Dan Walker

Side-scan sonar image registration for AUV navigation

The ability of an AUV to navigate an underwater environment precisely and for an extended period depends on its effectiveness at making accurate observations regarding its location and orientation. An AUV platform equipped with a side-scan sonar system has the potential to register the current sonar image with previously captured images for the purpose of obtaining information about the vehicles pose. Image registration is a procedure which transforms images viewed from different perspectives into a single coordinate system. The significance of using image registration techniques in a surveying or monitoring context comes from the fact that the registration parameters could provide the AUV with an indication of the discrepancy between its expected and observed pose vectors. As such, image registration provides feedback which can be used to compensate for drift in inertial sensors or to provide a standalone navigation solution in the event that the inertial navigation system fails. In order for image registration to provide an effective means for feedback a number of requirements on the performance of the image registration method employed must be met. Not only must the method be accurate in the face of possible image variations, but it must operate in real-time using the limited computing resources available within an AUV. In this paper, a number of key image registration techniques are applied to side-scan sonar images. These techniques include those based on the maximization of mutual information, log-polar cross-correlation, the Scale-Invariant Feature Transform (SIFT), and phase correlation. The performance of these techniques is assessed based on a number of metrics including execution time and registration accuracy. The challenges introduced by side-scan sonar imaging systems which degrade the performance of image registration are also discussed in detail.

High Resolution Seabed Sub-Bottom Profiler for AUV

Memorial University of Newfoundland (Memorial) is undertaking a novel and exciting area of interdisciplinary research and development related to Autonomous Underwater Vehicles (AUV). AUVs are an untethered, unmanned technology that enables a broad array of research, especially in hazardous underwater environments, that cannot be achieved by other means. In spring 2010, Memorial University commenced design work on a project that aims to provide a means to conduct high-resolution sub-bottom seabed surveys in water depths up to 1000 m (3281 ft), using a new imaging sub-bottom profiler technology with a 10 cm (3.9 in) resolution that has never been deployed on an AUV. The purpose of this project is to integrate a long-array sub-bottom profiler developed by PanGeo Subsea Inc. of Canada, into Memorial’s Explorer AUV by building a new vehicle section that resembles a thick airplane wing with a span of 3.5 m (11.5 ft). Memorial University is working to make the new equipment easily adaptable and removable from the Explorer AUV while in operation. The Explorer AUV equipped with this new sub-bottom profiler capability will be operational in 2012. In this paper, the underlying design criteria and challenges are discussed. A preliminary concept design is described and coarsely evaluated for technical feasibility.Copyright

The Memorial Explorer: Developing the role of AUVs in under-ice research

Autonomous Underwater Vehicles are a leading technology for under-ice deployment and research. Memorial University and its Explorer AUV, in cooperation with the University of Tasmania and the Australian Antarctic Division, has embarked on a multi-phase development program that will lead to a scientific mission in the Australian Antarctic. Completion of Phase I saw the AUV deployed in the Canadian high Arctic and acoustic data collection performed in the Australian Antarctic. Currently in Phase II, the AUV is being fitted with a full suite of survey tools and will see developments in the fields of navigation, positioning and mission completion. Phase III will see further developments in AUV technology for under-ice research, with the ultimate goal being the AUV deployed in the Antarctic for a scientific mission to explore the role of sea-ice in the Worlds climate.

Experimental Analysis of the Near Wake from a Ducted Thruster at True and Near Bollard Pull Conditions Using Stereo Particle Image Velocimetry (SPIV)

Thrusters working at low advance coefficients are employed in a wide range of offshore and marine applications on Floating, Production, Storage, and Offloading (FPSO) systems; shuttle tankers; tug boats; and mobile offshore units. Therefore, an understanding of the flow around the thrusters is of great practical interest. Despite this interest, there is lack of knowledge in the description of the hydrodynamic characteristics of a ducted thrusters wake at bollard pull and low advance coefficient values. This work was aimed at providing detailed data about the hydrodynamic characteristics of a Dynamic Positioning (DP) thruster near wake flow at different low advance coefficient values. Wake measurements were made during cavitation tunnel tests carried out on a ducted propeller model at the Italian Ship Model Basin (INSEAN), Rome, Italy. Through these experiments, the DP thruster near wake velocity components at different downstream axial planes, up to 1.5 diameters downstream, were obtained using a Stereoscopic Particle Image Velocimetry (SPIV) system. These experiments were carried out at different advance coefficient (J) values [bollard pull (J=0), J=0.4 and J=0.45]. Copyright

