Oyster 800 in operation

Oyster 800 – it does exactly what it says on the tin

In a recent discussion with Stephen Salter, Emeritus Professor of Design at Edinburgh University, we discussed the so-called ‘hostile marine environment’. We quickly concluded that the environment, in fact, wasn’t hostile at all; it was just that we humans didn’t understand it properly . After all, quipped Stephen, ‘dolphins don’t consider it hostile’. I agreed. At Aquamarine Power our aim is to understand this environment and to develop man-made technology to operate reliably and effectively in sympathy with it.

One such element we need to understand better is the dynamic and complex interaction of a Wave Energy Converter (WEC) with the ocean waves. This topic has a high degree of engineering uncertainty. Hence full scale testing of any WEC technology in the ocean environment is of paramount importance to minimise this uncertainty. It is the ultimate test to demonstrate that the technology actually works as well as being the best way of learning more about the environment. Such an ambitious and challenging mission is part and parcel of being at the pioneering forefront of technology development and is precisely what Aquamarine Power has achieved through full scale testing of the Oyster 800 prototype at the European Marine Energy Centre (EMEC) in Orkney, Scotland.

Over the last year we have gathered real-time operational data representing 94% of the energy capturing seas of Oyster’s power matrix at EMEC. The data includes periods where Oyster 800 was working in waves over eight metres high. These are among the largest waves that can occur at our machine’s location. We are now well advanced in analysing this data. The results show a very high correlation between Aquamarine Power’s predictions of how we thought Oyster 800 would perform and the machine’s actual performance. This, we believe, is a significant endorsement of the Oyster concept (‘a bottom-mounted, surface-piercing near shore oscillating wave surge converter’, to give it its full technical description), and also of Aquamarine Power’s numerical model and tank testing programme at Queen’s University, Belfast.

A fledgling industry

It is fair to say that on a global scale, the wave energy sector could still be classified as a fledgling industry. It is one with vast potential. If one looks at the size of the potential resource, the rate at which we are currently depleting the earth’s known fossil fuel reserves and continued demand for clean and renewable sources of energy then we believe the case for wave energy is clear. However, in order to facilitate technology roll out in the next decade we need to get our feet wet and understand the environment more. The availability of genuine concept validation data is sparse in the industry at present. However, we are one of the very few companies that has made the bold leap to full scale ocean trials of our device and have gathered data from some of the most energetic seas possible at our water depth.

Research tools

The development of the wave energy sector has been, and still is, heavily reliant on predictions from research tools such as small scale experimental wave tank laboratory tests and numerical models. The results from these tools not only shape development of the technology but also allows companies to develop accurate commercial and business plan projections.
Due to the complexity of the real ocean environment, sceptics could perhaps be forgiven for thinking that small scale laboratory testing allied to theoretical numeric models cannot come close to predicting how a full-scale machine might behave in the ocean. Thus there is an industry-wide appetite to establish confidence in the validity and accuracy of these research tools. There isn’t a more definitive and transparent test of their capability than direct correlation with the behaviour of a full scale prototype operation in the ocean.

During ocean trials of Oyster 800, Aquamarine Power has gathered a wealth of high quality prototype data across a wide range of wave and tidal conditions, or sea states. This data covers 86% of the annual average sea states that are expected to occur at the EMEC site, representing 94% of the energy capturing seas of Oyster’s power matrix.

The trials employed a targeted operational strategy which focused on operating the device in a standardised way, in a range of specifically targeted sea states to maximise knowledge capture and learning. This strategy led to pressurising the Power Take-Off (PTO) system (referred to as ‘damped operation’) and capturing power in some of the most energetic sea conditions containing waves over eight metres in height. It is good to be reminded what a sea state of this magnitude actually looks like; see our video here. In order to appreciate scale in this video note that the flap part of the device in this clip is almost 13 metres high. That’s the length of a typical articulated truck trailer from end to end.

Better than predicted

Detailed analysis of the damped operation events, where predictions from experimental and numerical research tools are correlated to the full scale prototype behaviour, has revealed that the performance of the real machine aligns extremely well with research estimates. In fact, in the larger more energetic sea states the prototype has consistently performed better than our initial predictions. We found that tank test results and numerical model predictions of performance were generally within a tolerance band of ±10%.
These initial findings show that the research techniques developed by Aquamarine Power to model the Oyster device are robust and have a high degree of accuracy. In addition to this, for the wider industry it also shows that small scale wave tank tests and numerical modelling techniques are indeed valid tools to aid the development of wave energy converter technologies.
The quality of data gathered has not only facilitated a comparison of statistical quantities such as average power capture in a given sea state, but has also enabled our researchers to precisely recreate the individual waves in each sea state within tank test and numerical modelling research tools. This wave-by-wave reconstruction unlocks analysis techniques where transient behaviour of the system can be interrogated and correlated to the prototype with every oscillation of the Oyster flap.
Analysis techniques with this elevated level of detail maximise the knowledge captured from full scale data. It also provides the definitive test of the capability of experimental and numerical research tools to accurately predict hydrodynamic performance.
Aquamarine Power is currently writing up the findings from the Oyster 800 analysis in a series of technical conference and journal papers for dissemination and knowledge sharing across the entire industry to help advance the development of the sector as a whole.
The first of these papers is titled “The Value of Full Scale Prototype Data: Testing Oyster 800 at EMEC, Orkney” and is due for submission to the 11th Wave and Tidal Energy Conference (EWTEC) to be held in Nantes, France, in September 2015.