Mooring Line Tension Research Articles

Investigation on Effects of Mooring Line Fractures and Connector Failures for A Hybrid Modular Floating Structure System

The present work reports a Hybrid Modular Floating Structure (HMFS) system with typical malfunction conditions. The effects of both fractured mooring lines and failed connectors on main hydrodynamic responses (mooring line tensions, module motions, connector loads and wave power production) of the HMFS system under typical sea conditions are comparatively investigated. The results indicate that the mooring tension distribution, certain module motions (surge, sway and yaw) and connector loads (Mz) are significantly influenced by mooring line fractures. The adjacent mooring line of the fractured line on the upstream side suffers the largest tension among the remaining mooring lines, and the case with two fractured mooring lines in the same group on the upstream side is the most dangerous among all cases of two-line failures in view of mooring line tensions, module motions and connector loads. Therefore, one emergency strategy with appropriate relaxation of a proper mooring line has been proposed and proved effective to reduce the risk of more progressive mooring line fractures. In addition, connector failures substantially affect certain module motions (heave and pitch), certain connector loads (Fz and My) and wave power production. The present work can be helpful and instructive for studies on malfunction conditions of modular floating structure (MFS) systems.

Localization and topological observability analysis of a moored floating structure using mooring line tension measurements

Mooring is a widely used stationkeeping approach for floating structures. The main contribution of this paper is to propose a novel localization scheme, which provides additional real-time position references and estimates the unknown anchor positions to a moored structure simultaneously using tension measurements from the mooring monitoring system. The proposed method enhances the safety of moored floating structures by extracting redundant position information from tension measurements instead of using satellite signals. A line-of-sight model that transforms tension measurements into range signals is developed. Fairleads, turret dynamics, and slow-varying current profiles are considered to provide a more realistic and robust tension-based localization scheme than early research. A distance-only sensor network is constructed, and graph rigidity theory is applied to analyze its topological observability. Two simplified models are presented for some specific applications: (1) to find the broken anchors offline with stored data for any vessels equipped with tension cells and (2) to position the moored structures with known anchor positions. Sensitivity studies show that the influence of unknown surface current is small, while the effects of unknown current profiles are larger. The positioning accuracy can be improved by increasing the number of virtual vessels.

Combined effects of aerodynamic and second-order hydrodynamic loads for floating wind turbines at different water depths

In this paper, fully coupled time-domain analysis of three different semisubmersible floating offshore wind turbines (semi-FOWTs) is performed. The focus of the present paper is to investigate the impact of second-order wave hydrodynamic loads combined with aerodynamic loads on the dynamic responses of semi-FOWTs under normal and extreme sea conditions. Three water depths (50, 100, and 200 m) are considered for examining the depth effect. The simulation results show that the second-order hydrodynamic loads play an important role in the motion and structural responses under extreme sea conditions irrespective of the inclusion of the aerodynamic loads in the analysis. Meanwhile, the aerodynamic loads significantly affect the dynamic responses of all semi-FOWTs under normal operational conditions. Among the three semi-FOWTs, the V-shaped semi-FOWT has the largest pitch motion responses under extreme sea conditions. Compared to the normal operation condition, the water depth has a larger impact on the motion and force responses of the three examined platforms under extreme sea condition. The surge and pitch motions decrease with water depth increases. The tower base loads tend to be lower for deep water depth, while the mooring line tensions are opposite. This findings could benefit the offshore wind turbine industries.

