The flow developed on a tidal site can be characterized by combinations of turbulence, shear flows, and waves. Horizontal-axis tidal turbines are therefore subjected to dynamic loadings that may compromise the working life of the rotor and drive train components. To this end, a series of experiments were carried out using a 0.9 m horizontal-axis tidal turbine in a tow tank facility. The experiments included two types of regular waveforms, one of them simulating an extreme wave case, the other simulating a more moderate wave case. The second regular wave was designed to match the peak period and significant wave height of an irregular wave which was also tested. Measurements of torque, thrust, and blade-bending moments were taken during the testing campaign. Speed and torque control strategies were implemented for a range of operational points to investigate the influence that a control mode had in the performance of a tidal stream turbine. The results showed similar average power and thrust values were not affected by the control strategy, nor the influence of either the regular or irregular wave cases. However, it was observed that using torque control resulted in an increase of thrust and blade root bending moment fluctuations per wave period. The increase in fluctuations was in the order of 40% when compared to the speed control cases.

The tidal energy industry is progressing rapidly, but there are still barriers to overcome to realise the commercial potential of this sector. Large magnitude and highly variable loads caused by waves acting on the turbine are of particular concern. Composite blades with in-built bend-twist elastic response may reduce these peak loads, by passively feathering with increasing thrust. This could decrease capital costs by lowering the design loads, and improve robustness through the mitigation of pitch mechanisms. In this study, the previous research is extended to examine the performance of bend-twist blades in combined wave–current flow, which will frequently be encountered in the field. A scaled 3 bladed turbine was tested in the flume at IFREMER with bend-twist composite blades and equivalent rigid blades, sequentially under current and co-directional wave–current cases. In agreement with previous research, when the turbine was operating in current alone at higher tip speed ratios the bend-twist blades reduced the mean thrust and power compared to the rigid blades. Under the specific wave–current condition tested the average loads were similar on both blade sets. Nevertheless, the bend-twist blades substantially reduced the magnitudes of the average thrust and torque fluctuations per wave cycle, by up to 10% and 14% respectively.

Wave loading on tidal turbines is of key concern for determining blade and drive train design loads and the fatigue life of components. Furthermore, irregular waveforms are likely to add complexity to the loading patterns, and represent more realistic conditions. To investigate this issue, a set of laboratory tests was conducted in a large wave-tow facility at CNR-INSEAN, Rome. A 0.9 m diameter three bladed horizontal axis turbine model was fixed to the tow carriage and tested under tow, regular wave-tow and irregular-wave-tow conditions at a range of turbine rotational velocities. Thrust and torque on the blades and rotor were measured dynamically during testing using strain gauges. The control mode was switched between constant speed and constant torque to understand how this influenced turbine power capture and thrust loading, and assess the potential to use control methods to mitigate loading fluctuations. It was found that average power and thrust values were not affected by the control mode or the addition of regular or irregular waves. However, using torque control resulted in increased thrust fluctuations per wave period of the order of 40% of the mean thrust compared to under speed control. Therefore, the operational mode must be taken into consideration.

Composite tidal turbine blades with bend-twist (BT) coupled layups allow the blade to self-adapt to local site conditions by passively twisting. Passive feathering has the potential to increase annual energy production and shed thrust loads and power under extreme tidal flows. Decreased hydrodynamic thrust and power during extreme conditions means that the turbine support structure, generator, and other components can be sized more appropriately, resulting in a higher utilization factor and increased cost effectiveness.

Load fluctuations caused by the unsteady nature of tidal streams can have severe impacts on turbine components. As seen in the wind industry, turbine blades can become misaligned due to a fault in the pitch mechanism or blade deformations arising over time. These misalignments will represent a loss of power capture and perhaps even premature failure of the components if not detected in time. Computational fluid dynamic (CFD) techniques can be used to predict the performance of a turbine with a misaligned blade. However, these numerical modelling techniques quickly become computationally expensive when modelling realistic, time-varying conditions. Blade Element Momentum Theory (BEMT) offers a quicker and simpler approach, although with several limitations. In this paper BEMT is adapted to predict the performance of a three bladed tidal turbine with one or two blades offset from the optimum pitch setting. This approach is compared with a CFD model to study the effectiveness of both methods to predict power and thrust when a rotor blade has an offset. The simulations were undertaken at three flow speeds (0.9, 1.0 and 1.1 m/s). Both numerical models are compared to experimental data that was obtained at a flume tank in similar flow conditions. The results showed that both BEMT and CFD are able to predict power coefficients when there is a small offset of one rotor blade. However, the predictions were poorer when two blades had two different offsets at the same time.

The main objective of this paper is to analyse extreme cases of wave-current interactions on tidal stream energy converters. Experiments were undertaken in the INSEAN tow tank facility where flow velocities of 0.5 and 1m/s were used with and without waves. The wave variations studied in this testing campaign were between wave heights of 0.2 to 0.4m with a 2s wave period. These wave conditions were considered extreme cases considering the use of a turbine with a rotor diameter of 0.5m. The turbine was equipped with a torque transducer, an encoder and a strain gauge to measure power coefficients and forces on a single blade root. Therefore, the results of this experiment are used to improve the understanding of wave effects on tidal stream rotors by analysing not only the temporal variations of power and blade loading but also the peak variations of them. Keywords— Tidal Turbine, Extreme Environment, WaveCurrent Interactions, Experiments.

