Very Large Floating Structure Research Articles

A discrete-module-beam hydroelasticity method with finite element theory in analyzing VLFS in different engineering scenarios

A numerical tool is developed in the framework of the Discrete-Module-Beam method to analyze hydroelasticity of very large floating structures (VLFS) such as floating offshore photovoltaic platforms (FOPV). The proposed method utilizes finite element theory to perform structural analysis. The 3D potential flow theory is used to perform a hydrodynamic analysis after macro-submodule division. For the structural analysis part, the platform is divided into macro-submodules and each macro-submodule is represented by a beam, which is then further discretized into micro beam elements with the nodes categorized into three groups, that is, the lumped mass, the boundary nodes and the inner nodes. Derivation of the lumped-mass stiffness matrix and recovery of the displacement distribution are given. Procedures to deal with interconnections, complex boundary conditions and unsteady loads are elaborated. Good agreement is obtained in different loading scenarios to the numerical and experimental results.

Hydrodynamic analysis of a novel box-type floating marine airport

Very Large Floating Structure (VLFS) plays a crucial role in mitigating land resource scarcity in developed coastal areas. This paper introduces an innovative VLFS concept consisting of numerous concrete box-type floats interconnected by steel tie rods, designed for a 4000-m-long floating marine airport. Based on three-dimensional potential flow theory, two numerical models for the VLFS are established: the single-float model and the multi-float model interconnected by joints, where each float treated as rigid. The research explores the impact of wave direction, wave period, and real sea conditions on the hydrodynamic response of the VLFS. Additionally, this paper focus on the discrepancies in dynamic characteristics between single-float and muti-float, and investigate the special response principles of the muti-float model that closely resemble the actual hydrodynamic features of VLFS. The findings show that local deformation in the VLFS results in elevated wave loads on the structure, posing a significant challenge in designing the VLFS mooring system. Higher structural stiffness correlates with decreased mean and pulsation values of mooring cable tension. Incident waves at 0° induce local deformation on the wave-facing side of the VLFS; however, the deformation rapidly dissipates, exerting minimal impact on the floats supporting the airport runway—thus, creating an indirect wave-absorbing mechanism. When waves incident at 90°, the VLFS bends, adversely impacting the operations of marine airports and causing uneven stresses on the connectors, making them susceptible to fracture and damage. Hence, it is advisable to install marine airports parallel to prevailing wave directions. Furthermore, the hydroelastic behaviour of VLFS contribute to alleviating internal stresses within the structure. The focus of future research lies in balancing the impact of VLFS hydroelasticity on both system motion performance and structural internal stresses. The VLFS concept presented in this study performs effectively under various real sea conditions. The rationality and feasibility of the proposed VLFS concept are substantiated. The study lays the foundation and presents ideas for the future development of marine space.

The Moving Element Method for the Dynamic Responses of Floating Double-Plate Structures on the Shallow Water

In this paper, the dynamic behavior of floating double-plate structures under a moving load is investigated in great detail with different factors utilizing the moving element method (MEM). The structures in this study are very large floating structures (VLFSs). The Kirchhoff–Love theory is used to model two floating plates connected by a Winkler-type elastic core, whilst the water surrounding the structure is simulated by the shallow linear water wave theory. In order to improve the computational efficiency of moving load problems, the MEM is adopted. Since the MEM uses fewer discrete elements than those of the traditional finite element method (FEM), the computational cost is reduced. The proposed two-plate model helps to reduce the influence of the surrounding water on the upper plate. Several numerical results are presented to investigate the influence of the ratio of thickness between layers as well as core stiffness on the displacements with different water depths, and load speeds.

Long-term extreme responses of torsional moments at two-directional hinges for moored very large floating structures

This paper presents an analysis procedure for estimating long-term extreme torsional moments at connections of very large floating structures (VLFS) with two-directional hinges and mooring lines under random waves. This procedure, compared to the common short-term estimation method, can accurately and quickly calculate the extreme response at a certain return period. Applying a coupled frequency-time domain approach, we simulate the hydroelastic response of VLFS with two-directional hinges and vertical mooring lines. In order to validate the proposed approach, numerical simulations are firstly carried out under the action of regular waves. The present approach demonstrates a high level of consistency with experimental data and numerical results from existing studies in terms of vertical displacement, bending moment and shear force. Then, on the basis of the short-term extreme response distribution and environmental model, a procedure for solving long-term extreme responses of the structure is introduced. The long-term extreme response characteristics of the torsional moment at hinges are evaluated and studied in depth using the procedure, providing a reference for the structural design of moored VLFS with two-directional hinges.

