In general, in the dry bulk cargo shipping industry there are two types of cargo instability that have been identified, so far. The first one is cargo liquefaction and the second one is cargo instability due to dynamic separation. We have two, it's not too many, but these two problems have caused a lot of safety issues to the seafarers and also the ship, that's carrying the cargo.
But what is cargo liquefaction?When a vessel goes into the sea loaded with cargoes, it will encounter motion at sea and, at sailing time, this cargo will go through a process of settlements, compactions and when they compact, they release the air in between the cargo space, or in between the pore area. this pore area when it is compacted the water pressurebetween the pore spaceswill increase and these take away the frictional contact between the particles and when the particles friction contact is being eradicated then the cargo becomes a problem because, at this time, the cargo has lost all the shear strength and when the cargo losses it's shear strength, is just like the cargo has become something like the meltdown ice cream.
Namely, cargo liquefaction describes the phenomenon whereby the partially saturated bulk cargo material (mineral cargoes) loaded within the cargo hold space loses it shear resistance due to sufficient rapid cyclic loading induced from ship vibration, motion and waves, wherein the cargo material that has a solid appearance initially behaves like a viscous liquid. Responding to the dynamic motion at sea, loosely loaded cargo on board ship that possesses adequate fine-grained material, moisture and air in between the pore spaces tend to be compacted and resettled during the ocean passage. Compaction and rearrangement of cargo cause cargo material confined in the hold space to be compressed expelling air, increases in pore water pressure and resulting the cargo grains to be saturated.
So, what will happen is that now this cargo could be flattened in the cargo hold and behave like a viscous fluid. When this happened, in fact, cargo liquefaction has occurred in the cargo. if cargo liquefaction occurs, what could happened in the dynamic sea? you might not be able to have your vessels sailing upright, and if your vessel is not sailing upright it's dangerous especially in a very bad weather condition.
The recent Global Bauxite Working Group (GBWG) has reported in May 2017 that cargo with a significant amount of fine particles and high moisture content may experiences cargo instability due to "dynamic separation" instead of liquefaction when exposed to sufficient cyclic loading due to ship motions at sea.
In the dynamic separation process, primarily the loosely loaded cargo pile in the cargo hold will experience compaction similar to liquefaction when encounters sufficient ship motions. Air and moisture between pore spaces will be compressed leading to an increase in pore water pressure but relatively below the normal contact force between grains causing moisture to be forced out and migrate above the cargo surface.
With the migration of moisture to the cargo surface and densification through compaction, the cargo moisture level below the interface of the free water pool and cargo surface become lesser and unsaturated. The cargo beneath the water pool is more compact than its original form and cargo is dynamically separated.
In the dynamic sea, the sloshing effect of the free water pool above the cargo can erode and slump the cargo surface to form a free slurry pool floating above the cargo surface. The free slurry pool and erosion process slumping on submerged cargo in the dynamic sea may pose catastrophic damage on ship stability due to free surface effect and a liable sudden cargo shift.
So, why is shipping of liquefiable bulk cargoes can be dangerous? Ship motion in the seaway may cause cargoes with high moisture content to be unstable and (more liable in rough sea condition) it may trigger the liquefied cargo/ unstable cargo due to moisture to flow or shift rapidly towards the side of the partially filled cargo hold causing the ship to list. Accumulatively, the ship may develop a threatening list and capsized, resulting in loss of lives and property.
In 2017, the INTERCARGO Bulk Carrier Casualty Report, has documented a tragic loss of 53 bulk carriers (over 10,000 dwt) and 202 seafarers over the years 2008 to 2017 (in a 10 year periods). Liquefaction incidents have resulted in the highest loss of life, adding to 101 lives (50% of the total life loss) and 9 ships lost (17% of the total ship loss) over the period. The recent sinking of Emerald Star laden with nickel ore cargo on 13th October 2017, resulting in 10 missing crews has again reminded the importance of safe carriage of liquefiable solid bulk cargoes within the threshold of transportable moisture limit (TML) at sea.
What can we learn from these particular cases that we have gathered, and try to understand the root causes of these incident?. Normally, for liquefaction cases to start the the cargo moisture level should be high and listing of vessel occurs only when heavy weather condition is encountered . Normally, they start with a small angle list and aggravated into an angle that is uncontrollable.
Countermeasures could be very less effective during the time of the liquefaction occurrence at heavy sea condition . The general relationship of cargo instability of liquefiable cargohave a general functional relationship of moisture content and sea condition. Cargo shift, also, having the same functional relationship of sea condition and unstable cargo. Capsizing of vessel has a general functional relationship of cargo shift and sea conditions. from this particular relationship derived, we can tell, or we can roughly guess, the sea condition is one of a very important criteria to prevent your vessel from sinking.
So, how can we apply this knowledge that we have acquired in loss and loss prevention? We can make sure that the cargo moisture level is below its TML level, transportable moisture levels, and we should make sure this by testing it by the competent laboratory and cargo loaded on board should be trimmed with grabs to prevent cargo sliding.
An alternate loading arrangement of cargo has better stability. We should, always, apply alternate loading if it's possible, if the strength of the cargo hold allows. Stability and strength check should, always, be the paramount thing to do before sea voyage. Professional and reliable weather forecasts or weather avoidance measures should be in place.
Sufficient place of shelter should be defined before a voyage for ship loaded with liquefiable cargo. Rolling motion is one of the very detrimental motion at sea, so, at all time, the vessel should avoid encountering rolling motion at sea. If rolling motion is too excessive, the vessels should be advised to change course.
Be watchful of the voyage and monitor the cargo condition and ship motion behavior, regularly. If free water pool develops it should be pump out and proceed to shelter when voyage weather deteriorated. What can we do after we have done all these, we have to make sure life-saving appliances (LSA) are functional and put on when the time we need them.
What can we learn from all these incident information, in general cargo instability is a two parameters problem. The first is transportable moisture limits and the second would be the voyage weather conditions. Therefore, before a quantitative understanding of the functional relationship between the main parameters responsible for cargo instability due to moisture, cargo shifting and the capsizing of the ship at sea is defined, rough weather avoidance or weather routing is suggest to be included as another preventive measure to lower the risk in transporting liquefiable cargo at sea.
Above text is an edited version of Mr. Lee Hiok Liang’s presentation during the SAFETY4SEA Conference in Singapore
View his video presentation herebelow
The views presented hereabove are only those of the author and not necessarily those of SAFETY4SEA and are for information sharing and discussion purposes only.
Lee Hiok Liang, Naval Architect, T&T Salvage
Lee Hiok Liang is a Naval Architect with T&T Salvage and has since participated in plan development for numerous significant emergency response and wreck removal cases around the world, both on the scene and remote engineering support. The notable emergency response case includes the developing of a salvage response plan for a handymax bulk carrier experiencing cargo liquefaction in extreme weather condition with a severe list, crew abandonment and eventual grounding. Before joining the T&T family, he worked for an international dredging company as a senior project engineer for ship newbuilding project, with consulting and salvage company as Naval Architect involved in marine salvage, offshore project and other engineering fields.