Heather Peng

Experimental Studies of Hydrodynamic Interaction of Two Bodies in Waves

There are challenges in the prediction of low-frequency load and especially the resonant free surface elevation between two bodies in close proximity. Most of the linear potential-flow based seakeeping programs currently used by the industry over-predict the free surface elevation between the vessels/bodies and hence the low-frequency loadings on the hulls. Various methods, such as the lid technique, have been developed to suppress the unrealistic values of low-frequency forces by introducing artificial damping coefficients. However, without the experimental data, it is challenging to specify the coefficients.This paper presents the experimental studies of motions of two bodies with various gaps and the wave elevations between bodies. Model tests were performed at the towing tank of Memorial University. The objective was to provide benchmark data for further numerical studies of the viscous effect on the free surface predictions. The experimental data were compared with numerical solutions based on potential flow methods. The effect of tank walls were examined. Preliminary uncertainty analysis was also carried out.Copyright

Discrete Element Modelling of Pack Ice Interaction With Floating Structures

This paper presents a two-dimensional numerical model for ship-ice interaction simulatiion using the discrete element method (DEM). The simulation was conducted for a broken ice field with hundreds of circular ice floes and various combinations of ice conditions. A viscous-elastic ice rheology was adopted to model the dynamic behavior of each individual ice floe. Both ship-ice and ice-ice contacts were considered in the interaction force. Environment forces, including wind force and wave force, were calculated by empirical formulas. An algorithm was developed to log each contact and solve motions of individual ice floe and the ship. The resistance of ship advancing in ice was predicted and compared with model test results.Copyright

Spurious Waves During Generation of Multi-Chromatic Waves in the Wave Tank in Shallow Water

Accurate generation of the primary waves and the reproduction of the group-induced second-order low and high frequency waves have been considered essential for physical i.e. model test in the laboratory. In the laboratory when multi-chromatic primary waves are generated the required bounded waves will be generated naturally at the difference frequencies. In addition to that several unwanted free waves are also generated. The free waves, having the same frequencies of the bounded waves are reproduced due to mismatch of the boundary conditions at the wave paddle. The other two types of free waves are due to the wave paddle displacement and the local disturbances. We carried out physical experiments to identify the second order spurious waves in shallow water in the Offshore Engineering Basin (OEB) at the Institute for Ocean Technology (IOT) of National Research Council (NRC) Canada. In the basin water depths in the range of 0.4m to 0.6m are used for the experiments. The peak wave periods also have varied from 1.133s to 2.145s. In the experiments multi-chromatic waves are used. The drive signals of the wave-makers are generated using first-order and second-order wave generation techniques. Total 14 wave probes are used to capture the data in the wave tank. A NRC-IOT code is used to isolate the primary waves, the bounded waves and the unwanted free waves from the measured data at each wave probe. The measured data are analyzed in this paper to illustrate the differences in the waves generated by two different generation techniques.Copyright

Hydrodynamic analysis and optimization of a hinged type wave energy converter

SeaWEED(Sea Wave Energy Extraction Device) is a multi-body floating wave energy converter (WEC) with hinged joints developed by Grey Island Energy Inc.(GIE) in Canada. Initial conceptual studies have been carried out to evaluate the performance of the first generation device by testing an 1:16 scale model in a wave basin. The experimental results were compared with the numerical solutions. Based on the experimental studies, improvements were made and a second generation model with a new geometry of the hull and a new connection structure was developed. This thesis is mainly focused on the numerical analysis and optimization of the second generation SeaWEED model. In the numerical studies, the hydraulic power take-off (PTO) system was simulated by a linear spring damper system coupled with the motion of the hinged bodies. The vertical hinge motion was computed at a series of wave periods using WAMIT. Optimization was focused on the PTO damping and the geometrical parameters in terms of the draft and the length of the truss structure between hinged bodies by using the response surface method. The optimization was conducted in regular waves and in irregular sea states. An optimal combination of length, draft and PTO damping was recommended for an intended operation location.

