Saturday, 4 October 2014

MOL Comfort- What Happened? (Part 1)

On 17th June 2013, MOL Comfort, a post Panamax container ship owned by Mitsui OSK Lines suffered a crack amidships and split off into two off the coast of Mumbai. It turned out to be one of the biggest structural failures in the history of container shipping, and the reason behind the same remained unknown unless a dedicated investigation was carried out into the matter by Class NK, who recently released their final investigation report on the case. 

This provides a great chance for designers to make reconsiderations and learn ship structure design from a different point of view. This series of articles are a detailed analysis on the investigation carried out by Class NK. The aim of launching this series is to give an interactive and informative insight into the entire investigation which fosters a better way of learning structural design than just what books and professors can do. It is also assumed that you have a basic touch up on ship structures before you read on.

The fracture generated from the bottom shell of the double bottom structure of the ship. The condition shown in Figure 1 is one of a later stage when the crack had propagated above the waterline. But the origination of the crack was investigated and found to be at 200 mm fore of Frame 151 (as shown in Figure 2), where there was a butt weld (remember this throughout the entire analysis). Also, the entire analysis was carried out on the double bottom structure at the hold corresponding to the area of crack generation and half of each hold fore and aft of the hold where the crack generated.

Fig. 1: The crack originated in the bottom shell and propagated above the waterline.
(Image Courtesy: Google Images)

Fig. 2: Actual position of generation of crack.
(Image Courtesy: Class NK)
The load on a ship not only depends on its cargo loading conditions but also on the environmental factors, which we collectively call as Sea State. The cargo loading conditions were obtained from the shippers (weight of each container along with the container loading plan). However, it can never be affirmed that the data obtained was correct, given the fact that many shippers practice in overloading the containers, which is often not a design standard in terms of the ship's strength. 

Estimation of Wave-Induced Loads (Considering Uncertainties)

The environmental conditions (wave induced loads) during the accident were recorded. But as ships always operate in an environment of periodically varying parameters, it is a common practice to consider certain deviations in the recorded data for a wider analysis. Class NK has pretty much done the same in their investigation. Considering certain deviations in the parameters of wave and wind induced loads, Class NK proceeded with the following data as shown in Table 1.

Table 1: Deviated values of wave induced load parameters during the accident.
(Image Courtesy: Class NK)

But how did they get to these values? Based on the recorded data during the accident, the sea state and load response on the ship was simulated probabilistically for 27 different scenarios. Since each load condition would subject the hull to a specific wave-induced vertical bending moment, a probability distribution was plotted against the possible values of the wave induced vertical bending moment and the frequency of occurrence of each, as shown in Figure 3. For each of the 27 conditions, 1000 waves (short term sea state) were considered for the simulation. Note how the frequency of occurrence of extreme maximum and minimum bending moment are lower than that of the occurrence of an averagely medium value.

Fig. 3: Frequency of occurrence of various wave induced vertical bending moments.
(Image Courtesy: Class NK)
For a better understanding, Table 4 shows the obtained parameters when the wave induced vertical bending moments were maximum and minimum. This is how the wave load parameters were considered with estimated deviations for more accuracy.

Table 2: Wave induced load parameters in case of maximum and minimum wave induced vertical bending moments.
(Image Courtesy: Class NK)

Uncertainty in Strength

Fig. 4: Factors that definitely affect the strength of the double bottom structure.

Fig. 5: Factors affecting uncertainty in strength on the structure.

What's in Part 2?

In Part 2 of this series, we will see how these uncertainties were used to estimate the strength of the double bottom structure probabilistically, rather than deterministically. The importance of probabilistic determination lies in the fact that ship structural failures may not occur in load scenarios which occur very frequently. It is the less frequent but most adverse conditions that lead to such failures. This makes it necessary to analyse the failure from a probabilistic point of view.LSD

Article By: Soumya Chakraborty