Mooring Tension Research Articles

Experimental study on dynamic responses of a spar-type floating offshore wind turbine

In this paper, a scaled model experiment of a spar-type floating offshore wind turbine (FOWT) is carried out. The scaled wind turbine model is designed based on the OC3-Hywind FOWT, and the pitch control mechanism is considered. An optimal arrangement scheme of anemometers deployment on a disc plane is proposed to improve the efficiency of wind field measurement. Three categories of load case tests corresponding to the wind-only, wave-only as well as wind-wave concurrent loads are carried out. The rigid-body motion of the FOWT model and the deformation of the tower as well as the mooring tension are captured. It is revealed that the yaw is greatly affected by the rotor, and the trend and amplitude of the structural rigid-body motion are dominated by aerodynamic loads and hydrodynamic loads, respectively. Moreover, the coupling effects of multi-degree-of-freedom (MDOF) rigid-body motions of the structure are quite obvious. As for the flexible tower, although its deformation is quite small, the resonant response caused by doubling frequency coupling effects is significant. Besides, large amplitude of stress can be induced in the flexible tower under wave-contained conditions. The present experiment provides a benchmark for the validation of numerical simulation tools of FOWTs.

Dynamic Response of SPAR-Type Floating Offshore Wind Turbine under Wave Group Scenarios

Numerical simulations are performed within the time domain to investigate the dynamic behaviors of an SPAR-type FOWT under wave group conditions. Towards this goal, the OC3 Hywind SPAR-type FOWT is adopted, and a JONSWAP (Joint North Sea Wave Project)-based wave group is generated by the envelope amplitude approach. The FOWT motion under wave group conditions, as well as the aerodynamic, hydrodynamic, and mooring performances, is simulated by our established in-house code. The rotating blades are modelled by the blade element momentum theory. The wave-body interaction effect is calculated by the three-dimensional potential theory. The mooring dynamics are also taken into consideration. According to the numerical results, the SPAR buoy motions are slightly increased by the wave group, while the heave motion is significantly amplified. Both the aerodynamic performance and the mooring tension are also influenced by the wave group. Furthermore, the low-frequency resonant response could be more easily excited by the wave group.

Sensitivity analysis of extreme loads acting on a point-absorbing wave energy converter

There are many uncertainties associated with the estimation of extreme loads acting on a wave energy converter (WEC). In this study we perform a sensitivity analysis of extreme loads acting on the Uppsala University (UU) WEC concept. The UU WEC consists of a bottom-mounted linear generator that is connected to a surface buoy with a taut mooring line. The maximum stroke length of the linear generator is enforced by end-stop springs. Initially, a Variation Mode and Effect Analysis (VMEA) was carried out in order to identify the largest input uncertainties. The system was then modelled in the time-domain solver WEC-SIM coupled to the dynamic mooring solver Moody. A sensitivity analysis was made by generating a surrogate model based on polynomial chaos expansions, which rapidly evaluates the maximum loads on the mooring line and the end-stops. The sensitivities are ranked using the Sobol index method. We investigated two sea states using equivalent regular waves (ERW) and irregular wave (IRW) trains. We found that the ERW approach significantly underestimate the maximum loads. Interestingly, the ERW predicted wave height and period as the most important parameters for the maximum mooring tension, whereas the tension in IRW was most sensitive to the drag coefficient of the surface buoy. The end-stop loads were most sensitive to the PTO damping coefficient.

Experimental and Numerical Simulation of a Symmetrical Three-Cylinder Buoy

The wave resistance of a buoy is affected by the mode of anchorage and the buoy structure. Combining the structures and the mode of anchorage of the existing buoys, designing a buoy with significantly improved wave resistance is a major challenge for marine environment monitoring. This work carried out experimental and numerical simulation studies on the hydrodynamic properties of a self-designed symmetrical three-cylinder buoy. The wave resistance of the buoy was analyzed using different wave conditions, and a full-scale simulation of the buoy was performed using the finite element method and lumped mass method. Experimentally, it was found that the symmetrical three-cylinder buoy stability was less affected by the wave height, but mainly by the wave period. Additionally, the effects of wave height and wave period on mooring tension were also studied, and the results showed that mooring tension was mainly affected by wave period, which was explained by the rate of change of the buoy momentum. Finally, a numerical model was proposed for the interpretation of these experiments. Results from numerical simulations for the trajectory of the buoy and the tension of the mooring cable correlated well with the experimental data.

