KUP 37 Have knowledge and ability to explain where to look for damage and defects most commonly encountered due to 1) loading and unloading operations, 2) corrosion, 3) severe weather conditions

Ships, while robust and designed for harsh environments, are not immune to damage and wear. This discussion will explore the most common types of defects and damage encountered on ships, focusing on three primary causes:
Loading and Unloading Operations: The frequent handling of cargo and operation of heavy machinery during loading and unloading processes expose the ship to significant stresses and potential impacts, leading to various forms of structural and cosmetic damage.
Corrosion: The constant exposure to seawater, humidity, and other corrosive elements accelerates the deterioration of metal components, leading to structural weakening, leaks, and potential failures.
Severe Weather Conditions: Storms, high waves, and extreme temperatures can exert tremendous forces on a ship’s structure, causing deformations, cracks, and damage to various systems and equipment.
Understanding the typical damage patterns associated with each of these causes is crucial for effective maintenance and repair strategies, ensuring the continued seaworthiness and safety of the vessel.

Learning objectives

Damages and defects during loading and unloading operations

When a ship floats in calm water, it’s subject to two forces: upward buoyancy, concentrated in the wider parts of the hull, and downward weight. These forces balance out, resulting in no net movement. However, the weight is not evenly distributed along the ship’s length, creating a difference between the downward weight distribution and the upward buoyancy distribution. This difference generates a “still water bending moment,” causing the hull to flex. If the weight is concentrated mid-ship, it causes “sagging.” If buoyancy is greater there, it results in “hogging.”

At sea, wave forces add to this effect, creating a “wave bending moment” and further flexing the hull. The combination of these two moments is the “total bending moment.” Both static and dynamic forces also induce shearing forces in the hull girder. The shearing force at any point along the ship’s length is the force that tries to move one section of the hull vertically relative to its neighboring section.

During hogging at the deck level, components like deck plating, longitudinal stiffeners, longitudinal hatch coamings, and sheer strakes experience tension, with maximum stress typically occurring around midship. Stress levels increase at corners of deck openings, brackets for longitudinal stiffeners, and hatch coaming brackets. Any transverse crack in these areas can spread quickly due to the high stress concentration. Effects of hogging on the side shell cause tensile and compressive stresses to increase on both plating and longitudinal stiffeners as you move towards the deck and bottom, respectively. High shear stresses occur around the neutral axis. Stress concentrations arise at corners of openings.At the bottom, compression is highest around midship on the bottom plating, bilge plating, and longitudinal stiffeners. Any wastage or thinning of these components can lead to increased compressive stresses and potential buckling.

During sagging, deck-level components like deck plating, longitudinal stiffeners, hatch coamings, and sheer strakes are compressed, peaking around midship. Wastage can lead to buckling due to increased compressive stress. Side shell is also affected. Tensile and compressive stresses intensify on plating and stiffeners towards the deck and bottom respectively. High shear stresses occur around the neutral axis, with stress concentrations at opening corners. In the bottom level, bottom plating, bilge plating, and stiffeners are under tension, highest around midship. Stress concentrates at bottom opening corners and stiffener brackets, and transverse cracks can spread quickly.

In general, longitudinal structural members, including plating and stiffeners, contributing to the longitudinal strength are continuous and not interrupted when crossing transverse members.

Torsional stress, the twisting force on a structure, occurs in a ship’s hull when forces act away from its centerline axis. This twists the hull along its length. It can be caused by cargo (static torsion) or wave action (dynamic torsion). While most ships aren’t significantly affected, container ships with wide decks, large hatches, and uneven cargo distribution are susceptible to severe torsional stress. Wave-induced torsion is considered during ship design. Classification societies set limits on allowable torsional moments in cargo bays. Static torsion is caused by unbalanced weight distribution across the ship’s width, such as unevenly loaded containers.

When a ship sails through waves, the strength and distribution of torsional forces depend on how the ship’s direction aligns with the waves. The most significant twisting effect happens when waves hit the ship at about a 45-degree angle on the bow. In this scenario, the wave crest might lift the bow while the trough lowers the stern, or vice versa. These wave-induced forces push the bow clockwise and the stern counterclockwise, creating a twisting force on the hull – known as dynamic torsion.

