Multi-point Mooring Research Articles

Analysis of Fluid Field in Fish Tank of Breeding Vessel with Perforated Broadsides under Wave Conditions

For a fish tank of breeding vessels with perforated broadsides, in order to analyze its fluid characteristics under wave conditions, this paper presents a numerical method and predicts the fluid field in a fish tank with opening holes. Therein, three critical problems were solved, which include wave generation, the fluid–structure interaction of waves and vessel motion, and the sloshing phenomenon in the tank. The feasibility of the method was validated through a comparison with theoretical and experimental results. On this basis, the fluid characteristics in the fish tank of a fixed breeding vessel with opening holes on side walls were discussed. Firstly, the effects of wave direction, wave height, and wave period on the flow field in the tank were mainly analyzed. Next, the fluid characteristics in the tank of a floating vessel with opening holes equipping a multi-point mooring system were investigated. Finally, the influence of motion responses of breeding vessels on the fluid field in the fish tank was examined.

Study on characteristics of mooring system of a new floating offshore wind turbine in shallow water by experiment

At present, researches on floating offshore wind turbine are mainly focused on deep water areas beyond 50 m, and less research has been conducted for shallow water. Considering that the floating fan still has many application values in 30 m–50 m area, it is necessary to study the floating offshore wind turbine in shallow water area. With the shallow design water depth and the effect of topography, the environmental condition is complex. Therefore, it is relatively difficult to design the mooring system. In this paper, a multi-point mooring system is proposed which based on PivotBuoy, a floating offshore wind turbine designed by X1-Wind Company. The characteristics of motion responses of the floating offshore wind turbine and mooring force under different mooring cable schemes are studied by physical model experiment. Through these results, it can be found that when the mooring system adopts the hybrid mooring cable scheme (anchor chain + polymer cable + anchor chain), the pitch and heave motion responses of the floating offshore wind turbine will increase by about 30% maximally, the mooring cable tension will decrease by about 27%. This paper also studies the influence of water depth and mooring cable pretension on the mooring system under the hybrid mooring cable scheme, and the characteristics of mooring system in 22.5 m shallow water. These results can provide certain theoretical guidance for the design of mooring system of floating offshore wind turbine in shallow water.

DRNN-MIMO-PID control strategy for multi-point mooring system

DRNN-MIMO-PID control strategy for multi-point mooring system

DRNN-MIMO-PID control strategy for multi-point mooring system

DRNN-MIMO-PID control strategy for multi-point mooring system

Dynamic response of a multi-point mooring cylindrical floating nuclear power platform carrying a small-scale reactor

An offshore floating nuclear power platform (FNPP) can provide stable energy and freshwater for remote islands. Compared with the ship-shaped FNPP, the cylindrical FNPP is suitable for small-scale lead-cooled reactors owing to its compactness and stability. Recently, there have been many studies on cylindrical FNPPs with a pressurized water reactor (PWR), however, the PWR's size limits the compact design of the FNPPs. Therefore, we designed a multipoint mooring cylindrical FNPP carrying a lead-cooled small-scale reactor (20 MWe). First, we performed a rolling decay test on a typical cylindrical FNPP model and obtained the damping coefficient by calibrating the corresponding simulation model. We then built a new cylindrical FNPP model with four mooring modes to study the configuration of the mooring line system using the simplified method and linear theory. Finally, we compared the transient response and mooring characteristics of the cylindrical FNPP when subjected to different directions of wind, waves, and currents. The results indicate that the mooring mode where the mooring line is divided into four groups is the optimal choice, with a maximum displacement of 16.79 m and a maximum roll angle of 7.24°. The safety factors of the mooring lines satisfy the requirements of the code.

