Mooring Line Dynamics Research Articles

Addition of dynamic mooring line force based on lumped-mass method in SPH

Smoothed particle hydrodynamics (SPH) method is widely recognized as the accurate approach to predict the fluid-structure interaction. However, the lack of mooring force in the program severely limits its research on the motion response of floating structures. In this paper, based on the dynamic analysis theory of the lumped-mass method, combined simultaneously with the characteristics of SPH method, a dynamic mooring force calculation method suitable for SPH is proposed as follows: The mooring line is divided into a series of virtual particles, which contains the inherent attribute parameters of the mooring line, and embeds it in the SPH program. The force of each virtual particle is calculated and then coupled with the motion equation for the hull at each time step. The numerical method has been validated with the results of the flume test. Numerical simulations are examined so as to determine the hydrodynamic performance and interaction of breakwater with surfaces waves. Finally, the effects of the structural width and wave characteristics on wave attenuating effect are studied.

An open-source comprehensive numerical model for dynamic response and loads analysis of floating offshore wind turbines

This paper presents the development of a comprehensive open-source numerical model to study the dynamic response and load analysis of floating offshore wind turbines. The model accounts for the wind inflow, rotor aerodynamics, multibody structural model of the system, wave and current kinematics, hydrodynamics, and mooring-line dynamics. This coupled simulation tool can be used for analysis, optimization and preliminary design to determine the technical and economic feasibility. Several verification and validation cases are performed to show the correctness of the numerical simulations. The results show that the proposed approach provides an accurate estimate of the wind turbine dynamics and loads. The simulation tool is then applied in the analysis of a 5 MW wind turbine aimed to characterize the dynamic response and to identify potential loads and instabilities resulting from the dynamic couplings between the turbine and the external conditions. This open-source fully coupled aero-hydro-elastic model provides a modular framework to enable investigating a variety of wind turbine configurations, support systems, and mooring lines. Therefore, it is expected that researchers and design engineers worldwide use the model to study, investigate and analyze different aspects of floating offshore wind turbine design, which results is the promotion and advancement of science and technology for floating offshore wind turbines.

Effect of Water Depths on the Hydrodynamic Responses of an FPSO Platform

Ship-shaped Floating Production Storage Offloading platforms (FPSO) are commonly used in the production of oil and gas in offshore deepwater regions. The vessel is held in place by mooring lines anchored to the seabed during operation, either in spread or turret mooring arrangement. When designing such systems, water depth is a main factor that needs to be considered. At greater depths, the hydrodynamic properties of mooring lines become important and may not be accurately predicted through traditional experiments or numerical quasi-static models. Numerical simulation using coupled dynamic analysis is thus recommended, as the hull-mooring behaviour is analysed simultaneously, and the damping and added mass properties of the entire mooring line system is taken into account. This paper investigates the motions and mooring line tensions of a turret-moored FPSO at various water depths ranging from 1000 m to 2000 m. The analysis focuses on numerical simulations in the fully coupled dynamic time domain. The study utilizes the commercial software AQWA, with the FPSO model subjected to a unidirectional random wave condition. The hull hydrodynamics is first solved using the 3D radiation/diffraction panel method, and the hull response equation is then coupled with the mooring line equation. The dynamic motions and mooring line tensions results are presented in terms of statistical parameters as well as response spectrum. The results highlight the significance of greater water depths on low frequency responses in surge motions and mooring line tensions, and provides insight on the increasing and decreasing trend of these responses.

A comparison of two fully coupled codes for integrated dynamic analysis of floating vertical axis wind turbines

