Poojitha D. Yapa

A model for simulating deepwater oil and gas blowouts - Part I: Theory and model formulation

A model developed to simulate the behavior of oil and gas accidentally released from deep water is presented. This model presents major modifications to a three-dimensional model developed earlier (Yapa and Zheng, 1997) that simulate the behaviour of oil from under water accidents (shallow water). In deepwater, the ultra-high pressure and cold temperature causes phase changes in gases. These combined with relatively strong currents in some deepwater regions presents extraordinary challenges to modeling jets/plumes from deepwater oil and gas blowouts. The present model incorporates the phase changes of gas, associated changes in thermodynamics and its impact on the hydrodynamics of the jet/plume. Hydrate formation, hydrate decomposition, gas dissolution, non-ideal behavior of the gas, and possible gas separation from the main plume due to strong cross currents are integrated with the jet/plume hydrodynamics and thermodynamics. This paper presents the complete model development and testing of various computational modules with available data. A companion paper presents the comparison of model results with three large-scale field experiments conducted in the Norwegian Sea.

Modeling gas dissolution in deepwater oil/gas spills

Gases in deepwater oil/gas spills can lose considerable amounts of the gas phase due to dissolution in water. Gas dissolution has a significant impact on the behavior of the oil/gas jet/plume because of its impact on the buoyancy. A method is presented in this paper for computing gas dissolution that covers a broad range of water depth, from shallow water where gases behave as ideal ones under low pressure to deepwater where gases behave as non-ideal ones under high pressures. The method presented also accounts for the spherical and non-spherical shapes of gas bubbles. The gas dissolution computations are validated by comparing the computed results with observed data from previously conducted laboratory experiments. The gas dissolution computation module is then integrated with a model for underwater oil/gas jets/plumes by Yapa and Zheng [J. Hydraul. Res. 35 (5) (1997) 673]. Scenario simulations are presented to show the impacts of gas dissolution on the behavior of jets/plumes. These scenarios show the impact of dissolution on the behavior of the jet/plume. The comparison of results using ideal gas conditions and non-ideal gas conditions is also shown.

A model for simulating deep water oil and gas blowouts - Part II: Comparison of numerical simulations with “Deepspill” field experiments

A companion paper (Part I-Zheng et al., 2003) presents the development and the module tests of a model, CDOG, developed to simulate the behavior of oil and gas accidentally released from deepwater. CDOG model incorporates the phase changes of gas. associated changes in thermodynamics and its impact on the hydrodynamics of the jet/plume. Hydrate formation, hydrate decomposition, gas dissolution, non-ideal behavior of the gas, possible gas separation from the main plume due to strong cross currents are integrated with the jet/plume hydrodynamics and thermodynamics. In this paper. CDOG model is used to numerically simulate the large-scale and unique field experiments conducted in Norway. The field experiments consisted of two oil and methane gas releases and one methane gas only release from a deepwater location (844 m water depth). Comparisons between the simulations and observations are discussed in detail. The comparisons between the simulations and the observations are good.

A model to simulate the transport and fate of gas and hydrates released in deepwater

Methane and natural gas, if released in deepwater, undergo physico-chemical processes as they rise through the water column. In deepwater, these gases are likely to be converted into hydrates. These are dissociated into gas upon reaching shallower regions. The present model accounts for plume thermodynamics and hydrodynamics, and is integrated with the associated physico-chemical processes such as hydrate formation/dissociation, gas dissolution, hydrate dissolution, hydrate shell crumbling and reformation, heat and mass transfer inside gas bubbles, multiple-sized bubbles and their size change, and possible gas separation from the main plume. The model simulations compare well with field data from Deepspill in Norway, and the gas bubble releases off the California coast. The scenario simulations show that the inclusion of multiple bubble sizes impacts the model results. Inclusion of hydrate dissolution is also important for deepwater releases. Without hydrate dissolution, the results tend to overestimate the hydrocarbon mass that will reach the water near the surface.

Simulation of oil spills from underwater accidents II: Model verification

A companion paper presented the development of a three-dimensional numerical model to simulate the behaviour of buoyant oil jets that result from underwater accidents. The numerical model was developed based on a Lagrangian integral technique. The model can simulate the behaviour of oil in stratified or unstratified ocean environments. The presence of a multi-directional ambient current is considered. The fluid in the buoyant jet can be a liquid, gas, or liquid/gas mixture, which is typical of many underwater oil-related accidents. The model formulation includes the diffusion and dissolution of oil from the jet to the ambient environment. In this paper, the numerical model is tested against a variety of conditions. First, the model results are compared with all available asymptotic results. Second, the model is run for cases in which experimental data (both small and large scale) are available, so that the numerical model results can be compared with the observed data. The experimental data includes buoyant jets in stratified and unstratified environments and relatively deep water experiments. They include cases both with and without ambient current. The cases compared include two-dimensional and three-dimensional jet trajectories. All comparisons show that the numerical model results match very well with the experimental data. In addition, the model is used to simulate buoyant oil jets for several cases of practical interest.