Cost-based mooring designs and a parametric study of bridles for a 15 MW spar-type floating offshore wind turbine

This paper highlights a cost-based design method for feasible and cost-effective mooring configurations. A parametric study of mooring designs considering bridle impact is performed for a 15 MW spar-type floating offshore wind turbine (FOWT). This site-specific FOWT is based on the preliminary model from EU funded H2020 Project COREWIND. Without running numerical simulations, the cost-based design method generates 1443 mooring configurations with normalized costs ranging from 1.00 to 1.667. Six configurations with the lowest normalized costs are selected for OpenFAST simulations. The ultimate test results verify the structural strength of mooring configurations. Bridle designs cause significant impact on the ultimate mooring tensions and floater motions in surge, sway and yaw, which is clearly influenced by the mass ratio of a bridle to a mooring line. When the mass ratio decreases from 13% to 5%, peak mooring tensions increase by up to 17%. The influence of bridle designs on mooring tension fatigue is less prominent and the effect of mass ratio on mooring fatigue varies with wind speed. Near the rated wind speed, deviations in mooring fatigue loads are within 5% for all six configurations. This cost-based method promotes feasible and economical mooring designs. The parametric study of bridle designs provides solid support for optimal mooring designs of spar-type FOWTs.

Nonsingular fast terminal sliding mode-based robust adaptive structural reliable position-mooring control with uncertainty estimation

This paper addresses a novel robust structural reliable position mooring scheme for a turret-moored floating production storage and offloading (FPSO) vessel subject to system uncertainties, environmental disturbances and unmeasurable velocity. A realistic mooring tension model considering buoys and different cable materials that can improve the capacity of the mooring system is derived. A sigmoid tracking differentiator (STD) is employed as a state observer to estimate the unmeasurable velocity. A structural reliability index is introduced to design a position mooring control law so that an FPSO vessel adequately uses a mooring system on the premise of safety to maintain position in an allowable region. It provides strong robustness and a fast transient response and suppresses chattering by merits of a backstepping method and an integral nonsingular fast terminal sliding mode surface (INFTSMS). An adaptive estimator is proposed to address the required unknown uncertainties and disturbances. Theoretical analysis proves the globally asymptotic stability of the whole system. Finally, comparative simulations on an FPSO vessel are conducted to illustrate the effectiveness and superiority of the proposed position mooring control scheme.

Effects of dynamic axial stiffness of elastic moorings for a wave energy converter

This work studies the effects of the dynamic axial stiffness of elastic moorings on the dynamic behaviour of a point absorber wave energy converter. Following two mooring analysis procedures, coupled dynamic analysis of a taut-moored WEC with three legs is performed using the FEM program DeepC in three irregular wave conditions. Two synthetic fibre rope materials are investigated, i.e. a normally stiff polyester and a wire-lay 3-strand nylon rope. The results of WEC motions and mooring tensions obtained from a quasi-static stiffness model and the dynamic stiffness model are compared and discussed. The former analysis applies the non-linear stiffness working curves of the ropes in the simulations, while the latter utilizes the dynamic stiffness expression with an iterative process following a practical mooring analysis procedure. For the nylon rope, the influence of the load amplitude on the dynamic stiffness and the WEC response is presented and analysed. It was found that the quasi-static stiffness model tends to underestimate the maximum mooring tensions, leading to 30%–40% lower results compared to the one accounting for the dynamic stiffness effects. For the studied WEC system, the nylon rope shows advantages over polyester, because of the lower mooring tensions and higher WEC motions.