Numerical analysis and comparison study of the 1:60 scaled DTU 10 MW TLP floating wind turbine

This paper presents numerical investigations and comparison to experiments for a scaled DTU 10MW Tension Leg Platform (TLP) floating wind turbine. Two state-of-the-art aero-servo-hydro-elastic models, HAWC2 and FAST, are used and compared with experimental results. This study applies the same controller, turbulent wind, and hydrodynamic input as in the experiments to the numerical simulations. First, detailed numerical model calibration and validation have been performed. Free decay experimental test results are compared. Next, the damping ratio is calibrated. The rotor thrust is matched to the measurements by adjustment of the blade pitch angle in both numerical models. The same controller used in the experiments is applied to the numerical model via a Dynamic Link Library interface. Detailed comparisons for regular waves, irregular waves, and a focused wave group impact are presented. Results for surge motion, nacelle acceleration, front mooring line tension, blade pitch and rotor speed are included. In general, both numerical models predict the floating wind turbine responses very well compared to the experimental results both in time and frequency domains. Owing to the use of a linear wave forcing model and Morison based viscous loads, however, the low-frequency surge motion is found to be under-predicted in both models.

Influence of emergency mechanical braking on the supporting platform performances of floating offshore wind turbines

The aim of this paper is to investigate the influence of Emergency Mechanical Braking (EMB) on the supporting platform performances of floating offshore wind turbines (FOWTs). This paper builds the dynamic model of the FOWT using coupled aero-hydro-servo-elastic model and further establishes the EMB model. The case studies of 5-MW FOWTs with three different offshore supporting structures are presented, and the Taguchi orthogonal array is employed as the method for designing the experiments to consider multiple scenarios with a wide variety of EMB control torques. Typical results of the effects arising from EMB on the supporting platform performance are presented to assess the effects of the EMB. As indicated by the results, the EMB control will normally disturb pitch angle control, and the mooring line tensions have sensitivity to different EMB torques. Accordingly, relatively large platform oscillations will be induced due to the increase in the EMB torques. This paper presents novel insights into FOWT designers and manufacturers on which EMB parameters should be assessed, or which EMB control methods can be properly used to reduce uncertainties/risks of FOWTs under emergency conditions.

Seabed bathymetry and friction modeling in MoorDyn

This paper presents the implementation and verification of a seabed bathymetry feature and seabed friction feature to the open-source, lumped-mass mooring system dynamics modeler, MoorDyn, which is part of the National Renewable Energy Laboratory’s aero-hydro-servo-elastic simulation tool, OpenFAST. Variations in seabed slope, as well as the frictional effects of mooring lines moving along the seabed, will affect the mooring line tensions of a floating platform and the consequent platform response. These new features are especially relevant for modeling mooring systems in deep-water coastal areas where seabed depth can change significantly over an entire mooring footprint. The bathymetry feature models the seabed as a rectangular grid of variable water depths in place of the existing, uniform water depth in MoorDyn. The friction force is primarily represented as a Coulombic friction force, or the product of a kinetic friction coefficient and the seabed contact normal force, with the ability to differentiate between transverse and axial motion of a line node on the seabed in any bathymetry grid. These capabilities were tested by running MoorDyn and OpenFAST simulations over a variety of seabed and environmental conditions; the resulting fairlead tensions, node tensions, and mooring line kinematics were verified against equivalent OrcaFlex simulations. The results match closely, meaning the features are verified, which will increase the overall fidelity of OpenFAST and FAST.Farm simulations.

A DNN-based approach to predict dynamic mooring tensions for semi-submersible platform under a mooring line failure condition

The station-keeping system of offshore platforms is susceptible to complex metocean environments. The failure of the mooring line is of utmost concern due to uncertainty in the mooring line's behavior and the severe aftermath of the accidents. Once the mooring line breakage occurs, the remaining mooring lines should provide sufficient tension to warrant the continual production operations and personnel safety on the platform. This paper proposes a deep neural networks (DNN) approach to predict the dynamic mooring line tension under one mooring line failure condition. Firstly, the tension change of mooring lines induced by mooring failure is investigated in two hydrodynamic models, and then select the tensions on the mooring line with maximum sensitivity to failure as the output objective. Secondly, using the responses of floating structures and mooring tensions to construct the two datasets of different hydrodynamic models, then train two DNN models and utilize grid search to determine the optimal network structure. Finally, some case studies with different mooring arrangements and sea state conditions are employed to verify the feasibility and adaptability of two established DNN models and compare their accuracy. According to the unseen data test results, the DNN-II model shows a better prediction performance compared to the DNN-I model in all test cases. Based on the results of the two types of the correlation coefficient, the main control variables in mooring tensions prediction are believed to be significantly increased sway and surge motion induced by mooring broken. This study provides an idea for practical engineering, through combining with the mature and stable Global Positioning System (GPS) in a platform or FPSO, the remaining mooring line tensions under mooring failure conditions can be monitored and controlled in real time.

