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

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