Methods and systems are for transferring fluid cargo between a first vessel and a second vessel at open sea in a Parallel configuration. The first vessel is equipped with a cargo connection point and the second vessel is equipped with cargo manifold. A tubular line is connectable between the cargo connection point and the cargo manifold. The method can include attaching a self-propelled buoy to the second vessel; connecting a cargo connection between the self-propelled buoy and the cargo manifold; connecting a cargo line between the cargo connection point and the self-propelled buoy; transferring cargo between the cargo connection point and the cargo vessel; and relying on the self-propelled buoy to keep the self-propelled buoy within predetermined distance boundaries from the first vessel also when the self-propelled buoy is attached to the second vessel.