Mooring Line Damping Research Articles

An improved method of mooring damping estimation considering mooring line segments contribution

The accurate estimation of mooring line damping is significant for the mooring system design and the global motion responses analysis. Aiming at the existing method of mooring damping estimation can only calculate the total damping contribution of mooring line, an improved method is proposed to accurate estimate the mooring line segments damping contribution to the floating structures. The dynamic responses analysis model of mooring line is established firstly. Secondly, the fluid drag force on the mooring line element and its working energy dissipation are obtained. Then, the mooring damping of each segment of mooring line could be calculated through piecewise integration combined with the indicator diagram method. Finally, the proposed method is validated by comparing with the experimental results of single catenary chain with the top end forced excitation in the wave tank. The results show that the proposed method could both accurately estimate the damping contribution of whole mooring line and each segment, which is suitable for the mooring damping estimation of single component and multi-components mooring line.

An Experimental Study of Mooring Line Damping and Snap Load in Shallow Water

The aim of this paper is to establish a simple approach to experimentally study the mooring line damping in shallow water, where snap loading may occur more frequently. Experimental measurements were conducted in a wave basin at a scale of 1:50, which corresponds to a full scale of 28 m water depth. A chain made by stainless steel was used, and the tension force at the fairlead was measured by tension gages. Moreover, the line geometry, touchdown point speed, and mooring line velocity were derived from image processing techniques. Surge motions at fairlead were driven from a programmable wavemaker. Regular surge motions with different frequencies and pretensions were tested in this system in order to investigate the quasi-static and dynamic behaviors of the mooring chain. In the quasi-static test, the mooring line keeps a typical catenary shape, and its indicator diagram exhibits a smooth-closed curve. In the dynamic test, the mooring line is fully lifted from the seabed, and it cyclically goes through the stage of semitaut and fully taut. We successfully reproduced a snap event in the laboratory scale, and the resulting mooring line damping can considerably increase in this manner. Two criteria for snap event were examined, and both of them were verified by the experiments.

An improved quasi-static model for mooring-induced damping estimation using in the truncation design of mooring system

The horizontal motions of floating oil and gas structures by a mooring system can be modeled as a resonant oscillator whose natural frequencies may be much lower than wave frequencies. Thus, the low frequency (LF) excitation caused by the irregular wave results in resonant motion responses in the horizontal plane. For instance, mooring lines may provide 80% of the total LF surge motion damping of a moored tanker in water depth of 200m, thus reducing the floater resonant excursion significantly. Therefore, when designing truncated mooring system for model testing, mooring damping equivalence should be considered. Thus, simple but effective method for predicting mooring line damping is required for designing truncated mooring system. Different with other quasi-static methods for estimating mooing line damping, besides normal drag forces, tangential drag forces and seabed friction are also considered in the improved quasi-static model presented in this paper. The analytical results based on the improved method are compared to other quasi-static and time domain results for a chain mooring line and a wire line. In addition, model tests of a 1:70 scaled catenary mooring line are conducted. Then, the improved quasi-static results, the AQWA results and the experimental results are compared.

Mooring line damping due to low-frequency superimposed with wave-frequency random line top end motion

The slow drift motions would lead to a serious influence on moored floating structures and cause the failure of mooring and riser systems. Mooring line damping which represents the transfer of energy is important for moored floating structures. In this paper, time domain finite element method was applied by using OrcaFlex. A series of mooring line top end motions was simulated to investigate the relationship between mooring line damping and low-frequency superimposed with wave-frequency random motion. A transformation method was introduced that wave-frequency random motion was transferred to an equivalent sinusoidal motion based on the spectral density of vessel motion. Then, the influence of equivalent sinusoidal motion and random motion on mooring line damping was compared. It can be found that mooring line damping could be reduced slightly if considering random motion. Finally, the influence of individual parameter which includes current speed, drag coefficient, added mass coefficient and pre-tension on mooring line damping was studied. The results showed that the significant status of drag coefficient and pre-tension on the predication of mooring line damping. But for current speed, the effect on mooring line damping cannot be overstated for considering random motion but the reverse is true for considering sinusoidal motion.

Numerical investigation of mooring line damping and the drag coefficients of studless chain links

The chain/wire rope/chain combination is a common choice for mooring offshore floating platforms. However, data of the drag coefficients of chain links are rather limited, resulting in uncertainties with the calculations of the drag force, and hence the damping of the mooring system. In this paper, the importance of the selection of the drag coefficient is first investigated. The computational fluid dynamics (CFD) method is then used to determine the drag coefficients of a studless chain under steady flows. Numerical model validation is first completed by simulating a smooth circular cylinder under steady flows. In particular, the performance of different turbulence models is assessed through the comparisons between the calculations and the experimental results. The large eddy simulation (LES) model is finally selected for the simulation of steady flows past a chain. The effects of the Reynolds number on the drag coefficient of a stud-less chain is also studied. The results show that the calculated drag coefficients of a stud-less chain are fairly consistent with the available experimental data.

