Ship designs are now routinely developed initially in the form of surface models representing the hull and major decks and bulkheads of the ship. This surface model can also be viewed as a topological model that organizes the three dimensional spaces of the ship, and defines the purposes of the spaces and the relationships between the spaces. Advanced topology models become the master ‘organizers’ of a ship design. The challenge for CAE models and analyses is to have a functional linkage or relationship with the surface-based topology model(s).
Hullform Sub-division & General Arrangement Parametric Structural Model Finite Element Models Structural Design Seakeeping & Hydro Loads High Performance Computers Structural Evaluation Loads Models FE Solvers Structural Optimization ITERATE Weights & Centers Cost Analysis Ship Signatures Hydrostatics Analysis Resistance & Powering
Ship Topology Model
Improved Integration with Overall Ship Design Process
Figure 4. Structural Design Coupled with the Ship Design Process
Figure 4 depicts:
Close coupling of ship surface topology with structural analysis and design models, including finite element models.
Next Generation Ship Structural Design
Automated generation and updating of structural models in response to changes in ship hull form, deck and bulkhead arrangements or other aspects of the ship design that affect structure, and feedback/updating of the ship design model(s) with changes in structure resulting from the structural analysis/design process.
Creating a parametric parent ship structural object model by defining structural attributions for the
Topology Model.
Spawning/automating multiple structural analysis models (including different detail levels of finite element models) from the parent structural object model.
Using open architecture software to facilitate interfacing structural analysis models with various load prediction analyses and tools, such as 2D/3D time and/or frequency domain hydrodynamic analyses.
Open architecture supports various special purpose analyses and different tools, such as Dynamic Load Approach, Spectral Fatigue Analysis, Underwater Shock, and forced vibration, some of which require the generation of input data for other analysis programs (Nastran, Ansys, etc.).
Automated structural panel evaluations (MAESTRO limit state sets; ALPS/ ULSAP; ALPS/Hull; Naval Vessel Rules; High Speed Naval Craft, etc.). Structural optimization to refine and improve the
structural performance and meet design requirements and objectives.
Coupling between the structure and the ship’s weights/centers and cost estimation models.
A further stage or phase of integration between the ship design topology and naval architecture analyses models and structural design and analysis is depicted in Figure 5. In fact, this same process can be applied not just to structural design but to the overall ship design as well. The emergence of novel ship concepts and advanced marine vehicles, as well as the refinement of competitive conventional ship designs, demand synthesis techniques that enable decision support problem (DSP) formulation as a basis for rational decision making.
“…the designer has at his disposal a large amount of information and possibilities which enable creation of a comprehensive picture of the design: the quality of satisfying the conditions of every particular attribute; the relation of attributes with corresponding attributes in other design solutions; and information on what should be considered with special attention in further phases of the design development.” (Zanic and Cudina, 2008)
Revise CAE Models CAE Hydrodynamics Stress Analysis Failure Evaluation Shock/Vibration Decision Support Tools: DeMak Design Changes D esig n M etric s Up dat ed M od els
Support different levels of design iterations, e.g.
changes in:
•Scantlings
•Frame spacing
•Topology
Ship Structure Design Synthesis
Figure 5. Design Synthesis Approach
6
DESIGN OF HIGHER PERFORMANCE
STRUCTURES
The trend towards higher performance ship structures continues with goals to provide greater safety and reliability while minimizing structural weight and cost. This trend is driven by economic and performance factors from the ship owners and operators, from safety and environmental concerns from society, and from competitive pressures between shipbuilders. Higher performance, safe and reliable structures can only be achieved by using progressively more rigorous and accurate design and construction processes. The following kinds of ship structural design process components are evolving today and form the basis of the next generation of ship structural design.
Hydrodynamic loads predictions interfaced to structural analysis models. As structural design methods have developed, so have tools for hydrodynamic load predictions. To make the application of predicted loads efficient, these loads need to be automatically interfaced and applied to the structural model(s). This type of loads interface is being developed and refined so that more comprehensive and more accurate loads can be incorporated into the structural design process. Figure 6 indicates how this type of interface initiates many stages of ship structural analysis and design.
