Elsevier

Powder Technology

Volume 326, 15 February 2018, Pages 255-264
Powder Technology

Modelling the stability of iron ore bulk cargoes during marine transport

https://doi.org/10.1016/j.powtec.2017.12.006Get rights and content

Highlights

  • Bulk solids flow property theory can model iron ore cargo slip in maritime transport.

  • Cargo slip prediction with flow properties theory is close to soil mechanics theory.

  • Cargo slip model results from DEM is close to theoretical results.

  • Precaution measures are required to prevent instabilities from cargo slip.

Abstract

The safe maritime transport of bulk commodities, such as iron ore, by large bulk carriers is vitally dependent on the stability of the cargo. During transport there is a propensity that cargo shift may be triggered under the vessel's rolling motion. The study presented in this paper aims to model the critical stress conditions within iron ore bulk cargoes from a bulk solids flow perspective, from which the maximum roll angle of the vessel prior to cargo slip can be predicted. Comparison of the new theoretical approach to the classic slope stability model was conducted with similar results achieved. The influence of the failed material mass after the cargo slip event on the overall cargo stability is then examined using the discrete element method. The new theoretical and numerical approaches provide a means to predict the stability and evaluate the potential safety hazards during maritime transport of iron ore bulk cargoes.

Introduction

A subject of particular importance to the resources industry concerns the safe trans-oceanic transport of large tonnages of iron ore [1]. It is most important that the stability of the loaded bulk cargo be guaranteed under all dynamic conditions due to the rolling and pitching motion of the vessel induced by waves. Historically, many vessels transporting iron ore bulk cargoes have listed or capsized, with cargo shift being the suspected cause [2], [3], [5], [6]. Therefore, safety precautions are urgently required during shipping the iron ore bulk materials. As shown in Fig. 1, there are two main failure modes that result in cargo shift, namely, liquefaction and cargo slip [7], [8].

Liquefaction occurs due to the cyclic motion of the ship and may lead to the loss of shear strength, and subsequent cargo shift [4], [9]. Liquefaction of an iron ore bulk cargo is a process where the bulk material flows in a manner resembling a liquid under the monotonic or cyclic ship motion. Under the regulation of the International Maritime Solid Bulk Cargo Code (IMSBC Code) [10], a Transportable Moisture Limit (TML) test shall be conducted on all eligible iron ore fines commodities to determine the upper moisture threshold for safe maritime transport. If an iron ore cargo is eligible for a TML, the gross water content of the material on board the vessel must not exceed the TML value to eliminate the risk of liquefaction.

In comparison, cargo slip involves a portion of the surcharge zone within the stockpile, and it is often argued that it poses less danger to the stability of the vessel [11]. The failure mechanism is predominantly related to the material's stress state under dynamic ship motion [12]. The cargo slip phenomenon alters the vessel's metacentre leading to instability of the bulk carrier. Kirby [13] suggests a circular shear failure surface for cohesive iron ore fines on the basis of the classic slope stability theory in soil mechanics. Recent studies [14] have investigated the motion experienced by various vessel classes, which provides further details to better predict and assess the cargo stability.

Based on the forgoing comments, the study presented in this paper aims to address the cargo slip phenomenon during maritime transport of iron ore using analytical and numerical methods to predict cargo slip and assess post-failure stability.

Section snippets

Cargo slip modelling — bulk solids flow theory

Generally, an iron ore bulk cargo is constrained on five boundaries with only the top surface free. The load profile is dependent on the shape of the hold, the hatch arrangement and the degree of fill, while the surface of the bulk material is often rilled to form a surcharge angle (θs). It is assumed that the moisture content of the bulk material is below saturation level and, therefore, the bulk solid can be described as a Coulomb friction, cohesive material [15]. Fig. 2 shows typical

Cargo slip model — the slope stability theory

Kirby [13] utilised the classical theory for slope stability in soil mechanics to model the cargo slip phenomenon. Considering a cohesive iron ore cargo shown in Fig. 7 (a), modelling was performed by considering the effect of gravity to shift the cargo when it is at a rolling level. The material burden is deemed to fail along a circular surface when the gravitational force due to rolling exceeds the resistant shear force. Despite that this method and the flow property theory were both

Dem modelling of the cargo stability

Discrete element modelling (DEM) is an ideal tool to study cargo stability post slip by analysing the post failure material redistribution. The DEM code used in this study was LIGGGHTS [21]. The Hertz-Mindlin model (Cundall & Strack, [22]) was used to compute the particle-particle and particle-wall contacts. The contact force between two particles includes a normal force (Fn) component and a tangential force (Ft) component,F=Fn+FtF=knδnijγnυnij+ktδtijγtυtijwhere

  • Fn is the normal contact force,

  • F

Cargo slip modelling comparison

The cargo slip model using the bulk solids flow property theory and the slope stability theory (Kirby [13]) is compared based on the Handy size cargo geometry in the forgoing discussion. The iron ore sample with material properties shown in Fig. 2 and Fig. 4 are also utilised. For Kirby's model, the cargo geometry yields a slope length of 6 m for a nominal stockpile surcharge angle of 30°. Following the assumption that the maximum roll angle is 20°, comprising of 15° static roll angle and 5°

Conclusion

The study presented in this paper investigated the load stability of iron ore cargoes during maritime transport. Two analytical approaches and a numerical method were compared with the results yielding the following major conclusions:

  • The bulk solids flow property theory can be utilised to model cargo slip during maritime transport of iron ore.

  • The cargo slip predicted using the classic slope stability theory and bulk solids flow property theory provide similar results.

  • The cargo slip predicted

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