Thesis

Drag coefficient and damping of moorings and their effect on FPSO's response

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Awarding institution
  • University of Strathclyde
Date of award
  • 2014
Thesis identifier
  • T13806
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Abstract
  • The exploitation of oil and gas is gradually moving to deep-water areas, where FPSOs are particularly effective and economical. Damping effect of moorings and risers is especially important to surge excitation of the FPSO vessel due to its lowdamped characteristic and hence coupled analysis is normally preferred to accurately predict the floater motions as well as the mooring and riser system response. The aims of this thesis are to investigate mooring line damping, drag coefficient of mooring chain and their effect on the motions of FPSOs. In particular the aim is to determine the CD values of a stud-less chain under different flows by numerical methods due to limited CD data for chains. A turret moored FPSO with 20 moorings operating in 400m water-depth is selected. Effects of environmental loads, including floater motions, current and waves, on the mooring line damping are investigated. Effect of random WF motions on mooring line damping, according to the study, can be represented by an equivalent harmonic WF motion if the variation of CD is negligible. So the estimated mooring line damping has more practical applications and can be used as a reference in uncoupled analysis. Mooring line damping is mainly caused by hydrodynamic drag force, so variations of CD would definitely affect the mooring line damping and line dynamic tension. Through the sensitivity study, it is found that at the top and town-down zone of a mooring line, where chain segment is normally used for a chain-wire-chain line, mooring line damping and dynamic tension are more sensitivity to the CD variations. This indicates the importance of CD selection for chains. Therefore, drag coefficients of a smooth stud-less chain under different flows are determined by CFD methods due to their recent advancement and the limitations of experiments. Validations of numerical models are completed by simulating steady and unsteady flows past a smooth circular cylinder. Performance of two different turbulence models, namely LES and k - ω SST models, is assessed by comparison of results obtained by these two models. Finally, by coupled analysis, damping effect of moorings on the motions of the moored FPSO system is investigated with CD variation taken into consideration. It is observed that a change of CD by a factor of 2.0 for mooring lines will decrease the vessel's LF surge motion and noticeably increase the dynamic WF tension of lines due to the increase of mooring line damping. Overall, this study has widened the knowledge about the hydrodynamic coefficients of chains under different flows. It also deepens the understanding of mooring line damping and its effect on the motions of the FPSO system. Through the study, it is known that mooring designers need to pay special attention to the touch-down zone for catenary moorings, not only due to the structure reliability but also because of the hydrodynamic implications.
Resource Type
DOI
Date Created
  • 2014
Former identifier
  • 1038260

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