Hydrodynamic modeling and ecological risk-based design of produced water discharge from an offshore platform

Mukhtasor (2001) Hydrodynamic modeling and ecological risk-based design of produced water discharge from an offshore platform. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This study has two major components: hydrodynamic modeling and ecological risk assessment (ERA) of produced water discharge. The genera objective was to develop a framework for ecological risk-based design of produced water discharge from an offshore platform. This consisted of six more specific objectives: (1) developing an initial dilution model; (2) integrating the developed initial dilution model with a far field dilution model; (3) developing a methodology for probabilistic hydrodynamic modeling; (4) identifying methodologies for ERA of produced water discharge; (5) developing a framework for ecological risk-based design of a produced water outfall; and (6) applying the framework to a case study dealing with the discharge from an offshore oil platform. -- Conceptual and numerical problems associated with presently available initial dilution models were elaborated in this study. A new approach to initial dilution modeling was proposed based on the hypothesis of additive shear and forced entrainment combined with nonlinear regression. Unlike the previous approach, which is typically "trial and error", the proposed approach is systematic and provides an objective means of evaluating the initial dilution model. Based on the proposed approach, an alternative initial dilution model was then developed. The developed model is more robust and justifiable conceptually and numerically. It gives a unique, continuous, solution of centerline dilution. A comparison with other available models shows that the proposed model is better in a number of ways: (1) it does not assume that the current has no effect in the buoyancy-dominated near field (BDNF), which other available models do; (2) in the buoyancy-dominated far field (BDFF) region the model has one parameter fewer than a previously available model yet it is no less accurate; (3) in the transition region it gives a unique solution which the asymptotic models do not; (4) unlike the previous models, the proposed model has approximately the same precision for all regions, i.e. the BDNF, the BDFF, and the transition; and (5) the proposed model can also be presented in a probabilistic form that permits calculation of failure probability for specified model inputs and a threshold dilution. -- Hydrodynamic modeling was carried out by integrating near and far field models. The developed initial dilution model was used as the near field model. The far field model and the control volume approach for connecting near and far field models were adapted from published methods. A comparison using a case study showed that the proposed hydrodynamic model and the Cornell Mixing Zone Expert System (CORMIX) model are generally in good agreement, particularly in estimating average effluent concentrations. However, the proposed model also provides the concentration field in the X-Y directions so that it may be applicable for analysis of mixing zones, which in some cases is defined in terms of the horizontal area around the discharge location. The proposed model can also be readily used in a probabilistic analysis to take into account the uncertainty associated with the model inputs, model coefficients and error term. The probabilistic analysis was carried out using Monte Carlo (MC) simulations. A comparison between random sampling and Latin Hypercube Sampling (LHS) showed that LHS-based MC simulations were typically about 15% more efficient than the random sampling MC simulations. -- In the context of produced water discharges, ERA has usually been directed at monitoring purposes. In the past, there is no consideration to the integration between ERA and engineering design of the produced water outfalls. In this research, an approach was identified to deal with specific problems relevant to design of produced water discharge in the marine environment. It consists of three phases, i.e. problem formulation, analysis, and risk characterization. A framework of ecological risk-based design was then developed by integrating the methodology of hydrodynamic modeling and ERA discussed above. The framework was presented systematically using a case study by evaluating design scenarios of produced water discharge relevant to an offshore oil production platform, the Terra Nova oil field, located on the Grand Banks, southeast of St. John's, Newfoundland, Canada. Instead of providing a solution for a particular problem of an existing oil production platform, the emphasis of the case study is to show how the risk-based design of produced water discharge could be undertaken.

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