COMPARISON OF UNSTEADY REYNOLDS-AVERAGED NAVIER-STOKES PREDICTION OF SELF-PROPELLED CONTAINER SHIP SQUAT AGAINST EMPIRICAL METHODS AND BENCHMARK DATA

The demand to increase port throughput has driven container ships to travel relatively fast in shallow water whilst avoiding grounding and hence, there is need for more accurate high-speed squat predictions. A study has been undertaken to determine the most suitable method to predict container ship squat when travelling at relatively high speeds (Frh ≥ 0.5) in finite water depth (1.1 ≤ h/T ≤ 1.3). The accuracy of two novel self-propelled URANS CFD squat model are compared with that of readily available empirical squat prediction formulae. Comparison of the CFD and empirical predictions with benchmark data demonstrates that for very low water depth (h/T < 1.14) and when Frh < 0.46; Barass II (1979), ICORELS (1980), and Millward’s (1992) formulae have the best correlation with benchmark data for all cases investigated. However, at relatively high speeds (Frh ≥ 0.5) which is achievable in deeper waters (h/T ≥ 1.14), most of the empirical formulae severely underestimated squat (7-49%) whereas the quasi-static CFD model presented has the best correlation. The changes in wave patterns and effective wake fraction with respect to h/T are also presented.

PHOTOGRAMMETRY MEASUREMENTS OF INITIAL IMPERFECTIONS FOR THE ULTIMATE STRENGTH ASSESSMENT OF PLATES

The objective of the present study is to develop a new approach to model the initial geometrical imperfections of ship plates by using Photogrammetry. Based on images, Photogrammetry is able to take measurements of the distortions of plates and to catch the dominant surface shape, including the deformations of the edges. Having this data, it is possible to generate faithful models of plate surface based on third order polynomial functions. Finally, the maximum load- carrying capacity of the plates is analysed by performing a nonlinear finite element analysis using a commercial finite element code. Three un-stiffened and four stiffened plates have been modelled and analysed. For each plate, two initial imperfection models have been generated one, based on photogrammetric measurements and the other, based on the trigonometric Fourier functions. Both models are subjected to the same uniaxial compressive load and boundary conditions in order to study the ultimate strength.

THE TRANSIENT EFFECTS OF FLOOD WATER ON A WARSHIP IN CALM WATER IMMEDIATELY FOLLOWING DAMAGE

The safety of a ship which is damaged below the waterline will depend on the way water floods into the internal compartments. The water will cause the ship to take on an angle of heel and trim which will further affect the flooding into the compartments. The ship’s equilibrium position in calm water can be predicted using hydrostatic theory, however at present it is difficult to predict the transient behaviour between the initial upright position of the ship and its final equilibrium. In some cases, the transient motion may cause a capsize prior to a possible equilibrium position being reached. This paper describes an investigation of this phenomenon using a model of a warship with simplified, typical internal geometry. With the model initially stationary, a rapid damage event was generated, and the global motions measured, along with the water levels in some of the internal compartments, as functions of time. Immediately after the damage occurred the model rolled to starboard (towards the damage). It then rolled to port (away from the damage) before eventually returning to starboard and settling at its equilibrium value. In all the tests conducted the equilibrium heel angle was less than that reached during the initial roll to starboard. This implies that the roll damping, and the way in which the water floods into the model immediately following the damage, could both have a very important influence on the likelihood of survival.

A REAL TIME SPEED MODULATION SYSTEM TO IMPROVE OPERATIONAL ABILITY OF AUTONOMOUS PLANING CRAFT IN A SEAWAY

This study focuses on developing a control system to enhance the seaworthiness of Autonomous high-speed Planing Crafts (APCs). APCs operating at high-speed in a seaway encounter very high vertical accelerations which pose a hazard to payload and crafts' structural integrity. Therefore, for safety operation of APCs in a seaway it is proposed to employ a system termed vision-aided speed modulation system (VSMS). The proposed VSMS employs an embedded analytical tool termed Motion Assessment of Planing Craft in a Seaway (MAPCS) for the prediction of vertical accelerations and angular velocities, the APC might encounter in the incoming waves. As a response to the MAPCS predicted values the VSMS speed setting module modulates the craft's forward speed. All modules of the VSMS are presented together with their validation and system's preliminary operational results. It is concluded that VSMS might be an essential tool to considerably enhance the operational ability of APCs.

REINFORCEMENT OF AGEING SHIP STRUCTURES

The objective of the present study is to investigate the possibility to recover the ultimate strength of a rectangular steel plate with a manhole shape opening subjected to a uniaxial compressive load and non-uniform corrosion degradation reinforced by additional stiffeners. Finite element analyses have been carried out to verify the possible design solutions. A total of four finite element models are generated, including 63 sub-structured models. The non-uniform corrosion has been generated by the Monte Carlo simulation. The reinforcement process covers three scenarios that include mounting of two longitudinal stiffeners, two longitudinal and two transverse stiffeners and the flange on the opening. The positioning of the stiffeners has also been studied. A total of 10 cases has been selected and tested for the numerical experiment. Three different assessments have been performed to evaluate the ultimate strength, weight and cost. Two additional studies on the effect of the plate thickness and slenderness have been also carried out.

