This paper describes advanced modelling that has been applied to the decommissioning of a major North Sea drill cuttings pile to support the decommissioning programme. The environmental impacts associated with North Sea drill cuttings piles were examined in the 1990s and 2000s with agreement to manage cuttings piles based on simple estimated oil loss and footprints. Options for managing the piles must be considered on decommissioning, and piles may be disturbed during removal operations and from the ultimate collapse of any remaining structures.

Advances in modelling techniques now allow us to predict long-term trends in cuttings pile characteristics and environmental risks quantitatively, providing firm direction in mitigating risks. This is key in determining appropriate decommissioning options for the piles and nearby structures. Modelling involving fluid dynamics, soil mechanics, scientifically verified 3D dispersion modelling, contaminant degradation and seabed recovery was used in several scenarios: the extant pile; moving the pile across the seabed using suction pumps; backflush discharges relating to retrieving the pile; and disturbance from remaining jacket footings ultimately falling into the pile after several hundred years.

The existing 15.3 metre high pile was re-created in the model domain with a good correlation using the original drilling discharge data from 54 wells and recent survey data. Long term characteristics and risk aspects were then modelled twenty years into the future, and extrapolated several hundred years to calculate persistence footprints, oil loss, and the contributing elements to environmental risk, such as toxicity, grain size change and dexoxygenation. This approach identified key risk factors and enabled quantitative option comparison. Water column impacts were also calculated.

The process generated valuable lessons regarding using survey data, generating input data, managing uncertainties and combining different modelling types. To date, there have been limited opportunities to evaluate cuttings pile impacts at the point of decommissioning and this will assist others and advance understanding and reasoning in this field.

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