The development of AUV strategies for multidisciplinary use

The Marine Environmental Research Lab for Intelligent Vehicles (MERLIN) operates an Explorer Autonomous Underwater Vehicle (AUV) that serves as a potential platform for ocean surveys over a broad range of disciplines. MERLIN undertook a four year development program to improve both the capacity and autonomy of the vehicle by acquiring multibeam, side-scan and sub-bottom profiling sonars and developing algorithms using these tools to improve the navigation of the vehicle as part of a project called Responsive AUV Localization and Mapping (REALM). The goal of REALM is to be able to pre-program the vehicle to both correct its own dead-reckoning position in a survey area and allow it to recognize Zones of Interest (ZOI). With such an improvement, programmed surveys could be modified in real time to allow the collection of additional detailed data in a ZOI. To develop an understanding of the types of data that might indicate a ZOI during a typical survey, a field program was developed for a site in Smith Sound, Newfoundland, Canada. Smith Sound was the site of the largest known overwintering inshore cod population in North America at a time when surrounding cod populations largely collapsed, creating interest in possible relationships between seabed properties and cod populations; the Sound is also of interest due to the potential presence of internal seiches and resonant tidal forcing; the area is the location of many documented, but unlocated shipwrecks; and, with depths over 200 m, is a region that is not easily explored without the use of technologies such as AUVs or other underwater vehicles. This paper presents the results of the preliminary Smith Sound survey, including an overview of the logistical planning aimed at collecting data with multidisciplinary interest. The individual data sets are presented, highlighting preliminary indications of ZOI for each topic of interest.

Hydrodynamic loads on ice-class propellers during propeller-ice interaction

Hydrodynamic loads on a propeller blocked with simulated ice were studied using a cavitation tunnel. Comparative predictions were made using a panel method. The propeller was a model of the Canadian Coast Guards R-class icebreake propeller, and the ice block was simulated using a solid blockage. Experimental results show the open water performance of the propeller, its performance behind a blockage, and the effects of cavitation in these conditions, as well as the loading on the simulated ice block. Panel method predictions were made of the time series propeller performance in the blocked flow. Cavitation during propellerice interaction resulted in a reduction of mean suction load on the ice block. Block load measurements indicated an increase in the oscillation about the mean value of the loads, with a variation in the phase of the loading with respect to blade position as compared with the non-cavitating results. Comparisons of panel method results with the measured block loads support the reliability of the dynamic measurements.

Hydrodynamic Performance and Cavitation of an Open Propeller in a Simulated Ice-Blocked Flow

Experiments were done on a 200-mm-dia open propeller behind a simulated ice blockage in a cavitation tunnel. The propeller performance in uniform flow and blocked flow is contrasted over a range of advance coefficients and at different cavitation numbers. Mean thrust and torque coefficients are presented. The types of cavitation, and its intermittent nature over a cycle of operation, are reported. The experiments indicate the likelihood of cavitation at full scale for blocked conditions and illustrate the effects of cavitation on mean values of thrust and torque.

Sheared Current Generation in Flume Tank for Experimental Research

Current velocity, profile, direction, and duration may affect hydrodynamic loads and VIM of offshore structure. It is often recommended that physical experiments are carried out in sheared current, in multiple directions and for sufficiently long period of time to investigate the hydrodynamic characteristics of deep draft offshore structures to obtain better correlation to the field measurements. This necessitates generating sheared current with acceptable turbulence level. This paper presents a recent advancement in generating sheared current in a flume tank facility. In this process, the test specimen remains moored and the water flows past with its velocity varied with depth as long as necessary.A combination of synthetic and wire meshes are used to provide the required amount of blockage onto the circulating channel flow of the flume tank to obtain specified current distribution across the cross-section and at the longitudinal center of the tank. The final set-up of the current screen provided a sheared flow distribution within 10% of the targets. Also, the measured turbulence level was below 10% in all the locations measured.VIM studies of a model spar were successfully carried out in the generated sheared current in the flume tank facility. The ability to accurately model the sheared flow essentially improves the accuracy of the measured VIM type response measurements. The generated sheared current can also be applied for other hydrodynamic experiments where sheared current is relevant.Copyright

MERLIN - A decade of large AUV experience at Memorial University of Newfoundland

Autonomous Underwater Vehicle (AUV) technology has significant research potential, specifically for harsh maritime environment operations. Memorial University of Newfoundland recognized this potential and in 2005 the University commissioned an International Submarine Engineering Explorer AUV to be built. The Marine Environmental Research Lab for Intelligent Vehicles (MERLIN) was established to manage, maintain and operate the Explorer for Memorial University and any other potential AUV users. Over the course of ten years, MERLIN capabilities have grown to a full service AUV research team with harsh maritime and polar experiences. The Explorer was born in the Pacific Ocean, lives in the Atlantic Ocean and has vacationed in the Arctic. AUV research efforts have evolved from dynamic vehicle studies with basic sensor technology to integration of cutting edge sonar systems that push the boundaries of AUV design and successful implementations of original research into advanced autonomous navigation.

Comparison of DP Performance Prediction Techniques for Scaled Models

Utilization of Dynamic Positioning (DP) systems for offshore exploration and production of hydrocarbons is increasing due to the need to exploit deeper water depths, where mooring becomes less feasible. In conducting analysis or predictions for DP System performance, there are three common techniques: experimental investigations at reduced scale, using a simplified mooring system without thrusters; similarly scaled experiments using active DP thrusters; or, time or frequency domain numerical simulations. This paper identifies differences in DP system performance estimates, provided by each of the three methods, by using each method to analyze the same system, in identical wind and wave environments. Experiments were completed using a 1:40 scale model of a typical 99,000t monohull drillship equipped with an active DP system consisting of six azimuthing thrusters. These experiments were repeated with the vessel unpowered on two mooring systems with different stiffnesses. Physical experimental results are then compared to time-domain numerical simulations completed using Oceanic Consulting Corporations DP simulation program. A comparison of system performance predictions provided by each method is presented.