Length Of Mooring Line Research Articles

Predictive Modeling of Semi-Submersible Floater Motion Using Bi-LSTM Model

Abstract Floating offshore wind turbines (FOWTs) are emerging as a promising renewable energy solution, tapping into the vast wind resources in deep-sea locations. Accurate predictions of heave, pitch, sway, roll, yaw, and surge are essential to ensuring the structural integrity and power generation efficiency of FOWTs. Traditional methods relying on potential flow theory to predict FOWTs’ hydrodynamic responses involve simplifications that may not capture real-world complexities. Although computational fluid dynamics (CFD) simulations offer accuracy, they are computationally expensive. This study explores the use of Bi-directional long-short-term memory (Bi-LSTM) neural networks as an alternative to address computational challenges in predicting FOWTs’ hydrodynamic responses. These networks excel at handling time series data, capturing temporal correlations crucial for understanding FOWT floater motion dynamics. They also effectively capture bidirectional dependencies, enhancing their suitability for this application. The model considers various input parameters, including wave amplitudes, periods, steepness, directions, and floater design characteristics such as stifness, pre-tension, and the free length of mooring lines. The Bi-LSTM model is trained using a subset of the CFD dataset generated for the scaled OC5 semi-submersible platform with mooring, reserving the remainder for validation and testing. The comparison between simulated and predicted motion demonstrated a strong correlation in surge but noticeable discrepancies in heave and pitch. FFT analysis highlighted consistent frequency components but variations in magnitude, suggesting areas for model sensitivity improvement and further investigation into wave interaction dynamics. The correlation coefficients revealed a strong positive correlation for the surge motion, while the heave and pitch motions showed slightly lower correlation coefficients, suggesting better performance in predicting the surge.

Hydrodynamic Characteristics Analysis and Mooring System Optimization of an Innovative Deep-Sea Aquaculture Platform

As nearshore aquaculture spaces become saturated, the development of fisheries aquaculture for deep sea has become an inevitable trend. This paper proposes an innovative deep-sea aquaculture platform that incorporates a vessel-shaped main structure and a single-point mooring system. The potential flow theory and the Morison equation are utilized to calculate the hydrodynamic loads on the main structure and the netting and mooring systems, respectively. The deformation and force of the netting in current are simulated, and the accuracy of the analytical methods used is validated based on experimental results. The influences of the netting system on the hydrodynamic characteristics of the platform are analyzed. Optimization on the single-point mooring system is conducted under static and dynamic conditions, considering the influences of various mooring parameters, including mooring line length, buoyancy of buoys, and mass of sinkers. The patterns of changes in motion response, mooring line tension, and minimum touchdown length under different mooring parameters are calculated and analyzed. The results indicate that changes in mooring line length have minimal impact on the dynamic response of the platform and mooring system. The addition of appropriate buoys or sinkers can reduce the motion response of the platform and the tension in the mooring lines. Moreover, compared to adding buoys, incorporating sinkers more effectively enhances the overall safety and stability of the platform system.

Numerical Calibration of the Mooring System for a Semi-Submersible Floating Wind Turbine Model

Abstract Numerical modeling of the floating offshore wind turbine (FOWT) dynamics plays a critical role at the design stage of a floating wind project. Still, there exist challenges for verification of efficient engineering models against experimental results. Recently, an experimental campaign was carried out for a 1:96 downscaled model of the OC4-DeepCWind semi-submersible platform with mooring lines made of fiber ropes and chains. Leveraging the results of this campaign, this paper focuses on the development and calibration of a numerical model for the semi-submersible platform with a focus on the dynamic responses under bichromatic waves. In the numerical model, the hydrodynamic loads are modeled based on the potential flow theory with Morison drag. The lumped mass method is applied to model the mooring system. Both free decay tests and bichromatic wave conditions are considered in the model calibration process, and key uncertain parameters (e.g., mooring line length) that affect the response have been identified and discussed. Using the proposed calibration procedure, we establish a reasonably good numerical model for prediction of the platform motion and mooring dynamics. The low-frequency responses of the platform under bichromatic waves are well-captured. These outcomes contribute to the development of efficient numerical FOWT models under experimental uncertainty.