A comparison study on the hydrodynamic performance of a buoyancy-distributed floating foundation in regular waves

The manufacturing and assembly costs have a great significance in floating foundations of offshore structures. This paper presents a novel buoyancy-distributed floating foundation (BDFF) based on modular design and validates its high hydrodynamic performance. A centralized ballast platform with a single-point mooring system and a distributed ballast platform with a multi-mooring system are designed for comparison. A series of tests with scaled models and numerical simulation work were conducted in regular waves. There is good agreement in the response amplitude operators (RAOs) for the experiments and the simulation with a bit of over-prediction. Additionally, the influence of wave frequencies and wave heights on the BDFF is also investigated. It is found that the model with a multi-point mooring system gives a moderate response and can be regarded as a linear system, while the model with a single-point mooring system has a large responsivity for high frequencies.

Time-domain Signal Compressing-Reconstruction Method for Underwater Inclination Data Transmission of a Multi-point Mooring System

Due to the obstacles to underwater acoustic transmission such as narrow bandwidth and frequent data loss, a novel time-domain signal compressing-reconstruction method is proposed for underwater inclination data transmission of a multi-point mooring system. First, a time-frequency analysis was carried out on the measured inclination data and a calculation method using the top tension force of the anchor chain was proposed. Then, to address problems like narrow bandwidth and easy data loss, a time-domain signal compressing-reconstruction algorithm was constructed. The data indicated that the reconstruction error was only 0.6957% with a compression ratio of 20%. Finally, the effects of compression ratios and loss rates on the reconstruction accuracy was analyzed to demonstrate the accuracy, feasibility and reliability of the compression-reconstruction algorithm.

Adaptive RBF -SMC control for multi-point mooring system

A stable adaptive control scheme for multi-point mooring system (MPMS) with uncertain dynamics is proposed in this paper. The control scheme is designed by a hybrid controller based on RBF (Radial Basis Function) NN (Neural Network) and SMC (Sliding Mode Control), which learns the MPMS dynamic changes, and the compensation of external disturbances is realized through adaptive RBFNN control. Meanwhile the RBF-SMC control parameters are adapted by the Lyapunov method to minimize squares dynamic positioning (DP) error. The convergence of the hybrid controller is proved theoretically, and the proposed mooring control scheme is applied to the “Kantan3” mooring simulation system. Finally, the simulation results are compared with the traditional PID controller and standard RBF controller to demonstrate the effective mooring positioning performance of the control scheme for the MPMS.

Innovative Method for Ship Navigation Safety Risk Response in Landslide-Induced Wave

When the bank of a reservoir slope slides along a weak structural plane at a high speed, “landslide slamming” will occur in the nearby water. The formation of landslide-induced waves is a serious threat to the safety of wharfs, shore marks, buildings in the water, and vessels navigating in reservoir areas. To ensure the safety of navigating ships, this study proposes a landslide-induced wave water ship navigation safety risk response technology. The propagation characteristics of landslide-induced waves are analysed based on a physical model experiment, and the characteristics of a ship's motion response and mooring cable tensions are studied under conditions of bow and stern mooring and multipoint mooring. The influences of the landslide-induced wave direction and ship navigation position on the ship rolling motion characteristics are discussed. The results of this study can further improve the navigation safety of ships in landslide-induced wave waters.

Multi-point Mooring PID Control Based on Kalman Filtering-Genetic Algorithm-Slow Disturbance Observer Compensation

An adaptive PID control technique based on the Kalman Filtering (KF) Genetic Algorithm (GA) and Slow Disturbance Observer Compensation (SDOC) is proposed for multi-point mooring system in this paper. The multi-point mooring model, which is simplified to two-point mooring, is established, its self-tuning PID mooring controller based on KF-GA-SDOC is designed. The multi-point mooring model is updated on-line with the KF and SDOC to filter and observe time-varying signal; the PID mooring control gains are adjustable parameters by selection, crossover and mutation operator of GA; the mooring control system based on KF-GA-SDOC PID is optimized. The simulation results show that the designed KF-GA-SDOC PID mooring controller can maintain good control performance under different conditions.

