Abstract

Sudden snap events on mooring lines and hanging cables can cause spikes in tension, resulting in reduced safety factors during extreme events. For example, the mooring system of a floating offshore wind turbine (FOWT) can be exposed to wave-induced motions making the former vulnerable to snap type impact. Suitable criteria to define snap events are still largely unclear, making current design practices overly conservative.

To understand the underlying physics of snap loads on a mooring line system, this paper presents a theoretical development and an experimental parametric study of snap events. The effects of the nonlinearity of bilinear line stiffness and hydrodynamic drag force, as well as the weight of payload on snap events are investigated using the vertical hanging cable model. This cable model includes two springs in series and a payload. The bilinear spring model is designed to create nonlinear dynamic tension. A total of 108 tests were conducted in the wave tank of Tainan Hydraulic Laboratory. The excitation amplitude ranges from 0.01 to 0.04m; excitation time period ranges from 0.5 to 2s; the weight of payload ranges from 6.13 to 18.95N. The tests carried out in water are compared to those conducted in air. It is seen that the hydrodynamic drag force together with the small pretension could result in larger normalized tension ranges.

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