This study applies the Buys-Ballot procedure to decompose and analyze the monthly turnaround time of vessels at Tincan Island Port, Lagos, Nigeria, covering the period from 2015 to 2024. The primary objective is to identify the appropriate time series model—additive, multiplicative, or mixed that best represents the data structure and seasonal behavior of vessel turnaround operations at the port. Monthly data comprising 120 observations were arranged into a Buys-Ballot table, and both graphical