Maneuverability of a combined two-ship unit engaged in underway transfer

Ship-to-ship transfer operations, primarily used to transfer crude oil, petroleum, and other chemical products between ocean going ships (i.e., lightering operations), are commonly conducted worldwide because a deep draft of a large ship limits its access to ports. In areas where the water depth is too great for anchoring, cargo is transferred to another rigidly moored ship running slowly. This operation is called “underway transfer.” During this process, the maneuvering service ship (SS) approaches and berths from the port side to the starboard side of the large ship to be lightered (STBL). After mooring, the SS stops its engine and behaves as a towed ship. Meanwhile, the STBL maintains its steerage on a steady course, responsible for all maneuvers. To reduce the possibility of accidents, the maneuverability of this unique combined two-ship unit should be further investigated. In this study, a very-large crude carrier (VLCC) and an Aframax tanker are used as the STBL and SS. Two scenarios are considered for the two-ship unit: the full-load STBL with the ballasted SS, and the half-load STBL with the full-load SS. These are assumed to be different stages of the cargo transfer process. Free-running model tests and captive model tests are conducted, and a mathematical model for maneuvering simulation of such a front-back and left-right asymmetric two-ship unit is presented. Additionally, full-scale simulations are performed. Based on the results of experiments and simulations, the change in maneuverability, according to the progress of the cargo transfer, is assessed. The combined two-ship unit needs a large advance and transfer when turning to port, especially in the case of the half-load STBL with the full-load SS. The difference of left- and right-turning abilities is significant. In terms of the directional stability, the combined unit condition during underway transfer would be preferable to running alone.