TUNRA Bulk Solids’ engineer, Dr Priscilla Freire, looks at some current trends in maritime trade of dry bulk and typical issues encountered in the storage and handling of bulk solids.
According to the United Nations Conference on Trade and Development (UNCTAD) Handbook of Statistics , a total of 7.9 billion tons of dry bulk cargo were loaded and 7.7 billion tons were unloaded world-wide during 2019. In the same year, Australia occupied the 20th position in terms of number of port calls, with 66,076 calls.
As an island nation, ports are a central part of the Australia’s supply chain, responsible for 98 per cent of its trade. Commodities such as iron ore, coal, bauxite, aluminium oxide and concentrates in general are amongst the highest tonnage exports; however, dry bulk ports are also central to the country’s domestic activities, connecting the west and east coasts or the mainland and Tasmania.
Australia has long held the position of world’s largest iron ore exporter, with 828 million tonnes exported in 2017, followed by Brazil (approximately 420 Mt) and China (about 200 Mt) . With the majority of its iron ore originating in the Pilbara region, Port Hedland, Dampier and Cape Lambert occupy the highest position in iron ore exports in Australia. Similarly, some of the largest coal ports in the world are also located in Australia, including the Port of Newcastle, Port of Hay Point and Port of Gladstone, among several other ports of major international relevance, totalling about 390 million tonnes of coal being exported in the coal terminals in NSW and QLD in 2019-2020 .
Regarding the destination of Australia’s export, the Office of the Chief Economist  has reported that the five key destinations of Australian resources and energy-based exports in 2020-2021 were China (AUD 149 billion), Japan (AUD 34 billion), South Korea (AUD 23 billion), India (AUD 12 billion) and the UK (AUD 10 billion).
Current trends and typical handling challenges in ports
The storage, handling and transportation of bulk solid materials are major activities for a variety of industries not only in Australia but worldwide, and efficient bulk solids handling operations are of vital importance in the supply chain. This becomes even more evident in ports, where the larger throughputs and number of materials often present unique challenges. From environmental and safety concerns such as dust emissions, risk of liquefaction, self-heating or explosions, to unplanned maintenance stops potentially leading to excessive demurrage costs, appropriate design of bulk handling equipment is the key to ensure operational efficiency.
The following trends have been observed in recent years in relation to international dry bulk trade, which naturally affect handling and storage operations :
– The average deadweight of dry bulk carriers is increasing: for Handymax vessels, it increased from 24,100 deadweight tons (dwt) in the 80s to 28,100 dwt in 2016. Panamax vessels increased from 67,800 dwt to 74,600 dwt in the same period, whilst Capesize and Ultra Large Capesize increased from 117,000 dwt to 196,000. This is not to mention the advent of the Valemax ships, which started being produced in 2011 and whose capacity ranges from 380,000 to 400,000 dwt.
– Further to the increase in average deadweight, there has also been a shift to using larger ships as opposed to smaller: in 1980, handy size vessels used to account for 54.3 per cent of bulk fleet capacity worldwide, whilst Capesize ships only accounted for 14.7 per cent. In 2000, these figures were close to 50 per cent, with an accelerated shift in carrying capacity in the past decade: In 2016, Handy size vessels accounted for approximately 17 per cent while Capesize corresponded to 38 per cent.
Such shifts mean that loading/unloading efficiency is growing in importance, as well as stockyard capacity and, in some cases, the increase in transhipment operations where ports do not have the infrastructure and size to handle large vessels. Furthermore, dry bulk typically has low value/volume ratio compared with other cargoes, such that the transport efficiency has great impact in the added value.
With more than 45 years of experience, TUNRA Bulk Solids has done its fair share of projects associated with bulk solids handling in ports, both in Australia and overseas. A recent example includes the redesign of a problematic shiploading spout/chute with excessive dust generation in a grain handling terminal: with the use of multiple analysis techniques such as the continuum mechanics approach and Discrete Element Modelling, combined with a prototype, the proposed modified design was based on minimising impact angle and maintaining dust encapsulated within a fast-moving material stream. The design was endorsed by operators, with visible dust reduction reported.
