In deep water – drilling discharges
A risk assessment model to identify and quantify the environmental hazards of disposing of drilling discharges in the marine environment is described by Paul Page of BP Exploration Ltd, UK, and Mark Reed and Henrik Rye, both of SINTEF Materials and Chemistry in Norway.
The environmental impacts caused by drilling discharges to the marine environment are of a more complex nature than those caused by produced water discharges as the former include both particulate and dissolved substances. Depending on the characteristics and sensitivity of the environment to the components and the duration of exposure, environmental effects might occur both in the water column and in the sediment. Therefore, the effects on biota and the surface of seabed sediments, as well as the water column, have to be evaluated.
Only a few models are available that predict the fate of chemicals and particles present in offshore drilling discharges. However, these do not include processes in the sediment nor calculate environmental risks. Typically, they only consider one component at a time.
The marine environmental modelling workbench (MEMW) tool can model hundreds of components simultaneously, include multiple release locations, and can consider essential processes and the resulting impacts in both water column and sediment.
The concept of environmental impact factor (EIF) is based on the predicted environmental concentration/predicted no effect concentration (PEC/PNEC) ratio approach, as described in the European Technical Guidance Document (2003) on environmental risk assessment. Its calculation is implemented in the DREAM (dose-related risk and effect assessment model) and ERMS (environmental risk management system) models.
Due to the particulate nature of drilling discharges, several additional non-toxic stressors to biota, in addition to toxicity, are encountered and evaluated for the EIF for drilling discharges (EIFDD). These are physical effects, such as increased sedimentation rates, oxygen depletion, change in sediment grain size and disturbances due to the presence of suspended particulate matter in the water column.
One of the main challenges in developing the ERMS model is applying the risk assessment principles, originally designed to evaluate toxic stress, to non-toxic stressors, and include their contribution in the EIFDD. To derive environmental thresholds for all stressors, an extensive literature study was carried out. Information resulting from decades of environmental monitoring round offshore platforms has also been used to determine thresholds and for validation.
A ‘setup utility’ carries data for each drilling section, such as hole diameter, washout, section length, rate of penetration, drill solids density and particle size distribution, as well as information on the mud package, release location coordinates, the date and time of release, and the temperature and salinity of the receiving environment. Properties of the chemical, solid and liquid components of each discharge are entered into the model’s chemical database. These include solubility, PNECchronic values, biodegradation data, density and octanol-water coefficients (Pow).
The system is flexible to allow various operational scenarios. Each drilling section can be modelled separately, or as a series of releases. The program can accommodate batch discharges and/or continuous discharges while drilling ahead.
Using 3D current and 2D wind time series data extracted from met-ocean databases and uploaded into the model, the physical-chemical fates of the various compounds in the discharges are calculated in three spatial dimensions and time. The model includes processes like near-field mixing, dilution in the sea due to currents and turbulence, and biodegradation of organic compounds in the discharge. The deposition of particulate matter on the sea floor is also calculated, including chemicals that are attached or adsorbed to particulates. In the sediment, the deposited matter is subject to bioturbation and biodegradation processes, which are also included.
The modelling results are displayed as cumulative and individual component concentration maps as a function of time (inmovie format) for both soluble and solid components in the discharge. Time series risk maps and EIFs for the water column and sediment are calculated and displayed. The sediment model allows for concentration and risk calculations to be extended over a much longer time period (years versus days) than for the water column. So the time it takes for restitution processes like chemical biodegradation, bioturbation and re-suspension to restore the sediment to the condition held before cuttings deposition occurred can be predicted and quantified. This allows the impact on sediment risk of employing different mud systems and cuttings treatment technologies to be compared and contrasted.
The model output can be downloaded to Excel and displayed as time series risk charts, as well as pie charts showing the contribution of individual components to the total EIF.
The model should prove useful for future evaluation of the results of drilling discharges on the marine environment, and assist in managing any detrimental effects.
Following the Norwegian authorities’ requirements in 1997 of ‘zero discharges to sea’, a joint industry project (JIP) was set up by Norwegian shelf operators to develop a modelling tool which could be used to guide management decisions and thereby reduce the potential harmful environmental effects associated with produced water discharges.
The JIP ran from 1998-2001 and resulted in the first version of the DREAM (dose-related risk and effect assessment model) project, which provided the framework for calculating the environmental impact factor for produced water (EIFPW). Thesuccess of the EIFPW as an environmental management tools. inspired the industry to further develop risk-based management tools. Initiated in 2002, the ERMS (environmental risk management system) JIP was established to enhance the DREAM model as a basis for calculating a similar EIF for drilling discharges (EIFDD).
These numerical models have been incorporated within a marine environmental modelling workbench (MEMW) tool, which provides the framework for performing modelling, simulation, analysis and presentation tasks related to releases to the marine environment.
The framework supports a common set of tools and resources, and provides a unified environment for the user to work with the different models and have them share common features, such as bathymetry and metocean input data, as well as mapping capability for defining the geographical domain within which the computations take place, and for presentating results.
The MEMW has been designed to:
• Assist operators in establishing cost effective mitigation measures for reducing potential harmful discharges to the marine environment. This may include assessing different treatment technologies and operational process parameters, as well as reviewing the impact of seasonal variations in currents and wind.
• Allow the potential environmental risks associated with produced water, drilling and other operational discharges (cooling water, hydro-test fluids, wellhead releases, oil spill releases, etc.) to be quantified and compared on an equal basis.
• Identify those components contributing most to environmental risk.
• Quantify the environmental benefits of alternate mitigating measures and justify ‘the right investments’ when selecting technologies for reducing environmental risk. It allows options to be ranked on the basis of EIF reduction/US$ used.
Paul Page firstname.lastname@example.org