Section 1.2 Longitudinal Strength, Transverse Strength, and High-Stress Areas

A bulk carrier, especially when laden with dense cargoes or navigating tumultuous seas, is subjected to enormous forces. Its structural design must ensure it can withstand these stresses without deformation or failure. Understanding the principles of longitudinal and transverse strength, and identifying areas prone to high stress, is critical for the Master to operate the vessel safely, particularly during loading, discharging, and ballasting operations, and in heavy weather.

1. Longitudinal Strength:

This refers to the vessel’s ability to resist bending forces along its length, akin to a long beam supported by the buoyancy of the water and loaded with the weight of cargo, machinery, and its own structure. The primary longitudinal stresses are bending moments and shearing forces.

1. Bending Moments:
   1. Hogging: Occurs when the buoyancy amidships is greater than the weight distribution, or when the ends of the ship are more heavily loaded/less supported by buoyancy than the midship section (e.g., wave crest amidships, wave troughs at ends; or more cargo concentrated at ends). The deck experiences tensile (stretching) stress, and the bottom plating experiences compressive (squeezing) stress. The vessel bends upwards in the middle, like a “hog’s back.”
   2. Sagging: Occurs when the weight amidships is greater than the buoyancy, or when the midship section is less supported by buoyancy than the ends (e.g., wave trough amidships, wave crests at ends; or more cargo concentrated amidships). The deck experiences compressive stress, and the bottom plating experiences tensile stress. The vessel bends downwards in the middle.
   3. Still Water Bending Moment (SWBM): The bending moment experienced by the vessel in calm water due to the distribution of its own weight, cargo, fuel, ballast, etc. This is calculated using the loading computer.
   4. Wave Bending Moment: The additional, dynamic bending moment imposed by waves.
   5. Total Bending Moment: The sum of SWBM and wave bending moment. The vessel’s structure is designed to withstand a maximum permissible bending moment.
2. Shearing Forces: These are vertical forces that tend to make one part of the ship slide vertically relative to an adjacent part. They are highest near the ends of the cargo section and at transverse bulkheads, where there are significant changes in load distribution.
3. Key Structural Members for Longitudinal Strength: The “hull girder” provides longitudinal strength. This includes:
   1. Deck plating and longitudinal stiffeners
   2. Bottom shell plating and longitudinal stiffeners
   3. Side shell plating (or inner hull plating in double-hull ships) and their longitudinal stiffeners
   4. Longitudinal bulkheads (if any)
   5. Keel
   6. Topside tank plating and hopper tank plating, which also act as longitudinal girders.

Analysis for the Master (Longitudinal Strength): The Master’s primary tool for managing longitudinal strength is the loading instrument (loading computer), which must be class-approved and regularly tested.

1. Loading/Discharging Sequences: It is imperative to follow approved loading/discharging sequences to keep SWBM and shearing forces within permissible limits at all stages. Incorrect sequences, such as loading all midship holds first or discharging end holds first without proper ballasting, can induce excessive hogging or sagging stresses, potentially leading to catastrophic failure. This is particularly critical for large bulk carriers and those carrying very dense cargoes.
2. Ballast Operations: Ballasting and de-ballasting must be carefully coordinated with cargo operations to manage stresses.
3. Heavy Weather: Reducing speed and altering course in heavy weather can significantly reduce wave-induced bending moments.
4. Awareness of Vessel Condition: The Master must be aware of any existing structural damage, corrosion, or repairs that might affect the vessel’s longitudinal strength and operate the vessel more cautiously if such conditions exist.

