A Contribution on Quasi-Static Mooring Line Damping

Abstract

Since the pioneering work of Huse (1986, “Influence of Mooring Line Damping Upon Rig Motions,” Proc., 18th OTC Conference), it is well known that mooring lines may account for a large fraction of the overall damping present in a moored floating structure. This paper is concerned with the mooring line damping induced by the low-frequency, quasi-static, horizontal motion of the mooring line fairlead. The main advantage of the quasi-static approach is that it is much faster than the more accurate finite element methods, and, secondly, that it does not require any finite element modeling skills. A new formulation is proposed and is compared to the results of Liu et al. (1998, “Improvement on Huse’s Model for Estimating Mooring Cable-Induced Damping,” Proc., 17th OMAE Conference), as well as to time domain results obtained with FLEXRISER. The improvement with respect to the previous quasi-static methods is quite notable and our results are closer to FLEXRISER predictions. Finally, quasi-static results are compared to mooring line damping values measured during model tests for full mooring systems. The agreement between the two is very encouraging and suggests that the simpler quasi-static approach may, in some circumstances, be a valuable substitute for the more complex and time-consuming numerical tools. [S0892-7219(00)00102-3]

An Improvement of a Semi-Coupled Analysis for a Turret Moored FPSO in Deep Water

“…Most of the works are based on estimations of energy dissipation by the quasi-static method (Huse and Matsumoto, 1988, Liu and Bergdahl, 1998, Bauduin and Naciri, 2000 or the dynamic method (Huse and Matsumoto, 1989, Webster, 1995, Brown and Mavrakos, 1999. Most of the works are based on estimations of energy dissipation by the quasi-static method (Huse and Matsumoto, 1988, Liu and Bergdahl, 1998, Bauduin and Naciri, 2000 or the dynamic method (Huse and Matsumoto, 1989, Webster, 1995, Brown and Mavrakos, 1999.…”

Mooring Line Damping Estimation for a Floating Wind Turbine

“…Hydrodynamic drag and friction damping are significantly affected by the motion of a floating platform aside from the internal damping caused by material properties. Many studies investigated the contribution of mooring line damping to the global responses of a deepwater floating platform, such as the quasistatic model series of mooring damping calculation [ 16 – 19 ]. Webster [ 20 ] used a time domain finite element approach for the parametric study of mooring line damping induced by both horizontal and vertical top end oscillations, and the parameters used in this analysis step from a dimensional analysis.…”

Mathematical Modelling of Mooring Systems for Wave Energy Converters—A Review

“…The mooring force on the WEC therefore consists only of the position dependent static restoring force, shown for example in Figure 8. Implementation of quasi-static models for offshore mooring applications, and a review of their development, can be found in [72,84,94,95] and, for the specific case of wave energy applications, in Nava et al [96].…”

“…Huse [70] had previously proposed that drag forces, due to the motion of the mooring lines through the water, were the main source of energy dissipation, the reason being that a small offset of the top end can result in large transverse displacements along the mooring line ( Figure 6), inducing considerable drag forces. A number of other authors, such as Liu [71], Bauduin and Naciri [72], and Lie et al [73], have built upon the early work of Huse [69,70], to improve the modelling of mooring line-induced damping.…”

“…The dynamic effects on the mooring system are ignored by this method, omitting the motion dependency of mass, damping and fluid acceleration on the system [20]. Quasi-static models derive the mooring line shape and tension from the catenary formulations, based on the assumption that the line is in static equilibrium in each time step, that inertia effects can be neglected and that the line profile is reasonably well described by the catenary equations [72]. The mooring force on the WEC therefore consists only of the position dependent static restoring force, shown for example in Figure 8.…”