1. Introduction
2. Measurement of Ice Load in an Ice Tank Test and Post-processing of the Data
2.1 Experimental Setup and Conditions
2.2 Post Processing of the Measured Data
3. Ice Load Generation in Time Domain Simulation
3.1 Process to Generate Random Value to an Arbitrary Heading Angle
3.2 Module for Ice Load Generation
4. Simulation Results
4.1 Setup for Simulation
4.2 Validation Test for the Ice Load Generated by the Developed Module
4.3 Derivation of Limit Status of DP Heading Control
4.4 Evaluation of Stationkeeping Performance
5. Conclusions
Although the simulated ice loads lacked statistical consistency with the experimental data, their validity was verified by creating ice loads that share some of the statistical characteristics and applying them to the simulation in the time domain. Although random variables were generated by assuming a normal distribution in this study, other distributions, such as a log-normal distribution, can be easily applied. Because the direction angle of the structure kept changing in the simulation, it is difficult to make a direct comparison between the simulation data and the values measured in the experiment while maintaining a particular angle. However, the analysis of the mean and maximum values, one of the main factors that characterize the ice load, is deemed acceptable.
Using the relative magnitude of the ice load and the DPS, it was found that the limit for maintaining the heading angle is approximately 4°. For the floating structure applied in the simulation, there were limitations in maintaining the position and controlling the heading angle simultaneously because the difference in thrust value between the thrusters installed at the bow and stern was significant.
Efforts were made to control the heading angle so it would match the ice load. As a result, the heading angle and the ice load could be matched, even when the contained angle was at 17°. The overall load acting on the structure is reduced by rotating the heading angle in the direction where the load is exerting the greatest force. As a result, it is possible to control the station keeping. This result, along with the previous simulation results, can be used as a reference for setting the acceptable heading angle in the DP-assisted mooring system.
Simulations were performed under a collinear condition, as well as under two different noncollinear conditions where the direction in which the ice load acts was different. The results showed that the station-keeping systems satisfied the performance requirements of the design phase. Because the mooring system is designed rather conservatively, the DPS did not seem to contribute to the station-keeping function. However, in a noncollinear condition, the difference in maximum tension between the stand-alone mooring system and the DP-assisted mooring system was 5,375 kN. Hence, the tension reduction effect of the DPS was shown. This result could be the basic data for redesigning a mooring system.