Making a hash of lashing

Submitted by Elsewhere on 2020-Sep-12 Sat 09:38
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Container losses – why do they keep happening? Asks Neil Gardiner, managing master mariner, one of the accident investigation team at Brookes Bell. Neil argues that while there are not as many container losses as is sometimes thought, those that do occur are often due to poor weight distribution, amongst other things.

Every year container ships lose containers, but it’s not as many as people think. The World Shipping Council which includes the big shipping lines, accounting for more than 80% of the global container shipping capacity, reports that only around 2,000 containers carried were lost in 2019.

That’s less than 0.001% of the estimated 226 million containers carried every year. But, they tend to be highly visible and high profile.

With 24 years of seagoing experience including considerable time on container ships and a degree in maritime technology and maritime law, I am in a position to give the scientific and technical insight into the transportation of all kinds of commodities carried in containers at sea.

Containers fall off ships for a number of reasons, but poor distribution of weight is one of the most common factors.  In recent years the IMO’s SOLAS regulations concerning the mis-declaration of container weights have tightened control in ports considerably. Mis-declaration of contents and weights does, however, still occur and when it does it poses a significant safety risk to the vessel and crew carrying the cargo.

Brookes Bell continues to be appointed by insurers and owners to investigate container losses. On arrival at the vessel, the appointed surveyor will conduct a thorough inspection of the vessel and paperwork. They will, thereafter, investigate and record the condition of the lashings, including twistlocks and the condition of the containers. They will, thereafter, assess the condition of the adjacent container bays, the arrangement of the remaining containers, the base locks, the configuration of the lashings to establish whether the arrangement was compliant with the approved Cargo Securing Manual.

This is where securing the containers is so important. In a boisterous seaway the vessel pitches and rolls, depending on the ship stability and the direction from where the seas are coming, these motions could translate adverse vessel motions resulting in excessive racking, compression and separation forces within the container stow.

It’s not just the overall weight of the containers that can lead to a movement of stowage, but the distribution of those weights within the stack, how the contents are distributed and secured inside the container and how the ship is handled in heavy weather are key factors too.

Domino effect
Each twistlock on a container vessel has been typically designed with a 25-tonne maximum securing load. In a bay filled with all rows of containers with similar weight distribution, the bay acts like a solid block with little relative movement.  But container stacks with significant weight variations act independently, each reacting differently to the vessel’s motions.

Lashing bars between the container stacks and deck are tensioned but over the course of a voyage they may loosen and need to be re-tightened.

Twistlocks and lashing bars need to be re-tightened, particularly during heavy seas when the cargo moves more.

All very well in calm weather, but in bad weather – when they are needed even more – it simply may not be possible for the crew to do this.  If the tension is not maintained, one stack may move more than the adjacent stack and a slapping motion may develop.  Over time, in the ‘right’ conditions this can have a domino effect, one stack leans against the next, overloading its securing components, until the whole bay becomes unstable, and overloading the securing arrangement such that the container stack(s) topple.

It might be that just one container at which a structural component has failed. The strength of a container is in the corner posts and structural rails and if one aspect fails, it can lead to the collapse of the entire stack. Any weak points in a container structure become critical when they have, at times up to 10 containers secured above.

Updating Class Rules
The container industry has changed significantly over the last ten years, with the trend being for larger and larger vessels. Classification Societies set the rules for vessel loading configurations, but with partial loading and discharging of the bays at various ports, vessel stability on a voyage can vary dramatically. This can and does on occasion translate into a sub-10 second roll period that can generate dynamic lashing forces that are far greater than what the lashing system was designed to withstand.

Where isolated container stacks may be a requirement between ports, the effect of severe weather can be more profound.  By example, in the North Sea, weather conditions can change quickly and the relatively shallow sea (30-35m deep) can at times produce treacherously short and steep waves, alternately in the Far East during typhoon season this can produce a whole range of extreme weather conditions and difficulties when trying to reduce movement of the vessel in a seaway.

In 2009, a paper published by Marin, Lashings@sea examined the forces experienced on a container ship using fitted sensors.  It revealed that many Class Rules are based on assumptions not reality. The modern, bigger ships are more flexible and container fixings need to be similarly responsive.

The Marin research found that with a ship in a head sea, the slamming motion can pass along the length of the vessel as a travelling wave through the hull.  When this wave reaches the aft end, the lighter structure shows peak accelerations of similar amplitude to the original slamming forward.  This can increase the loading on a container’s lashings by 40-50%, but it remains unclear if all the classification societies calculate or currently allow for such additional forces when approving the Cargo Securing Manual that the crew must use to assess the container stowage before each voyage.

Ship loading programmes are based on assumed vessel motions at a predetermined GM value by the approving classification society.  Unfortunately, when adverse weather is encountered it is not unusual for the parameters that the stowage is being assessed against are exceeded.

At the end of the day, it’s about getting the basics right. The container weight (VGM) needs to be verified before loading plans are developed and thereafter the vessel’s crew must follow the loading plan and check the loading weights are in line with the Cargo Securing Manual.

In every incident I have investigated, the ship has had loading software, but it has not always been used to its fullest extent. Before entering port, the vessel would likely receive the proposed loading plan.

This should be put into the loading computer and checked for compliance and where anomalies are found with stack weights, poor weight distribution and potentially excessive lashing forces against the Cargo Securing Manual criteria and/or stability these need to be immediately raised with shore planners and amendments to the loading plan made.

About the Author
Neil Gardiner, Managing Master Mariner, a Master Mariner with a combined Honours degree in Marine Technology and Maritime Law, joined Brookes Bell in April 2012 as a Managing Master Mariner. Prior to joining Brookes Bell, Neil worked for a leading P&I Club, inspecting their own vessels and investigated casualties for another UK-based marine consultancy.

Neil has approximately 24 years of seagoing experience, sailing as Master latterly on ice-classed panamax container ships trading between North Europe and Montreal. He has sailed extensively on container vessels, on reefer ships and high-speed ferries. Neil has investigated and advised on a wide range of matters including collision, grounding, fire/explosion, personal injury, port safety disputes, pollution, speed and performance, hull damage, aspects of the ISM Code and various cargo related matters. He has acted as expert witness in court and arbitration in the UK and abroad.

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