Section 1.5 Common Structural Issues and Defects in Bulk Carriers

Despite their robust construction, bulk carriers are subjected to immense operational stresses, harsh marine environments, and the abrasive/corrosive nature of many cargoes they carry. Over time, these factors can lead to various structural issues and defects. Early detection, proper assessment, and timely repair of these defects are critical for maintaining the vessel’s seaworthiness, preventing catastrophic failures, and ensuring a long operational life. The Master plays a vital role in overseeing the vessel’s structural condition through diligent inspections by the crew.

1. Corrosion:

Corrosion is arguably the most pervasive and relentless enemy of a ship’s structure. It is an electrochemical process that degrades steel, reducing its thickness and therefore its strength.

1. Types of Corrosion Affecting Bulk Carriers:
   1. General Corrosion: Uniform thinning of steel over a wide area. Often occurs in ballast tanks or on exposed deck plating if coatings fail.
   2. Pitting Corrosion: Localized, intense corrosion that creates pits or cavities in the steel. Can rapidly penetrate plating and is often found under failed coatings or in areas where moisture and corrosive agents collect.
   3. Grooving Corrosion: Occurs along welds or in areas of high stress concentration, often exacerbated by the flexing of the structure.
   4. Galvanic Corrosion: Occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte (seawater). Less common in the main hull structure but can affect fittings.
   5. Microbiologically Influenced Corrosion (MIC): Caused or accelerated by the activity of microorganisms, particularly in ballast tanks with stagnant water or under sludge.
2. Areas Prone to Corrosion:
   1. Ballast Tanks (Topside, Hopper, Double Bottom): These are frequently filled with corrosive seawater and can experience cycles of wetting and drying, accelerating corrosion if coatings are not maintained or cathodic protection (anodes) is depleted. MIC can be a significant issue.
   2. Cargo Holds:
      1. Under Cargo: Some cargoes are inherently corrosive (e.g., sulphur, some fertilizers, high-salt content ores). Moisture from the cargo can also create a corrosive environment against the steel.
      2. Side Shell Frames and Brackets (Single Hull): These are exposed to cargo and can suffer from abrasion (removing protective coatings) and corrosion. The undersides of brackets and horizontal stringers can trap cargo residues and moisture.
      3. Tank Top: Subject to abrasion from cargo and grabs, and corrosion from cargo residues.
      4. Underside of Hatch Covers and Coamings: Exposed to both cargo environment and external weather.
   3. Deck Plating: Exposed to seawater, rain, and cargo spillage.
   4. Void Spaces (in Double Hull vessels): If coatings break down or condensation occurs, these enclosed spaces can corrode unseen if not regularly inspected.
3. Consequences of Corrosion:
   1. Reduction in plate thickness, leading to loss of local and overall structural strength.
   2. Increased risk of cracking in weakened areas.
   3. Leakage (e.g., from ballast tanks into cargo holds, or water ingress from sea).
   4. Failure of structural members.

Analysis for the Master (Corrosion):

1. Coating Integrity: The primary defense against corrosion is the paint/coating system. The Master must ensure regular inspections of coatings and arrange for timely touch-ups and repairs.
2. Anode Condition: In ballast tanks, sacrificial anodes are often fitted. Their condition and renewal rate are critical.
3. Cargo Properties: Be aware of the corrosive nature of cargoes being carried and ensure holds are properly cleaned after discharge to remove corrosive residues.
4. Atmosphere Control: Proper ventilation of ballast tanks and void spaces can help reduce condensation and corrosion.
5. Systematic Inspection: Implement a rigorous inspection schedule as part of the PMS, paying close attention to known problem areas. Thickness measurement (gauging) by qualified personnel is essential during Class surveys and if significant corrosion is suspected.

2. Cracking:

Cracks are fractures in the steel structure. They can range from minor surface cracks to through-plate fractures that compromise weathertightness or structural integrity.

