Nonlinear Rock and Roll - Modelling and Control of Parametric Resonance in Wave Energy Converters

Project Information

ParaResWEC

Grant agreement ID: 867453

DOI

10.3030/867453

Project closed

EC signature date 24 April 2019

Start date 1 May 2019

End date 30 April 2021

Funded under

• SPREADING EXCELLENCE AND WIDENING PARTICIPATION

Total cost

€ 151 850,88

EU contribution

€ 151 850,88

Coordinated by

BUDAPESTI MUSZAKI ES GAZDASAGTUDOMANYI EGYETEM
Hungary

Periodic Reporting for period 1 - ParaResWEC (Nonlinear Rock and Roll - Modelling and Control of Parametric Resonance in Wave Energy Converters)

Reporting period: 2019-05-01 to 2021-04-30

Summary of the context and overall objectives of the project

The title of this project is ‘Nonlinear Rock and Roll – Modelling and Control of Parametric Resonance in Wave Energy Converters’. The over-arching aim is to catalyse increased research into parametric resonance within wave energy and other offshore renewable energy fields.

The concept of resonance is well known in the study of wave energy converters. In comparison, parametric resonance has received virtually no attention, and is often unexpected since it is a nonlinear phenomenon not predicted by the linear/frequency domain models traditionally favoured in wave energy research and analysis. Whereas normal resonance causes the oscillations of a system to grow linearly with time, parametric resonance causes an exponential increase in oscillation amplitude, and therefore has great potential to be either detrimental or beneficial to a wave energy converter performance. For certain classes of wave energy converters, parametric resonance can lead to instability and decreased performance, by transferring energy from the primary mode of motion into other modes. Alternatively, for other types of wave energy converters designed to extract power from the parametrically resonating modes of motion, triggering parametric resonance could result in increased energy capture and represent a game-changing approach to improving wave energy converter performance.

To achieve the over-arching goal, the project focuses on the following specific objectives:
1) Demonstrate novel control techniques to either mitigate or harness parametric resonance for different classes of wave energy converters.
2) Provide opensource tools for modelling and analysing parametric resonance in wave energy converters, to aide the wave energy community in overcoming the barriers surrounding the advanced nonlinear methods required to investigate parametric resonance.
3) Strong communication, dissemination and exploitation activities. Providing information to the wave energy community and connections with researchers from other disciplines with relevant knowledge and experience, such as Nonlinear Dynamics, Shipping and other Offshore Engineering fields.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project began by performing extensive literature reviews on the topics of modelling and control of parametric resonance in wave energy converters and other offshore systems. The review of modelling techniques has already received over 30 citations in the 17 months since it was published, highlighting the impact it is provides to the field. The review highlights the need for both high fidelity and computationally efficient modelling methods, which the project then focussed on developing and disseminating as open-source tools.

One major gap identified in the wave energy literature was the absence of an early warning system to detect the onset of parametric resonance. Such a detection system is vital in order to activate a control system designed to mitigate the occurrence of parametric resonance. A major result for this project was the development of the first real-time detection system for parametric resonance in wave energy converters. This result was published in a journal article and the code shared on the project Github page.

The application of methods from the field of nonlinear dynamics was explored and a comparison of dynamic vibration absorbers performed for the purpose of mitigating the occurrence of parametric pitch in a heaving point-absorber type wave energy converter. The results demonstrated that although nonlinear energy sinks have the ability to absorb energy from a wide frequency range and possess irreversible targetted energy transfer characteristics, tuned-mass dampers are able to eliminate the occurrence of parametric resonance more effectively. In addition the results showed that employing the vibration absorbers to act in the heave mode of motion is more effective at eliminating pitch resonance than attaching the vibration absorbers to the pitch mode of motion directly. This finding provides a techno-economic benefit to designing a parametric resonance mitigation system, since heaving point-absorber type wave energy converters already have a power take-off device implemented in the heave mode of motion, which could be leveraged by a control system, rather than having to install secondary vibration absorbers to both the pitch and roll modes of motion.

A case study was performed on the oscillating water column spar-buoy WEC, developed at Instituto Superior Técnico, Universidade de Lisboa, which had demonstrated high susceptibility to parametric pitch and roll motion during previous testing campaigns. After studying the device, a novel control system was proposed, comprising a relief valve system on the air chamber of the oscillating water column, which can be opened when the onset of parametric resonance is detected in order to detune the system. Experiments of a scale model prototype in a wave flume were performed which successfully demonstrated the potential of the proposed control method. The results were published in a journal article and the data from the experiments openly shared in an online repository.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project has:
• Demonstrated computationally efficient modelling techniques able to accurately capture the parametric resonance phenomena and contributed to the state-of-the-art and body of knowledge relating to numerical wave tank simulation of wave energy converters
• Proposed a novel control technique to mitigate the occurrence of parametric resonance in an oscillating water column spar buoy type wave energy converter, which works by using a relief valve at the top of the air chamber. The potential of this concept was experimentally verified through wave flume experiments with a scale model device
• Investigated the potential of utilising nonlinear energy sinks from the nonlinear dynamics community, comparing and contrasting their performance against more traditional tuned mass damper systems. With the results showing the ability of tuned mass dampers to resonanate at a targetted frequency range has a superior ability to eliminate the occurrence of parametric resonance compared to the nonlinear energy sinks.
• Developed and demonstrated the first real-time detection system for parametric resonance in wave energy converters

The project results have to potential to aide the succesful utilisation of the vast amounts of renewable energy present in the ocean, for the production of electricity, hydrogen and desalinated water. Developing a sustainable energy and water supply is vital for the socio-economic prosperity of humanity in the coming century. Without proper consideration of parametric resonance and appropriate design and implementation of mitigation methods, offshore renewable energy systems can be subjected to adverse effects ranging from decreased efficiency to catastrophic failure. The results from this project enable engineers and device developers to better understand the phenomenon of parametric resonance in floating bodies and provides them with the neccessary tools to model and control it.

OWC Spar Bouy (a) Normal heave motion operatiton, (b) Parametric roll and (c) Parametric pitch