Improper cargo handling can cause significant damage to ship structures. Common issues include:

• Grab and Payload Damage: Ships are often damaged during unloading, especially by grabs or payloaders that forcefully impact the ship’s structure.
• Local Overloading: When bulk cargo is loaded in ways not approved by the classification society or loading manual, it can lead to deck plating cracks near hatch covers and damage to plating between hatches.
• Faulty Weight Distribution: This can occur in several scenarios:
  • Tropical Loading with Low Bunkers: A ship loaded to its maximum tropical mark with less than full bunkers can overload some or all strengthened holds.
  • Overloading Specific Holds: Loading individual holds beyond their designed capacity can also cause damage.
  • Block Loading: Heavily loading consecutive holds while leaving adjacent ones empty, though reducing longitudinal stresses, can damage cross-deck structures between hatches.
• Insufficient Cargo Trimming: Cargoes with a low angle of repose (like grain) are prone to shifting during transport. Failure to properly trim and level such cargo, ensuring spaces are filled without overstressing nearby structures, can lead to structural damage.

Need to evaluate the condition of cargo spaces and identify any defects or damages:

• Maintaining the protective coatings in cargo holds and water ballast tanks is crucial. It’s essential for ship’s deck officers to inspect cargo holds and deck areas after cargo operations to identify any physical damage, corrosion, or coating deterioration.
• Any hull damage that might affect the ship’s structure or seaworthiness should be reported to the classification society.
• The internal hold structure and protective coatings are prone to damage from grab discharge, especially with careless handling. Grabs, made of hardened steel, can chip or buckle frames and brackets, potentially leading to cracks in the side shell plating and water ingress.
• Protective coatings in cargo holds can also deteriorate due to corrosive cargo, high temperatures, cargo settlement, and abrasion. Without these coatings, or when they break down, corrosion accelerates, particularly with corrosive cargoes like coal. This weakens the ship’s structure and can ultimately compromise its integrity. The severity of corrosion might not be apparent without close inspection until it causes major structural issues like frame collapse or detachment, resulting in cracks propagating through the side shell.

Corrosion

Hull of a ship can deteriorate mainly due to corrosion, damage to protective coatings, and the accumulation of microorganisms and fouling organisms on the surface. Corrosion can be caused by various factors, such as continuous failure of cathodic protection systems, the reaction of oxygen and hydrogen with the iron in the hull, and general wear and tear over time. Damaged protective coatings expose the hull to corrosion, especially at the damaged sites. Additionally, the attachment of microorganisms and fouling organisms can lead to severe corrosion, known as microbiologically influenced corrosion (MIC). These factors collectively reduce the lifespan of the hull, increase maintenance expenses, and pose potential safety hazards.

In the marine industry, mild steel is the favoured material for ship construction due to its affordability, strength, and ease of manufacture. However, its major drawback is its susceptibility to corrosion when exposed to saltwater. Without adequate protection, mild steel loses strength rapidly, potentially leading to structural failure. While repairing coatings at sea is an option, it can be up to 100 times more expensive than the initial application. National Association of Corrosion Engineers (NACE) International estimates the total annual cost of corrosion in the maritime sector to be a staggering 50 to 80 billion dollars globally. However, with careful initial planning and meticulous application of the first coating, shipowners can maximize the performance and cost-effectiveness of their vessels. If the cause of deterioration is inadequate surface preparation, the solution is straightforward: remove the existing paint and reapply it correctly.

Corrosion on ships typically manifests as surface corrosion (rust), pitting corrosion (small holes or deep penetration), and cracking corrosion (fine cracks on and beneath the surface). Surface corrosion is easily visible and leads to overall steel thickness reduction. Pitting, initially hard to detect, causes localized holes and can lead to leaks. Cracking is also dangerous due to its hidden nature and potential for widespread damage.

The main origins of corrosion are:

1. Contact corrosion
2. Stress-induced corrosion (from pressure, tension, or vibration)
3. Abrasive corrosion (due to friction, erosion, or cavitation)
4. Corrosion caused by hydrogen formation and diffusion

These processes can result in material loss through widespread wastage or weakened areas due to deep pitting or extensive cracks, particularly in high-stress zones. Tankers often experience a combination of these corrosion types, except for the unique case of hydrogen corrosion.

