Mooring Line Tension Research Articles

Experimentally validated simplified prediction model of unloaded spar-buoy wave energy converter motions’ responses in waves

The main objective of this study is to develop a fast and validated model to predict the motions’ responses of a spar-buoy wave energy converter based on oscillating water column principle. Experiments and former investigations highlighted the strong effect of the spar-buoy heave motion on the performance of the OWC whereas water oscillations slightly affect the spar-buoy motions. Therefore, a single degree of freedom model was adopted to simulate the structure’s surge, heave, and pitch responses separately. The prediction of the structure’s heave and pitch responses is a straightforward task and results were experimentally validated and compared to previous studies. Linear and quadratic damping coefficients were introduced for better agreement between measured and predicted pitch responses. In contrast, surge response prediction task was more complicated as it requires the consideration of the mooring system installation properties, while the mooring configuration was not experimentally investigated since the reduced-scale model was floating freely (un-moored). Therefore, the predicted surge response obtained from the proposed model was validated using the finite element method. Two different approaches were used to solve the equation of motion in surge direction considering multi-catenary mooring system. Static approach where constant mooring-lines stiffness was used and a quasi-static approach where reaction curves developed were used to estimate the instant resultant mooring line tension at different offsets. Load excursion and reaction curves of the full-scale device developed by the proposed model were finally validated.

Dynamic Behavior Analysis of an Engineering Ship With Mooring Line Failure

Abstract In the present study, a series of numerical simulations were conducted to analyze the dynamic behavior of an engineering ship with mooring line failure. For this purpose, a fully coupled dynamic analysis program was used to develop a nonlinear numerical model of the hull-mooring system. One of the four mooring lines was intentionally disconnected at a certain time. Under the environmental condition, the ship motions and mooring line tension were discussed in detail. It turned out that mooring line failure would have a significant influence on the performance of the ship, which may threaten the safety of the floating system. In addition, the transient effects of mooring line failure caused the transient overshoots of the ship motion and mooring line tension. Therefore, it is necessary to study the effects of mooring line failure on the floating system in advance.

Performance changes of an engineering ship with one mooring line failure

This paper analyzed the performance changes of an engineering ship with mooring line failure. The nonlinear numerical model of the hull-mooring system was established through a fully coupled dynamic analysis program - ANSYS/AQWA. At a certain time, one of the four mooring lines was intentionally disconnected. The dynamic responses of the ship, the mooring line tension and the transient effects of the slow-drift motions with mooring line failure were investigated in detail under the environmental conditions. After a series of numerical simulations were conducted, the results showed that the accidental failure of one mooring line changes the watch circle of the ship and causes the tension redistribution of the damaged mooring system. In addition, the results also indicated that the current practice should consider the transient effects after the mooring line breaks because it may cause the transient overshoot of the slow-drift motion. Consequently, it is necessary to evaluate the safety condition of the ship with mooring line failure in advance. This study could give a profound understanding of the influence of mooring line failure and have reference value in the design of the mooring system.

Numerical investigation on mooring line configurations of a Paired Column Semisubmersible for its global performance in deep water condition

The advances observed in the offshore-renewable industry include the recent development of a deep draft paired column semisubmersible platform developed for application on dry trees in Gulf of Mexico (GoM). These developments led to the recent innovation of the Paired Column Semisubmersible by Jun Zou of Houston Offshore Engineering. This paper presents a detailed investigation on the mooring line analysis for two different configurations on the hull of a Paired Column Semisubmersible (PCSemi). The numerical research of the PCSemi platform coupled with mooring lines was conducted, and the model was validated. Two different mooring configurations were investigated: Chain-Polyester-Chain (CPC) and Polyester-Polyester (PP) configurations, with dynamic coupling in frequency domain (FD) and time domain (TD) using ANSYS AQWA and Orcaflex. A comparative study of the mooring lines is also investigated in 2,438 m water depth at GoM, using 16 mooring lines with catenary mooring design. Results of this study gives the natural period of floating PCSemi's heave motion ranged from 21s to 22s. Also, the effect of a damaged mooring line increased the tension in other mooring lines. Lastly, the CPC configuration has a smaller amplitude for wave-frequency platform motions compared to the PP mooring configuration from the depicted PCSemi global performance.

