Effects of droplet size on intrusion of sub-surface oil spills

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, February 2013."February 2013." Cataloged from PDF version of thesis.Includes bibliographical references (p. 86-90).This thesis explores the effects of droplet size on droplet intrusion in sub-surface oil spills. Laboratory experiments were performed where glass beads of various sizes, which serve to simulate oil droplets in deepsea oil spills, were released vertically in a quiescent salinity stratified ambient and descended as multi-phase plumes. The two-tank stratification method was used to create linear density profiles for all experiments. The resulting radial concentration distributions of the dispersed phases were obtained by collecting the settled particles from the bottom of the tank. The radial distributions recorded were found to resemble Gaussian distributions, based on visual observations and analyses of kurtosis, which is consistent with particles being vertically well mixed in the intrusion layer. A new typology was proposed to describe plume structures with UN= us/(BN)¹/⁴ </= 1.4. For UN </=1.4 particle detrain from the plem, but only those with smallest slip velocity (UN </+0.3) intrude. An analytical model assuming well-mixed particle distributions within the intrusion layer was also used to predict the spread of the particle distribution based on initial buoyancy flux B, stratification frequency N, the particle slip velocity us, and the non-dimensional slip velocity UN. Comparison between experimental results and the analytical model suggested that the model accurately predicts the spread of the particles for UN </=1.4. Experiments with beads of difference sizes also suggested that the interaction between two particle groups has minimal effects on their radial particle spread. This indicates that particles of difference sizes can be treated independently when analyzing their radial plume spread. Chemical dispersants produce small oil droplets and the current experiments provide references on the minimum diameter needed for efficient particle spread (Type la* plume). By knowing the following parameters for a scenario - 1) initial buoyancy flux B; 2) the ambient stratification profile N; and 3) the slip velocities of the droplets u, - suitable amounts of dispersant can be determined and applied to reduce the size of the particles exiting the spill, allowing them to intrude and spread for a larger distance in the ocean column. A hypothetical example with conditions taken from the 'Deep Spill' experiment and Deepwater Horizon oil spill was also presented for reference.by Godine Kok Yan Chan.S.M

Computation of Nonlinear Hydrodynamic Loads on Floating Wind Turbines Using Fluid-Impulse Theory

A hydrodynamics computer module was developed to evaluate the linear and nonlinear loads on floating wind turbines using a new fluid-impulse formulation for coupling with the FAST program. The new formulation allows linear and nonlinear loads on floating bodies to be computed in the time domain. It also avoids the computationally intensive evaluation of temporal and spatial gradients of the velocity potential in the Bernoulli equation and the discretization of the nonlinear free surface. The new hydrodynamics module computes linear and nonlinear loads — including hydrostatic, Froude-Krylov, radiation and diffraction, as well as nonlinear effects known to cause ringing, springing, and slow-drift loads — directly in the time domain. The time-domain Green function is used to solve the linear and nonlinear free-surface problems and efficient methods are derived for its computation. The body instantaneous wetted surface is approximated by a panel mesh and the discretization of the free surface is circumvented by using the Green function. The evaluation of the nonlinear loads is based on explicit expressions derived by the fluid-impulse theory, which can be computed efficiently. Computations are presented of the linear and nonlinear loads on the MIT/NREL tension-leg platform. Comparisons were carried out with frequency-domain linear and second-order methods. Emphasis was placed on modeling accuracy of the magnitude of nonlinear low- and high-frequency wave loads in a sea state. Although fluid-impulse theory is applied to floating wind turbines in this paper, the theory is applicable to other offshore platforms as well.United States. Department of Energy (National Renewable Energy Laboratory. Contract DE-AC36-08GO28308)United States. Department of Energy. Office of Energy Efficiency and Renewable Energy. Wind and Water Power Technologies OfficeMassachusetts Clean Energy Cente

Computation of nonlinear hydrodynamic loads on floating wind turbines using fluid-impulse theory

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016.Cataloged from PDF version of thesis.Includes bibliographical references (pages 199-202).Wind energy is one of the more viable sources of renewable energy and offshore wind turbines represent a promising technology for the cost effective harvesting of this abundant source of energy. To capture wind energy offshore, horizontal-axis wind turbines can be installed on offshore platforms and the study of hydrodynamic loads on these offshore platforms becomes a critical issue for the design of offshore wind turbine systems. A versatile and efficient hydrodynamics module was developed to evaluate the linear and nonlinear loads on floating wind turbines using a new fluid-impulse formulation - the Fluid Impulse Theory(FIT). The new formulation allows linear and nonlinear loads on floating bodies to be computed in the time domain, and avoids the computationally intensive evaluation of temporal and spatial gradients of the velocity potential in the Bernoulli equation and the discretization of the nonlinear free surface. The module computes linear and nonlinear loads - including hydrostatic, Froude-Krylov, radiation and diffraction, as well as nonlinear effects known to cause ringing, springing and slow-drift loads - directly in the time domain and a stochastic seastate. The accurate evaluation of nonlinear loads by FIT provides an excellent alternative to existing methods for the safe and cost-effective design of offshore floating wind turbines. The time-domain Green function is used to solve the linear and nonlinear free-surface problems and efficient methods are derived for its computation. The body instantaneous wetted surface is approximated by a panel mesh and the discretization of the free surface is circumvented by using the Green function. The evaluation of the nonlinear loads is based on explicit expressions derived by the fluid-impulse theory, which can be computed efficiently.by Godine Kok Yan Chan.Ph. D

Effects of droplet size on intrusion of sub-surface oil spills

This paper explores the effects of droplet size on droplet intrusion and subsequent transport in sub-surface oil spills. In an inverted laboratory set-up, negatively buoyant glass beads were released continuously into a quiescent linearly stratified ambient to simulate buoyant oil droplets in a rising multiphase plume. Settled particles collected from the bottom of the tank exhibited a radial Gaussian distribution, consistent with their having been vertically well mixed in the intrusion layer, and a spatial variance that increased monotonically with decreasing particle size. A new typology was proposed to describe plume structure based on the normalized particle slip velocity UN=us/(BN)1/4, where us is the particle slip velocity, B is the plume’s kinematic buoyancy flux, and N is the ambient stratification frequency. For UN≤1.4 particles detrain from the plume, but only those with smaller slip velocity (UN≤0.3) intrude. An analytical model assuming well-mixed particle distributions within the intrusion layer was derived to predict the standard deviation of the particle distribution, σr=0.9−0.38(UN)0.24π−−−−−−−−−−√B3/8N5/8u1/2s and predictions were found to agree well with experimental values of σr. Experiments with beads of multiple sizes also suggested that the interaction between two particle groups had minimal effect on their radial particle spread. Because chemical dispersants have been used to reduce oil droplet size, this study contributes to one measure of dispersant effectiveness. Results are illustrated using conditions taken from the ‘Deep Spill’ field experiment and the recent Deepwater Horizon oil spill.Chevron-MITEI University Partnership ProgramBP/The Gulf of Mexico Research InitiativeNational Science Foundation (U.S.). Graduate Research Fellowship Progra