Ore carriers are designed to carry high-density iron ores on long-haul routes. To ensure their safe design and construction, DNV has developed state-of-the-art procedures and software for structural analysis taking these demanding operational challenges into account.

When VALE ordered its 12 VLOCs in August this year, see article on previous pages, one of its main requirements to the design of these ground-breaking vessels was that they should be designed with special attention to fatigue and ultimate strength. As a result, extensive analysis will be carried out in compliance with the procedures and requirements of the DNV class notations Nauticus (Newbuilding) and CSA-2. In addition to the DNV main class requirements, this extended calculation scope includes direct wave load analysis, global and local finite element analysis, and spectral fatigue calculations.
The Nauticus Hull Rule Check, Finite Element and Wave Load Analysis software modules provides an integrated solution covering the complete scope of these analyses.

Rule Check
The DNV main class requirements for hull structural design are covered by the basic Nauticus Hull Rule Check package. User friendliness and powerful modelling capabilities make the Rule Check package a generic design tool and a preferred system among ship designers for initial hull girder design and optimisation.

Cargo hold analysis
GeniE is the Finite Element module in Nauticus Hull. It represents the latest-generation design and analysis software for maritime and offshore structures. By concept modelling techniques, GeniE allows engineers to focus on the structure, loads and environmental conditions, rather than on nodes and elements. Combined with strong features for 3D visualisation, this significantly reduces the time spent on modelling and documentation and provides efficient verification.

Wave load analysis
Leading-edge software for prediction of hydrodynamic loads is available with the Nauticus Hull Wave Load Analysis package. With HydroD, the global responses and local loading of the ship are calculated based on the actual hull shape and mass distribution. Long-term design loads and fatigue loading are calculated based on scatter diagrams representing specific trading routes and operation profiles, and the uncertainties in the calculations are significantly reduced compared to the conventional rule-based approach.
Extreme hull girder shear forces and bending moments are established by non-linear time domain analysis including effects of forward speed and non-linear effects such as the integration of Froude-Krylov force and hydrostatics over exact wetted surface, finite rotation angles in equations of motions, quadratic pressure terms and quadratic roll damping.
Both linear frequency domain loads and non-linear design loads calculated
in HydroD can be transferred to the finite element models to calculate the structural response. The automatic load transfer includes a reduction of the dynamic
pressure range in the surface region
to account for intermittent wet and dry surfaces in the fatigue calculations.

Global finite element analysis
The procedure mandated by the CSA-2 notation require that global finite element analyses serve as basis for yield and buckling strength checks. The global finite element model can be modelled using the GeniE cargo hold model as basis and adding the fore and aft ship either directly to the model or as separate super-elements. Buckling strength calculations are carried out using the PULS software from DNV, with PULS Excel as the main tool supported by the PULS advanced viewer for more detailed evaluation and trouble shooting.

Local finite element analysis
Both in the Nauticus(Newbuilding) and the CSA-2 procedures, local finite element analyses are to be carried out for highly stressed structural details such as panel knuckles and structural discontinuities to determine the hot spot stresses for the spectral fatigue calculations. With GeniE, the local models can be easily generated from the global model by adjusting the mesh density. To limit the size of the models, submodelling technique should be used rather than including fine mesh areas in the global analysis.

Spectral fatigue calculations
Spectral fatigue calculations are based on the combination of direct calculated wave loads and finite element analysis. When the full set of frequency domain loads
calculated in HydroD, normally including 250 to 300 complex load cases, are transferred to the global and local models, stress transfer functions (or RAOs) are obtained directly from the structural analysis. The stress transfer functions are combined with the wave scatter diagram and spectrum in the Stofat software module, and the fatigue strength is calculated based on design S-N curves.
With Stofat, spectral fatigue calculations can be performed both for global screening purposes to identify fatigue-prone areas and for more detailed assessment of stress concentration models. Visualisation of the calculated fatigue damages and fatigue lives directly on the finite element models is a key feature supporting assessment and understanding of the fatigue results.

The benefit of concept technology
Productivity and quality are essential to manage such large and complex analysis as mandated in the CSA-2 procedure. Nauticus Hull provides such efficiency through an integrated analysis system based on concept technology and workflow templates.