As the main cause of vessel loss is sinking, the maintenance, testing and monitoring of watertight hatches, doors and multiple cable transits on vessels is essential. A watertight hatch cover is designed to prevent the passage of water in either direction under a head of water for which the surrounding structure is designed. Many mariners may think hatches are robust, monolithic structures, thereby failing to appreciate the small tolerances on panel alignment and gasket compression. It is better to think of hatches as complex, finely-made structures, to be handled with care. All types of seals, experience dynamic stresses as part of their operational lifetime. For example, 4mm wear on the steel-to-steel contact is sufficient to damage rubber sealing gaskets beyond repair; 5mm sag along the cross-joint can cause a large gap between the compression bar and gasket. The importance of continually maintaining seal integrity should take a more prominent position in ship maintenance scheduling.
The future of watertight integrity testing is with continuous monitoring. A lack of proper servicing of seals can lead to deterioration which endanger the lives of the crew, vessel and cargo. The large issue here is that ships are only tested before and after one or perhaps several journeys; yet a leak could occur at any point in between testing and continue unnoticed until the next inspection. A vessel generates its leak sites due to load states, sea states, wind states, and dynamic movement. The severity is amplified within a vessel structure constantly changing by varying sea, wind, load states, cargo types and dynamic stresses. There is a great deal of bending and deformation that naturally occurs in ships during travel. It was found that a comprehensive, autonomous continuous monitoring system for the watertight integrity of a ship’s cargo hatches, weathertight doors and other seals is possible to be developed. One that is capable of automatically detecting emerging leak sites, alerting officers and crew of the location and severity of the leak site and logging all data for future review. The developments in continuous monitoring technology being undertaken by Coltraco Ultrasonics will drive the industry towards ensuring that watertight integrity is never left to chance.
The case study of the Emma Maersk exemplifies the danger of improper servicing. A severe leakage occurred on the container ship in February 2013 when it was loaded with 14,000 containers. The leakage was caused by the mechanical break-down of a stern thruster, creating the shaft tunnel to flood, as well as leading to severe ingress of water in the aft part. This led to flooding of the main engine room. This was caused by non-effective cable penetration sealings: in a sudden blast, four cable penetration sealings in the watertight bulkhead gave way to the water pressure followed by a massive ingress of seawater. Shortly after this, the other three cable penetration sealings also failed, resulting in an even larger ingress of water into the engine room. This led to approximately USD 45 million worth of damages and towage cost.