Mooring Line Tension Research Articles

Analysis of Dynamic Characteristics of an Ultra-Large Semi-Submersible Floating Wind Turbine

An initial design of the platform for the moderate water depth (100 m) is performed by upscaling of an existing 5 MW braceless semi-submersible platform design to support the DTU (Danish University of Science and Technology) 10 MW wind turbine. To investigate the dynamic characteristics of the ultra-large semi-submersible floating offshore wind turbine (FOWT), an aero-hydro-servo-elastic numerical modeling is applied to carry out the fully coupled time-domain simulation analysis. The motion responses of the ultra-large semi-submersible FOWT are presented and discussed for selected environmental conditions. Based on the quasi-static and dynamic analysis methods, the influence of the dynamic effects of the mooring lines on the platform motion responses and mooring line tension responses are discussed. Subsequently, the difference in the motion responses and structural dynamics of the DTU 10 MW and NREL (National Renewable Energy Laboratory) 5 MW FOWT is studied due to the difference in turbine properties. The simulation results reveal that the excitation of the low-frequency wind loads on the surge and pitch motions, the tower-base fore-aft bending moments and the mooring line tension response becomes more prominent when the size of the wind turbine increases, but the excitation action of the 3P effect on the structural dynamics of the 5 MW FOWT is more obvious than those of the 10 MW FOWT.

A study on fully nonlinear wave load effects on floating wind turbine

The influence of fully nonlinear wave effects on floating wind turbine has been studied in this paper by comparing both floater motions and structural responses of tower base and mooring lines exposed to linear and fully nonlinear long-crested irregular waves. Wave kinematics of the linear and nonlinear wave are calculated in a 2D Harmonic Polynomial Cell wave tank. The wave kinematics are further processed by a polynomial fitting method to scale down the data size so that it fulfills the memory requirement of HAWC2 where coupled dynamic analysis is carried out. The external DLL used to provide wave kinematics to HAWC2 is extended from one dimensional (Wkin.dll 2.4) to two dimensional (Wkin.dll 2D) wave field so that fully nonlinear wave can be implemented on floating wind turbine through reading pre-generated wave kinematics manner. The whole work procedure including wave generation, polynomial fitting and implementation in HAWC2 has been verified with a linear regular wave case. Two extreme irregular wave conditions are focused to study the nonlinear wave effects regarding critical responses, such as wave elevation, floater motions and mooring line tension. The results have not only proved the accuracy and applicability of the polynomial fitting method and the extended Wkin.dll 2D for HAWC2 but also revealed the importance to consider fully nonlinear wave model in hydrodynamic analysis compared with linear wave theory especially for high sea states in shallow and intermediate water. As a result of development of computer capacity and numerical wave tank, this paper has demonstrated that fully nonlinear wave effect can be considered in an engineering manner with acceptable efficiency.

Hydrodynamic responses of longline aquaculture facility with lantern nets in waves

To prevent occurrences of structure failure, it is essential to study the mooring line tension and motions of the longline aquaculture facilities with lantern nets. This study describes a physical model experiment that attempts to determine an optimal structure design. The experiment investigated effects of three facility design factors on the mooring line tension and movement of the lantern net and mainline: incident wave angles, lantern net layout depths, and lantern net layout forms (horizontal arrangement/staggered arrangement). The experimental results indicate that the mooring line tension decreases with increasing wave periods. The amplitude and variation of windward mooring line tension are much larger and more complex than that of leeward. The vertical movement amplitude and variation range of the lantern net and mainline are much larger than that of horizontal movement amplitude. Decreasing the incident wave angle, deepening the arrangement depth of lantern nets or adopting staggered arrangement can reduce the mooring line tension in the case of longer wave period. The analysis indicates that increasing the layout depths of the lantern nets and applying the staggered layout form are beneficial in reducing the vertical motion amplitude and elliptical excursion of the mainline. The incident wave angle has a relatively weak effect on the motion of the main and lantern net. Increasing the layout depths of lantern nets will reduce the horizontal and vertical movement amplitudes and elliptical excursion of the lantern nets. Changing the layout forms of the lantern nets from horizontal to staggered will increase the horizontal motion amplitude and decrease the vertical motion amplitude and elliptical excursion of the lantern nets.