Drift simulation of a floating offshore wind turbine with broken mooring lines in a dynamic sea condition

This study investigates the aero-hydro-mooring coupled dynamics of the National Renewable Energy Laboratory (NREL) offshore 5-MW baseline wind turbine supported by the Offshore Code Comparison Collaboration Continuation (OC4) DeepCwind semisubmersible floating platform by considering the effects of suddenly broken mooring behaviour. The wind and wave loads can be selected as the Cummins time-domain equation inputs based on the Norwegian Petroleum Directorate wind and Joint North Sea Wave Project wave spectrums. The nonlinear viscous drag was estimated using the quadratic damping matrix rather than Morison's element. Additionally, the quadratic transfer function matrix was used to calculate the slow-drift force. Compared to the multi-segmented quasi-static mooring model, the lump-mass method is a dynamic mooring model that can solve real-time motion on the platform. Once a single or a pair of mooring lines is intentionally disconnected from the floating platform at a certain time, the transient responses of mooring line tensions and turbine performance will be evaluated. The novelty of this study is that the drift trajectories and power performances of OC4 DeepCwind semisubmersible floating offshore wind turbine under different conditions of environmental loads and broken mooring lines can be approximately estimated in the offshore wind farm. The analysis can be used for the environmental impact assessment in harsh sea conditions.

Estimation of top tensions in mooring lines by sensor fusion

The paper describes a sensor fusion method that provides reliable, uninterrupted and bias-free estimates of the top tension in a mooring line. The method exploits the geometric nonlinearity of mooring systems installed in shallow to moderate water depths: a change of line length (due to winching) affects the local dynamic stiffness of the mooring line. Based on measurements of fairlead displacements and of the dynamic part of the top tension, the line length and true (unbiased) mean tension can be inferred. The method combines the use of (1) a classical kinematic observer to derive fairlead motions, (2) the compression of the recent history of fairlead motions to a few parameters, (3) a bank of neural networks, each network modelling the response corresponding to a given line length/static tension, and (4) a heuristic approach to selecting the most promising model among the candidates. One major advantage of the method is its sparsity, making it computationally efficient so it can be applied both offline, on large sets of recorded historical data, and online running on lightweight embedded hardware. The paper presents in detail each component listed above, and the method as a whole is verified on a realistic case. Given that enough excitation is present, the estimator was found to converge towards the true value of the tension, and to cope well with transient conditions such as winching operations, and with the presence of oceanic current.

Structural Parametric Optimization of the VolturnUS-S Semi-Submersible Foundation for a 15 MW Floating Offshore Wind Turbine

The full exploitation of offshore wind resources can essentially satisfy the massive energy demand. The realization and application of ultra-high-power offshore wind turbines are crucial to achieving full use of deep-sea wind energy and reducing the cost of wind power. For the VolturnUS-S semi-submersible floating foundation of a 15 megawatt (MW) offshore wind turbine, the effect of structural parameters on hydrodynamic performance was investigated by controlling the variables described in this paper. Accordingly, the floating foundation was optimized and coupled to the 15 MW offshore wind turbine. The dynamic performance of the integrated 15 MW offshore wind turbine was analyzed under different operating conditions, by applying the aero-hydro-servo-elastic coupled method. The results show that for a wave in a 0-degree direction, a 5% increase of column spacing will reduce the peak value of the pitch transfer function by 33.61%, and that a 5% decrease of the outer column diameter will further reduce the peak value by 26.27%. The standard deviation of the time-domain surge responses was reduced by 19.78% for the optimized offshore wind turbine, and the maximum value of the mooring line tension was reduced by 13.55% under normal operating conditions.

