Nord Stream 2: impacts of construction in Swedish waters were minor, local and short-term

The construction activities carried out in the Swedish EEZ during 2019 included rock placement and mattress installation prior to pipelay, pipelay of both lines as well as post-lay trenching, and rock placement.

The monitoring of potential impacts of these activities focused on underwater noise and their potential impacts to marine mammals (specifically to harbour porpoises), turbidity in a Natura 2000 area, cultural heritage and ship traffic, as defined in the monitoring programme approved by the national authorities.

Underwater noise was monitored mainly due to concerns for disturbance of behaviour and masking effects on harbour porpoise in a Natura 2000 area. Also, the presence of harbour porpoises was documented by using acoustic devices.

The underwater noise monitoring results show that

• underwater noise generated by the pipelay vessels and their support vessels was comparable in level and frequency to that from commercial cargo ships in the area.
• Underwater noise from rock placement was slightly lower than that from the pipelay vessels and with comparable frequency spectrum. Harbour porpoises were detected both before and after pipelay. Overall, the recorded occurrence of harbour porpoises in the study area was relatively low.
• The integrity of the Natura 2000 site was not compromised by the construction of the pipeline. The pipelay for Nord Stream 2 was also faster than during construction of the Nord Stream Pipelines, thus the time spent inside the Natura 2000 area was shorter than that assumed in the impact assessment.
• The measurements obtained provide valuable new knowledge about the underwater noise from construction and other large vessels, but also on the general acoustic soundscape of the central Baltic Sea.

Turbidity* measurements were performed to verify that no detrimental sediment concentrations, resulting from the trenching activities (lowering the pipeline with a plough, would reach sensitive shallow banks within the Natura 2000 area of Hoburgs bank and Midsjöbankarna.

The monitoring of turbidity during trenching works showed that

• No sediment plumes with concentrations above the threshold value agreed with the authorities (15 mg/l) reached the shallow banks.
• The highest value measured directly behind the plough was 24.7 mg/l, hence making it extremely unlikely that high sediment concentrations were able to reach the sensitive areas more than 5 km away.
• The impacts were assessed to be local and of low intensity, and thus considered insignificant with respect to the protected areas.

Furthermore, construction activities did not have any impact to five cultural heritage objects identified before pipelay.

No accidents or incidents occurred during construction involving ship traffic, including fishing vessels. The impact on ship traffic was confirmed to be minor, local and short-term.

Monitoring, data processing and reporting were conducted by independent Swedish and Danish expert organisations. The report has been submitted to and accepted by the relevant national authorities.

The purpose of Nord Stream 2’s environmental monitoring is to verify the actual impacts of construction activities. It also confirms fulfilment of the national permit requirements and commitments made, and monitors the recovery of the environment after construction. By the end of 2020, the company will have invested over €100 million in environmental surveys, assessments, analyses, monitoring and conservation activities.

Read the article online at: https://www.worldpipelines.com/project-news/07092020/nord-stream-2-impacts-of-construction-in-swedish-waters-were-minor-local-and-short-term/