Impact of contemporary ship stability regulations on safety of shallow-draught inland container vessels
Introduction
Ships intended for international maritime trade are designed, built and operated in compliance with the rules of classification societies and the regulations established by International Maritime Organization (IMO). Seagoing ships are therefore subject to common safety standards valid across the globe. Unlike that, universal safety regulations for inland vessels presently do not exist, not even at the European level. Instead, a variety of technical regulations is imposed by the classification societies and national and regional administrations. As a consequence of different ship safety assessment methods and criteria contained in various rules, the certificates issued in one country may not be recognized in another state or region. For instance, inland vessels are not allowed to operate on the Rhine unless they comply with the technical standards set down by the Central Commission for Navigation on the Rhine (CCNR). Furthermore, each European country may impose its own regulations in certain navigation areas. Limited mutual recognition of regulatory bodies clearly hampers inland navigation in Europe. Therefore, there is an apparent need for harmonization of regulations for vessels operating on European inland waterways.
One of the milestones in the harmonization process is the Directive 2006/87/EC of the European Parliament and the Council (EC Directive, 2006). The Directive is to be adopted by EU Member States and transposed into national regulations. To a large extent, ship safety provisions of the Directive are exactly the same as in the rules of the Rhine Commission. This is in particular valid for ship stability regulations for inland container vessels. Obviously, as the inland waterway transport of containers mainly takes place on the Rhine, it is considered that the CCNR rules provide sufficient safety for this type of vessels. Nevertheless, present navigation conditions on the Danube are quite dissimilar from those on the Rhine, which affects the design and, consequently, the safety of the vessels, as it was demonstrated by Bačkalov et al. (2008). This study, therefore, focuses on implications for safety of inland container vessels intended for the Danube, if they are designed following the ship stability requirements of the Directive 2006/87/EC.
Instead of harmonization through common deterministic rules, the present paper promotes a novel, probabilistic approach to the safety of inland vessels. The proposed method follows the developments in the field of safety of seagoing ships as proposed by Bulian and Francescutto, 2004, Vassalos et al., 2004. Such an approach implies the estimation of the probability of a stability failure, based on the statistical analysis of roll motion of a vessel in realistic weather conditions. A vessel is considered to be safe if the probability of occurrence of a specified dangerous event does not exceed an adopted, required limit. So far, several studies that employed probabilistic analysis of inland vessel stability were carried out. The deficiencies of existing stability regulations were investigated by Hofman and Bačkalov, 2005, Hofman et al., 2006, Bačkalov et al., 2008, Hofman and Bačkalov, 2010. Additionally, Hofman and Bačkalov (2010) examined the risks related to transport of non-secured containers. The proper modeling of rolling of an inland container vessel exposed to stochastic wind was the subject of a paper by Bačkalov et al. (2010). A detailed overview of various stability regulations for inland freight and passenger vessels, presently in use in Europe, was given by Bačkalov (2013).
It should be noted that, in general, the research on safety of inland vessels is scarce and, for the most part, not published internationally. For instance, Ribar (1963) prepared a very comprehensive study on the stability of inland vessels, which also included a detailed overview of the wind statistics on the Middle Danube. Therefore, although rather old, this study still represents an important reference in the field. Egorov (2009) presented a risk-based approach to design of vessels intended for restricted navigation, with particular focus, however, on ship structure and strength related issues. Efremov (2011) made an extensive comparison between the rules of the Russian River Register and regulations used in Western Europe. Outside of Europe, Iqbal et al., 2008a, Iqbal et al., 2008b analyzed the safety of inland passenger vessels in Bangladesh and outlined a number of shortcomings in present regulations.
Section snippets
An insight into the navigation zone concept
Stability requirements for a particular type of inland vessel depend on the navigation zone, defined by the regulations. In general, European inland waterways are divided in three zones that are usually related to wave heights that may occur on the waterway. Most of the regulations indicate 2 m, 1.2 m and 0.6 m as limiting wave heights that correspond to navigation zones 1, 2 and 3, respectively. The only exception is the Directive 2006/87/EC that does not link the area of navigation to the waves.
Sample vessel
The investigation was carried out on a shallow-draught container vessel that would be suitable for navigation on the Danube, in particular on its Upper and Lower parts that are characterized by numerous shallow-water sectors. The hull form of the vessel, as well as her length and breadth (Table 2), is typical for European inland vessels of CEMT class Va. The vessel has a retractable wheelhouse and a single, box-shaped cargo hold without hatch covers. The body plan of the vessel is given in Fig.
Probabilistic approach to safety of inland vessels
In the previous section, the overview of environmental conditions used in the present regulations was made with an aim to emphasize the differences between existing rules. It was also shown that such disparities result in considerable discrepancies in stability evaluation. Therefore, an attempt is made to propose a different safety assessment procedure, based on a probabilistic analysis of relevant stability failures.
Concluding remarks
In the present regulations, safety requirements for inland vessels depend on the intended area of navigation. However, the concept of navigation zone itself is rather incoherent. The navigation zone is determined by the relevant wave height on the waterway, yet different statistical definitions of wave heights are used throughout the regulations. Furthermore, the weather conditions that are to be used in stability calculations may considerably differ even if they correspond to the same
Acknowledgments
The paper is part of the project “Development of Next Generation of Safe, Efficient, Ecological (SE-ECO) Ships” executed by Department of Naval Architecture, Faculty of Mechanical Engineering University of Belgrade. The project is partly financed by Serbian Ministry of Education, Science and Technology Development, Contract No. TR35009.
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