Towards the Understanding of the Steady Tilt Phenomenon in Semi-Submersibles

Dissatisfaction with the existing rules governing the intact stability of semi-submersibles has created one of the major research areas in recent years (post 1970). At that time several stability tests on models had shown that capsizing of a semi-submersible with minimum stability index in maximum environmental conditions had a very low probability due to its inherently good motion characteristics. This finding encouraged operators and designers to put pressure on the regulatory authorities and classification societies to relax the design rules by reducing the metacentric height (GM). This would provide more deck load and possibly improved motion characteristics. However, during several of these stability tests it was noted that, especially with low values of GM, the models developed a "steady tilt" in regular waves which could be as high as 10 - 15 and that it then rolled about this tilt angle. This tilt was worst in short and steep regular waves but could be observed in a confused seaway, although it was then periodic in that it occurred most commonly at certain wave frequencies in the spectrum. This phenomena was called "slowly-varying tilt". This behaviour was potentially dangerous since it could affect the motions non-linearly leading to large angles of inclinations and the deck edge becoming immersed; two conditions which could lead to dangerous stability problems, increased mooring tensions, structural damage due to slamming and operational difficulties with risers, helicopters, etc. The majority of the research studies were originally designed to explore various aspects of the dynamic behaviour of semi-submers-ibles and the tilt effect was merely observed incidental to these tests. Thus the data recorded were of limited scope and in some cases of a conflicting nature. No documental cases of tilt on semi-submersibles in service had been recorded. As a result the various theoretical approaches to the problem lacked good experimental verification and no clear guidance regarding the reasons for tilt had been developed. This thesis attempts to extend both the experimental and theoretical knowledge of this poorly understood and potentially dangerous phenomenon. The first chapter of the thesis is of an introductory nature where the existing rules which govern the intact stability of semi-submersibles are reviewed and attention is drawn to the need to explore some dynamic aspects of the stability of the semi-submersibles with emphasis on the tilt behaviour. The second chapter presents an historical review of past developments in the study of tilt behaviour. The results obtained from each tilt study are discussed with reference to the theory and experimental details which are provided in the appendix of this chapter. The conclusions drawn from this chapter determine that the primary requirement for the understanding of the phenomenon is some accurate experimental work devoted entirely to the tilt problem so that some of the conflicting reports in the early studies could be clarified and form a basis for a sound theoretical approach. The third chapter of the thesis presents the experimental work carried out with a twin-circular 4-columns per hull semi-submers-ible model. In order to provide a reliable database for the present and future studies systematic tilt measurements were obtained over a wide range of regular beam seas and varying GMs. Systematic force tests on the model hulls at various hull spacings and a number of other exploratory tests were carried out in order to clear up several hydrodynamic aspects some of which have been reported in previous studies. In the fourth chapter, a theoretical analysis is presented with reference to the previous theoretical approaches and the test results carried out in this thesis. It is concentrated on the wave-induced loads in terms of the oscillatory (first-order) and the steady (second-order) components which are believed to be mainly responsible for the tilt behaviour. The main emphasis is placed on a proper determination of the wave-induced tilting mechanism which causes the steady tilt response and the determination of a minimum GM needed to limit tilt to some specified angle. The theoretical methods for the prediction of oscillatory forces and resulting motions and the main components of the steady tilting moments are presented. (Abstract shortened by ProQuest.)

Estimating the impact of new-generation antifoulings on ship performance: the presence of slime

Due to the phase-out of TBT-SPCs imposed by the International Maritime Organization, new-generation antifoulings are set to replace 80% of the existing antifouling market. Two types of coatings are claimed to offer satisfactory performance over five years: tin-free SPCs and foul-release coatings, which were both commercially introduced in the mid 1990s. This paper gives an overview of the research at the University of Newcastle upon Tyne which compares the drag, boundary-layer and roughness characteristics of both coatings when newly applied. It was found that foul-release coatings offer less drag than tin-free SPC, by an amount which depends on the quality of application and which has been related to the respective differences in roughness characteristics. Assessments have shown that foul-release surfaces are very effective against macrofouling organisms, but that the surface is covered by slime films when the vessel returns to dry-dock. A literature review on the effect of slime films on ship resistance shows that slime films have a significant effect on drag, but in turbulent flows the effect is likely to remain limited because of detachment processes. Further research is underway to investigate this

