Dynamic Mooring Research Articles

A simulation technique for monitoring the real-time stress responses of various mooring configurations for offshore floating wind turbines

The concept of monitoring mooring stress response in real-time is not new, but its direct implementation in offshore wind energy activities is still a challenge. The study aims to present a simulation technique for controlling the hydrodynamic forces acting on the finite element mooring lines depending on a user-subroutine of an explicit finite element software. By controlling the mooring forces based on mooring nodal displacement in real-time, the mooring stress response is possible to be calculated using the finite element method. We introduced the theoretical background to develop the present method. In order to validate the present method, the five typical mooring configurations proposed by international standards were prepared: Catenary, Semi-taut, Taut, Lazy-S configuration type, and Catenary mooring with the inclined seabed type, respectively. The present method predicted the stress distributions along the mooring line where the upper end was excited by a typical wave frequency motion and the other end was anchored to the seabed. And the experimental and numerical results of five typical mooring configurations were compared to the results calculated from the present method. A good agreement was drawn from the comparative study of both quasi-static and dynamic mooring responses. The validation of the present method is helpful for further investigating the effects of the fluid loads, the mooring geometrical, and material nonlinearity on the real-time structural responses of mooring lines in service life.

Verification and Validation of Computational Fluid Dynamic Simulations of a FOWT Semi-Submersible Under Bichromatic and Random Waves

AbstractIn the present work, an extensive verification and validation study is performed to evaluate the accuracy and credibility for computational fluid dynamic (CFD) simulations of the hydrodynamic responses of a semi-submersible floating offshore wind turbine (FOWT) platform under bichromatic waves and random waves. A dynamic mooring model is coupled with the CFD code to accurately simulate the mooring system. For the bichromatic wave cases, the surge, heave, and pitch response amplitude operators (RAOs) at wave frequencies, mean surge offset and mean surge force of the semi-submersible platform are investigated. The numerical uncertainties of the above metrics are quantified, which are primarily sourced from the discretization uncertainty. For the random wave cases, the surge, heave, and pitch power spectral density (PSD) sums in wave frequency range and low frequency range are validated against the experimental results. The numerical uncertainty derived from the bichromatic wave cases is applied in the validation of the random wave cases. The PSD sums in wave frequency range have achieved the validation within the validation uncertainty. Though the PSD sums in low-frequency range are under-predicted, the results with the utilization of the CFD code agree more with the experimental value than the results from mid-fidelity tools.

Analysis of a Moored and Articulated Multibody Offshore System in Steep Waves

Abstract Wave-induced motions and loads on a moored and articulated multibody offshore structure are numerically analyzed, where a coupled mooring–joint–viscous flow solver is used to count for mooring dynamics, joint restrictions, nonlinear rigid body motions, and viscous flow effects. The considered concepts consist of two modular floating structures (MFSs) connected by two types of connections, namely, a rigid joint and a flexible joint, and positioned by four symmetrical catenary mooring lines. The analyzed responses comprised multibody motions as well as associated forces acting in the hinged joints and the mooring lines. Results indicate that surge motions of articulated bodies are almost identical to each other, whereas the effects of the joint on heave motions are not pronounced. However, highly dynamic pitch motions between two hinged MFSs are observed. Apart from motion responses, forces acting on the hinged joint and the mooring lines are estimated. The importance of wave nonlinearity and higher order components is identified by studying waves with different steepness. The coupled mooring–joint–viscous flow solver demonstrates its capability to predict wave-induced motions and loads on a moored multibody offshore structure articulated by various types of joints.

Experimental investigation on the mooring dynamics of the flexible trash intercept net under wave-current combined actions

Experimental investigation on the mooring dynamics of the flexible trash intercept net under wave-current combined actions

A hybrid numerical model for simulating aero-elastic-hydro-mooring-wake dynamic responses of floating offshore wind turbine

It demands many computational resources to model the coupled responses of a floating offshore wind turbine (FOWT), especially for its aero-elastic-hydro-mooring-wake dynamics and their interaction. In this paper, a new hybrid numerical model for FOWT systems is developed, which is based on the hybrid potential-viscous flow model called qaleFOAM. In this model, the aerodynamics of wind turbine are solved by the unsteady actuator line method (UALM); the elastic responses of the turbine blade are calculated by the Legendre spectral finite element model (BeamDyn); the hydrodynamics of the floating platform are dealt with by the combination of the fully nonlinear potential solver and a two-phase Navier–Stokes solver; the mooring dynamics are considered with the Lumped Mass Mooring Model (MoorDyn), and the turbine wake is solved with the large eddy simulation (LES) model. This newly formulated model can deal with wind, wave, mooring dynamics, platform motions, and turbine structural dynamics involved in the FOWT system. To demonstrate the capability of the present model, various cases with different complexities are investigated and compared with the experimental data and other numerical results. Then, the model is applied to simulation of a semi-submersible FOWT system, subjected to a regular wave and a uniform wind. The prediction of the aerodynamic performance, blade tip deflection, platform motion responses, and mooring line tension loads show good agreements with the results from other methods. In addition, the phenomenon of the coupled effects between the dynamic responses of platform, blade deformation and wake flow are captured reasonably well.

