Clarity for open-top transport

When multi-purpose vessels carry extremely large cargo that is too large to fit into cargo holds and cannot be placed on top of the hatch covers either, such as cranes or wind turbine towers and foundations, open-top transport is the only option. “Open-top” means that the vessel is operating with open hatch covers — a practice normally prohibited by freeboard regulations. The exemption for open-top container ships documented in MSC/Circ. 608 cannot be applied directly to multi-purpose vessels since MPV cargo holds are arranged quite differently. To close this gap, DNV GL engaged in an open dialogue with industry and flag state representatives, which led to the development of a new set of rules specifically for MPV open-top transport. These voluntary rules will enter into force on 1 July 2017. MPVs built according to these new rules will receive the “Hatchcoverless” class notation. Nevertheless, in every such case it will be necessary to liaise with the responsible flag state in a timely manner to request an exemption.

A number of key elements of the rules originally developed for container ships had to be modified for MPV operations.

Seakeeping model test

Model tests are required to establish how much green water can enter the open-top cargo hold under extreme wave conditions. To ensure meaningful test results, the number of test runs and the ambient conditions were adapted to reflect the specific properties of MPVs.

Intact stability at flooded cargo hold condition

Large quantities of rainwater can accumulate in an open cargo hold, which can severely affect ship stability and cause the vessel to capsize in extreme cases.

The rules for container ships require examination of a scenario where one of the open cargo holds is completely filled with rainwater, which is an extremely conservative assumption. Since MPVs frequently have just one very large cargo hold, this rule cannot be applied. Based on an analysis of global weather data, DNV GL proposed to limit the assumed fill level to a maximum of 2 metres. Where lower levels of water are found to have a significant negative impact on stability, that scenario must be accounted for as well. As part of the new rules a tailored approach for accounting for the effect of free liquid surfaces on intact stability has been established.

Damaged stability

The rules for container ships assume that, given the properties of the hypothetical container cargo, no more than 70 per cent of the cargo hold can be filled with water. Project load on an MPV, however, may have very large external dimensions but comparatively little displacement. Damaged-stability calculations must therefore assume the cargo hold to be filled to 90 per cent of capacity at both partially and fully loaded draught condition.

Firefighting systems

Standard CO2-based firefighting systems will not work with the cargo hold open, and systems required for open-top container ships will not work on MPVs because of their different cargo hold arrangement. The new rules recommend fixed fire monitors at both ends of the cargo hold ensuring sufficient coverage of the cargo hold area.

Dangerous goods

The requirements of MSC.Circ. 608 for the carriage of dangerous goods in packaged form in and above open-top cargo holds apply to MPVs as well. The carriage of dangerous solid bulk cargoes without weathertight hatch covers is normally not permitted but exceptions may be possible on a case-by-case basis.

With the new “Hatchcoverless” class notation DNV GL has developed a standard that carefully balances safety and commercial considerations, giving practical guidance and predictability.