Application of Flexible Structure in Marine Engineering

The aim of the Special Issue (SI) was to publish original and high-quality research papers that deal with model developments based on the mathematical, numerical, and experimental models associated with flexible floating or submerged structure applications in the discipline of marine engineering. Flexible floating or submerged structures are effective types of coastal and offshore structures that can be utilized in the sea environment for multi-use model applications, including breakwaters for coastal protection and fish cages for aquaculture.

Regarding the manuscripts published in this SI, different types of models associated with flexible structures for various applications have been developed and the effects of the associated design or physical parameters have been analyzed. For example, two mathematical hydroelastic models of wave interaction with horizontal floating and submerged flexible structures, based on the analytical and the numerical boundary element integral equation method (BIEM), were developed [1,2] based on linearized water wave theory. The submerged porous structure (modeled using the thin plate theory) connected to mooring lines was created and the analytical solutions were obtained by employing Green’s function technique and the matching technique. The accuracy of the theoretical solutions obtained in [1] are verified by comparing/validating against numerical and experimental datasets as well as existing published results in the literature that shown a good level of agreement. The effects of mooring stiffness, depth of submergence, porous-effect parameter, and incidence wave angle on the reflection coefficients and transmission coefficients, as well the structural deflections, were analyzed in detail. It can be observed that a suitable choice of mooring stiffness, incidence angle, flexural rigidity, porous-effect parameter, and submergence depth plays a vital role in wave energy dissipation regarding the construction of effective breakwaters and in modelling regarding the creation of wave energy conversion devices [1,2].

Three papers of the papers included in this SI deal with the study of fish cage modeling and the effects of mooring loads and environmental conditions based on numerical and experimental model tests. Liu et al. [3] presented a numerical model based on the finite element method that was aimed at comparing with analytical model solutions were developed in [3] and the effects of currents and mooring loads on the dynamics of floating flexible net cages. The authors observed that the comparison results of horizontal displacements of the cage for different values of mooring stiffness, cage heights, and cage diameters with a specific current speed value between the finite element and analytical model solutions had a good level of agreement. Furthermore, the analysis of the 3D cage displacements using the finite element model indicated that the cylindrical flexible net cage became more stable for the lower current speed values. Using a mass–spring model and 5th order Stokes wave theory, Lee et al. [4] studied various mooring conditions to minimize the mooring loads in a fish cage by including a damping buoy and wave conditions. In the numerical simulation, the following conditions were considered: a bridle-line length of 0.8–3.2 m; buoyancy of 2.894–20.513 N for the damping buoy; and a mooring-rope thickness of 0.002–0.004 m. The authors observed that the bridle-line length had the most significant effect on the mooring tension of the cage and the study concluded that a lower mooring-line load leads to the improvement of mooring stability for cage design.

Lastly, one paper deals with the performance and feasibility of a novel automated catch-hauling device using a flexible hose net structure by conducting a series of water tank experiments and catch-hauling tests [5]. The experiments were used to evaluate the performance of this hose net, the deformation shape of the hose net, the floating and sinking speeds, and fish school behavior. The authors observed that the fish were driven to the fish bag, as the fish escaped through the gaps and also became trapped in the hose net and were unable to move. In addition, the authors found that the floating speeds of the hose net could be controlled by the initial pressure of the air compressor and in practical application, sea currents and environmental conditions could cause substantial problems for the proposed novel automated catch-hauling device system.

We are aware that many other significant studies need to be carried out to better understand the effects of the design parameters and the incorporation of innovative components into the cage model associated with wave interactions with flexible structures. We hope that the articles published in this SI will encourage interest in these issues.