Nonlinear Dynamic Analysis of Multi-component Mooring Lines Incorporating Line-seabed Interaction

Abstract

In this study, a deterministic approach for the dynamic analysis of a multi-component mooring line was formulated. The floater motion responses were considered as the mooring line upper boundary conditions while the anchored point was considered as pinned. Lumped parameter approach was adopted for the mooring line modelling. The forces considered were the submerged weights of mooring/attachment, physical/added inertia, line tension, fluid/line relative drag forces and line/seabed reactive forces. The latter interactions were modelled assuming that the mooring line rested on an elastic dissipative foundation. An iterative procedure for the dynamic analysis was developed and results for various mooring lines partially lying on different soils were obtained and validated by conducting a comparative study against published results. Good agreement between numerical and published experimental results was achieved. The contribution of the soil characteristics of the seabed to the dynamic behaviour of mooring line was investigated for different types of soil and reported.

Offshore floating vertical axis wind turbines, dynamics modelling state of the art. Part II: Mooring line and structural dynamics

“…all force contributions from external forces, mass and internal reactions at a specific number of nodes along the length of the mooring line. This results in obtaining a set of differential equations by assuming dynamic equilibrium and stress/strain continuity at each node (see Kurian et al [19]). Its ability to accommodate large-amplitude, three-dimensional motions and hydrodynamic drag forces, whilst not being too computationally expensive, makes this approach very attractive.…”

“…More detailed dynamic models utilising the finite-element (FE) approach are also possible, but the finer discretisation and more complex mathematical formulation of the mooring lines may lead to more computational intensive simulations that is undesirable for FOWT coupled dynamics codes, as will be discussed in §4. As highlighted by Kurian et al [19], the FE approach makes use of interpolation functions to calculate an internal system state in terms of the mooring line nodal displacements within a generalised coordinate system. Hence by applying these interpolation functions to the established kinematic relations, and also dynamic equilibrium, the motion of each mooring line element may be obtained.…”

Evaluation and mitigation of modelling errors in numerical simulations of mooring line and seabed contact

“…The structural mass matrix is a diagonal matrix. Structural damping is often excluded as in References [50,66,[73][74][75], but may also be included [65,76] to improve numerical stability. For chain-link mooring lines, usually bending and torsion effects are excluded.…”

“…Buckham [78] noted that the lumped mass approach is simple, versatile and facilitates the assembly of multisegment, multicomponent line assemblies. Kurian et al [75] demonstrated this by generating a lumped mass discretization for a multicomponent line that includes buoys and clump weights for imposed fairlead motions at the fairlead.…”