Validation of a Tool for the Initial Dynamic Design of Mooring Systems for Large Floating Wave Energy Converters

Abstract

Mooring of floating wave energy converters is an important topic in renewable research since it highly influences the overall cost of the wave energy converter and thereby the cost of energy. In addition, several wave energy converter failures have been observed due to insufficient mooring systems. When designing these systems, it is necessary to ensure the applicability of the design tool and to establish an understanding of the error between model and prototype. The present paper presents the outcome of an experimental test campaign and construction of a numerical model using the open-source boundary element method code NEMOH and the commercial time-domain mooring analysis tool OrcaFlex. The work used the wind/wave energy converter Floating Power Plant as a case study, which is defined as a large floating structure with a passive mooring system. The investigated mooring consists of a three-legged turret system with synthetic lines, and it was tested for both operational and extreme events. In order to understand the difference between the model and experimental results, no tuning of the model was done, besides adding drag elements with values found from a simplified methodology. This resembles initial design cases where no experimental data are available. Generally good agreement was found for the tensions in the lines when the drag element was applied, with some overestimation of the motions. The main cause of difference was found to be underestimation of linear damping. A model was tested with additional linear damping, and it illustrated that a final analysis needs to use experimental data to achieve the best results. However, the analyses showed that the investigated model can be used without tuning in initial investigations of mooring systems, and it is expected that this approach can be applied to other similar systems.

KNSwing—On the Mooring Loads of a Ship-Like Wave Energy Converter

“…Work on numerical models of the KNSwing and its mooring system has previously been described and used in [13]. However, in [13], there was no validation against experimental data and the simplified methodology described in [14] was applied. The present paper continues the work on the numerical model and utilizes the present experimental data to validate and calibrate the model.…”

“…Nemoh does not account for viscous drag forces and hence, Morison drag elements [18] are added to account for the viscous forces on the structure. The methodology described in [14] is used to define the drag elements. Based on the geometry, coarse estimates of the drag coefficients are defined.…”

Cost Optimization of Mooring Solutions for Large Floating Wave Energy Converters

“…Often, the boundary element method (BEM) is used, which assumes inviscid, irrotational fluids and uses linear potential flow theory with its assumption of small steepness waves and small amplitude structure motion. In the ULS, this theory is stressed significantly, due to the extreme wave condition and compliant mooring, which allows large motions, but this work builds on the experience gained in previous work [15] where the applicability was validated for initial design and analysis.…”

“…The output of the software package is a time series of line tension and WEC motions. The construction of the numerical model follows the procedure presented in [15].…”

“…[12] Investigation of design procedures and tools [13][14][15] Optimization of mooring solutions Figure 1. Flow diagram of the previous and related work [11][12][13][14][15] in the Mooring Solutions for Large Wave Energy Converters (MSLWEC) project.…”