Design aspects of the hydrodynamic and structural loading on floating offshore platforms under wave excitation

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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

Investigation of side wall and ship model interaction

Due to the existence of the side wall of the towing tank, the measured hydrodynamic forces would present some discrepancies compared to the open sea results. This phenomenon is referred as side wall effect. The object of the present study is to investigate the parameters which determine the side wall effects. The method used in the present study involves a 3D panel method based on Rankine type Green function. A ship advancing in a towing tank with parallel side walls is simulated and the numerical results are validated against model test results carried out by Kashiwagi and Ohkusu (1991). The parameters including wave frequency and forward speed which determine the side wall effects are discussed

Full-scale unsteady RANS 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

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

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

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

Numerical study on wave run-up height and depression depth around a vertical circular cylinder at various froude numbers

The turbulent flow past a circular cylinder has been studied extensively by previous researchers due to its importance in many engineering applications. In particular, the wave run-up is one of the most significant design factors when offshore structures are operated. In this paper, the wave run-up height and depression depth around a vertical circular cylinder were numerically investigated. The commercial CFD solver “STAR-CCM+” has been used for the numerical simulations. The models of K-epsilon turbulence and volume of fluid (VOF) are utilised to solve the Reynolds Averaged Navier-Stokes equations (RANS) and continuity equations, respectively. Various Froude numbers and Reynolds numbers are utilised to observe the wave run-up height on the front of the cylinder and the depth of depression at the back. The results were compared with previous experimental data and theoretical values and were found to be in good agreement with other studies

A numerical investigation of the squat and resistance of ships advancing through a canal using CFD

As a ship approaches shallow water, a number of changes arise owing to the hydrodynamic interaction between the bottom of the ship’s hull and the seafloor. The flow velocity between the bottom of the hull and the seafloor increases, which leads to an increase in sinkage, trim and resistance. As the ship travels forward, squat of the ship may occur, stemming from this increase in sinkage and trim. Knowledge of a ship’s squat is necessary when navigating vessels through shallow water regions, such as rivers, channels and harbours. Accurate prediction of a ship’s squat is therefore essential, to minimize the risk of grounding for ships. Similarly, predicting a ship’s resistance in shallow water is equally important, to be able to calculate its power requirements. The key objective of this study was to perform fully nonlinear unsteady RANS simulations to predict the squat and resistance of a model-scale Duisburg Test Case container ship advancing in a canal. The analyses were carried out in different ship drafts at various speeds, utilizing a commercial CFD software package. The squat results obtained by CFD were then compared with available experimental data