Design Report Guided Missile Submarine SSG(X) - AOE - Virginia ...

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SSG(X) Design – VT Team 3 Page 66 spacing of one to two diameters. Based on this rule, an additional king frame divides the distance between the two bulkheads. Constraints provided by the arrangements fix the bulkhead and king frame locations. An aft section (38 ft. in length – Module 1) with external framing contains the machinery room; a watertight bulkhead separates this section from the next. Forward of this bulkhead are two sections (52 ft. and 30 ft. in length – Module 2 and 3, respectively) with internal framing which are separated by a king frame; a watertight bulkhead separates the third section from the forward most section. The forward most section (30 ft. in length – Module 4) with external framing contains the torpedo room. Figure 57 shows the modular divisions and frame locations. A rule for ring frame spacing suggests one tenth to one fifth of the diameter between ring frames. One tenth of the diameter is 32.4 inches. Initial optimization runs indicated that this is an optimum frame spacing. In order to achieve equal frame spacing along the length of the module, the frame spacing was calculated and fixed for the MathCAD calculations and within MAESTRO. HY-80 steel, which is the most common steel used in the construction of US Navy submarines, is used for the pressure hull shell plating and all framing. The framing consists of standard T-shapes. 4.4.2 Loads and Failure Modes The primary load on the pressure hull is the pressure at maximum operating depth. The MathCAD calculations use the nominal pressure in order to consider the following failure modes: shell yielding (SY), lobar buckling (LB), general instability (GI), frame yielding (fy), and frame instability (FI). MAESTRO assumes pressure is applied to plating on the side opposite of any frames attached to the plating. In order to correctly simulate pressure on the plating, artificial strakes with internal frames (with negligible scantlings) are placed at the same nodes as the actual strakes. Pressure is applied to the artificial strakes. As the nodes connected to the artificial strakes move in response to the pressure, the stress is transferred to the actual strakes via the displacement of the nodes. The axial compression caused by the pressure at the ends is simulated by placing equivalent point forces at the nodes on each end. MAESTRO considers the following failure modes: panel collapse membrane yield (PCMY), cylindrical local buckling (CCLocB), and cylindrical general buckling (CCGenB). 4.4.3 Safety Factors, Optimization Results, and Adequacy The following safety factors are used in determining adequacy in the MathCAD calculations: shell yielding (1.5), lobar buckling (2.25), general instability (3.75), frame yielding (1.5), and frame instability (1.8). Optimization is performed over continuous values; a final optimization with producible values is used to determine final adequacy values. Figure 56 shows the value of the weighted average buoyancy factor during the optimization; eliminating infeasible designs yielded Run #7 as the most efficient set of scantlings. Table 42 lists final adequacy parameters (normalized around one) by module and mode before and after conversion to standard size scantlings. Table 43 lists final frame scantlings. Figure 56 - MathCAD Optimization Results
SSG(X) Design – VT Team 3 Page 67 Table 42 - Module Adequacy Parameters Mode Module 1 Module 2 Module 3 Module 4 Final Adequacy Parameters Before Standardization Shell Yielding 1.28853 1.29033 1.2896 1.2896 Lobar Buckling 1.83517 1.85542 1.84717 1.84717 General Instability 1.99523 2.45622 1.415 1.415 Frame Yielding 2.11642 21.1956 1.24424 1.15088 Frame Instability 1.53919 1.43055 1.42653 1.54506 Final Adequacy Parameters After Standardization Shell Yielding 1.285 1.287 1.286 1.286 Lobar Buckling 1.840 1.860 1.852 1.852 General Instability 1.969 2.420 1.391 1.39 Frame Yielding 2.035 11.717 1.234 1.139 Frame Instability 1.552 1.443 1.439 1.558 Table 43 - Optimized frame scantlings Frame Type Flange Thickness Flange Width Web Thickness Web Height Ring Frame 5/8 (0.625) in. 6.0 in. 9/16 (0.5625) in. 12.0 in. King Frame 5/8 (0.625) in. 8.0 in. 5/8 (0.625) in. 24.0 in. The following safety factors are used in determining adequacy in the MAESTRO model: PCMY (1.5), CCLocB (2.25), and CCGenB (3.75). The initial result using the scantlings selected using the MathCAD calculations is an inadequate structure (see Table 44). Changes to the ring and king frame scantlings as well as frame spacing and shell thickness produce only minor changes to the adequacy parameters. In order to make the design feasible in MAESTRO, large changes have to be made which deviate from the results from the MathCAD calculations. Table 44 - Initial MAESTRO Adequacy Parameters Mode Module 1 Module 2 Module 3 Module 4 Mean CCGenB -0.602 -0.582 -0.565 -0.596 -0.586 PCMY 0.035 0.039 0.039 0.035 0.037 CCLocB -0.350 -0.351 -0.351 -0.351 -0.351 Because the MAESTRO results differ so greatly from the results from the MathCAD calculations and are only slightly affected by major changes in the hull geometry, the results from the MathCAD calculations are used until further work on developing a finite element model can be done. The scantlings listed in Table 43 are used to develop the structural profile in Figure 57. A midship section drawing is presented in Figure 58. Figure 57 - Longitudinal Structural Profile
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