Bruce Colbourne

Dynamics of a mussel longline system

Abstract The equations of the three dimensional motion of a submerged mussel longline system are formulated using Kanes formalism. The line is modelled using lumped masses and tension-only springs including structural damping. The mussel culture is modelled as lumped masses attached to the main line. Surface waves are described by Stokes’ second order wave theory. The hydrodynamic loads due to viscous drag are applied via a Morisons equation approach using the instantaneous relative velocities between the fluid field and the attached buoys and mussel masses. The detailed algorithm is presented and the equations are solved using a robust implementation of the Runge–Kutta method provided in matlab .

GPU-Event-Mechanics Evaluation of Ice Impact Load Statistics

The paper explores the use of a GPU-Event-Mechanics (GEM) simulation to assess local ice loads on a vessel operating in pack ice. The methodology uses an event mechanics concept implemented using massively parallel programming on a GPU enabled workstation. The simulation domain contains hundreds of discrete and interacting ice floes. A simple vessel is modeled as it navigates through the domain. Each ship-ice collision is modeled, as is every ice-ice contact. Each ship-ice collision event is logged, along with all relevant ice and ship data. Thousands of collisions are logged as the vessel transits many tens of kilometers of ice pack. The GEM methodology allows the simulations to be performed much faster than real time. The resulting impact load statistics are qualitatively evaluated and compared to published field data. The analysis provides insight into the nature of loads in pack ice. The work is part of a large research project at Memorial University called STePS2 (Sustainable Technology for Polar Ships and Structures). Introduction Ice class vessels are unique in a number of ways in comparison to non-ice class vessels. Hull strength, power, hull form and winterization aspects are all issues that raise special challenges in the design of ice class ships. This paper focuses on matters of local ice loads which pertain to hull strength in ice class vessels. More specifically, the paper examines the parametric causes of local ice loads and statistics that result as a ship transits through open pack ice. The issue of pack ice transit is of interest to those wishing to operate safely in such conditions. One key question is that of safe operational speeds. Consider the special case of open pack ice, where floes are relatively small, numerous and resting in calm water. A vessel moving through such an ice cover would experience a series of discrete collisions. As long as a vessel moved very slowly, the loads would be very low. In such a case the vessel could make safe and steady progress, even if it had a relatively low ice class. However, if the vessel attempted to operate more aggressively, impact speeds would increase and a higher ice class would be needed for safe operations. The investigation below provides some insight into the factors that influence the loads in this situation. These factors include hull form, speed, floe size and concentration, ice thickness, strength and edge shape. Most prior studies have tended to focus on ice thickness and strength as the primary determinants of load. This study shows that ice edge shape and mass, along with hull form and locations are also strong determinants of loads, and especially the load statistics. The simulations provide some interesting data, especially when compared to field trials data. A related focus for the study is to explore the use of the GPU-Event-Mechanics (GEM) simulation approach. The GEM approach represents the integration of a number of concepts. The physical space is described as a set of bodies. The movement (kinematics) of the bodies is tracked using simple equations of motion. Time is divided into relatively long ‘moments’, during which events occur. All variables in the simulation; forces, movements, fractures and other changes, are considered to be aspects of events. Some events are momentary, while others are continuing. Some events involve a single body and are termed solo events. Motion, for example, is treated as a solo event. Some events are two-body events. Impact is an example of a two-body event. The GEM approach lends itself to parallel implementation, which in this case is accomplished in a GPU environment. A GPU (Graphics Processing Unit) is a common element found in modern computer graphics cards. The GPU is primarily intended for making rapid calculations associated with the display. However, special software can access the GPU and enhance the computing power available to the user. See (Daley et.al. 2012) for further discussion of GPUs. The event models are the analytical solutions of specific scenarios. As a result, the events do not require solution (in the numerical sense) during the GEM simulation. The

Ice Loads in Dry and Submerged Conditions

This paper describes a study of the effects of submergence on ice crushing loads. Thirty three small scale indentation tests have been performed on cone-shaped ice samples in dry and submerged conditions using a material testing system (MTS machine) located in a cold room at −7°C. The indenter was a flat aluminum plate at the bottom of a container that was attached to the actuator of the MTS machine. Transparent windows facilitated visual observations and recordings using a high speed camera. In the submerged tests the container was partly filled with salt water. Testing was performed at rates of 1 mm/s and 100 mm/s. The specimens were ice cones with 25 cm in diameter and with 20° and 30° angles. Data recordings comprised time-penetration and time-force histories. Generally higher forces were obtained in submerged tests. Furthermore, the difference between dry and submerged condition was more pronounced at the high indentation rate.Copyright

