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 risk is worsened by the ageing nature of many bulk carrier ships in particular. There is also a degree of bending/deformation that naturally occurs in ships during travel, which puts pressure on hatch covers and can damage sealing. A recent wave of inexperienced crew members has swept across the shipping industry as a cost-saving mechanism, leaving vessel maintenance and hatch cover testing to decrease in quality. While hatch covers are often perceived as indestructible due to their large size and bulky exterior, in reality they are complex, finely made structures that need to be handled with care, a point that many mariners do not realize.
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 because of a 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. Eventually there was flooding of the main engine room. This was because of non-effective cable penetration sealing’s, in a sudden blast, four cable penetration sealing’s 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 sealing’s 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.
The use of UT is far more efficient than the traditional methods described in the previous section, taking less time and requiring no clean up while being in a portable, light-weight model for ease of use. Due to its convenience, UT tests can be conducted more frequently and can contribute to safety management and preventative maintenance procedures on board.