Jt Duffy

On the avoidance of parametric roll in head seas

Parametric rolling in head seas can cause serious safety issues for ships with the possibility of cargo shift, damage and difficult working conditions for crew. This work investigates the prediction of its occurrence and the factors that influence its onset and magnitude. It also seeks to establish the effect of bilge keels on containerships. The threshold boundaries for the inception of parametric roll, for a typical containership operating in Australian ports, are established using the Mathieu equation. Good agreement was found between the numerical predictions and results from a set of towing tank experiments. Fitting of bilge keels to the containership model was found to be an effective way of reducing the roll angles induced by parametric roll, and in some cases their presence can prevent its onset. A simplified method for use by masters to avoid the onset of parametric roll is also presented.

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.

Wave Wake: Focus on Vessel Operations within Sheltered Waterways

This article describes the development of an empirical tool that can rapidly and accurately predict the characteristics of the wave wake generated by vessels that typically operate within sheltered waterways, including small commercial craft and recreational vessels. A wave wake regulatory criterion is also proposed and incorporated within the prediction tool.

Influence of restricted water on the time domain interaction forces and moment on a berthed ship due to a passing ship

Abstract An investigation was conducted into the effect that different berthed ship bow and stern blockage arrangements has on the interaction forces and moment experienced by a berthed ship due to a passing ship. Physical scale model experiments were used to quantify the magnitude and form of the interaction forces and moments for five different blockage arrangements. The interaction force and moment traces from the experiments were compared to traces predicted using existing empirical formulae based on open water scenarios. Additionally, two methods were used to adjust idealised curves using the experimental results to better represent the form and magnitude of the interaction forces and moments. To demonstrate the effect that the changes in form and magnitude of the interaction forces and moments has on the predicted berthed ship motions the interaction forces and moment from the physical scale model experiments, empirical predictions and the adjusted idealised curves were extrapolated to represent a full scale ship and used as input to a commercially available time domain numerical simulation code to predict the berthed ship motions. The bow and stern blockage was shown to have a significant effect on both the form and magnitude of the interaction forces and moments, and hence, the predicted berthed ship motions. The empirically predicted interaction forces and moments correlated poorly with the experimental results.

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.

Tracking the vortex core from a surface-piercing flat plate by particle image velocimetry and numerical simulation

The wake flow around the tip of a surface piercing flat plate at an angle of incidence was studied using two-dimensional particle image velocimetry as part of benchmarking the particle image velocimetry technique on the moving carriage in the Australian Maritime College towing tank. Particle image velocimetry results were found to be in close agreement with those of the benchmarking work presented by the Hydro Testing Alliance, and a method of tracking the tip-vortex core near a free surface throughout numerical simulation has been demonstrated. Issues affecting signal to noise ratio, such as specula reflections from the free surface and model geometry were overcome through the use of fluorescing particles and a high-pass optical filter. Numerical simulations using the ANSYS CFX Solver with the volume of fluid method were validated against the experimental results, and a methodology was developed for tracking the location of the wandering vortex core experimentally and through simulation. The ability of the scale-adaptive simulation shear stress transport turbulence model and the shear stress transport model to simulate three-dimensional flow with high streamline curvature was compared. The scale-adaptive simulation shear stress transport turbulence model was found to provide a computationally less resource-intensive method of simulating a complex flow topology with large eddies, providing an insight into a possible cause of tip-vortex aperiodic wandering motion. At high angles of attack, vortex shedding from the leading edge separation of the test geometry is identified as a possible cause of the wandering phenomena. In this study, the vortex centre and point of extreme core velocity were found not to be co-located. The point of extreme stream wise velocity within the vortex core was found to be located within half the vortex radius of the vortex centre.

An experimental study of ship motions during replenishment at sea operations between a supply vessel and a landing helicopter dock

Hydrodynamic interactions during Replenishment at Sea (RAS) operations can lead to large ship motions and make it difficult for vessels to maintain station during the operation. A research program has been established which aims to validate numerical seakeeping tools to enable the development of enhanced operator guidance for RAS. This paper presents analysis of the first phase of scale model experiments and focuses on the influence that both the lateral and longitudinal separations between two vessels have on the interactions during RAS. The experiments are conducted in regular head seas on a Landing Helicopter Dock (LHD) and a Supply Vessel (SV) in intermediate water depth. The SV is shorter than the LHD by approximately 17%, but due to its larger block coefficient, it displaces almost 16% more than the LHD. Generally, the motions of the SV were larger than the LHD. It was found that hydrodynamic interactions can lead to large SV roll motions in head seas. Directions for future work are provided.

A Risk Assessment of a Novel Bulk Cargo Ship-to-Ship Transfer Operation Using the Functional Resonance Analysis Method

Risk assessments underpin a maritime operation’s safety management system. When applied to an untested concept a risk assessment can also assist with overcoming resistance to new technology. This paper proposes the functional resonance analysis method (FRAM) as a tool for developing design recommendations and fulfilling the safety management objectives of the ISM Code. The FRAM is applied to benefit the floating harbour transhipper (FHT), a novel concept for the transhipment of bulk commodities. The FHT acts as a large floating warehouse with an aft well dock that provides shelter for a feeder vessel. The FHT’s materials handling equipment transfers bulk cargo from the feeder vessel onto its own stockpile or directly to an export vessel moored alongside, or from its stockpile to the export vessel. Most risk assessment tools focus on identifying and addressing system components that can potentially fail. With the FRAM however, the scope, direction and recommendations are guided by a practical understanding of the variability of work undertaken rather than preconceived notions of potential failure modes. Adopting a method based on maximising resilience rather than minimising the causes of accidents promotes a shift from a blame culture to a safety culture. Applying the FRAM generated a deeper, broader and more transparent understanding of the FHT transfer operation than what would have been achievable using traditional risk assessment tools. This understanding was used to develop recommendations designed to improve the resilience of the FHT operation.

URANS prediction of berthed ship - passing ship interactions

In this paper, the unsteady Reynolds-Averaged Navier-Stokes computational method has been employed for investigating the hydrodynamic interactions between berthed and passing ships. Initially, simulations in model-scale were performed for validating the numerical modelling technique using available benchmark experimental test cases. A formal study of verification and validation was carried out for quantifying the numerical uncertainties. Based on the validated numerical setup, systematic computations were conducted for further investigations on the influence of varying passing ship speeds and lateral separations on the interaction forces and moments. The same conditions were repeated in full scale to quantify possible scale effects. The numerical results demonstrate that the interaction forces and moments are proportional to the square of the passing ship speed and inversely proportional to the lateral separation between the two ships, which agrees well with the findings by Remery (1974) and Kriebel (1995) respectively. In addition, when comparing model and full scale results, the overall differences are not very significant and are within the simulation uncertainty for most cases