Bulkheads are vertical steel walls inside a tanker that divide the hull into separate compartments. They serve three critical purposes: keeping the ship structurally rigid, preventing liquid cargo from sloshing freely, and stopping flooding from spreading if the hull is breached. Every tanker, whether it carries crude oil, refined fuel, or chemicals, relies on a network of bulkheads to stay stable and safe.
How Bulkheads Keep a Tanker Stable
The biggest danger of carrying liquid cargo is something called the free surface effect. When a partially filled tank is on a ship that tilts, the liquid inside shifts to follow gravity, moving the tank’s center of weight to one side. This acts like adding heavy weight high up on the ship, reducing its ability to right itself. In a large, undivided tank, this effect can be severe enough to capsize a vessel.
A single watertight bulkhead splitting a tank into two equal halves reduces this destabilizing effect by up to 75 percent. That one partition dramatically limits how far the liquid can travel when the ship rolls. This is why tankers carry their cargo in many smaller tanks rather than one giant hold. The more compartments, the less any single body of liquid can shift and threaten stability.
Transverse vs. Longitudinal Bulkheads
Tanker bulkheads run in two directions. Transverse bulkheads cross the ship from side to side, dividing the vessel’s length into separate tank sections. They carry vertical loads the ship encounters in calm water and rough seas, and they provide resistance against twisting forces on the hull. Classification societies like Lloyd’s Register set rules for how far apart transverse bulkheads can be spaced, based on the permissible length of cargo tanks.
Longitudinal bulkheads run parallel to the ship’s length, splitting the width of the vessel into port and starboard tanks (and sometimes a center tank as well). These are especially important in tankers and other vessels carrying liquid cargo, where controlling side-to-side liquid movement is essential. In dry cargo ships, you rarely see continuous longitudinal bulkheads, but in tankers they’re a defining structural feature. The longitudinally framed side shell, inner hull, and longitudinal bulkheads are all tied together by transverse support structures that distribute forces evenly through the hull.
Swash Bulkheads
Not every bulkhead inside a tanker is watertight. Swash bulkheads are perforated partitions, essentially steel walls with holes, placed inside cargo tanks to reduce sloshing without fully dividing the space. As liquid surges toward the bulkhead, the perforations force it through small openings, converting the energy of a large wave into turbulence and friction. This significantly lowers the impact forces on the tank walls.
The key design variable is the open-area ratio: how much of the bulkhead’s surface is open holes versus solid steel. Research has shown that the optimal ratio depends on how full the tank is. A lower open-area ratio works better to counter the primary back-and-forth sloshing mode, while a higher ratio is more effective against secondary sloshing patterns. Engineers tune the perforation pattern to match expected filling levels, and experimental studies dating back to the design of very large crude oil carriers in the 1960s and 70s confirmed that swash bulkheads are more effective at controlling sloshing than simple baffles or internal columns.
Cofferdams: The Empty Spaces Between Bulkheads
At the forward and aft ends of a tanker’s cargo area, you’ll find cofferdams. These are narrow empty spaces formed between two adjacent oiltight transverse bulkheads set at least 760 millimeters apart. Their purpose is to create a buffer zone between cargo tanks and other parts of the ship, like engine rooms or crew quarters. If cargo leaks through one bulkhead, the cofferdam catches it before it can reach non-cargo spaces.
Cofferdams also separate incompatible cargoes on chemical tankers, where mixing different substances could cause dangerous reactions. In some designs, cofferdams double as ballast tanks when approved by the classification society. For ships carrying extremely cold cargoes (below minus 55°C), cofferdams are mandatory to protect surrounding structures from thermal stress.
What Bulkheads Are Built From
Tanker bulkheads are constructed from structural steel plates, with the specific grade and thickness determined by the ship’s size, the bulkhead’s location, and the loads it must handle. For large tankers exceeding 250 meters in length, classification rules call for higher-grade steels (designated by letters like D, DH, E, and EH) in critical areas. The “H” grades are higher-strength steels that allow thinner plates while maintaining the same structural capacity.
Plate thickness isn’t uniform. Engineers calculate the minimum required thickness based on the span between stiffeners, the expected pressure from cargo and seawater, and buckling resistance. Corrugation webs, for instance, must pass shear buckling checks that account for both the steel’s yield strength and the geometry of the corrugation pattern.
Corrugated Bulkhead Design
Many tanker bulkheads are corrugated, meaning they have a zigzag or wave-like profile rather than being flat. This shape gives the steel plate much greater stiffness without needing to be excessively thick, similar to how corrugated cardboard is stronger than a flat sheet. Chemical tankers in particular use corrugated bulkheads because the profile achieves structural stability while creating smoother internal surfaces that are easier to clean between different cargo loads.
A joint feasibility study by the classification society DNV and industry partners explored simplified corrugated designs using 45-degree inclined plates at each corrugation end. The goals were practical: fewer individual steel pieces, less welding, cleaner surfaces with fewer shadow areas where cargo residue could hide, better drainage, and more usable tank volume by eliminating voids behind angular plates. Keeping surface stresses low also reduces the risk of paint cracking, which matters because the protective coating is the first line of defense against corrosion.
Forces Bulkheads Must Withstand
A transverse corrugated bulkhead in a tanker faces multiple simultaneous loads. Lateral pressure from the liquid cargo pushing against the wall is the most obvious, but it’s far from the only one. The bulkhead also absorbs “carry over” bending moments transferred up from the double bottom structure below, vertical loads transmitted from the deck above, and horizontal forces from the combination of external seawater pressure on one side and internal cargo pressure on the other. Sloshing adds dynamic impact loads on top of all these static forces.
The carry-over bending moment at the lower end of the corrugation can be substantial when adjacent tanks are loaded unevenly, a pattern called alternate loading. This is why the connection between a bulkhead and the ship’s bottom structure (called the stool) is one of the most heavily engineered details in the entire vessel.
Corrosion and Maintenance
Bulkheads deteriorate over time. Protective paint coatings break down from exposure to cargo chemicals, seawater, the warm humid atmosphere inside enclosed tanks, and physical damage from cargo handling equipment. Once the coating fails, the underlying steel corrodes, gradually thinning the plate and weakening the structure.
Corrugated bulkheads and their supporting stools are known corrosion hotspots, and classification societies require regular thickness measurements during surveys. Corrosion rates vary depending on what cargo the tanker carries, how often tanks are washed, ventilation conditions, and whether the ship operates in tropical or temperate waters. Combined with fatigue cracking from repeated loading cycles over years of service, corrosion is the primary reason aged tankers face increasingly strict inspection requirements.