This paper presents a comparison of two state-of-the-art codes that are capable of modelling floating vertical axis wind turbines (VAWTs) in fully coupled time-domain simulations, being the HAWC2 by DTU and the SIMO-RIFLEX-AC code by NTNU/MARINTEK. The comparative study focusses on the way aerodynamics, hydrodynamics and structural dynamics are treated for DeepWind’s 5MW Darrieus rotor mounted on a modified OC3 spar platform. The relevant modelling differences are described, followed by an introduction to the spar VAWT concept and selected load cases. Isolation of the aerodynamic model is achieved using an equivalent rigid land-based VAWT in steady wind-only environments. The added complexity in SIMO-RIFLEX-AC’s aerodynamic model has shown to increase aerodynamic torque at tip-speed ratios above 2.5. Differences in the hydrodynamic and structural models were brought forward through fully coupled analyses in turbulent wind and irregular wave climates. It is found that the simplified mooring system in HAWC2 introduces a 2P yaw response (1P in SIMO-RIFLEX-AC), stronger motion coupling in surge-heave and a largely reduced mooring line tension since the dynamics of mooring lines are not considered. Indications are given that a higher tower mode is excited by 4P aerodynamic loading; an effect that is significantly stronger in HAWC2.

Study on Design of Truncated Mooring Line with Static Similarity in Model Test Basins

In this study, a series of numerical simulations was conducted in order to design a truncated mooring line with a static similarity to the prototype. A finite element method based on minimizing the potential energy was utilized to describe the dynamics of mooring lines. The prototype mooring lines considered were installed at a water depth of 1,000 m, whereas the KRISO ocean engineering basin (OEB) in Daejeon has a water depth of 3.2 m, which represents 192 m using a scaling of 1:60. First, an investigation for the design of the truncated mooring line was carried out to match the static characteristics of the KRISO Daejeon OEB environment. Then, the same procedure was performed with the KRISO new deepwater ocean engineering basin (DOEB) that is under construction in Busan. This new facility has a water depth of 15 m, which reflects a real scale depth of 900 m considering the 1:60 scaling factor. A finite element method was used to model the mooring line dynamics. It was found that the targeted truncated mooring line could not be designed under the circumstances of the KRISO OEB with any material properties, whereas several mooring lines were easily matched to the prototype under the circumstances of the KRISO DOEB.

Establishing a fully coupled CFD analysis tool for floating offshore wind turbines

An accurate study of a floating offshore wind turbine (FOWT) system requires interdisciplinary knowledge about wind turbine aerodynamics, floating platform hydrodynamics and mooring line dynamics, as well as interaction between these discipline areas. Computational Fluid Dynamics (CFD) provides a new means of analysing a fully coupled fluid-structure interaction (FSI) system in a detailed manner. In this paper, a numerical tool based on the open source CFD toolbox OpenFOAM for application to FOWTs will be described. Various benchmark cases are first modelled to demonstrate the capability of the tool. The OC4 DeepCWind semi-submersible FOWT model is then investigated under different operating conditions.With this tool, the effects of the dynamic motions of the floating platform on the wind turbine aerodynamic performance and the impact of the wind turbine aerodynamics on the behaviour of the floating platform and on the mooring system responses are examined. The present results provide quantitative information of three-dimensional FSI that may complement related experimental studies. In addition, CFD modelling enables the detailed quantitative analysis of the wind turbine flow field, the pressure distribution along blades and their effects on the wind turbine aerodynamics and the hydrodynamics of the floating structure, which is difficult to carry out experimentally.

Dynamic response analysis of the equivalent water depth truncated point of the catenary mooring line

The real-time computer-controlled actuators are used to connect the truncated parts of moorings and risers in the active hybrid model testing system. This must be able to work in model-scale real time, based on feedback input from the floater motions. Thus, mooring line dynamics and damping effects are artificially simulated in real time, based on a computer-based model of the problem. In consideration of the nonlinear characteristics of the sea platform catenary mooring line, the equations of the mooring line motion are formulated by using the lumped-mass method and the dynamic response of several points on the mooring line is investigated by the time and frequency domain analysis method. The dynamic response of the representative point on the mooring line is analyzed under the condition of two different corresponding upper endpoint movements namely sine wave excitation and random wave excitation. The corresponding laws of the dynamic response between the equivalent water depth truncated points at different locations and the upper endpoint are obtained, which can provide technical support for further study of the active hybrid model test.

