This paper presents an application of modal expansion method to wave-structure interaction problem of Very Flexible Floating Structures (VFFSs), like Offshore Floating Photovoltaics (OFPV) membrane structures. In literature, the modal expansion method was applied to wave-structure interaction of Very Large Floating Structures (VLFSs), where wave amplitude and structure deformation amplitude were of the order of structure height. For VFFSs, wave and structure deformation amplitude could be of the order of ten times the structures thickness. In this study, we explore the applicability of the modal expansion method to this new field. The wave field is addressed by boundary integral equation using free surface Green’s function method and the structure deformation is approximated by structure vibration modes. Assuming small-amplitude wave and small structure motion, the coupling is achieved by solving the governing equilibrium equation based on the classic thin plate theory. The verification of the present method with respect to panel resolution and mode truncation is discussed. Structure displacements of numerical analysis are compared with experimental results. The comparison shows that the solution provides acceptable prediction for longer waves (wavelength is one-fifth of the structure length), whereas for shorter waves (wavelength is one-tenth of the structure length), obvious discrepancies occur, especially at the aft region of the structure.

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Thin continuous flexible floating structures have been shown to have technical and economic advantages for Offshore Floating Photovoltaic (OFPV) installations. In terms of large horizontal dimensions compared to the wave length, these structures are similar to sea ice as well as Very Large Floating Structures (VLFS), e.g. as proposed for floating airports. In this paper, we reviewed the hydroelastic theory for sea ice and VLFS and assessed its applicability to the newly envisaged flexible floating structures. While VLFS and sea ice motion in waves are dominated by elastic deformations, their motion amplitudes are limited to the order of the structure thickness. Thin and flexible floating structures were found to be able to follow the wave motion with amplitudes far exceeding their thickness. Nonlinear theories like Föppl–von Kármán plate theory are required to model these structures. The significant contribution of nonlinear effects in the structural response and the large deformations in waves far exceeding the structural thickness lead to the definition of the new category of Very Flexible Floating Structures (VFFS).

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With climate change and rising sea levels, the coastal zone’s flood risk is deteriorating. Previous researches have shown a gradually degrading capacity of traditional hard engineering structures (e.g., seawall, dikes) on flood mitigation due to problems such as land subsidence and insufficient maintenance. To remedy the defects, the “building with nature concept” for coastal protection with saltmarshes was examined by combining field measurements and numerical simulations. The advantages of saltmarsh over traditional seawall on flood protection was demonstrated from the perspective of both flood area mitigation and economic gain, based on scenario simulations. Results show that tidal wetlands are essential in mitigating significant wave heights (H_s) and current velocities even during storm conditions. The storm wave and current velocity reduction ratio (RR_w and RR_c) by saltmarshes on Chongming Dongtan Shoal (CMDS) during Typhoon 9711 is approximately 11% and 51%, respectively. The wave and current mitigation by Scirpus mariqueter are more efficient than Spartina alterniflora and Phragmites australis during measurements in 2010, which were approximately 0.3 m and 0.2 m/s, 0.125 m and 0.155 m/s, 0.086 m and 0.128 m/s per kilometer width, respectively. The summer saltmarsh area 54.2 km² on CMDS protects approximately 32 km² land area behind the seawall from being flooded, equivalent to the seawall heightening of approximately 0.42 m on equivalent flood mitigation. The performance of cost-and-benefit analysis shows a relatively higher (by 3%–7%) net present value (NPV) and a higher (by 1.5 times) benefit-cost ratio (BC) of nature-based solution (i.e., saltmarsh restoration) compared with traditional hard engineering solution (i.e., seawall construction). Thus, building seawall with nature, such as a hybrid flood protection measure, should be implemented in the future coastal redesign and maintenance.

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In lab-scale model tests of modular floating structures, motion measurement is sensitive to mechanical influence of contact-based measurement systems. To investigate alternatives, model tests of a modular flexible floating structure interacting with regular waves was carried out in the towing tank of Delft University of Technology. Non-contact 3D digital image correlation (3D-DIC) technique was employed to evaluate the structural displacement. Stereovision based system Optotrak Certus of Northern Digital Inc., which relies on cable connection to the model, was used as a reference system to validate the DIC results. Model motions were analyzed and compared between the two systems. Comparisons show that DIC results agree well with stereovision results, indicating that the DIC system is an accurate motion measurement of floating structures in waves.@en