Christopher Chin

Energy audit of fishing vessels

Recent dramatic rises in the price of diesel fuel have resulted in the questionable viability of sectors of the Australian commercial fishing industry. Many fishing vessels currently operating in the fleet are over 20 years old and operate in a way that is inefficient in terms of the energy input (combustible fuels) in contrast with the useful energy output (catch). Combined with the global need to reduce greenhouse gas emissions there is a clear need to introduce new efficiencies into fishing vessel operations. This paper reports on the development of an energy audit method for fishing vessels, based on similar systems for land-based industries. It enables the analysis of individual vessels to be made and allows the assessment of their energy usage and current level of efficiency. A sample energy audit has been conducted for an Australian fishing vessel that leads to recommendations for improved efficiencies to combat present and possible future fuel cost increases.

Sensitivity of vessel responses to environmental contours of extreme sea states

A comparative study of two methods for the generation of the environmental contours is presented investigating the sensitivity of the predicted extreme vessel responses to the type of the contour lines. Two approaches for the generation of environmental contours of the significant wave height and peak period are compared: the Inverse First Order Reliability Method (IFORM) and Constant Probability Density (CPD) approach. Case studies include several global responses of a ship-shaped weather-vaning vessel and a semisubmersible platform. The case studies reveal that the differences between the IFORM and CPD contours are more pronounced in the range of long wave periods. Vessel responses which are less sensitive to long wave periods exhibit less difference (less than 1.0%) in their maximum values between the two types of contours. In contrast, responses which are sensitive to long wave periods show significantly larger differences of up to 7.0%. Uncertainties also exist in the predicted extreme responses where the environmental contour and the response isoline behave tangentially. Differences between the extreme responses produced by the two contours generally decrease with an increase in return period; however exceptions exist due to the tangential behaviour. It is advised that these sensitivities should be taken into consideration when the environmental contours are used in the design.

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.

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.

Variability of extreme responses due to wave frequency motions of a weather-vaning FPSO

The variability of extreme responses of a flexible riser due to wave frequency motions of weather-vaning FPSO is investigated numerically. The objective of this study is to examine such variability in isolation from that caused by the low frequency (slow drift) vessel motions and vessel offsets.Investigation of the extreme value distributions of flexible risers provides the statistical foundation for flexible riser Response Based Analysis (RBA) for use in system design; the determination of the statistical properties of extreme flexible riser responses provides a method for the prediction of extreme responses of offshore systems in cyclonic conditions.A case study conducted in OrcaFlex included an FPSO vessel with a Lazy-S configured riser system. Five riser responses were selected in critical locations including tension, heave, and curvature responses. The environmental cases included two cyclonic storms consisting of multiple half-hour intervals. For each interval, time domain simulations included 40 wave realizations in order to provide a dataset for robust fitting of the extreme value distributions in the Gumbel format.Once the short term interval distributions were established, response distributions in a storm were generated by multiplying the short term distributions and the most probable maximum (MPM) response in a storm computed. A comparison of maximum interval, storm and 3-hour MPMs is presented, which indicates to what extent the MPM response in a storm exceeds the corresponding maximum interval response. Differences between the tension and heave responses are compared with those observed in the curvature responses.This study was limited to riser excitation by waves, current and wave frequency motions of a turret moored FPSO and it is expected that further inclusion of low frequency motions would contribute to the response variability. The inclusion of such variability will ultimately enable the storm-based statistical approach to be used for the development of long-term distribution of the riser responses.Copyright

Computational modelling of sloshing in liquefied natural gas tank

Sloshing in the tank of liquefied natural gas (LNG) carriers has recently attracted immense attention due to the rise in demand for LNG transportation. It occurs in partially filled tanks and is capable of inflicting severe damage to the tank’s interior. One effective method to dampen sloshing activities is by introducing baffles into the tank. In this paper, the nature of sloshing has been investigated using finite volume based unsteady Reynolds Averaged Navier-Stokes (URANS) method. Good correlation was achieved between the results obtained from the presented computations and past studies, demonstrating the feasibility of the established numerical modelling approach. Employing similar computational method, two-dimensional (2D) sloshing computations were performed or different baffle additions at varying filling levels. Observations were made in the baffled tanks where an increase in the number of baffles would cause the sloshing activities to magnify if the baffle height was significantly lower than the filling level. When comparing the 2D and 3D computational results, close resemblance of the average pressure profile and maximum impulsive loads had suggested that 2D simulations are feasible to model sloshing induced loads in a 3D tank.

