In the last issue of Bulk Carrier Update we looked into the general theory of steel coil loading and the first two questions put forward by Capt. Isbester in Seaways magazine. In this issue, we will continue with the questions and give some additional information about the DNV services relating to coil loading.
Question 3
What guidelines can be offered to a ship with a steel coil loading condition in her loading manual when the coils on offer:
a) are different in size from those specified?
If the loading manual includes a table of coils that may be carried by the vessel this should not cause any problems. The table gives a range of weights and lengths and also gives guidance as to the number of dunnage to be applied.
See Figure 1.
b) come in two or more different sizes?
This complicates matters somewhat. However, there is no problem provided we maintain the same dunnage positions on the tank top and do not increase the load on the individual dunnages. This will be illustrated below by two simplified examples.
Case 1, Coils of different weights:
Two tiers of 15t coils are approved in the loading manual, with a length of 1.5m and three dunnage per coil (loading is 10t per dunnage).
Hence one tier of 15t coils topped by two tiers of 7.5t coils may be carried, provided the length of the coils is the same and the number of dunnage is the same (loading will still be 10t per dunnage). See Figure 2.
Case 2, Coils of different sizes:
Two tiers of 15t coils are approved in the loading manual, with a length of 1.5m and three dunnages per coil (loading is 10t per dunnage).
Hence you could also load one tier of 15t coils with a length of 1.5m topped by one tier of 0.75m coils of 7.5t, i.e. there will be twice as many coils in the second tier (loading per dunnage will still be 10t).
These examples are very simple and in reality it will be a bit more complicated but the same reasoning will apply. Start with an approved arrangement, maintain the dunnage arrangement and make sure that the load per dunnage is the same or less than the approved arrangement.
As an option, a “maximum total mass of coils” table could be supplied to the ship. The diagram in figure 3 shows the allowable combined mass of all tiers of coil based on the size of the bottom coil and may thus be used as an option to the table shown in figure 1.
See Figure 3.
To compute the weight of a given arrangement, a simple spreadsheet is available for use onboard. This spreadsheet uses the same basic principles as those shown in figure 2 but handles more specialised arrangements.
Question 4
What factors govern the maximum tonnage of steel coils which can be loaded by a ship which is not provided with steel coil condition in her loading manual? Are the rules complex or can they be stated simply for the benefit of ships’ officers?
The coil loading capability is, as mentioned earlier, governed by:
• Inner bottom scantlings (plates and longitudinals)
• Coil weight and size
• Number of dunnage per coil
• Number of tiers
• Stress level in inner bottom (caused by hull girder and double bottom bending)
The rules are not very complex, as described in the last issue of Bulk Carrier Update, but the input data is not readily available to the ship’s officers.
DNV recommends including allowable loading tables, as shown in figures 1 or 3, as an addendum to the loading manual in order to avoid discussions and/or speculation.
Question 5
Does a ship which has no steel coil loading condition in her loading manual and which calculates maximum tonnage per hold on the basis of: [area of tank top (m2) x permissible loading tonnes per m2] remain within class?
No, this is not the correct way of doing this and may easily overstress the inner bottom structure. Whilst this is the correct calculation when dealing with homogeneous bulk cargoes which exert a uniform load on the tanktop, it is incorrect for coils because the load which they exert is not uniform. The crossing point of the coils and the dunnage will give rise to a point/patch load on the double bottom structure. This loading is very different from the uniform loading and will give rise to higher stresses in the tanktop plating as well as the inner bottom longitudinals. As mentioned earlier, DNV has calculation tools to compute loading tables so that this type of simplification can be avoided.
This concludes our description of the strength matters involved in steel coil loading. In future issues of Bulk Carrier Update we will return to this topic from a more operational point of view.
Anders.Gustafson.Swerke@dnv.com
Date: 12 February 2008