Decommissioning of currently existing offshore oil rigs is expected to become more frequent in the years to come, as old rigs must be removed or replaced by new ones. In the Ekofisk oil field in the North Sea, such decommissioning activities are also expected in the future. Underwater sound produced by different activities associated with decommissioning of an oil rig, such as noise from, cutting, drilling, shipping and occasionally even explosives, may have an adverse effect on animals in the area.
Four species of cetaceans (i.e. whales, dolphins and porpoises) are regularly observed in the central and northern North Sea: the harbour porpoise
(Phocoena phocoena), the white-beaked dolphin
(Lagenorhynchus albirostris), the killer whale
(Orcinus orca), and the northern minke whale
(Balaenoptera acutorostrata), a baleen whale. The area is also home to a wide variety of fish species. Of these, two are commercially important species of fish – the Atlantic herring (
Clupea harengus) and the Atlantic cod
(Gadus morhua).Hearing is the key modality for all these species for most aspects of their life. Harbour porpoises, white-beaked dolphins and killer whales all navigate and forage by emitting high intensity ultrasonic calls, and listening for the returning echoes.
Our client needed an assessment of such potential risks to marine animals, caused by decommissioning of an exemplary oil rig. This rig lies in the Ekofisk oil field, which is located in the central North Sea area. We investigated the relationship between the sound levels from decommissioning and the hearing capabilities and known response levels of cetaceans and fish. Based on these studies, we generated zones of impact with respect to detection, masking, behavioural response and hearing loss (temporary or permanent).
The results of our assessment indicate that the limits of each zone of impact are not sharp, but overlap to a large extent. They also critically depend on the background noise levels and on the age, sex and general physiological and behavioural states of the animals. The largest impact is expected from explosives. The client informed us that he was not planning to use explosives on his platforms. However, explosives are commonly used in decommissioning activities in other areas. As such, it was still important for us to provide the information on impact ranges as part of the risk assessment. We were also able to document additional cumulative effects from the other sound sources as well, that potentially could result in prolonged displacement effects.
With the help of our study, the client has the tools for reducing risks of injury and avoidance behaviour in marine life – thereby aiding in the protection of species that are of commercial and conservational importance.
Ekofisk Tank in the Northsea, prior to decommissioning