- The thesis report presents an overview of the cargo liquefaction problem which affects bulk carrier vessels.
- Liquefaction occurs due to contact forces between the individual solid particles are reduced, thereby softening and weakening the soil deposit.
- Liquefaction is produced by shaking forces or other rapid loadings in cargoes and soils, which contain a sufficient amount of water.
- The report suggests remedial factors, for instance, reduce the transport liquid mobility of the phenomenon, find methods and establish models that could solve the problem.
According to a thesis report submitted by the National Technical University of Athens, the project presents an overview of the cargo liquefaction problem which affects bulk carrier vessels.
What is cargo liquefaction?
Liquefaction is a rapid transition of the particles that form the cargo from a stable solid-state to a viscous fluid consistency. Through this phenomenon, the contact forces between the individual solid particles are reduced, thereby softening and weakening the soil deposit.
Therefore, the cargo will undergo a complete loss of strength and stiffness since there is no contact between particles. Liquefaction is produced by shaking forces or another rapid loading in cargoes and soils, which contain a sufficient amount of water.
Excess pore pressure
This phenomenon requires considerable excess pore pressure. Pore pressure refers to the pressure of groundwater held within a soil or rock, in gaps between particles. Excess pore pressure is defined as the difference between the actual pore pressure and the hydrostatic pressure for still water level. The contact forces are large when the void ratio i.e. the volume of voids per volume of solids, and subsequently the pore water pressure are low.
Energy input leads to gradual settling and compaction of the cargo into its optimum packing density with the volume of the cargo decreasing and the bulk density increasing. Inevitably, the gaps between the particles become smaller, reducing the available volume for air and water.
As a consequence, both the bulk density and the pore water pressure are rising. If the volume of the water is higher than the volume of the gaps, the pore water pressure will increase suddenly to a value equal to the applied confining pressure, and the particles will be pressed away from each other. In such cases, the cargo loses its strength owing to the particle’s loss of contact and behaves more like a liquid than a solid – hence, the name liquefaction.
Focus of the thesis
In particular, gaining a clear understanding of the problem and proposing early solutions, is the main focus of this thesis. The project is divided into several chapters approaching the phenomenon from the point of view of operation and design.
Ship safety compromised
The understanding of the nature of liquefaction as well as microscopic and macroscopic behavior of the material when liquefied, led us to identify the critical parameters that cause cargo liquefaction. The cargoes which are more susceptible to liquefaction are iron ore fines and nickel ore even though any granular cargo may liquefy.
Ship safety is affected by those cargoes when their form is changed during the voyage and the consequences may be catastrophic both for the vessel and the crew.
Detailed accident statistics in recent years indicate that small size ships, such as Handymax vessels are more at risk due to liquefaction than larger ships, where no accident has occurred so far.
Code of Safe Practice for Solid Bulk Cargoes
Progress in the Code of Safe Practice for Solid Bulk Cargoes (BC Code), which was adopted in the mid ’60s due to a spate of losses, led to the adoption of the International Maritime Solid Bulk Cargo Code (IMSBC Code). However, serious shortcomings in regulations still prevail, thus forcing a revision of the code.
Developments in different disciplines of engineering, like soil mechanics, offshore engineering and mechanics of materials helped us enhance understanding in the marine field. Similarities of the problem in those areas allowed us to transfer valuable knowledge to our field. Liquefaction is a phenomenon that initially appeared in soil mechanics many decades ago due to high-intensity earthquakes.
Procedure and remedial factors
The project includes the experimental procedure that should be followed. The results of experiments will show precisely the transient or the steady nature of the phenomenon and thus the spectrum of excitation forces responsible for cargo liquefaction. As a consequence, our aim was to suggest remedial factors, which for instance, reduce the transport liquid mobility of the phenomenon, find methods and establish models that could solve the problem.
A design solution that could be valuable for future research and its feasibility was studied through the AVEVA ship design software. This is air inflated devices. A Handysize bulk carrier was modeled to demonstrate the potential of these devices for improving the stability of the ship to any level required to resist capsize as a result of cargo liquefaction.
Based on the aforementioned research, a number of conclusions and recommendations are offered for further research.
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