Catenary Mooring Length Control for Motion Mitigation of Semi-Submersible Floating Wind Turbines

Besides improving the generator torque and blade pitch controller, incorporating additional control actuations, such as a vibration absorber and active ballast, into the floating offshore wind turbine (FOWT) system is also promising for the motion mitigation of FOWTs. This work aims to study the catenary mooring length re-configuration effect on the dynamic behaviours of semi-submersible FOWTs. The mooring length re-configuration mentioned here is achieved by altering the mooring length with winches mounted on the floating platform, which is in a period of minutes to hours, so that the mooring tensions could be adjusted to reduce the aerodynamic load induced platform mean pitch. Control designs for both single mooring line and multiple mooring lines have been described and studied comparatively. In order to assess the motion mitigation performance of the proposed mooring line length re-configuration methods, fully coupled numerical simulations under different environmental cases have been conducted. Results indicate that the catenary mooring length re-configuration is able to reduce the platform pitch motion by up to 15.8% under rated condition, while careful attention must be paid to the scenarios where the catenary moorings become taut, which may lead to large load variations.

Study on Mooring Design of 15 MW Floating Wind Turbines in South China Sea

Wind turbines and floating platform upsizing are major trends in the current offshore wind development. However, harsh environmental conditions increase the risk of anchor dragging and mooring failure when deploying large offshore floating wind turbines. Therefore, it is necessary to design a mooring system for the specific deployment site. This study aims to perform the mooring system design of a floating offshore wind turbine (FOWT) operated in the South China Sea, which is a combination of the IEA 15 MW wind turbine and UMaine VolturnUS-S floating platform. Hydrodynamic coefficients were calculated based on the potential flow theory, considering the environmental loads in the South China Sea. Additionally, the hydrodynamic coefficients were imported into AQWA to calculate the time-domain mooring tension. The mooring design parameters, such as mooring line length, nominal sizes, and anchor point, were determined using the criterion of anchor uplift, maximum breaking strength, and fatigue life, respectively. The design criterion required that the anchor uplift is not more than the allowable value, the long-term breaking limit of mooring with a 100-year return period should be less than the maximum breaking limit, and the fatigue damage accumulation in 50 years should be safe. The mooring design procedure provides a reference for mooring system design and safe operation of large floating wind turbines in the South China Sea.

Numerical modeling of a mooring line system for an offshore floating wind turbine in Vietnamese sea conditions using nonlinear materials

The offshore renewable energy industry has been developing farms of floating offshore wind turbines in water depths up to 100 m. In Vietnam, floating offshore wind turbines have been developed to increase the production of clean and sustainable energy. The mooring system, which is used to keep the turbine stable and ensure the safety and economic efficiency of wind power production, is an important part of a floating offshore wind turbine. Appropriate selection of the mooring type and mooring line material can reduce the risks arising from the motion of wind turbines. Different types of mooring line material have been simulated and compared in order to determine the optimal type with the minimum motion risk for a floating wind turbine. This study focused on numerical modeling of semi-taut mooring systems using nonlinear materials for a semi-submersible wind turbine. Several modeling approaches common to current practice were applied. Hydrodynamic analysis was performed to investigate the motion of the response amplitude operators of the floating wind turbine. Dynamic analysis of mooring systems was performed using a time domain to obtain the tension responses of mooring lines under the ultimate limit states and fatigue limit states in Vietnamese sea conditions. The results showed that the use of nonlinear materials (polyester and/or nylon) for mooring systems can minimize the movement of the turbine and save costs. The use of synthetic fibers can reduce the maximum tension in mooring lines and the length of mooring lines. However, synthetic fiber ropes showed highly nonlinear load elongation properties, which were difficult to simulate using numerical software. The comparison of the characteristics of polyester and nylon mooring lines showed that the maximum and mean tensions of the nylon line were less than those of the polyester line. In addition, the un-stretched length of the polyester line was greater than that of the nylon line under the same mean tension load. Therefore, nylon material is recommended for the mooring lines of a floating offshore wind turbine.

