Mooring Line Tension Research Articles

Dynamic response of floating offshore renewable energy devices: Sensitivity to mooring rope stiffness

The offshore renewable energy sector has seen a rise in floating devices, all of which require mooring and anchoring systems. Synthetic ropes have emerged as a promising technology for cost reduction in this system. However, characterising the behaviour of these materials, which exhibit complex non-linear, visco- elastic and plastic structural properties, presents challenges. Numerical modelling and tank testing are the available tools for developers to overcome these challenges, however, there is a lack of guidelines for test facilities regarding the design of tank-scale mooring systems. The present work focuses on the numerical design of a typical semi-taut mooring system using synthetic materials suitable for future-generation floating offshore wind turbines. A coupled time-domain hydrodynamic model was employed to explore the dynamic sensitivity of the device to changes in mooring rope stiffness. The results demonstrate that changes in line axial stiffness have a greater impact on platform surge and mooring line tension than on heave and pitch responses. These findings establish preliminary margins for target stiffness values, which are valuable for selecting mooring materials for scaled tank test models. Although the case study was floating wind, the results have broader applicability to wider floating marine energy device design.

Dynamic analysis of a 5 MW Barge-type FOWT with two-mooring failure of wind-wave misalignment scenarios

A floating offshore wind turbine (FOWT) experiencing two mooring failure can substantially escalate the risk of persistent damage to the remaining moorings. Industry standards mandate that FOWTs must demonstrate the capability of maintaining self-sustainability even in the occurrence of dual mooring failure. Therefore, it is importance to investigate the dynamic response of wind turbines in the case of two mooring line failure. In this study, a novel coupling method of FAST and AQWA was used to analyze the dynamic response of a 5 MW FOWT installed on a barge platform when two mooring lines fail at the same corner. The time-domain response processes of rated and extreme sea conditions were simulated for different failure time intervals (0s, 60s, and 1500s), and the dynamic response of the platform and the change of mooring line tension at each stage are regularly explored. The results indicate that a shorter interval between two mooring line failures results in a more unstable FOWT state. The center of gravity of the wind turbine presents an irregular spiral trajectory, and the platform response exhibits varying characteristics at each stage of the failure process. The transient effect is smaller as the wind turbine approaches the motion limit state.

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

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

Deep gated recurrent unit networks for time-domain long-term fatigue analysis of mooring lines considering wave directionality

Mooring lines are a crucial component of offshore mooring systems, and long-term fatigue damage assessment is a challenging task in mooring line design and structural health monitoring. Traditional methods require a large number of structural time-domain simulations. Because the sea state varies over time, the variations in wave parameters (e.g., wave heights, periods, and directions) must be considered in long-term fatigue damage assessment. Wave directions have usually been ignored in previous studies, while the wave direction sensitive analysis conducted in this study showed that the wave directions could not be neglected in the long-term fatigue damage assessment of mooring lines. Recent deep neural network (DNN) methods have shown great potential in mooring line/riser response prediction for offshore structures. However, the utilization of float responses as input data in these DNNs restricts their applicability during the structure design phase. To address this issue, a gated recurrent unit (GRU) network is proposed for mooring line tension modeling. The proposed GRU network surrogate models are capable of accurately predicting the mooring line tension using wave elevation components or float motion responses. The rain-flow counting algorithm, T-N approach, and Miner's rule were employed to estimate the structural fatigue damage. The results showed that the proposed method could replace the heavy cost numerical computation, assess the long-term fatigue damage with high accuracy and alleviate the data dependency of DNN methods.

Hydrodynamic characteristics of a 15 MW semi-submersible floating offshore wind turbine in freak waves

Dynamics of the floating offshore wind turbines (FOWTs) in harsh ocean environments gains increasing attention due to the high maintenance costs in case of structural damage. The freak wave occurring more frequently than believed poses a threat to FOWTs, but mechanism and influencing factors of the wave-structure interaction problem have not been fully explained. In this paper, dynamics of the VolturnUS-S semi-submersible platform designed for the IEA 15 MW wind turbine under freak waves is studied using a coupled computational fluid dynamics and finite element mooring line model. First, the hydrodynamic model is validated using the DeepCwind semi-submersible platform. Then, the validated model is employed for the analysis of the VolturnUS-S semi-submersible platform under freak waves described as a focused group of waves. Hydrodynamic behaviors of the semi-submersible FOWT are described in detail, and parametric analysis of the problem including the effects of focusing crest amplitude, focusing location, incident angle, gravity center location is conducted. Important results include the dramatic surge motion of the platform and fairlead tension of the upstream mooring line, and alleviation of these responses by varying the wave angle to 30° and moving the gravity center to the centroid of the platform geometry.

