The National Transportation Safety Board (NTSB) has released an investigation report on an incident where a 600-foot-long tanker struck a pier at the Joint Base Charleston’s Naval Weapons Station in South Carolina on September 5, 2022.
The incident
On September 5, 2022, about 1602 local time, the 600-foot-long tanker Bow Triumph was transiting outbound on the Cooper River near Naval Weapons Station, Joint Base Charleston, South Carolina, when the vessel struck Naval Weapons Station Pier B.1 The vessel’s bow sustained significant damage, and a 300-foot section of the pier collapsed. No pollution or injuries were reported. Damage to the vessel and pier was estimated at $29.5 million.
Analysis
On September 5, 2022, about 1602 local time, while the tanker Bow Triumph was transiting the JBC Channel on the Cooper River, the vessel attempted to turn to port when navigating a bend in the river and struck a pier. After making the right turn at 1553 onto Range D and transiting near the center of the channel, the Bow Triumph pilot began to favor the left side of the channel as the vessel approached the port turn (bend).
The Bow Triumph bridge team and the pilot told investigators the rudder had responded as ordered by the pilot as the vessel approached the bend and that there was no issue with the vessel’s steering, and the VDR showed that rudder response matched the pilot’s orders. When the pilot maneuvered the Bow Triumph closer to the left bank while approaching the turn, the vessel was susceptible to bank effect. Bank effect is experienced by ships maneuvering in confined waters (e.g., close to a canal bank, riverbank, or shoal).
While making headway, water flow down the side of a ship creates positive pressure forward of the pivot point and negative pressure aft. In a channel, the resultant forces can attract a ship’s stern toward the bank (bank suction) and yaw the bow away from the bank (bank cushion). Though bank effect is often experienced in waterways with steeply sided banks, The Shiphandler’s Guide explains: “To a ship running in shallow water, with adjacent but gently shelving mud or sand banks, such as low-lying estuarial areas … the effect can be far more insidious and violent.”
Generally, the faster the ship sails, the greater the suction at the stern. As it moved to the left side of the channel, the vessel traversed a section where shoaling had reduced the depth by more than 10 feet from the project depth. The Bow Triumph approached the turn running along the eastern (left) bank, and it had relatively open water to starboard. With the open water to starboard, there would not have been any opposing forces that offset the bank effect on the vessel’s port side. At the same time that the pilot was attempting to turn the vessel to port, the bank effect forces at the turn would have worked against the port turn by pushing the bow away (to starboard) and pulling the stern of the Bow Triumph toward the bank.
Additionally, as the bow of the Bow Triumph emerged from the shadow of the left bank, the roughly 1-knot current from the flood tide would have had an effect on the submerged portion of the vessel’s port bow—pushing it away from the bank and further working against the attempted port turn. All the other vessels of similar size and draft departing the INEOS Aromatics Terminal in the previous 12 months approached the bend nearer the center of the channel than the Bow Triumph did on its casualty transit, and all were able to safely navigate the bend. Transiting in the center of the channel is prudent to avoid the risks associated with bank effect.
Probable Cause
The National Transportation Safety Board determines that the probable cause of the contact of the Bow Triumph with Naval Weapons Station Pier B was the pilot’s decision to maneuver the vessel close to the left bank while approaching the turn immediately before the pier, exposing the tanker to bank effect, which the pilot’s subsequent rudder and engine orders could not overcome.
Lessons Learned: Planning for Hydrodynamic Forces in Areas Subject to Shoaling
Hydrodynamic forces reduce rudder effectiveness (squat and shallow water effect) and yaw the bow away from the closest bank and pull the stern in (bank effect). Shoaling can reduce the water depth in shallow waters, such as channels, below charted or expected, and therefore exacerbate the forces on a vessel. Bank effect can have an undesired effect on vessels, even for the most experienced shiphandlers. Pilots, masters, and other vessel operators should consider the risks in areas known for shoaling when planning transits.