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.

Reliability-based fixed-time nonsingular terminal sliding mode control for dynamic positioning of turret-moored vessels with uncertainties and unknown disturbances

This paper presents an area keeping and heading control method for a turret-moored vessel subject to uncertainties and unknown disturbances. Specifically, the nonlinear motion model of the vessel is firstly transformed into the form of reliability-heading error dynamics by introducing a reliability-based matrix, which builds the relationship between the mooring line tension and vessel motion. Then, a fixed-time fast nonsingular terminal sliding surface is constructed based on the states of reliability-heading error dynamics. Next, a fixed-time extended state observer (ESO) based on position-heading measurements is designed to estimate the uncertainties and unknown disturbances. Based on the sliding surface and ESO, a fixed-time nonsingular terminal sliding mode controller (NTSMC) is proposed to ensure that the reliability and heading errors converge to the sliding surface in fixed time. By adjusting the reliability to the desired value, the capacity of mooring system can be fully utilized to keep the vessel within the allowable area. Meanwhile, the safety of mooring lines can be guaranteed. Finally, simulations and comparisons demonstrate the effectiveness of the proposed fixed-time ESO-based NTSMC for dynamic positioning of a turret-moored vessel.

Robust output feedback control for dynamic positioning of turret-moored vessels based on bio-inspired state observer and online constructive fuzzy system

This paper investigates the dynamic positioning control of turret-moored vessels with uncertainties, unknown disturbances and velocities. Firstly, a bio-inspired state observer is proposed based on the bio-inspired neural dynamics model to estimate unknown velocities without vessel dynamics model. Meanwhile, an online constructive fuzzy approximator is designed to approximate uncertainties and unknown disturbances. To ensure fuzzy rules adequate and parsimonious, an online constructive scheme is proposed to adjust the fuzzy system structure without any prior knowledge. Based on the designed state observer and fuzzy approximator, a reliability-based robust output-feedback controller is proposed to make the reliability and heading converge to desired values. A reliability-based matrix is applied into the adaptive fuzzy parameter update law to simplify the stability analysis of closed-loop control system. The convergence of reliability can ensure full utilization of mooring system to maintain the vessel within allowable region. Finally, simulations and comparisons show the performance of proposed methods. • The bio-inspired state observer can make the velocity estimation errors converge to zero without the dynamics of vessels. • An online constructive scheme is proposed based on the distances between the current input and existing fuzzy set only. • The online constructive scheme can ensure the fuzzy rules adequate and parsimonious without any prior knowledge. • A reliability-based matrix is designed to build the relationship between the mooring line tension and vessel motion. • The reliability-based matrix is used in the adaptive fuzzy parameter update law to simplify system stability analysis.

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.

Research on Wave Attenuation Performance of Floating Breakwater

In this study, a new type of double-pontoon floating breakwater was designed to improve the wave attenuation performance through the addition of suspended Savonius propeller-blade. Its hydrodynamic characteristics were studied through numerical simulations and performance-testing experiment. The following investigations were performed in this study: Firstly, wave theory and hydrodynamic theory were combined to calculate the wave attenuation performance and motion response of double-pontoon floating breakwater under linear wave conditions. The numerical results showed that the wave attenuation performance was better under a specific wave period and height, the transmission coefficient reached a relatively small value, and the mooring line tension responded periodically and satisfied the condition of maximum breaking force. Secondly, three key geometric parameters of breakwater were researched, including the relative spacing of pontoons, the relative spacing between pontoons and blades, and the height–diameter ratio of Savonius blades. The calculation results showed that the pontoon spacing was closer to the wavelength and the breakwater wave attenuation performance was better. Lastly, experimental tests were also performed on the new double-pontoon floating breakwater and the results showed that the wave attenuation performance and numerical projections were basically the same, which verified the validity and effectiveness of the design method.

