CUVX Design Report - the AOE home page - Virginia Tech

CUVX Design – VT Team 2 Page 56AH36 steel was selected for the hull with HY-80 for the sheer and stringer strakes at deckedge. A standard catalogof shapes and plate thicknesses was selected using I-Ts, Ts and a limited number of fabricated shapes. The catalogwas kept as small as possible to maximize producibility. The geometry was modeled in MEASTRO, a coarse-meshfinite element solver with the additional ability to assess individual failure modes. After assessing adequacy, a fewiterations of scantling changes to correct inadequacies and reduce weight were performed.4.3.1 Geometry and Modeling ProcedureA three-dimensional mesh of CUVXHI3’s shell is created in FastShip. This mesh was imported into aMAESTRO file. The coordinate axes are adjusted such that the origin is coincident with the forward perpendicularof the imported mesh and the X-axis is positive in the aft direction, the Y-axis is positive vertically upward, and theZ-axis was positive in the port direction. Using the vertices of the imported mesh as reference points, the hull formis created in MAESTRO. Figure 45 shows the completed MAESTRO model.CUVX HI3 is a longitudinally stiffened ship with transverse frames every three meters. Initial scantlings werechosen based on similar designs. Figure 46 and Drawing D6 show the CUVX HI3 midship section. The structurebelow the damage control deck and in the middle two thirds of the ship uses an Advanced Double Hull design.Above this deck, the structure is a more traditional single hull design, with the decks and side shells supported bylongitudinal stiffeners, girders and transverse frames with tee-shaped cross-sections. Deep deck beams are used tosupport aircraft decks and the large transverse unsupported spans required over hangar decks.The Advanced Double Hull (ADH) is a cellular type of structural design that increases hull damage toleranceand producibility. A single average cell from the ADH section of the ship is approximately 9 meters long, 1.5meters in breadth, and 1 meter deep. In the right half of Figure 46, the cross sections of 17 of these cells are shownbelow the damage control deck. On the left side, a transverse web is shown below the damage control deck. Thesetransverse webs occur approximately every nine to twelve meters in the ADH vice the standard 3-meter framespacing used in the remainder of the structure. Figure 47 shows the portion of the hull that is constructed ofAdvanced Double Hull. The outer skin of the ADH is shown in the flat green color, and the inner skin is shown inyellow.Figure 48 shows the interior of the MAESTRO model. As shown in the figure, the Advanced Double Hullstarts and ends at 33 and 168 meters aft of the forward perpendicular, respectively. Note that there are very fewcenterline bulkheads in the ship, requiring that the hangar deck beams be very large. One of the ship’s aircraftelevators is visible in the aft section of this figure.The model includes eight substructures. There are short substructures in the bow and the stern, and six mainsubstructures between them, each averaging about 33 meters. Each of the six main substructures is divided intothree port and three starboard modules. There is also an additional module for each bulkhead4.3.2 LoadsFull Load and Minimum Operating weight distributions from HECSALV were applied in MAESTRO loadcases as illustrated in Figure 49. The resulting shear force and bending moment diagrams for the full load hoggingwave load case are shown in Figure 50 and Figure 51. The ship is balanced on hogging and sagging trochoidalwaves. Wave lengths in hogging (crest at midship) and sagging (crests at FP and AP) are equal to the ship length.The wave height is 1.1L 1/2 . Other quasi-static sea loads including passing wave (0.675L 1/2 ), wave slap, green seas,flooding and aircraft loads are also applied in a series of load cases.4.3.3 AdequacyMAESTRO calculates stresses for each load case and compares them to limit state values for various failuremodes. Stress divided by failure stress for various modes of failure results in a strength ratio, r. This value canrange between zero and infinity. An adequacy parameter is defined as: (1 – r)/(1 + r). This parameter is alwaysbetween negative one and positive one. A negative adequacy parameter indicates that the element will fail in aparticular failure mode and load case. A positive adequacy parameter indicates that it should not fail. An elementwith an adequacy parameter of positive one is over-designed. At this level of analysis, the main objective is to makeas many of the adequacy parameters positive as possible without too much over-design. In a more detailed analysis,the objective would be to adjust the scantlings such that all adequacy parameters were positive, but not much higherthan zero. A safety factor of 1.25 is used for serviceability limit states and 1.5 for collapse limit states.