Exploring the Evolution of Submarine Design

Submarine Design: No technology or information has been applied before in history, and this is particularly true when looking at the design and building of Cold War-era submarines. Modern naval submarines are equipped with state-of-the-art characteristics that come from Cold War technologies, which were themselves derived from 1930s Axis and Allied efforts.

The Soviet Union and the Western-allied countries commandeered numerous powerful, cutting-edge U-boats when Germany was defeated in 1945. Among them were the newest Type XXI boats, which at the time were the most advanced underwater combat vehicles to be deployed at sea.

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Since Soviet leader Joseph Stalin had given orders to dismantle all possible industrial infrastructure on German land for use in reconstruction in the Soviet Union, the occupying forces, especially the Red Army, also stripped entire German shipyard facilities and all of their production lines along with all of their surviving equipment, blueprints, design documents, prototypes, and unfinished boats. This is a quick overview of the major developments that still influence many aspects of submarine design and construction today.

Finding the loads that the structure will be subjected to is always the first step in the structural design process. The following categories apply to the cargoes carried by a submarine throughout its mission:

1. Pressure from Diving Causes Loading

One of the most crucial and determining factors in structural design is depth. The main structural component of the submarine is the pressure hull, which is made to withstand hydrostatic pressure from the outside. It is intended to collapse to a specific depth, within a very specific range, at which total failure is anticipated. In reality, the collapse depth is determined by multiplying the service depth or maximum operable depth (MOD) by a safety factor. The hydrostatic pressure at this depth is taken as the design pressure for all pressure hull calculations.

Submarines built to conventional design tolerances—which utilize safety factors of 1.5—should not be submerged beyond the service depth. On the other hand, they can go below the service depth in designs with greater safety factors (like 2.5), but only in an emergency.

2. Shock Weights

The load-bearing capacity of a submarine is intended to withstand underwater detonations, such as mine explosions and pressures caused by the burst of enormous underwater gas bubbles.

Underwater explosion physics is a fascinating field of study since it differs greatly from air explosion physics. Watch the video below to see how the explosion ball is made, how it contracts, and how it bursts once again to release a cloud of gas bubbles to grasp it.

A shockwave is produced at the moment of explosion, applying radial outward pressure to the surrounding water. This explosion ball grows until the water surrounding it causes the internal pressure on the ball’s inner wall to equal the hydrostatic pressure outside the ball. The pressure in the ball’s center is now less than the outside pressure because of its expansion. It collapses and contracts as a result of this. This implosion results in a cloud of gas bubbles that is expanding radially. The contraction, implosion, and expansion processes repeat in sequence until all of the explosion’s energy is gone. Every subsequent explosion is less in diameter and magnitude than the one before it.

Thus, the study aids in our conclusion that a submarine should be able to endure many shockwaves in the event of an explosion. Additionally, experiments have shown that the submarine tends to be drawn toward the center of the explosion cloud with each contraction of the cloud. The worst-case scenario is an explosion beneath a submarine, which causes a downward suction. If this occurs at the maximum service depth, it may force the submarine to be drawn into deeper water, increasing the structural risk because of hydrostatic pressure.

Every underwater explosion generates a shockwave that travels along the pressure hull in addition to the immediate shock load it causes. In addition to shortening the fatigue life, vibratory stresses have the potential to produce resonance and catastrophic structural failure.

3. Additional Loads

Similar to surface ships, submerged submarines experience torsional loads from wave action, longitudinal bending loads, and transverse shear forces on transverse structures.

Motor action produces local loads such as torsional and longitudinal vibrations. The structure must be designed in a way that keeps vibration levels substantially within permitted bounds.

Hull’s Pressure Strength

In a diving state, longitudinal compressive stress is applied on the cylindrical pressure hull. The circumferential or hoop stresses are twice as great as this longitudinal stress.

The hull thickness and stiffener scantlings needed to keep the pressure hull from buckling are determined using the following expression for the longitudinal stress on the pressure hull.

Hull's Pressure Strength

Thus, the pressure outside, the pressure-resistant hull’s radius, and the hull plate’s thickness all affect the longitudinal stress. How does a submarine designer fit into this equation now? The pressure hull radius is a fixed entity for the duration of the structural design since it is an input from the customer; that is, the submarine’s radius is defined along with a range.

In this computation, the external pressure is defined as the hydrostatic pressure at the collapse depth. The collapse depth is fixed because the contract specifies it as well. The pressure hull’s remaining vary in thickness. The yield strength of the material utilized now determines the maximum longitudinal compressive stress on the pressure hull. The minimal thickness necessary to maintain the stress within tolerances is what a designer determines for a given material.

Hull's Pressure Strength

From the aforementioned relationship, the following conclusions can be drawn:

  1. A fixed MOD necessitates a thicker pressure hull plate for a submarine with a bigger diameter than one with a smaller diameter.
  2. Using materials with higher yield strengths can result in a submarine’s pressure hull having a lower minimum thickness requirement. Although a thinner material might be better for weight reduction, it would cost more.

Transverse ring stiffeners are not necessary because the pressure hull shell absorbs all longitudinal forces. Nonetheless, ring stiffeners that are capable of absorbing the circumferential stresses brought on by buckling loads stiffen the shell. The pressure hull shell and the ring stiffeners, which are typically T profiles, are welded together, and the system functions as a single unit.

