An Analysis of MSC Zoe's Container Loss

On the night of January 1, 2019, MSC Zoe lost approximately 290 containers in heavy weather on the journey from Portugal to Bremerhaven. The loss of so many containers is an exceptional event and is the second largest known container loss of a ship due to heavy weather. Only Svendborg Maersk lost more, 517, in 2014.

This article is an attempt to analyze the cause but does not claim to be able to answer all the questions, because there are, of course, a number of unknown factors.

Where do I take my findings from? I have served as  as a Nautical Officer and Captain on container vessels of various sizes for many years, from feeder to very large container ships with 8,200+ TEU and vessel lengths of 340 meters, in all weather conditions worldwide. As Captain, I have sailed many large container ships and experienced numerous extreme weather conditions, without ever having container losses or damage. In addition, I have been working for years on issues of parametric rolling, its causes and its effects on container ships, in order to be able to address these in competent specialist decisions in my profession as Captain.

Weather conditions as a starting point

The six hourly published Sea Surface weather charts from 31.12.2018 00:00 UTC to 02.1.2019 06:00 UTC from metoffice.gov.uk are available to me and give an overview of the development of the weather during this period.

A depression (978 HPa) with its center over Iceland on December 31, 2018 moved within 24 hours in ENE-ly direction to the Vestfjord / Norway. The moving speed of 35 kt /hr showed that it was a fast-moving depression, which did not further deepen. A high-pressure area (1022 HPa) formed over South Greenland in the same period, shifted in SE-direction and on January 1, its center was W-ly of Reykjavik while its air pressure increased to 1035 HPa. In the course of that day, 12:00 UTC, the depression moved from the Vestfjord with its center (979 HPa) in SE direction over the Gulf of Bothnia, while the high pressure area with its center shifted in SE-ly direction via the Faroe Islands and thereby further increased its air pressure (1042 HPa).

In the afternoon and evening hours of January 1, both systems continued to relocate to the SE. On January 2, 00:00 UTC, the depression system had its center over the Baltic (Latvia / Estonia) and the high-pressure area had its center (1044 HPa) over NW Scotland (Outer Hebrides / Isl. Of Skye), where the depression began to slowly fill up. Until January 2 06:00 UTC, it could be seen that both pressure systems remained almost stationary, whereby the depression system continued to fill up (990 HPa) and thus a noticeable reduction of the wind forces occurred.

Such fast-moving depressions are not unusual in the winter season in the northern hemisphere. They are able to reach enormous wind speeds, Hurricane Force, due to extreme air pressure opposites. A kind of chimney effect develops by the opposite rotation of both systems. In addition, in the North Atlantic, the Norwegian mountain masses form a kind of wall and strengthen this effect, because the isobars are further compressed.

Wind condition analysis

During the period in question, over the North Atlantic between Iceland and Norway, winds from NW up to N prevailed, with winds speed of about 40 to 60 knots (8-11 Bft), and gusts with hurricane force (12 Bft). Wind speed is based on rough calculation of the distance between the 4 HPa Isobars. In the central North Sea, on the South Norwegian coast and the Skagerrak, in the period from January 1, 00:00 UTC until January 2, 00:00 UTC, wind from NW up to N with wind speed from 40 to 55 kt (8-10 Bft) prevailed.

In the area of the southern North Sea, the Dutch and German Frisian Islands, from December 31, 12:00 UTC, the surface analysis showed wind from WSW with 10-15 kt (3-4 Bft). Later it becomes apparent from the weather charts that the wind is from W-ly direction, with wind speed increasing 16 to 25 kt (5-6 Bft). After the passage of the cold front over the Frisian islands on January 1 at about 06:00 UTC, the wind turned noticeably towards NW, later N and increased significantly to wind speed from 35 to 45 kt (8-9 Bft) and in gusts up to 50 kt (10 Bf). The climax can be read from the weather charts at January 1 12:00 UTC to January 2 00:00 UTC, then a waning of the wind began.

