Extreme motion and response statistics for survival of the three-float wave energy converter M4 in intermediate water depth
Authors: Harrif Santo, Paul Taylor, Efrain Carpintero, Peter Stansby, Rodney Eatock Taylor, Liang Sun, Jun Zang
Journal: Journal of Fluid Mechanics
Publication Date: 19 January, 2017
Department of: Mechanical, Aerospace and Civil Engineering
Probing extreme motion of wave energy converters
Machines designed to convert the energy of waves into useful forms have to be able to survive extreme waves. Researchers at the University of Manchester have previously developed and patented a multi-body wave energy converter, the M4 machine which has excellent energy capture characteristics. In this work, the researchers, with collaborators in Oxford and Bath investigate its behaviour in extreme waves. Experiments show that extreme motion and structural loads are almost linear, that is proportional to wave height, up to the point of waves overtopping a float deck which then acts as a strong damper. It is shown that the response is limited (saturated) in the most extreme conditions, for example those in the waters around Orkney. Although the results are for M4, they also apply to floating systems based on interconnected cylindrical columns providing a relatively simple method of analysis for motions and internal stresses. Such floating structures including those without hinges are quite common in the oil and gas industry. Remarkably this has not been realised previously.
The researchers also uncovered another important finding that will allow them to define a ‘designer wave’. The average wave history, given an extreme system response and the average response history given an extreme wave, match in time, with time reversed for one of the signals. This relationship will provide an efficient and robust way of defining this ‘designer wave’, for both experimental testing and computational simulation for a wide range of wave-structure interaction problems.
- The multi-float machine M4 can survive west of Orkney, well known for its extreme wave climate
- Linear and nonlinear analysis shows that response is remarkably linear before ‘dunking’ or green water overtopping occurs which then provides strong nonlinear damping
- Such linear characteristics provide a way of defining an extreme designer wave
- While the analysis is for multi-float wave energy converters it will apply equally to any similar floating system such as those used in the oil and gas industry