Mooring Line Tension Response Research Articles

Effects of the second-order hydrodynamics on the dynamic behavior of the platform among the wind-wave hybrid systems

This paper presents three different wind-wave hybrid systems (HSs) with multiple point-absorption wave energy converters (WECs) integrated onto a floating semi-submersible wind turbine (WT). The focus of the present study is to explore the effects of the second-order wave forces on the dynamic behavior of the platform in the HS, including the platform motion and the mooring line tension, under both operational and extreme sea states. ANSYS/AQWA based on 3D diffraction/radiation potential theory is used to conduct numerical investigation on the second-order hydrodynamics of the HS. To ensure the reliability of the investigation, the validations of the semi-submersible platform and point-absorption WEC are done based on the available experimental data. Additionally, two models, "1st-order" and "1st&2nd-order", are set up for comparative analysis, to highlight the significance of the second-order forces. The simulation results show that the second-order hydrodynamics have a significant influence on the surge and pitch responses of the platform, but almost no effects on the heave response. In addition to this, ignoring the second-order hydrodynamics will significantly underestimate the mooring line tension response. Overall, this study suggests that the second-order hydrodynamics should be considered in the design of wind-wave HSs, in order to ensure their performance and safety, especially under the severe sea states.

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.

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

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

Moderate water depth effects on the response of a floating wind turbine

In the present paper, the mooring lines system of the V-shaped floating wind turbine (VSWT) at 50-m water depth is designed accounting shoaling effects and fully coupled aero-hydro-servo-elastic effects. The motions and mooring line tension responses under different load cases are analyzed at moderate water depth and are compared with revelant responses at deeper water depths for highlighting the rational response of the VSWT in shallow waters. The responses are dominated mainly by wave actions for the parked condition, while by wind actions for operational conditions. Moreover, comparisons between the onshore wind turbine (WT), monopile WT at 30-m water depth and VSWT at 50-m water depth are presented for different response and performance quantities including the generator power output and structural internal loads in order to study the rationality and efficiency of the mooring line design at moderate water depths and compare responses with alternative competitive possible solutions. It is found that the standard deviation of the generator power of the VSWT is larger than that of the onshore WT and monopile WT due to large platform motions induced by hydrodynamic loads. Moreover, the hydrodynamic loads on the VSWT have a great impact on the tower base loads, while do not significantly affect the rotor thrust force and blade root force. The results presented in the paper proves the efficient design of the VSWT at moderate water depths ready for use.

Probability Distribution of the Hull Motion and Mooring Line Tension of Two Floating Systems

The statistical and distribution characteristics of the responses of a floater and its mooring lines are essential in designing floating/mooring systems. In general, the dynamic responses of offshore structures obey a Gaussian distribution, assuming that the structural system, and sea loads are linear or weakly nonlinear. However, mooring systems and wave loads are considerably nonlinear, and the dynamic responses of hull/mooring systems are non-Gaussian. In this study, the dynamic responses of two types of floaters, semi-submersible and spar platforms, and their mooring lines are computed using coupled dynamic analysis in the time domain. Herein, the statistical characteristics and distributions of the hull motion and mooring line tension are discussed and compared. The statistical distributions of the dynamic responses have strong non-Gaussianity and are unreasonably fitted by a Gaussian distribution for the two floating and mooring systems. Then, the effects of water depth, wave parameters, and low-frequency and wave-frequency components on the non-Gaussianity of the hull motion, and mooring line tension are investigated and discussed. A comparison of the statistical distributions of the responses with various probability density functions, including the Gamma, Gaussian, General Extreme Value, Weibull, and Gaussian Mixture Model (GMM) distributions, shows that the GMM distribution is better than the others for characterizing the statistical distributions of the hull motion, and mooring line tension responses. Furthermore, the GMM distribution has the best accuracy of response prediction.

Analysis of Dynamic Characteristics of an Ultra-Large Semi-Submersible Floating Wind Turbine

An initial design of the platform for the moderate water depth (100 m) is performed by upscaling of an existing 5 MW braceless semi-submersible platform design to support the DTU (Danish University of Science and Technology) 10 MW wind turbine. To investigate the dynamic characteristics of the ultra-large semi-submersible floating offshore wind turbine (FOWT), an aero-hydro-servo-elastic numerical modeling is applied to carry out the fully coupled time-domain simulation analysis. The motion responses of the ultra-large semi-submersible FOWT are presented and discussed for selected environmental conditions. Based on the quasi-static and dynamic analysis methods, the influence of the dynamic effects of the mooring lines on the platform motion responses and mooring line tension responses are discussed. Subsequently, the difference in the motion responses and structural dynamics of the DTU 10 MW and NREL (National Renewable Energy Laboratory) 5 MW FOWT is studied due to the difference in turbine properties. The simulation results reveal that the excitation of the low-frequency wind loads on the surge and pitch motions, the tower-base fore-aft bending moments and the mooring line tension response becomes more prominent when the size of the wind turbine increases, but the excitation action of the 3P effect on the structural dynamics of the 5 MW FOWT is more obvious than those of the 10 MW FOWT.

