KUP 42 Ability to explain how to ensure reliable detection of defects and damages

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Reliable detection of defects and damages is paramount for ensuring the safety, integrity, and longevity of any vessel. In the demanding maritime environment, where ships are constantly exposed to corrosive elements and harsh weather conditions, early identification of structural weaknesses or potential failures is crucial to prevent catastrophic events.
This discourse will delve into the methodologies, tools, and best practices employed in the maritime industry to ensure reliable detection of defects and damages on ships. We will explore the various types of inspections, ranging from visual examinations to advanced non-destructive testing techniques, highlighting their strengths, limitations, and appropriate applications.

Detection of defects and damages

The vast storage spaces within a ship, known as tanks, are crucial for cargo transport and revenue generation. These steel tanks also play a vital role in maintaining the ship’s stability, especially when managing fuel or ballast water levels. Given their importance, these tanks require regular upkeep. Planned inspections, cleaning, and repairs are essential for ensuring the hull and its components remain in top condition. Over time, however, the steel structure of these tanks can deteriorate. If left unaddressed, this can escalate into serious structural defects, potentially leading to ship loss and even endangering lives.

To prevent such catastrophic consequences and avoid costly downtime, timely and effective repairs are crucial. This includes methodical visual inspections of the tanks. This guide aims to provide a comprehensive overview of how to inspect various ship tanks, including cargo holds (both dry and wet), ballast tanks, void spaces, fuel oil tanks, and freshwater storage tanks.

When inspecting steel tanks, pay close attention to the following:

Overall Condition: Upon entering, gauge the tank’s general health by observing the state of accessways, ladders, and paint coatings. Focus on corrosion-prone areas like weld joints.
- Rungs, steps, and ladders are often the first to show signs of rust due to oxidation, so check them for material loss.
- A thorough examination of the paint coating will help assess the tank’s overall resistance to corrosion. Remember, the paint applied on tank surfaces is typically light-colored for easier identification of issues. Re-coated areas should be carefully re-examined for any signs of coating failure, scaling, or pitting
Corrosion Levels: General corrosion, or non-protected oxidation, typically appears uniformly on uncoated internal surfaces. The corroded layer often flakes off, exposing the bare metal to further deterioration. In coated tanks, corrosion begins once the coating starts to fail. It’s difficult to determine the extent of steel thickness reduction unless significant shrinkage has occurred.
- For instance, in liquid cargo tanks like those on crude oil tankers, corrosion is primarily caused by a combination of corrosive gases, crude oil acids, and seawater (from crude oil washing). These factors, coupled with temperature fluctuations and structural flexing over time, gradually reduce the thickness of steel plating and supports, ultimately leading to structural failure.
  - Pay close attention to areas like:
  - Sounding pipes and striker plates
  - Air vent and tank gauging openings
  - Internal piping, including expansion joints, dresser couplings, and related fittings near valves
  - Bilges and tank top areas
  - The underside of hatch coamings and tank openings
  - Bulkheads and joints associated with girders and web frames
Sacrificial anodes, typically made of zinc and other materials, are key components in preventing corrosion within tanks, particularly ballast tanks. However, due to their sacrificial nature, they gradually wear down over time. Therefore, it’s crucial to regularly inspect anodes for excessive wear and tear. A record of material wastage should be kept to track their condition. Additionally, ensure that the anodes are securely fastened to their brackets.
Checking for Damage, Cracks, and Deformations: Proper lighting is essential for tank inspections to reveal any deformations or dents. Shadows are excellent indicators of buckling or cracks, but darker coatings like coal tar epoxy require maximum illumination. Deformations might not be easily visible over large areas. To spot them, use a high-beam flashlight parallel to the surface to highlight irregularities. If a flashlight doesn’t reveal straight lines, try the traditional method of using a string or rope to identify hidden deformations. Buckling, a type of severe deformation, can occur even with small load increases. Permanent buckling often results from overloading already weakened structures due to corrosion or contact damage.
Pitting corrosion: Pitting corrosion is frequently found in the bottom plating of ballast tanks, particularly near the bell-mouth, or adjacent to suction wells of submerged pumps within liquid cargo tanks. It typically starts with the localised breakdown of coatings, exposing bare metal to accelerated oxidation and galvanic corrosion. Blister formation commonly occurs in areas where surface preparation was insufficient before painting or where the coating has failed to adhere. These raised bumps on tank surfaces can be a warning sign of hidden corrosion underneath. Inspectors should be vigilant for these blisters, as they may indicate underlying structural issues.
Tank Gauging Systems: Carefully inspect all gauging systems, including gas measuring gauges, pressure gauges, temperature gauges, remote level sensors, sounding pipes, and striker plates, for any signs of malfunction. Check for rust under the tank top near the conduits housing the gauges. If possible, manually remove any debris like mud or oil deposits from the remote measuring sensors and test their operation. For instance, physically testing the remote gas measuring devices during the inspection is highly recommended. Ensure all gauges, both inside and outside the tank, are calibrated during major inspections (like dry docking) or at manufacturer-specified intervals.
Safety Devices: Safety devices installed in tanks are crucial for alerting crew members to potential hazards, like water accumulating in cargo hold bilges due to condensation or other causes. These devices are of paramount importance on board and should be prioritized for visual inspection. It is not common practice to manually test bilge high and low-level alarms in critical spaces like the chain locker, dry cargo holds, and void spaces. However, ensuring their proper operation is vital for early detection and response to potential flooding.
Mud or Sludge Buildup: Excessive mud and oily sludge in tanks can be harmful as it can conceal significant defects and foster structural deterioration beneath horizontal surfaces. Therefore, it is strongly advised to remove this debris before any tank inspection. This might involve thoroughly washing crude oil tanks to expose defects or using a hose to wash away mud buildup in ballast tanks. This process also helps reveal any pitting corrosion or deformations in the bottom shell.
Cargo Equipment: Inspect all cargo equipment inside the tanks, including heating coils, cargo pumps, crude oil washing machines, remote gauging systems, and temperature/heat sensors. Perform a leak test on heating coils using compressed air or steam. Visually inspect pipework and steam traps within the tanks for any defects or leaks.
The officer conducting the inspection should physically verify the proper operation of all cargo equipment within the tank. This can be done by remotely activating the system from the control room and confirming its functionality through feedback from inside the tank. Any malfunctions or irregularities should be noted and reported. Additional areas prone to damage and requiring frequent inspection include:
- Ballast tanks adjacent to the engine room (due to heat exposure).
- Ballast and void tanks near heated fuel or cargo tanks.
- Tanks near areas with high vibration levels.
- Side shell spaces between loaded and light draft lines.
- Tanks near external tug contact points.
- Spaces in the bow, especially after rough weather.

During ship inspections, various techniques are commonly employed:

Visual Inspection: This fundamental method, combined with sound judgment, is a crucial tool for assessing the ship’s condition. Taking detailed photos and videos provides valuable documentation for analysis.
Hammer Testing: Used to roughly estimate remaining steel thickness and remove rust, but should be avoided on pressurized pipelines unless proper precautions are taken.
Leak Tests: Chalk tests, hosing down, and ultrasonic leak testing are typically used to check hatch covers, access points, and watertight doors.
- Liquid dye penetrant: This method detects surface-level cracks or flaws in non-porous materials using a liquid’s capillary action. Capillary action is the tendency of a liquid to rise or fall when in contact with a solid, due to the balance between cohesive forces within the liquid and adhesive forces between the liquid and solid. This can be seen with a straw in a glass of water, where the water climbs up inside the straw. Cohesive forces hold the water together, while adhesive forces draw it up against the straw’s surface. In non-destructive testing, this principle is applied more complexly, as various surface conditions can affect the liquid’s movement. Penetrant inspection relies on a liquid’s ability to wet a solid surface. The cohesive forces between the liquid molecules create surface tension. Capillary action then causes the liquid to rise or fall in narrow gaps. However, the liquid’s viscosity is important because very thick liquids don’t flow quickly enough over the surface and into the cavities of the tested material. Penetrant materials consist of a dye (tracer) and a carrier fluid. The dye provides a color contrast against the background, aiding in the detection of flaws. There are two main types:
- Visible dye penetrant: Visible under natural light and can be inspected with the naked eye. It’s usually red for easy identification.
- Fluorescent dye penetrant: Requires a fluorescent lamp for inspection in a darkened environment. Penetrants with a visible dye are called color contrast penetrants, while those with fluorescent dye are simply fluorescent penetrants. Some penetrants offer dual sensitivity, combining both visible and fluorescent dyes.
Pressure Tests: Deck pipelines are examined under pressure along their entire length, with caution advised for hammer testing in pressurized states.
Ultrasonic Gauging: Having a calibrated gauge onboard is useful for estimating repairs, though it cannot be used for official classification purposes without a certified operator. Referencing the last UTG report is recommended for comparison. Ultrasonic waves are mechanical vibrations with frequencies exceeding the human hearing range of 20 to 20 kHz. Unlike electromagnetic waves, they require a medium for propagation, and their speed depends on the medium’s elasticity. The most common propagation modes are longitudinal, transverse, and surface (Rayleigh) waves. Advantages of Ultrasonic Inspection:
- Speed and Reliability: It’s a rapid and trustworthy method of non-destructive testing.
- High Sensitivity: It surpasses radiography in detecting flaws in metal objects.
- Small Flaw Detection: Can identify flaws as small as 0.1% of the distance between the probe and the defect.
- Systematic Scanning: Large welds can be scanned systematically to find major defects initially.
- Cost-Effective and Fast: Ultrasonic inspection is relatively inexpensive and quick to perform.
- High Sensitivity to Flaw Detection: The highest frequency waves offer the greatest sensitivity in detecting flaws.
Eddy current testing: Eddy current testing is applicable to any material that conducts electricity. It’s a versatile method used for crack detection, measuring the thickness of metallic and non-metallic coatings, detecting corrosion, assessing conductivity, and even analyzing chemical compositions. A major advantage is that it doesn’t require direct contact with the tested material. Eddy currents are generated within a conductive material by placing it near a coil carrying alternating current. This principle is similar to how a current is induced in the secondary coil of a transformer when the primary is energized. However, in transformers, eddy currents are often undesirable due to heat generation.

Hatch cover

Hatch covers face extreme conditions, constantly exposed to salty air, waves, and cargo during handling. Maintenance time is limited, as it cannot be done while hatches are loaded or during cargo operations. Even in ballast, maintenance is often weather-dependent. Insufficient maintenance leads to a gradual decline in performance, causing costly port delays when hatch covers become inoperable. This can also result in additional expenses from claims and port fees.

Maintenance to defects:

Replacing Hatch cover seals: Rubber seals typically last 4-5 years but can fail due to neglect, damage, paint overspray, or over-compression. If seal channels are corroded, they need to be cleaned (usually grit blasted) and coated with zinc before installing new seals. Seals are cut slightly longer (about 3%) to account for shrinkage. Joints should be cut square, smoothed, and glued on three sides, avoiding sharp tools. Corners, prone to leaks, need special attention. Replace the entire seal, not just sections, to maintain consistent thickness. Special grease is applied to the seal to reduce friction with the compression bar. Before installing a new seal, chip, grind, and paint the compression bars to prevent damage. Measure and record seal compression during fitting.
Hatch covers have drainage systems to handle leaks past the seal. Secondary channels guide leaked water back to the deck, preventing corrosion.
Cylinder ram seals are checked for leaks and replaced during dry dock.
Ultrasonic Leak Detection: Ultrasonic testing overcomes the limitations of hose testing. A transmitter is placed inside the closed hold, and a detector scans the outside for leaks. Classification society rules govern this method, ensuring equipment and surveyor competence. Only approved equipment and suppliers can be used for class surveys.
Survey Requirements: Hatch covers undergo an annual survey alongside the hull survey. A special survey with a hose test is required every 5 years at maximum.