Effect of Various Mooring Materials on Hydrodynamic Responses of Turret-Moored FPSO with Emphasis on Intact and Damaged Conditions

The behavior of different mooring line materials has a significant influence on the behavior of the mooring system and, consequently, the dynamic responses of the floating platform. Although there have been previous studies on FPSOs and their mooring systems, the influence of mooring line failure scenarios associated with different mooring materials has received less attention, particularly for turret-moored FPSOs with taut moorings. Thus, this paper investigates the behavior of different mooring line materials in intact, single-line, and double-line damaged conditions on the hydrodynamic responses of the FPSO, restoring behavior, mooring, and riser tensions considering wave conditions in the Gulf of Mexico. Mooring lines including Aramid, HMPE, polyester, and steel wire were considered in the middle segment, which was the segment of interest in this study. The restoring forces of the mooring system were found to increase with increasing mooring stiffness, and a higher stiffness resulted in a higher loss of restoring force in the case of single-line failure. In all cases, the submerged weight and material stiffness had a significant influence on dynamic responses, mooring tension, transient responses, riser tension, and especially on the ability of the mooring system to resist the case of single-line failure. Each material was observed to behave differently in each degree of freedom (DOF), showing the necessity to pay close attention to the selection of mooring material for specific objectives.

Influence of the power take-off damping of a dual chamber floating oscillating water column on the mooring fatigue damage

A series of 1:50 model tests are conducted to study the mooring dynamics of a dual-chamber floating oscillating water column device. This device consists of two chambers, namely one traditional oscillating water column chamber and one Backward Bend Duct Buoy chamber. To discuss the power take-off damping on system dynamics, the cap of each chamber is designed with three orifices. During model tests, one orifice is open and the other two are closed so that nine power take-off damping cases are studied for each wave condition. The mooring tension response amplitude operators are measured in regular wave tests. The irregular wave model tests with four different wave peak periods are conducted to measure mooring tensions. Based on the measured mooring tension time series, the rainflow flow counting technique, a spectral method and a spectral method taking the non-Gaussian behaviour into account are applied to study mooring fatigue damage. Furthermore, the spectral method combined with mooring tension response operators for mooring fatigue damage analysis is also studied. It is found that the power take-off damping can affect mooring fatigue damage to some extent. Spectral methods show better performance in predicting chain fatigue damage than in the application for fatigue damage analysis of hawsers.

A frequency domain approach for analyzing motion responses of integrated offshore fish cage and wind turbine under wind and wave actions

This paper presents a frequency domain (FD) approach for analyzing wind and wave coupling effect on motion responses of an integrated offshore fish cage and wind turbine named COSPAR. FD analysis was performed by ANSYS AQWA to obtain response amplitude operators (RAOs) for surge, pitch and heave motions as well as mooring tension RAOs. Wind thrust was estimated by using a linearization method based on the pre-defined thrust coefficients determined from a fixed-bottom wind turbine. In AQWA, wind force coefficients were specified to incorporate the thrust force and moment in the hydrodynamic response analysis of a floating structure. The present FD method was applied to a DeepCwind floating wind turbine problem for its validation. It was found that the AQWA-FD approach can furnish wind and wave induced motion responses that are comparable with published experimental and numerical results. Thereby, this approach can be used to readily obtain a preliminary design for subsequent refinements for an optimal design. Based on the FD analysis results, DeepCwind and COSPAR have a pronounced the coupling effect only in the pitch motion. Moreover, COSPAR shows not only moderate coupling effect, but steadier and less RAOs in surge, heave, pitch and mooring tension than those of DeepCwind. In addition, it is evident that COSPAR can provide a stable working platform for offshore fish farms from wind, wave and current loads, and has acceptable pitch angles for a steady wind turbine operation.