High-fidelity modelling of moored marine structures: multi-component simulations and fluid-mooring coupling

High-fidelity viscous computational fluid dynamics (CFD) models coupled to dynamic mooring models is becoming an established tool for marine wave-body-mooring (WBM) interaction problems. The CFD and the mooring solvers most often communicate by exchanging positions and mooring forces at the mooring fairleads. Mooring components such as submerged buoys and clump weights are usually not resolved in the CFD model, but are treated as Morison-type bodies. This paper presents two recent developments in high-fidelity WBM modelling: (i) a one-way fluid-mooring coupling that samples the CFD fluid kinematics to approximate drag and inertia forces in the mooring model; and (ii) support for inter-moored multibody simulations that can resolve fluid dynamics on a mooring component level. The developments are made in the high-order discontinuous Galerkin mooring solver MoodyCore, and in the two-phase incompressible Navier–Stokes finite volume solver OpenFOAM. The fluid-mooring coupling is verified with experimental tests of a mooring cable in steady current. It is also used to model the response of the slack-moored DeepCwind FOWT exposed to regular waves. Minor effects of fluid-mooring coupling were noted, as expected since this a mild wave case. The inter-mooring development is demonstrated on a point-absorbing WEC moored with a hybrid mooring system, fully resolved in CFD-MoodyCore. The WEC (including a quasi-linear PTO) and the submerged buoys are resolved in CFD, while the mooring dynamics include inter-mooring effects and the one-way sampling of the flow. The combined wave-body-mooring model is judged to be very complete and to cover most of the relevant effects for marine WBM problems.

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.

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.

Extension of a coupled mooring–viscous flow solver to account for mooring–joint–multibody interaction in waves

To account for nonlinear wave–structure interaction, mooring dynamics and the associated viscous flow effects, a coupled mooring–viscous flow solver was formerly developed and validated (Jiang et al. in Mar Struct 72:783, 2020a, Validation of a dynamic mooring model coupled with a RANS solver). This paper presents an extension of the coupled mooring–viscous flow solver to solve mooring dynamics interacting with an articulated multibody offshore system. The presently extended solver is verified by comparing the predicted motions of and loads on a moored floating box to those obtained from the formerly validated solver, which was aimed for solving mooring dynamics interacting with a single floating body. The almost identical results obtained from both solvers verify the presently developed multi-module coupling technique for solving the mooring dynamics and articulated multibody dynamics in a coupled manner. Apart from the code comparison and verification, the numerical predictions are also validated against experimental tank measurements both for a single body and an articulated multibody. The good agreements between the numerical predictions and the experimental measurements validate the presently extended solver, where wave-induced body motions together with loads acting on mooring lines and joint connections were examined. Developed as an open-source tool, the extended solver shows a potential of the coupled methodology for analyzing an articulated multibody offshore system, moored with various mooring configurations in extreme sea states, which goes beyond the state of the art.

Numerical coupling strategy using HOS-OpenFOAM-MoorDyn for OC3 Hywind SPAR type platform

The coupled (Potential theory and Navier–Stokes) solver is applied to simulate the interaction of sea waves with substructure of floating offshore wind turbines (FOWT) platform, notably similar to the OC3 Hywind SPAR structure. The intention is to develop a numerical tool that allows the study of the survivability of floating structures in extreme sea states. In this study, the moorings are modeled in two ways. One is by considering the mooring lines as a linear spring with defined spring stiffness, and another is by coupling the solver (foamStar) with a lumped-mass mooring dynamics model (MoorDyn). MoorDyn represents mooring line behavior subject to axial elasticity, hydrodynamic forces, and vertical contact forces with the seabed. The coupled model has been validated against the experiments carried out as part of the SOFTWIND project. The numerical model results of free surface elevation, floating body motions and mooring tensions are compared with the experiments. For wave cases with mild and moderate amplitudes, mooring in the form of a stiffness matrix is sufficient. However, dynamic mooring simulation (MoorDyn) is required for the extreme sea state conditions.

