A comparison of elastic mooring systems for floating wave energy converters
Typ
Examensarbete för masterexamen
Program
Naval architecture and ocean engineering (MPNAV), MSc
Publicerad
2020
Författare
Chen, Ho-Ann
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Wave energy is a widespread and abundant source of renewable energy, yet the
technology of harnessing energy from waves is still in the pre-commercial stage. One
of the key challenges in the development of wave energy converters (WEC) technology
is to ensure the long-term performance and reliability of the mooring system, which
must be designed to survice under cyclic loading, not fail due to fatigue, at a
commercially competitive cost. This thesis takes WaveEL, a heaving point-absorbing
WEC developed by Waves4Power, as the reference case to investigate the performance
of WEC systems with different elastic mooring systems in regards to fatigue life and
power absorption. As the electricity production of the heaving point-absorber WEC
relies on the cyclic heave motion of the WEC device, the long-term deployment of the
WEC systems under irregular ocean loads poses high fatigue damage on the mooring
lines. The snap loads that act on the mooring lines also decrease the mechanical life of
the mooring system.
Studies have shown that by adding tether components to the mooring system, the
fatigue life of the system can be increased, thus making the WEC systems more
commercially feasible. A tether component is a mechanical system that reduces the
responding force range and absorb the snap loads in mooring lines. It is this thesis’
objective to develop numerical models to represent tether components, and to evaluate
its impact on the WEC performance by integrating the elastic tether component into the
mooring system. This thesis investigates two mooring configurations: a 3-leg mooring
system with floating buoys between the WEC device and the anchor to decreases the
stress loading on the mooring lines, and a 2-leg mooring system with a deeper design
water depth so that the system can benefit from the elasticity of the mooring lines.
Comparisons are made between the two original mooring configurations and the same
mooring setup with the tether component implemented.
Coupled hydrodynamic-structural simulations are carried out with different mooring
configurations under operational loads. The heave motions of the WEC device and the
force response in the mooring lines under operation conditions are obtained from the
simulation results and used to calculate the fatigue life of the mooring system and the
absorbed power of the WEC system. It was found that the tether component greatly
improves the fatigue life of the mooring system, whilst not negatively affecting the
power absorption.
Beskrivning
Ämne/nyckelord
wave energy converter , mooring line , coupled simulation , elastic mooring tether , fatigue , power absorption , heaving point absorber