The ship’s loading condition required the ship to avoid roll angles exceeding 19.18° in order to stay within the stress load limits defined in the ship’s loading and stability computer..

Parametric rolling is the result of a dynamic stability failure mode where the main cause of extreme rolling lies within the ship’s oscillating system.. Thus, heavy rolling can occur in wave and swell heights that are not perceived as adverse to the ship..

Key findings in Maersk Essen investigation

1. Weather and sea conditions were not out of the ordinary for this sea area.. No extreme weather phenomena were observed, and the ship had encountered similar or worse weather and sea conditions earlier in the voyage, on 12 and 13 January, without experiencing heavy rolling..
2. Onboard weather routing focused only direction and height of the wave and swell and the risk of parametric rolling or pure loss of stability on a wave crest was not included in the onboard template for weather routing requests..
3. Weathernews Inc’s (WNI) voyage planning sheets and damage mitigation warnings visually and in text highlighted areas exceeding a predefined wave/swell height thres¬hold which the navigational officers did not perceive as a warning of the risk of parametric rolling..
4. Crew did not have reliable information or tools readily available onboard to calculate and monitor the risk of parametric roll resonance.. Cargo planning and stowage introduced variabilities that the crew had little influence on or opportunity to react on..
5. Simulations of the lashing stress loads by Loadstar (stability software) at the time of the accident showed that the limit was exceeded in most bays, but containers were not lost on all bays.. DMAIB did not find any clear indication of what caused the cargo to be lost from some bays and not others with similar conditions..
6. Variability in stowage and cargo securing might have influenced the stress loads and holding capacity.. However, the direct cause of the lashing failure was the excessive roll angles, which exposed the lashing gear to forces beyond their intended load limit..
7. Damage patterns indicated that bays stowed with ten containers in height (between 26-29 m high) were more exposed to acceleration forces than bays with lower stacks..

To understand parametric rolling further, I sought the view of an experienced and active Master Mariner for a technical explanation of how parametric rolling works..

Before we get into what parametric rolling is, a ship has 6 different motions when traveling in the ocean as shown below..