Keywords: finite element analysis, computational, modelling, historic, ship, composite, structure, Cutty Sark.

This paper details the computational methodology for analysis of the structural behaviour of historic composite structures. The modelling approach is based on finite element analysis and has been developed to aid the efficient and inexpensive computational mechanics of complex composite structures. The discussion is primarily focussed on the modelling methodology and analysis of structural designs that comprise of structural beam components acting as stiffeners to a wider shell part of the structure. A computational strategy for analysis of this type of composite structures that exploits their representation through smeared shell models is detailed in the paper.

The modelling approach has been applied to analyse the structural response of the historic Cutty Sark ship to conservation options as part of a major restoration program. The Cutty Sark ship, the last surviving clipper (type of sailing ship) in the world, marks the transition period in 1850's from wooden to iron ships [1]. Her design is a classical example of a composite structure. The design concept is based on the usage of wrought iron frames and deck beams to which the hull wooden planking and deck timbers of the vessel are fastened respectively.

Analyses of the ships structural response to various loading conditions have been undertaken using both local detailed models of sections of the vessel and a global model of the whole ship. The local detailed models have been analysed using continuum finite element analysis and the global model of the ship has been analysed through structural shell elements [2]. With the latter approach, the ship is modelled as a shell structure using the CAD tool Rhino [3]. The limitations of applying the conventional shell theory for the analysis of Cutty Sark are caused by the composite nature of the ship. The ship as a shell structure has different stiffness behaviour along the wooden planks and in direction of the iron ribs. Smeared shell definitions have been developed as part of the analysis methodology for Cutty Sark to make the structural shell approach applicable. The stiffness coefficients of the smeared shell elements are calculated independently and account for the simultaneous contribution to the shell stiffness from both the iron components and the wooden planking. The rules for this are described in the paper. These stiffness properties of the smeared shells are in the form of a direct input to the structural analysis tool [4]. This strategy simplifies the modelling work in terms of the time and effort required for model generation, particularly if complex geometries are involved, and results in a significant computational efficiency.

A brief outline of the modelling validation of the smeared shell approach is also given. It was found that although an approximate approach, the smeared shell analysis results compare favourably with the prediction from more accurate but time consuming models such as the full 3D continuum finite elements and the coupled structural beam and shell models. Experimental tests for further validation of the approach are also underway at present.

The computer models have been used to investigate the stress state of the ship and the future support structure. Demonstrations of the analysis approach emphasise the benefits of exploiting this modelling strategy as an efficient tool to test different conservation options. In its future state the ship will be raised above the ground of the dry dock and supported by external new steel structure at the level of the lower deck. Analysis results for the ship response with particular future support conditions are presented. We also detail how the modelling was used to get insight the effect of dismantling and re-assembling parts of the ship in the conservation process and to asses if there is any risk with this.

The effect of material damage of iron frames on the ships structural state due to the excessive corrosion processes is unknown and is being of primary concern; therefore the loss of strength due to corrosion is implemented accordingly in the finite element analysis.

Finite Element Analysis results from the local detailed model of the mid section of the ship with respect to the structural response of the new steel support members are briefly outlined. The simulations have captured all existing interactions between the news steel support structure and the ship. From these results, a design change in the support was identified to ensure improved stability of the whole structure.

1

B. Lubbock, "The Log of the Cutty Sark", Brown, Son & Ferguson Ltd, Glasgow, United Kingdom, 2004.

2

L. Kollar, G. Springer, "Mechanics of Composite Structures", Cambridge University Press, United Kingdom, 2003.

3

http://www.rhino3d.com

4

ANSYS, http://www.ansys.com

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