Effect of moorings drag and inertia on response of spar platform

Spar platform is a type of floating structure utilized for oil and gas exploration and production in deep and ultra-deep waters. Coupled analysis in present study considers Spar mooring lines as an integrated system. It incorporates the contribution of drag and inertia forces of mooring lines. A rigid classical Spar cylinder connected by four tensioned catenary mooring lines has been modeled and analysed using finite element approach. Mooring system has been modeled as hybrid beam elements. The Studies cover surge, heave, pitch and mooring line tension responses, highlighting the coupling effect. The non-linearities present in the coupled system leads to irregular behavior under regular sea states. The coupled model noticeably confirms its importance in terms of hydrodynamic damping on mooring system. There is a key variance in behavior found with and without drag and inertia forces on mooring system.

Experimental study of a hybrid mooring system

Mooring systems are essential to the floating structures including offshore platforms, wave energy converters and floating breakwaters. As floating structures propagate towards deeper waters, the design of the mooring systems needs to be revamped. In the present study, a new hybrid mooring system has been proposed. A series of weights and buoys are arranged to the different segments of each mooring line in order to improve the motion performance of the platform and eliminate the vertical forces at the lower end of the mooring lines. A series of experiments have been carried out at CSSRC wave basin for a semi-submersible platform moored by the mooring lines with or without the weights and buoys. The mooring lines’ tension and 6-DOF motions of the platform were measured. Discussions have been made on non-linear motion responses of the semi-submersible and non-linear mooring line tension responses for various mooring conditions.

Analysis of hydrodynamic behaviors of gravity net cage in irregular waves

Based on the lumped-mass method and rigid-body kinematics theory, a mathematical model of a gravity cage system attacked by irregular waves is developed to simulate the hydrodynamic response of cage system, including the maximum tension of mooring lines and the motion of float collar. The normalized response amplitudes (response amplitude operators) are calculated for the cage motion response in heave and surge, and the mooring line tension response, in regular waves. In addition, a statistical approach is taken to determine the motion and tension transfer functions in irregular waves. In order to validate the numerical model of a gravity cage attacked by irregular waves, numerical predictions have been compared with the experimental observations in the time and frequency domain. The effect of wave incident angle on the float collar motion, mooring line tension and net volume reduction of the gravity cage system in irregular waves is also investigated. The results show that at high frequencies, the cage system has no significant heave motion. It tends to contour itself to longer waves. The variation amplitude of mooring line forces decreases as the wave frequency increases. With the increasing of wave incident angle, the horizontal displacement of the float collar increases.

Numerical simulation of hydrodynamic behavior of gravity cage in irregular waves

Compared with net cage used in sheltered area, the net cage in open sea will resist more severe wave loading. But the open sea area is more suitable to keep the environment condition to avoid pollution. For this reason it is necessary to understand the behavior of net cages as they are exposed to large sea-loads from waves and current. In this paper, the hydrodynamic behavior of gravity cage in waves is systematically analyzed by numerical methods. The lumped-mass method and the principle of rigid-body kinematics are used to establish the governing equations of motion, which are solved by the Runge–Kutta–Verner sixth-order method. In order to validate the numerical model, a series of physical model tests in regular waves are conducted. The results of numerical simulation show a good agreement with the experimental observations. Thereafter, the irregular wave fields are generated according to the random phase method, and then the hydrodynamic behaviors of gravity cage in irregular waves are simulated and compared with the results of regular waves. The statistical approach and Fourier transform are applied to analyze the cage motion response and the mooring-line tension response in both time and frequency domain. The numerical results indicate that the mooring-line forces decrease as the wave frequency increases. At low frequencies, the heave motion response is synchronized with the wave elevation.

Moored Fish Cage Dynamics in Waves and Currents

Recent work in the area of open ocean aquaculture system dynamics has focused separately upon either the response of fish cages in waves or the steady drag response due to ocean currents. In reality, however, forcing on these open ocean structures is a nonlinear, multidirectional combination of both wave and current profiles. At the University of New Hampshire-operated Open Ocean Aquaculture site, data were collected from a wave measurement buoy and a downward-looking Acoustic Doppler Current Profiler to characterize the surface elevation and water velocity profiles during an extreme northeast storm event. In addition to waves and currents, fish cage motion response in heave, surge, and pitch was inferred from accelerometer measurements during the same storm. The environmental data sets obtained during the peak of the storm were processed, analyzed, and used as input to a dynamic finite-element model. Simulations were performed using three load case scenarios: 1) in both waves and currents; 2) in waves only; and 3) in currents only. Model motion response results in both the time and frequency domain were compared with data obtained in situ . In addition to the motion response tests, the wave and current forcing influencing the mooring line tension response was also investigated. Analysis shows that in this case, the currents do not severely influence the oscillatory motion response, but do cause the cage to tilt, layback, and sink. The wave and current interaction effect did, however, influence the anchor line loads with a portion being attributed to nonlinear effects.