Optimising the decommissioning of pipeline protection structures in the North Sea
Pipeline protection structures, including concrete mattresses, grout bags and rockdump, are used to protect and support pipelines on the seabed. The OSPAR 98/3 decision to ban offshore disposal of decommissioned installations does not cover pipeline protection structures, although current UK guidelines state that pipeline protection structures should be removed and disposed of onshore. Undertaking formal comparative assessment (CA) of alternatives to landfill such as reuse or recycling could reduce costs for taxpayer and industry, whilst promoting a circular economy around decommissioning. Before investigating potential decommissioning scenarios, it is necessary to understand the spatial context and contamination status of mattresses. The overall aim of this thesis was to assess pipeline protection structure decommissioning in the North Sea by 1) contributing evidence to enable CA of decommissioning programmes; and 2) providing regional-scale recommendations to policy-makers and industry. Spatial data and ROV footage from four main oil and gas operators were collated and analysed at a regional scale. Decommissioned concrete mattress samples from two oil and gas fields were analysed for polycyclic aromatic hydrocarbons (PAHs) and metal concentrations. Variation in contaminant concentrations was quantified in concrete and polypropylene rope samples from decommissioned mattresses. In some cases, contamination (e.g. PAHs) was discovered in high concentrations with a petrogenic source. The results drive towards developing an ecological risk assessment in decommissioned concrete mattresses, especially with regard to polypropylene rope. A method was developed to assess atmospheric emissions and energy use for decommissioning options, including a contamination risk scenario. Reuse options onshore are available for decommissioned concrete mattresses and could be more suitable to manage than offshore options under the current waste regulatory framework. Such an evidence-base should be incorporated into future decision-making for decommissioning regulations, along with the inclusion of additional studies across the North Sea.
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