Another challenge often observed in ports is the design of transfer chutes able to efficiently handle multiple materials at very high throughputs. This is the case in several of Australia’s mining ports, which export ore or coal from various different mines, whose materials often behave very differently. In such cases, material characterisation, coupled with advanced engineering practices such as computational modelling, becomes even more essential for effective design.
Bringing the Industry Together
In December 2021, TUNRA Bulk Solids’ specialists and guest speakers presented a 1-day workshop for industry on the typical handling challenges encountered in ports. The event took place online and attracted over 40 delegates from Australia and internationally, including participants from Brazil, Peru, Canada and China.
The technical event started with a keynote presentation by Associate Professor Dingena Schott, from Delft University of Technology, where participants had the opportunity to hear about “Increasingly Sustainable Bulk Handling Operations”.
The research led by Dr Schott mostly focuses on machine-cargo interaction for logistics, with four main aspects being discussed:
1. Port accessibility: with a focus on more efficient material handling operations, increasing uptime (decreasing downtime) and overall reduction of time at port;
2. Energy Transition: new fuels and design of infrastructure, industry adapting to new energy carriers (both liquid and granular), non-fossil cargo fuels;
3. Process control and Design: industry 4.0 including Internet of Things and increasing inter connectivity at ports; system and equipment control; development of fast and accurate models (metamodels) for cargo behaviour; focus on higher degrees of autonomy;
4. Environmental sustainability: search for more energy-efficient systems; reduction in air, water, land and noise pollution; health and biodiversity.
Among several areas of interest, the research conducted by Dr Schott’s group includes the MAGPIE Project (sMArt Green Ports as Integrated Efficient multimodal hubs), which is coordinated by the Port of Rotterdam and involves 45 partners, including TU Delft. In 2021, the project was awarded nearly 25 million euros of funding by the European Union to be used in 10 pilot projects and demonstration projects focusing on sustainable and smart logistics in port operations, mainly in relation to the use of new fuels and energy carriers not yet tested in practice . Some examples include an electric battery-powered locomotive that uses power from an overhead line for both motive power and for recharging its battery, allowing it to work in areas that lack an overhead line such as marshalling yards, for instance. Other examples of projects include bunkering ammonia as a transport fuel, or electrical power from shore for ships moored offshore to a mooring buoy. The consortium will also aim to design and implement digitalisation and automation solutions in the context of the energy transition . Even though the primary focus of the MAGPIE research is on container terminals, it is believed that the outcomes of the research will also be of great relevance to dry bulk handling terminals worldwide.
Dr Schott’s presentation was followed by Dr Jayne O’Shea and Dr Dave Bradney, two of TUNRA Bulk Solids’ consulting engineers, who presented on material characterisation and dust respectively. Dr O’Shea introduced the typical flow properties characterisation techniques available with a focus on efficient design of materials handling facilities such as transfer chutes, bins, hoppers and belt conveyors, all of which of critical importance in dry bulk handling ports.
Dr Bradney’s presentation consisted of specific characterisation and modelling techniques with a focus on dust control. The topic is of special relevance given that port facilities are typically situated in close proximity to the general public, and exposure to dust is a risk both from a health and environmental point of view.
When it comes to dust management, active measures are commonly taken, especially with the use of water for dust minimisation. However, water use must be balanced in the context of sustainability, not to mention that excess water can also cause handling issues. This leads to the need for finding the right balance, as well as applying “smart design” techniques, which aim at minimising dust without having to increase water use.
It is recommended that material characterisation for dust start with particle size distribution, which reveals valuable information about the potential for dust generation. Furthermore, the risk to health is associated with the size and nature of the dust particles. Mechanical sieving is typically used to characterise particle sizes in the range of 100 mm to 45 µm, whereas laser diffraction can be used for powder-like materials, ranging from 0.1 to 1000 µm.
