Drill cuttings piles in the North Sea: management options during platform decommissioning

Abstract

From the Executive Summary: “This report was commissioned by Amoco UK. It identifies, describes and partially evaluates a number of solutions for managing cuttings piles in the North Sea environment. The main options considered are:

· Leave in situ (including possible bioremediation and capping)

· Dispersal

· Removal from the sea bed, followed by reinjection

· Removal from the sea bed, followed by treatment on land

The comparative assessment of the different options uses four impact categories:

· Technical feasibility

· Ecology

· Energy and Emissions

· Health and Safety

The NWH cuttings pile is used as a case study. This is one of the larger piles in the North Sea, with a volume of 25 225 m3 and an estimated mass of 42 126 tonnes. It is also one of the piles on which most information is available. Although there are still considerable uncertainties to be resolved, technical feasibility and ecological impacts1 are likely to be of greatest importance in making choices among the different options.

Based on a review of contaminant concentrations known to cause ecological effects and those present in drill cuttings, it is likely that hydrocarbons rather than metals present the greatest ecological risks. If the cuttings pile is left in situ, some hydrocarbons will slowly leach into the North Sea, contributing to the overall contamination burden. However, the total inventory of hydrocarbons in cuttings piles is similar to the annual input to the North Sea from all sources. Broad field impacts are therefore not a major concern, and the most significant ecological effects are the local effects on the ecology of the seabed around the platform.

In the case of NWH, hydrocarbon concentrations in sediments within an area approximately 500 m in diameter, centred on the platform were between two and five percent (equivalent to 20,000 to 50,000 parts per million (ppm)) in 1997, six years after the use of oil-based muds ceased. As no direct comparisons with previous surveys is possible making a judgement about the speed of biodegradation is problematic. However, from what is known about the physical nature of the pile it appears that in the most contaminated sediments, biodegradation is relatively slow. Thus, if left alone, the central area is likely to remain severely contaminated for a relatively long period. This contamination will produce a marked and long-term ecological impact, lasting for several decades at minimum. The spatial and temporal extent of the ecological impacts outside of this area are more difficult to predict. The spatial extent of contamination in 1992 was similar to that described for other North Sea sites. However, the 1989 survey found hydrocarbon concentrations of 60-80 ppm out to 5 000 m along the orientation of the strongest tidal current (100°-135°). The 1997 survey reported 27 ppm at 7 500 m on a bearing of 288° (‘Platform North’) although the characteristics of oil at this location indicate that it is probably not derived from drilling muds but may arise from other man-induced or natural sources.

Hydrocarbon concentrations at these distances are relatively high in comparison with background concentrations for this part of the North Sea (NSTF 1993a) and several other sites in the North Sea, such as Veslefriff, and Gyda (Olsgard and Gray, 1995). These concentrations may decrease over time as bacterial degradation takes place. However, this may be outweighed by dispersal of contaminated sediment outwards from the main body of the pile which all available data indicates to be very contaminated.

The rate at which contaminated material disperses outwards may be increased if piles are disturbed during platform decommissioning or by fishing activity after platforms are removed. Further work is required combining sediment dispersion modelling and a full chemical and ecological survey to allow assessment of how the ecological impacts have altered over the seven years since the 1992 survey.

The study found no convincing evidence that bioremediation of the cuttings pile in situ would be practicable. Capping the pile with concrete mats offers little environmental benefit and may not be technically feasible. Capping the pile with sand may offer environmental benefits, but requires further study.

There are also issues related to residual liability and impacts on other users of the sea that have to be considered for in situ optons. Dispersing the cuttings pile would produce ecological impacts over an unacceptably large area of seabed. It could be viewed as a form of dumping at sea, in which case it would be foreclosed as an option.

There are significant questions about the technical feasibility of all other options. Burial of material is feasible on a small scale, but there are questions as to whether it would be feasible for large cuttings piles, such as that at North West Hutton. Like dispersal, this could be viewed as a form of dumping at sea.

Removing cuttings from the seabed would involve technology that has not yet been demonstrated as feasible in 150 m of water. If feasible, the ecological impacts will depend on the amount of pile sediment stirred up, and on the amounts of contaminants released during the operation. It will be possible to use modelling to examine how cuttings material disturbed during removal will disperse. However, no data are currently available on the amounts of cuttings pile material likely to be disturbed. More precise assessment of the environmental impacts of removal is therefore unlikely to be possible without more accurate modelling and a field trial of the technology.

There are significant energy costs associated with transporting cuttings to shore. However, the energy estimates cover a wide range. A more detailed specification of the options is needed before energy calculations can be usefully included in the decision-making process.

Landspreading of untreated cuttings is not thought to be an acceptable option. Even if treated this option may prove to be unacceptable.

Existing technology for the processing of cuttings is on too small a scale to readily deal with the volume of cuttings from NWH, but treatment to remove the majority of the oil is possible in principle. The development of new facilities is required. Even after this treatment, the bulk of the cuttings would probably have to be disposed of in landfill sites. Current treatment methods generally result in material that is still classified as special waste, so disposal would have to be in one of a limited number of landfill sites which can accept special waste. An alternative option would be to dispose of the recovered cuttings directly to landfill without treatment.

Reinjection has been carried out at a number of other locations, and has the lowest environmental impacts of any of the cuttings disposal options. However, reinjection may be a much slower process than the lifting of material from the seabed so there may be logistical difficulties. More significantly, in most cases it would be carried out before platform decommissioning, and would require the cuttings pile to be removed from within the platform jacket. There may be also some issues about the political acceptability of reinjection.

In conclusion, the preferred option on environmental grounds is likely to be a combination of:

· Leave in situ

·Remove and reinject

· Remove and bring to land for treatment and disposal or re-use”