Section 7.3 Trimming of Cargo: Importance and Methods

Once the loading sequence is underway and cargo is flowing into the holds, the manner in which it settles and is distributed within each hold is of paramount importance. “Trimming” of bulk cargo refers to the process of leveling or partially leveling the cargo within the hold to minimize the risk of it shifting during the voyage, to ensure even distribution of weight on the tank top, and to maximize the amount of cargo safely loaded. Improper or insufficient trimming, especially with certain types of cargo, can lead to severe stability problems, including capsize. The IMSBC Code provides specific guidance on trimming, and the Master, through the Chief Officer, must ensure these requirements are met.

1. Definition and Fundamental Importance of Trimming:

Definition: Trimming is the operation of leveling a bulk cargo within a cargo space, either partially or fully, to:

Reduce the height of the cargo cone that forms naturally under the loading spout.

Fill the void spaces in the wings and ends of the hold.

Create a relatively flat or profiled surface to minimize the likelihood of the cargo shifting.

Why is Trimming Critical?

Preventing Cargo Shifting and Ensuring Stability: This is the primary safety reason. If a bulk cargo is loaded and left as a tall cone (i.e., untrimmed or poorly trimmed), it has a high center of gravity within that hold and large void spaces around it. During the voyage, particularly in heavy weather, the ship’s rolling and pitching motions can cause this cone to collapse and the cargo to shift transversely (towards one side) or longitudinally.

Transverse Shift: A significant transverse shift of a dense bulk cargo can:

Cause the vessel to develop a dangerous list.

Drastically reduce the vessel’s effective metacentric height (GM) due to the shift in the cargo’s center of gravity and the creation of a void on one side.

Lead to capsize if the list is severe and stability is critically impaired.

Longitudinal Shift: Can alter the vessel’s trim, potentially affecting maneuverability, propeller immersion, and even longitudinal strength if the shift is substantial.

Maximizing Cargo Intake (Stowage Optimization): By leveling the cargo and filling void spaces in the wings and ends of the hold, trimming allows more cargo to be loaded into the available space, thus better utilizing the vessel’s cubic capacity. This is particularly important for cargoes with a high stowage factor (light cargoes).

Even Distribution of Weight on Tank Top: Trimming helps to distribute the weight of the cargo more evenly over the tank top structure, preventing localized overstressing, especially with dense cargoes.

Facilitating Discharge: A relatively level cargo surface can sometimes make discharge operations (especially with grabs) more efficient, although this is a secondary consideration to safety.

Accurate Ullage/Sounding Measurements: A level surface allows for more accurate measurement of cargo ullages (the empty space above the cargo), which is important for calculating the quantity of cargo loaded.

Reducing Dust (for some cargoes): Leveling the cargo can sometimes help to reduce the surface area exposed to airflow, potentially minimizing dust generation during the voyage for very dusty cargoes, although this is a minor benefit.

2. IMSBC Code Requirements for Trimming:

The IMSBC Code (Section 5 – Trimming Procedures) provides mandatory requirements and guidance on trimming.

General Requirement (Section 5.2.1): “To minimize the risk of a bulk cargo shifting, the cargo shall be trimmed reasonably level, as necessary.”

Specific Requirements based on Angle of Repose: The Code links trimming requirements to the cargo’s angle of repose (the angle between the horizontal and the slope of a cone of freely poured bulk material).

Cargoes with an Angle of Repose ≤ 30° (Highly prone to shifting): These cargoes must be trimmed “reasonably level.” The void spaces (ullages) in the hatchways must be filled as much as practicable.

Cargoes with an Angle of Repose > 30° to 35°: Must also be trimmed “reasonably level.”

Cargoes with an Angle of Repose > 35°: Trimming may be less critical, but the Code still advises that the surface of the cargo should be trimmed to an angle significantly less than its angle of repose to prevent local slumping or shifting. The aim is to minimize the formation of steep slopes or cliffs of cargo.

Shipper’s Declaration: The shipper is required to declare if trimming is necessary and, if so, provide trimming procedures. However, the Master always retains the responsibility to ensure adequate trimming for safety, irrespective of the shipper’s declaration if the Master deems it insufficient.

Exemptions (Section 5.2.2): Trimming may not be required for some cargoes in specially designed ships (e.g., self-unloaders with holds designed to control cargo movement) or for certain cohesive cargoes that are known not to shift (this must be properly documented and assessed). Such exemptions are rare for typical bulk carriers loading common bulk commodities.

Definition of “Reasonably Level”: The Code doesn’t give a precise geometric definition, but it implies that the peak of the cargo cone should be significantly reduced, and void spaces in the wings and ends of the hold should be substantially filled to minimize the potential for a surface slide or shift.

3. Methods of Trimming:

The method used for trimming depends on the cargo, the loading equipment, port facilities, and cost considerations.

A. Spout Trimming (Natural Trimming / Gravity Trimming):

Method: Utilizing the loading spout or conveyor to distribute the cargo across the hold as much as possible during loading. This might involve:

Moving the shiploader’s boom and spout systematically across the hatch opening.

Using rotating or telescoping spouts that can direct the cargo flow into different parts of the hold.

Effectiveness: Limited, especially for filling the extreme ends and wings of large holds, or for cargoes with a high angle of repose that tend to form steep cones. It rarely achieves a truly “level” stow for most bulk cargoes without supplementary methods.

Common Practice: Most bulk carriers rely on the terminal’s shiploader to do the primary distribution, but this often requires supplementary trimming for a safe stow.

B. Mechanical Trimming (using Shore-based or Ship’s Equipment): This is the most common method for achieving proper trimming, especially for cargoes requiring a well-leveled surface.