Experimental Study of a Moored Floating Oscillating Water Column Wave-Energy Converter and of a Moored Cubic Box

This paper describes experimental research on a floating moored Oscillating Water Column (OWC)-type Wave-Energy Converter (WEC) carried out in the wave flume of the Coastal Engineering Research Group of Ghent University. This research has been introduced to cover the existing data scarcity and knowledge gaps regarding response of moored floating OWC WECs. The obtained data will be available in the future for the validation of nonlinear numerical models. The experiment focuses on the assessment of the nonlinear motion and mooring-line response of a 1:25 floating moored OWC WEC model to regular waves. The OWC WEC model motion has 6 degrees of freedom and is limited by a symmetrical 4-point mooring system. The model is composed of a chamber with an orifice on top of it to simulate the power-take-off (PTO) system and the associated damping of the motion of the OWC WEC model. In the first place, the motion response in waves of the moored floating OWC WEC model is investigated and the water surface elevation in the OWC WEC chamber is measured. Secondly, two different mooring-line materials (iron chains and nylon ropes) are tested and the corresponding OWC WEC model motions and mooring-line tensions are measured. The performance of these two materials is similar in small-amplitude waves but different in large wave-amplitude conditions. Thirdly, the influence of different PTO conditions is investigated by varying the diameter of the top orifice of the OWC WEC model. The results show that the PTO damping does not affect the OWC WEC motion but has an impact on the water surface elevation inside the OWC chamber. In addition, an unbalanced mooring configuration is discussed. Finally, the obtained data for a moored cubic model in waves are presented, which is a benchmarking case for future validation purposes.

Experimental Study on the Hydrodynamic Characteristics of a Submersible Fish Cage at Various Depths in Waves

Submersible fish cages can be submerged under the water to mitigate the negative effects that arise from severe sea conditions and improve the growing environment for the farmed fish. Thus they are increasingly applied in offshore aquaculture. To ensure both safety and economic efficiency of submersible fish cages, it is important to determine the optimum submergence depth. In this study, a series of physical model experiments were conducted to investigate the hydrodynamic performance of a submersible fish cage at various submergence depths (1/6, 1/4, 1/3, and 1/2 of the water depth as well as the floating condition for reference) with a model scale of 1:20. The results of the physical model experiment for the different depths were compared to analyze the effects of submergence depths on the mooring line tension and the movement of the floating collar. The results showed that the mooring line tension and the floating collar movement significantly attenuated with increasing submergence depth. However, the attenuation tendency became stable when the fish cage reached a certain depth. According to the results, 1/3 of water depth was determined as the optimal submergence depth of the fish cages. Deeper submergence depths showed no significant advantage from a perspective of the hydrodynamic characteristics of the fish cage. The determination of the optimum submergence depth is beneficial for the structural design and operation safety of submersible net cages.

Dynamic Response for a Submerged Floating Offshore Wind Turbine with Different Mooring Configurations

The paper discusses the effects of mooring configurations on the dynamic response of a submerged floating offshore wind turbine (SFOWT) for intermediate water depths. A coupled dynamic model of a wind turbine-tower-floating platform-mooring system is established, and the dynamic response of the platform, tensions in mooring lines, and bending moment at the tower base and blade root under four different mooring configurations are checked. A well-stabilized configuration (i.e., four vertical lines and 12 diagonal lines with an inclination angle of 30°) is selected to study the coupled dynamic responses of SFOWT with broken mooring lines, and in order to keep the safety of SFOWT under extreme sea-states, the pretension of the vertical mooring line has to increase from 1800–2780 kN. Results show that the optimized mooring system can provide larger restoring force, and the SFOWT has a smaller movement response under extreme sea-states; when the mooring lines in the upwind wave direction are broken, an increased motion response of the platform will be caused. However, there is no slack in the remaining mooring lines, and the SFOWT still has enough stability.

