Characterising underwater noise and changes in harbour porpoise behaviour during the decommissioning of an oil and gas platform

Many man-made marine structures (MMS) will have to be decommissioned in
the coming decades. While studies on the impacts of the construction of
MMS on marine mammals exist, no research has been done on the effects of
their decommissioning. The complete removal of an oil and gas platform in
Scotland in 2021 provided an opportunity to investigate the response of
harbour porpoises to decommissioning. Arrays of broadband noise recorders
and echolocation detectors were used to describe noise characteristics
produced by decommissioning activities and assess porpoise behaviour.
During decommissioning, sound pressure levels in the frequency range 100
Hz to 10 kHz were 30-40 dB higher than baseline, with the presence of
vessels being the main source of noise. The study detected small-scale
(< 2 km) and short-term levels of porpoise displacement during
decommissioning, with porpoise occurrence increasing immediately after
this. These findings can inform the consenting process of future
decommissioning projects.