Treacherous turn: beware the bank effect

The Cooper River has some significant bends, requiring precise manoeuvring. But as the pilot explained during his interviews with the NTSB, he had assessed the ship’s manoeuvrability, rudder and engine power on a previous turn near a submarine base that he described in his experience as the “most significant turn” on the river. He noted: “The ship came around without any discrepancies, very well managed, and very well tracked.” The pilot even commented about his previous experience with such vessels and their propulsion and steering.

“These ships that I’ve been on in my 30-year career, are extremely good-handling vessels. They have Becker rudders, which are rudders that will break at a certain area and give you more rate of turn. And their engine power is usually very, very responsive.”

But as explained by the NTSB, after navigating the ship through a right turn on the channel, the pilot steered the tanker closer to the left bank as he approached the next turn, a left turn around a bend. The manoeuvre caused Bow Triumph to experience bank effect — when a ship’s bow is pushed away from the bank and the stern is pulled toward the bank while transiting in confined, shallow waters.

“There was no indication that the ship was not going to handle properly”

“There was no indication that the ship was not going to handle properly. So, I went to mid ship for a second to ease my rate of turn, and then to…complete the manoeuvre, I put her to port 20 again, and kicked it to full ahead to make the turn, which is a normal procedure for us, especially with a ship with that type of power structure,” the pilot told investigators. What happened next, however, was “totally unexpected” and something he had “never seen before,” said the pilot. The vessel’s rate of turn was not increasing.

Hard a-port and full ahead, dropping anchor and the pilot’s other subsequent rudder and engine orders all failed to overcome the bank effect, and the tanker struck the pier on the opposite side of the river.

The pilot, Bow Triumph’s master, and bridge watch standers told investigators that there was no issue with the vessel’s steering. Investigators reviewed voyage data recorder data (audio and parametric) and found that the rudder matched orders given by the pilot up to the moment of the contact with the pier.

One of the key takeaways shared by the NTSB report was that hydrodynamic forces reduce rudder effectiveness (squat and shallow-water effect), pushing the bow away from the closest bank and pulling the stern towards the bank in the so-called ‘bank effect’.

Shoaling can reduce the water depth in shallow waters, such as channel, below chartered or expected norms, exacerbating the forces on a ship. “Bank effect can have an undesired effect on vessels, even for the most experienced ship handlers,” cautioned the NTSB, adding “pilots, masters, and other vessel operators should consider the risks in the areas known for shoaling when planning transits.”

Treacherous turn

The turn on the river has proved treacherous for other tankers, too. On 14 January 2024, the tanker Hafnia Amessi departed the INEOS Aromatics Terminal, attempted to make the same turn at the bend, and contacted Pier B. The NTSB is investigating the incident and will issue a report when its investigation is complete.

Working with stakeholders, the Coast Guard Sector Charleston Captain of the Port is looking to determine appropriate regulatory and non-regulatory long-term solutions to reduce the risk of similar incidents with Pier B. Until such long-term solutions are in place, ships that are 10,000 gross tons or more, or ships with a draughts exceeding 7.6 m, are required to employ a tethered two-tug escort while transiting the Joint Base Charleston Channel segment of Cooper River.