1. Causes of Cracking:
   1. Stress Concentration: Cracks often initiate at points of high stress (hatch corners, frame connections, weld toes) due to design features or operational loads.
   2. Fatigue: Repeated cyclical loading (e.g., from wave action, engine vibration, or repeated loading/discharging stresses) can lead to fatigue cracks, even if the peak stress is below the steel’s yield strength. Bulk carriers, with their long service lives and exposure to dynamic loads, are susceptible to fatigue.
   3. Corrosion: Corrosion reduces the effective thickness of steel, making it weaker and more prone to cracking under load. Pitting corrosion can act as a stress raiser, initiating cracks.
   4. Poor Weld Quality: Defects in welds (e.g., lack of penetration, porosity, slag inclusions) can be initiation points for cracks.
   5. Overloading / Incorrect Loading: Exceeding permissible bending moments or shear forces, or improper loading sequences, can overstress the structure and cause cracking or buckling.
   6. Impact Damage: Collisions, contact with quay, or heavy grab impact can directly cause cracks or create stress points for later cracking.
2. Common Locations for Cracks:
   1. Side Shell Frames and their Brackets (especially toe of bracket connecting to tank top or hopper tank): This is a classic problem area in single-hull bulk carriers due to high stresses and potential for corrosion/grab damage.
   2. Hatch Coamings and Hatch Corners: Due to stress concentrations around the large deck openings.
   3. Transverse Bulkheads: Especially at connections to the hull, hopper tanks, or topside tanks, and in areas of corrugated plating.
   4. Welds: Cracks often initiate at the toe or root of welds.
   5. Areas of High Local Stress: Near chocks, bitts, or crane pedestals if not adequately reinforced.
3. Consequences of Cracking:
   1. Loss of structural integrity.
   2. Propagation of cracks, potentially leading to major failure.
   3. Water ingress if the crack penetrates the hull or hatch covers.
   4. Failure of supporting members, leading to deformation of adjacent structures.

Analysis for the Master (Cracking):

1. Vigilant Inspection: Crew members, particularly deck officers and the bosun during routine rounds and hold inspections, must be trained to look for cracks. Good lighting is essential.
2. “Stop-Drilling”: For minor cracks, drilling a small hole at each end of the crack can sometimes temporarily arrest its propagation by blunting the sharp crack tip and reducing stress concentration, pending proper repair. This is a temporary measure and requires Class approval/guidance for significant cracks.
3. Proper Repair: Cracks should be properly investigated (e.g., using dye penetrant or magnetic particle inspection to determine their full extent), gouged out, and re-welded by qualified welders according to approved procedures. Simply welding over a crack is often ineffective.
4. Addressing the Root Cause: It’s important to understand why a crack occurred. If it’s due to a design issue or persistent high stress, simply repairing the crack may not be a long-term solution. Structural reinforcement might be needed.
5. Reporting to Class: Significant cracks or patterns of cracking must be reported to the Classification Society for assessment and approval of repair procedures.

3. Deformation and Buckling:

Deformation is a permanent change in the shape of a structural member (e.g., bent frames, dented plating). Buckling is a specific type of deformation where a plate or stiffener under compressive stress distorts or “cripples.”

1. Causes:
   1. Impact Damage: Contact with quay, tugs, other vessels, or heavy objects (e.g., grabs hitting frames, heavy cargo dropping onto tank top).
   2. Overstressing: Exceeding design loads can cause permanent deformation. For example, tank top plating can buckle if the permissible load density is exceeded by heavy cargo.
   3. Corrosion Wastage: Thinned plating is more susceptible to buckling under compressive loads.
   4. Improper Cargo Handling: Careless grab operation is a major cause of damage to side shell frames, brackets, and tank tops in cargo holds.
2. Common Areas for Deformation:
   1. Side shell frames and brackets.
   2. Tank top plating.
   3. Hatch coamings (can be bent by swinging grabs).
   4. Deck plating (e.g., from dropped cargo or equipment).
3. Consequences:
   1. Reduced strength of the affected member.
   2. Potential for initiation of cracks at points of deformation.
   3. Difficulty in fitting equipment (e.g., a deformed hatch coaming may prevent proper hatch cover sealing).
   4. Interference with cargo operations (e.g., bent frames snagging cargo).

Analysis for the Master (Deformation & Buckling):

1. Prevention: Strict supervision of cargo operations to prevent grab damage is crucial. Adherence to loading limits prevents overstressing.
2. Assessment: Dents and deformations should be assessed for their severity. The Class surveyor will often have specific criteria for acceptable levels of deformation.
3. Repair: Repairs may involve cropping out the damaged section and renewing it (insert plate) or, for less severe damage, fairing (straightening) the plating, often with heat.

4. Other Issues:

1. Weld Defects: Poor quality welds from construction or previous repairs can be points of weakness.
2. Loose or Missing Rivets/Bolts (older vessels or specific fittings): Can compromise connections.

Role of the Enhanced Survey Programme (ESP): For bulk carriers (and oil tankers), the IMO has mandated an Enhanced Survey Programme (ESP) through SOLAS. This involves much more detailed and rigorous inspections by Class surveyors at specific intervals, focusing on known problem areas, close-up surveys, and thickness measurements. The Master and ship’s staff play a crucial role in preparing the vessel for these surveys, including providing safe access and having all relevant documentation and history available. The ESP aims to detect and address structural defects before they become critical.

Conclusion for the Master: The Master’s diligence in overseeing structural inspections, fostering a culture where crew members proactively report defects, ensuring timely and proper repairs, and operating the vessel within its design limits are all fundamental to preventing serious structural issues. A well-maintained vessel is a safe and commercially viable vessel. This understanding is also critical when dealing with Port State Control, charterers’ surveyors, and P&I club inspections, where structural condition is always a key focus.