Shore-based Equipment (provided by Stevedores/Terminal):

Bulldozers / Front-End Loaders (Bobcats): Small bulldozers or skid-steer loaders are lowered into the hold to push the cargo from the central cone into the wings and ends, leveling the surface. This is very common for many bulk cargoes.

Safety: Requires careful operation to avoid damaging the ship’s structure (frames, ladders, tank top), and strict safety procedures for personnel working in the hold with machinery (ventilation, communication, designated walkways).

Mechanical Trimming Machines (Throwers/Slingers): Specialized machines that use high-speed conveyor belts or rotating paddles to throw or sling the cargo into the void spaces. More common for certain cargoes like grain or fertilizers where a very even fill is desired.

Ship’s Equipment (Less Common for Trimming on Modern Bulkers):

Ship’s Grabs (if geared vessel): In some cases, if the ship’s cranes are used for loading (rare for high-volume bulk), the grabs might be used to try and spread the cargo, but this is inefficient and not true trimming.

Specialized Self-Trimming Vessels: Some bulk carriers are designed with hold configurations (e.g., significantly sloped topside tanks, specific hopper angles) that promote self-trimming of certain free-flowing cargoes to a degree, reducing the need for mechanical trimming. However, even these may require some final mechanical trimming for optimal stowage or specific cargoes.

C. Manual Trimming:

Method: Using shovels and manpower to level the cargo.

Applicability: Extremely labor-intensive and slow for large bulk quantities. Generally only used for:

Very small parcels of cargo.

Final “touch-up” trimming in corners or hard-to-reach areas after mechanical trimming.

Trimming residues during hold cleaning.

Not a primary method for loading entire holds on modern bulk carriers.

4. Factors Affecting Trimming Requirements and Effectiveness:

Angle of Repose: The lower the angle of repose, the more easily the cargo will flow and potentially shift, hence the greater the need for thorough trimming to a level surface.

Cohesiveness of Cargo: Some cargoes are cohesive (particles tend to stick together) and are less prone to shifting even if not perfectly trimmed. Others are non-cohesive (free-flowing) and require careful trimming.

Particle Size and Shape: Fine, rounded particles tend to flow more easily than coarse, angular particles.

Moisture Content: Can affect the angle of repose and flowability.

Hold Configuration:

Smooth-sided holds (double-hull): Easier to achieve a good trim.

Holds with prominent frames/brackets (single-hull): More difficult to fill all voids behind frames.

Presence of hopper tanks and topside wing tanks: These are designed to aid in self-trimming to some extent by directing cargo towards the center and filling wing spaces.

Type of Loading Equipment: The outreach and maneuverability of the shiploader spout significantly impact how well the cargo can be initially distributed.

Loading Rate: Very high loading rates can make it difficult for trimming operations (especially mechanical trimming within the hold) to keep pace, potentially leading to untrimmed cones building up quickly.

5. Consequences of Improper or Insufficient Trimming:

Cargo Shifting: The most severe risk, potentially leading to:

Dangerous list.

Loss of stability and capsize.

Structural damage due to dynamic impact forces of shifting cargo.

Structural Overstressing: Uneven piles of dense cargo can create localized overloading on the tank top or uneven longitudinal weight distribution.

Inaccurate Cargo Quantity Measurement: An untrimmed, conical surface makes it very difficult to accurately measure ullages for calculating the loaded quantity. This can lead to disputes over cargo figures.

Difficulties and Delays During Discharge: Unevenly stowed or shifted cargo can be more difficult and time-consuming to discharge, especially for grab operations.

Safety Hazards for Personnel: If personnel need to enter a hold with poorly trimmed cargo (e.g., for sampling or inspection), they face risks from unstable slopes or potential cargo collapse.

Commercial Claims: For cargo shortage (due to poor stowage factor utilization) or damage caused by shifting.

6. Master’s and Chief Officer’s Responsibilities Regarding Trimming:

Ensure Compliance with IMSBC Code: The Master is responsible for ensuring that trimming is carried out in accordance with the Code’s requirements for the specific cargo being loaded.

Review Shipper’s Trimming Instructions: Assess if the shipper’s provided trimming procedures are adequate for safety. If not, the Master must insist on more thorough trimming.

Supervise Trimming Operations: The Chief Officer must closely supervise the trimming process, whether it’s done by shore personnel (stevedores with bulldozers) or by adjusting the loading spout.

Liaise with Stevedores/Terminal: Clearly communicate the required standard of trimming to the stevedore foreman. Ensure they have the necessary equipment and understand the vessel’s requirements.

Safety During Mechanical Trimming: If machinery like bulldozers is used in the hold:

Ensure adequate ventilation.

Confirm operators are competent.

Ensure clear communication between machine operators and deck personnel.

Take precautions to prevent damage to the ship’s structure.

Prohibit personnel from being in the hold unnecessarily while machines are operating.

Monitor Cargo Surface Profile: Regularly observe the cargo surface during and after loading to ensure it is trimmed to a safe profile.

Document Trimming Operations: Note in the deck logbook or cargo log the method of trimming used and that it was completed to satisfaction. Photographic evidence of the trimmed surface can be beneficial.

Final Decision: The Master has the final say on the adequacy of trimming. If not satisfied that the cargo is safely trimmed, loading should be stopped, or further trimming insisted upon before sailing.

In conclusion, proper trimming of bulk cargo is not an optional extra; it is a fundamental safety requirement. It directly impacts the stability of the vessel and the prevention of cargo shifts, which can have catastrophic consequences. The Master and Chief Officer must ensure that all trimming operations are carried out diligently, in accordance with the IMSBC Code, and to a standard that guarantees a safe voyage.