The angle of the skewed line in the drawing reflects as good as possible the figures given in the Scientific Analysis. It gives an overview of the acceleration forces versus the roll angle.
From the report:
According to crew statements, at 23.00 hours on the evening of 1 January 2019, the MSC ZOE suddenly started to roll violently which felt like rolling angles of 20° till 30°, for a period of around 30 seconds. These movements were so violent that the equipment in the fitness area shifted and on the bridge various items, including the printer, flew through the air. According to the crew, after this period of violent rolling, the movements of the vessel returned to the previous 5 to 10° roll.
From personal experience I know this phenomenon as once on a tanker, in the middle of the night during stormy weather, I was catapulted out of my berth to the bench on the other side of my cabin. That was my luck, otherwise… All the drawers from underneath my berth also flew through the cabin and that while they had a lock system to prevent it from just doing that. We had a lot of damage on deck, also part of the ‘chicken walk’ as we called it, your cat walk, was swept away. Now we had to walk over the deck to get to the mess room aft.
The cause of the violent motion could be two or more waves that add up algebraically, a very high wave preceded by a deep trough may occur. This may be referred to as an ‘episodic wave’, i.e. a random large wave, noticeably of greater height than its precursors or successors, which occurs when one or more wave trains fall into phase with another so that a wave or waves of large amplitude is/are produced giving rise to sudden steep and violent rolling and/or pitching of the ship. These are popularly, and incorrectly, referred to as ‘freak’ wave.
The 30° roll, which amounts to 7.84 m/sec² or 0.8 g, is doubted by the Scientific Analysis and is indeed a very high value. It is a pity that on such a state of the art ship no trustworthy electronic inclinometer was available. The excessive acceleration criterium R1 (m/sec²) in the formula is the standard for lateral acceleration and the value of standard is defined as R1 = 7.848 m/s2 for ships with the length between perpendiculars greater than 250.0 m. Note that the ship’s value of 7.84 m/sec² is a fraction below the standard.
According to the graph the lashings must hold to at least a roll of 18.74°. That I don’t understand since that figure can be reached in bad weather rather easily…