Zhiming Yuan

Ship-to-Ship Interaction During Overtaking Operation in Shallow Water

Hydrodynamic interaction continues to be a major contributory factor in marine casualties and hazardous incidents, in particular, in the case of overtaking operations. The situation becomes even worse when the overtaking operation occurs in shallow and narrow channels, where the interaction can cause the vessels to collide and, in one case has caused the capsizal of the smaller vessel with loss of life. The aim of this article is to propose a methodology, as well as to discuss the development of a numerical program, to predict the ship-to-ship interaction during overtaking operations in shallow water. Since the vessels involved in this study have different forward speeds, an uncoupled method will be used to solve the boundary value problem. The in-house multibody hydrodynamic interaction program MHydro, which is based on the 3D Rankine source method, is used and extended here to investigate the interactive forces and wave patterns between two ships during an overtaking operation. The calculations given in this article are compared with model test results as well as published computational fluid dynamics (CFD) calculations. Very satisfactory agreement has been obtained, which indicates that the proposed methodology and developed program are successfully validated to predict the hydrodynamic interaction between two ships advancing in confined waters. The discussions also highlight the speed effects.

Manoeuvring prediction based on CFD generated derivatives

This paper presents numerical predictions of ship manoeuvring motions with the help of computational fluid dynamics (CFD) techniques. A program applying the modular concept proposed by the Japanese ship manoeuvring mathematical modelling group (MMG) to simulate the standard manoeuvring motions of ships has been initially developed for 3 degrees of freedom manoeu- vring motions in deep water with regression formulae to derive the hydrodynamic derivatives of the vessels. For higher accuracy, several CFD generated derivatives had been substituted to replace the empirical ones. This allows for the prediction of the maneuve- rability of a vessel in a variety of scenarios such as shallow water with expected good results in practice, which may be significantly more time-consuming if performed using a fully CFD approach. The MOERI KVLCC2 tanker vessel was selected as the sample ship for prediction. Model scale aligned and oblique resistance and Planar Motion Mechanism (PMM) simulations were carried out using the commercial CFD software StarCCM+. The PMM simulations included pure sway and pure yaw to obtain the linear manoeuvring derivatives required by the computational model of the program. Simulations of the standard free running manoeuvers were carried out on the vessel in deep water and compared with published results available for validation. Finally, simulations in shallow water were also presented based on the CFD results from existing publications and compared with model test results. The challenges of using a coupled CFD approach in this manner are outlined and discussed.

Coupled analysis of floating structures with a new mooring system

As the exploitation of hydrocarbon reserves moves towards deeper waters, the floating structures are becoming more and more popular, and the catenary and taut mooring systems are two widespread mooring systems which are used for these floating structures. However, both of them have their inherent drawbacks. The aim of the present work is to develop and validate a new mooring system which will overcome these shortcomings. To this end, the motion performance of a semi-submersible platform is simulated by employing a full time domain coupled analysis method. It is shown that the new mooring system yields very good motion performance when benchmarked against the taut mooring system, and the reasons for this improved performance are discussed. Also, the new mooring system is compatible with the characteristic of catenary mooring system, which eliminates the requirement of anti-uplift capacity of the anchors. The second aim of this paper is to explore the proper water depth in employing this new mooring system. For this purpose, several typical water depths are simulated. It is found that the new mooring system works well both in deep water and ultra-deep water. But, as the water depth becomes deeper, the advantages of the new mooring system are reduced.Copyright

Wave force prediction effect on the energy absorption of a wave energy converter with real-time control

Real-time control has been widely adopted to enlarge the energy extraction of a wave energy converter (WEC). In order to implement a real-time control, it is necessary to predict the wave excitation forces in the close future. In many previous studies, the wave forces over the prediction horizon were assumed to be already known, while the wave force prediction effect has been hardly examined. In this paper, we investigate the effect of wave force prediction on the energy absorption of a heaving point-absorber WEC with real-time latching control. The real-time control strategy is based on the combination of optimal command theory and first order-one variable grey model. It is shown that a long prediction horizon is beneficial to the energy absorption, whereas the prediction deviation reduces extracting efficiency of the WEC. Further analysis indicates that deviation of wave force amplitude has little influence on the WEC performance. It is the phase deviation that leads to energy loss. Since the prediction deviation accumulates over the horizon, a moderate horizon is, thus, recommended.

Optimum Distance Between Two Advancing Ships Arranged Side by Side

The hydrodynamic interaction between two advancing ships is very important. Because of the hydrodynamic interactions, even relatively small waves can induce large motions of the smaller ship due to the proximity of the larger ship. The aim of this paper is to develop a method to optimize the spacing between two advancing ships, in order to minimize the hydrodynamic interactions. The optimization method is based on the far-field wave patterns produced by a translating and oscillating source point. For values of the parameter τ > 0.25 ( τ = ωeu/g) there is a fan-shaped quiescent region in front of the vessel. As τ increases, the range of the fan-shaped quiescent region will be expanded. It can be supposed that if the two ships are located in each other’s fan-shaped quiescent region, the hydrodynamic interactions can be minimized. This assumption was validated through the numerical simulation, which was based on a 3-D Rankine source panel method. We calculated and compared the wave exciting forces and wave patterns of two Wigley hulls advancing in waves side by side. The numerical results were consistent with our theoretical assumption.

Investigation on long-term extreme response of an integrated offshore renewable energy device with a modified environmental contour method

Considering the massive simulations required by the full long-term analysis, the environmental contour method is commonly used to predict the long-term extreme responses of an offshore renewable system during life time. Nevertheless, the standard environmental contour method is not applicable to the wind energy device due to the non-monotonic aerodynamic behaviour of the wind turbine. This study presents the development of a modified environmental counter method and its application to the extreme responses of a hybrid offshore renewable system. The modified method considers the variability of the responses by checking multiple contour surfaces so that the non-monotonic aerodynamic behaviour of the wind turbine is considered. The hybrid system integrates a floating wind turbine, a wave energy converter and two tidal turbines. Simulation results prove that the modified method has a better accuracy.

Ultimate structural and fatigue damage loads of a spar-type floating wind turbine

ABSTRACT This study addresses the ultimate structural and fatigue damage loads of a spar-type offshore floating wind turbine under joint excitations of wind and wave. Aero-hydro-servo-elastic coupled analysis is performed in time-domain to capture the dynamic responses of the floating wind turbine. Based on the mean up-crossing rate method, the short-term ultimate structural load is estimated. The cumulative fatigue damage load is computed with the S-N method. It is shown that the low-level ultimate load is mostly influenced by wind forces whereas the high-level ultimate load is more closely related to wave forces. The wave excitations dominate the fatigue damage at tower top and tower base, whereas the mooring line fatigue damage is more sensitive to the wind forces.

Hydrodynamic Interaction Between Two Ships Arranged Side by Side in Shallow Water

The hydrodynamic interaction between two ships with side-by-side arrangement is analyzed by using 3-D Rankine source panel code. The source points are distributed over the mean wetted body surface as well as on the free surface. The shallow water effect has been taken into consideration. Moreover, the influence of the distance between the vessels is also investigated. To verify the present code, two Wigley III hulls are simulated both in beam sea and head sea conditions. The wave pattern and the motion RAOs of 6-DOF are calculated by present code and compared with WADAM program which is based on Green function method. From the comparisons, good agreement is found between present calculation and Wadam results. It is found that the hydrodynamic interactions are generally important especially in beam sea case. The resonant frequency is greatly influenced by the distance between the vessels.