Lessons learned: A slightly uneven distribution of heavy cargo can twist the vessel

A Panamax bulk carrier developed a significant torsional twist after unevenly distributed iron ore cargo caused the vessel’s hatch coamings to distort, preventing three forward hatch covers from closing.

The incident, presented by The Nautical Institute, demonstrates how even relatively small differences in the distribution of heavy cargo can create sufficient torsional forces to distort a vessel’s structure. It also highlights the importance of considering cargo distribution, ballasting and the need to close hatch covers during cargo operations.

What happened

The six-hold Panamax bulk carrier had loaded a full cargo of iron ore when the crew found that three of the forward hatch covers could not be closed. Each cargo hold was fitted with four hinged hatch panels, with two panels forward and two aft. The covers were standard McGregor-type hydraulically operated sliding panels.

As the hatch covers were being closed, the leading rollers repeatedly tended to skew off the guiding trackway in the hatch coaming. Once off the track, the rollers could not move further and the hatch covers could not be closed. The crew suspected that the problem was related to distortion of the vessel.

The hatchway, which should have remained rectangular, had effectively become a parallelogram, while the hatch panels themselves remained undistorted.

The vessel was suspected of having slightly twisted along its length. To establish whether this was the case, freeboard measurements were taken at various points on both sides of the vessel. The readings clearly confirmed that the vessel had developed a torsional twist.

Diagonal measurements of the hatch coamings were also taken and confirmed that the vessel had twisted. The crew then inspected the cargo stow in the holds to determine whether uneven cargo distribution could have contributed to the distortion.

The inspection found that the iron ore in the three aft holds was positioned slightly to starboard of the vessel’s centreline, while the cargo in the three forward holds was slightly to port of centreline.

This uneven distribution had created torsional forces along the vessel. The crew used bulldozers to level the cargo in each hold. Diagonal measurements of the hatch coamings were taken periodically to determine whether the cargo levelling was correcting the distortion.

As the cargo distribution was corrected, the torsional deformation was gradually reduced.

Eventually, the vessel’s torsion was corrected and the hatch covers could be successfully closed.

What went wrong

The primary issue was the uneven distribution of the heavy iron ore cargo between the forward and aft sections of the vessel.

Although the difference in cargo position was relatively small, the weight and density of iron ore meant that it generated sufficient forces to twist the vessel.

The three aft holds had cargo slightly to starboard of centreline, while the forward three holds had cargo slightly to port. The opposing distribution created a torsional effect along the vessel rather than simply producing a conventional list.

The vessel’s design also contributed to its susceptibility to twisting. It had broader-than-normal hatch openings because it was a combination bulk/container vessel.

The wider hatch squares reduced the structural resistance to torsional deformation compared with a vessel with narrower hatch openings.

The effect became apparent when the hatch covers were being closed. The hatch panels themselves remained undistorted, but the hatch coamings had changed shape sufficiently for the leading rollers to leave their intended track.

This prevented the hydraulically operated covers from being closed normally.

The incident also demonstrates that cargo distribution needs to be considered in three dimensions. It is not sufficient to consider only overall cargo weights, trim and heel.

A vessel can remain apparently level while still experiencing torsional forces caused by differences in the transverse distribution of heavy cargo between different sections.

The problem was ultimately corrected by physically redistributing the cargo. Bulldozers were used to level the iron ore, while repeated diagonal measurements provided a means of monitoring whether the vessel’s deformation was being reduced.

The big picture

The wider lesson is that heavy cargo does not need to be dramatically misaligned to produce significant structural effects.

Iron ore is particularly dense, meaning that relatively small changes in its distribution can generate substantial forces. Where cargo is positioned differently on either side of the centreline in different holds, these forces can contribute to torsional deformation.

The vessel’s broader hatch openings made this effect particularly important. Hatch coamings form part of the vessel’s structure, and distortion can affect the operation of hatch covers even when the covers themselves remain structurally intact.

The incident also highlights the importance of monitoring the vessel during loading rather than treating the loading plan as a static document.

If uneven cargo distribution is identified early, corrective action can be taken before structural deformation becomes significant enough to interfere with hatch operations.

Loading and discharge plans should therefore consider not only the final cargo condition, but also intermediate conditions.

This is particularly important where hatch covers may need to be closed during cargo operations, for example because of rain or other operational requirements.

The case also shows why measurements can be useful when structural distortion is suspected. Freeboard readings and diagonal measurements of hatch coamings provided objective evidence of the vessel’s condition and allowed the crew to monitor the effectiveness of cargo redistribution.

What could have reduced the risk?

The first safeguard is to ensure that heavy cargo is distributed as evenly as practicable throughout the vessel, both transversely and longitudinally, in accordance with the approved loading plan.

Cargo loading plans should account for the potential torsional effects of uneven cargo distribution, particularly on vessels with broad hatch openings or other structural characteristics that may make them more susceptible to deformation.

The vessel’s loading and ballasting arrangements should also be considered together. Proper ballast distribution can help control the vessel’s condition and reduce unwanted structural deflections during loading and discharge.

Loading should be monitored continuously rather than relying only on the final cargo condition. Where heavy cargo is being loaded into multiple holds, crew should remain alert to changes in freeboard, trim, heel and other indications of uneven loading.

Hatch covers should also be considered during cargo planning. If there is a possibility that hatch covers will need to be closed during loading or discharge, the loading sequence should allow this to be done safely and without creating excessive structural deformation.

The vessel’s manuals and operating procedures should provide guidance on cargo trimming and ballasting, including the need to avoid hatch coaming deflections caused by uneven loading.

Finally, where hatch covers become difficult to operate, crew should not attempt to force them closed. The condition of the vessel and hatch coamings should first be assessed, with appropriate measurements taken to establish whether structural deformation is responsible.

Ask your team

• How do we monitor the transverse and longitudinal distribution of heavy cargo during loading?
• Does our loading plan account for torsional forces as well as trim, heel and longitudinal strength?
• Are there specific precautions for vessels with broad hatch openings or other characteristics that make them more susceptible to torsional deformation?
• Do our procedures explain how cargo distribution and ballasting should be managed to minimise hatch coaming deflections?
• How would we identify torsional deformation before it prevents hatch covers from operating?
• Does our cargo plan consider situations where hatch covers may need to be closed during loading or discharge, for example because of rain?

Key takeaways

• The vessel had broader than normal hatch squares: Being a combination bulk/container type, it was thus more susceptible to torsional forces.
• Even a slightly uneven distribution of heavy cargo was sufficient to twist the vessel: This resulted in some hatches being unable to close.
• Cargo trimming and ballasting guidance is important: Manuals usually mention maximum trim and heel conditions, but a note on the need for proper cargo trimming/ballasting the vessel to avoid coaming deflections during loading is equally useful.
• Consider the potential need to close hatch covers: When making the loading/discharge plan, consider the potential need to close hatch covers at any time, for example if it rains. Continuous proper distribution of heavy cargo is a good practice to avoid nefarious twisting effects.
• source : safety4sea