DNV GL revised its GRAB notation for cargo hold reinforcements that will save owners money, meaning that smaller ships which use lighter grabs will need less massive reinforcements.
Accordingly, the revised notation, which took effect in July 2020, specifies a minimum grab weight of 10 metric tonnes (mt). This means that as long as lightweight grabs are used exclusively, a lower reinforcement steel plate thickness is allowable under the class notation.
DNV GL’s Jan Rüde explained that all these technical advancements have made it possible to revise the GRAB notation since state-of-the-art lightweight grabs are efficient and also reduce wear on the steel plates in cargo holds.
To remind, the class notation GRAB attests to the cargo hold reinforcements in place to protect a ship from the impacts of cargo grabs during discharge operations.
Jan Rüde, Ship Type Expert MPV, Hull Structure & Outfitting at DNV GL, commented that
The new version of this class notation will save owners money. It means that smaller ships which use lighter grabs will need less massive reinforcements. It also gives owners more flexibility to determine the design grab weight.
Grabs can cause damage
According to DNV GL, when a grab is lowered into a cargo hold, it will eventually hit the bottom or occasionally even the side walls. This is unavoidable, and it can cause dents in the steel plates or even structural damage. Also, a large, heavy grab is more likely to cause damage than a smaller, lighter one.
Moreover, at the end of the discharging process, one or two bulldozers are often lowered into the hold to push the remaining cargo into the middle so the grab can pick it up. The bulldozers likewise put a strain on the bottom and the lower side walls of the hold.
Therefore, these challenges must be highlighted for when designing a ship in order to protect its structural integrity. The bottom and side walls of a cargo hold must be strong enough to withstand the impact forces of grabs and bulldozer shields.
Smaller ships
The size development of ships and grabs put owners of smaller vessels at a disadvantage. Mr. Rüde explains that “mega-sized grabs designed for today’s large bulkers cannot be used in the smaller cargo holds of MPVs. Nevertheless, in the past, the same reinforcement rules applied to small MPVs as to large bulk carriers.” He added that “earlier versions of the GRAB notation from DNV GL, which is based on the shipbuilding rules in DNVGL-RU-SHIP Pt. 6, Ch. 1, Sec. 1, assumed that an empty grab weighs at least 20 metric tonnes. This assumption turned out to be too conservative for MPVs.”