A new approach to predict dynamic mooring tension using LSTM neural network based on responses of floating structure

The prediction of dynamic mooring line tension using the responses of floating structures is of great significance for the safety monitoring of the station-keeping. Due to the mooring line tension usually has the typical low-frequency and wave-frequency characteristics, this paper proposes a method of Low-frequency adds wave-frequency responses (LAWR) to predict the mooring line tension. Firstly, the optimal Long-short term memory (LSTM) neural network structure is determined through conducting the parameter sensitivity analysis on the performance of the LSTM model. Secondly, the low-frequency and wave-frequency tension are predicted using the corresponding low-frequency and wave-frequency responses as the input data, respectively. Then, the total mooring line tension is predicted by superposing the low-frequency tension and wave-frequency tension. Finally, the feasibility of the LAWR method is verified by considering the different data relevance between the training sets and validating sets, including the sea state condition contains different current direction, wave height and spectral peak frequency. The method proposed in this paper could further improve the prediction accuracy of mooring line tension more than 30%, which has great engineering significance.

Characterization of tropical cyclones to identify the response based design metocean conditions for an FPSO mooring system

Design metocean conditions (DMC) for a single-point moored weathervaning vessel exposed to the cyclonic environment are obtained by characterising the tropical cyclones on the basis of extreme responses induced in its mooring system. Determination of design metocean conditions, as combinations of wave, wind and current associated with the extreme response, is a multivariate problem that does not have a unique solution. Its probabilistic nature dictates that the design metocean condition should be a short-term sea state which is most probable to occur while at the same time inducing the extreme N-year response of the system. This study aims at the development of a methodology for obtaining the DMC for a mooring system of a weathervaning vessel by characterizing the tropical cyclonic environment based on the mooring responses. First, a method for identifying the DMCs within the metocean history is described, along with its limitations. Then, the storm characterisation approach is introduced, based upon the maximum responses of the system rather than the extreme sea states in a particular event. The metocean conditions at a specific location during the passage of a cyclone are described by a set of dimensionless shape functions, with their main parameters linked to the most probable mooring tension experienced during each cyclonic event. It is further determined that for a weathervaning floating system, cyclones should be separated into two distinct types according to the relative wave-wind direction highly affecting the mooring response. The cyclone shape functions are generalized to describe the characteristic cyclones at the N-year return period response, which are determined by considering a limited group of storms inducing the mooring responses close to the N-year response. By averaging the dimensionless shape functions for each of the two cyclone types, the characteristic cyclones can be synthesized to describe two types of events, where the N-year mooring response is most likely to occur. The robustness of the method is checked through a case study, in which the characteristic 5-year, 10-year and 100-year DMCs are produced for a mooring system of a floating production, storage and offloading vessel (FPSO) permanently moored offshore Australia. By using a 44-year hindcast time history of cyclonic metocean conditions, the DMCs were developed from the response-based synthesized N-year cyclones and compared with the specific short-term conditions retrieved from the same time history. An alternative averaging method for the cyclone shape functions is also considered and results are discussed.

Analysis of Mooring Line Tension in Floating Collar Net Cage

World fish production is increasing every year. This is mainly because of the trend to use floating net cages in aquaculture. One of the common net cage types is the collar cage. The net cage system must withstand the environmental and accidental loads, particularly on the mooring system as its function to maintain the position. Thus, this study focuses on analysis in mooring tension of the fully scaled net cage using numerical methods. The model scale data obtained from a previous experimental study and then scaled up to obtain the fully scaled net cage. After validation, current and wave data at Pangandaran bay, Indonesia is adapted to the simulation. Morison’s hydrodynamic force formula is used. Configuration of the mooring system is a rectangular array with a variation of spread angles of mooring lines between 90°, 60°, and 30°. The load cases used for simulation considering the operation and extreme conditions also the directions in lines and between lines. The result shows that the smallest mooring tension and offset is the configuration 30° in which the mooring lines spread evenly in each direction.

