from the September 1999 issue of Marine Log
Kvaerner Masa Marine (KMM), through Kvaerner Masa Yards - Technology (KMYT), is
a part of the team working with Ingalls Shipbuilding, Pascagoula,
Miss., a unit of Litton Ship Systems, to build two 72,000 gt cruise ships for American Classic
Voyages.
Drawing on its cruise ship experience, KMYT is providing design and construction expertise to help Ingalls adapt its military shipbuilding background to this commercial program. While military vessels require shipyard facilities and personnel well suited to the building of cruise vessels, there are differences in the approach and utilization of these resources.
Kvaerner Masa Marine will complete the structural design of the vessels, performing all structural calculations, develop all structural plans and build Finite Element Models for submission to and approval by class. KMM believes these ships will be the first constructed by a U.S. shipyard built to dual class under both Lloyd's Register and ABS rules.
The structural design began in Finland at KMYT with the development of the Midship Section. The unique characteristics of 840 ft cruise ships make optimizing the structure to reduce weight significantly more complicated than for a typical cargo vessel.
The use of the Finite Element Analysis can reduce the structural weight by as much as 2,500 tons for a vessel of this size. The structural weight is in the order of 50% of the vessel's lightship weight, of which a third can be superstructure. The ability to minimize the structural weight enables the owner to maximize the passenger capacity and earning potential of the vessel.
The scantlings for the complete vessel are being completed by KMM in Annapolis using Lloyd's Rules. The structural arrangements are developed in AutoCAD and provided via e-mail to prepare two global structural models, one for longitudinal strength and one for racking. Breaking down the analysis into two distinct modeling tasks (Racking & Longitudinal Strength) requires marginally more man-hours than preparing one, fully detailed model. This is offset by the ability to advance one task independently of the other and therefore, ultimately reduce the total time required to complete the tasks.
The longitudinal strength model is being built using the Maestro software produced by Proteus Engineering, Stevensville, Md. Girders, transverses, pillars and floors are all modeled as beams. Plating and the associated longitudinals or beams are modeled as orthotropic plate panels. This approach provides the level of detail required for the global longitudinal strength analysis without burdening the modeling process with excessive, inconsequential detail. The complete hull model will have in excess of 35,000 nodes and 177,700 degrees of freedom. The model complexity makes it one of the largest models to be constructed in Maestro. The applied loads and limiting stresses are all as defined in Lloyd's "Large Passenger Ships Direct Calculation Procedure."
The racking model is being built entirely within COSMOS; a general finite element modeling package produced by Structural Research and Analysis Corp. KMM has written a number of routines that reduce the production time of stiffened plate panel models by 40%. These specialized routines allow rapid updates to be made to the model's geometry as the structural design evolves. Using these tools, all of the main fire zone bulkheads have been modeled using shell elements for plate and imbedded 3D Beam elements for stiffeners. As the bulkheads are the main load-carrying members in racking,
the level of detail in the bulkheads is quite high. The decks have been modeled in sufficient detail to approximate the load sharing between bulkheads and to accurately apply the loads to the bulkheads and side transverses. The model will have in the order of 50,000 elements, which will generate a stiffness matrix with over 250 million matrix elements. Once again, bulkhead openings have been simplified by eliminating corner radii and local inserts. Any areas of concern will be modeled in detail to confirm the adequacy of the structure.
Structural analysis is a major task on a vessel of this size and as a result it is expected that KMM staff will be working for over six months to complete the analysis and subsequent reports to the satisfaction of the classification societies. ML