Motion Of Floating Body Research Articles

Development of 6-DOF Floating Body Motion Reduction System (1st Report)

Development of 6-DOF Floating Body Motion Reduction System (1st Report)

Floating body motion coupling study and economic evaluation of different floating photovoltaic arrangement schemes under hybrid solar-wind power generation

Abstract Reducing the number of anchor points and moorings for floating platforms is one of the ways to reduce the integration and maintenance costs of floating offshore wind and solar farms. This paper focuses on the technical feasibility and economic viability of reducing the number of anchor points and moorings for floating offshore PV platforms by using a monopile of a stationary wind turbine as a mooring anchor point and introducing a shared line system. In this paper, three different floating solar layouts and their corresponding mooring systems are numerically evaluated and optimized under different environmental loading conditions using the commercial software OrcaFlex. The results of the study show that the implementation of a shared mooring solution maximizes the cost-effectiveness economically while technically ensuring stable operation.

Dynamic response strength of integrated foundation for multiple wind turbines

The focus of this paper is on the conceptual design of offshore multi wind turbine integrated floating foundation (MWF) and the strength analysis method of floating structures considering motion response. Firstly, the maximum environmental load and ultimate motion attitude of MWF were used as boundary conditions to solve the problem of imprecise constraint loads in traditional analysis methods. Then, a stress solving equation based on motion state and inertia release was established to solve the equivalent accuracy problem of floating body motion state. Finally, through theoretical analysis, numerical simulation, and experimental testing, it was found that the stress solution method considered the motion state of the floating body in this paper to be more accurate and reliable, with an error of less than 10% compared to the stress results of experimental testing. Improved calculation accuracy by 30%. These studies provide more accurate analytical methods and theoretical references for predicting the strength of floating structures in actual oceans.

A Simplified Method for the Evaluation of Floating-Body Motion Responses over a Sloping Bottom

Recently, many floating renewable energy platforms have been deployed in coastal regions, where sloping bottoms are an important factor when evaluating their safety. In this article, a simplified method coupling an eigenfunction matching method (EMM) and a finite-depth Green’s function (FDGF) is developed to evaluate floating-body motion responses over a sloping bottom for which bathymetry is homogeneous in the longshore direction. We propose an extended EMM to create an incident wave model over the sloping bottom, thereby obtaining the Froude–Krylov (F–K) force and Neumann data on the wet surfaces of the floating body for the diffraction problem. An equivalent depth is introduced to account for the interaction between the sloping bottom and floating bodies when dealing with the diffraction and radiation problems. The accuracy of the present method is validated through a comprehensive comparison with numerical and/or experiment results for a liquefied natural gas (LNG) ship and a floating hemisphere from the literature. Generally, the present, simplified method can give predictions with sufficient accuracy.

Forward prediction of surface wave elevations and motions of offshore floating structures using a data-driven model

Accurate predictions of surface waves and subsequent wave-induced vessel motion have the potential to improve safety and efficiency for a wide range of offshore operations, such as active control of wave energy converters and floating wind turbines. In this paper, an Auto-Regressive model is proposed to predict surface waves and vessel motions based on the measured time sequences at a particular location. Based on numerically synthesized wave data, it is shown that band-pass filtering with a single cut-off frequency can improve the accuracy of predictions. These prediction results suggest that the narrower the spectral bandwidth, the more accurate predictions. However, digital filters cannot be implemented in the context of real-time prediction due to non-causality. For practical forecast, prediction of offshore floating structures could be treated in much the same as a physical filter as it only responds significantly over a limited range of frequencies. Analysis of wave basin test data confirms this hypothesis, making the forecasting of floating body motions promising.