Vibration control of submerged floating tunnel in waves and earthquakes through tuned mass damper

In this study, a passive tuned mass damper (TMD) is investigated to dampen resonant motions of a submerged floating tunnel (SFT) in waves and earthquakes. TMD is adopted to control resonant lateral motion. A time-domain dynamics simulation model that considers the elasticity of the tunnel and mooring lines is first established, which is based on the lumped mass method and Morison equation, and the SFT-TMD interaction is considered through springs and dampers. Next, by using the harmony search (HS) algorithm, TMD's spring and damping coefficients are optimized; the dynamics simulations under white noise seismic excitations are continuously carried out with updated spring and damping coefficients produced by HS rules until the designed maximum iteration number. The influence of TMD's mass is individually investigated through a parametric study. After the optimization process is completed, the global performances of SFT with and without TMD are systematically evaluated in both waves and earthquakes. It is found that TMD plays a crucial role in controlling SFT vibrations when environmental loads are close to the system's fundamental lateral natural frequency under both wave and seismic excitations. TMD also enhances the comfort of passengers and reduces static and dynamic mooring tensions. • A time-domain dynamics simulation model is built for the interaction of a submerged floating tunnel and a tuned mass damper. • The harmony search algorithm shows good optimization performance for the tuning mass damper parameters. • A tuned mass damper effectively reduces resonant motions and mooring tensions under wave and seismic excitations.

Experimental and Numerical Investigation on the Mooring Tension of a Gravity Net Cage Under Waves

Experimental and Numerical Investigation on the Mooring Tension of a Gravity Net Cage Under Waves

Prediction of dynamic and structural responses of submerged floating tunnel using artificial neural network and minimum sensors

This paper presents a machine learning method to predict the dynamic and structural behaviors of the submerged floating tunnel (SFT) based on an artificial neural network (ANN) and numerical sensors. The training and testing data are generated by the tunnel-mooring coupled time-domain hydro-elastic simulations under various random wave excitations. Then, ANN is constructed with an input layer, hidden layers, and an output layer. The input layer consists of the numerical angle and acceleration signals while the output is the tunnel's estimated displacements, bending moments, and mooring tensions. The number of hidden layers, neurons in each layer, and epochs for stable performance are selected based on the parametric study. For high prediction accuracy, Rectified Linear Unit (ReLU) and Root Mean Square Propagation (RMSProp) are adopted as activation functions and optimizers. In addition, a cost-effective sensor combination at an optimal location is investigated to achieve good performance while using a minimal number of sensors. The optimized sensor combination with one angle sensor and one accelerometer successfully predicts the dynamic and structural responses based on not only R-squared and root-mean-square errors but also representative time histories, spectra, and prediction accuracy plots.

Experimental validation on the dynamic response of a novel floater uniting a vertical-axis wind turbine with a steel fishing cage

A floating vertical-axis wind turbine (VAWT) concept that combines a VAWT with a steel fishing cage has been proposed by lead authors. To probe kinetic characteristics of this floater, this study performs a series of model tests at a 1/40 scale. The comprehensive calibration tests, including wind/wave field quality validation tests, hammer tests, quasi-static mooring tests, and decay tests, are performed for various identification purposes. After that a series of basin tests for the floater under regular waves, irregular waves, and irregular waves with steady/turbulent wind are performed. The experimental data of motions and loads are compared with numerical predictions from an in-house fully coupled simulation tool. Good agreements are observed, affirming the accuracy of the developed simulation tool. Because the main dimension of cage coincides with wavelength and , multi-peaks appear in surge and pitch spectra in the wave frequency band (0.3–1.3 rad/s). The second-order wave effects are clearly observed in tests, but they induce a much smaller low-frequency motion response (<0.3 rad/s) than that by aerodynamic effects. The mooring tension response is influenced by surge motions more than by first-order wave actions. Third-per-revolution (3P) effects are dominant in tower-top shear forces but insignificant to the platform motions.

