Mohammed Islam

Hydrodynamic Performance Evaluation of an Ice Class Podded Propeller Under Ice Interaction

Fluid-structure interaction between an ice sheet on the water surface and a podded R-Class propeller was examined and analyzed in terms of numerical simulation using a newly enhanced unsteady time-domain, multiple body panel method model. The numerical model was validated and verified and also checked against various previous in-house experimental measurements. The simulation was performed in a real unsteady case, that is, the ice piece stands still and the podded propeller moves and approaches the ice piece until collision occurs. Experimental data were taken from a previous cavitation tunnel test program for a bare R-Class ice breaker propeller under open water conditions, for the R-Class propeller approaching a blade-leading-edge contoured large size ice block under the proximity condition, and from an ice tank test program for a tractor type podded/strutted R-Class propeller under open water conditions. Comparison between experimental and numerical results was made. A general agreement was obtained. The magnitude of force fluctuations during the interaction increased significantly at the instant immediately before the impact between the propeller blades and the ice piece.Copyright

Performance study of podded propulsor in static azimuthing conditions

This paper presents a comprehensive experimental study on variations of propulsive characteristics of puller and pusher podded propulsors in static azimuthing open water conditions. A custom designed experimental apparatus consisting of a six-component global dynamometer and a three-component pod dynamometer was used to measure the propulsive performance of a model pod unit in pusher and puller configurations in a towing tank. The pod model was tested to measure the forces on the whole unit as well as thrust and torque of the propeller shaft for a range of advance coefficients combined with a range of static azimuth angles from +30° to −30°. The variations in forces and moments of the propulsor unit with change of azimuth angle and advance speed are presented in non-dimensional forms. The results illustrate that the propeller thrust and torque as well as the unit axial, side forces and the steering moment are complex functions of the azimuth angle and propeller loading.

Design and Optimization of an Ice Class Propeller Under Shallow Water, Semi-Tunnel Hull and Heavy Load Conditions

A design and optimization procedure developed and used for a propeller installed on a twin-semi-tunnel-hull ship navigating in very shallow and icy water under heavy load conditions is presented. The base propeller was first determined using classical design routines under open water condition utilizing existing model test data. In the optimization process, a panel method code (PROPELLA) was used to vary the pitch values and distributions and take into account the inflow wake distribution, tunnel gap and cavitation effects. The optimized propeller was able to numerically achieve a ship speed 0.02 knots higher than the desired speed and 0.06 knots higher than the classical B-series propeller. The analysis of the effect of inflow wake, hull tunnel, cavitation and blade rake angle on propulsive performance will be the focus of this paper.© 2006 ASME

Investigation of Hydrodynamic Loads and Flow Patterns Near an Escort Tug in Oblique Flows

The present research investigates the hydrodynamic forces and moments and the flow pattern near the hull of an escort tug at steady oblique flow conditions. An escort tug is modelled and numerical simulations have been carried out using a commercial RANS solver. In the simulations, the viscous flow field is calculated by the finite volume method, adapting the k-e turbulence model. Free surface is modeled using the Volume of Fluid (VOF) approach for calm water condition. The hull is assumed fixed in space with an even keel. Grid dependency studies are conducted to obtain insight into the reliability and accuracy of the results. Flow velocities around the escort tug as well as integral variables are computed at different Froude numbers and compared to the corresponding measurement data available in the public domain. The measurements was completed using a Planar Motion Mechanism (PMM) apparatus to measure the resistance, side force and yaw moment of the tug hull. Additionally, the velocity distributions on the upstream and downstream sides of the tug were measured by Particle Image Velocimetry (PIV).The measurements and simulations have been completed at one draft, and at 15°, 30° and 45° inflow conditions. A reasonable agreement has been obtained between the predicted axial and lateral forces and the corresponding measurements. The flow pattern and the velocity distribution at different drift angle are comparable with the measurements. The variation of the pattern of flow separation at the suction side of the hull at different yaw angle is also analyzed and presented.Copyright

