The weight- and fuel-saving possibilities of composite materials mean they have potential for use in hatch covers
Composite materials are lightweight, have excellent fatigue properties and do not corrode as steel does. Through life maintenance costs can be significantly reduced where steel is replaced with composite material.
This has made them particularly of interest as a material for hatch covers. In 2011, composite hatch covers were first developed and introduced by Oshima and DNV for the Oshima ECO-Ship 2020 concept design. A similar design for a Panamax bulk carrier was awarded an AiP from DNV in 2013, followed by Panama flag approval for fire safety in 2013.
Composite hatch covers were first approved for fitting to a 225 m bulk carrier in 2014. The concept was developed by Hansen Engineering.
The design for the 17 m x 8 m FRP hatch covers had several benefits. It reduces weight (typically 35-40% of steel), resulting in fuel saving and/or increased cargo, as well as easier crane handling and lighter motors. No corrosion means better seal performance, reducing the risk of damage to cargo.
Approval for conversion was given for a 225 m x 32 m cargo vessel owned by Danish shipping company Nordic Bulk Carriers AS. Hansen worked with classification society DNV GL and fire experts at SP Technical Research Institute of Sweden to provide the design and risk assessment for the conversion-project specification developed by Oshima Shipbuilding.
The IMO Solas regulations are based on steel and require structural materials to be non-combustible. Although an alternative design approach with risk analysis has been possible since 2002, this was the first time a composite part has been approved using the alternative design approach.
A 2016 study by the Society of Naval Architects of Korea titled ‘Weight reduction and strengthening of marine hatch covers by using composite materials’ found that a vessel can load more cargo while using the same draft it was using before converting its hatch covers to composite material.
In this study, steel hatch covers of a bulk carrier were replaced by composite covers and a weight reduction of 44.32% was achieved, leading to many benefits including fuel savings.
Equally, because the materials are so much lighter than steel, composite hatch covers are easier to handle using the ship's own cargo gear or using smaller port facilities. As a matter of fact, vessels equipped with composite hatch covers can be designed with smaller and lighter cargo-handling equipment, which will lead to more weight savings and additional cargo capacity.
Furthermore, basic lifecycle cost analysis proved that composite hatch covers are economically competitive through their economic life time. This is because, while steel hatch cover acquisition costs are less expensive than those for a composite cover, the extra payload and reduced maintenance costs through the time will compensate for the difference in initial cost until a break-even point occurs where the cost of the two design alternatives (steel and composites) are equal.
Most recently (in May 2017), Oshima Shipbuilding and DNV GL returned to the theme, launching a new open-hatch general cargo carrier design, which includes composite tween decks to save weight.
This design can also be fitted with composite hatch covers, which offer similar advantages to tween decks.