A semiparametric Bayesian method with birth-death Markov Chain Monte Carlo algorithm for extreme mooring tension analysis

A series of 1: 50 model tests have been conducted to study the dynamics of a taut mooring system and two hybrid mooring systems attached to a deep-water semi-submersible in the 100-year wave condition of the South China Sea. Three typical environmental conditions were tested, including head sea, beam sea and quartering sea. The mixture Gamma- Generalized Pareto distribution (GPD) model is applied to study short term extreme dynamic mooring tensions based on the measured data, the results are compared with the estimations from mixture Gamma and two-parameter Weibull models. A birth-death Markov chain Monte Carlo (MCMC) sampling procedure is developed to fit the parameters of a mixture Gamma- GPD via Bayesian inference, and an extensive discussion of the de-clustering procedure, parameters of the prior distribution for threshold and MCMC iteration steps on Bayesian inference are carried out. It is found that the Weibull model underestimates the most probable largest dynamic mooring tensions. On the contrary, the mixture Gamma model overestimates the extreme dynamic mooring tension significantly. The mixture Gamma- GPD can present good estimations of extreme dynamic mooring tension, and its performance is robust to the sample definition, MCMC iteration steps and parameters of threshold prior distribution.

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

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

Hydrodynamic response of a FOWT semi-submersible under regular waves using CFD: Verification and validation

In this work an extensive verification and validation study is performed to evaluate the accuracy and credibility for CFD simulations of hydrodynamic response of a Floating Offshore Wind Turbine (FOWT) with semi-submersible platform under regular waves. A dynamic mooring model is coupled with the CFD code to accurately simulate the mooring system. The surge, heave and pitch RAOs at wave frequency and double wave frequency, mean surge offset and zero frequency surge QTF of the semi-submersible platform are investigated. The numerical uncertainties of the above metrics are quantified based on three sources - statistical, iterative and discretization uncertainty. Amongst them, the discretization uncertainty is identified as the dominant source of numerical uncertainty. The validation uncertainty is finally estimated combining the numerical, input and experimental uncertainties. Most of the metrics have achieved the validation against the experimental measurement within the validation uncertainty except for the mean surge offset and zero frequency surge QTF. The utilization of the coupled CFD-FEM code is proved to be accurate for simulations of FOWT under regular waves. Guidelines for best-practices of the procedure for verification and validation study of CFD simulations of floating structures under regular waves are also given.

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

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

Experimental 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.

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.

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

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

A 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.

Numerical Study on the Mooring Force in an Offshore Fish Cage Array

The mooring force in a fish cage array subjected to currents and waves is investigated using the finite element method. Firstly, the numerical model of a fish cage array with six gravity cages is built by Ansys/APDL. Collars and bottom rings are simulated with pipe and beam elements while the rest structure is simulated with link elements, including the net and mooring cables. Thus, the weight and hydrodynamic load on the cables can be considered. The initial shape of the mooring ropes is calculated based on mooring dynamics. Since each component is a slender structure in the cage array, the Morison equation is used to calculate the hydrodynamic load. Secondly, the mooring forces are assessed for the system in different sea states. The locations of the maximum mooring force on different parts in the mooring system are found. The mean values and amplitudes of maximum mooring forces on different parts are calculated. The main ropes have the maximum mooring forces under all sea states. The mean values of the maximum mooring forces increase with the current velocity and wave height. When the attack angle is 0° and 90°, the two adjacent bridle ropes do not play the role of pulling the cage together. One is pulled tight and the other one is slack.

Physical model test validation of dynamic similarity truncation method for an internal turret mooring system

The mooring system needs to be truncated in physical model test to adapt to the limited dimensions of offshore basins. Especially, the drag force and inertial force are required to be included in the dynamic mooring truncation methods, and the method should be verified by model test comparison. Here an effective dynamic truncation method is proposed based on slender rod theory and multi-objective cuckoo search optimization algorithm. The line tensions in time domain are transferred into frequency domain to implement the dynamic similarity. The proposed process can promote optimization efficiency in a great extent. In order to demonstrate the accuracy of the proposed method, a horizontal truncation is carried out to an internal turret moored FPSO working in 420 m water depth. The model test in deep-water basin is performed with static tests and irregular wave tests. It can be found that model test and numerical results show consistence on the static stiffness characteristics and dynamic response. The comparison verifies the reliability and accuracy of the proposed truncation method well.