Coupled analysis between catenary mooring and VLFS with structural hydroelasticity in waves

The rapid growth of marine renewables has led to the development of very large floating structures (VLFS) that are designed to operate in deep seas. It is significant to understand the mechanism of the coupled effects between deformable VLFS and catenary mooring system. This paper presents a time-domain hydro-elastic-moored model developed by integrating a quasi-static mooring module into a fully coupled Computational Fluid Dynamics (CFD) - discrete-module-beam (DMB) approach. The model is used to investigate the coupled effects between structural hydroelasticity and loose-type mooring systems on a deformable VLFS in waves. The mooring and hydroelasticity codes are validated separately and show favourable agreement with other numerical and experimental results. Then the coupled effects between the mooring system and structural hydroelasticity are evaluated by assigning various design parameters, i.e., VLFS structural stiffness and mooring stiffness. The numerical results, including dynamic motions, longitudinal vertical bending moments (VBMs) and mooring tension forces are presented and analysed. These results can be used to design a VLFS with mooring in medium-deep sea, and help with the conventional mooring design for a less-stiffness VLFS due to hydroelastic response.

Artificial neural network framework for prediction of hydroelastic response of very large floating structure

The response of a very large floating structure (VLFS) must take into consideration the elastic deformation of the structure (commonly termed hydroelastic response) under wave action. Conventionally, the hydroelastic response could be computed by using the coupled finite element-boundary element (FE-BE) method, where the mat-like structure is modelled using plate theory and the water modelled using the potential theory. The FE-BE method requires the structure to be discretised into finer elements and the wetted surface boundary to be represented by smaller panels to accurately capture the hydroelastic response of the structure. Thus, the coupled FE-BE method could be computationally expensive when the structure gets larger or when subjected to waves of smaller wavelengths. To accelerate the computational time in predicting the hydroelastic response of the VLFS, a surrogate model trained using the feed-forward neural network is proposed. The hydroelastic responses under different wavelengths, structural stiffnesses and wave directions are first generated where these data are split into three groups for training, validation, and testing (prediction) purposes. The accuracy of the prediction in terms of correlation coefficient R is compared for the different train datasets, the number of neurons and hidden layers as well as the optimisation techniques. The finding shows that an accuracy of close to 99% to the ground truth could be achieved with only 80% of the train dataset. The hydroelastic response under irregular wave conditions predicted using the feed-forward neural network framework is also presented.

Research on the Connector Loads of a Multi-Module Floating Body with Hinged Connector Based on FMFC Model

VLFSs (Very Large Floating Structures) are often assembled by multiple modules through connectors where the connector structure is prone to stress concentration. Therefore, the loads at the connectors have become a significant focus in VLFS design. In this paper, the FMFC (Flexible Module Flexible Connector) method, which can account for the elastic deformation of each module and the connector, is established in order to predict the coupling response of the connector and the platform. The finite element model of a dual-module transfer platform with a hinged connector is established and the modal shape characteristics of the entire structure are analyzed. The accuracy of the method is verified through a model test, where the model was scaled as 1:50. Furthermore, the RAOs (Response Amplitude Operators) of connector loads were calculated. It was found that the horizontal loads (along the x and y directions) were more sensitive to the wave direction. Furthermore, the wave direction where the maximum short-term significant value appears was not consistent with the direction of the maximum RAO. The loads response law of the VLFS with a hinged connector is summarized in this paper and provides a reference for the design of connectors.

Application of CFD–FEA coupling to predict hydroelastic responses of a single module VLFS in extreme wave conditions

Freak waves, as a kind of strong nonlinear waves, have a great threat to the safety of Very Large Floating Structures (VLFS). Freak wave produces enormous wave height, which caused the hydroelastic responses on the VLFS to increase significantly. Nevertheless, due to the complexity of the highly nonlinear interactions between waves and VLFS, studies of wave-induced loads and the hydroelastic responses of VFLS in freak waves are rare. In this paper, a fluid-structure interaction technique that strongly connects the CFD and FEA solvers is proposed to evaluate the wave-induced loads and hydroelastic responses of a single module VLFS under freak wave circumstances. The accuracy and reliability of the CFD–FEA coupling approach is validated through the comparisons of the results of hydroelastic responses of VLFS in regular waves with those from other numerical methods. Finally, the global motion and vertical bending moment (VBM) of VLFS in freak wave conditions are investigated. The result shows that the freak wave will lead to the global motion of VLFS being multiplied several times and largely increasing the instantaneous maximum VBM. The novel findings in this study will provide a reference for the structural design of VLFS under survival conditions.