Application of Frequency Domain Methods for Response Based Analysis of Flexible Risers

Response Based Analysis (RBA) is an advanced method for the prediction of long term distributions of critical responses in offshore floating systems. For complex non-linear systems such as flexible risers, RBA requires time domain simulations that form the core data to which probabilistic models are applied. Because RBA requires significantly larger amounts of data than traditional short term analysis approaches, running the required number of simulations in the time domain can quickly become unfeasible if the system’s physics being modelled are exceedingly complex. In addition, flexible risers are complex composite structures with highly dynamic, non-linear responses which further limit the feasibility of application of the RBA process to these systems.As an alternative, frequency domain solvers, such as that used in the OrcaFlex software, are potential substitutes for portions of datasets due to their processing times being significantly faster than time domain solvers.A comparison of extreme responses generated by frequency and time domain solvers was performed over the duration of two storms. An upper threshold limit for the frequency domain’s accuracy was found by comparing the differences of the two solver’s responses as the storm progressed; where the differences became too large the threshold limit was set. For environmental conditions smaller than this threshold, the frequency domain solver may provide a quicker method for predicting the riser responses. Conditions that exceed this threshold require full time domain analysis for accurate responses to be generated.Limitations of the frequency domain solvers include their reduced ability to deal with non-linear mechanics such as bending/curvature responses. As a result, curvature component results from the frequency domain are limited in their direct usability, especially when exposed to more extreme metocean conditions and locations along the riser that are subject to larger curvature (generally where risers are connected to structures with greater stiffness).Although these limitations exist, the frequency domain solver may still provide reasonable insight into metocean conditions that potentially cause extreme responses.A method is proposed for the use of both frequency and time domain simulations in the flexible riser flowline RBA process. Screening, filtering and ‘stitching’ methods utilizing the speed of the frequency domain solver are presented in order to compensate for the time domain’s extensive computation times. The proposed method of stitching, when applied to an example storm history, required 39% of the processing time when using only the time domain solver.Copyright

Articulated Concrete Mattress block size stability comparison in omni-directional current

Articulated Concrete Mattresses (ACMs) are scour protection method widely used for subsea pipeline protection and stability. In constructing ACMs more economical a stability analysis must be conducted for the required minimum weight estimate with safety factors. To date there are not theory accurate hydrodynamic coefficients for uses in determining stability analyses for full mattress. This research paper investigates the stability of ACMs through several experimental investigations into their hydrodynamic coefficients at all horizontal flow angles. The ACMs used for this investigation comprised of three block types the 300 series, the 500 series and the 400 series which has the bottom half of the 300 series but the top half of the 500 series. The main finding of the block comparison was that the 400 series in comparison to the 500 series has either 0.02m/s lower or 0.2 m/s failure flow rate higher depending on the failure criteria. This was also observed with the less favourable failure criteria for the 400 series. This implies no justifiably economic advantage for deployment of the 500 series over the 400 series to increased stability in high current flow.

Experimental study of wave induced loads and motions on FLNG in head and oblique sea waves

In the past decade, an innovative concept, the floating liquefied natural gas (FLNG) system has been developed as a more effective solution over conventional pipelines for exploiting offshore natural gas resources. Understanding the hydrodynamic behaviour of such a mega structure in a real seaway is essential for determining its performance as well as evaluating the operabilities of on-board facilities and safe offloading. In this paper, experimental study on the hydrodynamic performance of a generic FLNG hull form has been presented. The 1:100 scale model was tested in the Australian Maritime College model test basin for head sea and oblique sea conditions at zero forward speed. The wave induced loads and motions were measured by load cells and linear variable differential transducers (LVDTs) respectively. Experimental uncertainties on each of the measured variables were studied by taking partial differentiations on the uncertainty sources. The time history measurements were decomposed by Fourier series for obtaining frequency domain force/moment and motion transfer functions. The results were compared with numerical solutions from potential flow and Reynolds-Averaged NavierStokes (RANS) solvers. A good correlation between the experimental and numerical results has been demonstrated

The use of Echo360 generated materials and perceived student performance

Recent research on the impact of educational technology such as lecture capture has paid much attention to the possible effects of using technology in improving student performance. Using a combination of an online questionnaire and focus group interviews involving three different disciplinary areas, this study showed that a majority of respondents agreed that Echo360 generated materials had positive impact on their learning outcomes and improvement of their performance. The results revealed differences among the two age groups in relation to the perceived extent of the effect of using Echo360 generated materials on their learning performance.