Preliminary study of moored power cables (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
Dynamic Model for Catenary Mooring: Experimental Validation of the Wave Induced Load
This note presents a model for the dynamic simulation of a catenary mooring line (or a submarine power cable). In general, mooring lines are subject to a direct wave load (e.g. drag, inertia) in addition to the induced load due to movement of the vessel to which they are linked. Specific aim of this note is to present, calibrate and validate the numerical response to direct wave action, by comparisons with physical model tests. Tests were carried out at the wave flume of the Maritime Laboratory of IMAGE Department, Padova University, within the framework of a Master thesis. Wave induced loads were measured at the fairlead of a compliant chain, with the peculiarity that the fairlead was hinged to a fixed point, the chain being simply subject to the load induced by regular (slightly non-linear) waves. The equations describing the dynamic movements of the chain in Comsol Multiphysics are written in weak form. It is concluded that the model well represents the tests when the drag coefficient is equal to 1.2-1.4 and applied to an area equal to the maximum apparent width.
Dynamic Analysis of Mooring Cables with Application to Floating Offshore Wind Turbines
Journal of Engineering Mechanics, 2016
Floating offshore wind turbines are recently being considered widely for adoption in the wind power industry, attracting interest of several researchers and calling for the development of appropriate computational models and techniques. In the present work, a nonlinear finite element formulation is proposed and applied to the static and dynamic analysis of mooring cables. Numerical examples are presented, and in particular, a mooring cable typically used for floating offshore wind turbines is analyzed. Hydrodynamic effects on the cable are accounted for using the Morison approach. A key enabling development here is an algorithmic tangent stiffness operator including hydrodynamic coupling. Numerical results also suggest that previously empirical hydrodynamic coefficients could be obtained by fully coupled fluidstructure interaction. Convergence rate and energy balance calculations have been used to demonstrate the accuracy of computed solutions. The introduction of the developed cable model in a framework for the study of the global behavior of floating offshore wind turbines is subject of current work. Source code developed for this work is available as online supplemental material with the paper.
International Journal of Steel Structures, 2018
An effective numerical method is proposed for the three-dimensional nonlinear analysis of mooring cables using catenary theory. In this method, the mooring line is divided into finite number of catenary elements. In addition to self-weight, each catenary element is subjected to drag force due to steady ocean currents. The proposed procedure is validated by comparing the results with those of the shooting optimization and the finite element methods. Finally, a parametric study is conducted to study the effect of extensibility on the static response of mooring cables. The effects of fluid drag forces and cable extensibility on mooring cable tension, equilibrium configurations, and stressed lengths are illustrated for two-and three-dimensional mooring cable problems. From the numerical results, the method is found to be numerically stable, and it provides a more rational static response for mooring cables.
Three-dimensional behaviour of elastic marine cables in sheared currents
This paper presents the formulation and solution of governing equations that can be used to analyse the three-dimensional (3D) behaviour of either marine cables during installation or the response of segmented elastic mooring line catenaries as used for floating offshore structures when both are subjected to arbitrary sheared currents. The methodology used is an extension of one recently developed for analyses of marine cables when being installed on the seabed or being towed. The formulation describes elastic cable geometry in terms of two angles, elevation and azimuth, which are related to Cartesian co-ordinates by geometric compatibility relations. These relations are combined with the cable equilibrium equations to obtain a system of non-linear differential equations, which are numerically integrated by fourth and fifth order Runga-Kutta methods. The inclusion of cable elasticity and the ability to consider arbitrary stored currents are key features of this analysis. Results for cable tension, angles, geometry and elongation are presented for three example cases-the installation of a fibre optic marine cable, the static analysis of a deep water mooring line and the response of a telecommunications cable to a multi-directional current profile. ᭧
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.
Dynamic Response of Coupled Mooring Lines and Floating Wave Energy Devices in Waves / Currents
International Journal of Innovative Technology and Exploring Engineering, 2019
Various global studies have shown that ocean waves energy have large potential in renewable energy sector. Their role within renewable energy gets high priority in the future by the government of United Kingdom. The principle concept of wave energy is when wave energy is converted into potential energy by the wave energy devices to generate electricity. An understanding of the dynamic response of the devices and mooring lines is important for this paper. This paper deals with the analysis of the various effects that influence the different design of wave energy converter devices. The mooring design idea is also analyzed to show which mooring layout is suitable to fulfill the requirement. The design of mooring configuration also influence how wave power is extracted and how such system are operated and maintained. The effects investigated in this paper are regular and irregular waves, motion @ six degrees of freedom, maximum and minimum mooring tension, different waves direction, wav...
OMAE2003-37465 MODELLING OF SEABED INTERACTION IN FREQUENCY DOMAIN ANALYSIS OF MOORING CABLES
Mooring cables under wave loading interact dynamically with the seabed; this interaction is nonlinear and can be modelled in full only by performing lengthy time integration of the equations of motion. However, time domain integration is far too computationally expensive to be carried out for all load cases. A new method of modelling the interaction between a cable and the seabed in the frequency domain, but without considering fric-tional effects and impact, is therefore proposed. The section of cable interacting with the seabed is truncated and replaced with a system of coupled linear springs, with stiffnesses linearised from static catenary equations. These springs would model the behaviour of the truncated cable and hence the time-varying boundary condition at the touchdown. The entire cable-spring system is then analysed in the frequency domain with a centred finite difference scheme. The proposed method has shown to increase the accuracy of frequency domain analysis in certain cases with affordable computational overhead.
Mooring Dynamics: Computer Models and Experiments at a Sixty Foot Scale
Ihree cases from the experiment conducted in the hydroballistics tank of the Naval Surface Weapons Center in 1976 are compared to the SNAPLOAD and SEADYN computer models. Two of theruns simulate the anchor-last deployment of a mooring; the third shows the relaxation of a mooring displaced laterally, then released. The quality of the experimental data is evaluated by comparing each case to the static, elastic catenary equations at the start and finish of each run. The measured positions of points along the static catenaries are found typically to agree with the catenary calculations within 1 to 2 percent of the cable length. Tension measured at the fixed end typically agrees with the calculated value within about 12 percent. The SEADYN and SNAPLOAD computer models are found to reproduce all the significant motion and forces observed in the experiment. The "handbook" drag coefficients programmed in these models allow the cable motion sometimes to lead the data, sometimes to lag behind. More specific coefficients must be used when the rate of the dynamic motion is critical. Neither model included elastic hysteresis. The SEADYN program gave somewhat erratic tension values in the mooring line because tension waves were not damped by hysteresis. The SNAPLOAD model eliminated the tension variation through artificial damping. wie A lieC un.e Uq I. .°. .... . UNCLASSIFIED SgCURITY CLASSIPICATION OF 'MIS P446Mlhen Onto tntofed) 12