Sea state analysis

The wave heights in the area of the Frisian Islands were reported up to 10 meters. Under consideration of wind fetch, wind direction, wind duration of effect, prevailing water depth in the area of the Dutch Frisian Islands and the Borkum Reef is difficult to envisage. The average water depth of TSS Terschelling - German Bight is between 17 and 26 meters. It should also be noted that the striking 40-meter depth line runs approximately 40 - 50 nm northerly of the TSS and Borkum reef (N-tip of the new wind farm "Veja Mate"), which equates to a natural breakwater and thus the wave length from the approaching waves from the North Atlantic will reduce.

With the concomitant reduction of the wavelength, the wave period is also reduced, which in turn can be expressed in steeper waves in the transition to the 40-meter depth line. However, that is only a limited effect, which should not have a major impact on the TSS Terschelling. From my experience, I assume that the wave height by the NW up to N swell was not more than eight meters, with a wave period of 8-11 seconds. In the area of the accident, eight seconds is realistic due to the above water depths. In addition, it is difficult to estimate the wave height in the dark. Whereas eight meter waves are always high enough to develop resonances.

These wind and sea conditions are not really a major problem for ships of this size. Wind speed of up to 40 kt and related swell fields with wave heights of 6 -10 meters are not an uncommon rarity, especially in the winter and transitional months on the North Atlantic. They are by no means pleasant, but manageable, if you follow the basic principles and safety criteria for going to sea in heavy weather.

Certainly, voices will now come up that say that wave heights of 25 meters and more can occur in the North Sea. This is not denied, but it requires several factors: longer-lasting storms in storm / hurricane force, several superimposed waves from different directions, long wind fetch, greater water depths, strong ocean currents. They are the prerequisite for the now undoubtedly recognized and by Prof. Alfred Osborne (2010) scientifically proven phenomenon about nonlinear wave systems, based on the Schrödinger equation from quantum mechanics. This is now considered as one of the main prerequisites for the formation of freak waves. Until then, the assumption had always been that there are only linear wave systems. All these factors can be excluded with great certainty for the sea area of the Dutch Frisian Islands and the TSS Terschelling - German Bight that night.

 The passage along the coast of The Netherlands via TSS Off Vlieland with a true course of 024° includes a course alteration by 50° to the eastbound lane of TSS Terschelling to a true course of 073° until Borkum Riff, from Borkum Riff, then 076° to buoy Jade Weser. From the course in TSS Terschelling it becomes apparent that for MSC Zoe the sea initially came from WNW, later NW to N direction that means from portside abaft beam to port beam of the ship.

This constellation is interesting in that it can cause resonance phenomena especially in stern quarter seas and beam seas, which means that the period of encounter cycles between sea and ship coincides with its own rolling period.

 Resonance phenomena and their effects

In the literature, a distinction is made between simple (1:1 resonance) and double-natural roll period (2:1 resonance). In the 2:1 resonance a greater danger comes since each roll period coincides with two pitch periods, which has the effect that the wave crest is always at the main frame line amidship, when the ship is floating upright. Due to the resulting loss of stability, the ship has the desire to roll immediately to one side. When the maximum roll angle is reached, the wave is at the front and aft end of the ship and the ship will be uprighted very quickly. The background to this is the fact that in longitudinal waves all ships based on their shape have their lowest stability on the wave crest and their greatest stability in the wave trough. If the sea is now irregular, extreme roll angles are possible within a short time. Thereby the wave-induced lever arm fluctuations between wave trough and wave crest act as essential influencing factors.

Basically, it is necessary that the lever arm variability must take critical values in order to achieve the effects described. This is the case when the wavelength equals 0.7 -2.0 times the ship's length (the data in the technical literature varies in values), with the shorter wavelengths often representing the more dangerous wavelengths.