A Numerical Study of the Hydrodynamics of an Offshore Fish Farm Using REEF3D

Abstract In this article, the hydrodynamics of and nonlinear interaction between the large offshore fish farm “ShenLan 1” and regular waves are investigated using the open-source computational fluid dynamics (CFD) toolbox REEF3D. The framework consists of a rigid body dynamics solver for the frame structure coupled to a fluid solver including the shielding effects of the nets. The solver and grid independence are validated using a 2D numerical wave tank, a free decay test, and a study of the wave loads on a rigid net panel. Then, the effects of regular wave parameters, the thickness of the vertical outer columns of the structure, and the variations of the aspect ratios on the loads, responses, and maximum mooring tension forces are studied. It is concluded that the response motion is sensitive to the wave period rather than the wave height due to the longer duration of unidirectional wave loads acting on the frame. The frequent events of partial submersions and wave overtopping in rather steep waves are confirmed through the capturing of the free surface. The net system accounts for about 30% of the total drag but does not influence the structural response to a larger extend. The effect of the aspect ratio on the hydrodynamics is more distinct than that of the frame thickness. As a result of the study, the first step toward a systemic evaluation of the importance of different structural parts of an offshore fish cage for the expected responses is provided.

Conceptual Design and Hydrodynamic Performance of a Modular Hybrid Floating Foundation

The comprehensive utilization of offshore renewable energies is an effective way to solve the intermittency and variability of power supply. This paper aims to present a hybrid floating system (HFS) based on a modular buoyancy-distributed floating foundation (BDFF) that can be equipped with a horizontal-axis wind turbine, solar panels, and wave energy converters (WEC). A simplified test model with a Froude scale ratio of 1/10 is employed to perform the experiments in a deep-water basin to validate the numerical results computed from the code program ANSYS AQWA based on the potential flow theory. The Response Amplitude Operators (RAOs) under regular waves are compared to evaluate the hydrodynamic performance. There is a good agreement in the surge, pitch, and heave RAOs for experiments and the numerical simulation, with a maximum of 6.45 degrees per meter for the pitch motion. Furthermore, the mooring tensions in the time domain are analyzed under different wave conditions.The tension RAOs from simulations are slightly higher than those from measurements with a maximum value at the period of 3.416 s. The mooring line on the windward side has a more considerable mooring tension that is far less than the allowable tensile strength, especially under the wave height of 2 m and the wave period of 2.873 s. The influence of loaded weight representing solar panels is weak, and the impact of winds is acceptable, as the platform deviates 1.3 degrees from the equilibrium state under the test wind speed. Eventually, the effect of irregular waves on the HFS is presented with the critical parameters of mooring tension and pitch motion. The results show that the HFS has a good motion performance.

Exploring inflow wind condition on floating offshore wind turbine aerodynamic characterisation and platform motion prediction using blade resolved CFD simulation

The present study is aimed at investigating the effect of turbulent wind and shear wind on the floating offshore wind turbine (FOWT) structure by using a high-fidelity computational fluid dynamics (CFD) method. This method is believed to resolve the wind field around the turbine blades, wake and the near air-wave free-surface regime, allowing us to have a more in-depth examination into both aerodynamic and hydrodynamic of the FOWT. In the present study, the modelling of a coupled aero-hydro-mooring FOWT system is focused on a temporal and spatial variable turbulent wind field by using a time-varying spectrum, which has not been examined for a floating wind turbine. The turbulent wind in the study is generated with Mann's wind turbulence model, while the Von Karman wind spectrum is used to represent wind turbulence. In addition, different wind shears were also examined. We can conclude from this study that, when turbulent wind is present, there are fluctuations in both the rotor thrust and power outputs associated with the non-uniform wake region although the time-mean magnitude is almost the same. In addition, turbulence wind lead to a quicker wake diffusion than time-independent inflow wind. Furthermore, the existence of wind shear results in an even larger decrease in the local minimum thrust/power about 2–6% when the turbine blade is passing in front of the tower. Despite this, under the present wind inflow conditions, the inline surge force, dynamic motion, and the mooring tension of the floater are not significantly affected by either the turbulence wind or the wind shear.