OPERATIONAL ANALYSIS IN SYSTEM DESIGN OF SUBMARINES DURING THE EARLY PHASES

This paper presents a new method for operational analysis (OA) as a tool in simulation based design (SBD) for Naval Integrated Complex Systems (NICS), here applied to the submarine domain. An operational analysis model is developed and described. The first step of the design process is to identify and collect the needs from the customer and stakeholders, from which requirements can be deduced and designed in an organized way, i.e. requirement elucidation. It is important to evaluate the benefits or penalties of each requirement on the design as early as possible during initial design. Thus the OA-model must be able to evaluate requirements aggregated in synthesised ships such as initial concepts, i.e. Play-Cards, as representations of a submarine concept in the functions domain where the first set of requirements are designed, and establish their Measure of Capability (MoC) and Measure of Effectiveness (MoE). The work has resulted in an OA-model for submarine design that can be used during the development and for evaluation during the life cycle of a submarine system. The purpose of integrating OA in the design process is to explore the design space and evaluate not only technical solutions and cost but also the system effect in the early phases and thereby find and describe a suitable design room. This will generate a more rapid knowledge growth compared to the classic basic ship design procedures which focus on technical performance and cost. It is expected that we not only reach a higher level of knowledge about the design object but also achieve higher precision in the compliance to needs and deduced and designed requirements by the use of an OA-model as an integrated tool during initial design. This approach also invites customer participation within the framework of integrated project teams.

RANS-BASED CFD PREDICTION OF THE HYDRODYNAMIC COEFFICIENTS OF DARPA SUBOFF GEOMETRY IN STRAIGHT-LINE AND ROTATING ARM MANOEUVRES

Computational Fluid Dynamics (CFD) simulations using Reynolds Averaged Navier-Stokes (RANS) equations are increasingly adopted as an analysis tool to predict the hydrodynamic coefficients of underwater vehicles. These simulations have shown to offer both a high degree of accuracy comparable to experimental methods and a greatly reduced computational cost compared to Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS). However, one of the major challenges faced with CFD simulations is that the results can vary greatly depending on the numerical model settings. This paper uses the DARPA SUBOFF hull form undergoing straight-line and rotating arm manoeuvres at different drift angles to analyse the hydrodynamic forces and moments on the vehicle against experimental data, showing that the selection of the boundary conditions and turbulence models, and the quality of the mesh model can have a considerable and independent effect on the computational results. Comparison between the Baseline Reynolds Stress Model (BSLRSM) and Shear Stress Transport with Curvature Correction (SSTCC) were carried out for both manoeuvres, showing that with a sufficiently fine mesh, appropriate mesh treatment, and simulation conditions matching the experiments; the BSLRSM predictions offer good agreement with experimental measurements, while the SSTCC predictions are agreeable with the longitudinal force but fall outside the experimental uncertainty for both the lateral force and yawing moment.

PRESSURE DISTRIBUTION DUE TO STERN TAB DEFLECTION AT MODEL SCALE

Trim tabs form an important part of motion control systems on high-speed watercraft. By altering the pitch angle, significant improvements in propulsion efficiency can be achieved by reducing overall resistance. For a ship in heavy seas, trim tabs can also be used to reduce structural loads by changing the vessel orientation in response to encountered waves. In this study, trials have been conducted in the University of Tasmania hydraulics laboratory using a closed- circuit water tunnel to measure model scale trim tab forces. The model scale system replicates the stern tabs on the full- scale INCAT Tasmania 112 m high-speed wave-piercer catamaran. The model was designed for total lift force measurement and pressure tappings allowed for pressures to be measured at fixed locations on the underside of the hull and tab. This investigation examines the pressures at various flow velocities and tab deflection angles for the case of horizontal vessel trim. A simplified two-dimensional CFD model of the hull and tab has also been analysed using ANSYS CFX software. The results of model tests and CFD indicate that the maximum pressure occurs in the vicinity of the tab hinge and that the pressure distribution is long-tailed in the direction forward of the hinge. This accounts for the location of the resultant lift force, which is found to act forward of the tab hinge.

SLAM CHARACTERISTICS OF A HIGH-SPEED WAVE PIERCING CATAMARAN IN IRREGULAR WAVES

Slam characteristics of a 112m INCAT wave piercing catamaran in a range of realistic irregular sea conditions are presented in this paper. Towing tank testing of a 2.5 m hydroelastic segmented catamaran model was used to gather a database of slam events in irregular seas. The model was instrumented to measure motions, centrebow surface pressures and forces, encountered wave elevations and wave elevations within the bow area tunnel arches. From these measurements characteristics of the vessel slamming behaviour are examined: in particular relative vertical velocity, centrebow immersion, archway wave elevations and slam load distributions. A total of 2,098 slam events were identified over 22 different conditions, each containing about 80 to 100 slam events. The data, although inherently scattered, shows that encounter wave frequency and significant wave height are important parameters with regard to centrebow slamming. Relative vertical velocity was found to be a poor indicator of slam magnitude and slams were found to occur before the centrebow arch tunnel was completely filled, supporting the application of a two-dimensional filling height parameter as a slam indicator.

OPERATIONAL BEHAVIOUR OF AN OFFSHORE MULTIPURPOSE SUPPORT VESSEL IN THE EASTERN MEDITERRANEAN SEA

The objective of this work is to analyse the operational behaviour of an offshore multipurpose support vessel designed to operate in the Eastern Mediterranean Sea. First, the seakeeping analysis is performed in a regular wave condition for different heading angles estimating heave and pitch motions through the strip theory. After that, the effects of the vertical acceleration on the bow, occurrence of slamming or hydrodynamic impact of the hull on the surface of the water; wetted deck, occurrence or invasion of water on the deck of the vessel and propeller emersion, motion sickness and wave-induced additional resistance are analysed. The present analysis is extended in an irregular sea condition, and the estimated seakeeping criteria are compared to the acceptable levels. In defining the most suitable operational mode of the offshore support vessels, multi-criteria decision techniques and probabilistic approach are employed to perform an adequate evaluation of the seakeeping performance accounting for different hazardous events through the service life.