Quasi-dynamic mooring line model

The quasi-dynamic mooring line model is a new approach to assess the mooring line tension that considers the static solution of the catenary shape, but updates the distributed vertical force along the mooring line as a function of the external forces, drag and inertial, acting on the line. In this way, an updated apparent weight is applied to the static solution of the catenary shape that improves significantly the solution obtained by the quasi-static approach, with an equivalent computational cost. Then, the model improves the tension solution of the quasi-static models by using that updated weight of the mooring, achieving results much closer to the dynamic models than the standard quasi-static ones. To appraise the improvement of the results, a parametric analysis within a wide range of mooring physical parameters is conducted. The error of the quasi-static tension of one cycle is less than 20% only for the 18% of the simulations, while the quasi-dynamic model produces an error less than 20% for the 84% of simulations. In addition, the model can predict the slack phenomenon.

Prediction of dynamic mooring responses of a floating wind turbine using an artificial neural network

Numerical simulations in coupled aero-hydro-servo-elastic codes are known to be a challenge for design and analysis of offshore wind turbine systems because of the large number of design load cases involved in checking the ultimate and fatigue limit states. To alleviate the simulation burden, machine learning methods can be useful. This article investigates the effect of machine learning methods on predicting the mooring line tension of a spar floating wind turbine. The OC3 Hywind wind turbine with a spar-buoy foundation and three mooring lines is selected and simulated with SIMA. A total of 32 sea states with irregular waves are considered. Artificial neural works with different constructions were applied to reproduce the time history of mooring tensions. The best performing network provides a strong average correlation of 71% and consists of two hidden layers with 35 neurons, using the Bayesian regularisation backpropagation algorithm. Sea states applied in the network training are predicted with greater accuracy than sea states used for validation of the network. The correlation coefficient is primarily higher for sea states with lower significant wave height and peak period. One sea state with a significant wave height of 5 meters and a peak period of 9 seconds has an average extreme value deviation for all mooring lines of 0.46%. Results from the study illustrate the potential of incorporating artificial neural networks in the mooring design process.

Effect of mooring line layout on the loads of ship-shaped offshore installations

An offshore mooring system stations a ship-shaped offshore installation in place while withstanding incoming loads from the marine environment with short-term and long-term uncertainties. This study aims to develop a novel framework for analysing the loads on floating systems, namely mooring line tension, mooring line fatigue damage, and hull bending moment, as a function of the mooring layout design variables and environmental random variables. The nonlinear influence of those variables is assessed by means of advanced techniques using response charts, response divergence charts, and Sobol's total-effect sensitivity indexes. The developed procedure includes a probabilistic selection of mooring scenarios, station-keeping numerical analyses, and metamodel selection to define input loads. An example of a hypothetical floating production storage and offloading (FPSO) unit with taut legs in the Gulf of Mexico illustrates the procedure. The details of the computations are documented, and the findings show that the mooring line top-tension has a high total-effect index for the wave-induced bending moment and the total mooring line tension, whereas the fatigue damage is mostly affected by the chain diameter. The results of this research offer useful insights to designers and propose the use of a surrogate model to be used in the reliability-based design of mooring systems.

Effects of strain- and strain rate- dependent nonlinear mooring line stiffness on floating platform motion

The synthetic fiber rope is a mooring material that is increasingly used, but few commercial codes are known to reliably implement the nonlinear stiffnesses of this material. Through the development of a floating platform-mooring line coupled simulation code that allows simultaneous consideration of the strain- and strain rate-dependent nonlinear stiffness, this paper aims to examine the effects of the nonlinear stiffnesses on platform motion and mooring tension responses. We introduced theoretical background to develop a new code. We compared the experimental and numerical results of two floating platforms with the results calculated from the code. A very good agreement was drawn from the comparative study for the two verification platforms. We analyzed the effect of the nonlinear mooring line stiffness on the platform motion and mooring line tension responses by assuming three mooring materials with the linear, strain-dependent nonlinear, and strain rate-dependent nonlinear stiffnesses. When applying the catenary mooring system, the nonlinear stiffness little affects the motion and line tension responses. Considering the nonlinear stiffness in the taut-leg mooring system, the slowly varying responses in the surge motion and mooring tension were observed in the response spectra. The strain rate-dependent nonlinear stiffness should be considered for the taut-leg mooring system.