Investigation on ship mooring forces including passing ship effects validated by experiments

Additional loads on the moored system are generated due to the interaction effects of a passing ship in the proximity of a moored ship and, in the worst case, can even cause the breaking of mooring lines. Existing studies on mooring lines considering passing ship forces and the effects of passing ship parameters on mooring line tension are limited. A mooring analysis is performed using OPTIMOOR to evaluate the impact of passing ship forces and environmental loads on mooring line tension. Results indicate a maximum mooring line tension increment of around 40% due to passing ship forces. The passing ship forces are obtained from the experiments conducted on scaled tanker models. A wide range of significant parameters, such as the passing ship size, passing ship speed, the separation distance between the ships, water depth and moored ship's length, are considered for the study. It is observed that the Froude number based on length (FrL) has the most significant impact on the passing ship effects compared to displacement ratio (DR) and Froude number based on water depth (FrD). Also, FrD influences the interaction effects more when compared to DR. The experimental results are compared with numerical simulations.

Preliminary design and dynamic analysis of constant tension mooring system on a 15 MW semi-submersible wind turbine for extreme conditions in shallow water

Floating offshore wind in China faces the challenge of shallow water depths and frequent typhoons, which are very different from the environmental conditions in European waters. The performance of the mooring system directly affects the overall cost of the floating offshore wind turbine and its survivability in extreme conditions. Under extreme conditions in shallow water, line dragging or breaking can frequently occur when the floating platform restrained by a conventional chain catenary mooring system drifts with waves. A constant tension mooring system concept is proposed to solve the design challenges of shallow water mooring systems in harsh conditions for floating wind turbines. The newly proposed mooring system consists of clump weight and winch wires, which release kinetic energy by increasing the platform's moving distance and ensuring that the instantaneous maximum mooring force never exceeds the breaking strength of the chain. Based on the time-domain potential flow theory, the coupling dynamic response of the UMaine VolturnUS-S semi-submersible reference wind turbine moored by a constant tension mooring system is studied. The mooring line tension and the platform motion are analysed. By comparing with the traditional catenary mooring system, it is found that the constant tension mooring system can significantly reduce the length of the mooring line used and the maximum mooring line tension. It completely prevents the mooring chain from being tightened and the turbine from being damaged by excessive mooring loads, effectively improving the survivability of a floating offshore wind turbine in extreme conditions in shallow water areas.

Dynamic analysis of a porous wall fencing offshore fish cage subjected to regular waves

Global aquaculture is in exponential trend to fulfil the demand for seafood due to the rise in world population. Most countries have implemented nearshore farming and reached their limits, which impacts water quality parameters. Offshore farming is the alternative option to counteract this nearshore farming issue and balance the aquaculture demand and supply. The present study construes on the numerical study of the porous wall fencing offshore fish cage subjected to regular waves. The numerical analysis is carried out for four cages by varying porous hole diameters from 0.5 to 0.7 m and without porosity. All the cages are placed at the same water depth of 200 m, interacting with a constant wave height of 6m with wave periods ranging from 6.92 to 19.05 s. Both frequency and time domain analysis are conducted to study the variation of hydrodynamic parameters, namely added mass, wave excitation forces, radiational potential damping, motion responses, and mooring line tension. Among all cage configurations, the cage with 0.5 m diameter porous hole fencing performs better for all wave conditions considered. Also, a scaled model of 1:75 was considered in both experimental and numerical studies for the purpose of validation. It is learnt that experimental parameters such as motion responses and mooring line tension are in good agreement.

