The inside of a block is not one empty box

During ship construction, a block is not yet a completed vessel, but for workers it is already a complex internal space. Bulkheads, longitudinals, stiffeners, double-bottom structures, tank boundaries, manholes, and narrow passages divide the airflow. Once outfitting components, temporary platforms, cables, gas hoses, and welding machines are added, even a space shown as one area on a drawing behaves in practice like several small compartments. If access is restricted, natural ventilation is unfavorable, and a hazardous atmosphere can develop, the first step is to determine whether the space is subject to confined-space or hazardous-atmosphere management under applicable law and workplace procedures. It cannot be deemed uniformly safe or hazardous merely because it is called a “block.”

The goal of ventilation design is not simply to switch on a fan. Clean air must reach workers, and generated fumes, gases, and vapors must be captured or diluted along a route that does not cross the workers’ breathing zones and then discharged to a safe location. Measurements must verify that this route actually exists throughout the space. OSHA’s shipyard ventilation guidance warns that placing a fan only at a tank entrance may create short-circuiting, in which air circulates through a small area around the entrance and provides almost no protection to workers deeper inside. This is why the end of the duct must be brought close to the actual work location.

First, map the space and the work together

Before determining ventilation fan capacity, mark the air path on sectional and plan drawings of the block. Overlay on one sheet the location where clean outdoor air will be drawn in, supply openings, work points, openings in bulkheads, expected discharge points, worker travel routes, and emergency escape routes. Mark the areas behind stiffeners, puddles and bilges at the bottom, overhead pockets, dead ends, compartments with temporary covers, and duct bends as separate candidates for stagnant zones. Also check whether exhaust from an adjacent block or painting area could return to the supply-air side.

Next, record the sources generated by each task. Depending on working conditions, welding and cutting can produce metal fumes, ozone, nitrogen oxides, carbon monoxide, and other contaminants. In processes using inert gases, leaks can reduce oxygen, whereas an oxygen-hose leak can create oxygen enrichment and increase the severity of combustion. Painting, cleaning, bonding, and resin work generate solvent vapors and mists identified in the safety data sheets. Include exhaust from engine-powered equipment, decomposition products generated by heating existing coatings, and contamination entering from adjacent compartments. Even in the same space, morning fit-up work and afternoon painting require different ventilation and different measurement parameters.

This information is input for a competent person’s risk assessment. The actual airflow rate and equipment specifications can be established only after reviewing the target substances, generation rates, block volume and geometry, openings, personnel and processes working simultaneously, outdoor conditions, applicable exposure limits, explosion-protection requirements, and duct pressure losses. The placement principles or general air-change rates in this article must not be used unchanged as field design values. Actual design and work authorization must be finalized after a competent person or industrial hygienist familiar with the applicable law and workplace procedures evaluates the site.

Supply and exhaust are not just an inlet and an outlet—they form one airflow path

Supply ventilation is effective for delivering clean outdoor air through a duct close to the work area, diluting contaminants, and creating a relatively clean flow on the worker’s side. But if no path is defined for the supplied air to leave, contaminated air may be pushed into another compartment or toward an access opening. Exhaust ventilation is effective for drawing contaminants out near the source, but without a makeup-air path, contamination may enter through unintended gaps or the required airflow may not develop. In a large, complex block, supply and exhaust ventilation may need to be used together to create an intentional airflow path.

The basic principle is to make air flow from the clean side toward the contaminated side. Place the supply intake in clean outdoor air away from painting exhaust, welding fumes, and vehicle exhaust. Do not merely hang the end of the supply duct at the manhole; extend it close to workers or into the farthest compartment. Place the exhaust opening as close as practicable to the source, while discharging to a safe outdoor location so that exhaust does not accumulate near another workplace, ignition source, supply intake, or pedestrian route. OSHA 1915.51 requires contaminated air to be discharged outdoors or away from the source of intake air, and makeup air to be clean and respirable. Oxygen must not be used for ventilation.

If supply and exhaust openings are placed immediately next to each other at the same manhole, incoming air may short-circuit directly back out. Use an opening on the opposite side or extend the duct deep into the compartment so that air passes through the work area. In a compartment with only one opening, separate the terminal position of the supply or exhaust duct from the outdoor fan location, and verify airflow with a visualization method allowed by site procedure, such as a smoke test, together with concentration measurements. Where a flammable atmosphere may occur, separately review whether the fans, motors, ducts, and static-electricity controls are suitable for the work conditions.