Short-crestedness effect on the dynamic response of offshore floating wind turbines

ABSTRACT In practice, the ocean waves are short-crested and three-dimensional. The primary objective of the present research is to form a better understanding of the dynamics of offshore floating wind turbines in multi-directional waves. The short-crested waves are modelled using a spreading exponent function. The wave directionality effect on the dynamic performance of a semisubmersible offshore floating wind turbine is studied. In short-crested waves, the longitudinal response is reduced whereas the lateral response is amplified. The tower base fore-aft bending moment and the mooring line tension force are reduced. In the meanwhile, the fatigue damages of critical points at the tower base are also reduced. It is concluded that short-crestedness is beneficial to the structural integrity. The present research reveals the importance of wave short-crestedness effect on the performance of offshore floating wind turbines.

Wind–Wave Coupling Effect on the Dynamic Response of a Combined Wind–Wave Energy Converter

There is a huge energy demand from offshore renewable energy resources. To maximize the use of various renewable energy sources, a combined floating energy system consisting of different types of energy devices is an ideal option to reduce the levelized cost of energy (LCOE) by sharing the infrastructure of the platform and enhancing the power production capacity. This study proposed a combined concept of energy systems by combing a heave-type wave energy converter (WEC) with a semisubmersible floating wind turbine. In order to investigate the power performance and dynamic response of the combined concept, coupled aero-hydro-servo-elastic analysis was carried out using the open-source code F2A, which is based on the coupling of the FAST and AQWA tools by integrating all the possible environmental loadings (e.g., aerodynamic, hydrodynamic). Numerical results obtained by AQWA are used to verify the accuracy of the coupled model in F2A in predicting dynamic responses of the combined system. The main hydrodynamic characteristics of the combined system under typical operational conditions were examined, and the calculated responses (motions, mooring line tension and produced wave power) are discussed. Additionally, the effect of aerodynamic damping on the dynamic response of the combined system was examined and presented. Moreover, a second fully coupled analysis model was developed, and its response predictions were compared with the predictions of the model developed with F2A in order for the differences of the calculated responses resulted by the different modeling techniques to be discussed and explained. Finally, the survivability of the combined concept has been examined for different possible proposed survival modes.

Dynamic Responses for WindFloat Floating Offshore Wind Turbine at Intermediate Water Depth Based on Local Conditions in China

Offshore wind energy, a clean energy resource, is considered to be a possible alternative to fossil energy. Floating offshore wind technology is considered to be a proper concept to develop abundant wind energy in deep water. Considering the reality of offshore wind energy development in China, the floating offshore wind turbine concept is expected to be developed at moderate water depths. In this paper, a mooring system of the WindFloat semisubmersible floating offshore wind turbine (SFOWT) at a water depth of 60 m is designed. The dynamic responses of the WindFloat SFOWT under different wind–wave combination conditions are investigated using the coupled method and the simplified method, which do not include the effect of the tower top motion in the aerodynamic calculation. The results show that the dynamic responses of the WindFloat SFOWT, including the platform motions, tower loads, and mooring line tensions, perform fairly well at a moderate water depth. A comparison between the coupled method and simplified method shows that the calculated results are slightly different between the different conditions for the time domain results, response spectra results, and fatigue results. In addition, mooring line 1 (ML 1) suffers higher fatigue damage than ML2, which should be paid more attention.

