One cost factor is certainly cargo securing which must always be designed in such a way that it offers more restraining force than the expected accelerations. If lower accelerations are applied, it is possible to compensate these lower values with less cargo securing equipment. Less material on the one hand means reduced costs for purchasing of the necessary material, but above all it saves time for the shipping company. The fitting of additional cargo securing material takes more time and results in higher labour costs and port fees. If the ship acceleration values are calculated according to the methods of a classification society, the acceleration values are generally lower than if they are calculated according to the values used in the CSS Code. As a result, some shipping companies prefer to have their ships certified with a classification society whose conditions of certification match the tight pre-calculated cost framework, or, in some cases, allow other standards of equipment, whereas others use other methods of calculation as a basis.
The definition of the regulatory requirements themselves provides the incentive for such deviations: The (not perfect, but nevertheless helpful) CSS Code has thus far always assumed a calculated worst case scenario. This does not result in an excessive level of safety, but at least one that covers a wide range of margins.
The analysis method propagated by some parties in this business sector is based on measured ship values or computer models. These, in turn, take into account a vessel’s command that always acts befitting of the circumstances, i.e., avoids sailing into a storm at all if possible, or acts completely freely in the choice of course and speed of the vessel without time constraints or deadlines.
It is obvious, which of the two approaches is advantageous for the individual parties. However, the method which is ultimately the better approach from the point of view of the overall economy, the insurance industry or environmental protection is a different matter.
4
Can calculation models cover the situations that occur in practice with sufficient accuracy?
If we look at the theoretical calculation models, we come up against a number of restrictive marginal conditions:
4.1
Vertical accelerations (common approach so far)
The CSS Code states that vertical accelerations, for example, are only to be taken into account in combination with longitudinal accelerations. (The basic transverse acceleration parameters are already designed in such a way that they take into account the corresponding vertical accelerations).
From practical experience, however, it is known that there are sometimes considerable vertical accelerations with simultaneous strong roll angles. If these are not taken into account separately and included in other considerations, they can have considerable effects.
Additional vertical accelerations reduce the stability of components at risk of tipping. At the same time, frictional forces are reduced, which in turn means that other measures against slipping have to withstand stronger forces.
4.2 Vessel speeds
Vessel speeds undoubtedly have a considerable effect on the transverse acceleration of the vessel. In the formulas of some classification societies for calculating transverse acceleration, it is therefore a marginal condition that a certain ratio of speed to length of the vessel is maintained. Some calculation methods are only comprehensible to the extent that the calculated “minimum speeds” must be regarded as obligatory rather than as a marginal condition for the applicability of the calculation methods.
However, it may be questioned as to whether the vessel will consistently steam at the stated minimum speeds later on. For example, prior to the economic crisis from 2007 to 2010, ships sailed considerably faster. In the course of and after the economic crisis, the actual speed of the vessels was considerably reduced. According to the MSC Zoe investigation report, the ship was travelling at a speed of between 8kn and 10kn at the time of the most violent movements. According to press reports (www.
gcaptain.com), the APL England was travelling at a speed of approximately 7kn at the time of a recent incident in which it lost containers. According to a maritime tracking service during a recent incident in the Pacific, a vessel was steaming at a speed of approximately 12kn.
It can be seen from these two examples that international shipping no longer occurs at speeds of more than 20 knots, as was the case before 2007. It begs the question as to whether the reduction in speed ought to lead to corrections to the calculation of the transverse acceleration of the vessel.
56 | The Report • June 2021 • Issue 96
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