Hydrodynamic performance analysis of a new hybrid wave energy converter system using OpenFOAM

In this research study, a newly proposed hybrid device of Wave Energy Converters (WEC) is investigated by considering computational fluid dynamic (CFD)-based numerical wave tanks (NWTs). The open-source CFD code solver, OpenFOAM (Open Field Operation and Manipulation) is implemented, which is numerically solved the Reynolds-averaged Navier–Stokes (RANS) equations to simulate the two-phase flow. The hybrid system consists of an Oscillating Water Column (OWC) and a point absorber (Wavestar) device installed in a shared platform. The main goal is to recognize how wave diffractions caused by the adjacent floating body could affect the rate of power absorption by the Fixed-OWC. This aim is followed by a 2D numerical analysis of three different installation configurations, variable intervals between the Wavestars' buoy and the Fixed-OWCs' front wall, in four different wavelengths with and without Power Take-Off (PTO). Finally, the efficiency characteristics of the integrated system such as free surface velocity and air pressure within the chamber, besides floating body motions are investigated and compared for the hybrid system. Although the overall assessment for 28 different case studies reveals an efficiency reduction in some cases, the superiority of this hybrid plan is recording several incremental efficiency rates.

Optimization Design Method of the Umbilical Cable Global Configuration Based on Representative Fatigue Conditions

Umbilical cable is one of the key equipment for deep sea oil and gas exploration. To ensure structural safety and normal functioning, the umbilical cable is often presented with a specific global configuration for compensating for the floating body motion due to the harsh environmental load during operation. Considering the reciprocating periodicity of the dynamic response of the umbilical cable under ocean wave loads, the fatigue analysis is required to ensure the safe operation. However, the numerous sea conditions lead to a huge amount of calculation. Aiming at the global configuration design of the lazy-wave umbilical cable, this study carried out a global configuration optimization design method based on a few representative fatigue conditions. The design variables were the control parameters of the global configuration, and the total length and the cumulative fatigue damage were the two objective functions. By resolving the optimization formulation, an optimal global configuration design for a dynamic umbilical cable with antifatigue performance and economic cost was obtained.

Nonlinear harmonics of gap fluid resonance with floating body motions

Resonant response of water waves in a narrow gap, the so-called gap resonance, is a hydrodynamic phenomenon with practical applications. When coupled to body motions, the physics becomes rather complicated, involving body motions, fluid resonance and damping that determines the response amplitude. Gap resonances between two identical elongated bodies, with one held fixed and the other floating, are investigated experimentally for unidirectional waves with broadside incidence. Transient wave groups with different maximum surface elevations are generated, to examine the nonlinear physics. Sub- and super-harmonics, which can be substantially larger than the linear component, are observed in the responses. When going from the diffraction problem (both bodies fixed) to the coupled problem (allowing one body to move), the gap responses are significantly amplified, i.e. by a factor of 2. The first resonant mode that dominates the diffraction problem disappears in the coupled problem. A new resonant mode, which is shown to arise from body motions, is excited inside the gap through nonlinear processes. This mode features a double-humped ‘camel-back’ shape, leading to remarkably large responses. The floating body is observed to oscillate in sway with significant amplitudes at two distinct natural frequencies that are far apart. This is of great interest for the design of mooring lines connecting the two bodies, as it appears to cause resonances at the wave frequencies in addition to the low frequencies. A semi-analytical model is developed to investigate the multiple natural frequencies in sway, yielding further insight into gap resonances.

Numerical coupling strategy using HOS-OpenFOAM-MoorDyn for OC3 Hywind SPAR type platform

The coupled (Potential theory and Navier–Stokes) solver is applied to simulate the interaction of sea waves with substructure of floating offshore wind turbines (FOWT) platform, notably similar to the OC3 Hywind SPAR structure. The intention is to develop a numerical tool that allows the study of the survivability of floating structures in extreme sea states. In this study, the moorings are modeled in two ways. One is by considering the mooring lines as a linear spring with defined spring stiffness, and another is by coupling the solver (foamStar) with a lumped-mass mooring dynamics model (MoorDyn). MoorDyn represents mooring line behavior subject to axial elasticity, hydrodynamic forces, and vertical contact forces with the seabed. The coupled model has been validated against the experiments carried out as part of the SOFTWIND project. The numerical model results of free surface elevation, floating body motions and mooring tensions are compared with the experiments. For wave cases with mild and moderate amplitudes, mooring in the form of a stiffness matrix is sufficient. However, dynamic mooring simulation (MoorDyn) is required for the extreme sea state conditions.

