Rolling in the deep
When a ship lost hundreds of its containers overboard after rolling heavily in benign seas, accident investigators found the incident was caused by a common yet fundamentally misunderstood maritime phenomenon — parametric resonance.
In January 2021, the Danish container ship Maersk Essen lost hundreds of its containers due to heavy rolling as it sailed 450 nautical miles off Hawaii, whilst on passage from China to Los Angeles.
The incident began when the 366m (1,200ft) vessel developed an initial series of slow, heavy rolls that stopped quickly, leaving the crew unconcerned.
However, two hours later, the heavy rolling started suddenly again. The ship’s course was immediately adjusted three degrees to starboard, which re-stabilised its motion. But, within a couple of minutes, the rolling began yet again — this time with even greater rolling angles.
The crew couldn’t keep their balance, items flew around, and the alarms started to sound.
An echo was then detected on the radar close by the ship’s stern. From the bridge wing, the master could see containers hanging over the ship’s side with others now floating in the sea.
The Maersk Essen diverted to Mexico to remove the remaining cargo and carry out repairs.
In addition to the 689 containers lost overboard, a further 258 onboard were deemed to be damaged (Credit: Maersk A/S)
It was assessed that 689 containers had been lost overboard and a further 258 damaged.
Many of the container stacks had collapsed and toppled over, causing extensive damage to the deck structures, blocking passageways and tearing off safety railings. Accommodation ladders on both the port and starboard side of the ship were also damaged. Items that had not been secured were hurled around, including office equipment, furniture and galley equipment. In the engine room, items of heavy equipment, including a lifting device and a spare electrical motor, became loosened and slid across the floor.
Data analysis by the Institute of Ship Design and Ship Safety at the Technische Universität Hamburg (TUHH) revealed that the Maersk Essen experienced roll angles of up to 30° while on an easterly course at around 11 knots in 7m (23ft) swells coming onto its port quarter.
High wave attack and synchronous roll resonance were ruled out as the cause of the incident because conditions for those two phenomena were not present at the time.
Accident investigators from the Danish Maritime Accident Investigation Board (DMAIB) therefore determined that the heavy rolling was most likely to have been a result of parametric resonance rolling — a dynamic stability failure where extreme rolling is caused, not by the direct impact of waves, but by a build up in the frequency of the ship’s oscillating motion resulting from a complex interaction between the length of the waves, the wave periods, the ship’s natural roll period, the ship’s length, and the ship’s course and speed.
Predicting a risk of parametric resonance rolling is difficult because it can occur in wave and swell conditions that would not normally be perceived to be adverse in any way.
The DMAIB report points out that parametric resonance effect is considered to be a rare phenomenon by navigational officers — something it describes as a ‘false assumption’.
The report states that parametric resonance is actually experienced a lot more frequently than reported, because it tends to only result in an occasional heavy roll, and can be negated by a change in either the ship’s speed and course or the state of the sea.
The officers aboard the Maersk Essen told the investigators that, although they had good knowledge of how to react to stop heavy rolling caused by parametric resonance, they had no way of predicting the risk of it or pre-empting its onset.
The DMAIB investigators found that the acceleration forces acting on the container stacks during the rolling events exposed the securing equipment to stress loads they were not designed to withstand, even though the conditions triggering the parametric rolling were well within the spectrum of normal operational conditions for Maersk Essen and sea conditions for the area at the time would not be considered abnormal or problematic.
The triggering conditions for parametric rolling, the report explains, were not particularly rare for a container ship in the sea area where the incident took place. Indeed, it is likely that the ship had actually experienced resonance effects earlier on in its voyage.
On board calculator
Maersk Essen’s owner, Maersk A/S, says it has now provided the ship with an ‘On Board Parametric Roll Calculator’ in the format of an Excel spreadsheet. According to the guidance notes in the document, it is not feasible to use the calculator for continuously calculating the risk of parametric resonance in real time, but the master and crew are encouraged to use the calculator to help calculate the overall risk that might occur during an upcoming voyage.
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Dennis O’Neill is a freelance journalist specialising in maritime.