The authors regret an omission in the Acknowledgments and would like to revise it as follows: Project funded under the National Recovery and Resilience Plan (NRRP), Italy, Mission 4 Component 2 Investment 1.3 - Call for tender No. 1561 of 11.10.2022 of Ministero dell’ Universitá e della Ricerca (MUR); funded by the European Union – NextGenerationEU Award Number: Project code PE0000021, Concession Decree No. 1561 of 11.10.2022 adopted by Ministero dell'Universitá e della Ricerca (MUR), Italy, CUP, Italy E13C22001890001, Project title “ Network 4 Energy Sustainable Transition – NEST”. The authors would like to apologise for any inconvenience caused.

This study presents an experimental investigation focusing on the performance of a taut mooring systems for Floating Offshore Wind Turbines (FOWTs) under accidental limit state scenarios. These tests are part of the data obtained during the experimental campaign conducted at the Ship Hydromechanics Laboratory of Delft University of Technology, aimed to assess the behavior of FOWTs under various conditions, including operating, extreme, and failure scenarios. All the data related to the experiments are freely downloadable from this link. The experimental analysis conducted in this work focuses on simulating various failure scenarios, such as the rupture of mooring lines and turbine blade actuators considering two different mooring configurations. Results show that in such scenarios, taut mooring systems possess design characteristics that mitigate the risk of additional damage to the floating system and underscores the importance of understanding and mitigating risks associated with accidental limit states in offshore platforms, particularly by optimizing mooring configurations and materials to enhance overall safety and resilience.

In the offshore wind energy field, reducing energy costs involves optimizing and analyzing each system component. A key component influenced by the installation site is the mooring system, which can be designed using various concepts. This study focuses on examining a taut-leg mooring system and its impact on the overall system behavior. Due to a lack of experimental data on taut-leg mooring systems for Floating Offshore Wind Turbines (FOWTs) in the existing literature, our work aims to provide the scientific community with an extensive experimental dataset to validate various numerical models and support the design process of a taut-leg mooring system for a selected installation site. The full-scale mooring system was designed, scaled down, and evaluated through experiments at a 1:96 scale using a mooring configuration realized with springs. Springs offer a constant axial stiffness, reflecting the ideal structural behavior. Our paper highlights significant observations for this configuration, even under off-design conditions with modified pre-tension levels. Regular and irregular waves were tested to establish a baseline hydrodynamic response, assess the wind turbine's impact on the floater, and evaluate operating conditions. An environmental contour (EC) was defined to analyze the system's behavior in ultimate and accidental limit states. System identification (ID) waves streamlined the characterization process by reducing the number of required waves. Additionally, free decay tests were performed to assess the system's dynamic characteristics at resonance. The analysis of experimental data reveals that pre-tension variations minimally influence the dynamics of the floating structure. Results showed that the tested mooring system exhibits stability during power production and withstands ultimate and accidental limit states.

Creating Resilient River Systems by Mainstreaming and Upscaling Nature-based Solutions or ‘ResiRiver’ for short, is an EU Interreg project focusing on the creation of Resilient River Systems through the mainstreaming and upscaling of Nature-based Solutions (NbS). Today, most North-West European (NWE) river systems face significant challenges, largely resulting from past anthropogenic riverine alterations (regulations and interventions) which have greatly exacerbated the negative effects of climate change like flooding and drought. These changes also put additional pressures on existing land-use-claims, and other societal/economic interests like shipping, agriculture and water quality and cause continued biodiversity-loss. In recent years, NbS have emerged as a novel and sustainable approach in river management, cap able of mitigating these threats while also strengthening climate resilience. They fit well within the EU’s ‘Blue Economy’ ambition and provide undeniable benefits for both human well-being and biodiversity. However, the widespread integration (mainstreaming) and implementation (upscal ing) of NbS in riverine systems has yet to become common practice. The ResiRiver project there fore aims to accelerate the upscaling and mainstreaming of NbS through a curated partnership of full-scale North West Europe NbS projects by gathering, transferring, and synthesizing know ledge and experiences, whilst ensuring that this is done at all relevant partner/national/EU levels.

Increasing utilization of ocean space and a global push for renewable energy solutions has spurred interest in wave behavior around Very Large Floating Structures, like floating photovoltaic (PV) systems. Flexible PV modules may be more suitable for the varying wave conditions found in offshore environments. However, while viscoelastic models are commonly used for wave prediction, they show notable discrepancies with experiments, likely due to untested assumptions of inviscid flow. This experimental study aims to fill that gap by investigating both the wave characteristics and velocity fields underneath flexible and rigid structures using simultaneous Particle Image Velocimetry (PIV) and wave elevation measurements. Wave attenuation is observed for short wavelengths over the flexible structure length. The 2nd order Stokes wave theory provides a good approximation of the wave-induced horizontal velocity profiles under the flexible structure but underestimates the velocities under the rigid one which further lacks the typical exponential decay with water depth. The presence of a wave boundary layer is showcased and compared to an adaptation of the Stokes 2nd problem.

This study examines the potential of a new type of floating seawall, made up of retired large-scale oceangoing vessels, to be used in open water and exposed coastal areas. The main objectives of the research are to assess the effectiveness of the floating seawall concept, to determine the contribution of the gap resonance to wave attenuation, and to compare the results of physical tests with those obtained numerically using ANSYS-AQWA. The use of end-of-life ships in this way provides a unique opportunity to extend their life cycle and reduce the environmental and human health risks associated with the current practice of shipbreaking. The research focuses on a multimodule floating seawall configuration, where each module is composed of two hulls that are rigidly connected side by side, with a small gap to induce gap resonance. The results suggest that end-of-life ships can be used as a resource for the construction of floating seawalls for various marine applications. Furthermore, the results demonstrate the positive influence of the gap resonance on the wave attenuation capacity of the seawall, as well as the limitations of the numerical tool in providing realistic values in this region.