Rewinding the waves: tracking underwater signals to their source

Analysis of data, recorded on March 8th 2014 at the Comprehensive Nuclear-Test-Ban Treaty Organisation’s hydroacoustic stations off Cape Leeuwin Western Australia, and at Diego Garcia, reveal unique pressure signatures that could be associated with objects impacting at the sea surface, such as falling meteorites, or the missing Malaysian Aeroplane MH370. To examine the recorded signatures, we carried out experiments with spheres impacting at the surface of a water tank, where we observed almost identical pressure signature structures. While the pressure structure is unique to impacting objects, the evolution of the radiated acoustic waves carries information on the source. Employing acoustic–gravity wave theory we present an analytical inverse method to retrieve the impact time and location. The solution was validated using field observations of recent earthquakes, where we were able to calculate the eruption time and location to a satisfactory degree of accuracy. Moreover, numerical validations confirm an error below 0.02% for events at relatively large distances of over 1000 km. The method can be developed to calculate other essential properties such as impact duration and geometry. Besides impacting objects and earthquakes, the method could help in identifying the location of underwater explosions and landslides.

The Influence of External Boundary Conditions on Ice Loads in Ice-Structure Interactions

Design ice loads are generally derived from field measurements or laboratory experiments. The latter commonly neglect the circumstance that most ice-structure interactions occur underwater, despite the fact that studies report higher ice loads if water is present. Other than a few studies on ice extrusion processes, most investigations on ice loads also do not specifically consider the presence of snow or granular ice at the ice-structure interface. To elucidate the influence of water, snow and crushed ice, as external boundary conditions, on ice load magnitude, 71 small-scale laboratory tests were carried out. Testing involved a hydraulic material testing system (MTS machine) located in a cold room at -7°C. Ice specimens were conical shaped with 25 cm in diameter and with 20° and 30° cone angles. Those were impacted with a flat indentation plate at 1 mm/s, 10mm/s and 100 mm/s indentation rates. Timepenetration and time-force histories from the MTS machine, as well as qualitative contact area and local pressure measurements from tactile pressure sensors were collected. Tests were also recorded with a high speed camera and monitored with still photos. The effect of submergence was most evident at high indentation rate, yielding high ice loads. Snow and granular ice caused comparably high ice loads at the high indentation rate. Moreover, the snow and granular ice conditions also dramatically increased loads at the low indentation rate. In all cases, higher ice loads were associated with increased effective contact areas. INTRODUCTION Icebergs still pose a significant risk of damage to marine structures. In 2000, Hill [1] introduced an iceberg collision database with information on environmental conditions, and damage severity. The database comprises 670 events between 1810 and 2004 involving fishing boats, passenger ships, tankers, bulk carriers and freighters. In just above one quarter of the events, the vessel sank or had to be abandoned. The numbers of accidents and extent of damage reveal the need for measures to reduce risks and damage. Field studies that naturally involve ice impacts in water are most often conducted with instrumented ships. For instance, Masterson and Frederking [2] examined local pressures and forces on icebreakers that rammed ice floes. Later, in 2001, the Canadian Coast Guard Ship Terry Fox was equipped with strain gauges and 178 ice impacts with bergy bits (up to 20,000 t) were accomplished (Ritch et al. [3]). However, those studies do not allow a comparison with loads derived under dry circumstances to directly assess the effect of the water. Most other field indentation tests focus on the influence of different indenter shapes (e.g. Frederking et al. [4], Masterson et al. [5], Kennedy et al. [6]) but not on external boundary conditions. This is despite the fact that ice-structure interactions most likely occur underwater, or at least partially underwater. Ice strength information is often derived from laboratory dry tests and laboratory experiments are still essential to investigate the processes involved in ice-structure interactions. There are only a few publications that address the influence of submergence during an ice impact. For example a layer of spray water was found to yield higher ice loads compared to a dry impact (Varsta [7]). A first approach was taken in a recent laboratory study (Sopper et al. [8]) on ice impacts that provides clear evidence that submergence significantly influences ice loads, particularly at high indentation rates. Furthermore, little information exists on the difference that snow or granular ice at the ice-structure interface cause in ice load magnitude, and how this compares to other external boundary conditions. Most studies focus on ice

