Many masters/operators have discovered a conflict between the damage stability requirements (SOLAS Ch II-1 Reg.25) and the need to have a reasonable GoM value for the securing of the cargo (avoiding excessive transverse accelerations). So the question one has to ask is whether this is a problem related to contradictory regulations, a design problem or not a problem at all!

To illustrate this, we will take a look at the limiting GoM diagram, as found in the Trim and Stability booklet, in Figure 1. (The main dimensions of the example vessels are given in Table 1.)
The green curve is the minimum GoM curve with respect to intact stability (IMO Res. A749). The blue line represents the minimum GoM curve with respect to damage stability. The damage stability is computed based on the so-called probabilistic method. This means that an attained index “A”, reflecting the ship’s capability of surviving collision damage, is computed and found to be above the required index “R” as stipulated by the regulations. The attained index is calculated by carrying out systematic damage stability calculations for two drafts, the partial and full loads stipulated by the rules, while the GoM values at these drafts may be chosen during the design work based on the intended loading conditions.
IMO resolution A.715(17) “Code of Safe Practice for Ships Carrying Timber Deck Cargoes” recommends, in relation to the carriage of timber on deck, that the GoM value should not exceed 3% of the vessel’s beam. This is based on the fact that higher GM values will give rise to high transverse accelerations and may thus cause a shift or even loss of the deck cargo. The red line in Figure 1 is positioned at 1m GoM, which represents 3% of a 31m beam.
Since the loading timber loading condition needs to be on the right-hand side of the blue “damage” curve and should preferably be to the left of the red “acceleration” curve, this means that the timber carrying “area” for this vessel is “limited” to the yellow triangle in Figure 1. The yellow dots represent typical timber conditions, as shown in the Trim and Stability booklet provided onboard this vessel by the yard , which shows much too high GoM values for a comfortable journey.
In practice, this means that this vessel may have problems with timber cargoes since it is unlikely that she can take a full deadweight cargo of timber and thus obtain this large draft. The solution to this problem could be to carry ballast water but, since most bulk carriers nowadays have connected double bottom and topwing tanks so that the GoM value would be increased at the same time as the draft was increased, this may be a problem. Hence, it is important at the newbuilding stage to ensure that the top wing tanks can be filled independently of the double bottom tanks in order to control the GoM when timber carriage is intended (thus increasing draft while reducing GoM).
The curve above could be adjusted for timber carriage during the design of the vessel by choosing more appropriate loading conditions as a basis for the damage stability calculations. This is illustrated in Figure 2 below, where the blue line represents the minimum GoM curve for a similar vessel.
The attained index A and hence the limiting GM values are mainly a function of the watertight subdivision, the watertight integrity in general and the position of openings such as air pipes, ie, improvements in the design directly influence the limiting GM values and thus provide a larger operational area for timber carriage.
The pink line in Figure 2 represents the minimum GoM curve where the IACS Unified Interpretation SC161 (IMO MSC/Circ.998) has been applied. This Unified Interpretation is based on the recognition that the timber load on deck will provide additional buoyancy even under damaged conditions. In short, this means that under certain conditions the buoyancy of the timber deck cargo may be taken into account when doing the damage stability calculations. Hence the pink GoM curve may only be used in relation to the carriage of timber and when loaded in accordance with the provisions of the Timber Code. This increases the area in the diagram where timber cargoes can be carried, as illustrated by the yellow triangle. Also, for this vessel we have added a few typical timber conditions from the loading manual.
So what can be done to improve the timber carrying capability of existing vessels? In some cases it might be possible to obtain more suitable limiting GoM curves by simple means, such as by altering the positions of the air-pipes. However, since it is quite costly to rebuild the vessel, the best option may be to add another GoM curve for timber carriage where the IACS UI SC161 has been taken into account.

For newbuildings there are more possibilities, such as:

1. Make sure that the shipyard includes timber deck loading conditions in the preliminary loading manual that fulfil the damage stability criteria and the 3% rule of thumb.
2. Check that these conditions are realistic.
3. Apply IACS Unified Interpretation SC161. When SC 161 is applied, the vessel may have two sets of GoM limiting curves onboard, e.g. one for when timber deck cargo is stowed as stipulated in SC 161 and the other for any other type of cargo.
4. Top wing tanks should preferably be separated from double bottom tanks for bulk carriers intended to carry timber deck cargo frequently, so that the GoM can be lowered.

Date: 12 February 2008