Parametric Modeling and Correlation Analysis of the Substructure Design Variables for a Single-Column Floating Offshore Wind Turbine

The design and optimization of substructure of a floating offshore wind turbines (FOWT) is an extremely challengeable work. Before this work, it is necessary to investigate the correlation of design variables of the FOWT substructure. The correlation analysis can help to reduce the dimensions of design parameters and output results, and provide insight into the nature of the design space. The subsequent design and optimization workload can therefore be reduced. This paper aims to provide a framework of parametric modeling and correlation analysis of the substructure design variables for a single-column FOWT with a slack catenary mooring system. The NREL offshore 5 MW baseline wind turbine is selected in this study, and various floating foundation configurations are designed to adapt to this wind turbine. In order to evaluate the main dynamic performances of the FOWT well and quickly, a reduced-degree-of-freedom model of the FOWT is constructed and verified. Then, the substructure design parameters are parameterized to facilitate modeling, and simulations based on design of experiments are performed to save evaluation time and cost. In simulations, three sets of steady wind speeds are applied to eliminate the influence of controller dynamics. The results showed that the correlation of dynamic performances of the FOWT is significant, indicating that it is enough to consider several outputs to represent the dynamic behavior of the system. Regarding the designing parameters, the draft generates the most significant influence on the FOWT dynamic performances. The column radius has different influences for the cases with and without heave plates. Additionally, whether or not the heave plate is added, the mooring line length mainly influences the 95th percentile of platform horizontal surge displacement and standard deviation of fairlead tension of the downwind mooring line.

Effect of Mooring Parameters on Dynamic Responses of a Semi-Submersible Floating Offshore Wind Turbine

Based on a new semi-submersible floating offshore wind turbine (FOWT), a coupling aero-hydro-flexible model was established to study its dynamic behaviors, as well as the corresponding mooring system, under complicated sea scenarios. The aerodynamic load, the wave load, the current load, and the mooring load were taken into consideration. To further investigate the influence of the mooring parameters on the floating system, the diameter and the total length of mooring lines, which are the most critical parameters in mooring line design, were chosen to be analyzed. Particularly, five diameters and seven lengths were adopted to establish the FOWT mooring system, and a time-domain simulation was carried out for each cases. Based on the numerical simulations, their influences on the mooring system stiffness and the dynamic responses of FOWT were studied. The results show that the diameter has little influence on the static shape of the mooring line. The mooring system stiffness can be effectively increased by reducing the length and increasing the diameter of mooring lines. Moreover, the surge motion of floating foundation can be effectively controlled by increasing the mooring line diameter and decreasing mooring line length under the rated sea scenario. From this aspect, the dynamic response features of the FOWTs could be improved.

Preliminary Study on an Integrated System Composed of a Floating Offshore Wind Turbine and an Octagonal Fishing Cage

To maximize the utilization of ocean resources, shorten the return period of investment and directly supply energy to the fishing cage, this paper performs a preliminary study for a state-of-the-art concept integrating a floating offshore wind turbine with a fishing cage. An octagonal semisubmersible rigid fishing cage with a slack catenary mooring system is designed to match the NREL 5 MW offshore baseline wind turbine. Combined with the blade pitch controller, fully coupled aero-hydro-elastic-servo-mooring simulations are performed through FAST and AQWA to explore the dynamic performance of the integrated system. Free decay conditions, uniform wind with irregular and regular waves, and turbulent wind with irregular waves are tested. The results showed that the integrated system works normally at the operating conditions and exhibits different dynamic characteristics for various scenarios. Additionally, the study on the influence of mooring line length indicates that the increasing line length can significantly affect the cage surge motion and the maximum and mean values of the upwind line tension at fairlead. Specifically, the maximum surge motion with a 924-m-long line is 404.8% larger than that with an 880-m-long line. When the line length increases by 5%, the maximum and mean line tensions decrease by 45.7% and 47.7%, respectively, while when the line length increases by 10%, the maximum and mean line tension decrease by 52.9% and 54.2%, respectively. It should be noted that the main purpose of this work is to conduct a preliminary study on this integrated system, aiming to provide an idea for the conceptual design, modeling and simulation analysis of this integrated system.