There are three ways that the pressure hull can fail, and the stiffener configuration determines how likely it is for each mode to occur, as will be covered below:

  • Failure Mode 1: When the ring stiffeners are positioned closely together and exhibit high scantling, this is the first mode of failure. The shell plate yields as a result between two successive frames. As seen in the picture below, the yielding, which takes place across the shell’s circumference between two frames, is also known as symmetrical buckling.
  • Failure Mode 2: When there is a greater frame spacing between frames, the scantling of the frames is too low and they are positioned too far apart. In this instance, the shell plate buckles in the shape of a wave at the edges of two successive frames. The schematic picture below illustrates how one buckle will be oriented inward and the other outward.

Failure Mode 1
Because there are five waves produced in the example above, it is referred to as a five-lobe buckling. Depending on the distance between the stiffeners and the load, two to five lobes may be formed. An actual picture of shell buckling in between frames may be seen in the accompanying figure.

  • Failure Mode 3: Since the first and second modes of failure were local failures, there would be no immediate threat to the integrity of the pressure hull’s entire structure from such an occurrence. However, failure in mode three results in the pressure hull buckling over its whole length, which bends the transverse rings off-axis, as seen in the picture below. The pressure hull’s length between two heavy transverse structures, such as bulkheads or heavy web frames, would be affected. Another name for this is complete pressure hull collapse. This happens when the submarine descends deeper than the depth of collapse or when the material is not strong enough.

In addition to the three failure mechanisms mentioned above, the following are some other pressure hull failure modes:

  • If you do not weld or scantle correctly, the pressure hull shell, circular frames, bulkheads, and decks can become generally unstable, which can lead to failures in specific areas or across many frames.
  • Because the aft conical bulkhead and the forward elliptical bulkhead (dome) experience different compressive loads, snap-through buckling may happen at these locations.
  • Cracks on the pressure hull structure may begin to appear and spread as a result of low cycle stresses.
  • Failures brought on by the accumulation of stress at areas where the shape discontinues. For instance, there are large strains at the joint connecting the pressure hull’s cylindrical part with its forward elliptical and aft conical ends.

The types of failures that a pressure hull can experience and how they affect the structure’s geometry are summarized in the accompanying figure.

Fact

Fact about submarine design

Submarine pressure hulls can be reinforced from the inside or the outside. However, for the following reasons, external stiffening is more desirable:

  • It has been noted that external stiffeners with an equivalent scantling to interior stiffeners yield five percent greater strength.
  • There is a lot of usable space within the pressure hull since external stiffening takes up the space between the pressure hull and outer hull.
  • However, for certain designs, particularly those in which the pressure hull serves as the outer hull for the majority of the submarine’s length, internal stiffening is the only practical solution.

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The following figure depicts the structural elements of a double-hull submarine at a portion that houses the sail, just like a surface ship has drawings of its midship and all transverse frames. The arrangement at an ordinary frame is shown in the right half of the drawing, while a web frame (often at three to five frame intervals) is shown in the section to the left of the centerline.

Pressure Cracks in Hulls

With atmospheric pressure inside its contained volume, the pressure hull is a pressure-tight enclosed structure. Even yet, there must be a way to get from the interior to the outside in both surfaced and submerged situations. Circular hatches are installed for personnel access, with a conning tower at the center and a single hatch each for the forward and aft directions. Access points are made available for cables and pipelines that link equipment that is stored outside the pressure hull but is operated from within.

Pressure Cracks in Hulls

There are many penetrations in the forward elliptical dome bulkhead, two of which are for the passage of torpedo tubes and the other of which gives access to pipelines for the weapon compensating tanks. The graphic below depicts penetrations on a submarine’s forward bulkhead.

These are extremely important structures because the borders of the penetrations, whether round or elliptical, constitute concentrated areas of high stress because they are inevitable discontinuities on the pressure hull. Therefore, the welding of pressure hull penetrations is a highly regulated operation, and the welds of pressure hull penetrations typically undergo multiple types of non-destructive testing (NDT).

Category of Submarine Structures

Category of Submarine Structures

Submarine structures can be roughly classified into three groups based on the potential consequences of their failure for the submarine.

  • Class I Structures: These are the kinds of structures that, in the event of destruction, would not only make it impossible for the submarine to operate or stay afloat but would also endanger the personnel’s safety. The pressure hull’s shell and stiffeners together make up its whole basic structure, which is classified as Class I structure. Strict NDT guidelines are adhered to certify the calibre of these constructions. Before the installation of additional structural components, the pressure hull is additionally pre-tested to its design pressure through a vacuum inside it.
  • Class II Structures: Should these structures sustain damage or fail, they would only impair a portion of the submarine’s mission-fulfilment capacity. Damage to Class II structures would impact a system, or a portion of a system, that performs an essential function within the submarine, even while it does not make the submarine inoperable. Often, these damages can be fixed on board or by dry docking the submarine. Strict NDT standards also apply to Class II structures. Ballast tanks, trim tanks, regulating and compensating tanks, and pressure hull penetrations are a few examples of these.
  • Class III Structures: Damage to Class III structures wouldn’t be dangerous or significantly affect the submarine’s ability to navigate the seas. While the submarine is in service, these damages can be fixed. Equipment supports and knee brackets are a few examples of Class III buildings.

Conclusion

All submarine design firms carry out comprehensive finite element evaluations for a variety of load scenarios that the structure might encounter. The fact that submarines, unlike many surface ships, are not tested projects limits the breadth of research in this sector. Since every design is unique and depends on the navy and other interconnected project needs, there is ample opportunity for the designers to get better with each subsequent design.

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