Ways to prevent and control corrosion on ship

• Ship designers and operators strive to mitigate corrosion in order to extend the lifespan and safety of vessels. This can be achieved through incorporating certain design elements that minimize corrosion rates and decrease maintenance costs throughout the ship’s operational life.
• Strategically positioning scuppers and drains helps remove water from decks and bilge areas, directly preventing corrosion.
• To minimize galvanic corrosion, insulation should be used where dissimilar metals meet. An impressed current system can also be installed to actively monitor and counteract corrosion.
• Insulation is crucial in areas with temperature fluctuations to prevent thermal fatigue.
• Anti-vibration measures, such as fitting turbine machinery with sliding feet, reduce metal fatigue. Sacrificial anodes made of magnesium, aluminum, or zinc can further protect against corrosion.
• Employing corrosion-resistant alloys or stainless steel, adding rubbing strakes or doubling plates to absorb wear, and designing structures for easier maintenance and coating applications can all contribute to extending a ship’s lifespan and managing corrosion effectively.
• Implementing these design elements during construction significantly reduces maintenance expenses and mitigates corrosion over time.
• A highly effective method for protecting ships from corrosion is the application of specialized paint coatings. These coatings act as a barrier between the metal surface and the corrosive marine environment. The hull and open deck, constantly exposed to water and sea air, benefit from this extra layer of protection. It prevents moisture and salt from reaching the metal, thus reducing corrosion risk. Additionally, hull coatings deter marine life, such as algae and mollusks, from attaching to the hull, which can otherwise expose metal to seawater and accelerate corrosion. These coatings also smooth the hull surface, reducing drag and improving fuel efficiency. Common coating types include epoxy, polyurethane, and zinc-rich coatings. Epoxy coatings are favored for their high resistance to water and chemicals, along with excellent adhesion to metal. Polyurethane coatings provide a robust, long-lasting finish capable of withstanding harsh conditions. Zinc-rich coatings, containing a high percentage of zinc, act as sacrificial anodes, effectively protecting the metal surface from corrosion.
• Cathodic protection is a corrosion prevention method employing sacrificial anodes, which are metals more prone to corrosion than the protected metal. These anodes corrode (sacrifice themselves) instead of the ship’s metal, thus safeguarding it. There are two main types:
  • Galvanic cathodic protection: Sacrificial anodes directly connected to the protected metal. As the anodes corrode, they release electrons, flowing to the metal and inhibiting corrosion.
  • Impressed current cathodic protection: An external power source supplies electrons to protect the metal surface. Both methods effectively protect ships, but the impressed current is preferred for larger areas and allows for precise adjustment to the ship’s needs. Sacrificial anodes are used for smaller areas or machinery handling seawater.
• Corrosion inhibitors are chemical compounds applied to metal surfaces. While not a preventive measure, they significantly slow down the corrosion rate of materials like steel or alloys. This is a cost-effective approach, as their use can reduce corrosion-related losses on ships by up to 35%. A major benefit of corrosion inhibitors is that they enable the use of less expensive metals in harsh environments. When treated with these chemicals, metals can last beyond their expected lifespan, resulting in lower maintenance costs due to prolonged rust protection.
• Anodic protection is an alternative method for preventing corrosion. However, it hasn’t gained widespread use in the shipping industry due to its dependence on a continuous power supply, which can be unreliable during electrical outages.
• Joining aluminum and steel plates can lead to bimetallic corrosion, a problem addressed by using a transition sheet. This sheet is made of pure aluminum and mild steel, separated by polystyrene and bonded together through an explosive process. It acts as a barrier between the aluminum and steel sections. The transition sheet is then welded to both sections using a weld nugget process – steel-to-steel on one side and aluminum-to-aluminum on the other, with the aluminum side welded first.
• Explosive bonded transition plates are used in various applications, such as attaching aluminum deckhouses to steel decks, connecting aluminum tubing to steel or stainless steel tube sheets in heat exchangers, and creating arc welded joints between aluminum and steel pipelines.
• To prevent bimetallic corrosion when joining dissimilar metals, an inert neoprene joint is used to separate them. This joint acts as a barrier to prevent direct contact between the two metals. When a rivet or bolt is used, it passes through a neoprene tube, isolating it from the metals. Similarly, the bolt and nut surfaces are also separated from the dissimilar metals using neoprene washers. This approach ensures that the metals do not come into direct contact, thus mitigating the risk of corrosion.
• Car makers pioneered the adhesive bonding route which is increasingly being used in shipbuilding. An electrically inert adhesive agent is used to bond the metal and steel surfaces together, while a sealant keeps out electrolyte from seeping into the joint.

Severe weather conditions

Ships in rough seas experience various impacts from waves, causing vibrations throughout the hull. These impacts involve short, intense pressure surges due to hydrodynamic forces acting on the ship’s surface.

“Slamming” and “pounding” are terms used to describe these wave impacts. Pounding refers to a wave slap causing a sudden but not severe shuddering load. Slamming typically refers to an impact at the bow, either on the flare or the bottom. It generally describes the forward bottom impact when a ship’s bow rises above water during heavy pitch and heave motions and then forcefully re-enters, causing intense pressures and whipping forces that lead to high-frequency stresses and local structural damage.

