CFD simulations of vertical ship motions in shallow water

The seakeeping behaviour of a vessel in shallow water differs significantly from its behaviour in deep water. In shallow water, a vessel’s motion responses to incident waves will be affected by hydrodynamic effects caused by the presence of a finite depth. Given that a vessel will sail in shallow water at various times during its service life, such as when entering harbours, it is important to have an understanding of the influence of shallow water on ship motions. In this study, using a commercial unsteady Reynolds-Averaged Navier-Stokes solver, a numerical study of ship motions in shallow water was carried out. Firstly, the characteristics of shallow water waves were investigated by conducting a series of simulations. Then, a full-scale large tanker model was used as a case study to predict its heave and pitch responses to head waves at various water depths, covering a range of wave frequencies at zero speed. The motion results obtained were validated against related experimental studies available in the literature, and were also compared to those from 3-D potential theory. The results were found to be in good agreement with the experimental data. Finally, it was shown that vertical motions were significantly affected by shallow water

On the importance of antifouling coatings regarding ship resistance and powering

This paper aims to introduce one of the latest investigations on development of marine antifouling coatings and also to demonstrate the importance of the type of antifouling coatings on fouling accumulation and ship resistance/powering. First, marine biofouling and fouling prevention methods are reviewed. A recent research study (EU FP7 FOUL-X-SPEL Project) concerning a novel and environmentally friendly antifouling coating is presented and discussed. Next, a case study is carried out to assess the effect of fouling on ship resistance and powering. A vessel is selected and the roughness on the hull surface induced by different level of fouling is considered. The increase in frictional resistance and effective power is evaluated for each particular case by using boundary layer similarity law analysis and experimental data. The results emphasise that the type of antifouling coatings has a great importance on the amount of fouling accumulation, hence on ship performance especially in low speed

CFD simulation of vortex-induced vibration of a vertical riser

Three-dimensional fluid-structure interaction (FSI) simulations are conducted on a vertical riser with a length-to-diameter ratio L/D = 481.5. Important vortex-induced vibration (VIV) parameters including the amplitude responses, orbital trajectories, oscillation frequencies and vorticity contours are presented. The computational fluid dynamics (CFD) simulation results are in good agreement with published experimental data. The riser exhibits a dual-resonant response. Two different vortex shedding mode is observed, i.e., 2P and 2S modes. 2P mode is associated with the maximum transverse amplitude and 2S mode is observed elsewhere along the riser

Three-dimensional numerical simulation of two-degree-of-freedom VIV of a circular cylinder with varying natural frequency ratios at Re = 500

The two-degree-of-freedom (2DOF) vortex-induced vibration (VIV) of a circular cylinder with varying in-line to cross-flow natural frequency ratios (f* = fnx/fny) is studied using a three-dimensional (3D) computational fluid dynamics (CFD) approach. Numerical simulation is carried out for a constant mass ratio of 2 at a fixed Reynolds number Re = 500. The reduced velocity ranges from 2 to 12. Three natural frequency ratios are considered, i.e., f* = 1, 1.5 and 2. The structural damping is set to zero to maximise the response of the cylinder. The main objective of this study is to investigate the effect of f* on the 2DOF VIV responses and the 3D characteristics of the flow. It is discovered that there is a significant increase in the vibration amplitude, and the peak amplitude shifts to a higher reduced velocity when f* increases from 1 to 2. A single-peak cross-flow response is observed for the identical in-line and cross-flow mass ratios when f* = 2. Dual resonance is found to exist over the range of f* studied. The preferable trajectories of the cylinder in the lock-in range are counterclockwise figure-eight orbits. Oblique figure-eight trajectories appear at Vr = 6, 7 and 8 when f* = 1. The third harmonic component which is observed in the lift fluctuation increases with f*. The correlation decreases in the lock-in range and reaches its minimum value around the transition region between the lock-in and post-lock-in ranges. Three vortex shedding modes (2S, P + S and 2P) appear in the present simulation. A dominant P + S mode is associated with the oblique figure-eight trajectories. Variation of vortex shedding flows along the cylinder is observed leading to the poor correlation of the sectional lift forces

Seafarers’ current awareness, knowledge, motivation and ideas towards Low Carbon – Energy Efficient operations

Worldwide there is increasing concern regarding green house gases, in particular carbon emissions and their detrimental effects to our earth’s atmosphere; resulting in climate change. International and National pressure requires the shipping industry to play its’ role in reducing the 3.3% of total global carbon emission that it currently emits into the atmosphere. On the 1st January 2013 the IMO are expected to enforce mandatory measures to reduce shipping carbon emissions and these measures will directly and indirectly affect the daily operations of seafarers, onshore performance staff, and managerial personnel with influence over operational procedures. It is therefore imperative that these personnel have the awareness, knowledge, skills, and motivation necessary to successfully implement the operational changes that are needed. A questionnaire has been distributed to investigate seafarers’ and onshore personnels current levels of awareness, knowledge and motivation towards carbon emissions in general and towards shipping carbon emissions. The questionnaire also asked participants to contribute which level of personnel have the most influence over carbon changes and what are the most important operational improvements that can be made. 317 questionnaire responses were collected in total and the analysis of the results is discussed within this paper. The primary benefit of this study has been to support the development of a specific Low Carbon – Energy Efficiency maritime education and training program, by identifying target group needs and attitudes, and key areas for focus