The size of the particle has a direct relationship with its terminal velocity (vt), as per the following equation:
Where g is acceleration due to gravity, рs and рa are the solid and air density respectively, d is the particle diameter (assuming spheric), CD is the drag resistance factor and n is the drag index, typically considered 2. By plotting the equation above, the following relationship between terminal velocity (vt) and particle diameter (d) is obtained:
The use of water or surfactants as dust suppression methods looks to increase the resistance of a particle to air, such that they bind smaller particles together, making them “heavier”. Further to this water also has surface tension which increases the resistive force. Given that water is a precious commodity that should be used with wisdom, the following question arises: how much water is enough? To answer this question, the Dust Extinction Moisture test can be performed, which aims to determine the dust/moisture relationship. Although the test was originally designed for Australian coal (AS 4156.6), it has been applied to other bulk commodities with adjustments in the material quantity used in the test.
Also of great relevance to the ports industry is the assessment of dust lift-off propensity across material surfaces such as stockpiles and train wagons. Such assessment can be made using wind tunnels, and is useful to assess the effectiveness of different methods of dust suppression such as water or surfactants. The dust lift-off test can also determine threshold wind speeds and interaction between wind speeds and spraying regimes for instance.
As a final point in Dr Bradney’s presentation, some principles of optimal chute design for dust control were presented, with a focus on passive dust control, such as minimising impact angles between the material and the chute walls, avoiding unconstrained or dispersed flow especially during free-fall, incorporating convergence and curvature of the material stream through deflectors and spoons wherever possible and maintaining streamlined flow with minimal impact losses to ensure dust particles are entrained in the stream.
After looking at moisture from a dust point of view, another topic of great relevance to the ports industry was presented by Shaun Reid, focusing on the effects of excessive moisture. Shaun introduced the concept of cargo liquefaction and the Transportable Moisture Limit (TML), as well as an overview of the International Maritime Organisation’s IMSBC Code (International Maritime Solid Bulk Cargoes Code) and material classifications .
The Code sets out requirements for shipping bulk cargoes both from an operational point of view and a safe shipping perspective, and classifies materials into three categories:
• Group A cargoes are materials that may liquefy if shipped at a moisture content in excess of the Transportable Moisture Limit;
• Group B cargoes possess a chemical hazard which could give rise to a dangerous situation on a ship; and
• Group C cargoes are neither liable to liquefaction nor chemical hazards.
It is worth noting that certain materials, such as bauxite, only present risk of liquefaction at specific particle sizes, such that characterising the material’s particle size distribution is often the first step in the testing process.
Liquefaction is a phenomenon that can happen to certain bulk materials after being loaded in granular form: in some cases, the induced stresses during shipping can cause the material to settle and compact, leading to liquefaction. This represents a hazard due to shifts in the centre of gravity of the liquefied cargo potentially leading to capsizing, with serious economic and environmental consequences, not to mention loss of lives. The Transportable Moisture Limit is then defined as the maximum moisture content at which a bulk cargo prone to liquefaction may be safely shipped, and, with several accidents occurring even in recent times, research on the topic is still ongoing.
It is worth noting that the liquefaction phenomenon is not restricted to shipping, but it may also be observed in other systems. More generally referred to as “moisture migration”, it can happen in train wagons and even on conveyor belts, and although the safety risks are not as impactful as in vessels, it still poses significant operational challenges, such that moisture control is of utmost importance along the handling chain.
Specifically at ports, a thorough understanding of the material’s behaviour in relation to moisture may be combined with other relevant variables to form a Moisture Management Plan. In these plans, the Dust Extinction Moisture is considered the lower bound, whereas the Transportable Moisture Limit is the upper bound which should never be exceeded. In addition to these, optimum moisture levels can be determined through flow properties testing, to ensure that the cargo is handled efficiently, minimising material hang-ups and blockages.
The final part of Shaun Reid’s presentation involved an introduction to the different methods for TML testing, which are conducted by certified labs and in accordance with specific guidelines. The IMSBC Code contains 3 broad categories of testing with sub-categories for specific materials, summarised in Table 1:
Progressing on the topic of safe handling of cargoes, Dr Peter Robinson, Research Associate with the Centre for Bulk Solids & Particulate Technologies (University of Newcastle), was invited to present on self-heating.
Self-heating materials generate heat through an exothermic reaction with atmospheric oxygen, consuming oxygen and typically producing carbon oxides. Some examples of self-heating materials include (not limited to): charcoal, linseed oil, biofuels, Direct Reduced Iron (DRI) and seed cake.