Behavior of container ships in the stern quarterly sea

According to present knowledge, shipbuilding experts and scientists take the view that a 2:1 resonance when riding with the stern sea can only occur at "relatively low speeds" and low stability. This is partly reflected in extreme roll angles. It is important to know that in the stern-sea state the ship's natural roll period is subject to great fluctuations, because the lever arms acting in calm water can no be used; the wave crest and wave trough are levers only.

The consequence is that the natural roll period adjusts to the respective excitation, the more the stability changes between waves crest and wave trough. Special attention must be paid to cases where negative stability occurs on the wave crest. 

Unfortunately, the term "relatively low speed" is not defined further. From practical experience in extreme weather conditions and sea conditions, especially in seas from aft, I would consider there the range of six to 12 knots as realistic, depending on how fast the sea from aft is moving, which would be assigned as definition for "relatively low speed." But it can certainly be discussed. I emphasize once again that I'm particularly referencing my background in container shipping and especially Post Panmax container ships.

It follows, also confirmed by practical experience on board, that in the irregular aft seas no sharp resonance prevails, in contrast to regular sea state. It has been repeatedly found at sea that there are areas with courses or speeds where large roll angles can occur. The dimension of this areas increases with decreasing stability on the wave crest, because, as previously stated, then the ship increasingly tends to modulate its roll behavior to the sea state excitation.

Parametric Rolling

The term "Parametric Rolling" is often used, but it should be noted that to many nautical officers, the meaning of this term and especially the causes and characteristics of parametric rolling are not known. This means that in such situations wrong decisions may not be excluded which can cause loss of cargo, damage to the ship or even total loss of the ship. So here is an explanation of what "Parametric Rolling" means. I rely on Prof. Dr.-Ing. Stefan Krüger from the Hamburg University of Technology, Institute for Ship Design and Ship Safety.

Prof. Dr.-Ing. Stefan Krüger has it in his interesting article:

Zur Frage des Erkennens von gefährlich großen Rollwinkeln im praktischen Bordbetrieb (On the question of recognizing dangerously large roll angles in practical on-board operation)

It is also explained to non-professionals very understandable and I would like to quote him:

"In heavy seas, ships are essentially at risk when the sea comes from the front or aft. These lies essential (but not alone) at the periodically in sea changing lever arms. The ship will thereby not directly excitated to roll motions (how e.g. in beam-sea), but indirectly over the periodically changing lever arms. However, it is important to note that in addition to the parametric excitation, always a direct sea excitation through in the ship introduced sea moments take place, which be superimposed on the parametric excitation. Only in the case of the ship traveling exactly in the longitudinal direction of regular waves, the direct sea excitation is not present. 

“Put simply, the direct sea excitation changes the current width center of gravity of the displacement and thus directly introduces a moment into the ship, whereas the parametric component changes the height center of gravity of the displacement and therefore does not directly introduce a moment into the ship, but rather via a parameter. Both effects are inherently forced vibrations, and in practice it depends on which effect is dominant in which situation. "

(Source: Prof Dr. Ing. Stefan Krüger, Hamburg University of Technology, Institute for Ship Design and Ship Safety in Hansa 2007: Zur Frage des Erkennens von gefährlich großen Rollwinkeln im praktischen Bordbetrieb)

It must also be pointed out that "parametric rolling" is a very special behavior of container ships, almost exclusively of container ships.

It has been known since the 1990s that container ships have a special feature due to their underwater design, which is unique compared to any other type of ship in this dimension. This refers to the phenomenon of parametric rolling. Container ships were optimized in their underwater ship design in that they were geared primarily for speed. Especially the end of the 90s and in the 2000s the initiated competition, who crossed the ocean the fastest, reached the next port first, was a trademark of container shipping for a long time.