CFD simulation of floating body motion with mooring dynamics: Coupling MoorDyn with OpenFOAM

It is increasingly popular to use Computational Fluid Dynamics (CFD) models to study floating structures subjected to ocean waves, especially when it comes to applications of floating offshore wind turbines and Wave Energy Converters (WECs). Mooring dynamics are currently lacking in most of these applications. This paper presents a coupled simulation study of moored floating body motion by coupling two open-source libraries: a finite volume CFD toolbox, OpenFOAM, and a lumped-mass mooring model, MoorDyn. The instantaneous floating body position and velocity are passed from the body motion solver in the CFD model to the mooring model to calculate the fairlead kinematics. The mooring reaction forces, which are calculated by MoorDyn after updating the mooring system states, are then returned to the body motion solver to update the floating body motion. Both mesh deformation and overset mesh methods are used as the mesh motion solver in the CFD model to account for the floating body motion. The coupled model was validated against experimental measurements for a floating box moored with four catenary lines under the action of regular waves, which came from a preliminary test campaign for WECs. Apart from the lumped-mass mooring model, the present work also coupled a quasi-static mooring model and a finite element model with the floating body motion solver in OpenFOAM. The mooring line tensions predicted by these models were compared. The coupled model equipped with three mooring line codes may be further used to carry out survivability studies of FOWTs and WECs subject to severe sea states.

An implementation of Three-Dimensional Multi-Component Mooring Line Dynamics Model for Multi-Leg mooring line configuration

Multi-component mooring line (MCML) system is widely used and expected to be escalated for deep water mooring operation since it has several advantages comparing with conventional mooring line. The modelling of MCML involving its dynamic effects is strongly required to predict the dynamic behaviours of the mooring line precisely. In this paper, a proposed three-dimensional dynamics model of MCML system is presented and implemented to single point multi-leg mooring line configuration operated in deep water condition. An extending three-dimensional lumped mass method is elaborated to develop the model while the combination of Manoeuvring Modelling Group (MMG) and conventional floating body motion equations is used to perform simultaneous motion between floating structure and mooring lines. The simultaneous motion is performed with respect to wind, wave, and current. The motion of the floating structure and mooring line as well as mooring line tension can be analysed properly by using the presented model. The simulation results of the model show reasonable results for both floating structure and mooring line.

Experimental dataset and numerical simulation of floating bodies transport in open-channel flow

Abstract Numerical models trying to faithfully represent the movement of floating bodies transport in open-channel flow require experimental data for validation. In order to provide an adequate dataset, flume experiments were carried out to analyse the transport of singular and grouped rigid bodies floating on the water surface. Both cylindrical and spherical samples were employed: they were released in a rectangular channel under steady conditions in one-dimensional (plain channel) and two-dimensional (2D) configurations using one rectangular side obstacle, one smooth side obstacle or two rectangular alternate obstacles. The outcomes of the experiments are the planar displacement and the rotation of the samples, which are related to the flow field in the different configurations. The detailed experimental analysis of the floating body motion provides information for the calibration of numerical models simulating floating bodies transport. This dataset is thus employed for the validation of the Eulerian–Lagrangian model ORSA2D_WT, highlighting its strengths and improvable aspects. Similar applications could be carried out with any 2D model which performs the simulation of discrete elements moving on the water surface.

Numerical re-creation of multi-directional waves in a circular basin using a particle based method

Numerical wave basins are a powerful tool for the study of fluid-structure interaction (FSI) problems in coastal and ocean engineering fields. Once well validated, numerical wave basins can offer significant advantages over experimental testing in terms of the number and type of available data-streams, and the associated cost. However, conventional numerical wave tanks tend to be limited to the generation of long-crested conditions, and struggle to properly model large fluid deformations along with wave-induced floating body motions and pressure distribution on structures. Here, for the first time we create, and validate, the short-crested wave fields in a circular wave basin, using a particle-based method to solve the full 3D Navier-Stokes equation. The model is based on the FloWave facility at the University of Edinburgh: the geometry enables the generation of multi-directional waves, and the particle-based-approach enables large fluid deformations to be automatically modelled. Multi-directional waves are generated with differing values of steepness and directional spreading, and simulated surface elevations are compared to experimentally-obtained data.Acceptable agreement is found between measured and modelled surface elevation values. Better performance is confirmed for lower-steepness wave cases; where r2 values of greater than 0.7 are found for all cases, and errors in significant wave height range between −8% and +1%. Greater under-production is found for higher-steepness conditions with a −10% to −16% error in significant wave height. It is concluded that a greater number of particles with smaller radius is required to accurately capture the smaller higher frequency disturbances. It is suggested, however, that as the lower-frequency components are well produced for all cases that the model is already suitable for the majority of FSI problems in both uni- and multi-directional wave fields.