Predicting the effect of biofouling on ship resistance using CFD

This paper proposes a Computational Fluid Dynamics (CFD) based unsteady RANS model which enables the prediction of the effect of marine coatings and biofouling on ship resistance and presents CFD simulations of the roughness effects on the resistance and effective power of the full-scale 3D KRISO Container Ship (KCS) hull. Initially, a roughness function model representing a typical coating and different fouling conditions was developed by using the roughness functions given in the literature. This model then was employed in the wall-function of the CFD software and the effects of a typical as applied coating and different fouling conditions on the frictional resistance of flat plates representing the KCS were predicted for a design speed of 24 knots and a slow steaming speed of 19 knots using the proposed CFD model. The roughness effects of such conditions on the resistance components and effective power of the full-scale 3D KCS model were then predicted at the same speeds. The resulting frictional resistance values of the present study were then compared with each other and with results obtained using the similarity law analysis. The increase in the effective power of the full-scale KCS hull was predicted to be 18.1% for a deteriorated coating or light slime whereas that due to heavy slime was predicted to be 38% at a ship speed of 24 knots. In addition, it was observed that the wave resistance and wave systems are significantly affected by the hull roughness and hence viscosity

Nondestructive testing of marine protective coatings using terahertz waves with stationary wavelet transform

Terahertz wave propagation in marine protective coatings and its non-destructive testing (NDT) capability were studied by the finite difference time domain (FDTD) method. The FDTD model was used to calculate the propagation and reflection of THz radiation from marine protective coatings. The reflected terahertz waves could be employed in coating thickness analysis of the paint layers. In order to clearly identify the interface between antifouling and anticorrosive coatings, stationary wavelet transform (SWT) approach was applied to decompose the obtained terahertz impulse functions into approximation and detail coefficients; SWT detail coefficients were used for the feature extraction of the coating thickness. SWT provides a more accurate identification of salient features in a signal, such as the weak feature between antifouling and anticorrosive coatings. We found that the developed model and SWT-based algorithms could be used to evaluate the occurrence of defects beneath the coatings (e.g., paint-off and corrosion defects). The proposed method provides the solution for coating thickness of marine protective coatings and it would benefit the effective maintenance to avoid coating failure and facilitate marine protective coating design. Therefore, non-destructive testing and evaluation of marine protective coating system by terahertz waves with SWT could be recommended for engineering applications

Investigation on motion responses of a semi-submersible platform and its mooring system

More and more floating structures are used in both offshore and coastal engineering, and also under assessment for wind energy. Mooring systems are needed by floating structures for station-keeping. In this paper, motion responses of a semi-submersible platform in regular waves are investigated numerically by a viscous flow solver naoe-FOAM-SJTU based on OpenFOAM. Influence of the mooring system on the motion responses of platform is evaluated via the study on (a) effect of each element length while maintaining the overall length as a constant; and (b) the cross angles between mooring lines

Numerical studies on added resistance and motions of KVLCC2 in head seas for various ship speeds

In this study, numerical simulations for the prediction of added resistance and ship motions at various ship speeds and wave steepnesses for the KVLCC2 are presented. These are calculated using URANS CFD and 3-D potential methods, both in regular head seas. Numerical analysis is focused on the added resistance and the vertical ship motions for a wide range of wave conditions at stationary, operating and design speeds. Firstly, the characteristics of the CFD and the 3-D potential method are presented. Simulations of various wave conditions at design speed are used as a validation study, and then simulations are carried out at stationary and operating speed. Secondly, unsteady wave patterns and time history results of the added resistance and the ship motions are simulated and analysed at each ship speed using the CFD tool. Finally, the relationship between the added resistance and the vertical ship motions is studied in detail and the non-linearity of the added resistance and ship motions with the varying wave steepness are investigated. Systematic studies of the numerical computations at various ship speeds are conducted as well as the grid convergence tests, to show that the numerical results have a reasonable agreement with the available EFD results

Investigation of ship-bank, ship-bottom and ship-ship interactions by using potential flow method

The authors were inspired by the benchmark model test data in MASHCON [1, 2] and carried out some numerical studies on ship-bank, ship-bottom and ship-ship interactions based on potential flow method in the last few years. In the confined waterways, many researchers question the applicability of the classical potential flow method. The main objective of the present paper is to present some validations of the 3D boundary element method (BEM) against the model test data to exam the feasibility of the potential method in predicting the hydrodynamic behaviour of the ships in confined water. The methodology used in the present paper is a 3D boundary element method based on Rankine type Green function. The numerical simulation is based on the in-house developed multi-body hydrodynamic interaction program MHydro. We calculate the wave elevations and forces (or moments) when the ship is manoeuvring in shallow and narrow channel, or when the two ships is travelling side by side or crossing each other. These calculations are compared with the benchmark test data, as well as the published CFD results. Generally, the agreement between the present calculations and model test and CFD results are satisfactory, which indicates that the potential flow method and developed program are still capable to predict the hydrodynamic interaction involved in ship-bank, ship-bottom and ship-ship problem