Feature 1 | METHODS & MATERIALS LASS is more for composites
As a part of the LASS* project a study concerning the use of composite materials in selected parts of small cargo vessels has been performed by Kockums AB.
analysed the likely consequences of using composites in the cargo hatches, a grain bulkhead, and the deckhouse onboard a notional bulk carrier. Part of the LASS (‘Lightweight
W
construction applications at sea’) project, the study was devised to result in a “base line for composite structures”, from which it will be possible to estimate the weight, the material cost and the manufacturing cost. No detailed design concerning battening or interlinking of hatch sections was performed. Known requirements were that the
cargo hatch and the grain bulkhead must have a robust design. Te surfaces must be flat in order to achieve easy cleaning. Te deformation when loaded must be limited in order to maintain the water tightness of the cargo hatch. Te grain bulkhead is not water tight, but it may be unpractical with large deformations for this structural element. For the deckhouse, the prime goal was to
decrease the structural weight. All composite details had to be easy to
manufacture and to repair and the material cost was “low”. Design loads were based on the DNV Rules for Ships, while design of the composite structures was based on the DNV Rules for High Speed, Light Craſt and Naval Surface Craſt.
Laminates hatched A typical hatch is assumed to be 10.4 x 6.5m, with the height 0.4m. Te proposed cargo hatch design is of a traditional stiffened single skin structure, consisting of a number of joined square hollowed sections. The
*Te full LASS report can be downloaded from
www.lass.nu while the 1st International Conference on Light Weight Marine Structures will be held 7-8 September 2009 at the Dept. of Naval Architecture & Marine Engineering, Universities of Glasgow and Strathclyde, Henry Dyer Buliding, 100 Montrose Street, Glasgow (see
www.liwem.com).
The Naval Architect July/August 2009 Web laminate shear stress.
sections can be pultruded or manufactured by vacuum infusion on a mould. Te laminate design can be optimised
when vacuum infused, saving weight. Te size of the cross section may be limited when pultruded. In the context of the study, to avoid local
deflections and to create a tough upper skin laminate a layer of high absorbent fibres was used - “Lantor Soric XF6”. Te layer thickness was 6mm and the fibres located in the middle of the skin laminate. Tis increased the thickness and the bending stiffness of the skin laminate. A FEM analysis was performed to obtain
knowledge of the hatch deflections. The hatch was built up with a number of similar sections, with each section designed to carry itself and the design pressure. Te
Type of vessel studied.
ith a view to reducing structural weight and thus increase payload, Kockums AB has
FEM model consisted of only one single square hollowed section. Te analysis was performed with ANSYS and the element used in the FEM analysis was Shell 181, a four-node finite strain shell. Te largest local bending deflection was
found in the top side laminate - 3.33mm. Te maximum allowed local deflection
was set to w=2t, where t = the laminate thickness. In this case t=14.94mm, including the Lantor soric ply of 6mm. Te allowable local bending deflection will then be 29.88mm. Tis means that the deflection was within the rule limitations. A shred section at the mid-span of the
Typical cargo hatch cover.
hatch showed the bending stress at the mid span in the top layer of the laminates. Te compression stress in the top side laminate was 51MPa. The maximum allowed compression stress had been set at 118MPa. Te tensile stress in the bottom side laminate was 14MPa, where the maximum allowed tensile stress was set at 158MPa. Te shear stress corresponded to the well
known transverse force distribution over a beam subjected to a uniform pressure over the span. Maximum occurs at the ends and no shear at mid span. Maximum shear stress was 57MPa and the maximum allowed shear stress was 82MPa. The estimated weight of each square
hollowed section in the cargo hatch was approximately 180kg. One cargo hatch section will be built up by 16 of these, so the total weight for one hatch section could be seen to be approximately 2880kg. Te equivalent steel hatch would weigh
approximately 6114kg. Te ship envisaged features nine hatches, and therefore the total weight saved would be 29tonnes. In this example a single skin design has
been studied for the cargo hatch. Kockums said it may be possible to use a sandwich design with a suitable core, but that this fell outside the scope of the study.
Grain bulkhead Turning to a grain bulkhead made of glass fibre reinforced polyester, the study envisaged an 11.6m x 1.93m bulkhead section that was
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