2. Transverse Strength:

This refers to the vessel’s ability to resist forces acting across its breadth, tending to distort its cross-sectional shape. These forces arise from:

1. External water pressure on the hull.
2. The weight of cargo pressing outwards on the sides of the holds and downwards on the tank top.
3. The weight of deck cargo (if any, though less common on typical bulkers).
4. Forces from rolling in a seaway.
5. Impact forces during berthing.
6. Key Structural Members for Transverse Strength:
   1. Transverse Frames (Ribs): These are the primary structures resisting transverse forces. In single-hull bulkers, these are prominent within the cargo holds. In double-hull vessels, they are located within the double-hull spaces.
   2. Transverse Bulkheads: These divide the ship into compartments (holds) and are crucial for transverse strength, as well as for limiting the extent of flooding in case of damage.
   3. Deck Beams and Carlings: Support the deck plating.
   4. Floor Structures (Floors): Transverse members in the double bottom, supporting the tank top.
   5. Tank Top Plating: The inner bottom, forming the floor of the cargo holds.
   6. Hopper Tank Structures and Topside Tank Structures: These also contribute significantly to transverse rigidity.
   7. Brackets: Connect frames to the tank top, deck beams, hopper plates, etc., and are vital for distributing stresses at these connections.

Analysis for the Master (Transverse Strength):

1. Cargo Loading: Uneven transverse loading within a hold can strain framing. While bulk cargoes tend to self-level to some extent, ensuring cargo is reasonably trimmed across the hold is important.
2. Ballast Distribution: Maintaining symmetrical ballast distribution in wing tanks and hopper tanks is important.
3. Inspection: Regular inspection of transverse frames, bulkheads, and their connections (especially brackets) for damage, corrosion, or deformation is crucial. Grab damage to frames in single-hull ships is a common issue.

3. High-Stress Areas:

Certain areas of a bulk carrier’s structure are inherently subjected to higher concentrations of stress due to changes in geometry, load application, or structural discontinuities. These areas require particular attention during inspection and operation.

1. Hatch Corners and Coamings: The large openings in the deck for cargo hatches create discontinuities. Hatch corners are classic stress concentration points, prone to cracking if not properly designed and maintained. The hatch coamings themselves are also critical structural elements contributing to longitudinal strength.
2. Connections of Longitudinal Stiffeners to Transverse Members: Where longitudinal frames or girders pass through or connect to transverse bulkheads or web frames, stress concentrations can occur.
3. Frame and Beam Connections (Brackets): The brackets connecting side shell frames to the tank top, hopper tanks, deck beams, and topside tanks are vital. Failures or cracks in these brackets can lead to a loss of support for the frames and subsequent deformation or failure of the side shell. This is a notorious problem area, especially in older single-hull vessels.
4. Bulkhead Stools (Upper and Lower): The connections of transverse bulkheads to the deck (upper stool) and inner bottom (lower stool) are high-stress areas, especially when adjacent holds have different loading or ballast conditions.
5. Areas Around Openings: Any opening in plating (e.g., for manholes, sea chests, pipe penetrations) can create stress concentrations if not properly reinforced.
6. Fore and Aft Ends of the Vessel: These areas experience significant dynamic forces from pitching, slamming (fore end), and propeller/rudder vibrations (aft end).
7. Areas of Corrosion or Previous Damage/Repair: Corroded plating is thinner and weaker. Areas that have been previously damaged and repaired may also be more susceptible to future issues if the repair was not perfectly executed or if the underlying cause of the initial damage persists.

Analysis for the Master (High-Stress Areas):

1. Targeted Inspections: The Master must ensure that the ship’s Planned Maintenance System (PMS) and routine inspections specifically target these known high-stress areas. Close visual inspection for cracks, deformation, and corrosion is essential.
2. Operational Care: Avoid abrupt changes in loading/ballasting that could shock-load these areas. Careful handling of the vessel in heavy weather is also key.
3. Reporting and Repair: Any defects found in high-stress areas must be promptly reported, assessed by competent personnel (e.g., Class surveyor), and properly repaired. Ignoring small cracks can lead to much larger failures.

A comprehensive understanding of these structural principles allows the Master to not only use the loading computer effectively but also to visually assess the vessel’s condition and make informed decisions that safeguard its integrity throughout its operational life. This knowledge is also invaluable when liaising with surveyors, superintendents, and repair facilities.