The authors thank the discussers for their interest and comments.The aims of the published work were 1.To examine the time development and mechanics of scour under simultaneous changes in water depth, speed and direction; and 2. To focus on the changes to flow intensity and the effect of these on the scouring process.The exact definition of equilibrium scour depth and generation of empirical formulae was not the aim of the study.The authors suggest that the discussers may have misinterpreted the aim of the paper and thus the applicability of the data within it.The discussers argue that the flattening of the curve in Fig. 12 cannot indicate approaching equilibrium since it does not account for the observed continuation of scour in the live-bed time steps.It is made clear that it is an approximate inference of equilibrium due to the limited number of tidal cycles.Continuing scour during the live-bed stages is present towards the end of the test, but the rate clearly reduces within each half-cycle (Fig. 12).

Laboratory studies form an integral part of scour research, yet there is no standard technique for monitoring scour depth development during testing. This paper investigates the advantages and disadvantages of a variety of techniques, and compares results from three methods: photogrammetry, an echosounder, and a calibrated pile. A novel system involving an underwater camera is presented. This produces results in close agreement with the other techniques, and has the advantage of providing accurate measurements over the full scour hole while at the same time enabling qualitative information to be gained from the photographs.

In order to improve the understanding of scour processes in layered conditions for offshore foundations, a laboratory study was conducted regarding equilibrium scour depth vs. time curve and its adjustment to hyperbolic and exponential/linear combined functions. The present paper provides a mathematical study describing a set of scour tests in order to clarify the scour depth evolution for complex, non-cohesive soil configurations.

This paper describes the extension of an engineering model (Harris et al., 2010) developed to predict the time evolution of scour around a monopile in uniform granular soils in the marine environment to layered granular soils. The model is capable of looking at cylindrical, square and rectangular structures. It is capable of representing both the scouring and backfilling process and has been modified to incorporate the ability to predict the effect of scouring in layered soils making use of the results from recent experimental studies into scouring in non-uniform sediment beds. Output from the model is compared with the laboratory results and extended to look at potential implications at field-scale.The model is of use on those occasions when knowledge of the likely variation in depth of scour is important. Examples include predictions of scour development to inform the evaluation of the duration of time after pile driving for a fully developed scour hole to form for installation of scour protection, or to determine the likely exposure of cables entering and exiting offshore turbine foundations.

The 24 papers and posters presented at the eighth Young Coastal Scientists and Engineers Conference held at Bangor University in April 2012 were of a high standard, typical of that expected at a leading international conference. They gave a good indication of the breadth of basic and applied research currently being pursued in UK universities and consultancies. Papers covered a wide range of topics, including coastal management, shelf sea turbulence, renewable energy, cliff erosion, sediment transport, morphological modelling, nutrient and pollution transport, and scour protection, to mention but a few. This briefing reproduces the abstracts of the two Keynote lectures and of three presentations judged by the organising committee to be of particular merit. The next UK Young Coastal Scientists and Engineers Conference will be organised by Marine Scotland in Aberdeen in spring 2013.

Marine soils are often variable in their make up and bed sediments of silty sand or sandy clay do not respond in the same way as sands without fines. Their resistance to erosion and rate of erosion is still an area of uncertainty and requires further examination so that a clearer understanding of scour for multi-modal sediment distributions can be achieved. A series of simplified laboratory experiments have been performed to investigate the rate and extent of scour development in layered sediments. These initial tests, part of a much more extensive four year research project, are being undertaken in the reversing current flume in the Department of Civil, Environmental & Geomatic Engineering, UCL. The soil bed is layered with materials of different diameter, differentiated for these tests by coloured dye. Hydrodynamic conditions are sufficient to mobilise the sediment in both soil layers. The scour development is measured through time using both an echo sounder and the photogrammetry technique. The latter is a non-intrusive approach and uses the method of photographing one object from two angles at the same time to obtain full spatial coordinates of the object. This paper presents initial results from these studies.

This paper presents a method to obtain the pressure distribution across the surface of a tidal turbine blade, but without the extensive computational time that is required by 3D CFD modelling. The approach uses a combination of blade element momentum theory (BEMT) and 2D CFD modelling, where the inflow velocity vector for each blade element computed from the BEMT model is input to a 2D CFD model of each of the blade sections. To assess the validity of this approach, a comparison is made with both a BEMT and a 3D CFD model for three different blade profiles at full scale (NACA 63-8xx, NREL S814 and Wortmann FX 63-137). A comparison is also made of the NREL blade at smaller scale to investigate any Reynolds number effects on the model performance. The agreement is shown to be very reasonable between the three methods, although the forces are consistently slightly over-predicted by the BEMT method compared to the 2D-CFD-BEMT model, and the 2D-CFD-BEMT model over-predicts the pressure along the leading edge compared to the 3D CFD results. The proposed method is shown to be particularly useful when conducting initial blade structural analysis under dynamic loading.