The Huangpu River tidal barrage is in a navigable reach where the quay wall greatly restrains the flow. Earlier studies focused on ship-structure interaction, but there aren’t many hard numbers on safe passing distance near the barrage pier. The purpose of this paper is to determine the lowest safe lateral gap for large vessels and to provide an engineering means of ascertaining the clear width of the main channel. A 20,000-ton oil tanker is used as the test vessel. Using the boundary element method (BEM), a 3D model is constructed to simulate the interactions between the ship and the pier under varying speeds, draft depths, and lateral offsets. Simulations were conducted for different speed levels, draft depths, and lateral offsets to observe the flow and the ship’s movement. Changes in side forces, yawing moments, and bow angles were recorded over time, and their maximum values were determined. Sensitivity and uncertainty analyses were also performed to evaluate how input variations affect the results and to assess model stability. Based on these results, simple limits were established for the three measures to serve as safety rules. Using these rules, the clear width of the main channel was calculated. The results indicate a minimum safe lateral distance of 14 m and a suggested channel width of 202 m, deviating by only 0.5 m, or approximately 0.25%, from the theoretical value of 202.5 m. These findings can also be applied in practice for pier layout and the determination of speed limits in narrow channels.