Construction of Numerical Simulation Model and Behavior Evaluation for Slewing Motion Under the Initial Stage of Dragging Anchor

Abstract

　In recent years, risk of dragging anchor is increasing by typhoon that is becoming stronger. Evacuation advisory for the typhoon is announced in each harbor. However, this is for large vessels and not for the small vessels that are not correspond to the criteria. These vessels need to moor at suitable area in the harbor. Counterplan of the dragging anchor for the small vessel needs to be established immediately. The slewing motion is specific motion under the initial stage of the dragging anchor. By stabilizing the motion, the risk of the dragging anchor is decreased.

　From these backgrounds, construction of the high accuracy simulation model for the slew motion is paid attention. Numbers of research about the slewing motion are conducted until now. However, those focus only wind load as the environmental load and are constructed by the maneuvering equation. In this research, wind and wave load were considered by using the Cummins equation that was the basic linear equation for seakeeping in time domain simulation. The linear and non-linear maneuvering effect were simulated by adding on the model as the external forces. The models were constructed for coastal tanker and ferry. Model tests were conducted to evaluate the accuracy of this model and parametric simulations were also conducted to evaluate the impact force acting at top of the mooring line.

　From the model test, the accuracy of the model was confirmed by comparing the trajectory of center of the gravity. Line top tension was also compared and confirmed the accuracy. Moreover, cause of top tension’s two peaks in yawing half period were studied by evaluating of cross-correlation function between the top tension and motional parameter. As a result, it is found that the main cause of first peak is multiple of the sway velocity and yaw rate. Additionally, the second peak’s cause is also found that the position of the moored ship. These knowledges are believed to be useful to reduce the top tension and the dragging anchor’s risk.

　Through the parametric simulation, the impact force ratio that is the value between the maximum value of the horizontal mooring force and frontal wind load was evaluated using the single anchored tanker and the ferry model. The simulations were conducted by changing the wave height, the maximum instantaneous wind velocity. From the result, it was shown that increasing of wave height under the condition of low wind velocity induce an increase the ratio. Moreover, the value greatly exceeded the usual ratio got by the frontal wind load. These knowledges also are believed to be useful to estimate the anchor holding power.