Conceptual design, parameter optimization and performance investigation of a 10MW semi-submersible floating wind turbine in shallow water

There is lacking of floating wind turbine supporting large wind turbine, i.e. 10 MW, aiming at shallow water applications. In this paper, a 10 MW semi-submersible floating wind turbine named OUCwind is proposed for shallow water, and a scale optimization procedure is proposed during the preliminary design stage. It offers an engineering-practical method to determine the main parameters of a new-proposed floater, and critical floater characteristics, such as floater stability, natural period, platform mass etc. are chosen as important design criteria. The hydrodynamic performance with different scale parameters are investigated and it is found that enlarging distance between columns is a cost-effective way to get better stability and hydrodynamic performance. The structural performance of the new-designed OUCwind floater are compared with other two well-known braceless 10 MW platforms, 10 MW-CSC, and OO-star. From the frequency-domain analysis results, the surge linear transfer function, mean drift force and quadratic transfer function near the surge natural frequency of OUCwind is relatively small compared with other two platforms. In the time domain, the new-designed OUCwind also performs better in surge motion and mooring line tension response compared with 10 MW-CSC and OO-star, especially under extreme condition. Nevertheless, for the heave and pitch motions, the wave-frequency response of the OUCwind platform is relatively large under extreme load conditions.

Comparative study of spread and vertical mooring configuration in floating collar net cage under irregular waves and current

Offshore aquaculture is one of the national strategic programs of the Ministry of Fishery and Ocean Affairs of Indonesia to increase national fish cultivation capacity. One solution is using a simple flexible floating collar net which is suitable for small to medium cage volume. In designing such floating net, its mooring line configuration is vital to maintain fixed position of structural part of the cage volume. The purpose of this study is to compare design aspect of two types of mooring systems, i.e., spread and vertical configuration, by investigating selected parameters: mooring line tension, collar offset, and volume reduction. The mooring configuration and floating collar net cage are analysed through a numerical model based on Morison-force equations, under environmental condition adopted from Kangean field East Java. Load cases from collinearly in-line and between-line directions under extreme waves and current condition are checked against relevant safety factor. The dynamics analysis using the time domain simulation is based on 900 seconds cut-off according to its maximum wave elevation profile to generate the hydrodynamics forces to the mooring systems.

DYNAMIC SIMULATION OF AN OFFSHORE AQUACULTURE STRUCTURE SUBJECTED TO COMBINED WAVE AND CURRENT CONDITIONS

Abstract The reliable design of offshore aquaculture structure (OAS) for fish farming in the open ocean is vital to the marine aquaculture industry in the future. However, the lack of easy-to-access numerical tools for the dynamic analysis of OAS challenges the development of marine aquaculture. This article presents a newly developed numerical library under an open-source, finite-element analysis code, Code_Aster, enabling the dynamic analysis of OAS. A numerical model of OAS is first developed using the present numerical library, and then validated against published experiments. The validation shows a good agreement in terms of structural motions and tensions in mooring lines. Subsequently, the dynamic responses of this model are analyzed under irregular waves and current conditions from field measurements on an offshore fish farm site. The results indicate that a negative mean pitch angle will occur when the current velocity is large.

Experimental Investigation of the Hydrodynamic Characteristics of Longline Aquaculture Facilities under Current and Wave Conditions

In this study, a longline aquaculture facility with lantern nets off the coast of northern China was modelled to conduct hydrodynamic tests starting from the culture unit to the entire facility under various current and wave conditions. The experimental results indicated that the drag coefficients of the lantern net model with weights of 0.5, 0.75, and 1.0 kg were 0.75, 0.83, and 0.91, respectively, in the Reynolds number range of 1 × 104–1 × 106. The current-driven upstream mooring line was more dominant than the wave-driven tension, and a simplified model of the longline facility accurately predicted the mooring line tension under the current conditions. The scope of the mooring line (defined as the length of the mooring line related to the water depth) played an important role in eliminating an order of magnitude difference in mooring tension under the wave conditions. The amplitudes of the vertical movement of the longline facility were smaller than the wave height when L/Lm was less than 1.5. Therefore, detailed information is needed to better understand the hydrodynamic characteristics and motion response of longline aquaculture facilities for the safe operation of longline structures in offshore environments, in order to process high-quality oyster products.

