EXPERIMENTAL AND NUMERICAL ANALYSIS OF A MOORED FLOATING STRUCTURE RESPONSE TO WAVES (original) (raw)
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On the design of multi-component cable systems for moored offshore vessels
Energy Conversion and Management, 1991
A mooring system for an offshore vessel must meet certain prescribed requirements imposed by factors such as the environment at site, the operational constraints and the vessel employed. Its adequacy, survival and ability to stay on site must, therefore, be checked out using proper analytical methods. In this paper, the equations of motion of a moored vessel are presented and methods of generating the restoring forces from the mooring cables are discussed. Equations for computation of environmental loadings due to wind and ocean current on the vessel are also cited and a procedure for generating a wave force time history from a given random sea spectrum outlined. Numerical results for a typical construction vessel moored by a multi-leg mooring system in a random sea environment are generated with, as well as without, the effect of cable dynamics included in the analysis. Since the inclusion of vessel added mass is an important consideration in the dynamic analysis of a moored structure, its effect on vessel station keeping response is also investigated. Finally, some conclusions toward designing a mooring system are drawn on the basis of the numerical results obtained and the observations made. Offshore vessels Mooring system design Multi-component cables Station-keeping dynamics Cable NOMENCLATURE Bxx, Byy, B~ = Damping coefficients of vessel resistance to velocity
The stability and dynamic positioning of any floating structure is essential during offshore activities. It is therefore necessary to carry out offshore mooring system analysis in order for it to withstand extreme environmental forces of wind, wave, and current that will act on the structures. This project is focused on designing a fit-for-purpose catenary mooring system to achieve the stability and dynamic positioning of a 5000 tonnes Offshore Work barge with an Helicopter landing Platform and a crane carrier. The mooring design is based on engineering and scientific principle (numerical method), in which elastic catenary equation are derived and applied to determine the dynamic response; the degree of environmental force in the floating structure and the minimum line required for mooring. Classification society of Det Norske Veritas DNV regulations were adopted.
On the equilibrium configuration of mooring and towing cables
Applied Ocean Research, 2008
Marine compound mooring and towing cables are usually formed by cables, chains, buoys and underwater bodies. They are exposed to the action of the water in relative motion, and hence the need to predict the static configuration of these complex mechanical systems under typical conditions from the early stages of the design process. This paper introduces a new mathematical model and a matching numerical method based on finite differences, in order to predict the static configuration of mooring or towing compound cables. The model is validated by analyzing some case studies using a computer program that was specially written for the purpose. The same program is then used for the analysis of a lazy S riser application. The conclusion is that the new model provides a coherent and efficient means to analyze moored/towed systems.
Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment, 2004
A combined experimental/numerical investigation of a moored floating structure response to incoming waves to incoming waves is proposed. The floating structure consists of three bodies, equipped with fenders, joined by elastic cables. The system is also moored to the seabed with eight mooring lines. This corresponds to an actual configuration of a floating structure used for ships and submarines in special docking operations. The dynamic wave response is investigated by performing experiments in a towing tank equipped with a wave maker. Experimental results are compared with numerical simulations in regular and irregular waves, showing a good agreement. In regular waves the predicted time histories of pitch, heave and surge motions of the three-body structure and of the mooring line forces, bear very satisfactorily the experimental results. The case of irregular waves is also encouraging, since the statistics of the response is correctly kept up to the fourth order statistical moments. This confirms that the theoretical model proposed in this paper is a suitable tool to predict the actual behaviour of a complex moored structure at sea.
Effect of Mooring Lines Pattern in a Semi-submersible Platform at Surge and Sway Movements
Exposure to environmental conditions at sea for floating structures is inevitable. Environmental conditions that waves are most important of them will enter forces on structure of semi-submersible platforms. Therefore such structures should be deployed in the operational capability of their own, that one of these methods is mooring them. In this condition, structure shows different behavior compared with unmoored structure. Wave force cause motions of structure and subsequently produce tension force on mooring lines. Hence, investigation of structure movements and selection an appropriate mooring system to minimizing the structure motions must have been discussed. semi-submersible platforms mooring systems results restoring force in horizontal plane, and thus control degree of freedom on Surge, Sway and Yaw movements. This study estimated Surge and Sway movements of a semi-submersible platform when that it has been exposed to 0, 45 and 90 degrees of sea wave direction with the environmental conditions of the Caspian Sea using Flow-3d (version10.0.1) software. Also the seven symmetric mooring systems in the form of 4 and 8 numbers of mooring lines' systems have been used to investigate the best modes.
A Review of Ship Mooring Systems
Brodogradnja, 2017
The physical principle that governs how ships are moored to a port has changed little over the years. Nevertheless, in recent decades, there have been developments in maritime transport towards increased vessel dimensions and operations in specialist terminals. These trends mean that offshore ports and mooring systems have to face more challenging conditions in terms of the waves, wind and drift current. At the same time, pier side port loading and unloading systems place demands on the mooring system, which must immobilise ships better. In this situation, the mooring system's own equipment, such as lines, deck fittings and mooring winches, must also evolve to work alongside new port devices. It is also necessary to point out that changes in mooring will take place in subsequent years. These innovations in attaching the ship to the pier will be highlighted here as they mark a significant change in mooring and pier components.
Dynamics of mooring systems in ocean engineering
Archive of Applied Mechanics (Ingenieur Archiv), 2003
The computation of restoring forces on floating platforms caused by mooring systems in ocean engineering is usually performed by means of quasi-static procedures. Thereby, nonlinear phenomena produced by the motion of the mooring line or the interaction between fluid and line are not considered. For lines in deep water, these effects may cause a tremendous increase in the tension force, which cannot be predicted by simplified models. Therefore, it is mandatory to determine the dynamics by means of a more precise mathematical model. In this paper, a model is presented for the analysis of the dynamic behavior of mooring systems taking into account the hydrodynamic forces exerted by the surrounding fluid. The mathematical description of the real mooring line is performed using a multibody system approach. Furthermore, a comparison of results of a quasi-static and a dynamic analysis is presented in order to stress the importance of a dynamic analysis of mooring lines.