Assessment of the disturbance of drill cuttings during decommissioning
Studies into drill cuttings pile management in recent years have concluded that leaving the piles in situ to degrade naturally is generally the best option. However, disturbance during decommissioning may be unavoidable and there is therefore a need to better understand the options available to manage drill cuttings during decommissioning and how they could impact the environment.
Estimate of volume of cuttings piles likely to be disturbed during decommissioning
The total volume of cuttings in UK waters is estimated at 1,150,086 m3, giving an average volume of a single cuttings pile in UK waters (ERT, 2009) of around 6,610 m3. Some of these cuttings piles are located around installations that are likely to be derogation cases, and therefore any disturbance is likely to be minimal. There are, however, around 129 installations in UK waters likely to require removal with the potential for disturbance of associated cuttings piles. There are a further 72 installations on the Norwegian Continental Shelf likely to require removal, amounting to potentially 201 installations in the OSPAR region to be removed from the seabed in the next 20 to 30 years, with the potential for disturbance of associated drill cuttings piles.
Impacts of cuttings piles on the seabed
The time needed for the seabed to recover following the deposition of cuttings is influenced by the rate of biodegradation of hydrocarbons and other contaminants, the resuspension and redistribution of matter on the seabed due to currents and wave action and the time for recolonisation of the biota.
During the natural degradation of cuttings piles, aerobic biodegradation of hydrocarbons typically occurs only in the upper few centimetres of the pile. Anaerobic degradation may take place down to at least 20 - 50 cm but will occur very slowly, with oil and other contaminants in the deeper parts of the pile remaining essentially unchanged. If the cuttings piles remain undisturbed then the release of hydrocarbons and other contaminants is likely to be limited.
Environmental impacts resulting from deposition or re-deposition of cuttings include smothering, grain size changes, deoxygenation and toxicity, which in turn can result in localised changes to the plankton, the benthos and other organisms. Benthic fauna, including crustaceans and molluscs, appear to be at most risk from persistent oil in the sediments as they can accumulate chemical contaminants in their tissues.
Based on the case studies reviewed, the majority of impacts from cuttings piles are noted within 100 m of the centre of the pile; and, generally, beyond 500 m there is little discernible impact. When cuttings piles are disturbed, the pile is aerated allowing some additional degradation to take place. However, this disturbance results in additional, albeit generally short-term and localised impacts on the water column, and in some (not all) cases could cause contamination of the seabed outwith the areas impacted by the original cuttings discharge.
Fishing may be able to resume over cuttings piles previously contained in a 500 m safety zone where fishing activities would have been excluded. Where cuttings are left in situ or relocated on the seabed there is the potential for trawling activities to disturb the cuttings pile resulting in the release of contaminants contained within the cutting pile into the water column, as well as the potential for the nets and catch to be contaminated.
Options for disturbance of historical cuttings piles during decommissioning
The disturbance of drill cuttings piles during decommissioning may be unavoidable if infrastructure is to be removed. Therefore, there may be some localised disturbance of cuttings piles prior to the preferred option of leaving piles in situ to allow natural degradation to take place. This report focusses particularly on the potential impacts of dispersing cuttings offshore in the vicinity of the existing pile.
To date the most commonly used methods for relocating cuttings on the seabed have been suction dredging and water jetting. The choice of technology used for relocating cuttings can help reduce potential impacts. For example, suction dredging will usually result in a more localised impact on the water column and seabed sediments than jetting, and impacts will be even more localised if other methods such as physical ploughing are used. However, in many cases physical constraints (i.e. the configuration of the cuttings pile and installation infrastructure) may dictate the selected method and physical ploughing is unlikely to be suitable at congested locations. Relocation of cuttings inevitably causes short term impacts but can also increase degradation rates and recovery, and a key consideration is whether cuttings should be deliberately dispersed widely to speed recovery, or whether cuttings should be relocated to distinct new ‘piles’ to limit the spatial coverage of the contaminated sediments. The optimum solution is likely to depend on the local balance of risks to fishing and ecology, but there is little field data at present on actual uptake by biota of contaminants.
Mitigation and monitoring
Where historical contamination is disturbed and there is a high impact risk profile (e.g. larger scale disturbance, a more sensitive environment, designated species nearby, or the potential for susceptible seabed fisheries) it is recommended that pre and post disturbance monitoring is undertaken. Monitoring should include sampling of the water column, seabed sediments, and the benthos, in order to assess potential impacts on the environment. Where relevant, uptake in food species should also be considered. In addition, any monitoring should not be limited to the hydrocarbon content of the cuttings pile and should consider other potential contaminants such as heavy metals.