Resource assessment parameterization impact on wave energy converter power production and mooring loads

Assessing and quantifying a wave energy resource is difficult and complex. Parameterizing the spectral wave information, the distribution of winds and current directions and magnitudes, and the flow of tides, inherently discards valuable information and introduces uncertainty. The impact of this uncertainty on wave energy converter power production, mooring loads, and survivability metrics is still vague and unresolved. This novel work quantifies the uncertainty, and ranks the relative impact, of differing parameterizations of the wave and environmental conditions on wave energy converter power and mooring line tension predictions. The results shows a higher fidelity representation, when compared to the International Electrotechnical Commission Technical Specification requirements, reduces mean power predictions by 11.6%, but increases maximum predictions by 328%. For mooring tension (one metric for survivability), a higher fidelity representation indicates the frequently applied specification methods overpredicts the mean and maximum tensions by 182% and 729%, respectively. This research identifies wave groupiness and wind speed as dominant additional parameters to include for improved power estimation; and the difference between wave and current direction, and current speed for improve mooring line tension predictions. By quantifying the relative impact and ranking of environmental conditions on both power and mooring tension predictions, this novel work highlights the necessary and sufficient fidelity within the parameterization of environmental conditions for minimizing uncertainty in assessments of wave energy technologies.

Analysis of the Motion Response on the Cylindrical Platform of the Deep Sea

The strategic point of potential tidal energy in Indonesia can be a solution to the energy crisis, and as the development of renewable energy. Several concepts of the floating marine current turbine (MCT) have been offered, one of them is the cylindrical platform. The object of the conducted study is 33 m cylindrical platform with twin horizontal axis MCT installed underneath the platform. The analysis begins with the platform properties which include Centre of gravity, gyration radius and inertial. The environmental loads are 1 m/s and 2 m/s current speed, 2.4 m wave height, and 22 knots wind. The properties will be used as input for motion analysis and mooring line tension. The results of the analysis with turret configuration of three lines and four lines show that the response amplitude operator (RAO) on the largest translational motion on surge motion as far as 4 m and rotational motion of 4.5 degrees. For the analysis of the mooring system the average maximum tension of the mooring chain reaches 2000 kN, with the maximum proof load at the value of 2245.82 kN which resulted to a safety factor of 1,73 and it still complies to API RP 2SK Standard.

Hydrodynamic Analysis of a Semi-submersible Wind-Tidal Combined Power Generation Device

Energy shortages and environmental pollution are becoming increasingly severe globally. The exploitation and utilization of renewable energy have become an effective way to alleviate these problems. To improve power production capacity, power output quality, and cost effectiveness, comprehensive marine energy utilization has become an inevitable trend in marine energy development. Based on a semi-submersible wind–tidal combined power generation device, a three-dimensional frequency domain potential flow theory is used to study the hydrodynamic performance of such a device. For this study, the RAOs and hydrodynamic coefficients of the floating carrier platform to the regular wave were obtained. The influence of the tidal turbine on the platform in terms of frequency domain was considered as added mass and damping. The direct load of the tidal turbine was obtained by CFX. FORTRAN software was used for the second development of adaptive query workload aware software, which can include the external force. The motion response of the platform to the irregular wave and the tension of the mooring line were calculated under the limiting condition (one mooring line breakage). The results showed that the motion response of the carrier to the surge and sway direction is more intense, but the swing amplitude is within the acceptable range. Even in the worst case scenario, the balance position of the platform was still in the positioning range, which met the requirements of the working sea area. The safety factor of the mooring line tension also complied with the requirements of the design specification. Therefore, it was found that the hydrodynamic performance and motion responses of a semi-submersible wind–tidal combined power generation device can meet the power generation requirements under all design conditions, and the device presents a reliable power generation system.

A prediction on structural stress and deformation of fish cage in waves using machine-learning method

Generally, the hydrodynamics of a fish cage are investigated using numerical simulation, physical model experiments, and field measurements. However, these traditional research methods are time consuming and low in efficiency. In this study, an artificial neural network (ANN) model is built such that the hydrodynamic characteristics of a fish cage in waves can be predicted rapidly. The training data of the ANN model are generated by a well-developed numerical model from our previous studies. The parameters in the hidden layer of the ANN model are determined considering the prediction accuracy of the hydrodynamic results of the fish cage. The ANN model is validated against a well-developed numerical model with satisfactory accuracy. Using the proposed ANN model, the hydrodynamic results of the fish cage including the maximum tension in mooring lines, minimum effective-volume ratio, and maximum stress of the floating collar are predicted for various waves. Overall, the predicted results indicate a trend consistent with that of the previous studies. The present model can potentially forecast disasters for an oncoming typhoon, which is important for the hazard prevention of a fish farm.