Structural Dynamic Analysis of Semi-Submersible Floating Vertical Axis Wind Turbines

The strong and stable wind at offshore locations and the increasing demand for energy have made the application of wind turbines in deeper water surge. A novel concept of a 5 MW baseline Floating Vertical Axis Wind Turbine (FVAWT) and a 5 MW optimised FVAWT with the DeepWind Darrieus rotor and the optimised DeepWind Darrieus rotor, respectively, were studied extensively. The structural responses, fatigue damages, platform global motions and mooring line dynamics of the FVAWTs were investigated comprehensively during normal operating conditions under steady wind and turbulent wind conditions, using a coupled non-linear aero-hydro-servo-elastic code (the Simo-Riflex-DMS code) which was developed by Wang et al. for modeling FVAWTs. This coupled code incorporates the models for the turbulent wind field, aerodynamics, hydrodynamics, structural dynamics, and generator controller. The simulation is performed in a fully coupled manner in time domain. The comparison of responses under different wind conditions were used to demonstrate the effect of turbulence on both FVAWTs dynamic responses. The turbulent wind condition has the advantage of reducing the 2P effects. Furthermore, comparative studies of the FVAWTs responses were undertaken to explore the advantages of adopting the optimised 5 MW DeepWind Darrieus rotor over the baseline model. The results identified the 5 MW optimised FVAWT to having: lower Fore-Aft (FA) but higher lower Side-Side (SS) bending moments of structural components; lower motions amplitude; lower short-term fatigue equivalent loads and a further reduced 2P effects.

Experimental study on the hydrodynamic behaviour of an FPSO in a deepwater region of the Gulf of Mexico

As offshore oil and gas exploration moves progressively toward greater water depths, it becomes more challenging to predict the environmental forces and global responses of floating production storage and offloading (FPSO) systems and the dynamic behaviour of the mooring lines and risers. The validation of complex numerical models through scale model experimental testing is restricted by the physical limits of the test facilities. It is not feasible to install the equivalent full length mooring lines and riser systems and select an appropriate scale model for reducing the uncertainties in the experimental test programme for deepwater and ultra-deepwater conditions. The combination of an appropriate scale FPSO model with a suitable level of equivalent effect reduced depth using a hybrid passive truncated experimental methodology for the mooring lines and risers is a practical approach.Following recent discoveries, FPSO has been proposed for a portion of the planned development in the southern Gulf of Mexico (GOM) in water depth ranging from 1000 to 2000m. Based on a scale model and a hybrid passive truncated experimental method for mooring lines and risers, this paper investigates the global response of an FPSO, as well as the dynamics of mooring lines and risers in the context of prevailing environmental conditions for field development in a specific deepwater location in GOM. The experiments revealed that the main horizontal motion response of the FPSO (surge) under non-collinear loading condition is almost two-times that of the collinear loading condition. The mooring lines in the non-collinear condition are more sensitive to the dynamic response and risers appear to have an important influence on the low frequency damping.

Impacts of moving catenary mooring-lines on the dynamic response of floating wind turbine

The dynamic response of a floating wind turbine with 5-megawatt power and large size undergoing different ocean waves is examined by the numerical simulations based on a modified approach which combines the 3D flexible catenary theory with the finite element method. By use of our modified approach, the additional mooring dynamic behaviors including the structural inertial effect and fluid drag, compared to the previous quasi-static model, are involved in this study; what’s more, the integrated system of which the dynamic interaction between the flexible components such as blades, tower, SPAR platform and catenary mooring system is considered too. The influences of catenary mooring-line inertia and fluid damping force on tension and motion characteristics of catenary mooring system are presented, and the impact of mooring-line dynamics on the integrated system of wind turbine response is studied and compared with the quasi-static method which only includes the static restoring force of the catenary. Our numerical results show that the dynamic characteristics of mooring-line may significantly increase the top tension, particularly, the peak-trough tension difference of snap tension may be more than 10 times larger than the quasi-static result owing to the occurrence of taut-slack during the dynamic response on certain situations. When the wave frequency is much higher than the system, the dynamic effects of the mooring system will accelerate the decay of transient items of the dynamic response; when the wave frequency and the system frequency are close to each other, the dynamic characteristics of mooring systems can significantly reduce the response displacement of the floating SPAR, i.e. the amplitude is reduced by 20%.