Dynamic Response of 15 MW Floating Wind Turbine With Non-Redundant and Redundant Mooring Systems Under Extreme and Accidental Conditions

AbstractThis work focuses on examining the dynamic behavior of large floating offshore wind turbine (FOWT) exposed to extreme (wind and wave with 50-year return period) and accidental (mooring line breakage) loadings. The FOWT considered is 15 MW reference turbine supported on semi-submersible platform stationed using catenary moorings. As the mooring configuration greatly affects the response of FOWT, two different mooring configurations namely non-redundant (three-line) and redundant (six-line) systems were studied and compared. The coupled multi-body dynamic system was solved using Openfast. When simulating the mooring line failure, both the operating and extreme loading conditions were considered. Failure of one mooring was considered at a time. The response of the coupled system due to breakage of the mooring indicates high displacements in surge and sway directions in comparison to the intact system especially for the non-redundant mooring system. Furthermore, change in platform yaw angle and increased tension in the other intact mooring lines were observed. The findings from this study will be helpful in accidental limit state design and preventing failure of similar large FOWT systems. In addition, insights into using non-redundant and redundant mooring configurations for such large structures with respect to extreme and accidental loadings were also discussed.

Coupled aero-hydro-mooring dynamic analysis of floating offshore wind turbine under blade pitch motion

Accurate prediction of the dynamic responses of the floating offshore wind turbine (FOWT) under the blade pitch motion is quite challenging because of the strong nonlinear effects. In this study, a fully coupled and highly elaborated model was established based on the computational fluid dynamics, with the dynamic fluid body interaction method. The multi-stage movements consisting of the six degrees of freedom motions of the platform, the rotation of the rotor, and the blade pitch motion were defined by the superposition motion technologies. The blade pitch control module was created through the user-defined function to regulate the blade pitch motion. Then, several coupled dynamic simulations of the full-configuration DeepCwind floating wind turbine system were performed in power production, shutdown, and startup cases. The simulation results in the power production case indicate that the blade pitch motion decreases the generated aerodynamic loads and amplifies the response amplitude of the platform as negative damping is introduced in the FOWT system. The simulation results in the shutdown and startup cases indicate that the extreme motion responses are enlarged, and the mooring line tension oscillates dramatically when it is in high-tension states. In addition, the nonlinear interference effects in the unsteady flow fields, such as the shedding vortices broken by the blade pitch motion, are visualized and investigated in detail.

Experimental and numerical investigation of the dynamic responses of longline aquacultural structures under waves

Mariculture is an important activity in the exploitation of marine resources, of which longline raft culture accounts for a large part of the production. The flexibility due to the structural characteristics and the front-to-back shielding effect between suspended culture units have great influence on the dynamic response of the structure under waves. There are relatively few systematic studies on the impacts of this flexibility and shielding effect. Laboratory experiments are carried out in our work to investigate the dynamic response of different forms of aquacultural structures under the action of regular and irregular waves, in which the structural flexibility is reasonably considered and modeled. Then based on the experimental results, a numerical model developed in our previous work is optimized to take the shielding effect into account. Through the improved numerical model, the influence of structural parameters and wave parameters on the mooring line tension is analyzed. The results indicate that the shielding effect reduces with the increase of wave frequency. The maximum mooring line tension generally decreases with the decrease of culture density, and the decreasing trend is gradually weakened. The increase of anchoring angle results in a smaller maximum mooring line tension in most cases due to the reducing anchoring system constrains. The maximum value of the mooring line tension under the action of regular and irregular waves generally appears around the relative structure length B/L=1.15∼1.35. The conclusions of the experiment and analysis can be used as a reference for the design of similar longline aquacultural structures to avoid structural failure.

Integrity assessment of the mooring lines and risers of floating production systems based on numerical models generated by retroanalysis of measured data: PART II – Integrity Assessment

This work describes an integrity assessment methodology that considers field monitoring and inspection activities that gather information regarding the actual configuration of the mooring lines and risers of floating production systems (FPS). The first stage consists of calibrating numerical models representing the updated as-laid configuration, comprising “digital twins” of the FPS. Then, in the remaining stages, these models are employed to perform a series of uncoupled, hybrid and fully coupled analyses that provide the platform offsets, and the structural response of the mooring lines and risers. These are expressed in terms of offset diagrams (polar diagrams comprised by the envelope of maximum platform excursions); and the safe operational zone (SAFOP) for the risers (polar diagrams defining the envelope of safe horizontal excursions within which the riser top connections must remain to avoid violation of any riser design criterion). The integrity is assessed by comparing the SAFOP and offset diagrams, and comparing the maximum mooring line tensions with their corresponding limit values. Mitigation and contingency procedures are then recommended whenever the integrity of one or more mooring lines or risers is compromised, involving redesign steps and field interventions to adjust the as-laid configuration and return the system to a safe operational condition.