Enhancement the Fatigue Life of Floating Breakwater Mooring System Using Tuned Liquid Column Damper

Abstract Safety is a key design criterion for floating structures. A high rate of mooring accidents has been reported over the past decades. Preventing mooring line failure is a key design objective for floating breakwater systems. The mooring system comprises sets of mooring lines anchored to the seabed. These components are exposed to highly cyclic nonlinear load fluctuations induced by an irregular wave climate during their service life. DNVGL-OS-E301 classifies the mooring lines for floating breakwaters as long-term elements that should be evaluated according to the fatigue limit state. Fatigue of mooring lines needs to be monitored and evaluated to warrant the station keeping and integrity of overall system. Applying an additional control device to a floating breakwater mitigates the structural response and hence mobilized tension in mooring system. The focus of present study was to examine the effect of an additional control device on fatigue life of mooring lines for floating breakwaters. To evaluate the effect of a control device on the fatigue behavior of mooring lines, a floating breakwater was simulated with a tuned liquid column damper (TLCD) attached. A time-dependent approach based on S-N curves in conjunction with the Palmgren-Miner rule was employed to evaluate the mooring line fatigue. This paper presents a further parametric study focused on the effect of TLCD on fairlead point displacement, mobilized tension, damage rate, and fatigue life of mooring lines. The results showed that TLCD increased the fatigue life of mooring line and thus dramatically decreased the likelihood of the mooring system being damaged by fatigue. This would reduce the maintenance costs and increase the lifetime and operational safety of floating breakwater. In addition, the presented case study showed that failure probability of mooring lines against fatigue damage was also reduced and was acceptable for the safety factor defined in DNVGL-OS-E301. This proposed approach of applying a TLCD is a practical tool for designing the components of a floating breakwater more efficiently.

Coupled dynamic responses of a semisubmersible under the irregular wave and turbulent wind

A coupled dynamic analysis of a semisubmersible-type FOWT has been carried out in time domain under the combined action of irregular wave and turbulent wind represented respectively by JONSWAP spectrum and Kaimal spectrum. To account for the turbine-floater motion coupling in a more realistic way, the wind turbulence has been incorporated into the calculation of aerodynamic loads. The platform model was referred from the DeepCwind project and the turbine considered here was the NREL 5MW Baseline. To account for the operationality of the turbine, two different environmental conditions (operational and survival) have been considered and the aerodynamic effect of turbine-rotation on actual responses of the FOWT has been studied. Higher mean offsets in surge and pitch responses were obtained under the operational condition as compared to the survival condition. The mooring line tensions were also observed to be sensitive to the rotation of turbine due to the turbulence of wind and overestimated responses were found when the constant wind was considered in the analysis. Additionally, a special analysis case of sudden shutdown of the turbine has also been considered to study the swift modification of responses and tension in the mooring cables.

Wind spectral characteristics on strength design of floating offshore wind turbines

Characteristics of a turbulence wind model control the magnitude and frequency distribution of wind loading on floating offshore wind turbines (FOWTs), and an in-depth understanding of how wind spectral characteristics affect the responses, and ultimately the design cost of system components, is in shortage in the offshore wind industry. Wind spectrum models as well as turbulence intensity curves recommended by the International Electrotechnical Commission (IEC) have characteristics derived from land-based sites, and have been widely adopted in offshore wind projects (in the absence of site-specific offshore data) without sufficient assessment of design implications. In this paper, effects of wind spectra and turbulence intensities on the strength or extreme responses of a 5 MW floating offshore wind turbine are investigated. The impact of different wind spectral parameters on the extreme blade loads, nacelle accelerations, towertop motions, towerbase loads, platform motions and accelerations, and mooring line tensions are presented and discussed. Results highlight the need to consider the appropriateness of a wind spectral model implemented in the strength design of FOWT structures.