Frequency domain approach for the coupled analysis of floating wind turbine system

ABSTRACTThis paper presents a simple frequency domain approach to predict the coupled response of floating wind turbine based on AQWA, and then investigates the effect of damping induced by wind, current and mooring system on the floating foundation response. This approach is validated against the experiment of DeepCwind floating offshore wind turbine in literatures. The results show reasonable agreement, and also indicate that the drift damping should be elaborated for the drift response. Based on this approach, the effect of wind and current on the floating wind turbine response is parametrically investigated. These aspects have rare effect on the wave frequency response, but affect the drift response due to the introduction of damping. The comparison between quasi-static and dynamic mooring line models reveals that the dynamics and added mass of mooring line play negligible roles, but the drag force significantly damps down the surge drift response.

The Dynamic Coupling Effects of a MUFOWT (Multiple Unit Floating Offshore Wind Turbine) with Partially Broken Blade

In this study, an advanced numerical simulation tool has been developed for the rotor-floater-mooring coupled dynamic analysis of a Multiple Unit Floating Offshore Wind Turbine (MUFOWT) in the time domain including aero-blade-multiple tower dynamics and control, mooring dynamics, and platform motion. In particular, the developed numerical tool is extended from the single turbine analysis tool FAST, which has been developed by the National Renewable Energy Laboratory (NREL) for years. For the linear and nonlinear hydrodynamics of a floating platform and the generalized coordinate-based Finite Element Method (FEM) mooring line dynamics, CHARM3D is used. Therefore, the entire dynamics of a floating offshore wind turbine were obtained by coupling FAST-CHARM3D in the time domain, which has been further extended to include additional coupled dynamics among multiple turbines. The global coefficient matrix, including one floating platform and a number of turbines, is built at each time step of the simulation and is solved simultaneously to obtain the responses of the entire degrees of freedom (DOFs) of the MUFOWT system. To investigate the dynamic coupling effects between the platform and turbines, a five-turbine semisubmersible is modeled and simulated. Then 1P excitation from one turbine is intentionally generated by the partial breaking of a blade, and the loads transferred to the platform and other turbines are investigated. The analysis shows that the dynamic load imbalance of one turbine in the MUFOWT may induce appreciable changes in the performance of the other turbines or the floating platform.

Verification of a hybrid model test method for a deep water floating system with large truncation factor

The hybrid model test method has been regarded as the most reliable method to investigate the dynamics of deepwater floating systems when the wave basin is not large enough to conduct the full water depth model tests at reasonable model scales. In order to get the reliable experimental results, the truncation factor should be less than 5. However, as the offshore oil and gas industry moves towards deeper waters, the model tests of floating structures carried out at the wave basin with reasonable model scales need to be revamped. The objective of the present study is to investigate the feasibility of largely truncated hybrid model test for the deep water floating systems. The hybrid model test techniques and the dynamic behaviors of the truncated mooring lines are also studied. In order to achieve the static and dynamic similarities easily in the largely-truncated model tests, an optimization design method of the truncated mooring system is proposed, and the similarities of platform motion responses as well as the mooring lines dynamics are investigated in the largely-truncated model tests. To verify the present method, a deepwater semisubmersible system operated in 1500m and 1000m water depth is truncated to 200m water depth based on the static equivalent principle (the truncation factors are 7.5 and 5 respectively), and the dynamically similar design is implemented to the model tests which are carried out at China Ship Scientific Research Center wave basin. The motion responses of the platform and the mooring line dynamics are obtained. These experimental results are used to compare with the numerical simulation, which is based on the full time domain coupled dynamic analysis method. The comparisons show that the numerical results are in good agreement with the model measurements.

Coupled dynamic analysis of multiple wind turbines on a large single floater

The present study has developed a numerical simulation tool for the coupled dynamic analysis of multiple turbines on a single floater (or Multiple Unit Floating Offshore Wind Turbine (MUFOWT)) in the time domain including multiple-turbine aero-blade-tower dynamics and control, mooring dynamics, and platform motions. The numerical tool developed in this study was designed based on and extended from the single-turbine analysis tool FAST to be suitable for multiple turbines. For hydrodynamic loadings of floating platform and mooring-line dynamics, the CHARM3D program developed by the authors was incorporated. Thus, the coupled dynamic behavior of a floating base with multiple turbines and mooring lines can be simulated in the time domain. The developed MUFOWT analysis tool can compute any type of floating platform with multiple horizontal-axis wind turbines (HAWT). To investigate the dynamic coupling effect between platform and each turbine, one turbine failure case with a fully broken blade was simulated and checked. The aerodynamic interference between adjacent turbines, including wake effect, was not considered in this study to more clearly demonstrate only the dynamic coupling. The analysis shows that some damage-induced excitations from one turbine in MUFOWT may induce appreciable changes in the performance of other turbines or the floating platform.

