Fate and transport of contaminants in drill cuttings piles - Use of Klif's risk assessment guidelines for contaminated sediments to estimate leaching rates from oil-based drill cuttings piles.
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The North Sea contains several oil-based drill cuttings piles which have accumulated on the seabed as a result of oil and gas production. An increased awareness of the contaminants within these piles has grown over the years since several platforms are to be decommissioned. Possible relocation of cuttings piles might result in release of contaminants from the drill cuttings, possibly affecting local organisms. It has been of interest to determine rates at which contaminants, total hydrocarbon (THC) in particular, are leaching out to the environment from these piles. This can be done by physical analyses in the laboratory, such as shaking tests, or by mathematical models. The Norwegian Climate and Pollution Agency (Klif) has prepared risk assessment guidelines for contaminated sediments in coastal areas and fjords which includes diffusion calculations. The objective of this thesis is to examine if these guidelines can be used to calculate leaching rates from oil-based drill cuttings piles in the North Sea. Data from drill cuttings piles at Statfjord A was used as basis for the calculations in this study. THC is the main contaminant of concern, determining the leaching rate of THC would therefore be preferred. This was not possible however, due to the fact that THC is not included in the guidelines. The leaching rates are thus calculated based on poly aromatic hydrocarbons (PAH), which often represent the most toxic part of drill cuttings piles. The calculated maximum and mean PAH leaching rates, based on equations given in Klif’s guidelines, was determined to be 41,2 kg/year and 3,40 kg/year respectively. Several factors of uncertainty with regards to these calculations, such as extensive use of default values and the possibility that processes not included in the guidelines might contribute to the spreading, and lack of comparison values, makes it difficult to evaluate the accuracy of these leaching rates. However, the estimated time to drain the sediment for contaminants was rapid for some of the PAHs, which might indicate overestimations in the calculations. The environmental impact results show that several of the PAHs are exceeding their respective limit values given in Klif’s guidelines. This might indicate that leaching of PAHs from the drill cuttings piles might affect the species living in, on or possibly near the drill cuttings piles. This study shows that there are several limitations when using Klif’s guidelines to determine leaching rates from oil-based drill cuttings piles. Critical parameters are deficient from both the guidelines and the drill cuttings characterization reports to be able to get reliable results. In addition, uncertainty arises due to the fact that the guidelines are based on conditions in harbors and coastal areas and not conditions present in the North Sea. The sum of these uncertainties indicates that the calculated results cannot be considered reliable, and that the present guideline is not feasible to use on oilcontaminated drill cuttings piles. However, the study might provide useful information on factors that should be present to calculate leaching rates from oil-based drill cuttings piles.
Master's thesis in Environmental technology