Advantages of twin rudder system with asymmetric wing section aside a propeller : the new hull form with twin rudders utilizing duct effects

This study presents a new twin rudder system with asymmetric wing section, aside a propeller, as a new category energy saving device (ESD) for ships. The energy saving principle of the new ESD, which is called "Gate rudder", is described and its applicability on a large bulk carrier is explored using experimental and numerical methods. The study makes emphasis on the cost-effectiveness of the proposed ESD and presents a potential energy saving up to 7–8 % with the new device as well as an attractive return investment in 0.37–0.9 year. These estimations are based on the conventional powering methods, whereas the accuracy of the ESD design method is confirmed by model test measurements

An investigation into computational modelling of cavitation in a propeller's slipstream

This paper reports on the ongoing developments of cavitation modelling so far which include preliminary validation studies for simulating the performances of two benchmark model propellers: i.e. PPTC propeller with inclined shaft; and E779A propeller, in non-cavitating and cavitating conditions. The main purpose of this study is to estimate the propeller’s performance in cavitating conditions particularly developing tip vortex cavitation. The simulations in open water and cavitating conditions were carried out in uniform flow using a commercial CFD package. Firstly, the validation studies were conducted for non-cavitating condition. The comparison with the benchmark experimental data showed good agreement for the thrust and torque coefficients as well as for the open water efficiency. Next, the cavitation developed on the propeller was simulated using a numerical model based on the Rayleigh-Plesset equation. Propulsion coefficients (KT, KQ) and the cavity patterns on the benchmark propellers’ blades showed very good agreement with the experimental data. However, the tip vortices off the blades could only be traced for E779A propeller by using a new mesh refinement approach

An improved tip vortex cavitation model for propeller-rudder interaction

The paper starts with the computational modelling of the tip vortex cavitation in uniform flow conditions with an isolated propeller in detail and provides experimental validation. It then moves onto further modelling to include the effect of non-uniform flow and the presence of a rudder placed in the propeller slipstream. The propeller-rudder arrangement of the Newcastle University research vessel, The Princess Royal, and associated experimental data were used for Experimental Fluid Dynamics (EFD) analysis to validate the modelling. The cavitation simulations were conducted using commercial CFD software, Star CCM+. A new meshing technique, which utilizes a Mesh Adaptive Refinement approach for Cavitation Simulations (MARCS), recently developed by the authors, has been applied successfully to simulate the tip vortex cavitation, particularly to trace its extension up to the rudder in the propeller slipstream. The comparison of the CFD and EFD methods for the isolated propeller in cavitation tunnel conditions showed very good agreement in terms of the thrust and torque coefficients of the propeller as well as the sheet and tip vortex cavitation patterns observed. The cavitation simulations have been extended for the same propeller by using the new mesh refinement approach to include the effect of the hull wake and the presence of the rudder. Although the latter simulations fall short of the EFD results and hence they are still under development, the paper presents the developments and results so far to achieve the ultimate aim of this study, i.e. computational modelling of cavitating tip vortices of a propeller interacting with a rudder

Humpback whale inspired design for tidal turbine blades

This study is to further improve the hydrodynamic performance of tidal turbines by applying leading-edge tubercles to the blades inspired by the humpback whales. The study first focused on the design and optimisation of the leading edge tubercles for a specific tidal turbine blade section by using numerical methods to propose an "optimum" design for the blade section. This optimum design was then applied onto a representative tidal turbine blade. This representative 3D blade demonstrated significant benefits especially aft er stall. The experimental measurements were further validated and complimented by numerical simulations using commercial CFD software for the detailed flow analysis. Following that, a set of tidal turbine models with different leading - edge profiles was manufactured and series of model test campaigns were conducted in the cavitation tunnel to evaluate their efficiency, cavitation, underwater noise, and detailed flow characteristics. Based on these experimental investigations it was confirmed that the leading edge tubercles can improve: the hydrodynamic performance in the low Tip Speed Ratio (TSR) region without lowering the maximum power coefficient; constrain the cavitation development to within the troughs of the tubercles; and hence mitigating the underwater noise levels

