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The Deepwater Horizon Mobile Offshore Drilling Unit (MODU) was one of several classes of floatable drilling systems. The explosion on April 20, 2010 led to fatalities and the worst oil spill in the U.S. We present an independent estimate of the oil-flow rate into The Gulf caused by the drill-pipe rupture. We employed the NASA Moderate-Resolution Imaging-Spectroradiometer (MODIS) satellite photographs, starting from the days immediately following the disaster, to determine the magnitude of spill. From these images, we obtained the surface area of the spill and calculated the oil flow rate by two different methods based on contrasting luminance within that area. The first assumes a constant thickness for the total area with upper and lower bounds for the thickness. The second separates the area into different patches based on the luminance levels of each. The probability density function (PDF) of such luminance plots showed natural groupings, allowing patches be identifiable. Each patch maps to a specific thickness. This second approach provides a more accurate average thickness. With the assumption that evaporation and other loss amounted to ~40% of the spill, we obtained, from the first method, a flow rate ranging from 9,300 barrels per day (BPD) to 93,000 BPD. A value of 51,200 BPD was obtained using patch-separation method. This latter estimate was a plausible value, obtained from the current analysis, but with no details presented in an Extended Abstract in OMAE2012, is remarkably consistent with the "official U.S.-Govt. estimates". [ABSTRACT FROM AUTHOR]

Model-predictive control (MPC) has shown its strong potential in maximizing energy extraction for wave-energy converters (WECs) while handling hard constraints. However, the computational demand is known to be a primary concern for applying MPC in real time. In this work, we develop a cost function in which a penalty term on the slew rate of the machinery force is introduced and used to ensure the convexity of the cost function. Constraints on states and the input are incorporated. Such a constrained optimization problem is cast into a Quadratic Programming (QP) form and efficiently solved by a standard QP solver. The current MPC is found to have good energy-capture capability in both regular and irregular wave conditions, and is able to broaden favorably the bandwidth for capturing wave energy compared to other controllers in the literature. Reactive power required by the power-take-off (PTO) system is presented. The effects of the additional penalty term are discussed. [ABSTRACT FROM AUTHOR]

The prediction of roll motion of a ship with bilge keels is particularly difficult because of the nonlinear characteristics of the viscous roll damping. Flow separation and vortex shedding caused by bilge keels significantly affect the roll damping and hence the magnitude of the roll response. To predict the ship motion, the Slender-Ship Free-Surface Random-Vortex Method (SSFSRVM) was employed. It is a fast discrete-vortex free-surface viscous-flow solver developed to run on a standard desktop computer. It features a quasi-three-dimensional formulation that allows the decomposition of the three-dimensional ship-hull problem into a series of two-dimensional computational planes, in which the two-dimensional free-surface Navier-Stokes solver Free-Surface Random-Vortex Method (FSRVM) can be applied. In this paper, the effectiveness of SSFSRVM modeling is examined by comparing the time histories of free roll-decay motion resulting from simulations and from experimental measurements. Furthermore, the detailed two-dimensional vorticity distribution near a bilge keel obtained from the numerical model will also be compared with the existing experimental Digital Particle Image Velocimetry (DPIV) images. Next, we will report, based on the time-domain simulation of the coupled hull and fluid motion, how the roll-decay coefficients and the flow field are altered by the span of the bilge keels. Plots of vorticity contour and vorticity isosurface along the three-dimensional hull will be presented to reveal the motion of fluid particles and vortex filaments near the keels. [ABSTRACT FROM AUTHOR]

This paper evaluates the theoretical application of nonlinear model predictive control (NMPC) to a model-scale point absorber for wave energy conversion. The NMPC strategy will be evaluated against a passive system, which utilizes no controller, using a performance metric based on the absorbed energy. The NMPC strategy was setup as a nonlinear optimization problem utilizing the interior point optimizer (IPOPT) package to obtain a time-varying optimal generator damping from the power-take-off (PTO) unit. This formulation is different from previous investigations in model predictive control, as the current methodology only allows the PTO unit to behave as a generator, thereby unable to return energy to the waves. Each strategy was simulated in the time domain for regular and irregular waves, the latter taken from a modified Pierson-Moskowitz spectrum. In regular waves, the performance advantages over a passive system appear at frequencies near resonance while at the lower and higher frequencies they become nearly equivalent. For irregular waves, the NMPC strategy leads to greater energy absorption than the passive system, though strongly dependent on the prediction horizon. It was found that the ideal NMPC strategy required a generator that could be turned on and off instantaneously, leading to sequences where the generator can be inactive for up to 50% of the wave period. [ABSTRACT FROM AUTHOR]

