Artificial reef creation using decommissioned pipeline protection structures - environmental performance in the North Sea.

Abstract

An increasing number of subsea oil and gas (O&G) pipelines in the North Sea (NS) require decommissioning. Protection structures are installed over pipelines to safeguard them from damage, these include the placement of concrete blocks linked together with polypropylene rope (“concrete mattresses”). NS decommissioning policy requires complete removal of mattresses. However, with political impetus toward circular economies, the reuse of mattresses, for example as artificial reefs (ARs), has not been investigated. This thesis provides novel evidence regarding the potential environmental performance of these structures as ARs. Ecological assessments of O&G infrastructure are increasingly being conducted using industry collated video footage from remotely operated vehicles (ROVs), but limitations in taxonomic accuracy have been highlighted. A survey of 105 participants found that higher accuracy was exhibited by observers with prior identification experience, at lower altitudes, and following training. These results informed the methods of an ecological analysis using industry ROV footage of pipelines and protections in the NS. The structures act as ARs, and mattresses exhibited higher abundance of most taxa compared to bare pipelines and rock dump, with habitat complexity identified as a potential driver. The habitat complexity offered by mattresses, at different scales, and in different decommissioning scenarios was calculated from three-dimensional models of the structures. Mattresses broken up and randomly deployed as individual blocks provided the highest level of complexity at all scales, with the potential to enhance fisheries and provide habitat. To assess suitability of reuse, heavy metal contamination of decommissioned mattresses was examined. Mattresses exhibited metal concentrations below historic averages in the NS, with no variation observed depending on distances from contamination sources. The evidence from this thesis can assist decommissioning decision makers, inform regulatory changes, and support AR creation in other regions that currently allow reuse of structures and in situ decommissioning.

Date of Award	21 Mar 2022
Original language	English
Awarding Institution	University of the Highlands and Islands
Supervisor	Tom Wilding (Supervisor) & Sally Rouse (Supervisor)