A novel multi-dimensional reliability approach for floating wind turbines under power production conditions

Floating offshore wind turbines (FOWT) generate green renewable energy and are a vital part of the modern offshore wind energy industry. Robust predicting extreme offshore loads during FOWT operations is an important safety concern. Excessive structural bending moments may occur during certain sea conditions, posing an operational risk of structural damage. This paper uses the FAST code to analyze offshore wind turbine structural loads due to environmental loads acting on a specific FOWT under actual local environmental conditions. The work proposes a unique Gaidai-Fu-Xing structural reliability approach that is probably best suited for multi-dimensional structural responses that have been simulated or measured over a long period to produce relatively large ergodic time series. In the context of numerical simulation, unlike existing reliability approaches, the novel methodology does not need to re-start simulation again each time the system fails. As shown in this work, an accurate forecast of the probability of system failure can be made using measured structural response. Furthermore, traditional reliability techniques cannot effectively deal with large dimensionality systems and cross-correction across multiple dimensions. The paper aims to establish a state-of-the-art method for extracting essential information concerning extreme responses of the FOWT through simulated time-history data. Three key components of structural loads are analyzed, including the blade-root out-of-plane bending moment, tower fore-aft bending moment, and mooring line tension. The approach suggested in this study allows predicting failure probability efficiently for a non-linear multi-dimensional dynamic system as a whole.

Nonlinear mooring system for a ‘Sharp-Eagle’ wave energy converter

This numerical study investigated the effects caused by the UoE Tether shock absorber (in the following named UoE tether) on the motion and load characteristics of the 500 KW sharp-eagle WEC. Both 3-leg and 4-leg mooring configurations with different line combinations were selected in the share-eagle WEC project. The line combinations include a chain line and two hybrid mooring lines, i) a UoE Tether and ii) a nylon rope section. The UoE tether is a passive damper that has been proven in recent field demonstrations to reduce peak tensions in mooring lines during extreme environmental conditions. This paper discusses the line tension and motion characteristics of the WEC and compares the variations of mooring configurations and line combinations with respect to its capability to stabilise the device and reduce tension. Compared with the other two line combinations, the UoE tether could significantly reduce the peak tensions under the extreme wave conditions. Under the same mooring configuration, the UoE tether can reduce up to 31% and 67% peak tension. On the other hand, the UoE could allow a shorter mooring line design with a similar safety factor. Compared to the single Chain design, the length of the mooring line could be reduced by around 25%. It was also found that the UoE tether would not significantly influence the motion characteristics of the WEC, which means it would not affect the device installation characteristics. This work will be helpful for practitioners and researchers working in the wave energy industry, particularly decision-makers in mooring system design.

The influence of internal solitary wave on semi-submersible platform system including mooring line failure

As a primary catastrophic factor, an internal solitary wave (ISW) is typically characterized by a large amplitude and a long duration, which can cause severe damage to deep-sea platforms due to large drift or the rupture of mooring lines. In this paper, a secondary development of ANSYS AQWA was performed to study the dynamic response of the platform system under the combined action of surface wave load and ISW load. Then, mooring line failure under the ISW was analysed, and the differences between the simultaneous and progressive failure of the two mooring lines were compared. Numerical simulation results showed that under the action of the ISW, the movements of the platform and the tensions of mooring lines markedly increased. The low-frequency slow-drift motion under the combined action of the ISW and a regular wave was less than that under the action of the ISW. Mooring line failure worsened the movement of the platform and the mooring line section tension under the ISW. However, the transient responses of the platform caused by the progressive failure of two mooring lines were less than those caused by the simultaneous failure.

