Shuhong Chai

Transportation Risk Analysis Framework for Arctic Waters

Arctic waters have historically been relatively inaccessible for marine transport. Lately, climate change has made more of this region ice-free in the summer season. This has reduced the difficulty of marine transport in Arctic waters. Further, exploration and development of natural resources is increasing in Arctic regions, as is destinational shipping. The unique risk factors of this region, such as extremely low temperature, ice conditions and drifting icebergs, continue to pose threats to transportation. Potential impacts associated with marine transportation accidents warrant contingency plans that recognize that preventative measures may fail. To plan effectively, a transportation accident risk assessment model for Arctic waters is helpful. There is limited work on the development of such models. A new cause-consequences based risk assessment model is proposed here. The model estimates the probability of a transportation accident and also the related consequences during navigation in Arctic waters. To illustrate the application of the methodology, it is applied to a case of an oil-tanker collision on the Northern Sea Route

Probability of Occurrence of Extreme Waves in Three Dimensional Mechanically Generated Random Wave Fields: A Comparison With Numerical Simulations

Experimental and numerical investigations reveal that nonlinear modulational instability can significantly affect the probability of occurrence of extreme waves, especially if waves are sufficiently steep and narrow banded both in the frequency and directional domain. However, it is not yet completely clear whether numerical simulations can provide an accurate quantitative estimate of experimental results. Here the potential Euler equations are used to assess the ability of numerical models to describe the evolution of statistical properties of mechanically generated directional, random wave fields and in particular the evolution of the kurtosis. Results show that simulations provide a good quantitative estimate of experimental observations within a broad range of wave directional width. Copyright

URANS Prediction of Ship Hydrodynamics in Head Sea Waves at Zero Forward Speed With Model Testing Validation

The paper presents computations on predicting the hydrodynamics of a generic floating liquefied natural gas (FLNG) hull form in regular head sea waves using unsteady Reynolds-Averaged Navier-Stokes (URANS) solver StarCCM+. Initially, model scale simulations were conducted at model test basin water depth (d=0.8m), with detailed verification and validation study performed to estimate numerical uncertainties. The simulation results were compared with potential flow solutions and validated against experimental studies. Using the verified numerical setup, ship hydrodynamics including wave induced loads, moments as well as ship motion responses in deep water waves(d=8.0m) have been studied. The computed time history results were decomposed by Fourier series to obtain force/moment and motion transfer functions on the frequency domain. From the obtained results, the presented URANS approach demonstrates slightly better accuracy compared with potential flow (PF) solutions. It is also found that water depth has great influences on the computed wave force and ship motion transfer functions for certain range of wave frequencies.

THE EFFECT OF BENDING STIFFNESS ON HYDROELASTIC RESPONSE OF VLFS

Owing to the flexibility of ocean structures with large dimension, the hydroelastic theory is more applicable than traditional method of treating the floating structure as a rigid body. To study the factors that influence the hydroelastic responses, a very large floating structure (namely, VLFS) model is chosen to conduct numerical calculations in regular waves with the aid of three dimensional linear hydroelastic code concerning varied bending stiffness and wave frequency. It is found that bending stiffness and wave frequency have a critical but complex influence on relevant hydroelastic results, including generalized displacement, vertical response amplitude and bending moment. More specifically, the effect of bending stiffness on hydroelastic parameters above can be categorized into different phases, and quite different tendencies are observed in each phase.

Conceptual design of a Submersible Remotely Operated Swimming Dredger (SROSD)

Increasing use of deep-water dredging and mining vehicles has been anticipated for resource collection, engineering construction and environmental protection. Existing deep-dredging or mining equipment can be classified as i) diver-assisted dredging tools, ii) surface-floating dredgers with deep-dredging capability and iii) submersible dredgers. Diver assisted dredging tools have limited capacity and involve human risk. Surface floating dredgers can work to a specific dredging depth controlled by their ladder length, but modification is limited by their large size and significant cost. Submersible dredgers are deployed for sub-sea operations and are the focus of this research. Submersible crawlers and walkers work in a submerged terrain-contact condition and depend on their apparent weight and ground reactions to counteract the excavation forces. Crawlers are inefficient in negotiating difficult sub-sea terrain and walking submersibles are slow moving over long-distances. Considering the constraints of dredging depth, negotiation of uneven terrain, slow motion, interchange ability of excavation or transport sub-system components and station keeping during operation, a new type of submersible dredger or miner was conceived. In working mode, it imitates a walking motion by spuds that are also used for station keeping during dredging. For longdistance travel, the vehicle can swim by means of vector thrusters. The vector thrusters also help in position-keeping and motion-control during swimming. To offset higher forces generated during excavation of hard materials, spuds, variable buoyancy tanks and control planes are included as secondary station-keeping devices. The paper describes the general arrangement and the distinguished sub-systems of the conceptualised vehicle. Special attention was given to working and swimming locomotion and the methods of station keeping during operation. Investigations about the station-keeping, propulsion and controlling conditions of the vehicle are in progress. Experiments to measure the cutting forces from the cutter design are described. It is expected that the new design will significantly contribute to the evolution of existing deep-dredging equipment with improved efficiency, increased mobility and location control while minimising larger environmental disturbances.Copyright

Performance Evaluations of Taut-Wire Mooring Systems for Deepwater Semi-Submersible Platform