Slamming pressures last mere milliseconds and are usually localized. Flare slamming, on the other hand, occurs when the bow flare hits an oncoming wave, generating slower impacts but larger total forces compared to bottom slamming. Unlike bottom slamming, flare slamming doesn’t necessarily involve the bow rising out of the water, and the impact duration is relatively longer.

Classification society rules mandate strengthening the forward bottom structure if the forward draft is below a certain percentage of the ship’s length, to withstand bottom-pounding pressures. Damage from hydrodynamic impacts can vary, including deformed shell plating, buckled longitudinals and frames, and fatigue cracks in connections. This damage can result from both the direct impact forces and the accompanying high-frequency whipping forces. Therefore, hydrodynamic loadings are periodic forces that cause dynamic stresses. Two major stresses when a ship is underway are pounding and panting. Pounding occurs when the bow rises over a wave crest and slams back down, particularly in heavy seas and light ballast conditions. This stress extends beyond the forward collision bulkhead, requiring reinforced bow plating. Panting happens when waves hit the bow and stern, causing pressure fluctuations that push the plating in and out. This necessitates reinforced framing at the ship’s ends to prevent excessive hull movement.

Operating in severe weather conditions

Navigating through harsh weather conditions demands careful consideration. With the recent trend of ships opting for routes like the Straits of Magellan and around the Cape of Good Hope to save costs, it’s crucial for Masters to prioritize safety in adverse weather. Before allowing crew on deck in heavy weather, the Master must determine if the task is essential for the ship’s safe operation. If not, it’s advisable to wait for calmer conditions. If deck work is unavoidable, all ISM safety procedures, including a comprehensive risk assessment and a permit to work with a heavy weather checklist, must be followed as per the ship’s Safety Management System.

Code of Safe Working Practices(CoSWP), Chapter 11.12

The Code of Safe Working Practices for Merchant Seaman (CoSWP) should also be consulted in heavy weather conditions. It recommends rigging lifelines, postponing non-essential tasks, inspecting and tightening cargo lashings, securing anchors, and conducting risk assessments before authorizing deck work in adverse weather.

Crew members working on deck during rough weather should wear life jackets, safety harnesses, waterproof protective gear, and carry water-resistant radios. Working in pairs or teams under a competent person’s supervision is recommended.

To minimize exposure, all stores and spares should be secured before leaving or soon after departing port. Cargo lashings should be inspected daily and tightened as needed, and anchors should be secured with closed and waterproofed spurling pipes.

By following these guidelines, ships can significantly enhance crew safety during challenging weather conditions.

Precautions to be taken in sever weather conditions on a ship

When facing rough weather at sea, here are essential precautions for steering control:

• Switch to Manual Steering: Change from autopilot to manual control to prevent excessive rudder movement.
• Check Steering Gear: Verify oil levels, linkages, and other crucial parameters of the steering gear.
• Activate Both Motors: If only one steering motor is running, start the other to maximize rudder torque.
• Increase Bridge Personnel: Ensure enough manpower, including senior officers, is present on the bridge to handle the challenging conditions.
• Man the Engine Room: If the engine room is in UMS mode (unmanned machinery space), ensure it is manned with sufficient personnel.
• Monitor Parameters: Continuously observe the vital signs of the main propulsion plant and auxiliary power plant machinery.
• Secure Spares: Lash and stow all spare parts in the engine room after receiving a rough weather warning.
• Adjust Engine Settings: Reduce propeller RPM or switch to rough weather mode to handle the fluctuating engine load caused by propeller emergence from the water.
• Maintain Sump Levels: Keep sump levels optimal to prevent false alarms due to ship rolling, which could trip machinery and create a dangerous situation.
• Tank Levels: Ensure adequate levels in important tanks to avoid pump suction loss during rolling.
• Standby Generator: Keep the standby generator running until the rough weather subsides.
• Close Watertight Doors: Secure watertight doors in machinery spaces to prevent flooding.
• Secure Openings: Close skylights and other openings to protect against water ingress.
• Prevent Spills: Secure trays to prevent spills during rough conditions.
• During rough weather, crew members should avoid going on open decks.
• Secure all deck items like mooring ropes, lashing equipment, and drums after use.
• Ensure all openings on deck for cargo and other spaces are closed.
• Close all openings to the accommodation.
• Use the shaft tunnel and other internal passages to access the steering room or other compartments.
• All crew members must be aware of their assigned duties as listed in the muster list.
• Turn off the elevator to prevent accidents during rolling and pitching.
• Always wear Personal Protective Equipment (PPE) and use handrails for support while moving around the ship to prevent falls.
• Stay alert and work in teams for safety.

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