Study on Array Floating Platform for Wind Energy and Marine Space Optimization

The concept of multiline anchor, whose application is mainly considered in water depths beyond 100 m and analyzed only by numerical simulation, has been discussed for half a decade, yet previous studies have not conducted the wave basin experiment. Thus, this paper set this concept firstly with a shallow water mooring system designed for a Taiwan offshore water area, where the suitable water depth for floating offshore wind turbine is located from 50 to 100 m, and then conducted a 1:144 scaled model wave basin experiment to validate the results from numerical simulation. In this paper, the numerical model simulated and analyzed three identical DeepCwind OC4 semi-submersible platforms equipped with NREL 5MW wind turbines in OrcaFlex and the experiment carried out by using three 1:144 scaled semi-submersible platforms with equivalent disks which simulated different operations of wind thrusts. To consider the possible influence of the wake effect, the minimum turbines spacing was set at 750 m in a full scaled model and the length of mooring lines was redesigned according to the catenary theory. This paper utilized OrcaWave to calculate hydrodynamic parameters and input it into OrcaFlex to simulate the line tension and the three degrees of freedom (surge, heave, and pitch) of the platforms under regular and irregular wave tests, and coordinate with scaled model tests carried out in Tainan Hydraulics Laboratory (THL). In addition to the reduction in the number of anchors, the concept of multiline anchor was also discussed in this study for the spatial configuration of offshore wind farms. It shows that the wind farm composed of three floating wind turbines can reduce the ocean space by roughly 24% compared to that with a single-line anchor. According to the comparison of numerical and experimental results, this study finally optimized the mooring lines by changing the diameter to increase the stability and the threshold of Minimum Breaking Load (MBL) and proposed a multiline anchor configuration for shallow offshore water area in Taiwan based on the results obtained.

Proposal of a Novel Mooring System Using Three-Bifurcated Mooring Lines for Spar-Type Off-Shore Wind Turbines

Floating wind turbine vibration controlling becomes more and more important with the increase in wind turbine size. Thus, a novel three-bifurcated mooring system is proposed for Spar-type floating wind turbines. Compared with the original mooring system using three mooring lines, three-bifurcated sub-mooring-lines are added into the novel mooring system. Specifically, each three-bifurcated sub-mooring-line is first connected to a Spar-type platform using three fairleads, then it is connected to the anchor using the main mooring line. Six fairleads are involved in the proposed mooring system, theoretically resulting in larger overturning and torsional stiffness. For further improvement, a clump mass is attached onto the main mooring lines of the proposed mooring system. The wind turbine surge, pitch, and yaw movements under regular and irregular waves are calculated to quantitatively examine the mooring system performances. A recommended configuration for the proposed mooring system is presented: the three-bifurcated sub-mooring-line and main mooring line lengths should be (0.0166, 0.0111, 0.0166) and 0.9723 times the total mooring line length in the traditional mooring system. The proposed mooring system can at most reduce the wind turbine surge movement 37.15% and 54.5% when under regular and irregular waves, respectively, and can at most reduce the yaw movement 30.1% and 40% when under regular and irregular waves, respectively.

The Quasi-Static Response of Moored Floating Structures Based on Minimization of Mechanical Energy

It is essential to design a reasonable mooring line length that ensures quasi-static responses of moored floating structures are within an acceptable level, and that reduces the cost of mooring lines in the overall project. Quasi-static responses include the equilibrium position and the line tension of a moored floating structure (also called the mean value in a dynamic response), etc. The quasi-static responses derived by the classic catenary equation cannot present mooring–seabed interaction and hydrodynamic effects on a mooring line. While a commercial program can predict reasonable quasi-static responses, costly modeling is required. This motivated us to propose a new method for predicting quasi-static responses that minimizes the mechanical energy of the whole system based on basic geometric parameters, and that is easy to implement. In this study, the mechanical energy of moored floating structures is assumed to be the sum of gravitational–buoyancy potential energy, kinetic energy induced by drag forces, and spring potential energy derived by line tension. We introduce fundamental theoretical background for the development of the proposed method. We investigate the effect of quasi-static actions on mooring response, comparing the proposed method’s results with those from the catenary equation and ABAQUS software. The study reveals the shortcomings of the catenary equation in offshore applications. We also compare quasi-static responses derived by the AQWA numerical package with the results calculated from the proposed method for an 8 MW WindFloat 2 type of platform. Good agreement was drawn between the proposed method and AQWA. The proposed method proves more timesaving than AQWA in terms of modeling of mooring lines and floaters, and more accurate than the catenary equation, and can be used effectively in the early design phase of dimension mooring lengths for moored floating structures.