A Coupled Hydrodynamic–Structural Model for Flexible Interconnected Multiple Floating Bodies

Evaluating the structural safety and seakeeping performance of very large floating structures (VLFS) using the rigid module flexible connector (RMFC) method remains challenging due to the complexity of the coupled hydrodynamic–structural responses in this system. In this study, a coupled hydrodynamic–structural frequency–time domain model is developed based on the RMFC method employing the planar Euler–Bernoulli beam elements to investigate the dynamic responses of multi-module floating systems. To reveal the dynamic characteristics of the systems, the coupled hydrodynamic–structural responses are investigated using a frequency–time-domain numerical model with viscous correction, in which the mass and stiffness attributes of connectors are incorporated into the system. Given the effects of hydrodynamic interaction, consideration is given to the case of three modular boxes connected by flexible beams aligned in series in shallow water to validate the present model. Higher efficiency and accuracy can be found in the system using viscous correction in potential flow theory and introducing state–space model to replace the convolution terms in the Cummins equation for the time domain. Moreover, this model can be extended to a considerable number of floating modules, which provides possibilities to analyze N-module floating systems.

Hydroelastic analysis and structural design of a modular floating structure applying ultra-high performance fiber-reinforced concrete

This paper proposes a hydroelastic and structural analysis method for modular floating structures (MFS) constructed from fiber-reinforced polymer (FRP) reinforced ultra-high performance concrete (UHPC) for floating photovoltaic (FPV) systems. Initial structural design, hydroelastic analysis of an equivalent plate model as well as structural analysis are performed in the approach. We investigate the hydroelastic responses of a continuous very large floating structure (VLFS) and three hinge-connected modular floating structures. The floating module is designed to be carbon fiber-reinforced polymer (CFRP) reinforced UHPC box-pontoon. Hinge connections are applied to neighboring modules to form the floating structure. In the hydroelastic analysis, an equivalent plate model was conducted, and the effects of the number of hinge connections and incident wave conditions on the bending moment and vertical deflection were studied. We found that the hinge connection had a great effect on reducing the bending moment. The aspect ratio of the module should also be considered. Cross-sectional analysis was performed to estimate the bending capacity. Nine cross-sectional trials were compared to verify their safety probability in the specific static analysis. Prestressing was found very effective in enhancing the bending capacity. Non-prestressed UHPC is not recommended owing to its limited bending capacity.

Dynamic responses of serially connected truss pontoon-MOB – A numerical investigation

Very large floating structures (VLFS) are typically made up of several modules of offshore platforms, such as semi-submersibles, connected by mechanical connectors. The proposed Truss Pontoon Mobile Offshore Base (TP-MOB) is a new type of offshore platform based on the truss pontoon semi-submersible design. The design of the connectors is crucial for the functionality of the structure. In this study, the flexible connectors are modeled with a 3D source distribution method and the wave forces are estimated with 3D linear potential theory. The open-source finite element-based numerical model, HYDRAN-XR, is used to analyze the structural responses and connector forces in both regular and irregular wave conditions. The results indicate significant differences in responses between single and multi-module cases, especially in extreme waves. Loads on the connector change greatly for three- and five-module cases, with maximum values at specific wave angles.

Feasibility of integrating a very large floating structure with multiple wave energy converters combining oscillating water columns and oscillating flaps

The installation of wave energy converters (WECs) along the facing-wave side of in-development or pre-existing offshore structures provides an effective cost-sharing solution. In this study, a time-domain numerical model based on the modal expansion theory and the nonlinear potential flow theory, is applied to optimize the size and layout of an in-line array of oscillating water column (OWCs) and oscillating flaps (OFs) deployed along the facing-wave side of a very large floating structure (VLFS). The front/back-wall end of each OWC chamber is hinged by an OF device, and thus the number of OWCs is identical with that of OFs. Comparison with a typical OWC-VLFS integration shows that the addition of OFs can significantly reduce the vertical hydroelastic response of the VLFS. Additionally, wave energy extraction from the design of OFs hinged at the back-wall end of OWCs is enhanced due to more wave energy gathered in the chamber. Under the premise of the same space, the array layout demonstrates both higher conervsion efficiency and hydroelastic reduction. The energy conversion is generally enhanced by increasing the gap, the OWC width, the OF height. The maximum efficiency increases firstly with wave height for weak wave nonlinearity and then decreases for strong wave nonliearity.

