UPLIFT ON HATCHES
In today’s competitive market, ship owners and brokers are always looking to make sure their ships are fully booked before transporting cargo. With respect to project cargo, this can mean shipping cargo up on the weather deck or hatch covers, especially when the cargo is too large to fit below deck in the hold.
Failure to recognise any structural defects in the hatch covers can ultimately endanger the cargo, the ship and its personnel. If hatch covers are not maintained properly, the strength of the hatch will be reduced and, furthermore, leaks can occur which can lead to underdeck cargo being exposed to sea-water and in the long term, corrosion of the ships own internal structure. It is vital to carry out regular examinations of hatch covers to identify any corrosion, and subsequently rectify any faults or damaged parts prior to cargo transportation.
The primary concern with shipping large cargoes on the hatch covers is the strength of the hatches under heavy loading. Cargoes stowed on the weather deck are subject to higher forces than those under deck, therefore more likely to experience ‘uplift’ where the cargo is physically tipping due to centripetal and transverse forces acting on the cargo. After the uplift connection to the hatch cover itself is checked and adequately designed, consideration should be given to the adequacy then of the hatch cover’s connection to the vessel coaming.
The problem arises when the tipping moment (including cargo inertia) is significant but only restrained on one or two hatch covers. In such circumstances, there can be an over reliance on the assumption that the combined weight of the cargo and hatch covers suitably lowers the combined centre of gravity and this, coupled with the support span between coamings, removes the uplift load on the hatch connections. In cases where this has not been checked and there is still an uplift load, typically the only thing restraining the hatch cover from lifting off the hatch coamings are the securing cleats. It is more than likely that there will only be two cleats per hatch cover –port and starboard – and these are normally not more than a threaded bolt and nut. Should these fail due to insufficient strength, the result could be catastrophic.
In the past, some of the possible solutions or mitigations that we have found practical include:
- • By passing the hatch covers with respect to uplift lashing by lashing the cargo to the upper deck structure
• Adding welded sea-fastening between ship’s hatch covers and hatch coaming structure
• Increasing the quantity of uplift lashing to improve the distribution of the lashing over multiple hatch covers
In conclusion, uplift load paths on cargoes should be followed right into the principle structure of the ship itself, ensuring that there are no unanalysed failure modes.