In order to investigate the effect of taut-wire mooring system on the motion performance of semi-submersible platforms, parametric studies of coupled motion responses are conducted using a time domain analysis in this study. The nonlinear dynamic characteristics of mooring lines and the interactions of platform and mooring lines are investigated. The parametric studies consist of investigating the effects of the hydrodynamic coefficients CA and CD of mooring line, tension dip angle, mooring line pretension, different taut-mooring arrangements and total number of mooring lines on the motion performance of a semi-submersible platform in water depth of 1500 meters, which is subjected to a 100 year return significant wave height of 13.3 meters, a peak period of 15.5 seconds, a current speed of 1.97 meters per second and wind speed of 55 meters per second. The wind and current both act in the same direction as the ocean waves in this study in order to estimate the maximum mooring line loads. The environmental load direction is varied from 0° to 90 °at the interval of 15 degrees. Seven directions are calculated in total. The research results show that the different parameters, such as the hydrodynamic coefficients of the mooring line, tension dip angle, pre-tension, arrangement angle of mooring lines and total number of mooring lines, have different effects on the coupled motion responses. In particular, the arrangement angles of mooring lines have significant effect on motion responses and dynamic loads of mooring lines. The motion performance of semi-submersible platform and mooring line dynamic loads can be controlled effectively when these parameters are selected reasonably throughout parametric studies carefully designed and conducted. Copyright

Hydrodynamics of a conceptual FLNG system in side-by-side offloading operation

ABSTRACT This paper investigates the hydrodynamics of an floating liquefied natural gas (FLNG)–liquefied natural gas (LNG) offloading system in a side-by-side configuration using potential flow solver AQWA. The system is moored by an internal turret mooring system allowing itself to weathervane under external disturbances and is designed to operate in a sea environment close to that on the coast of Western Australia. Initially, validations are presented for the computed wave loads, representative motion responses and mooring loads. The results are compared against experimental data and those gathered from literatures. Time domain analyses are carried out for the FLNG–LNG system coupled with hawser, fender and mooring systems under the combination of wind, current and waves. The relative motions between the FLNG and LNG in the horizontal plane as well as the mechanical loads on the hawsers, fenders and moorings are computed. Furthermore, the effects of varying hawser pretension and stiffness on the hydrodynamic performance are investigated.

URANS prediction of berthed ship–passing ship interactions

ABSTRACT This paper investigates berthed ship–passing ship hydrodynamic interactions using unsteady Reynolds-averaged Navier–Stokes (URANS) solver StarCCM+ in both model scale and full scale. A double-body approximation method is adopted to investigate the hydrodynamic effects on the berthed ship when in close proximity with the passing ship. A benchmark experimental test condition is replicated for the verification and validation of the numerical simulation. Based on the validated numerical model, the interaction forces and moments are predicted for different passing ship speeds and lateral separations. The same conditions are repeated in full scale to quantify possible scale effects. The trends seen in the numerical results correlated well with past authors findings. The difference in the model-scale and full-scale interaction force and moment predications are not significant. Full scale berthed ship–passing ship interaction forces and moments can therefore be approximated (for similar cases) by model-scale tests with confidence. HIGHLIGHTS (1) URANS computations are conducted to study berthed ship–passing ship interactions in both full-scale and model-scale conditions.(2) Numerical results are validated against benchmark experimental data.(3) Relationship between interaction forces and moments acting on the berthed ship with respect to passing ship speed and lateral separation are identified.(4) Scale effects in berthed ship–passing ship interaction forces and moments are quantified.

AUVSIPRO – A Simulation Program for Performance Prediction of Autonomous Underwater Vehicle with Different Propulsion System Configurations

Autonomous Underwater Vehicle (AUV) is a growing technology with a great potential to both military and civilian applications. Extensive developments and advanced innovations of AUV have been introduced in recent years from various research centres around the world. Among the fundamental modules of an AUV, the propulsion system strongly affects the vehicle performance. The increasing complexity in missions and operational environment require the propulsion system to offer high efficiency and excellent manoeuvrability. In this study, an AUV simulation program named AUVSIPRO is proposed in the preliminary design stage to predict and compare the AUV manoeuvrability equipped with different propulsion configurations. A series of primary manoeuvres standard for underwater vehicles are presented to investigate the system feasibility. In order to derive the mathematical model in the simulator, the propulsor models are experimentally conducted in the Towing Tank, the hull hydrodynamic coefficients are calculated using analytical, and system identification approaches. The system outputs are achieved by numerical method. The simulation program provides an effective platform to examine different the propulsion system configurations to an AUV as well as a torpedo shaped submarine.

Modelling of a surface vessel from free running test using low cost sensors

Identification of hydrodynamic coefficients and accessibility of accurate mathematical model to predict actual responses of vessels has practical significance to design computer-based simulators and apply new control algorithms, thus effective methods and proper devices should be investigated to do the modelling. The aim of this study was to estimate hydrodynamic coefficients of a surface vessel from the free running test using an experimental modelling method. Working as the embedded platform and data acquisition card, myRIO was utilized to control the scaled model, namely ‘P&O Nedlloyd Hoorn’, and measure her motion states using low cost sensors including a Global Positioning System (GPS) receiver, accelerometer, gyroscope and digital compass. System identification was conducted utilizing the processed experimental data with Kalman filter to estimate the hydrodynamic coefficients of a mathematical model in four degree of freedom (DOF) including surge, sway, yaw and roll. The developed mathematical model of the scaled model was validated through the comparison between the experimental data and simulation results. It has demonstrated that the proposed low cost hardware and system identification algorithm is capable of estimating hydrodynamic coefficients of the proposed mathematical model of the scaled surface vessel.