Dynamic deformation of the floating collar of a net cage under the combined effect of waves and current

In this study, a numerical model was established to simulate the dynamic behavior of a floating net cage in waves and current, based on the finite element method. To validate the numerical model, we considered a case based on the motions of the net cage under regular waves and current, where we calculated and compared the simulated results. The comparison of the results indicated good agreement. We calculated the dynamic deformation of the floating collar of the net cage under regular/irregular waves and current, and we analyzed the effects of the net, strong waves and current, and the length of the mooring line on the cage collar deformation. The results indicated that the deformation of the floating collar with a net was much greater than that without a net. The collar deformation became larger as the wave height or current velocity increased. Therefore, in order to reduce the risk of local collar failure due to plastic deformation, adding a casing pipe in the position of the mooring line connection point on the floating collar could greatly enhance the bearing capacity of the cage collar under severe sea conditions. In addition, increasing the length of the mooring line would help to decrease the mooring line tension and further reduce deformation of the cage collar.

Research on Dynamic Response Characteristics of 6MW Spar-Type Floating Offshore Wind Turbine

A 6MW spar-type floating offshore wind turbine (FOWT) model is put forward and a fully coupled aero-hydro-servo-elastic time domain model is established in the fatigue, aerodynamics, structures and turbulence (FAST) code. Influence rules of wind load and wave load on the characteristics of 6MW spar-type FOWT are investigated. Firstly, validation of the model is carried out and a satisfactory result is obtained. The maximal deviations of rotor thrust and power between simulation results and reference values are 4.54% and −2.74%, respectively. Then the characteristics, including rotor thrust, rotor power, out-of-plane blade deflection, tower base fore-aft bending moment, and mooring line tension, are researched. The results illustrate that the mean value of dynamic response characteristics is mainly controlled by the wind-induced action. For characteristics of tower base fore-aft bending moment and platform pitch motion, the oscillation is dominated by the wave-induced action during all conditions considered. For characteristics of out-of-plane blade tip deflection and mooring line tension, the oscillation is commanded by combination effect of wave and wind loads when the wind speed is lower than the rated wind speed (hereinafter referred to as below rated wind speed) and is controlled by the wave-induced action when the wind speed is higher than the rated wind speed (hereinafter referred to as above rated wind speed). As to the rotor thrust and power, the oscillation is dominated by the wind induced action at below rated wind speed and by the combination action of wind and wave loads at above rated wind speed. The results should be useful to the detailed design and model basin test of the 6MW spar-type FOWT.

The Hull Strength of Marine Current Turbine Platform

Strength analysis of hull structure of marine renewable energy is crucial. The hull structure must be able to whitstand the harsh environmental load such as wave, wind and the weight of the payload and the structure itself. This paper analyzies the structural hull strength of marine current turbine platform. The platform is held by 4 mooring lines in 200 m water depth. The hull material is AISI1050 with yield stress 530MPa, the analysis consist of stress analysis which consider the dynamic motion of the platform due to wave, current, win and mooring line tension. The dynamic motion shows the maximum gravity acceleration is 1.3496G. The gravity acceleration creates additional loads due to weight multiplication of payload and structure. From the dynamic analysis using numerical software, the maximum mooring line tension due to dynamic analysis is 300.74 kN and this tension will creates stress in the moring line connection structure in the platform hull. The maximum stress of the is 126.045Pa, The mooring line tension is the highest loads compared to other factors such as weight. The safety factor of the structure based on the analysis is 4.2 which can be calculated by comparing the yield stress of the hull material and the maximum stress occurred due to dynamic load

Assessment of performance enhancement of a semi-submersible vertical axis wind turbine with an optimized Darrieus rotor

