Trans RINA, Vol 157, Part A3, Intl J Maritime Eng, Jul-Sep 2015
VERTICAL AND SLOPED BANK EFFECTS ON DIFFERENT SHIP TYPES (DOI No: 10.3940/rina.ijme.2015.a3.340)
Dong-Taur Su, Department of Shipping Technology, National Kaohsiung Marine University, Kaohsiung, Taiwan SUMMARY
This study employed computer design software to completely draft 3D ship models; then, computational fluid dynamics were used to establish numeric navigation channels and simulate fluid hydrodynamic analysis of ships navigating along shore banks. The parameters considered comprised bank type (vertical and sloped), ship model (two types), velocity, ship-to-bank distance, and navigation time. Figures and tables were used to present the distribution of ship stern eddy current, flow field pressure, and velocity, and the comparison of center of mass deviation, sway force, and yaw moment. Results showed that ships navigating along embankments and channels produced asymmetric flows, which draw the bow away from the shore. Larger ships are substantially more influenced by bank effects than smaller ships. Large sway forces and yaw moments are produced in large ships, drifting the bow away from the bank and the stern towards the bank, increasing the risk of collision with the embankment. From the study results, the characteristics of bank effects are understood and can be used for assisting the safe navigation of ships in restricted waters.
1. INTRODUCTION
Watchkeeping (STCW 2010) from the 26th International Towing Tank Conference (ITTC) [1], mariners at the management level must be able to immediately handle emergency conditions and appropriately control ships; thus, bank effects are worthy of in-depth investigation. Bank effects are caused when the water flow between a ship and a bank increases, which causes the formation of a low pressure zone. Meanwhile, the inward suction and outward flow of water around the
Ships navigating in narrow channels are influenced by restricted boundaries such as the influence of ship–ship and ship–bank interactions. Asymmetric flows are produced in the ship navigation zones interfering with the stability of ship navigation directions, and bank suction and cushion substantially affect ship control, influencing ship navigation safety. According to the mariners’
Standards of Training, Certification, and propeller blade
surface facing the shore are not sufficient for replenishing the displaced water on time; therefore, water level decreases and pressure becomes lower than that in the outboard, drawing the ship stern toward the bank and producing a suction phenomenon, resulting in bank suction. Additionally, when ships advance, water flow is displaced toward the two sides of the ship. The shore side is obstructed by the bank where water cannot diffuse, forming a higher water level. By contrast, water diffuses faster on the opposite side, forming a lower water level, which results in an outward deviation of the bow, which is called bank cushion. Bank suction and bank cushion are collectively called bank effects. Ship velocity and ship-to-bank distance (distance to bank, BS) are the primary factors influencing the bank effects (Lo et al. [2]). When ships navigate along a bank, a vertical or sloped embankment will influence the magnitude of bank effects on a ship. In addition, ship size and tonnage will also directly influence bank effects.
This study investigated various types and sizes of ships navigating along vertical or sloped banks to explore and discuss the influences of
©2015: The Royal Institution of Naval Architects the bank effects. A ship
operator must understand the interaction between ship and shore
to properly control ships. When a ship
navigates along a bank, the hull is close to the bank, and the bank side of
the midship and stern has a small
cross-sectional flow area, which increases the water flow rate and reduces pressure; consequently, a pressure difference forms between the two sides of the ship, pushing the ship toward the shore. This lateral suction force is called bank suction. If the ship deviates toward one side of the channel in close proximity to the bank, the obstruction of the bank to the side of the ship decelerates the discharge and diffusion effect of water; subsequently, the
hydrodynamics beneath the ship
bottom are retarded because of the squat effect. Thus, high water levels form at the bow side near the bank, producing yaw moment that pushes the bow toward the center of the channel. This phenomenon is called bank cushion. Regarding factors that influence bank effects, model and actual ship experiments have been conducted to demonstrate that bank effects are related to the following factors: Strong bank effects are produced when (1) a ship is close to the shore and far off-course from the center of the channel, (2) the channel is narrow, (3) the ship is traveling at a high speed, (4) the ship hull is corpulent, (5) the ship has a large tonnage, and (6) the water depth is shallow.
In general, by studying ship hydrodynamics in confined waters, the relationship between a ship and a bank can be analyzed. When two ships are within a specific distance, the movements of the two ships interact; the operation of both ships will be influenced during fronting and passing. Ship models and experimental towing tanks can be used to simulate relevant ship motions. Norrbin [3] conducted experimental studies on the bank effects in 1974 and discovered that banks critically influence fluid dynamic coefficients. Ch’ng [4] and Ch’ng and Renilson [5] have studied the effect that varying bank effect factors such as ship type, water depth, ship speed, bank slope, and propeller speed had on the operability of the ships. High dependencies were found on the degree and direction of sway force and yaw moment produced when the depth to
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