Numerical simulation on ship to ship transfer operation in rough sea

In recent years, a ship-to-ship (STS) transfer operation of crude oil is increasing and has become a common practice in the offshore marine industry. Furthermore, the operation is expanded to the LPG-transfer offshore operation and is also applied to the offshore LNG-terminal projects. Up to now, the fender used for absorbing the impact energy between both ships has been selected by the berthing energy derived from virtual mass and relative berthing speed of two ships, VLCC (ship to be lightered, STBL) and service ship (SS) in accordance with OCIMF Guideline. The whole operation is mainly dependent on the empirical and traditional rules of mooring masters or ship's captains. The practical and advance guideline is required to verify and keep safe operations in the real sea. In this study, time domain analyses to assess the dynamic behavior and the operational capability of the STS transfer operation with forward speed in rough sea are made. This paper presents an analysis tool to predict the dynamic motion response and non-linear connecting and mooring forces on a STS transfer operation due to short crested irregular wave and fluctuated wind load actions. Simulation for the connecting ropes and fenders are also investigated. The three-dimensional source-sink technique has been applied to obtain the radiation forces and the transfer function of wave exciting forces. The hydrodynamic interaction effect between both ships is included to calculate the hydrodynamic forces. Forward speed of STS transfer operation is considered as encounter current. For the simulation of a random sea and also for the generation of fluctuated wind velocity, a fully probabilistic simulation technique has been applied. Wind and current loads are estimated according to OCIMF. The effects of variations in wave, wind and current loads are also investigated in this paper. Finally, some conclusions are drawn based on the numerical results obtained from the present time domain simulations. These will be helpful for the practical and advance guideline for STS transfer operation in sea.