Numerical Study of a Single-Point Mooring Gravity Fish Cage With Different Deformation-Suppression Methods

Abstract Compared with the multipoint mooring fish cage, the single-point mooring (SPM) fish cage can spread out the fish wastes and uneaten feeds in a larger area, and it can also prevent the local environment from being overwhelmed. Thus, it has attracted much attention recently. In this research, different deformation-suppression methods are applied to the SPM system with a typical Norwegian fish cage aiming to increase the cultivation volume under the action of current and/or wave loads. A well-validated software, fhsim, is used to conduct the full-scale numerical study. The effects of the three deformation-suppression methods, i.e., (i) adding the lower bridles, (ii) adding the frontal rigid frame and (iii) adding the trawl doors, are analyzed under pure current and combined wave–current conditions. The results indicate that all the three deformation-suppression methods can improve the cultivation volume at least by 32% compared to the original SPM fish cage when the current velocity is larger than 0.5 m/s. In addition, moving the conjunction point close to the bottom ring can bring a positive effect on the cultivation volume maintaining with an only small increment in the tension force. This study can provide practical advice and useful guides for the SPM fish cage design.

Numerical design study of multipoint mooring systems for the floating wave energy converter in deep water with a sloping bottom

The use of a floating wave energy converter (WEC) in the deep water zones around islands is increasing, but conventional mooring configurations often do not provide suitable station keeping options. This paper focus on the assessment of a suitable mooring design for floating WECs in deep water locations, with a sloping seabed. The mooring study considers a 50-year return period to assess the survival rate for three mooring designs i) catenary mooring systems, ii) synthetic cable (polyester) taut mooring system, and iii) suspended anchoring point mooring system. Furthermore, the variations of bathymetry conditions around the oceanic islands are considered, as well as factors directly influencing these deep-water mooring installations (e.g. anchor uplift forces, weight of mooring system). The coupled motion dynamics of the WEC and the mooring system are studied using a potential theory in a time-domain model, and the power production is also considered. The Chinese 100 kW Sharp Eagle WEC [22] is used as the reference design. The deployment site has a water depth of 200–700 m and the angle of the slope ranges from 30° to 90°. The paper is investigating the effects of (i) the features of the seabed condition; (ii) the mooring line material, size and layout; and (iii) the installation and maintenance costs. The outcomes of the studies for the three different mooring designs are compared and an additional fatigue analysis has been implemented to obtain fatigue damage criteria for the mixed catenary mooring system. A cost analysis has been implemented, and recommendations provided discussing the most suitable mooring design configuration.

Design and Comparison of Catenary and Taut Mooring Systems for New Concept FPSO IQFP in Shallow Waters

In order to design an effective and economical multi-point mooring system for the new concept FPSO IQFP (fillet inverted quadrangular frustum pyramid shaped FPSO, presented by the lead author), the effects of the mooring parameters on the performance of catenary mooring system and taut mooring system were investigated according to the initial design plans given in advance according to the steady environment forces and practical experiences. Then, the optimal mooring systems were developed on the basis of the static analysis results combined with the coupled calculations in time domain. Finally, the merits and demerits of the two mooring schemes were compared based on the mooring and static characteristic parameters as well as the curves of restoring force against horizontal displacement of the catenary and taut mooring systems respectively. The results verify that the taut mooring system is preferable to the catenary mooring system for IQFP deployed in shallow waters.

Stability Analysis of Two-Point Mooring System in Surge Oscillation

Nonlinear surge response behavior of a multipoint mooring system under harmonic wave excitation is analyzed to investigate various instability phenomena such as bifurcation, period-doubling, and subharmonic and chaotic responses. The nonlinearity of the system arises due to nonlinear restoring force, which is modeled as a cubic polynomial. In order to trace different branches at the bifurcation point on the response curve (amplitude versus frequency of excitation plot), an arc-length continuation technique along with the incremental harmonic balance (IHBC) method is employed. The stability of the solution is investigated by the Floquet theory using Hsu’s scheme. The period-one and subharmonic solutions obtained by the IHBC method are compared with those obtained by the numerical integration of the equation of motion. Characteristics of solutions from stable to unstable zones, chaotic motion, nT solutions, etc., are identified with the help of phase plots and Poincaré map sections.