Experimental Study on Application of an Optical Sensor to Measure Mooring-Line Tension in Waves

Moored floating platforms have great potential in ocean engineering applications because a mooring system is necessary to keep the platform in station, which is directly related to the operational efficiency and safety of the platform. This paper briefly introduces the technical and operational details of an optical sensor for measuring the tension of mooring lines of a moored platform in waves. In order to check the performance of optical sensors, an experiment with a moored floating platform in waves is carried out in the wave tank at Changwon National University. The experiment is performed in regular waves and irregular waves with a semi-submersible and triangle platform. The performance of the optical sensor is confirmed by comparing the results of the tension of the mooring lines by the optical sensor and tension gauges. The maximum tension of the mooring lines is estimated to investigate the mooring dynamics due to the effect of the wave direction and wavelength in the regular waves. The significant value of the tension of mooring lines in various wave directions is estimated in the case of irregular waves. The results show that the optical sensor is effective in measuring the tension of the mooring lines.

Bivariate statistics of floating offshore wind turbine dynamic response under operational conditions

FOWT (Floating offshore wind turbines) belong to the modern offshore wind energy industry generating green renewable energy. Accurate extreme loads and response prediction during power generation is an important design concern. More accurate and reliable estimations of extreme responses are significant for the offshore wind industry as it advances the design, manufacturing and deployment of large FOWTs in the coming decade. In this study, the OpenFAST code was used to model offshore wind turbine mooring line tension force and blade bending moment due to environmental loads, acting on a site-specific FOWT under realistic local environmental conditions. This paper presents an efficient Monte Carlo based method to study bivariate extreme dynamic response statistics. The ACER2D (bivariate average conditional exceedance rate) method is briefly discussed. The ACER2D method enables robust estimation of bivariate statistics, efficiently utilizing available data. Large return period 2D (two-dimensional) probability contours were obtained using the ACER2D method. Based on the studied performance of the presented methodology, it was seen that ACER2D provides accurate and efficient predictions of extreme return period contours.The described approach may be utilized at the design stage, defining optimal FOWT design values to minimize potential structural damage due to extreme environmental loads. It should be noted that the bivariate design point is less conservative than the classic univariate one; therefore, this study advocates a design method leading to lower structural production costs.

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

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

Research on Mooring System Design for Kulluk Platform in Arctic Region

Mooring system design of a floating offshore structure in the arctic region is considered to be extremely important. This paper aims at investigating an optimal mooring system for the Kulluk platform operating in the Beaufort Sea, which has ice-free and ice-covered conditions during the whole year time. In order to complete the layout design of the mooring system to satisfy the year-round operation, both the effect of wave loads and ice loads should be considered. The research establishes a coupled numerical production system composed of the Kulluk platform and mooring system. Wave load is solved by potential flow theory. The slender finite element method is used to compute the tension of the mooring system. The nonlinear finite element method, discrete element method, and empirical formula are compared to analyze ice load. Finally, the discrete element method is selected for the analysis of the Kulluk, and the simulated results are compared reasonably with the field data. When studying the mooring line configurations, quantitative time-domain analysis is carried out, including tension of mooring lines and the motions of the platform under different working conditions. The research work in this paper will provide a reference for the optimal design of the mooring system of the platform operating in the Arctic Sea.

Evaluation of Dynamic Tensions of Single Point Mooring System under Random Waves with Artificial Neural Network

Real-time monitoring of the mooring safety of floating structures is of great significance to their production operations. A deep learning model is proposed here, based on the long short-term memory (LSTM) artificial neural network. Firstly, the numerical simulation is carried out with the single-point mooring system of a Floating Production Storage and Offloading (FPSO) as the training data of LSTM. Then the proposed LSTM is performed. Finally, taking the motion of FPSO which is not encountered by LSTM neural network model as input, we predict the mooring line tension with this model. Here, one FPSO in the South China Sea is taken as a research case, hydrodynamic and mooring models are established, and the network structure and hyper-parameters of the LSTM model are determined. The prediction results of the LSTM under different combinations of wind, wave, and current are compared with the calculation results of AQWA software. The model constructed here can well predict the mooring line tension of different combinations of wind, wave and current.

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.