Design and analysis of combined floating wave and wind power facilities : with emphasis on extreme load effects of the mooring system
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- Institutt for marin teknikk 
A rapid increase in energy demand and the global warming problem has increased the focus on renewable energy resources. Among renewable sources, ocean wave and offshore wind are the topic of this thesis because both resources are considered to have high potential and they are highly correlated. However, the main challenge is whether they can be exploited to produce energy at a reasonable cost. To reduce the cost of power from such energy systems, we need to reduce the cost of the structures and/or increase their power production. The latter is becoming the focus of many studies of wave energy systems through the use of control. However, some studies show that this does not necessarily reduce the cost of power due to the extra costs needed for the control system, especially for wave energy systems. This thesis focuses on global analyses to estimate the responses during non-operational conditions, including extreme environmental conditions that will be used for the design of such system. Additionally, several survival strategies for renewable energy systems were studied in the present thesis to reduce the structural load during extreme environmental conditions. As for other floating structures, floating wave energy converters (FWEC) require mooring systems to remain stationary. In addition to being one of the main cost contributors of a FWEC project, a catenary mooring system could affect motion and therefore the power. Different mooring configurations have been considered in one of the case studies in the present thesis to study the effect of the mooring system on the power capture of a two-body FWEC. The analyses performed indicate that the influence of the mooring increases as sea severity increases and as line length decreases. As long as the length of the mooring lines are sufficient to accommodate the motions of the device due to first order wave forces, especially in heave, the effect of mooring on the power performance of a two-body axi-symmetric FWEC will be insignificant. The mooring lines used in the studies were designed to withstand the relevant extreme environmental conditions (ultimate limit state (ULS) criterion). To capture the short-term variability of the 3-hour response extreme value, especially due to different strategies applied for operational and non-operational conditions, the ULS level response for a two-body FWEC should ideally be estimated by performing the full long term analysis by taking into account all sea states. However, such an analysis is time consuming; the simplified contour line method that takes into account only a few sea states along a return period environmental contour is suggested. Referring to the practices in the oil and gas industry, the present thesis has explored the feasibility of the contour line method to predict the ULS response level (100-year) in mooring lines of a two-body FWEC. Commonly, a short-term extreme is obtained by a high percentile level. In principle however, large numbers of extremes from simulations are required to obtain an adequate asymptotic Gumbel distribution with a targeted response with a high percentile level. Therefore, an alternative method using the expected maximum value multiplied by a factor has been explored and was found to be a more robust method for this application. To reduce wave loads on the floater in extreme sea states, the over-topping Wave Dragon FWEC considers lowering the floater to a certain submergence level. A laboratory model test has shown that this strategy reduces the mooring line tension significantly. The numerical model developed in the present thesis yields motions and mooring line tensions that are in good agreement with the model test observations. Extracting energy from waves and wind simultaneously may be profitable to reduce the cost of power from the renewable systems. For that reason, a combined concept involving the combination of a spar-type floating wind turbine (FWT) and a torus-shaped point-absorbertype FWEC, referred to as the Spar Torus Combination (STC), has been introduced. The effect of adding a FWEC system onto a spar-type FWT for STC has been investigated in terms of power production in operational conditions and in terms of extreme responses in non-operational and extreme environmental conditions. Analyses indicate that the additional FWEC system on the FWT increases the stability of the spar. It was found that this addition results in higher wind power production compared to the spar FWT alone for wind speeds lower than the rated one. However, the additional WEC also increases the structural responses in extreme conditions. To reduce these responses, some relevant non-operational modes have been considered and investigated. The analyses performed indicate that the structural responses of the STC are highly dependent on the applied non-operational mode. As a very large floater, Wave Dragon could easily support wind turbines (WTs). This combination is interesting because the power production from wind and waves will be approximately the same order of magnitude. Simulation results indicate that additional WTs will not affect the wave power performance of the Wave Dragon. This means that the average power from a Wave Dragon with 2 WTs could be estimated based on a linear summation of Wave Dragon wave power and wind turbine wind power.
SerieDoctoral thesis at NTNU;