Experimental analysis of an air cavity concept applied on a ship hull to improve the hull resistance

At the forefront of ship design is the desire to reduce a ship׳s resistance, thus being the most effective way to reduce operating costs and fulfil the international criteria for reduction in CO2 emissions. Frictional drag is always proportional to the wetted surface of the vessel and typically accounts for more than 60% of the required propulsive power to overcome; hence the desire to reduce the wetted surface area is an active research interest. An initial full-scale sea trail on a vessel by introducing air as a lubricating medium has indicated 5–20% propulsive energy savings (DK-GROUP, 2010). Following the report of the fundamental tests with the air cavity concept applied on a flat plate, which was conducted in the Emerson Cavitation Tunnel of Newcastle University (Slyozkin et al., 2014), this paper explores the same concept only this time applied on an existing container ship model to investigate whether it benefits in frictional drag reduction, whilst producing a net energy saving. The middle section of this 2.2 m ship model was modified to accommodate a 0.43×0.09 m2 air cavity in the bottom of the hull and then various model scale tests have been conducted in the towing tank of Newcastle University. The model experiments produced results ranging from 4% to 16% gross drag reduction. Upon applying scaling factors, it is estimated from the experimental results that around 22% gross energy could be saved in a full-scale application with just a 5% reduction in the wetted surface area. Further complementary model tests were also conducted to explore the effect of the air cavity on the stability of the model and on the vertical motion responses in a regular head and following wave. While the cavity did not affect the vessel stability the motion response behaviour seemed to be affected non-linearly by the effect of the air cavity

An investigation into the effect of biofouling on the ship hydrodynamic characteristics using CFD

To reduce the fuel consumption and green-house gas emissions of ships, it is necessary to understand the ship resistance. In this context, understanding the effect of surface roughness on the frictional resistance is of particular importance since the skin friction, which often takes a large portion in ship drag, increases with surface roughness. Although a large number of studies have been carried out since the age of William Froude, understanding the roughness effect is yet challenging due to its unique feature in scaling. In this study, a Computational Fluid Dynamics (CFD) based unsteady Reynolds Averaged Navier-Stokes (RANS) resistance simulation model was developed to predict the effect of barnacle fouling mainly on the resistance and hull wake characteristics of the full-scale KRISO container ship (KCS) hull. Initially, a roughness function model was employed in the wall-function of the CFD software to represent the surface conditions of barnacle fouling. A validation study was carried out involving the model-scale flat plate simulation, and then the same approach was applied in full-scale flat plate simulation and full-scale 3D KCS hull simulation for predicting the effect of barnacle fouling.The increase in frictional resistance due to the different fouling conditions were predictedand compared with the results obtained using the boundary layer similarity law analysis of Granville. Also, a further investigation of the roughness effect on the residuary resistance, viscous pressure resistance and wave making resistance was carried out. Finally, the roughness effect on the wave profile, pressure distribution along the hull, velocity distribution around the hull and wake flows were examined

Investigation into the propulsive efficiency characteristics of a ship with the Gate Rudder propulsion

Following the first successful application of the Gate Rudder® propulsion system on a 2500GT container ship (Lpp=102m) in Japan, excellent manoeuvring performance was reported with a significant fuel saving over her sister ship fitted with a conventional rudder propeller arrangement. Based upon the investigations carried out by using model tests, CFD simulations and the full-scale data of two container vessels, this paper discusses the details of the propulsive efficiency characteristics of a vessel fitted with the GATE RUDDER® propulsion system in comparison those of the same vessel with the conventional rudder-propeller arrangement. In the paper the evolution history of the GATE RUDDER® concept is presented by tracing the development of the state-of-the-art energy saving devices (ESD) involving ducts since the GATE RUDDER® exploits the advantage of the duct effect. The components of the propulsive efficiency parameters, with an emphasis on the thrust deduction and effective wake parameters, are explored and discussed highlighting the differences for the hull with the GATE RUDDER® and the conventional rudder arrangements