On Mooring Design of Wave Energy Converters: The Seabreath Application

Abstract

The design of a mooring system of a Wave Energy Converter is a challenging process that points out several unsolved technical problems, mostly related to the highly non-linear hydrodynamic phenomena occurring when high waves (e.g. 8 m high with 200 m wavelength) propagate in relatively shallow waters (e.g. 20 m). The aim of this note is to point out the relevance of the non-linear response of a WEC anchored in relatively shallow waters (shallow in the “non-linear” sense) in terms of loads applied to the mooring lines. Further, the effects of this cyclic load on the anchors is investigated. Note that to some extent it is like checking the importance of geotechnical and coastal engineers in the design process of the WEC structure and its mooring system (typically carried out by naval architects). The whole mooring design process is first outlined and then it is schematically applied to a specific case, namely a promising Italian device named SeaBreath (www.seabreath.it), in view of a possible deployment in the Adriatic Sea. The main concern of mooring designers is related to resonance effects induced by the second order drift. Therefore specific tests have been carried out in the 36 m long x 1.0 m wide x 1.3 m high wave flume of Padova University. Tests focused on the forces on the mooring lines induced by the sum of two regular waves of similar frequency. The mooring design is still far from complete: the physical model proved the relevance of the aforementioned effects but a numerical investigation (not yet performed) is required to draw final conclusions.

Cost Optimization of Mooring Solutions for Large Floating Wave Energy Converters

“…According to [4,5], several parameters can be improved and take a considerable part in the cost reduction of wave energy. Despite different evaluations of the importance, station keeping moorings are listed as a driver towards lower cost, as they are estimated by [6,7] to compose 20-30% of the total structural cost of a WEC. In [8], the mooring is estimated to take up 8% of the CAPEX cost.…”

“…In addition to the cost, by now, mooring has also taken part in the failure of several WECs due to insufficient durability of the mooring system [7,9,10]. Consequently, a Danish research project entitled "Mooring Solutions for Large Wave Energy Converters" (MSLWEC) was initiated in 2014, which aimed at reducing the cost of the system, improving the applied design procedure and increasing the durability of the systems.…”

Mathematical Modelling of Mooring Systems for Wave Energy Converters—A Review

“…Fitzgerald and Bergdahl [22] and Martinelli et al [23] also detail many important aspects of mooring design for WECs. For example, Fitzgerald and Bergdahl [22] uses MMs to compare different alternatives for the design of the mooring cables of a point absorber WEC in 50-m water depth.…”

“…For some classes of directional floating WECs, such as terminator or attenuator devices, the mooring system must allow the WEC to weathervane into the predominant wave direction, while for axisymmetric point absorber type WECs the wave directionality is not important. Mooring systems have been classified into three categories, depending on their relevance to the WEC operation [15,[23][24][25]; passive, active and reactive:…”

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

“…In addition, the cost of moorings has proven to take up a large part of the total structural cost [5,6] and is, therefore, vital to investigate and improve in order to lower the LCOE. Despite a large experience in mooring design from other offshore sectors, a large number of the WECs deployed offshore have failed due to insufficient moorings [5]. This can partly be explained by a tendency to apply mooring principles from the traditional oil and gas sector with a catenary system of mooring chains.…”