Fluid viscosity is known to influence hydrodynamic forces on a floating body in motion, particularly when the motion amplitude is large and the body is of bluff shape. While traditionally these hydrodynamic force or force coefficients have been predicted by inviscidfluid theory, much recent advances had taken place in the inclusion of viscous effects. Sophisticated Reynolds-Averaged Navier-Stokes (RANS) software are increasingly popular. However, they are often too elaborate for a systematic study of various parameters, geometry or frequency, where many runs with extensive data grid generation are needed. The Free-Surface Random-Vortex Method (FSRVM) developed at UC Berkeley in the early 2000 offers a middle-ground alternative, by which the viscous-fluid motion can be modeled by allowing vorticity generation be either turned on or turned off. The heavily validated FSRVM methodology is applied in this paper to examine how the draft-to-beam ratio and the shaping details of two-dimensional cylinders can alter the added inertia and viscous damping properties. A collection of four shapes is studied, varying from rectangles with sharp bilge corners to a reversed-curvature wedge shape. For these shapes, basic hydrodynamic properties are examined, with the effects of viscosity considered. With the use of these hydrodynamic coefficients, the motion response of the cylinders in waves is also investigated. The sources of viscous damping are clarified. [ABSTRACT FROM AUTHOR]

This paper evaluates two aspects of enhancements made to a generic ocean-wave energy extraction device, developed recently at University of California (UC)-Berkeley with features reported in Yeung et al. (2010, "Design, Analysis, and Evaluation of the UC-Berkeley Wave-Energy Extractor," ASME J. Offshore Mech. Arct. Eng., 134(2), p. 021902). First, the differences in hydrodynamic performance between flat- and hemi-spherical bottom floaters were investigated theoretically using the LJC Berkeley 2D viscous-flow solver: FSRVM (Seah and Yeung, 2008, "Vortical-Flow Modeling for Ship Hulls in Forward and Lateral Motion," Proceedings of the 27th Symposium on Naval Hydrodynamics, Seoul, Korea). The predicted enhancement was compared with experimental results, demonstrating that an increase in motion of over 50% was realizable. Second, important modifications to the design, fabrication, and material of the rotor and stator of the permanent magnet linear generator (PMLG) were made with the aim to increase both power output and mechanical-to-electrical conversion efficiency, ηel. Increased power extraction and efficiency were achieved, doubling what had been previously reported. The nonlinear relationship between the generator damping and the magnet-coil gap width was also investigated to verify that the conditions for optimal power extraction presented in Yeung et al. (2010, "Design, Analysis, and Evaluation of the UC-Berkeley Wave-Energy Extractor," ASME J. Offshore Mech. Arct. Eng., 134(2), p. 021902) were achievable with the PMLG. Experimental results, obtained from testing the coupled floater and PMLG systems in a wave tank, revealed that realized capture widths were more than double those from the previous design. These results further confirmed that matching of the generator and floater damping significantly increased the global efficiency of the extraction process. [ABSTRACT FROM AUTHOR]

This work, presented at the OMAE-2002 Special Symposium in honor of John V. Wehausen, is dedicated to him, an admirable teacher, colleague, and scholar. A general open boundary condition for time-dependent wave-body interaction problems is developed. The technique is based on domain decomposition and a use of the unsteady free-surface Green function in an outer region. The boundary condition results in a shell condition which may be applied to a variety of interior solution methods. When the interior solution method is a boundary-integral method, an implicit matching can be done. When a volumediscretization method is used in the interior region, a newly developed explicit matching procedure is performed. Demonstrations of the transparent properties of this shell are made for several types of unsteady problems. [ABSTRACT FROM AUTHOR]

The powering issue of a high-speed marine vehicle with multihulls and air-cushion support is addressed, since there is an often need to quickly evaluate the effects of several configuration parameters in the early stage of the design. For component hulls with given geometry, the parameters considered include the relative locations of individual hulls and the relative volumetric ratios. Within the realm of linearized theory, an interference-resistance expression for hull-to-hull interaction is first reviewed, and then a new formula for hull-and-pressure distribution interference is derived. Each of these analytical expressions is expressed in terms of the Fourier signatures or Kochin functions of the interacting component hulls, with the separation, stagger, and speed as explicit parameters. Based on this framework, an example is given for assessing the powering performance of a catamaran (dihull) as opposed to a tetrahull system. Also examined is the wave resistance of a surface-effect ship of varying cushion support in comparison with that of a base line catamaran, subject to the constraint of constant total displacement. [ABSTRACT FROM AUTHOR]