Coupled Surge-Heave Motions of a Moored System. I: Model Calibration and Parametric Study

Model calibration and parametric studies of the coupled, complex surge-heave motions of a medium-scale experimental, nonlinear, submerged, moored structural system are presented here. The experimental system excited by periodic wave fields consists of a spherical buoy and attached multipoint mooring lines. Sources of nonlinearity include complex geometric restoring forces and coupled fluid–structure interaction exciting forces. The sphere moves mainly in a two-dimensional fashion [or two degrees of freedom (2DOF)] of surge and heave, with negligible pitch. Characteristic experimental results include harmonic, subharmonic, and superharmonic responses. As an extension of single-degree-of-freedom, independent-flow-field (IFF) models, a 2DOF, IFF model is derived and employed. Good agreement is shown between the analytical predictions and experimental results. Existence of complex nonlinear responses, including chaos and multiple coexisting steady states, are numerically identified when the coupling is strong...

Experimental investigation of response stability and transition behaviour of a nonlinear ocean structural system

This study experimentally investigates the nonlinear response stability and transition behaviour of a submerged, moored ocean structural system which consists of a spherical buoy and attached multi-point mooring lines. The system is excited by a periodic wave field in a closed channel. System nonlinearities include complex geometric restoring (stiffness) force and coupled fluid–system interaction exciting forces. Experimental set-up, operating procedures and analysis of the measured results are presented. Characteristic motions observed include harmonic, subharmonic and ultraharmonic responses, which demonstrate a signature of the intricate pattern of the nonlinear global behaviour. Good agreements between the measured and most predicted responses are demonstrated in both time and frequency domains. These results confirm the validity of the analytical model presented and calibrate the accuracy of the existing numerical predictions. Primary and secondary resonances in the response are identified via frequency response curves. Response bifurcation cascades are observed in the experimental results and the possible existence of higher-order nonlinear responses is inferred.

Experimental Calibration of Bifurcation Superstructure of Nonlinear System

Results of a medium-scale experimental study revealing the underlying superstructure in bifurcation sets of a submerged, moored ocean system are reported. The experimental model, assimilating nonlinear moored structural responses, consists of a spherical buoy and attached multipoint mooring lines. The model is excited under a periodic wave field in a closed channel. Sources of system nonlinearity include complex geometric restoring force and coupled fluid-structure interaction exciting forces. Experimental setup, configuration, and classification of the measured results are described. Characteristic motions observed include harmonic, subharmonic, and ultraharmonic responses. Nonlinear primary and secondary resonances in the response are identified in frequency response diagrams, which also indicate the existence of intricate patterns of the nonlinear global behavior.

Drag-induced instabilities and chaos in mooring systems

Abstract The influence of a quadratic viscous drag nonlinearity in multi-point mooring systems is examined in this paper. These systems are characterized by a nonlinear restoring force and a coupled wave-structure exciting force. Stability analysis of system response and its corresponding Poincare map define domains of primary and subharmonic resonances and reveal the existence of coexisting nonlinear solutions. Local and global tangent and period doubling bifurcations identify possible routes to chaotic motion and their controlling parameters. Thus, complex dynamics are obtained semi-analytically resulting in identification and control of the drag-induced instabilities which are not attainable by equivalently linearizing the hydrodynamic drag force.

Nonlinear oscillations, bifurcations and chaos in a multi-point mooring system with a geometric nonlinearity

A taut multi-point mooring system with a large geometric nonlinearity under wave and current excitation is examined in this paper. The system is found to exhibit local instability and global bifurcations leading to complex nonlinear and chaotic responses. A semi-analytic method based on stability analyses of approximate solutions to the exact nonlinear system is shown to be an efficient predictor of bifurcations and chaos. Thus the method may supplant or significantly reduce the effort of a numerical parametric analysis for the strongly nonlinear ocean system.

Application of extensible catenary theory to determine the displacement of a moored ship

A mathematical model of a ship moored by several cables is developed taking into account the extension of the cables. The equations for a single cable are rigorously developed and then solved numerically for a complete multipoint mooring system. Results are presented to show the relationship between the lateral displacement of the ship, the corresponding restoring force and the length of each cable remaining on the sea bed for a representative system.