Initial Design of a Novel Barge-Type Floating Offshore Wind Turbine in Shallow Water

The studies on floating offshore wind turbines (FOWTs) have been increasing over recent decades due to the growing interest in offshore renewable energy. The present paper proposes a barge platform with four moonpools to support the Technical University of Denmark 10 MW wind turbine for a designed water depth of 60 m. A 4 × 2 mooring system with eight mooring lines is also proposed for the barge platform. The main dimensions of the barge platform are optimally selected with respect to its preliminary hydrodynamic properties and potential financial benefit. The proposed barge-type FOWT is then demonstrated to be aligned with the DNV standard requirements in terms of its intact and damage stability. Furthermore, coupled time-domain simulations are conducted for the proposed barge FOWT with mooring under the selected environmental and operational conditions by using Simo-Riflex-AeroDyn (SRA). Through decay test simulations, the natural periods of the barge-type FOWT are demonstrated to be within the DNV recommended ranges. The proposed mooring system is also benchmarked with the 3 × 3 mooring concept that was used for a 3 MW barge-type FOWT installed in Kitakyushu. The response magnitudes of the barge platform and mooring line tension are similar to both mooring systems, and thus the 4 × 2 mooring system is preferred due to its lower cost. In addition, the proposed barge platform is preliminarily demonstrated to be able to survive for the 50-year extreme environmental conditions under parked wind turbine status, as well as the normal environmental conditions under the operating status.

Normal Operating Performance Study of 15 MW Floating Wind Turbine System Using Semisubmersible Taida Floating Platform in Hsinchu Offshore Area

This study predicted the motion response and power performance of a floating wind turbine system equipped with a semisubmersible Taida platform, an IEA 15 MW wind turbine, and a 3 × 2 mooring design in the Hsinchu offshore area in the Taiwan Strait. The hydrodynamic properties were calculated using ANSYS-AQWA and STAR-CCM+. The motion equations were solved by OrcaFlex to obtain the motion response and generator power, as well as the dynamics of the mooring system and aerodynamics of the wind turbine. The waves were assumed to share the same direction as the wind. This study compared the mean values and standard deviations of the motion response, generator power, and mooring line tension between the potential- and viscous-flow approaches by considering the combination of seven wind directions and four current directions under two wave conditions in the Hsinchu offshore area. The numerical prediction shows that the viscous effect has a larger impact on the hydrodynamic properties in the heave, roll, and pitch motions. The angle between the leading mooring line of the system and dominant wind direction in the Taiwan Strait, which comes from the northeast, should be from 120° to 180° in order to deliver a relatively favorable performance of the system.

Predicting future mooring line tension of floating structure by machine learning

Predicting future mooring line tension of floating structure by machine learning

Impact of Limited Degree of Freedom Drag Coefficients on a Floating Offshore Wind Turbine Simulation

The worldwide effort to design and commission floating offshore wind turbines (FOWT) is motivating the need for reliable numerical models that adequately represent their physical behavior under realistic sea states. However, properly representing the hydrodynamic quadratic damping for FOWT remains uncertain, because of its dependency on the choice of drag coefficients (dimensionless or not). It is hypothesized that the limited degree of freedom (DoF) drag coefficient formulation that uses only translational drag coefficients causes mischaracterization of the rotational DoF drag, leading to underestimation of FOWT global loads, such as tower base fore-aft shear. To address these hydrodynamic modeling uncertainties, different quadratic drag models implemented in the open-source mid-fidelity simulation tool, OpenFAST, were investigated and compared with the experimental data from the Offshore Code Comparison Collaboration, Continued, with Correlation (OC5) project. The tower base fore-aft shear and up-wave mooring line tension were compared under an irregular wave loading condition to demonstrate the effects of the different damping models. Two types of hydrodynamic quadratic drag formulations were considered: (1) member-based dimensionless drag coefficients applied only at the translational DoF (namely limited-DoF drag model) and (2) quadratic drag matrix model (in dimensional form). Based on the results, the former consistently underestimated the 95th percentile peak loads and spectral responses when compared to the OC5 experimental data. In contrast, the drag matrix models reduced errors in estimates of the tower base shear peak load by 7–10% compared to the limited-DoF drag model. The underestimation in the tower base fore-aft shear was thus inferred be related to mischaracterization of the rotational pitch drag and the heave motion/drag by the limited-DoF model.