Structural control of the floating breakwaters by implementation of tuned liquid column damper

The growing maritime activities have led offshore engineers to the application of floating breakwaters to protect coastlines from erosion and to provide a layer of support to ports in the case of strong wave actions. In these applications, minimizing the floating part responses to wave-induced excitations as one of the most critical operational characteristics would be a key factor. In this article, the structural responses of the floating breakwaters are studied through modeling and simulation. To mitigate the floating part wave-induced vibrations, an attached tuned liquid column damper is integrated into the simulation. In the first stage, the numerical model of the floating breakwater system containing the floating body and the mooring system is created. Then, by proposing an iterative coupling procedure, the joint tuned liquid column damper-floating breakwater model is developed. Several rigorous study cases are defined to evaluate the effect of tuned liquid column damper on controlling the floating body unwanted vibrations. In this application, the Airy wave theory and the Joint North Sea Wave Observation Project wave spectrum are used for wave excitations. In order to evaluate the effect of tuned liquid column damper on floating breakwaters, some design parameters like floating body motions and mooring system-mobilized tensile stresses are evaluated. The simulations are conducted in both the time domain and the frequency domain for cases with and without tuned liquid column damper. The rain flow counting method is applied to perform the mooring system fatigue analysis. The study results clearly proved that implementation of tuned liquid column damper would mitigate the floating body responses compared to the original floating breakwater system. In addition, it is indicated that the fatigue-induced failure risks in tuned liquid column damper-floating breakwater mooring system is decreased that would reduce the procurement costs and increase the operational safety of the floating breakwater system.

Numerical Study on the Coupled Responses of the Steel Lazy Wave Riser (SLWR) Based on the Basis of Design and Moored FPSO

A coupled analysis was performed between the riser to develop oil and gas in ultra-deepwater and the moored floating body. In general, the safety of the riser is conservatively evaluated by considering the maximum offset excluding the coupled analysis with the floating body. In this study, the safety of the riser was analyzed by considering the coupled motion analysis of the moored floating body. The riser is considered steel lazy wave riser (SLWR) applied in the deep sea, and the floating body is determined to FPSO. The methodology was presented on coupled and uncoupled analysis. The coupled effects were analyzed according to the incident wave headings in intact and damaged conditions of mooring lines. The tension of mooring lines, the motion of the floating body, and riser responses were analyzed according to the loading conditions.

Freely floating body motion responses induced by wave and current in seabed conditions

Two dimensional freely floating body motion responses induced by wave and current are investigated in seabed conditions. This problem is solved by boundary integral method, which is formulated by a combination of steady, diffraction/radiation and reflection potential problems accounting for current, incoming wave and seabed effects respectively. All the three potential problems are solved using a boundary element model involving a continuous Rankine source method subject to appropriate boundary conditions. The presence of incoming wave, current and seabed causes an asymmetric fluid domain. Numerical simulations show that water depth, seabed slope angle, current speed, current direction and cross coupling effects are significantly important for surge, heave and roll motion responses.

Nonlinear Time-Domain Theory for the Simulation of Moored Floating Body Motion

Nonlinear wave loads can induce low-frequency and high-frequency resonance motions of a moored platform in deep water. For the analysis of the nonlinear response of an offshore platform under the action of irregular waves, the most widely used method in practice is the Cummins method, in which the second-order exciting forces in the time domain are computed by a two-term Volterra series model based on incident waves, first-order body motion response, and quadratic transfer functions (QTFs). QTFs are bichromatic waves acting on a body and are computed in the frequency domain in advance. For moving bodies, QTFs are related to the first-order body response, which is to be determined in the simulation process of body motion response but is unknown in the computation procedure of QTFs. In solving this problem, Teng and Cong (2017) proposed a method to divide the QTFs into different components, which are unrelated to the body response. With the application of the new QTF components, a modified Cummins method can be developed for the simulation of the nonlinear response of a moored floating platform. This paper presents a review of the theory.