Increasing efforts are currently being devoted to develop floating vertical axis wind turbines (VAWTs), due to their potential to reduce the cost-of-energy. Optimization of rotors is favorable to increase the power capture and mitigate structural loads. In the DeepWind project, a baseline Darrieus type vertical axis wind turbine (VAWT) was first developed, followed by an optimized rotor. In this study, these two different rotors are adapted to a semi-submersible platform to achieve two floating VAWTs. The performance enhancement of the floating VAWT with the optimized rotor is evaluated by performance comparison with the floating VAWT with the baseline rotor. Numerical simulations are conducted using a fully coupled aero-hydro-servo-elastic simulation tool in time domain, under turbulent wind and irregular waves. Numerical analyses indicate that the floating VAWT with the optimized rotor greatly improves the performances with respect to power production, global platform motions, mooring line tensions, and especially tower base bending moments. The annual power production of the floating VAWT with the optimized rotor is improved by 11.3% as compared to that of the floating VAWT with the baseline rotor. However, the variations of flapwise and edgewise bending moments along the blades are not improved for the optimized rotor, which might cause severe fatigue damage, and should be considered during the optimization of blades for VAWTs.

Numerical and Experimental Study on Hydrodynamic Performance of A Novel Semi-Submersible Concept

Multiple Column Platform (MCP) semi-submersible is a newly proposed concept, which differs from the conventional semi-submersibles, featuring centre column and middle pontoon. It is paramount to ensure its structural reliability and safe operation at sea, and a rigorous investigation is conducted to examine the hydrodynamic and structural performance for the novel structure concept. In this paper, the numerical and experimental studies on the hydrodynamic performance of MCP are performed. Numerical simulations are conducted in both the frequency and time domains based on 3D potential theory. The numerical models are validated by experimental measurements obtained from extensive sets of model tests under both regular wave and irregular wave conditions. Moreover, a comparative study on MCP and two conventional semi-submersibles are carried out using numerical simulation. Specifically, the hydrodynamic characteristics, including hydrodynamic coefficients, natural periods and motion response amplitude operators (RAOs), mooring line tension are fully examined. The present study proves the feasibility of the novel MCP and demonstrates the potential possibility of optimization in the future study.

On mooring line tension and fatigue prediction for offshore vertical axis wind turbines: A comparison of lumped mass and quasi-static approaches

Despite several potential advantages, relatively few studies and design support tools have been developed for floating vertical axis wind turbines. Due to the substantial aerodynamics differences, the analyses of vertical axis wind turbine on floating structures cannot be easily extended from what have been already done for horizontal axis wind turbines. Therefore, the main aim of the present work is to compare the dynamic response of the floating offshore wind turbine system adopting two different mooring dynamics approaches. Two versions of the in-house aero-hydro-mooring coupled model of dynamics for floating vertical axis wind turbine (FloVAWT) have been used, employing a mooring quasi-static model, which solves the equations using an energetic approach, and a modified version of floating vertical axis wind turbine, which instead couples with the lumped mass mooring line model MoorDyn. The results, in terms of mooring line tension, fatigue and response in frequency have been obtained and analysed, based on a 5 MW Darrieus type rotor supported by the OC4-DeepCwind semisubmersible.

Spar-Type Vertical-Axis Wind Turbines in Moderate Water Depth: A Feasibility Study

The applications of floating vertical-axis wind turbines (VAWTs) in deep water have been proposed and studied by several researchers recently. However, the feasibility of deploying a floating VAWT at a moderate water depth has not yet been studied. In this paper, this feasibility is thoroughly addressed by comparing the dynamic responses of spar-type VAWTs in deep water and moderate water depth. A short spar VAWT supporting a 5 MW Darrieus rotor at moderate water depth is proposed by following the deep spar concept in deep water. A fully coupled simulation tool, SIMO-RIFLEX-DMS code, is utilized to carry out time domain simulations under turbulent wind and irregular waves. Dynamic responses of the short spar and deep spar VAWTs are analyzed and compared, including the natural periods, wind turbine performance, platform motions, tower base bending moments, and tension of mooring lines. The statistical characteristics of the thrust and power production for both spars are similar. The comparison of platform motions and tower base bending moments demonstrate a good agreement for both spars, but the short spar has better performance in surge/sway motions and side–side bending moments. The 2P response dominates the bending moment spectra for both spars. A significant variation in tension of Mooring Line 1 and a larger corresponding spectrum value are found in the short spar concept. The results indicate that the application of short spar VAWTs is feasible and could become an alternative concept at moderate water depth.