades. The results from this structural design need to ensure that the blades can withstand the extreme environment, extract the maximum energy from the tidal flow and also contribute to minimising costs for the electricity generated.
Cost-effectiveness
The case study presented here was carried out by Gurit’s structural engineering team and seeks to highlight key structural design drivers and quantify their effect on overall blade design and structural efficiency which then impacts on overall turbine cost-effectiveness.
The hypothetical blade analysed in this study was for a horizontal axis tidal turbine blade engineered for typical load cases and site conditions and manufactured using Gurit’s epoxy SPRINT® glass multiaxials and SparPreg® carbon and glass unidirectional materials.
Blade stiffness limits are often a key design driver imposed either to maintain satisfactory clearance to supporting structure or for hydrodynamic efficiency. Such requirements can have a significant effect on blade mass, cost and feasibility. Where no such limit is imposed, the stiffness of the blade is purely governed by the minimum stiffness required to maintain laminate strains below failure levels, often resulting in a lower cost and mass and increased design space but lower margins on material strength.
Optimisation
Several design iterations were performed using Gurit’s in-house Blade Design Program. This software has been used extensively for wind turbine blade structural design and now incorporates features that allow rapid optimisation of tidal turbine blades.
In this study, S-N curves and safety factors from Germanischer Lloyd’s ocean current turbine guidelines [1] were applied. Also, an additional knockdown factor for fatigue in the sub-sea environment was used based on results from mechanical testing of saturated laminates. A Miner’s damage summation was then performed for the spar caps. Load cycles due to wave, wake and tide for a typical device and flow regime were used but no account taken of free stream turbulence, which has also been shown to have a significant effect[2].
The result of this damage summation for 107 cycles showed that, in this case, for both carbon and glass unidirectional spar caps, the blade design was not found to be fatigue critical. One of the main reasons for this was found to be the severity of the assumed extreme loads (runaway conditions) compared to the mean stress state. This creates relatively low mean stress state fatigue strains. Whilst it is likely that relatively high extreme loads will be seen on tidal turbine devices (compared to wind turbines) we cannot extrapolate this result to all tidal turbine blades as the Miner’s damage summation results are very dependant on the severity of the extreme static case as shown in figure 4.
Performance
The effect of the increased fatigue performance of carbon fibre laminates compared to glass fibre laminates can be seen by comparing the results of the damage summations. The glass laminate was found to become fatigue critical at a 22 per cent lower severity of fatigue load case than the carbon spar caps.
Importance
This study gives valuable insight into relative importance of fatigue versus extreme loads, and acceptable thickness/chord ratios for a single turbine installation. However, the immense variations between turbine designs and site conditions mean that a ‘standard’ design case is impossible to create. This is where the experience and skill of the engineering team makes the difference between a successful project and one filled with unnecessary challenges.
Gurit has been engineering large and highly loaded structures for the Marine industry for over 20 years and has been active in Wind Energy for 12 years, both in parallel with the requirements of the certifying bodies. This combination ensures that Gurit has the technology to engineer an effective structure first time, every time and hand the project over to process engineers and the prototyping team to turn the designs into a cost effective reality. Blade cost is of course a key driver in blade design and engineering and Gurit’s expertise in this field is second to none.
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