Glen Rice

Estimating oil concentration and flow rate with calibrated vessel-mounted acoustic echo sounders

As part of a larger program aimed at evaluating acoustic techniques for mapping the distribution of subsurface oil and gas associated with the Deepwater Horizon-Macondo oil spill, observations were made on June 24 and 25, 2010 using vessel-mounted calibrated single-beam echo sounders on the National Oceanic and Atmospheric Administration ship Thomas Jefferson. Coincident with visual observations of oil at the sea surface, the 200-kHz echo sounder showed anomalously high-volume scattering strength in the upper 200 m on the western side of the wellhead, more than 100 times higher than the surrounding waters at 1,800-m distance from the wellhead, and weakening with increasing distance out to 5,000 m. Similar high-volume scattering anomalies were not observed at 12 or 38 kHz, although observations of anomalously low-volume scattering strength were made in the deep scattering layer at these frequencies at approximately the same locations. Together with observations of ocean currents, the acoustic observations are consistent with a rising plume of small (< 1-mm radius) oil droplets. Using simplistic but reasonable assumptions about the properties of the oil droplets, an estimate of the flow rate was made that is remarkably consistent with those made at the wellhead by other means. The uncertainty in this acoustically derived estimate is high due to lack of knowledge of the size distribution and rise speed of the oil droplets. If properly constrained, these types of acoustic measurements can be used to rapidly estimate the flow rate of oil reaching the surface over large temporal and spatial scales.

Instrumentation for open ocean aquaculture monitoring

Fudning was provided by National Oceanic and Atmospheric Adminstration for the Open Ocean Aquaculture Project under Contract No. NA86RG0016 to the University of New Hampshire and under Subcontracts 00-394 and 01-442 to the Woods Hole Oceanographic Institution.

Engineering Overview of the University of New Hampshire's Open Ocean Aquaculture Project

Aquaculture products are projected to play an important role infilling the global demand for seafood in the world marketplace. In the US, stiff resistance to near shore aquaculture sites (where most farms are located) will drive the industry to more exposed locations. In an effort to better understand open ocean aquaculture challenges, the University of New Hampshire (UNH) has been investigating the biological, engineering, environmental and economical issues. This overview focuses on the engineering approach utilized by UNH to determine aquaculture system loads, motions and operational logistics by utilizing a variety of tools including numerical and physical models and field experimentation. Numerical modeling is performed with Aqua-FE, a finite element analysis (FEA) program developed to study aquaculture type systems, MSC.MARC/Mentat, a FEA structural modeling program, and FLUENT, a computational dynamics program. Scaled physical model tests are performed in the UNH wave/tow tank. In addition, an extensive field program experiments with the use of biofouled net panels, telemetry and control systems, feed buoys, scaled cages and various environmental monitoring equipment. Biofouled net panels were tested to determine the blockage effect due to the biological growth. Feed buoys, with telemetry and control options, have been deployed and tested. A new 20 ton capacity feed buoy has been designed and is currently under construction. A scale, experimental, submersible net pen has been designed, built and deployed to determine the feasibility of various components. Environmental measurements are collected with a surface buoy and the data is transmitted to shore. The resulting information from these experiments can help move the near shore aquaculture industry to more exposed locations

Practical Applications of Numerical Modeling using Aqua-FE: A Case Study

A numerical model of an American Soybean Association (ASA) cage system was constructed using a finite element program developed at the University of New Hampshire (UNH) called Aqua-FE. The small volume, high density aquaculture system was modeled to determine how the system will operate in normal and extreme environmental conditions. The goals of the study were to determine the maximum loads in the system during tropical storm conditions and determine a similar cage systems response under specified environmental criteria. The cage is currently deployed in Weitou Bay, China. The system consists of a 100 m3 cage (2 m times 4.5 m times 7 m) secured in a single point mooring. The rigid HDPE cage is held to the mooring by two sets of bridle lines, attached to the upper and lower cage framework. Chain ballast hangs below the lower cage rim providing a restoring force. A deadweight anchor secures the system to the seafloor. A 90 kg float suspends the single point mooring and serves as a tie-up location for servicing vessels. Aqua-FE can apply wave and current loading on truss and buoy elements by utilizing the Morrison equation adopted for analysis of aquaculture net pen systems. The algorithm employs a nonlinear Lagrangian formulation to account for large displacements of structural elements. In addition, the unconditionally stable Newmark direct integration scheme is adopted to solve the nonlinear equations of motion. Hydrodynamic forces on the structural elements are calculated using the Morison equation modified to account for relative motion between the structural element and the surrounding fluid. Maximum loads in the mooring gear approached 56 kN during the storm events. When various current velocities were applied, the cage submerged to a maximum depth of 16.4 meters

An extended surface target for high-frequency multibeam echo sounder calibration

An extended calibration target has been developed for calibrating the intensity output of a multibeam echo sounder (MBES). The target was constructed of chain links arranged similar to a curtain, providing an extended surface target with a mean scattering strength of -17.8 dB at 200 kHz. The target was used to calibrate a 200 kHz MBES, and the MBES was subsequently used to collect seafloor backscatter over sand and gravel seafloors. Field results were compared with calibrated split-beam echo sounder measurements at an incidence angle of 45°. The results suggest that the chain target is a viable MBES calibration tool.

