There seems to be some confusion on how to calculate or estimate the possible cargo intake with respect to coil loading. When looking into the literature or talking to operators you will get varying answers, and the methods applied are often based on general assumptions and experience rather than the physical characteristics of steel coil loading.

In May this year, an article by Capt. J Isbester was published in Seaways. This article highlighted a number of unanswered questions with respect to steel coil loading. In a series of articles, we will look more closely into the matter of steel coil loading and give answers to these questions from a DNV point of view.
When discussing the matter of steel coil loading, we need to look at three separate strength matters (as in all types of loading) – longitudinal strength, double bottom strength and local strength of inner bottom.

Longitudinal strength
For vessels where flooding calculations are required according to IACS UR S17, the coil loading will give rise to very large global bending moments. This is due to the fact that, since the coils occupy a relatively small portion of the cargo hold volume, there is more space left for the water, thus giving a very large combined mass in the hold. It is imperative that the loading computer can handle this type of calculations automatically and in addition the loading manual should include load cases with coil loading.

Double bottom strength
The double bottom strength is governed by the cargo hold loading diagrams (for newer ships also incorporated in the loading computer, ref. UR S1A.2.2 a & b) and reflects the strength of the double bottom structure (i.e. bending of the bottom structure between the bulkheads). These local load diagrams should correspond to the allowable tank top load (t/m2), hence at full draft the allowable mass according to the load diagrams should give the allowable tank top loading. There is no special consideration with respect to coil loading.

Local inner bottom strength
The crossing point of the coils and the dunnage will give rise to a point/patch load on the double bottom structure, see figures 1 and 2. This loading is very different from the uniform loading and will give rise to higher stresses in the tanktop plating as well as in the inner bottom longitudinals. This is similar to the effect experienced with wheel loading on RoRo decks. The local inner bottom strength is often a limiting factor for coil loading.
Due to this, alternative arrangements have been proposed to avoid exerting load on the inner bottom. One such arrangement is to position dunnage directly on top of the floors. On these dunnages, steel billets are placed in the longitudinal direction (provided these are long enough to span between the floors) and on top of these coils of any length may be positioned. The steel billets will act as a “loading platform” for the coils and carry the load directly to the floors. Hence no force will be exerted on the inner bottom plating and longitudinals between the floors.
So then what determines the allowable coil loading with respect to local inner bottom strength?
This is a combination of the coil weight, coil length, number of tiers, number of dunnages per coil and the arrangement of the inner bottom structure.
For many years, class has based the approval of steel coil loading on the Japanese standard (the 166th Committee of the Shipbuilding Research Association of Japan, March 1977).
Recently, some class societies have adjusted this procedure to fit their rule format and included it into their rules. Below you will find a brief description of the DNV calculation procedure.
The following assumptions have been made:
• Longitudinal stiffening is assumed with the coils stowed with the axis in the longitudinal direction.
• The distance between two adjacent steel coils is a fraction (Cs) of the steel coil length, 0.2 in general, however Cslc is not to be taken as larger than 0.3m, see figure 3.
• The dunnage is placed equidistantly within one coil at a distance sd, see figure 3.
• The distance between the end of the steel coil and the nearest dunnage is half of the distance between two adjacent dunnages.

Inner bottom longitudinals
For a given number of patch loads and the assumptions concerning spacing of coils and dunnage, we can compute the bending moment of the longitudinals based on simple beam theory.
Based on the bending moment and the allowable stress, the required section modulus of the longitudinal can be calculated.
The allowable stress takes into account the effects of hull girder bending and double bottom bending.
As long as the inner bottom plating and longitudinals fulfil the requirements above, the total mass in hold is according to the hold loading diagrams and the hull girder moments and shear forces are within allowable limits (including flooding) the steel coils can be carried.
Now that we have been through the theoretical parts of this topic, we will take a closer look at the questions put forward by Capt. Isbester.

Question 1
Can the tonnage specified for steel coils in the ship’s loading manual be exceeded in any circumstances (for example if sufficient specified dunnage is used)?
Yes, in special cases. The loading condition in the manual is for a given coil size, usually 15t x 2tiers, coil length 1.5m and 3 dunnage per coil. Since this is a “standard” size of coil, it is not certain that this coil is the maximum coil that can be carried. Hence DNV recommends that a table of allowable coil sizes and weights is included in the loading manual, see figure 7. (These diagrams will be discussed further in the next issue of Bulk Carrier Update.) These can quite easily be set up for ships in operation and should preferably be included in the specification for newbuildings. It should also be noted that this “standard” size coil is somewhat outdated, the widely shipped coil weighs 25t and has a length of around 1.2m.

Question 2
What dunnage is most effective in distributing the load from a cargo of coils uniformly over the tanktop?
It is not so much the type of dunnage used (steel or wood, neither will have any significant load spread) but rather the number of dunnages that sets the limit for the coil loading. The more dunnage applied, the more the loading will resemble a uniform loading.
In the next issue we will look further into the rest of Capt. Isbester’s questions and give some further information about the guidance DNV can give to ship operators with respect to coil loading.
To be continued in next issue.
Anders.Gustafson.Swerke@dnv.com

Date: 12 February 2008