Average ventilation does not eliminate stagnant zones

The theoretical number of air changes obtained by dividing block volume by the fan’s rated airflow is only a starting point. Actual airflow is reduced by long flexible ducts, small diameters, sharp bends, crushing, damage, filters, and leakage at connections. Air also takes the path of least resistance, so it may flow quickly only between large openings while leaving areas behind stiffeners or lower pockets untouched. A Korean Occupational Safety and Health Research Institute study of confined-space geometry found that even simple models required different ventilation volumes for displacement depending on geometry and the internal gas composition. Entry must therefore not be authorized solely through a time calculation based on the claim that “the space has received a certain number of air changes”; actual readings must confirm the condition.

When searching for stagnant zones, change both the drawing view and eye level. Treat pockets beneath the upper deck, the central work level, the double bottom and low floor points, vertical ladder passages, the far side of bulkheads, dead ends, and areas behind large equipment as independent points. Check not only locations far from the duct end but also places where the supply jet strikes a stiffener and turns back. If a worker welds while lying down or crouching, the breathing zone in that posture may differ from “middle height.” Even when the average concentration across the block is low, a high peak can occur around one person’s face.

KOSHA research on ventilation for shipyard welders notes that mobile local exhaust systems can be difficult to use because the work moves frequently and because of hull structures, and that flexible ducts inside blocks may be avoided because they obstruct access and are prone to damage. This does not mean ducts should be omitted; it means they must be installed to fit the work flow. Flame-retardant, durable ducts must be protected, bends reduced, and the duct end and measuring points moved whenever the work location moves. To prevent ducts from narrowing emergency escape routes or blocking ladders and manholes, consider ventilation saddles, separate openings, or another arrangement.

Distinguish source control from general ventilation

If welding fumes or solvent vapors can be captured close to their source, local exhaust ventilation is the preferred control. OSHA 1915.51 requires a movable hood to be positioned as close as practicable to the work point so that fumes and smoke are removed at the source and concentrations in the breathing zone remain within safe limits. Capture effectiveness falls if the hood is too far away or a strong supply-air stream pushes fumes toward the worker’s face. Conversely, the process and ventilation teams must jointly adjust location and airflow so as not to create cross-drafts strong enough to impair weld quality.

General ventilation dilutes contamination not captured by local exhaust and carries it out of the compartment. However, it must not be assumed that general ventilation alone can address highly toxic substances, high generation rates, solvents continuously evaporating from surfaces, or gas leaks that can create oxygen deficiency. The process may need to be isolated or the substance substituted, the leak source shut off, and local exhaust combined with respiratory protection. Continuing work in an inadequately ventilated IDLH atmosphere while wearing an ordinary particulate or gas-filtering respirator is not an acceptable alternative.

For painting, both the generation rate during work and off-gassing after work are especially important. OSHA 1915.35 requires sufficient exhaust ventilation to maintain solvent vapors below 10% of the LEL and frequent testing by a competent person during work with paints containing highly volatile, toxic, or flammable solvents. If ventilation fails or the concentration reaches that level, painting must stop and the compartment must be evacuated; ventilation must continue after painting until the space is gas-free. This figure comes from a specific provision of the U.S. shipyard standard and should not be applied mechanically to Korean work. Korean law, the paint SDS, exposure limits, and workplace criteria must be applied together.

Connect measurements at upper, middle, and lower levels with the work location

Pre-entry remote testing is performed from outside to prevent workers from being exposed to a hazardous atmosphere before it has been tested. Check oxygen, flammable gases and vapors, and anticipated toxic substances in that order, while accounting for whether the selected sensors respond to the actual substances and for pump-and-hose transport time. Do not treat one sample as representative of the entire space. Divide the space into upper, middle, and lower levels, and at each level measure the far end, closed compartments, areas around piping and hoses, low points containing residue, and gaps connected to adjacent spaces. Deep vertical compartments must be checked step by step along the travel path.

A gas’s relative density is a clue for selecting measurement locations, not the answer. Warm gases and welding fumes may rise by buoyancy, while cold vapors and some solvents may remain low. But supply jets, heat sources, human movement, bulkheads, and temperature differences can mix layers or create unexpected pockets. Do not measure at only one level by applying the one-line rule “light gases above, heavy gases below.” Reflect expected density but verify upper, middle, and lower levels and actual stagnant zones through multilayer measurements.