Boundary control for floating beam system in irregular waves under one end pinned to reduce hydroelastic response

The structural stability is of great importance in research and development of very large floating structure (VLFS). The vibration reduction on VLFS would not only prevent the structural fatigue but also improve the platform serviceability. In order to reduce the hydroelastic response on VLFS in the actual ocean environment for better land use, especially on the area close to boundaries, we investigate a boundary control method for a floating beam system with irregular waves in this paper. Through utilizing Lyapunov direct method, a boundary controller is proposed to reduce the hydroelastic response of the floating beam system restrained at its downstream end. The numerical simulation with experimental calibration is given to verify the effectiveness of the suggested control strategies on the floating beam system. Then, we further investigate the hydroelastic performance of floating beam system under boundary control with various wave amplitudes and bending stiffness. The results demonstrate that the boundary control method could be a potential solution to improve the serviceability of the system in the actual uncertain ocean environments.

A Simplified Method to Analyze Dynamic Response of VLFS Based on the Kane Method

Abstract A two-dimensional (2D) simplified model of the very large floating structure (VLFS) is formulated based on Huston's interpretation of the Kane methodology. In this proposed model, the VLFS is considered as a series of discrete floating bodies connected by elastic hinges. The rotation stiffness of elastic hinges has a great influence on the dynamic responses of VLFS and its value is determined based on the vertical displacements equivalent between the simply supported beam model and the elastically hinged multiple bodies model with the same boundary conditions on a concentrated load. Reduced Kane equations are used in the actual dynamic analysis, once initial conditions and mechanical analysis of the system have been formulated. Validation of the Kane-based method and the reliability of the corresponding program developed are established by several comparative studies on a continuous structure and a hinged structure with three parts. The predictions based on the proposed method are essentially identical to the model test data and calculation results provided by related literature.

A monolithic finite element formulation for the hydroelastic analysis of very large floating structures

AbstractIn this work we present a novel monolithic Finite Element method for the hydroelastic analysis of very large floating structures (VLFS) with arbitrary shapes that is stable, energy conserving, and overcomes the need of an iterative algorithm. The new formulation enables a fully monolithic solution of the linear free‐surface flow, described by linear potential flow, coupled with floating thin structures, described by the Euler–Bernoulli beam or Poisson–Kirchhoff plate equations. The formulation presented in this work is general in the sense that solutions can be found in the frequency and time domains, it overcomes the need of using elements with continuity by employing a continuous/discontinuous Galerkin approach, and it is suitable for finite elements of arbitrary order. We show that the proposed approach can accurately describe the hydroelastic phenomena of VLFS with a variety of tests, including structures with elastic joints, variable bathymetry, and arbitrary structural shapes.

Hydrodynamic response and energy analysis in a very large floating structure supporting a marine airport under typhoon-driven waves

Very large floating structures (VLFSs) can be used to support offshore airports and seaports. Most existing studies on VLFSs have analyzed the dynamic responses of their rigid modules with flexible connections under regular sea conditions. None have examined the hydrodynamic response characteristics of flexible marine airports under typhoon-driven waves. For this, a novel method is proposed to model a flexible module rigid connection (FMRC) VLFS. The measured wave spectrum of Typhoon Megi was finely simulated using Jonswap spectral characteristic parameters. The overall and local hydrodynamic response characteristics of the VLFS under typhoon-driven waves were analyzed, and the mechanisms of energy conversion between the airport and environmental loads were determined. The FMRC model was found to accurately model the hydrodynamic responses of VLFSs. The responses were significantly nonlinear due to structural flexibility and the inhomogeneity of the typhoon loads and wave field. Displacement, rotation angle, and hydroelastic deformation of marine airport occurred mainly along the flow direction, spanwise direction, and vertical direction, respectively, with extreme stress mainly distributed near the VLFS's struts. The environmental load energy and structural gravitational potential energy were mainly converted into potential energy of the moorings in the initial stage of typhoon-driven waves, and structural kinetic energy and elastic potential energy in the stable stage of marine airport.

