Abstract
This paper analyses the numerical outcome of applying three different well-known mooring design approaches to a floating wave energy converter, moored by means of four catenary lines. The approaches include: a linearized frequency domain based on a quasistatic model of the mooring lines, a time domain approach coupled with an analytic catenary model of the mooring system, and a fully coupled non-linear time domain approach, considering lines’ drag and inertia forces. Simulations have been carried out based on a set of realistic combinations of lines pretension and linear mass, subject to extreme environmental conditions. Obtained results provide realistic cost and performance indicators, presenting a comparison in terms of total mooring mass and required footprint, as well as the design line tension and structure offset. It has been found that lines’ viscous forces influence significantly the performance of the structure with high pretensions, i.e., >1.2, while there is acceptable agreement between the modelling approaches with lower pretensions. Line tensions are significantly influenced by drag and inertia forces because of the occurrence of snap loads due to the heaving of the floater. However, the frequency domain approach provides an insight towards the optimal design of the mooring system for preliminary designs.
Highlights
The early stage design of mooring systems for offshore floating wave energy technologies is affected by high uncertainty of the estimated required investment
The approaches include: a linearized frequency domain based on a quasistatic model of the mooring lines, a time domain approach coupled with an analytic catenary model of the mooring system, and a fully coupled non-linear time domain approach, considering lines’ drag and inertia forces
In order to compare the influence of the non-linear effects, the quasistatic-frequency domain method (QSFD) results have been considered as a baseline whilst the results of both dynamic time domain method (DynTD) and quasistatic-time domain method (QSTD) models are compared with the baseline
Summary
The early stage design of mooring systems for offshore floating wave energy technologies is affected by high uncertainty of the estimated required investment. This approach is denoted here as quasistatic-frequency domain method (QSFD) It is recommended [3] to simulate the floating structure low frequency motions in the time domain coupled with the analytical catenary equations, which provides a force in all degrees of freedom every time step of the simulation. This method introduces the non-linear geometric stiffness in floaters’ motions, unlike the previous model in which it was linearized. The most sophisticated model, and in turn time consuming, consists in accounting for the non-linear geometric stiffness as well as lines’ drag and inertia forces in the time domain, fully coupled with the structure This approach is denoted here as the dynamic time domain method (DynTD)
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