ATSB: Complex Interaction Forces Can Cause Breakaways in Fast Currents

 

Pilots and port officials at Brisbane have revised their rules for ship movements after two dangerous breakaways, caused by a combination of extreme river currents and hydrodynamic forces from passing ships. 

In May 2022, during an unprecented high rainfall event, a series of controlled dam releases upstream on the Brisbane River caused strong currents at the Port of Brisbane's container terminal. The currents put extra strain on the mooring lines on the boxships that were berthed alongside at the port. 

On May 16, the container ship OOCL Brisbane broke away from its berth at the port, just as it was passed by the container ship Delos Wave. All of Brisbane's mooring lines parted or paid out, and it drifted off into the Brisbane River before it was corralled and brought under control by harbor tugs. 

Four days later, CMA CGM Bellini's forward mooring lines parted just as it was passed by the APL Scotland, and its bow drifted off the wharf before tugs brought it safely back to the pier. 

CMA CGM Bellini breaks away (Maritime Safety Queensland / ATSB)

In both incidents, the ships were moored uneventfully for more than 20 hours without issue, then broke away as soon as a second container ship pulled alongside and berthed just ahead. Suspecting interaction forces, the Australian Transp Safety Board (ATSB) commissioned a hydrodynamic study to examine the currents and the effects of the nearby moving ships. The expert study found that the currents alone should not have exceeded the holding power of the Brisbane and Bellini's mooring arrangements, indicating that interaction forces created by the second vessel were at play. 

Delos Wave and APL Scotland both transited to their berths at normal operating speed over ground - but with the high current, they were moving faster than normal through the water. This created a larger surface displacement wave than usual, and this enhanced wave caused the moored container ships to surge and yaw at the pier - much like a moored dinghy hit by an excessive wake. 

When the approaching boxships moored just ahead, the complex current flow around their hulls and against the pier structure, combined with the wash from their propellers, added to the forces on Brisbane and Bellini until their mooring systems failed. 

“Fortunately, the ships were brought under control in both cases, and there were no injuries or substantial damage in either incident,” ATSB Chief Commissioner Angus Mitchell said. “But breakaways can have serious outcomes. These breakaways highlight the importance of robust, properly structured and clearly defined emergency and risk management arrangements for managing port shipping movements outside of normal operating conditions."

Brisbane's pilots and port authorities have adjusted their operating procedures to account for flood events, and advised that ship movements may be restricted for safety when necessary during extreme environmental conditions.