Abstract
Catenary mooring lines experience touchdown and liftoff from the seabed as the applied fairlead tension varies with time. The numerical modelling of touchdown/liftoff effects requires a finer discretisation at the touchdown point to avoid the production of spurious line tension fluctuations. The disparity in element sizes within and outside the local refinement zone gives rise to a stiff dynamical system. As the touchdown point changes with time, the locally refined zone has to shift in tandem to limit the spatial extent of the refined domain. This work introduces an approach for applying adaptive discretisation to a numerical mooring cable model with a non-uniform mesh, and dual-rate time integration for the resultant stiff dynamic system.
Highlights
A catenary mooring line provides a restoring force to the floating structure it is connected to at its fairlead primarily through varying its suspended weight
The time-varying line tension leads to accumulated fatigue damage, which tends to be most critical at the fairlead for catenary mooring lines (API, 2005; Yang et al, 2016; Wu et al, 2015)
This paper presents an adaptive discretisation method applied to a lumped-mass mooring line model, limiting the refined region close to the touchdown zone where nodal grounding and liftoff occurs
Summary
A catenary mooring line provides a restoring force to the floating structure it is connected to at its fairlead primarily through varying its suspended weight. Applying the rainflow counting method, Yang et al (2016) showed that the evaluated fatigue damage is sensitive to the appearance of possibly spurious fluctuations in the line tension time history Such tension fluctuations have been found to be associated with impact loads from seabed contact (Thomas, 1993; Wang et al, 2009; Palm et al, 2017), and can be mitigated by reducing the element size in a numerical mooring line model (Thomas, 1993; Palm et al, 2017; Low et al, 2018). The quality of the results obtained and the computational cost of the solution using the proposed multi-rate, adaptive discretisation method, in comparison with uniformly coarse and fine element density meshes are presented
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