Next Generation Ship Structural Design
Flowchart of Integrated Structural Analysis
STRUCTURAL OPTIMIZATION: MAESTRO with DeMak (Multiple Methods)
Hydrodynamic Load Prediction
STRUCTURAL INTEGRITY: Dynamic Load Approach Spectral Fatigue Analysis Underwater Shock
Vibration Automated Structural Panel Evaluation (IACS CSR, ALPS/ULSAP, MAESTRO Native)
Hydrodynamic/Structural Model Interface
Structural Changes? No Yes Hydro Model S tr u ctu ral M o del STRUCTURAL LIFECYCLE: Corrosion Damage Recoverability Safe Operating Envelope
Structural Design Complete
Figure 6. Structural Design Process
Open architecture structural design toolsets allow special purpose analyses such as Dynamic Load Approach analysis, Spectral Fatigue Analysis, underwater shock response analysis for warships, free and forced vibrations, to be introduced as requirements as defined by ship classification societies and other safety authorities. “During the last few decades, methods useful for ultimate limit state assessment of marine structures have been developed in the literature. It is considered that such methods are now mature enough to enter day-to-day design practice.” (Paik, et al., 2007) An open architecture hosts multiple sets of structural integrity analysis and evaluation capabilities that can be invoked by the design team on a basis customized to meet a specific set of ship requirements. The open architecture further enables the efficient introduction of new analysis technologies as they transition from research to applied practice.
Structural optimization methods provide capabilities to move the structural design toward objective goals such as reduced weight and cost, while ensuring that all the necessary structural integrity constraints and safety margins are maintained. Hybrid solvers such as DeMak (Zanic et al., 2009) have been developed that organize multiple optimization procedures that can be applied to specific aspects of the structural design/optimization problem.
DeMak includes five methods: 1) multilevel multi criteria
search strategy; 2) fractional factorial design; 3) cross section optimizer; 4) genetic algorithms; and, 5) multi- objective particle swarm optimization. These methods are controlled via a ‘sequencer’ that gives the design team direct control over the application of the different optimization methods to different aspects of the structural design.
Structural lifecycle considerations including corrosion, fatigue, damage recoverability, and structural Safe Operating Envelope determination, comprise another set of complex ship structural performance elements which must be addressed as integral aspects with the design process.
These areas evolve from research and development, safety authority procedures, and owner/operator guidance and requirements. An interesting source of these requirements has been the development of ship classification rules for naval vessels.
“The rapidity and extent of the post-Cold War downsizing has caught many navies by surprise, forcing a global re-think of policies regarding acquisition, operations and maintenance of warships on a scale not seen since the Second World War. These navies are beginning to look to classification societies as an important element in preserving the technical standards of their current and future fleets, through the development of Rules, certification and classification procedures for design, construction and through-life maintenance.” (Ferreiro et al., 2001)
Feedback loop to the ship design model to return changes in the structural design to the baseline ship design model(s) for re-analysis and evaluation. As Figure 6 indicates the ship structural design process will evolve toward a more unified set of modeling and analysis capabilities and a more efficient and more effective set of computer-based tools for performing the design development.
7
SUMMARY AND CONCLUSION
Next generation ship structural design tools and methods must further unify structural design process sub-elements into a more efficient and higher fidelity process that supports the realization of engineering integrity with optimized performance for the owner/operator. Advances in design tool architecture, geometry and topology modeling, loads analysis, and structural evaluation must be better unified in order to achieve progress toward these objectives. Strategies for implementing these improvements have been in place for several decades now, and elements of the early strategies, for example the tenants of rationally-based structural design, have borne the test of time. On the other hand, the degree of complexity of ship structural design continues to grow driven by the results of scientific development coupled with the ever-competitive environment of ship owners and operators. As presented herein, the vision of next generation ship structural design requires more complete unification with both the basic ship topology design and with the multiple aspects of ship loading and structural design. Furthermore, decision support technologies and methods are here to stay and are becoming more widely applied and accepted. Next generation structural design will depend more on these technologies to effectively explore the design space and generate the best designs for ships of tomorrow.
Next Generation Ship Structural Design
REFERENCES
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EU FP6 project IMPROVE-Final Conference IMPROVE 2009, Dubrovnik, CROATIA, 17-19 Sept. 2009