Assessment of mooring configurations for the IEA 15MW floating offshore wind turbine

To achieve cost-effective deployment of floating offshore wind farms, it is necessary to reduce mooring costs of Floating Offshore Wind Turbines (FOWTs). Beyond the cost, in terms of environmental impact, the seabed disrupted area due to mooring lines should be mitigated with care. The objective of this paper is to shed light on design parameters for cost- effective and low-footprint mooring configurations for FOWTs using coupled dynamic analyses. A design space is explored for mooring configurations with different pretension ratios, laid down length ratios and clump weight sizes. Ultimate and fatigue load cases are simulated in OpenFast to compute the floater motions, mooring line tensions and fatigue damage. With constant pretension ratio of 0.15 and adding clump weights of 40t, mooring line length, mooring footprint and peak tension can be reduced by 14%, 15% and 9% respectively, while maximum surge and fatigue damage increase by 25% and 12% respectively. This paper will serve as a basis for further work on mooring design in the EU H2020 funded project COREWIND and provide a practical reference for the mooring system design for FOWTs.

Open center tidal turbine: How a new mooring system concept affects the performances

The huge manufacturing and installations costs, together with the adverse impact of the mooring systems and structures on the environment, flora and fauna have become one of the major constraints for the application of many marine technologies. An innovative mooring system coupled to an open centre turbine has been developed, adopting a full floating concept, which allows a kite like connection between the turbine and the coast. Replacing the huge underwater mooring structures with such kite flying system dramatically reduces the environmental impact and effects, meanwhile the lightweight open centre turbine coupled to this mooring system, assures a self-adaptive optimal orientation, achieving a new equilibrium arrangement at given depth and distance to the coast, following only the fluid dynamics rules. In addition, the synergy between the two machine components makes also suitable the adoption of low aspect ratio blades, characterized by lower specific loads, and leads to a reduction of the manufacturing cost. To optimize the kite concept, the connecting rope has been replaced with heavy and light nodes connected by tubular elements, where the light ones accommodate the deflectors to increase the lateral force responsible of the machine positioning, facilitating the machine transients, and manoeuvring and leading to improve the output reliability, reducing also the mooring line length. A case study shows that a 12 m diameter open centre turbine with the new mooring line produces, at Messina Strait site (Italy), an output of 362 MWh/year, 17.5% higher than, at same operating conditions, a classic 3 blades turbine moored on the seabed. It confirms the high suitability of this solution for marine purposes.

Analysis on snap load characteristics of mooring line in slack-taut process

The mooring system may appear slack-taut alternating transformation with the motion excitation of floating structure, resulting in the dynamic tension amplitude of mooring line significantly increase. Based on the dynamic finite element analysis, a series of sinusoidal excitations with different amplitudes and frequencies are applied at the top of mooring line, and the results show that the slack-taut phenomenon would occur when the top excitation reaches a certain amplitude and frequency. The minimum value of tension is close to zero, and the maximum value can reach several times of pre-tension. The spectrum analyses show that the components of high frequency appear besides the excitation frequency, and the main factor affecting slack-taut phenomenon of mooring line is the component of excitation along axial direction of mooring line. Through improving the initial pre-tension, selecting the cable with large elastic modulus and small cross-section diameter, and making the length of mooring line as short as possible, these settings would have benefit for keeping mooring line in the taut state and avoid the occurrence of snap load. The results of this paper would have significant reference for further exploring the mechanism of snap load in the mooring system and its design and safety assessment.

A shallow water mooring system design methodology combining NSGA-II with the vessel-mooring coupled model

In this paper, a mooring system design methodology combining the Non-dominated Sorting Genetic Algorithm-II (NSGA-II) with the vessel-mooring coupled model is first proposed. In the proposed methodology, the multi-objective optimization problem for mooring system design which involves several design variables, objective functions and constraints is established and solved by the combination of the NSGA-II algorithm and the vessel-mooring coupled model, and the initial feasible range of design variables is narrowed in advance by combining the NSGA-II with the quasi-static mooring system model. A case study is presented to demonstrated the proposed methodology, in which a catenary mooring system is designed from scratch for a semi-submersible very large floating structure (VLFS) module. It can be concluded from the obtained results that, owing to the combination of the NSGA-II algorithm and the vessel-mooring coupled model, the accurate dynamic responses of vessel and mooring systems can be used as the objective functions; thus, the proposed methodology is able to attain an optimal mooring system considering more practical design requirements, such as the vessel motion response, the safety factor of the mooring system, the length of mooring line lay down part and mooring system layout.

