of the alternative design is equal to that based on prescriptive rules. The main benefit of these regulations is expected for cruise vessels and ferries, as the alternative design approach allows large passenger and car deck spaces beyond what is possible with the prescriptive rules. In principle, ‘engineering analyses’ could also
mean fire experiments, but these are too costly and time consuming to support ship design. This leaves computer simulation as a suitable option. At present, zone models and CFD tools are considered for fire simulations in ships. Zone models are suitable for examining more complex, time-dependent scenarios involving multiple compartments and levels, but numerical stability can be a problem for multi-level scenarios, for scenarios with Heating, Ventilation and Air Conditioning (HVAC) systems, and for post-flashover conditions. CFD models can yield detailed information on
temperatures, heat fluxes, and species concentrations; however, the time penalty of this approach currently makes CFD unfeasible for long periods of real time simulations or for large computational domains. While reproducing several typical fire
characteristics, fire simulations are not yet mature, and more progress can be expected in the next decade. For example, results are not grid-independent with the currently employed typical grid resolutions, but finer grids appear out of reach for present computer power and algorithms. Despite such short-comings, fire simulations already appear suitable as a general support both for fire containment strategies and for design alternatives.
Structural analyses The reliable computation of loads is crucial for an accurate global FE strength analysis of a ship. In its “Guideline for the global strength analysis for container vessels” (2006), GL used the design wave approach to find those load combinations which were most relevant for the dimensioning of the structure. In contrast to the loading approaches in the common structural rules for bulkers and tankers, the hydromechanic pressure and the ship accelerations were taken from first principle hydrodynamic computations for regular waves. As an aid in applying the loading procedure, the software GL ShipLoad has been developed. GL provides a user-friendly computer application
for the efficient load generation for global FEA of ship structures. The graphical user interface facilitates the convenient application of ship and cargo masses to the FEA model. Hydrostatic and hydrodynamic computations are integrated into the program. GL ShipLoad supports the generation of loads from first principles (realistic inertia and wave loads for user supplied wave parameters), but the program also aids in the selection of relevant wave situations for the global strength assessment based on bending moments and shear forces according to Germanischer Lloyd’s rules. The result is a small number of balanced load cases that are sufficient for the dimensioning of the hull structure. Until 1998, the SOLAS regulations on subdivision
and damage stability specified damage stability requirements only for cargo ships longer than 100m. Since 1998, this limit has been lowered to 80m for new cargo ships. Additional transverse bulkheads to fulfil damage stability requirements are costly and restrict operations. However, new SOLAS regulations permit for
some ships alternative arrangements, provided that at least the ‘same degree of safety’ is achieved. This
THE NAVAL ARCHITECT FEBRUARY 2007 CFD is by now the most appropriate tool to support practical rudder and podded drive design.
notation allows some flexibility of structural designs supported by advanced simulations. E.g. a structural design having increased collision resistance thus reducing the probability of penetration of the inner hull could eliminate the need for additional bulkheads. Based on extensive FEA simulations for ship
collisions, GL developed an approval procedure which provides the first such standard for evaluation and approval of alternative solutions for design and construction of these ships. The basic philosophy of the approval procedure is to compare the critical deformation energy in case of side collision of a strengthened structural design to that of a reference design complying with the damage stability requirement described in the SOLAS regulation.
Vibration prediction Ship vibrations are increasingly important due to several design trends: lightweight construction (with low stiffness and mass), arranging living and working quarters near the propeller to optimise stowage space, high propulsion power, small tip clearance of the propeller (to increase propeller efficiency), and fuel-efficient, slow-running main engines. It has become standard practice to regulate vibration aspects for a new building on a contractual basis. Therefore, vibration predictions are performed already during the preliminary or structural design stage for many ship types. As complex structural arrangements can be reflected comfortably, 3-d FEA is today the standard tool used for this purpose. However, modelling of the stiffness distribution
is all but trivial and requires a certain amount of experience. This is specifically true if the dynamic properties of large deck panel structures, as often met on cruise ships or mega yachts, shall be considered in a realistic way. It must always be kept in mind that this represents one paramount prerequisite for an accurate prediction of the vibration level distributions to be expected on the deck structures.
However, the mass distribution must also be
accounted for carefully. This includes the light ship weight, the cargo and the hydrodynamic 'added' mass, which reflects the effect of the surrounding water. The matrix representing this hydrodynamic mass effect typically couples all wet degrees of freedom and is therefore complex to compute and handle. For a long time, the approach of Lewis (1929) – a frequency dependent diagonal matrix simplification – was used. To account for 3D effects accurately, in particular in
the case of appendices, a boundary element method can be used to compute the full matrix. A comparison of this procedure with the Lewis approach and full- scale measurements indicates superior results when using the full mass matrix. Use of empirical methods is still considered justified
for the determination of the propeller vibration excitation forces in the early design phase. However, with progressing design more accurate methods should by used, i.e. cavitation tank tests and CFD computations. In the last decade special software has been
developed to simulate the vibration excitation forces stemming from slow and medium speed diesel engines. By integrating a model of the engine frame into the FE-model this allows for the computation of the coupled vibration of hull and main engine structure. Direct calculation methods come to their limits
in calculating the vibration of local structures such as plate fields and stiffeners. Despite modern pre- processors with parameterised input possibilities and graphic support the required mesh density, the number of structural details to be considered and structures to be analysed makes the use of FEA methods more or less impossible for this purpose. Consequently, Mumm (2005) recommends a 2- step approach. At first the local vibration design is checked by calculating the natural frequencies of the local structures and comparing them to the relevant excitation frequencies. If resonance is likely, the scantlings of the local structures are changed. These
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