Design of a 20-Ton Capacity Finfish Aquaculture Feeding Buoy

A design for a 20-ton capacity buoy was developed to feed fish in four submerged cages at an exposed site south of the Isles of Shoals, New Hampshire, USA. The buoy was designed to contain all the equipment necessary to accomplish the feed dispensing tasks as well as have the strength and stability to remain on location in a variety of sea states. New feed handling and distribution systems were developed and tested. To evaluate seakeeping response a Froude scaled physical model was constructed and tested at the Ocean Engineering wave/tow tank at the University of New Hampshire (UNH). The mooring system was designed using the UNH developed finite element analysis program called Aqua-FE. The prototype buoy is now under construction, and is scheduled for deployment in late summer 2006

Computationally Efficient Variable Resolution Depth Estimation

Abstract A new algorithm for data-adaptive, large-scale, computationally efficient estimation of bathymetry is proposed. The algorithm uses a first pass over the observations to construct a spatially varying estimate of data density, which is then used to predict achievable estimate sample spacing for robust depth estimation across the area of interest. A low-resolution estimate of depth is also constructed during the first pass as a guide for further work. A piecewise-regular grid is then constructed following the sample spacing estimates, and accurate depth is finally estimated using the composite refined grid and an extended and re-implemented version of the cube algorithm. Resource-efficient data structures allow for the algorithm to operate over large areas and large datasets without excessive compute resources; modular design allows for more complex spatial representations to be included if required. The proposed system is demonstrated on a pair of hydrographic datasets, illustrating the adaptation of the algorithm to different depth- and sensor-driven data densities. Although the algorithm was designed for bathymetric estimation, it could be readily used on other two dimensional scalar fields where variable data density is a driver.

Utilizing an extended target for high frequency multi-beam sonar intensity calibration

There is an interest in expediting intensity calibration procedures for Multi-Beam Echo-Sounders (MBES) to be used for acoustic backscatter measurements. To this end, a target was constructed of irregularly oriented small chain links arranged in a “curtain” simulating an extended scattering surface, such as the seafloor. Tests with a 200-kHz, 7°, SIMRAD EK60 Split-Beam Echo-Sounder (SBES) were performed in a tank in order to investigate the targets angular- and range-dependent scattering strength. These tests suggest that the amplitude envelope of the scattered signal is Rayleigh distributed and that the backscatter strength depends linearly on the number of active scattering elements. Following these initial validation tests, a 200 kHz Reson SeaBat 7125-SV2 MBES was calibrated in the tank with the same extended target. During the calibration, the MBES was rotated so that every beam was incident on the target. This calibration test was performed once when the target was at normal and once at oblique (45°)...

Operational mapping of the DWH deep subsurface dispersed oil

Mapping of the deep dispersed oil feature from the blowout of the MC252 wellhead was organized by the subsurface mapping unit within the Unified Area Command starting in early August, 2010. The operational process employed and the challenge presented by the response situation are reviewed. Colored dissolved organic matter fluorescence, used to establish existence of the subsurface oil prior to this time, had largely fallen below background levels for the sensors by this time. Dissolved oxygen (DO), deficits in which were assumed to be related to consumption of oil by microbes, was the only routinely observed variable in vertical profiles that displayed a persistent and obvious anomaly. The DO anomaly was therefore used to identify the presence and magnitude of the dispersed oil impact. An adaptive sampling plan employing daily review of DO profiles to provide vessel guidance was established and permitted a coarse mapping of the feature within 4 weeks. The DO anomaly extended from the wellhead to the WSW for more than 350 km, bounded to the north by the upper slope (approximately 1000 m isobath), with a cross-slope extent of 60-100 km, and was also present to the ENE of the wellhead out to 60 km.

Regional seabed backscatter mapping using multiple frequencies

The frequency dependence of seabed backscatter has previously been assessed at static sites. While dependencies have been identified, the restricted range of seabed types and issues of absolute calibration have limited inferences. Until recently seabed backscatter measurements from underway mapping sonars (sidescans and multibeams) have predominantly been at a single center frequency, dictated by the range versus resolution compromise best suited for the sonar altitude. With improved range performance using FM pulses, the depth range over which a specific frequency is usable have expanded. Taking advantage of that, two national seafloor mapping programs have switched to routine collection of seabed backscatter with wavelength differences of almost an order of magnitude. The NOAA ship Thomas Jefferson and the Irish government vessel Celtic Explorer now are acquiring data at 45 and 300 kHz and 30 and 200 kHz, respectively. Even though absolute calibration remains a concern (particularly for multi-sector sys...