A Comparison of Numerical Approaches for the Design of Mooring Systems for Wave Energy Converters

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.

Frequency domain modelling of a coupled system of floating structure and mooring Lines: An application to a wave energy converter

“…Two diffracting bodies have been modelled, the first body (dofs 1 to 6) represents the geometry of the spar, shown in Figure 2. The outer diameter specified in Figure 2 refers to the larger diameter of the floater both at the top and at the bottom, the OWC diameter indicates the diameter of the internal water column, all dimensions are defined in [15], model 'K' and also used as a case study in [5]. The second body in the numerical model (dofs 7 to 12) is a massless surface at the internal water surface to model its motions.…”

“…An initial approach can be the linearization of the nonlinear geometric stiffness at the mean horizontal position of the structure as suggested in [4]. Through this procedure main horizontal motions are acceptably reproduced whilst just the order of magnitude of line tensions can be estimated, as pointed out in [5] for a wave energy converter (WEC).…”

Dynamic Stability and Protection Design of a Submarined Floater Platform Avoiding Typhoon Wave Impact

“…Davidson and Ringwood [5] reviewed the mathematical models for mooring systems for the wave energy converters. Touzon et al [8] investigated three different well-known mooring design methods applied to a floating wave energy converter system moored by four catenaries. The three research methods that have been used are linearized frequency domain, based on the quasi-static model of mooring cables, time-domain methods for analyzing catenary models combined with mooring systems, and fully coupled nonlinear time-domain methods.…”

Numerical Approaches for Loads and Motions Assessment of Floating WECs Moored by Means of Catenary Mooring Systems

“…Several standardization bodies provide guidelines on how to apply such methods for traditional offshore structures based on quasistatic mooring lines 11 . However, wave energy conversion structures are very dynamically excited and further assumptions may be necessary in order to use the mentioned quasistatic methods, as suggested in 12 .…”