Nonlinear analysis of sloshing and floating body coupled motion in the time-domain

The aim of this paper is to develop a coupled nonlinear time-domain simulation scheme for nonlinear interactions among sloshing flows and floating body motion for both regular and irregular wave excitation. The contributions of a variety of nonlinear factors, outside waves and inside sloshing induced forces, as well as their influences on body coupled sway and roll motion were investigated. The induced forces are due to the changes in the transient wet surface of the floating body and full nonlinear sloshing. The effects of tank fill ratio and excitation wave height on the nonlinear coupled motion, as well as the relationship between sloshing and floating body nonlinear coupled motion under large wave amplitudes and severe sea conditions were also investigated and the results are presented. Finally, the numerical solutions are compared with existing experimental result. The fully nonlinear sloshing and floating body coupled motion are simulated based on the potential flow theory, with the transient position hydrodynamic assumption. The boundary value problem is solved by the B-spline higher-order panel method. The ISITIMFB (iterative semi-implicit time integration method for floating bodies) is applied to solve for the body's velocity and displacements. The sloshing energy dissipation is modeled by changing the boundary condition on the tank's solid boundaries. An extended principle to determine the energy dissipation coefficient for both regular and irregular cases is extracted. Then, the sloshing and floating body nonlinear coupled motion under large wave amplitudes and severe sea conditions are investigated, and the numerical solutions are compared with existing experimental results. The effects of tank fill ratio and excitation wave height on the nonlinear coupled motion is also investigated.

Motions and safety of a floating liquefied natural gas and shuttle tanker during side-by-side offloading operations

In recent years, the use of natural gas has remarkably increased due to growth in energy demands and concerns about the environment. Generally, pipelines are used for liquefied natural gas transportation; however, there are technical problems and cost increases in constructing liquefied natural gas pipelines in the deep sea. For this reason, floating liquefied natural gas system has attracted the attention of the industries, governments and academia. One of the major issues to be overcome in realisation of floating liquefied natural gas is the motion prediction of close-proximity floating bodies including the gap resonance between the two bodies. In this article, three steps were carried out in order to understand the ship motions and the gap resonance and their effects to the side-by-side offloading operations. First, the two-dimensional gap resonance between the two bodies was investigated using a viscous flow numerical method. Second, the three-dimensional gap resonance and the floating body motions were studied using a potential flow method and validated by the experimental model test. Finally, the limits of environmental conditions for the side-by-side offloading operations were examined by the results of numerical calculation.

RETRACTED ARTICLE: Numerical simulation of twin-barge float-over installation on AQWA software

Float-over installation for the deep water platform is widely used all over the world. As one of float-over installation methods, twin-barge float-over installation has become a hot topic for offshore platform installation for it’s being unlimited by size of platform opening and draught as well as being capable for installation of larger topsides. By using AQWA software according to characteristics of weight loading transfer during installation of topsides by means of twin-barge float-over and taking the model of Cell Spar applicable for reference establishes a hydrodynamic model of objective platform on different stages during the installation process. By two connection methods of rigid and hinge connection by using the AQWA-Drift software simulates the movement response of float-over barge, topsides and platform in an ocean environment. Movement characteristics of upper modules of Spar platform is analyzed in installation of twin-barge float-over by different connection methods on four aspects of contrast of relative motion between barge and platform, contrast of single floating body motion response upon rigid and hinge connection, contrast of stress status of joint touch point and contrast in stress of mooring rope. The conclusion is that when the environmental loading incidence is transverse, the relative motion between barge and platform is most intense during the upper loading of float-over barge’s transferring to main body of the platform by 50%. And it is obvious that two connection methods of rigid and hinge connection have a certain influence on roll motion response of barges. Meanwhile, the support reverses force and cable tension upon rigid connection is significantly greater than the ones upon hinge connection.