Viscous damping levels in mooring computations and the effect of increasing water depth

Typ
Examensarbete för masterexamen
Master Thesis
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2013
Författare
Carlsson, Mikael
Eriksson, Robert
Modellbyggare
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With an increased demand for operations in deeper water, the requirements of vessels to withstand the environmental loads and stay in position become more important. The increased water depth puts higher requirements of the mooring system; the weight of the lines is an important factor since it affects the amount of payload. The mooring system can be made less heavy if the line damping contribution is included in the mooring calculations and so reduces the motions as well as the line tensions. In the classification rules there is limited information regarding the damping contributions from the mooring lines and the guidance note in DNV-OS-E301 gives conservative and rather low damping levels for mooring systems. The aim with this thesis is to achieve a better understanding of when and how the viscous damping of mooring lines and risers should be accounted for. In addition to this, the aim is to establish realistic levels for viscous damping contribution from mooring lines and to compare the time-domain and frequency-domain methods with regard to the mooring-line damping. The study has been performed on a generic semi-submersible drilling platform at 5 water depths ranging from 100 to 1,250m. The study shows that the performed simulations in time-domain compared to frequency-domain methods give a greater contribution of mooring line damping to the system. The damping levels in surge from mooring lines are between 8 and 28 per cent of the critical damping and in sway 10 and 36 percent, depending on water depth and heading. The damping in absolute numbers decreases with increasing water depth, although the relative percentage of critical damping remains at the same order of magnitude. For further investigation a different system is recommended with a higher pre-tension in order to achieve a stiffer system and to not allow such large static displacements, and, in addition, also to include risers to study the influence on the damping. Moreover, the damping is dependent on sea state and it is of interest to perform the study in another sea state than the extreme sea state used.
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Transport, Farkostteknik, Transport, Vehicle Engineering
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