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

Response Surface Models for Analyzing Sinkage and Trim Effects on Planing Hull Motions in a Vertical Plane

Further to the studies presented by Sayeed et al. (OMAE2014-23489), response surface models have been improved by including the effects of sinkage and trim to the vertical plane motions of planing hull in head sea. This was achieved by including longitudinal center of gravity (LCG) as an additional factor to the existing model and change in LCG eventually incorporated the effects of sinkage and trim. The validated non-linear mathematical model, Planing Hull Motion Program (PHMP) can predict the heave and pitch motions and bow and center of gravity accelerations with reasonable accuracy at planing and semi-planing speeds. This paper again illustrates an application of modern statistical design of experiment (DOE) methodology to develop response surface models to assess planing hull motions in a vertical plane in head seas. Responses were obtained from PHMP based on a multifactor uniform design scheme. Results showed that the simple one line regression models provided adequate fit to the generated responses and provided valuable insights into the behaviour of planing hull motions in a vertical plane. The new response surface models includes all possible influential factors that affect the vertical plane motions of planing hull in head sea. The simple surrogate models can be a quick and useful tool for the designers during the preliminary design stages.Copyright

Effect of Line Integral on the Computation of Forward-Speed Ship Motions

Computations have been performed to predict motions ships at forward speeds. The radiation and diffraction problems of a ship with forward speed are solved with the panel-free method in the frequency domain. In this paper, the effect of the line integral on the solutions are investigated using three ships, including a Series 60 ship, the S175 ship and the DTMB 5512 hull. Computed motions were compared with experimental data and those by other numerical methods.Copyright

RESPONSE SURFACE MODELS FOR ANALYZING PLANING HULL MOTIONS IN A VERTICAL PLANE

A non-linear mathematical model, Planing Hull Motion Program (PHMP) has been developed based on strip theory to predict the heave and pitch motions of planing hull at high speed in head seas. PHMP has been validated against published model test data. For various combinations of design parameters, PHMP can predict the heave and pitch motions and bow and center of gravity accelerations with reasonable accuracy at planing and semi-planing speeds. This paper illustrates an application of modern statistical design of experiment (DOE) methodology to develop simple surrogate models to assess planing hull motions in a vertical plane (surge, heave and pitch) in calm water and in head seas. Responses for running attitude (sinkage and trim) in calm water, and for heave and pitch motions and bow and center of gravity accelerations in head seas were obtained from PHMP based on a multifactor uniform design scheme. Regression surrogate models were developed for both calm water and in head seas for each of the relevant responses. Results showed that the simple one line regression models provided adequate fit to the generated responses and provided valuable insights into the behaviour of planing hull motions in a vertical plane. The simple surrogate models can be a quick and useful tool for the designers during the preliminary design stages.Copyright

Identifications of Spurious Waves in the Wave Tank With Shallow Water

Accurate generation of the primary waves and the reproduction of the group-induced second-order low and high frequency waves have been considered essential for physical model test in the laboratory. In the laboratory when bi-chromatic primary waves are generated the required bounded waves will be generated naturally at the difference frequency. In addition to that several unwanted free waves are also generated. The free waves, having the same frequency of the bounded wave are reproduced due to mismatch of the boundary conditions at the wave paddle. The other two types of free waves are due to the wave paddle displacement and the local disturbances. We carried out an extensive experimental program to identify the second order spurious waves in shallow water in the Offshore Engineering Basin (OEB) at the Institute for Ocean Technology (IOT) of National Research Council (NRC) Canada. In the experiments water depths are used in the range of 0.3m to 0.8m. The wave periods also have varied from 0.9s to 2.22s. In the experiments mono- and bi-chromatic waves are used. The drive signals of the wave-maker are generated using first-order and second-order wave generation techniques. Total 14 wave probes are used to capture the data in the wave tank. A NRC-IOT code is used to isolate the primary waves, the bounded waves and the unwanted free waves from the measured data at each wave probe. The measured data are analyzed in this paper to illustrate the differences in the waves generated by two different generation techniques.Copyright

Numerical Modeling and Evaluation of Wave Energy Converters

Wave energy converters use the motion of floating or submerged bodies to extract energy from the waves. Power absorption can be simulated using a simple linear damper with a resistance to motion which is proportional to velocity. Because of the interaction between energy production and motion, there will be an optimum rate of energy production for each wave frequency. Too much damping or too little damping can cause little energy produced. The wave absorption range also depends on the tuned frequency. In this paper, the maximum rates of energy absorption for submerged and floating wave energy converters are evaluated by employing the panel-free method for the motions of the converters in the frequency domain. A general expression for the wave power absorption is described. Numerical studies show that the optimal energy efficiencies of wave energy converters can be well predicted by employing the panel-free method for motion computations.Copyright