Hydroelastic responses of a Truss Pontoon Mobile Offshore Base – An experimental investigation

The present study focuses on the details of 1:150 scaled model tests conducted on a Semisubmersible type Very Large Floating Structures (VLFS), namely Truss Pontoon Mobile Offshore Base (TP-MOB) under regular waves. The laboratory tests were performed in a deep water wave basin at the Indian Institute of Technology Madras (IIT Madras). The model, governed by Froude and Cauchy scaling laws, was constructed of thermoplastic polyurethane (TPU) sheet with internal steel stiffener plates. The top deck was constructed using thicker TPU to bring in the elasticity effect. The responses of the model were measured for regular waves with different wave frequencies at a constant wave height of 0.04 m and various heading angles. Three different accelerometers recorded the responses at three different locations for different heading angles. From the recorded time series Response Amplitude Operators (RAOs) of vertical displacements, roll, and pitch are obtained. The characteristics frequency response curve obtained from the experiments are consistent with that reported in the literature by Suzuki et al. (1996). The results indicate the presence of resonant elastic modes in the wave excitation ranges. The uncertainty analysis carried out indicated that the uncertainty in the frequency responses is generally small for kd values greater than 0.2.

A coupled frequency and time domain approach for hydroelastic analysis of very large floating structures under focused wave groups

This paper presents a Python-based scheme for simulating the hydroelastic response of very large floating structures (VLFS) under extreme conditions which are represented by focused wave groups. Applying the modal expansion method, we decouple the fluid-structure problem into a hydrodynamic problem in terms of velocity potentials for the fluid and a mechanical problem for the structure. The hydrodynamic problem is solved by the higher-order boundary element method in the frequency domain, while the structural analysis is performed by a time-domain finite element model. In order to validate the proposed scheme, numerical simulations are carried out on a scaled model of VLFS in regular and random waves. Both analytical and discrete numerical modal shape functions are used for representing the elastic deformations. The Mindlin plate theory is used for the finite element model. Extensive convergence studies of finite element model meshes, hydrodynamic model meshes and elastic mode numbers are carefully carried out. Good agreements of the vertical displacement, bending moment and shear force amplitudes of the floating plate are found between the predictions of the present model, and experimental data and numerical results in the literature for regular and random waves. The hydroelastic responses of the scaled model in focused wave groups are then investigated. A modified NewWave model is proposed to generate focused pressure in linear wave groups, and the results show the effectiveness of the proposed model. Influence of the focal position, water depth, incident wave angle, and peak spectrum frequency and bandwidth of the focused wave groups on the hydroelastic response of the plate is also studied. The proposed approach is shown promising for hydroelastic analysis for more complex VLFS in realistic sea states.

Scattering of Water Waves by Very Large Floating Structure in the Presence of a Porous Box

Abstract We investigate the effectiveness of a porous box in attenuating the structural response of a very large floating structure (VLFS). Assuming the water depth to be finite and small amplitude water wave theory, the physical problem is formulated by employing Darcy’s law for flow past a porous structure. The boundary value problem is reduced to a system of linear algebraic equations with the aid of matched eigenfunction expansion method. Further, these simultaneous equations are solved numerically to compute physical quantities. The mathematical model is validated through a comparison with the theoretical results available in the literature. The reflection, transmission, and dissipation coefficients, elastic plate deflection, forces acting on the box, and free-surface elevation are computed. The reflection, transmission, and dissipation coefficients exhibit an oscillatory pattern for large values of wavenumber irrespective of the structural parameters of the box. It is highlighted that a porous box with moderate values of the porous-effect parameter is effective in reducing strain and shear force on the VLFS. Further, the results on elevation and VLFS deflection depict that the suitable porous-effect parameter values for the box reduce the resultant wave amplitude of the elastic plate deflection as well as the amplitude in the lee side of the VLFS. The study reveals that the width and height of the porous box are critical toward the trapping of incident waves inside the box and dissipating the maximum amount of incident wave energy in reduction of wave transmission in the lee side of the structure and thereby attenuating the structural response of the VLFS.

Full-coupled analysis of offshore floating wind turbine supported by very large floating structure with consideration of hydroelasticity

A multi-bodies hydroelastic model is developed and coupled to the full aero-hydro-servo-elastic analysis of a VLFS (very large floating structure) floating wind turbine. The proposed hydroelastic model discretizes the VLFS into a set of interconnected submodules. The hydrodynamic load acting on the entire VFLS is lumped to the mass center of submodules whilst the structural flexibility is represented by the finite element model used to connect submodules. The multi-bodies character makes the developed hydroelastic model robust to the floating structure and convenient to be embedded into coupled dynamic modelling of offshore floating wind turbines. The established model is validated against model test first and subsequently coupled to the numerical model of a moored VFLS floating wind turbine. The wind turbine power production, VFLS flexible deformation and structural loads are investigated. The analysis results reveal the significance of hydroelasticity on wind turbine power production and structural dynamics.