Flexible Fluid-Structure Interaction (FFSI) has emerged as an important, but challenging research direction in modern ocean engineering. This line of research gradually evolved in response to the pressing need to model the dynamic responses of ships and marine structures to sea loads; to predict the performance of flexible marine propellers, energy converters, and coastal protection systems; and to understand the mutual interactions between sea ice, marine vegetation, and mud with oceanic and coastal processes occurring near the surface and seabed. This review presents the state of knowledge and art of modelling of FFSI in the maritime environment, tracing research progress from early physical tests to high-fidelity computational ones emerged recently. Flexible wave–structure interaction, global ship hydroelasticity, hydroelastic slamming, flexible marine propellers, vegetation dynamics, and wave–mud interactions are covered. Limitations and strengths of existing models, and the challenges that remain are discussed in-depth, and it is concluded that FFSI-based research in ocean engineering has very well grown, though some gaps are still open. In specific, hydroelastic effects are still overlooked in the design practices and classification rules do not fully incorporate them, and there are still concerns regarding uncertainties related to FFSI modelling of flexible slamming, dynamic of flexible marine vegetation, and wave-mud interactions. Hence, future research must bridge computational modelling with real-world applications, expand benchmarking coverage for marine engineering problem, and incorporate AI-based methods for modelling FFSI problems, predicting related dynamic responses, or accelerating simulations.

An important trend exhibited by the offshore wind market is the increasing size of wind turbines, leading to longer and stiffer monopiles with larger diameter-to-thickness ratios. Current transport analysis is focused on loads resulting from hydrodynamic accelerations, without taking into account the loads resulting from differences in bending deflection between the vessel and cargo. This investigation examines the structural response of a monopile and sea-fastening system subjected to displacement-based loads. The load case follows from a vessel excited using a regular wave leading to bending deflections and rigid body accelerations. The intermittent contact between the saddles and monopile is modeled by representing the saddle with a unilateral spring. This requires the use of a nonlinear solution method to obtain structural responses. The harmonic nature of hydrodynamic-based loads led to the selection of the harmonic balance method (HBM) to model the cargo-sea-fastening system. A novel understanding is gained of how cargo properties, sea-fastening properties, and sea-fastening arrangements influence the structural response of the coupled cargo-sea-fastening system. Various parametric studies are performed to identify behaviors related to the total structural response. Based on this study, the conclusion can be drawn that a large number of saddles in combination with a low stiffness is desired to minimize the structural response of the cargo and sea-fastening system. Furthermore, the influence of lashing stiffness and pretension is limited with respect to the total response. Both these conclusions also hold for an increase in cargo length and diameter.

Catamarans are popular in the offshore sector as they combine good transverse stability and ample deck space with low wave resistance. However, their slender hull shape results in low restoring qualities in heave and pitch motions. The large motions in rough weather can often result in water impacting the underside of the deck connecting the two hulls, a phenomenon called wet deck slamming. The impulse excitation from wet deck slamming can then produce a transient hydroelastic response of the structure called whipping. Whipping excites mode shapes that would not normally be present in the response, as their natural frequencies are significantly higher than the wave encounter frequency. This results in detrimental contributions to fatigue life through high-amplitude cyclical bending moments. Both the calculation of slamming loads and the prediction of resulting structural responses have been a challenge for several decades. The highly nonlinear and three-dimensional character of the phenomenon, combined with the strongly coupled fluid-structure interaction means that it is unpredictable, and even the definition of slamming events has been a matter of disagreement among researchers. Experiments are still a vital part of these investigations, for validating ever-improving numerical techniques. An essential issue with experiments is the extent to which mode shapes and natural frequencies can be emulated in model scale. Traditional hydroelastic models are segmented and use either a flexible backbone or flexible joints to introduce stiffness. This often results in an excellent description of the 2-node bending mode, but an increasing error for higher modes leads to stress inaccuracies. In this investigation, a continuous model of a catamaran is designed and produced for hydroelastic experiments. The advantages and limitations of the concept are identified, the verification against structural models is presented, and the calibration of the measurements is discussed.

Hydroelasticity of ships has been established as a necessary form of investigation for both slender ships and high-speed craft. Experimental investigations have spanned various topics, including symmetric and antisymmetric, harmonic and transient, linear and nonlinear responses. Models have varied in size and the way the structure is modelled, depending of the focus of the investigation. The multitude of interacting physical mechanisms introduce almost-impossible-to-resolve scaling issues, and the eventual compromises depend on the aim of the investigator. This publication provides a comprehensive review of the evolution of these experimental techniques, from the first appearances of the field to the modern state-of-the-art and potential future directions.

Experimental hydroelasticity has not followed the rapid evolution of its computational counterpart. Hydroelastic codes have changed significantly in the past few decades, moving to more detailed modelling of both the structure and the fluid domain. Physical models of ships are, even today, manufactured with a very simplified structural arrangement, usually consisting of a hollow rectangular cross section. Appropriate depiction of the internal structural details ensures that properties relevant to antisymmetric vibration are scaled accurately from the real ship to the model. Attempts to create continuous, ship-like structures had limited success, as manufacturing constraints did not allow for much internal structural detail to be included. In this investigation, the first continuous model of a ship with a detailed internal arrangement resembling a container ship is designed, produced using 3D printing and tested in waves. It is demonstrated that the global responses of the hull in regular head waves agree well with theory and past literature, confirming that such a model can represent the behaviour of a ship. Furthermore, it is found that the model is capable of capturing local responses of the structure, something that would be impossible with “traditional” hydroelastic ship models. Finally, the capability of the model to be used to investigate antisymmetric vibrations is confirmed. The methodology developed here opens a whole new world of possibilities for experiments with models that are tailored to the focus of the investigation at hand. Moreover, it offers a powerful tool for the validation of modern state-of-the-art hydroelastic codes. Ultimately, it creates the next step in the investigation of dynamic responses of ship structures, which contribute significantly to accumulating damage of the hull. Better understanding of these responses will allow designers to avoid over-engineering and use of big safety factors to account for uncertainties in their predictions.

Hydroelasticity of ships and studies in coupled antisymmetric vibrations have become increasingly important with container ships becoming faster and more slender. In this investigation, a ship-like structure is modelled and an equivalent backbone with a U-shaped cross section is designed. Their responses are compared, and limitations of various modelling approaches are discussed. It is demonstrated that scaling of the natural frequencies is insufficient to ensure scaling of the antisymmetric mode shapes and the relevant differences are quantified. Consequently, the backbone model should be viewed as a separate structure for validation purposes rather than a scaled model of a ship.

The use of flexible ship models to determine the dynamic behaviour of full-scale ships in waves and to compare the accuracy of numerical predictions has increased in the past few years. Segments attached to a flexible uniform backbone of suitable but simple cross section is the preferred solution. Although such models are relatively easy to manufacture with conventional processes, they do not represent accurately the structural detail, for example, of a container ship. The limitations of conventional manufacturing constraints can be potentially overcome by use of modern technologies such as additive manufacturing. Designing detailed elastic ship models requires the determination of dynamic material properties, in addition to the manufacturer mechanical properties. In this investigation, a detailed but easy-to-implement method is developed, and applied to a uniform container ship-like model, to identify the material properties that are relevant to the calculation of the natural frequencies of 3D printed thin-walled structures. It is demonstrated that modal testing of 3D printed specimens, combined with FEAmodelling, can be used to accurately predict the natural frequencies of much more complex thin-walled structures. This method allows investigators to acquire all information necessary during the design stage of 3D printed structures without having to resort to full material characterisation.