Dynamics of large-truncated mooring systems coupled with a catenary moored semi-submersible

With the floating structures pushing their activities to the ultra-deep water, model tests have presented a challenge due to the limitation of the existing wave basins. Therefore, the concept of truncated mooring system is implemented to replace the full depth mooring system in the model tests, which aims to have the same dynamic responses as the full depth system. The truncated mooring system plays such a significant role that extra attention should be paid to the mooring systems with large truncation factor. Three different types of large truncation factor mooring system are being employed in the simulations, including the homogenously truncated mooring system, non-homogenously truncated mooring system and simplified truncated mooring system. A catenary moored semi-submersible operating at 1000 m water depth is presented. In addition, truncated mooring systems are proposed at the truncated water depth of 200 m. In order to explore the applicability of these truncated mooring systems, numerical simulations of the platform’s surge free decay interacting with three different styles of truncated mooring systems are studied in calm water. Furthermore, the mooring-induced damping of the truncated mooring systems is simulated in the regular wave. Finally, the platform motion responses and mooring line dynamics are simulated in irregular wave. All these simulations are implemented by employing full time domain coupled dynamic analysis, and the results are compared with those of the full depth simulations in the same cases. The results show that the mooring-induced damping plays a significant role in platform motion responses, and all truncated mooring systems are suitable for model tests with appropriate truncated mooring line diameters. However, a large diameter is needed for simplified truncated mooring lines. The suggestions are given to the selection of truncated mooring system for different situations as well as to the truncated mooring design criteria.

Structural Reliability Based Dynamic Positioning of Turret-Moored FPSOs in Extreme Seas

FPSO is widely used during the deep-sea oil and gas exploration operations, for which it is an effective way to keep their position by means of positioning mooring (PM) technology to ensure the long-term reliability of operations, even in extreme seas. Here, a kind of dynamic positioning (DP) controller in terms of structural reliability is presented for the single-point turret-moored FPSOs. Firstly, the mathematical model of the moored FPSO in terms of kinematics and dynamics is established. Secondly, the catenary method is applied to analyze the mooring line dynamics, and mathematical model of one single mooring line is set up based on the catenary equation. Thereafter, mathematical model for the whole turret mooring system is established. Thirdly, a structural reliability index is defined to evaluate the breaking strength of each mooring line. At the same time, control constraints are also considered to design a state feedback controller using the backstepping technique. Finally, a series of simulation tests are carried out for a certain turret-moored FPSO with eight mooring lines. It is shown in the simulation results that the moored FPSO can keep its position well in extreme seas. Besides, the FPSO mooring line tension is reduced effectively to ensure mooring lines safety to a large extent in harsh sea environment.

Evaluating the importance of mooring line model fidelity in floating offshore wind turbine simulations