Driven by population growth and rural migration toward the cities, the demand for affordable housing continues to increase. However, due to the scarcity of urban development space—especially in coastal areas, the supply is limited. As increasing land availability is one of the most effective ways to reduce real estate costs, this interdisciplinary research explores the alternative of urban expansion toward the adjacent marine environment of coastal cities. It focuses on floating residential dwellings from both technological and urban planning perspective, aiming to include the waterfront of coastal cities as viable, sustainable, and affordable alternative for urban development. The research takes on one of the most expensive cities in the world, Tel Aviv-Yafo, as a case study for increasing the supply of affordable housing in addition to vital sustainable future growth in the adjacent marine environment.

For Offshore Floating Photovoltaics (OFPV) applications, thin-film PV panels on lightweight floating support structures gain increasing scientific and commercial interest. Over the past years, several different concepts of thin-film OFPV have been proposed, with the common denominator of floating mattress or blanket-like support structures with very little draft in the order of centimeters compared to their width and length in the order of several tens to hundreds of meters. Mostly made from polymer foam materials, these floating support structures are more flexible than the conventional Very Large Floating Structures (VLFS) investigated in 1990s. The flexibility of a floating structure is expressed by the characteristic length derived from the ratio of the structural bending stiffness and the hydrostatic stiffness of the support. For conventional VLFS, this characteristic length is usually longer than the dominant wavelength of the ocean waves, resulting in only moderate structural deflections of the order of 1/10 of the wave height and the total thickness of the structure. The newly proposed structures have characteristic lengths of less than the wavelength of ocean waves. This allows the structures to move with the waves and follow the wave elevation like a floating blanket. Therefore, these structures are classified as Very Flexible Floating Structures (VFFS). Despite the growing interest in VFFS, little is still known about their hydroelastic deformation and their influence on the surrounding wave field. To start the experimental VFFS research at Delft University of Technology, Digital Image Correlation (DIC) measurements were carried out in this study to investigate the vertical deflection of a VFFS at model scale in a small towing. The model’s characteristic length was 1/3 of the shortest wavelength and it was tested in long-crested regular longitudinal waves. The wavelength varied between 1/10 and 1/5 of the structure length. The measurements showed that the structure indeed mostly followed the wave elevation and revealed 3D effects across the structure, which require deeper investigation into wave scattering of VFFS.

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.

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).

Very flexible floating structures have been proposed for offshore floating photovoltaics installation. Characterized by having structural lengths much longer than wavelengths, small thickness, and low bending stiffness, these structures are prone to large vertical deflections and strong hydroelastic interactions. Experimental information on these structures is scarce. In this study, we employed digital image correlation (DIC) to investigate the hydroelastic interaction of a flexible floating sheet with a length-to-height ratio of 1,000 in regular long-crested head waves. The wavelength was one-tenth and one-fifth of the structure length, with a wave steepness of 0.04. The repeatability of wave conditions and measurement results was demonstrated, and measurement errors were quantified. Surface elevations showed that the sheet followed a local wave elevation in long waves. In shorter waves, strong hydroelastic interactions led to wave lengthening underneath the floating structure and three-dimensional (3D) effects across the structure width. Wave lengthening agreed well with prediction from the hydroelastic dispersion relation. Observed 3D effects necessitate further research into the possible influence of viscoelastic effects. It was shown that the DIC technique is suitable to measure flexible floating structures in waves with low error and good repeatability. Experimental data are publicly available.

Flexible floating structures received increasing attention in recent years as support structures for floating offshore solar installations and other forms of oceans space utilization. An early example for such structures was the Mega-Float structure proposed as floating airport runway for Tokyo Bay. More recent examples can be found in the large inland floating solar parks where interconnected pontoons form a flexible floating structure. The common denominator of these structures is their small height compared to their length and width resulting in low bending stiffness in the vertical direction. Structural length being much longer than the wavelength and low bending stiffness result in large vertical deflections of the floating structures and strong hydroelastic interaction with the waves. Similar behavior can be observed for sloshing mitigation measures with flexible membranes. In this study, we investigated the wave structure interaction of a floating flexible sheet with a length to height ratio of 1000 in regular long-crested head waves in the small towing tank of Delft University of Technology. Wavelength was varied between 1/20 and 1/5 of structure length with wave steepness in the range of 0.02 to 0.05. Digital Image Correlation (DIC) was used to measure the surface elevation of the entire structure and wave elevation was measured in three different locations to provide reference data. The results show that the floating sheet mainly followed the local wave elevation and a reduction of motion amplitude was observed over the length of the structure. Further, the results reveal 3D effects of different elevation amplitude across the width of the sheet, which suggests strong interaction with the waves.

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.

The intricate physical mechanisms involved during sloshing impacts in LNG tanks lead to biases in sloshing model tests when the impact loads are predicted. In order to increase the understanding of these biases, a new state-of-the-art facility dubbed the Multiphase Wave Lab (MWL) has been established. In the MWL, impact tests are performed within an autoclave (15 m long x 2.5 m diameter), whose purpose is to provide an accurately controlled environment in which the pressure, temperature and gas composition can be controlled and monitored. Wave impact tests are performed by generating waves in a flume which is located inside the autoclave. In this paper, we present the capabilities of the MWL to control the temperature, ullage pressure and gas composition in the autoclave. We study also the quality of the global flow repeatability by means of a breaking wave which is created with a wave-focusing technique. We quantify the repeatability of the waves with a Sobolevnorm-like criterion on the frequency domain and evaluate the repeatability for different ullage pressures. Preliminary experiments show a good degree of repeatability, in accordance with high-speed recordings of the impacting waves.