LARGE SCALE TESTING FOR MOORING LINE HYDRODYNAMIC DAMPING
Han mei Lin, David T Brown and Geoffrey J Lyons, Santa Fe Laboratory for Offshore Engineering, Department of Mechanical Engineering, University College London, Torrington Place, London WC1E7JE, United Kingdom
ABSTRACT
Design of the mooring system required to hold a vessel within a specified watch-circle depends on an understanding of the imposed static and dynamic environmental loads. The low frequency excitation caused by random waves and wind loading results in resonant motion responses in the horizontal plane leading to high mooring line forces. Many of the loading mechanisms are well understood, however until recently the fluid induced forces acting on the moorings were assumed to have little influence on the vessel dynamic response. Recent work has shown that the mooring system may under certain circumstances provide up to 80% of the total damping available thus significantly reducing the peak line tensions, however design data is currently sparse.
A severe drawback of much of the previous experimental work performed to measure line damping is associated with the relatively small geometric scale at which the model tests were performed (typically 1/ 100th), and the fact that it is difficult to apply the results to non-specific field scenarios.
This paper presents continuing model testing work at University College London as part of a JIP study. The objectives of the testing are to provide added mass and drag contributions from the mooring line. The tests are being carried out at full to 1/3rd scale on elements of line to ensure that the Reynolds number flow regime is as close as possible to that encountered at full scale. Line segments are tested at various inclinations and orientations to the flow to represent sections of single catenary mooring lines.
Because of the complex nature of the flow around oscillating lines it is essential to consider the hydrodynamic loading interaction due to the combined low drift frequency and wave frequency induced motion as it is known that simply superposing the two effects gives erroneous results. To this end a computer controlled hi-planar carriage to carry line specimens has been designed and installed in the wave tank at UCL to simulate these motions. Sectional added mass and drag forces are derived from measurements of the hydrodynamic loads on a range of segments. These can be used within a finite-element formulation of mooring line behaviour to estimate the damping contributions for all lines allowing results to be applied to different field scenarios and weather conditions.