The risk a self-heating material presents is primarily dependent on three factors:
• Material properties such as activation energy, and the pre-exponential factor of the reaction,
• Volume of material, larger volumes will self heat at lower temperatures due to ineffective heat dissipation, and
• Storage temperature.
Additional factors such as sample age, moisture, packing density and atmospheric oxygen content also affect self-heating rate.
Dr Robinson provided an overview of the regulations used for self-heating cargoes, including the IMSBC Code and IMDG Code (International Maritime Dangerous Goods Code) for bulk maritime transport, both of which are fundamentally centred around the United Nations Manual for Tests and Criteria.
The IMSBC Code classifies a self-heating material as a Group B cargo, that is one that presents a chemical hazard. These cargos may be classified as a Dangerous Good (DG), which are typically classified in accordance with the IMDG Code, Materials Hazardous only in Bulk (MHB) or neither, and these classifications result in different conditions for transport. Materials classified as Dangerous Goods may be further classified into several sub-categories (packing groups), based on the proposed transport conditions for that material.
Group B cargoes are initially characterised using the United Nations N.4 test, which subjects a 100mm sample cube to a test temperature of 140oC. When measured at the centre of the sample, if the temperature of the material exceeds the oven by 60oC or greater, the material is classified as Class 4.2 – Dangerous Goods, and further testing specified in the UN Manual is required to determine the Packing Group (PG). If the material is not deemed as a Dangerous Good, further testing is required as specified in the IMSBC Code to determine its MHB status (if any).
The N.4 test is based on a kinetic scaling model developed by David Frank-Kamenetskii, and research into bulk carrier conditions by P. C. Bowes in the 1960s . For bulk carriers transporting charcoal (on which the N4 test is based), Bowes observed sustainable mean ambient temperatures as high as 38°C, with the temperature in the cargo space up to 10°C higher, due to radiation from the steel hold. At that time, charcoal was typically shipped in a 10-tonne stow, equating approximately to a 3m-sided-cube (27 m3). This results in the need for charcoal to present no self-heating potential in a 27m3 volume at 50°C. Frank Kamenetskii theory, in short, depicts similarities in the self-heating behaviour of a material, for different temperatures and volumes. Applying this model to charcoal, the self-heating behaviour of a 27 cubic meter stow of charcoal at 50°C is equivalent to a 100mm cube at 140°C, resulting in the N4 test.
Lastly, Dr Tim Donohue presented principles for adequate transfer chute design, and analysis tools such as the Continuum Mechanics approach and Discrete Element Modelling. Dr Donohue’s presentation included some applications of DEM, assumptions, and limitations.
Critical thought should be given to what problem is being analysed and whether DEM is the best tool available to replicate it, considering that DEM is a tool to build confidence regarding the design solutions being analysed. Dr Donohue proposed the following considerations when defining the “problem”:
• Can the correct mode of flow be captured, including internal shear and wall friction.
• What is the scale of the problem, including particle scale and geometry scale?
• What is the volumetric throughput?
• Under what circumstances does the specific behaviour occur? Considerations such as flow rates and wall friction are of relevance here.
• Are the effects of the problem time-dependant? If so, how much simulation time would be needed to capture the behaviour?
The presentation also included TUNRA’s 10 Commandments for Reliable Transfer Chute Design, and practical examples and design considerations.
To conclude the workshop, the final session, led by Dr Bin Chen and Dr Jens Plinke, included a series case studies with special focus on wet and sticky material, dust and wear issues.
The presentations have been recorded and are available on TUNRA Bulk Solids’ website.
 2020 Handbook of Statistics, United Nations, 2020.
 Maritime Transport Infrastructure Discussion Paper, Transport Australia Society, 2020.
 Department of Industry, Science, Energy and Resources, Commonwealth of Australia Resources and Energy Quarterly September 2021.
 Song, D. W., & Panayides, P. (Eds.). Maritime logistics: A guide to contemporary shipping and port management. Kogan Page Publishers, 2015.
 International Maritime Organisation, 2011. International Maritime Solid Bulk Cargoes Code.
 Bowes, P. C., The Marine Observer, 1969.