Fatigue Life of Mooring Lines on External Turret Floating LNG for Different Pretension and Water Depth

This paper studies fatigue life of mooring lines applied on turret-moored Floating LNG (Liquefied Natural Gas). Several case studies were designed to investigate the influence of mooring pretension and water depth on the fatigue life of mooring lines. Floating LNG with permanent external turret mooring system consists of 12 catenary lines arranged in 3 group with 4 lines each. Time domain simulation applied to calculated coupled dynamic response and mooring tension. Fatigue life is assessed using rainflow counting method, T-N curves and Palmgren-Miner rule. Analysis results for mooring pretension case show that case study 2 with lowest pretension (14%MBL) yields shortest mooring fatigue life of 1814.20 years compared to case study 1 (18%MBL) and 3 (22%MBL) with 2034.61 years and 2983.33 years respectively. Despite has the lowest dynamic line tension, case study 2 has larger tension range that results in the increased of fatigue damage. The increase in water depth will increase the mooring line length and its weight, so it results in the increase of fatigue damage. This is reflected in case study 5 (903m water depth), which has shortest mooring fatigue life of 1842.65 years compare to case study 1 (602m water depth) and 4 (301m water depth) with 2983.33 years and 3363.62 years respectively.

Comparison of pilot-scale floating offshore wind farms with shared moorings

As floating wind turbine technology matures and wind farms are considered at larger scales in deeper waters, moorings may take a larger share of the costs. In response, floating wind farms could be designed such that the moorings are shared between turbines, using interconnecting mooring lines between adjacent floating offshore wind turbine platforms and also possibly connecting multiple mooring lines to the same anchor. This may reduce the total length of mooring line, the wind farm footprint, and the number of anchors; worthwhile advantages if not overcome by increases in mooring strength requirements.A design algorithm is presented for preliminary sizing of shared mooring systems for pilot-scale floating offshore wind farms. Quasi-static modelling is used to analyze the performance of shared-mooring systems produced by the algorithm. The model accounts for steady wind thrust forces and nonlinear mooring line reactions. A four-turbine floating wind farm is designed and analyzed with three alternative shared-mooring configurations and four water depths. Results are presented in terms of platform displacements, mooring tensions, and cost, providing an initial comparison of shared mooring options and their potential for cost savings over conventional individually-moored configurations.

Numerical study of a novel flex mooring system of the floating wave energy converter in ultra-shallow water and experimental validation

In order to cope with design difficulties of the mooring system of the floating wave energy converter (WEC) in ultra-shallow water under typhoon sea state, while remaining economically feasible and reliable, new mooring system design strategies need to be implemented. The present paper aims at summarizing these design strategies of mooring system and proposing a novel mixed superflex wire rope design for WEC mooring system. By comparing with the traditional low elastic polyester ropes and hybrid catenary chain mooring system, it has been found that adding high elastic cable such as superflex rope and buoy can effectively improve the reliability of the mooring system and the energy storage of the WEC. The numerical model demonstrated a good, reliable and effective agreement with the experiment with the present superflex design. The present mixed superflex rope mooring system can effectively reduce the mooring radius, the length of mooring line and material usage as well as max mooring tension. Thanks to the good elasticity of the mooring system, the transient extreme mooring tension can be effectively reduced and the survivability of the device can be improved. This experience proposes significant improvements for other mooring system design of floating WEC design in ultra-shallow water.

A coupled analysis of motion and structural responses for an offshore spar platform in irregular waves

ABSTRACTA coupled numerical analysis has been carried out for the structural responses and motion behaviour of a classical spar structure subjected to irregular waves represented by JONSWAP (Joint North Sea Wave Project) spectrum. The motion of the floating body is restrained by the four catenary mooring lines with the generation of tension due to change in their nonlinear profile. The surface of the spar is represented by hydrodynamic pressure panels while cables are discretised using a series of Morison elements. A comprehensive sensitivity analysis under two sea depths is carried out by changing (1) the length of mooring lines, (2) the vertical position of fairlead point, (3) incident angle of long crested unidirectional single spectrum, and (4) the number of short crested sub segmented spreading spectra. The effect of second-order hydrodynamic loading on the structure is also taken into account in the analysis procedure by using quadratic transfer function.