Measurements during work serve a different purpose from the approval measurements taken before entry. A portable monitor in the worker’s breathing zone tracks changes in the atmosphere the person actually inhales. Place area monitors near the source, in the most unfavorable stagnant zone, and where concentration would rise first if ventilation failed. Consider continuous monitoring when oxygen deficiency or a flammable atmosphere could develop rapidly. If the welding location, painted surface, or supply and exhaust ducts move, reposition the monitors as well. Avoid a situation in which a fixed monitor is still watching yesterday’s work point while today’s worker is in another compartment.

To verify ventilation performance, examine four types of points together. The first is the supply intake, which must remain clean; the second is the worker’s breathing zone; the third is the expected worst stagnant zone; and the fourth is the exhaust opening and its surroundings. Measurement at the supply intake finds recirculation of contamination, the breathing zone confirms personal exposure, and the stagnant zone finds high concentrations hidden by averages. Measurement around the exhaust checks whether discharged vapors reaccumulate in another workplace, a low area of the dock, or an adjacent block. OSHA 1915.35 also requires periodic testing to ensure that discharged vapors do not accumulate in other areas around the vessel or dock.

Instrument capability matters as much as instrument location

The presence of O2, %LEL, CO, and H2S sensors in a multigas monitor does not mean it measures xylene in paint or a particular metal fume. Identify target substances before work from the SDS and process information, and select methods suited to the purpose, such as direct-reading sensors, PID, detector tubes, particulate measurements, or personal sampling. Direct-reading instruments are strong at detecting rapid changes and providing alarms, while personal sampling may be required to assess time-weighted exposure. Neither replaces the other.

For pumped measurements, consider hose length and inside diameter, adsorption, condensation, leaks, and the response time of each sensor. If a long hose is lowered to several levels and read immediately, air from the previous position may remain at the sensor and lead to a wrong decision. At each point, allow enough time for the sample to reach the sensor and the indication to stabilize, and follow the manufacturer’s instructions. A bump test or calibration check before use, confirmation of battery status and pump flow, and inspection of filter condition are also necessary. Match alarm setpoints and units (ppm, %vol, %LEL) to the work permit, and record the time, point, height, work status, and ventilation status with the results.

When the work changes, redesign the ventilation and measurements

The atmosphere inside a block is not static. Air paths change when painting starts or the number of welders increases, when a duct is crushed or fan power is disconnected, when a manhole cover is closed, or when a new compartment is connected. OSHA 1915.15 requires work to stop and the space to be reinspected and retested when a change can alter conditions in a tested space, and it requires testing as often as necessary to maintain safe conditions. The ILO code of practice on safety and health in shipbuilding also recommends mechanical ventilation when natural ventilation does not maintain a safe atmospheric composition, local exhaust at the point of generation when necessary, and pre-entry testing for oxygen and flammable, explosive, and toxic substances whenever a hazardous atmosphere could accumulate.

At the pre-job meeting, do not check only the fan number and rating. Specify the supply and exhaust directions, duct-end positions, worst-case point, person responsible for measurement, conditions requiring remeasurement, evacuation upon an alarm, response to a power failure, and rescue plan. Fan failure, a monitor alarm, a rapid change in wind direction, the addition of a work process, and re-entry after a long break are signals for reassessment. Do not assume that conditions remain safe after ventilation stops. Work such as painting, where residual vapor may continue to be released, may require remeasurement after the equipment is stopped and a prescribed stabilization period has elapsed.

A good layout is one that is adjusted based on measurements

The success of ventilation inside a block depends less on the number of fans than on the actual path of air. Clean air must be delivered to the work point; contaminants must be captured at the source or carried through stagnant zones and discharged safely outdoors; and short-circuiting at access openings and recirculation must be prevented. Measurements at upper, middle, and lower levels, the worker’s breathing zone, the source, the farthest compartment, and the surroundings of the exhaust must be connected to show that this path is working.

A standard layout therefore cannot simply be copied to every block. A competent person must assess the space geometry and substances generated by the work, establish a temporary ventilation plan, and then adjust duct and monitor locations based on multipoint pre-entry measurements and monitoring results during work. The objective is not a check mark labeled “fan operating,” but a verifiable condition in which every worker continues to breathe safe air for the entire duration of the work.

Sources

Cover photograph

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