The OSPAR convention requires that almost all offshore oil and gas installations have to be removed when they are no longer in active use. There are currently approximately 832 oil & gas production facilities in the North Sea many of which will be abandoned in the coming decades.

In the decommissioning of offshore oil and gas installations, there is a continuous focus on keeping environmental and safety risks as low as reasonably practicable (ALARP). This paper will describe the assessment of decommissioning solutions from an environmental point of view, as a basis for submitting a decommissioning programme. The paper is primarily based on experiences gained from ongoing and completed decommissioning projects in the North Sea (Ekofisk tank, Brent Bravo/Delta and Frigg & MCP01). The focus of this paper is restricted to the decommissioning of concrete substructures.

Decommissioning concrete structures typically involves larger environmental and safety considerations than steel structures, as they tend to be more polluted from trapped oil, sludge, etc. Furthermore, several of these structures have not been designed with the intention for later removal and may involve large safety risks during a refloating operation.

Some of the major conclusions with regard to decommissioning of concrete substructures are:

  • Ensuring a reliable decision-basis must not be under-estimated, as this is vital to support the difficult decommissioning options selection. This could be made by means of e.g. offshore sampling / surveys

  • Onshore preparation, such as pilot-scale testing of tools and laboratory testing, can in some cases be beneficial and save time offshore

  • The environmental and safety risk considerations play an important part in the fine balance between the company's wish to ensure a good reputation by removing all traces of their activities, and substantial practical safety and technical safety challenges associated with a ‘removal all’ operation.

  • In some cases, partial removal or ‘leave in place’ could be the best decommissioning solution in relation to HSE.

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