Make sure you understand your ship’s motion monitoring and forecast tools
Container ship CMA CGM G. Washington lost 137 containers overboard as a result of parametric roll. The Nautical Institute points out that a major contributing factor was that the crew was not fully conversant with the ship’s motion monitoring, forecasting and decision support tool.
The incident was covered in a recent Mars Report. These reports are compiled (anonymously) by The Nautical Institute to prevent other accidents from happening. A link to the official investigation report reveals that the ship involved is the G. Washington. On 20 January 2018, this ship lost 137 containers when experiencing heavy seas in the North Pacifc Ocean while on passage from Xiamen, China, to Los Angeles, USA. A summary of the incident:
A large container vessel was underway in the open ocean when the weather began to deteriorate. The wind was force 6 and the ship was rolling between seven and twelve degrees in a 3-metre swell. The deck crew began the daily lashing checks as usual, but the weather conditions meant that they were only able to inspect the bays forward of the bridge. Later that day, the swell increased to 4.5 metres. That evening, the ship experienced a sudden large roll of approximately 16 degrees. Later on, it started to roll routinely to 15 degrees.
The master reviewed the data provided by the ship’s electronic motion monitoring and forecasting system and instructed the officer of the watch (OOW) to switch to hand-steering and alter course from 088 to 082 degrees. Following the alteration of course, the ship’s rolling reduced to less than 10 degrees. Later, the master told the OOW to return to automatic steering as he was now confident that the autopilot was up to the task. About an hour later, the ship unexpectedly rolled twenty degrees to starboard, paused for a few seconds, then made a similar roll to port.
The deck lights were turned on after the large roll, but no damage was seen from the bridge and the container stows appeared to be intact. At daylight, an officer went to the weather deck with the bosun to investigate. They found that bays 18, 54 and 58 had collapsed. It was later determined that 137 containers had been lost overboard and 85 damaged.
The official investigation found, among other things, that:
- It is likely that the forces generated when the vessel rolled twenty degrees to port and starboard initiated the collapse of the container stows at bays 18, 54 and 58.
- The amplitude of the ship’s rolling exceeded the limits set by the company for the class of vessel.
- It is almost certain that the vessel experienced parametric rolling prior to and at the time of the container collapses.
- The master and his bridge team were familiar with, but did not fully understand, the functionality of the ship’s motion monitoring, forecasting and decision support tool. As a result, they did not appreciate the imminent risk of parametric roll.
- The cause of the collapse at bay 18 could not be determined. It is most likely that this collapse was initiated following the structural failure of one of its containers, brought about by a combination of factors including: excessive stack loads as a result of mis-stowed or overweight containers; excessive racking loads or contact between containers due to loose lashings; and/or existing damage or poor material condition of a container.
Advice from The Nautical Institute
- Parametric rolling is where a ship experiences larger than expected roll behaviour when the primary sea wavelength is similar to the ship’s length with either the wave crest amidships and the bow and stern in wave troughs, or the ship is supported by a crest at the bow and stern with the trough amidships.
- IMO guidance suggests that parametric rolling may occur when either the period of roll equals the period of encounter, or the period of encounter is approximately half the roll period.
- The risk of parametric roll in a following sea is very sensitive to minor changes in the relative direction of the sea. Large container ships are particularly vulnerable to parametric rolling due to their length and fine hull form.
- If you work on a vessel equipped with a motion monitoring, forecasting and decision support tool, ensure you are fully conversant with its functionalities.
This accident was covered in the Mars Reports, originally published as Mars 202033, that are part of Report Number 332. A selection of this Report will also be published in SWZ|Maritime’s July/August 2020 issue. The Nautical Institute compiles these reports to help prevent maritime accidents. That is why they are also published on SWZ|Maritime’s website.
More reports are needed to keep the scheme interesting and informative. All reports are read only by the Mars coordinator and are treated in the strictest confidence. To submit a report, please use the Mars report form.
Picture: The container bays that collapsed.