Speed and maximum payload resulted in an underwater design which was very slim to minimize water resistance and a hull design that was in the width outer drawn. This made it possible to create large cargo storage possibilities in holds and on deck. Slim long drawn bulb bow designs should additionally optimize the fast forward movement. While stern sections above the waterline are designed as mirror and wide overhanging side walls to maximize storage capacity in holds and on deck, so is the design of the underwater stern section very slim. Such an extreme design we find only on container vessels and it explains why with head / stern sea this type has such roll/ pitch motion problems in heavy sea. 

Even in the technology euphoria of increasing gigantism in container shipping, voices from engineers and scientists rose who demonstrated in their shipbuilding research institutes, simulation channels and maritime research facilities that these ships pose a risk that should not be underestimated, the risk of parametric rolling.

The first time I heard about parametric rolling, I felt like many others, I had no idea what that meant. One reason to deal with it is because it frames a very important, essential basis for working as a nautical officer aboard container ships to correctly assess ship behavior on the basis of existing stability criteria and the associated risks in bad weather under the influence of heavy seas and associated decisions to avoid extreme situations.

In technical scientific publications there are numerous graphics, e.g. polar diagrams from tests and simulations to sea effects under different stability states in ship technical test plants and simulation centers. They show the significant effects of wave height at different wavelengths, on container ships with small and large GM, different speeds, different seaward angles of inclination to the ship on rolling motions and roll angles and which resonances come. For this reason, I abstain from making representations of polar diagrams, which are usually used for this purpose, and give references to relevant engineering research and articles.

 Conclusions

The facts I have described, at least in broad terms, show that in the case of MSC Zoe, when cruising with wind and waves abaft beam at low/ medium speed and wave heights of up to eight meters and an angel of encounter from port 0° to 90°, there is a real danger the ship may be exposed to extreme roll angles, which lead to enormous centrifugal forces, especially in the high container tiers on deck. This may lead to container damage or loss of containers caused by broken lashing rods / turnbuckle or twistlocks when the effecting forces exceed many times over the safety force.

The maximum transverse accelerations acting on a container are usually introduced when the vessel is at an extreme roll angle. Maximum rolling normally occurs when the vessel encounters heavy beam or quartering stern seas.

 Effects by large roll angle and fast uprighting moment on the forces acting

Newton's law: F = m * a, is also valid in seafaring. The acceleration "a" defined as a speed change in a certain time interval illustrates which enormous forces can act at large roll angles and quick-uprighting moment in a short time interval. It is interesting to consider the effective weight force G, which is calculated from G = m * g, the mass of the object and the acceleration due to gravity (g = 9.81 m/s2 = 1 g). This can be explained by the example of a 20t container. If a 20t container has a weight force G in 1 "g" of about 20 tons and if now "g" will multiplied five times, this results in a weight of 100 tons. Since this calculation must be taken into account for each individual container, it becomes apparent which weight forces in a row, with eight tiers are acting. 160t become 800t weight. This notion is necessary to understand how tremendous powers multiply when large roll angles and quick upright moments coincide. The consequence: The load securing, is no longer able to absorb these forces and the lashing material is literally torn apart. 

No man on land has any idea of what incredible power the sea can unleash. Especially considering that today's VLCS / ULCS are stiff ships due to their width, which means that they have a high stability, a big GM, hence they have a strong uprighting moment. To reduce the stability effect, heavy containers are therefore not only stowed in the holds, but also on deck, even at higher altitudes, to lift the center of gravity and thus to reduce the GM. If we talk about lifting forces, compression forces and tension forces in container shipping, then experienced loading officers and captains know what forces they talking about that can be in the multiple G range. If they do occur, you have no way of effectively encountering them. Only course and speed alterations are helpful measures in such cases.

But that's only half truth. Improper load securing, i.e. not lashed or insufficiently tightening lashing rods and turnbuckles, not placing lashing rods according Cargo Securing Manual corresponding, not closing twistlocks, invite themselves to be destroyed in extreme situations and thus lead to container damage or losses and to damage to ships too.

Further details are available here.