Extracting wave energy while reducing hydroelastic response of very large floating structures

Abstract

Extracting wave energy is a promising way for renewable energy production because wave energy intensity is higher than energy intensity of other renewable energy resources such as wind energy and solar energy. On another front of ocean utilization, very large floating structures (VLFSs) provide an alternative solution to the conventional land reclamation technique for creating artificial land from the sea. The integration of wave energy converters (WECs) with VLFS brings many benefits, including: (i) reducing costs of installation, mooring/foundation, operation and maintenance owing to cost-sharing; (ii) increasing WEC reliability; (iii) limiting negative environmental impact; (iv) reducing hydroelastic response of VLFS; (v) providing power supply for operations on VLFS.Motivated by these enormous benefits, this thesis proposes four WEC-type attachments to VLFS, namely: (i) the modular raft WEC-type attachment comprising multiple independent auxiliary pontoons connected to the fore edge of VLFS with hinges and linear PTO systems; (ii) the oscillating wave surge converter (OWSC)-type attachment comprising a submerged vertical flap connected to the fore edge of VLFS with hinges and linear PTO systems; (iii) the two-mode WEC-type attachment comprising floating horizontal and submerged vertical plates connected to the fore edge of VLFS with hinges and linear PTO systems; (iv) the heaving WEC-type attachment comprising a linear PTO system connected to VLFS at one end and to the seabed at the other end. This thesis focuses on the power capture factor (i.e. the power produced / the incident wave power) and the reduction in hydroelastic response of VLFS by using WEC-type attachments.In terms of solution methodology, this thesis extends the finite element n boundary element (FE-BE) method for the hydroelastic analysis of VLFS to incorporate WEC-type attachments. In addition, this thesis proposes single-objective and bi-objective optimization processes based on the differential evolution to obtain optimal PTO damping coefficients for maximizing the power capture factor and/or the reduction in hydroelastic response of VLFS under action of regular and irregular waves. The proposed optimization processes can also be used for optimization of other design parameters such as the auxiliary pontoon length for the modular raft WEC-type attachment, the number of VLFS module connections and connection stiffness.It is found that when compared to the raft WEC-type attachment comprising a single wide auxiliary pontoon connected to the fore edge of VLFS with hinges and linear PTO systems, the modular raft WEC-type attachment comprising multiple independent narrow pontoons possesses several advantages, including: (i) larger power output and hydroelastic response reduction for oblique waves if the overall width of the attachment is comparable to the wavelength; (ii) more flexibility for expansion and downsizing; (iii) higher redundancy in the system; (iv) lower manufacturing and installation costs.With regard to the hydroelastic response reduction, the OWSC-type attachment is better than the corresponding raft WEC-type attachment for wave period T l 7 s, but it is worse for longer waves. In terms of wave energy extraction, the OWSC-type attachment is better than the raft WEC-type attachment for Tngn7 s, but it is worse for shorter waves. For maximizing power production, the required flap length for the OWSC-type attachment is smaller than the required pontoon length for the raft WEC-type attachment (about l/10 as compared to about l/3 where l is the incident wavelength).The two-mode WEC-type attachment combines the superior performances of floating horizontal plates in extracting wave energy for T l 5 s and of submerged vertical plates in extracting wave energy for T g 7 s. When deployed in sites with 5 s lnTnln7 s, the two-mode attachment yields a larger power production than the sole usage of floating plates or the sole usage of submerged vertical plates. Interestingly, the two-mode attachment also combines the superior performance of submerged vertical plates in reducing hydroelastic response for T l 7 s and the superior performance of floating plates in reducing hydroelastic response for Tngn9 s. Moreover, the two-mode attachment furnishes a good balance between extracting wave energy and reducing hydroelastic response of VLFS for the same wave period.For VLFS with heaving WEC-type attachments, the power capture factor is found to generally increase with respect to wave period and incident wave angle. For Tngn12 s and large oblique waves, heaving WEC-type attachments are generally more effective in extracting wave energy than the other kinds of WEC-type attachments.