Monitoring the condition of Marine Renewable Energy Devices through underwater Acoustic Emissions: Case study of a Wave Energy Converter in Falmouth Bay, UK

Abstract

Maintaining the engineering health of Marine Renewable Energy Devices (MREDs) is one of the main limits to their economic viability, because of the requirement for costly marine interventions in challenging conditions. Acoustic Emission (AE) condition monitoring is routinely and successfully used for land-based devices, and this paper shows how it can be used underwater. We review the acoustic signatures expected from operation and likely failure modes of MREDs, providing a basis for a generic classification system. This is illustrated with a Wave Energy Converter tested at Falmouth Bay (UK), monitored for 2 years. Underwater noise levels have been measured between 10 Hz and 32 kHz throughout this time, covering operational and inactive periods. Broadband MRED contributions to ambient noise are generally negligible. Time-frequency analyses are used to detect acoustic signatures (60 Hz-5 kHz) of specific operational activities, such as the active Power Take Off, and relate them to engineering and environmental conditions. These first results demonstrate the feasibility of using underwater Acoustic Emissions to monitor the health and performance of MREDs.

Modelling the propagation of underwater acoustic emissions for condition monitoring of marine renewable energy

“…This was briefly described in Section 2.2 and is fully detailed in Walsh et al 2016. The acoustic signature was broadband (100 Hz -1 kHz) with tonal components at 30, 60, 80 and 100 Hz.…”

“…Detailed time-frequency analyses showed that the AE signature of the active PTO system during the WEC operation were 0.5-second duration bursts up to 90 dB loud, mostly between 100 Hz and 1 kHz. Loud tonal components at 30, 60, 80 and 100 Hz, reaching 90 dB were also recorded and attributed to the device generator (Walsh et al 2016). Table 1.…”

“…Underwater AE has started to emerge as a viable monitoring method and early publications suggest that useful information can be found at frequencies below 1 kHz (Walsh et al 2015). Table 1 outlines a recent literature review, which together with measurements in the field has guided laboratory experiments investigating the underwater acoustic emission of breaking of polyester fibre ropes.…”

“…Scotrenewables Tidal Power Ltd WEC, Pelamis WEC, Verdant Tidal Turbines and two point absorbers at Lysekil have all reported an increase in emitted noise from devices due to unexpected stresses from anchors, broken blades and incorrect assembly (Walsh et al 2015).…”

“…Table 1. Summary quality matrix of AE of components relevant to MRE devices including the maximum frequency of the broadband signal detected (Walsh et al 2015 …”