VIV Response of a Subsea Jumper in Uniform Current

Subsea jumpers are susceptible to in-line and/or cross-flow vortex induced vibration (VIV) fatigue damage due to sea bottom currents. However, there is no proven industry standard design analysis methodology currently available specifically for assessing subsea jumper VIV response.In 2012, ExxonMobil conducted a jumper VIV model test to assess the validity of potential jumper VIV prediction approaches. A towing test rig was used to expose a small scale jumper model to flow conditions simulating uniform bottom currents. The jumper model was instrumented to acquire acceleration, bending strain and end connection load data. Several accelerometers and strain gauges were installed to enable reconstruction of static and dynamic deformations and bending deflections along the jumper model. Towing tests at different orientations and tow speeds were performed on both a bare pipe model and a straked pipe model. The data were analyzed to examine the frequencies and amplitudes of the jumper vibration. The data from these experiments provide a benchmark for validating jumper VIV prediction approaches.In this paper, the model test program is presented including model testing philosophy, jumper design and fabrication, and high level model test results.Copyright

The Use of Additive Manufacturing Techniques in the Construction of Model-Scale Propellers

Historically, it has been expensive for model test basins to supply custom model-scale propellers for ship propulsion experiments, due to the high cost of traditional propeller fabrication methods. This paper presents a new method of manufacturing model propellers in a cost effective and time efficient manner.A new computer aided design tool has been developed to quickly develop propeller geometry, which can then be built using additive manufacturing processes. These processes reduce the cost of manufacturing propellers by an order of magnitude compared to machined metal propellers.The results of a study comparing propeller open water and self-propulsion experiments using traditional metal propellers to newly developed plastic propellers are presented. This study also includes an evaluation of the effect of surface roughness on the open water performance coefficients. The measured propulsive performance coefficients of the plastic propellers were within 2% of that of geometrically similar metal propellers with the same blade roughness.Copyright

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

Performance of dynamic azimuthing podded propulsor

This paper presents results and analyses of an experimental study into the effects of static and dynamic azimuthing conditions on the propulsive characteristics of a puller podded unit in open water. The model propulsor was instrumented to measure thrust and torque of the propeller, three orthogonal forces and moments on the unit, rotational speed of the propeller, azimuthing angle and azimuthing rate. The model was first tested over a range of advance coefficients at various static azimuthing angles in the range of-180° to 180°. These tests were followed by tests in which the azimuthing angle was varied dynamically at certain azimuthing rate and propeller rotational speed. A comparative study of the performance coefficients at static and dynamic azimuthing conditions in the range of-180° to 180° is presented. The performance coefficients of the propeller and the pod unit showed a strong dependence on the propeller loading and azimuthing angle. The coefficients in static azimuthing conditions fit well with a 10th order polynomial fit of the data obtained in the dynamic azimuthing condition in the corresponding azimuthing angles and advance coefficient. An uncertainty analysis of the measurements is also presented.

Modelling Details of Fenders in Float-Over Installation Experiments

Fenders are a key element of float-over topsides installations protecting the vessel and jacket legs by absorbing impact energy. Fender capacity is defined in accordance with expected vessel motions, vessel hull strength, and anticipated impact energy. Translating those dependencies from full-scale float-over scenarios to model-scale experiments provides added challenges when devising experimental approaches.This paper describes the design and development of model-scale fenders and their application in float-over topsides installation experiments. Current model fenders use multi-stage cantilevered leaf springs to develop the required non-linear stiffness curve which is calculated from full-scale fender data. Spring material and geometry, relative position, and loading location are key variables used to match the target fender stiffness as changes in these parameters directly affect the force-deflection characteristics of the model-scale fenders.The force-deflection characteristics (stiffness) of the model-scale fenders were checked before, during, and after wave experiments, using both isolated bench tests and in situ verifications. Multiple iterations of stiffness checks were completed during the stiffness curve matching process prior to the experimentation. The achieved load-deflection characteristics of the fender units were within 2% of the target stiffness curve. The damping characteristics of the fenders were not specifically modeled, but were measured for each of the stiffness stages. The model fenders have successfully been used in multiple float-over experimental campaigns.© 2012 ASME

Simulation of Surface Pressure Induced by Vortex/Body Interaction

When a strong vortical wake impacts a structure, the pressure on the impacted surface sees large variations in its amplitude. This pressure fluctuation is one of the main sources causing severe structural vibration and hydrodynamic noise. Economical and effective prediction methods of the fluctuating pressure are required by engineers in many fields. This paper presents a wake impingement model (WIM) that was incorporated into a panel method code, Propella, and its applications in simulations of a podded propeller wake impacting on a strut. Simulated strut surface pressure distributions and variations are compared with experimental data in terms of time-averaged components and phase-averaged components. The pressure comparisons show that the calculated results are in a good agreement with experimental data.