Some geotechnical considerations for subsea decommissioning

Whilst decommissioning of offshore oil and gas infrastructure has been taking place in the North Sea since the early 1990s, a significant number of topside and subsea structures, pipe¬lines, umbilicals and cables are scheduled to be decommissioned between now and 2040. As such, decommissioning activities will represent an increasingly substantial proportion of offshore operations and associated engineering. According to a recent survey by Oil & Gas UK, nearly £17bn is forecast to be spent on scrapping 79 North Sea platforms and plugging 1200 wells over the next ten years. The latest forecast is up from a total of £14.6bn recorded in 2014. This represents around one-sixth of the 470 installations that will require decommissioning over the next 30 to 40 years. The removal of subsea and top¬side infrastructure presents a significant challenge to operators, especially infrastructure installed prior to the 1992/1998 OSPAR agreements, where full decommissioning may not have been explicitly considered during the original design phase. Most offshore infrastructure is bespoke, designed for the specific requirements of the produc-tion philosophy, marine environment setting, with the available technology and best practice of the day. Each decommissioning project will present challenges to the operators and the contractors tasked with removal of the infrastructure. The greatest decommissioning costs are associated with the removal of large fixed concrete and steel installations. However, given the large population of subsea equipment and pipelines to be decommissioned, a significant proportion of the £30bn to £40bn estimates required to decommission the UKCS will be associated with removal of subsea infrastructure. As all subsea infrastructure inter¬acts with the seabed in some manner, understanding the geotechnical aspects and managing geotechnical risk will be of significant benefit to executing decommissioning projects successfully.