Modeling oil spills in a river—lake system

Abstract A general model shell, ROSS3, is developed for simulating oil spills in complex river systems using techniques which have not been previously exploited in oil spill models. ROSS3s new approach has several advantages over the approach to model oil spills in the past: (a) The use of a time-varying boundary-fitted coordinate system that allows accurate accounting for complex river/lake boundary as well as the river boundary changes as its water levels fluctuate; (b) The ability to confine two-dimensional hydrodynamic computations to a limited river reach; (c) The ability to interactively layout the channel networks for setting up the model, define extra cross sections to increase the accuracy if needed, in addition to the traditional data entry and visualization interfaces. ROSS3 is a two-layer two-dimensional oip spill model that can simulate the mechanism of advection, horizontal diffusion, mechanical spreading, shoreline deposition, evaporation, dissolution, vertical mixing, resurfacing and sinking. In ROSS3 spilled oil may be a surface slick or suspended oil droplets, or a combination of both. Both free surface and ice cover conditions can be simulated. The flow of conditions can be varied and the unsteady flow model can be run within ROSS3 to simulate the flow conditions in both the river and the lake. The ice conditions can be added or removed from the model input using easy interactive procedures.

A Model for Deepwater Oil/Gas Blowouts

When gas is released in deepwater, the high pressure and low temperature can convert the gases into hydrates, which are buoyant. As these hydrates travel upwards they will encounter regions of lower pressure and can decompose into free gas. The presence or absence of hydrates has a significant impact on the behaviour of the jet/plume due to the alteration of the buoyancy. The free gas may dissolve in water. This paper describes a computer model developed to simulate the behaviour of oil and gas released from deepwater locations in the ocean. The model integrates the hydrodynamics and thermodynamics of the jet/plume with kinetics and thermodynamics of hydrate formation/decomposition. Model formulation and comparison of results with laboratory data for hydrates is presented. Scenario simulations show the behaviour of oil/gas under different deepwater conditions.

Bubble Sizes, Breakup, and Coalescence in Deepwater Gas/Oil Plumes

Bubble size distribution (BSD) plays a major role in transport and fate of gas or oil released in deepwater. However, no reliable method is available to estimate gas or oil BSD after a deepwater spill. Breakup and coalescence have been identified as key processes controlling BSDs in turbulent jets. The present work introduces bubble breakup and coalescence processes for deepwater gas or oil spill models. A population balance equation representing bubble volumes is used to model the evolution of bubble sizes caused by breakup and coalescence. Existing theories for bubble breakup and coalescence rates in bubble columns are adopted to deepwater plumes. The advantage of the present model is that the BSD is generated as a result of breakup and coalescence; and therefore, a predefined BSD is no longer necessary for simulations. The comparison of model-computed results with laboratory and field data shows a good agreement. Scenario simulations show that the seed diameter given to start computations affects only for a short distance from the release point. Simulations also show that bubble breakup and coalescence is important only during the early stages of the plume where turbulence is dominant. The importance of accounting for gas bubble breakup and coalescence in estimation of gas dissolution is also demonstrated.

Modelling river oil spills: a review

The risk of oil spills in rivers has increased due to oil storage facilities along rivers, inland navigation, major oil transport pipelines that cross rivers. Inland oil spills are more frequent than ocean oil spills but are usually of smaller volumes. Oil spills in inland waterways can have enormous environmental and economical impacts because of their closeness to populated areas and economic centres. Previous review studies on oil spill modelling have concentrated on ocean oil spill modelling. In this paper all existing major river oil spill models are reviewed. The specific needs of the river oil spill models that are different from the ocean oil spill models are identified. The physico chemical oil spill processes which form the model are discussed. A comparison of the different models are presented. Simulations are presented to demostrate state-of-the-art in river oil spill models.

An Integrated Computer Model for Simulating Oil Spills in the Upper St. Lawrence River

Abstract An integrated oil spill model MICROSS2 has been developed for simulating the fate and transport of spilled oil in rivers and is applied to the upper St. Lawrence river bordering the United States and Canada. This model considers the physico-chemical mechanisms of advection, horizontal diffusion, mechanical spreading, evaporation, dissolution, vertical mixing, and shoreline deposition in simulating the oil slick transformation. It can simulate the oil slick transformation on the water surface and in the water column, in transient flow conditions with varying wind and air temperature. The integrated model consists of the following modules: a) a menu based interface for interactive data preparation, and execution of other modules; b) flow model to compute the velocity distribution in the river; c) a two-dimensional two layer oil spill model; d) a graphics interface for visualizing the results from the flow model and the oil spill model. The integrated model is operational on a micro-computer, and can be used as a part of an oil spill response program to assist cleanup, environmental impact assessment, and contingency planning.