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LBR5 - Least Cost Optimisation of ship structures

 

To be attractive for shipyards, scantling optimisation has to be performed at the preliminary design stage. It is indeed the most relevant period to assess the construction cost, to compare fabrication sequences and, to find the best frame/stiffener spacing and most suitable scantlings to minimize the production costs. The LBR-5 softawre package performs such early design least cost optimisation.


Application

 

The application domain concerns lock gate, barge FSO, LNG carriers, passengers vessels (mono- and multi-hull).


Aims of the software

 

LBR5 is a rationally based optimisation module that, in the preliminary stage, allows for:

  • A 3D analysis of the general behaviour of the structure (usually one cargo hold),
  • To explicitly take into account all the relevant limit states of the structure (service limit states and ultimate limit states) thanks to a rational analysis of the structure based on the general solid-mechanics theory,
  • An optimisation of the sizing/scantling (profile sizes, dimensions and spacing) of the structure's constituent elements,
  • To include the unitary construction costs and the production sequences in the optimisation process (through a production-oriented cost objective function).

Design variables are the dimensions of the longitudinal and transversal members, plate thickness and spacing between members.


Optimization

 

LBR-5, a tool for least cost scantling optimisation

 

LBR5 is built around three basic modules, respectively, OPTI, CONSTRAINT and COST. The OPTI module contains the mathematical optimisation algorithm to solve non-linear constrained optimisation problems. The CONSTRAINT module includes:

  • Technological constraints that provide the upper and lower bounds of the design variables;
  • Geometrical constraints that are generally based on good practice rules to avoid local strength failures,
  • Structural constraints that represent limit states in order to avoid yielding, buckling, cracks, etc. and to limit deflection, stress, etc.

The COST module the objective function is the construction cost that includes labour costs and material cost. LBR-5 is also an efficient tool to assess and compare different alternatives. For instance Fig. 3 and 4 gives the cost and the weight as functions of the web-frame spacing. A major capability of the method is to quantitatively assess a change of the production technology on the construction cost. For instance, effect of an improved welding procedure (lower unitary welding cost) can be assessed by comparing the least cost optimum scantling obtained with and without the improvement.

Principia Marine (France) and University of Liège (Belgium) have furthermore engaged in cooperation to jointly develop their software and integrate them as part of an early design suite. Fig. 1 shows the simple and fast mesh modelling methodology used by the LBR-5 software to optimise a fast ferry, which may latter be modelled using standard finite elements (for advanced analysis).



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