Dispersion of Drilling Discharges : A comparison of two dispersion models and consequences for the risk picture of cold water corals

Abstract: One of the ocean’s greatest resources is the coral reefs, providing unique habitats for alarge variety of organisms. During drilling operations offshore many activities maypotentially harm these sensitive habitats. Det Norske Veritas (DNV) has developed arisk-based approach for planning of drilling operations called Coral Risk Assessment (CRA) to reduce the risk of negative effects upon cold water corals (Lophelia pertusa) on the Norwegian Continental Shelf (NCS). In order to get a good risk assessment amodelled dispersion plume of the drilling discharges is recommended. This study concerned a drilling case at the Pumbaa field (NOCS 6407/12-2) on the NCS, and used two different dispersion models, the DREAM model and the MUDFATE model in order to investigate how to perform good risk assessments. In the drill planning process a decision to move the discharge location 300 m north-west from the actual drilling location and reducing the amount of drilling discharges, was made inorder to reduce the risk for the coral targets in the area. The CRA analysis indicated that these decisions minimised the risk for the corals, and showed that the environmentalactions in the drill planning processes are necessary in order to reduce the risk for the coral targets and that the analysis method is a preferable tool to use. The amount of discharges, the ocean current data, the discharge location and the condition of the coral targets are the factors having the most important impact on the CRA results. From monitoring analysis from the case of study, it can be seen that a pile builds up around the discharge location. The dispersion models do not seem to take into account this build-up of a pile and thereby overestimate the dispersion of drilling discharges. This observation was done when modelled barite deposit was compared with barium concentrations measured in the sediment after the drilling operation. The overestimationis the case for the DREAM model, but has not been seen in the simulations with the MUDFATE model. Results from the modelling also indicated a higher overestimation for the DREAM model when using a cutting transport system (CTS) to release thedrilling discharges compared to release the discharges without using the CTS.