Accurate computer modelling is critical in achieving cost-effective floating offshore wind turbine designs. Although a range of modelling fidelities are available for all parts of the simulation, a lower-fidelity quasi-static approach that neglects inertia and hydrodynamics is often used for the mooring line model. The loss of accuracy from using this approach has not been thoroughly studied across different support structure designs. To test the adequacy of this widely used simplified mooring line modelling approach, the floating wind turbine simulator FAST (National Renewable Energy Laboratory, Golden, Colorado) was modified to allow the use of a high-fidelity dynamic mooring line model, ProteusDS (Dynamic Systems Analysis Inc. of Victoria, BC, Canada). Three standard floating wind turbine designs were implemented in this new simulator arrangement and tested using a set of steady and stochastic wind and wave conditions. The static equivalence between the built-in quasi-static mooring model and the dynamic mooring model is within 0.6% in terms of fairlead tension. Tests of the systems’ responses in still water indicate that the hydrodynamic damping of the mooring lines can constitute anywhere from 1% to 35% of the overall system damping in pitch, depending on the design. Tests in steady and stochastic operating conditions show that for very stable designs with slack moorings, or designs with taut moorings, a quasi-static mooring model can in many conditions predict the platform motions and turbine loads with reasonable accuracy. For slack-moored designs with larger platform motions, however, a quasi-static model can lead to inaccuracies of as much as 30% in the damage-equivalent and extreme loads on the turbine. An important observation is that even in situations where the platform response is predicted reasonably well by a quasi-static model, larger inaccuracies can arise in the response of the rotor blades. These inaccuracies are more severe in the time series (with instantaneous discrepancies as high as 50% of the mean load) than in the corresponding damage-equivalent and extreme loads calculated over multiple stochastic simulations. Consequently, differences in damage-equivalent and extreme load metrics should be considered a floor to the measure of inaccuracy caused by a quasi-static mooring model. Copyright © 2013 John Wiley & Sons, Ltd.

Application of the Contour Line Method for Estimating Extreme Responses in the Mooring Lines of a Two-Body Floating Wave Energy Converter

The ultimate limit state (ULS) is one of the design criteria used in the oil and gas industry in mooring system design for floating platforms. The 100 year level response in the mooring line should be applied for the ULS design check, which is ideally estimated by taking into account the dynamic mooring line tension in all sea states available at the operational site. This approach is known as a full long-term response analysis using the all-sea-state approach. However, this approach is time consuming, and therefore, the contour line method is proposed for estimation of the 100 year response by primarily studying the short-term response for the most unfavorable sea states along the 100 year environmental contour line. Experience in the oil and gas industry confirmed that this method could yield good predictions if the responses at higher percentiles than the median are used. In this paper, the mooring system of a two-body wave energy converter (WEC) is considered. Because this system involves the interaction between two bodies, the estimation of the ULS level response using the all-sea-state approach may be even more time consuming. Therefore, application of the contour line method for this case will certainly be beneficial. However, its feasibility for application to a WEC case must be documented first. In the present paper, the ULS level response in the mooring tension predicted by the contour line method is compared to that estimated by taking into account all sea states. This prediction is achieved by performing coupled time domain mooring analyses using Simo/Riflex for six cases with different mooring configurations and connections between two bodies. An axisymmetric Wavebob-type WEC is chosen for investigation, and the Yeu site in France is assumed as the operational site. Hydrodynamic loads including second-order forces are determined using Wamit. Finally, the applicability of the contour line method for prediction of the ULS level mooring tension for a two-body WEC is assessed and shown to yield accurate results with the proper choice of percentile level for the extreme response.

NUMERICAL INVESTIGATION ON THE EFFECTS OF WAVE GROUPING ON THE MOTION OF MOORED DDMS PLATFORM

The random wave groups with the same wave parameters, such as significant wave height, period and overshoot parameter, but with different wave groupiness are simulated by an empirical wave envelope spectrum involved the group height factor GFH and group length factor GLF based on field measured sea waves. A geometrically nonlinear finite element method based on the total Lagrangian formulation is developed to calculate the mooring-line dynamics. Coupled dynamic analysis of DDMS (Deep Draft Multi-Spar) platform and the attached mooring lines under the action of wave groups with different groupiness in deep water is executed in time domain. The effects of groupiness parameters on wave surfaces, motions of DDMS and tensions in the mooring lines are detailed in this paper.

Dynamic response analysis of ddms platform subjected to actions of wave groups and current fources

Coupled dynamic analysis of the Deep Draft Multi-Spar (DDMS) platform and the mooring system under the action of waves and current is carried out in the time domain. Using a geometrically nonlinear finite element method, the mooring-line dynamics is simulated based on the total Lagrangian formulation. Wave groups are obtained by the JONSWAP spectrum and an empirical wave envelope spectrum involving two envelope-based factors Group Height Factor (GFH) and Group Length Factor (GLF). The results show that the wave groups have a significant effect on the motion responses of the platform and the mooring line tensions.