NON-LINEAR DYNAMIC ANALYSIS OF COUPLED SPAR PLATFORM

Spar platforms are treated as cost-effective and resourceful type of offshore structure in deep water. With increasing depth there are significant changes in its structural behaviour due to coupling of spar hull-mooring line along with radical influence of mooring line damping. So these phenomena should be precisely counted for accurate motion analysis of spar mooring system. In present study, spar platform are configured as a single fully coupled integrated model in ABAQUS/AQUA. Non-linear dynamic analysis in time domain is performed adopting Newmark-β automatic time incrementation technique. Non-linearities due to geometric, loading and boundary conditions are duly considered. Displacement and rotational responses of spar and mooring tensions are obtained during long-duration storm. spar responses get significantly modified and mean position of oscillations gets shifted after longer wave loading. The surge, heave and pitch responses are predominantly excited respectively. The energy contents of PSDs of these responses reduce considerably after long wave loading. Mooring tension responses are significantly different reflecting the damping effect of mooring lines. The pitch response is fairly sensitive to the wave loading duration. After long duration of storm the wave frequency response increases. However, low frequency and wave frequency responses may simultaneously occur due to synchronising sea states.

Measurements of static and dynamic mooring line damping and their importance for floating WEC devices

The dynamic response of the mooring line will be a dominant factor to consider in their use for the station keeping of a wave energy converter (WEC). Due to the relatively small size of WECs and their being moored in relatively shallow waters the effect of waves, tide and current can be of greater significance than for other floating offshore systems. Axial line stretching and high-frequency ‘top-end’ dynamics can importantly modify damping and top-end loading. If a ‘farm’ of devices is to be considered then limitations in sea space may necessitate that the devices be relatively densely packed. This will mean that the ‘footprint’ of the mooring should be constrained, to ensure that the moorings from each device do not interfere and this will have great significance for the loading experienced by the line. One must also consider how the mooring system might change the response of the WEC and so alter its ability to extract power from the waves. Unlike a typical offshore system, the design of moorings for a WEC device must consider reliability and survivability, and the need to ensure efficient energy conversion. The design and operation of a chain mooring for a WEC is considered here. Generic experimental measurements of mooring line damping were conducted in the Heriot-Watt University wave basin at a scale of 1:10. The measurements were conducted on a single mooring line for surge motions and include the study of axial stretching and high top-end dynamics. The laboratory procedures were designed to resemble tests undertaken earlier at ‘full’ scale in 24 m water depth. The measurements were also compared with numerical studies. The experimental findings for WEC devices, supports the conclusion that dynamic mooring line motion will be an important variable, needing to be considered carefully within the design.

Effects of mooring line drag damping on response statistics of vessels excited by first- and second-order wave forces

The drag-induced damping in a mooring cable due to combined first- and second-order wave excited motion of a moored vessel has been determined by statistical linearisation. A dynamic stiffness approach developed elsewhere is used to deal with the dynamics of the mooring cables. The power spectral densities of low- and wave-frequency responses are obtained which clearly show the influence of mooring line damping. The non-Gaussian probability density functions (pdf) and expected crossing rates of vessel responses and dynamic cable tensions are determined using the Kac–Seigert technique, and the influence of drag damping is highlighted.

Slow motion dynamics of turret mooring and its approximation as single point mooring

The mathematical model for the nonlinear dynamics of slow motions in the horizontal plane of Turret Mooring Systems (TMS) is presented. It is shown that the TMS model differs from the classical Single Point Mooring (SPM) model, which is used generally to study the TMS dynamics. The friction moment exerted between the turret and the vessel, and the mooring line damping moment resulting from the turret rotation are the sources of difference between TMS and SPM. Qualitative differences in the dynamical behavior between these two mooring systems are identified using nonlinear dynamics and bifurcation theory. In two- and three-dimensional parametric design spaces, the dependence of stability boundaries and singularities of bifurcations for given TMS and SPM configurations is revealed. It is shown that the static loss of stability of a TMS can be located approximately by the SPM static bifurcation. The dynamic loss of stability of TMS and the associated morphogenesis may be affected strongly by the friction/damping moment, and to a lesser extent, by the mooring line damping. Nonlinear time simulations are used to assess the effects of these properties on TMS and compared to SPM systems. The TMS mathematical model consists of the nonlinear horizontal plane fifth-order, large drift, low speed maneuvering equations. Mooring line behavior is modeled quasistatically by submerged catenaries, including nonlinear drag and touchdown. External excitation consists of time independent current, steady wind, and second-order mean drift forces.

THE EFFECTS OF MOORING LINE DAMPING ANDWAVE DRIFT DAMPING ON MOORED TANKERDYNAMICS

This paper is concerned with the analysis of the dynamic behavior of a tanker, acting as a FPSO, moored through a differentiated compliance anchoring system (DICAS). It is presented, initially, the mathematical models of the moored tanker dynamics, with special consideration on the description of the interaction between waves and current, and the mooring line damping The mathematical formulation is used to simulate the performance of a 130kDWT tanker in the DICAS configuration. The simulation results are compared with model test data in order to evaluate the accuracy of the models proposed to represent the wave drift and mooring line damping.

Advances in Mooring Line Damping

This paper describes recent fundamental and applied work investigating the damping forces induced on floating vessel and in particular catenary mooring lines. A description of new tests being performed on large scale sections of mooring line to reveal damping contributions is also presented.