Digital twins of the mooring line tension for floating offshore wind turbines to improve monitoring, lifespan, and safety

The number of installed floating offshore wind turbines (FOWTs) has doubled since 2017, quadrupling the total installed capacity, and is expected to increase significantly over the next decade. Consequently, there is a growing consideration towards the main challenges for FOWT projects: monitoring the system’s integrity, extending the lifespan of the components, and maintaining FOWTs safely at scale. Effectively and efficiently addressing these challenges would unlock the wide-scale deployment of FOWTs. In this work, we focus on one of the most critical components of the FOWTs, the Mooring Lines (MoLs), which are responsible for fixing the structure to the seabed. The primary mechanical failure mechanisms in MoLs are extreme load and fatigue, both of which are functions of the axial tension. An effective solution to detect long-term drifts in the mechanical response of the MoLs is to develop a Digital Twin (DT) able to accurately predict the behaviour of the healthy system to compare with the actual one. Moreover, we will develop another DT able to accurately predict the near future axial tension as an effective tool to improve the lifespan of the MoLs and the safety of FOWT maintenance operations. In fact, by changing the FOWT operational settings, according to the DT prediction, operators can increase the lifespan of the MoLs by reducing the stress and, additionally, in the case where FOWT operational maintenance is in progress, the prediction from the DT can serve as early safety warning to operators. Authors will leverage operational data collected from the world’s first commercial floating-wind farm [the Hywind Pilot Park (https://www.equinor.com/en/what-we-do/floating-wind/hywind-scotland.html.)] in 2018, to investigate the effectiveness of DTs for the prediction of the MoL axial tension for the two scenarios depicted above. The DTs will be developed using state-of-the-art data-driven methods, and results based on real operational data will support our proposal.

Hydrodynamic Sensitivity of Moored and Articulated Multibody Offshore Structures in Waves

Within the framework of Space@Sea project, an articulated modular floating structure was developed to serve as building blocks for artificial islands. The modularity was one of the key elements, intended to provide the desired flexibility of additional deck space at sea. Consequently, the layout of a modular floating concept may change, depending on its functionality and environmental condition. Employing a potential-flow-based numerical model (i.e., weakly nonlinear Green function solver AQWA), this paper studied the hydrodynamic sensitivity of such multibody structures to the number of modules, to the arrangement of these modules, and to the incident wave angle. Results showed that for most wave frequencies, their hydrodynamic characteristics were similar although the floating platforms consisted of a different number of modules. Only translational horizontal motions, i.e., surge and sway, were sensitive to the incident wave angle. The most critical phenomenon occurred at head seas, where waves traveled perpendicularly to the rotation axes of hinged joints, and the hinge forces were largest. Hydrodynamic characteristics of modules attached behind the forth module hardly changed. The highest mooring line tensions arose at low wave frequencies, and they were caused by second-order mean drift forces. First-order forces acting on the mooring lines were relatively small. Apart from the motion responses and mooring tensions, forces acting on the hinge joints governed the system’s design. The associated results contribute to design of optimal configurations of moored and articulated multibody floating islands.

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.

Experimental and numerical investigations of a mooring line failure of an aquaculture net cage subjected to currents

This study investigates the impact of a mooring line failure on an aquaculture net cage through a laboratory experiment and numerical simulations. An eight-point mooring net-cage model with two load cells in its upstream mooring lines was installed in a water flume. An experiment was conducted to measure the upstream mooring force acting on the net cage exposed to currents in the intact and failed mooring conditions. A device equipped with a pneumatic cylinder and a blade was designed to cut the mooring line. The obtained results revealed that as one of the upstream mooring lines was cut, the mooring load in the other line increased greatly. However, the net-cage system reached a new force equilibrium within a few seconds. The mooring line tension ratios between the failed and intact states observed in this study were between 1.5 and 2.0. In addition to the strong mooring tension response, a significant yaw rotation of the floating collar of the net-cage system was also observed, but the motion stopped within a second after the failure occurred. In addition, a comparison of mooring forces between the experiment and numerical simulations exhibited good agreement for current velocities less than 25 cm/s. The simulated results showed that after the failure of one upstream line, the mooring load in selected lateral mooring lines increased substantially, and the net-volume change showed a sharp peak at the time of failure. Moreover, when the cage system regained balance after failure, the net-cage volume returned to its pre-failure value. Finally, the reduction in the safety factor resulted from the mooring line failure was discussed for practical engineering applications.