Hydrodynamic Study of Submerged Ice Collisions

Most of the research done on ice-structure interaction deals with the ice at the sea surface. Whereas majority of ice-strengthened regions of ships and offshore structures are well below the waterline. The aim of this research is to examine the mechanics of ice loads caused by submerged ice blocks colliding with the structure. The kinematics is an essential determinant of the energy that is available to drive the ice crushing process during the collision. The present research aims to develop a model to represent the mechanics of such collisions and set a direction for future work. This study includes experimental and numerical components. Various physical experiments have been conducted using a submerged ice model moving solely due to its buoyancy. Using high speed camera the experiments are recorded and analysed to determine the kinematics of collision. These include location, velocity and acceleration of the model ice as a function of time. In parallel, numerical simulations have being conducted using FLOW 3DTM software. The results of the experiments are used to validate the numerical model of the underwater collision. The results shows that added mass plays an important role during the underwater impact collisions. The paper presents some preliminary results obtained during this research.Copyright

Harmonic wave and steady current effects on plane fishnets

Individual and interaction effects of parallel current and waves on three plane nets were empirically examined. A current opposing the direction of the waves was shown to shorten the wavelength while a current in-line with the direction of the waves stretched the wave length. Due to these wave modifications, the combined loads produced by a current and waves were significantly less than the sum of current and wave forces applied individually. Applying a vector approach, the unsteady loads were split into drag forces and inertia components. Both components contributed considerably to the hydrodynamic loads for wave-only cases. For combinations of waves and current, the inertia force was significantly greater than the drag force. Further insights were also provided into the concepts of effective thickness and the modified Keulegan-Carpenter number as parameters quantifying inertia force and drag force for fishnets.

Steel-Fiber Self-Consolidating Rubberized Concrete Subjected to Impact Loading

This investigation was carried out to evaluate the combined effect of crumb rubber (CR) and steel fibers (SFs) on improving the impact resistance of self-consolidating concrete (SCC) mixtures. Seven SCC mixtures were developed with varied percentages of CR (0–15% by volume of sand) and SF’s volume of 0.35%. The performance of the developed mixtures was evaluated by testing the fresh properties, compressive strength, splitting tensile strength (STS), flexural strength (FS), and impact loading (drop weight on cylindrical and beam specimens). The results indicated that inclusion of CR decreased the compressive strength, STS, and FS of the tested mixtures, while the impact resistance obviously increased. Reinforcing CR mixtures with 0.35% SFs could compensate the reduction in the tensile strength resulting from adding rubber and further increase the resistance of mixtures to impact loading, achieving mixtures with promising properties for multiple structural applications.

Experimental Investigation of Ice Mass Hydrodynamic Interaction With Offshore Structure in Close Proximity

Iceberg/bergy bit impact load with fixed and floating offshore structures and supply ships is an important design consideration in ice-prone regions. Studies tend to divide the iceberg impact problem into phases from far field to contact. This results in a tendency to over simplify the final crucial stage where the structure is impacted. The authors have identified knowledge gaps and their influence on the analysis and prediction of iceberg impact velocities and loads (Sayeed et. al (2014)). The experimental and numerical study of viscous dominated very near field region is the main area of interest. This paper reports preliminary results of physical model tests conducted at Ocean Engineering Research Center (OERC) to investigate hydrodynamic interaction between ice masses and fixed offshore structure in close proximity. The objective was to perform a systematic study from simple to complex phenomena which will be a support base for the development of subsequent numerical models. The results demonstrated that hydrodynamic proximity and wave reflection effects do significantly influence the impact velocities at which ice masses approach to large structures. The effect is more pronounced for smaller ice masses.Copyright

Effect of Ship Speed on Level Ice Edge Breaking

This paper presents a numerical model of ship ice-wedge interaction to study the effect of ship speed on level ice edge breaking. The interaction process is modeled using LS-DYNA. The developed model considers ice crushing, ice flexural failure and the water foundation effect. For the ice, two different plasticity-based material models are used to represent ice crushing and ice flexural behaviors. The water foundation effect is modeled using a simple linear elastic material. The analysis is performed for a ship speed range of 0.1 to 5 ms−1 and ice thickness of 0.5 to 1.5 m. The analysis indicates that both ship speed and ice thickness significantly affect the ice breaking process. The model results are in good agreement with a number of analytical and empirical models. The model can be useful in establishing a rational basis for safe speed criteria, improving ship structural standards and tools for ice management capability assessment.Copyright