Section 7.2 Loading Sequence and Stress Monitoring (Use of Loading Computers)

A well-considered final stowage plan is essential, but it only represents the vessel’s condition after all cargo is loaded. The process of getting to that final state – the loading sequence – is equally critical for the safety and structural integrity of a bulk carrier. Loading thousands, or hundreds of thousands, of tonnes of cargo significantly alters the vessel’s weight distribution, buoyancy, stability, and stresses, often rapidly. Without a carefully planned and monitored loading sequence, the ship can be subjected to unacceptable bending moments, shear forces, or stability conditions even if the final loaded condition is perfectly safe. The ship’s approved loading instrument (loading computer) is the indispensable tool for planning this sequence and for continuous monitoring throughout the operation. The Master and Chief Officer must ensure this process is managed with utmost diligence.

1. Why a Detailed Loading Sequence is Critical:

Preventing Overstressing During Intermediate Stages: As cargo is loaded into different holds, the distribution of weight along the ship’s length changes dynamically. Loading heavily in the midship holds first can induce excessive sagging, while loading heavily in the end holds first can cause excessive hogging. A planned sequence aims to distribute the load progressively, keeping stresses within permissible limits at all times.

Maintaining Adequate Stability Throughout Loading: Similarly, stability (GM) can fluctuate significantly during loading, especially if combined with de-ballasting operations. The sequence must ensure that stability remains positive and meets minimum criteria at all intermediate stages.

Coordinating with De-Ballasting Operations: On most bulk carriers, de-ballasting is carried out concurrently with loading to save time. The loading sequence must be synchronized with the de-ballasting plan to manage drafts, trim, stability, and stresses effectively. Rapid de-ballasting without corresponding cargo loading can lead to a dangerously light condition or excessive trim.

Managing Draft and Trim: The sequence helps control the vessel’s draft and trim during loading, ensuring it remains within terminal limits and that loading equipment (shiploaders) can maintain proper outreach and clearance.

Optimizing Port Time: A well-planned sequence, communicated effectively to the terminal, can contribute to a more efficient and faster loading operation by minimizing delays caused by shifting loaders or waiting for the ship to adjust ballast.

Preventing Damage to Loading Equipment or Ship Structure: Sudden, large lists or trims due to improper sequencing can pose risks to shore-based loading arms or conveyors, and to the ship’s structure if contact occurs.

2. Developing the Loading Sequence:

The Chief Officer, under the Master’s supervision, is primarily responsible for developing the detailed loading sequence. This is typically done once the final stowage plan is agreed upon and detailed information about the terminal’s loading capabilities is available.

A. Inputs for Sequence Planning:

Approved Final Stowage Plan: Quantities per hold.

Terminal Information:

Number of shiploaders/spouts available and their designated working holds.

Maximum loading rate per loader and overall.

Shiploader travel time between hatches (if applicable).

Any terminal preferences or restrictions on loading order (though ship’s safety always takes precedence).

Ship’s De-ballasting Plan: Planned sequence and rates for discharging ballast from various tanks.

Loading Instrument: Essential for simulating and verifying each step.

B. General Principles for Sequencing:

Incremental Loading: Load cargo in stages or “passes” across multiple holds rather than completely filling one hold before starting another (unless specifically allowed by the loading manual for certain conditions, e.g., block loading).

Maintain Even Keel or Slight Stern Trim (Generally): Avoid excessive trim by the head during loading, as it can strain the hull and affect stability.

Symmetrical Loading (Port/Starboard): If multiple loaders are used in the same hold or adjacent holds, try to load symmetrically to avoid inducing a list.

Start Midships or Spread Load: Often, loading starts in the midship holds or is spread across several holds to manage initial stresses, gradually working towards the ends, but this depends heavily on the ship’s design, the cargo density, and the final stowage plan (e.g., for alternate hold loading, the sequence is very specific).

“Pyramid” Loading (for some cargoes): Building up cargo in a pyramidal fashion within a hold can help with trimming and even distribution.

Coordinate with De-ballasting: De-ballasting should ideally keep pace with cargo loading to manage drafts and stresses. Avoid having the vessel become too light or develop excessive trim due to rapid de-ballasting ahead of cargo loading.

Minimize Shifting of Loaders: Plan the sequence to reduce the number of times shiploaders need to be moved between hatches, if possible, to save time.

C. Using the Loading Instrument for Sequence Planning:

Initial Condition: Start with the vessel’s arrival condition (ballast, fuel, etc.) in the loading computer.

Define Stages/Steps: Break down the entire loading operation into a series of manageable steps or stages (e.g., “Load 2,000 tonnes into No. 3 Hold,” “Discharge 1,500 tonnes of ballast from No. 2 Topside Tanks P&S,” “Load 3,000 tonnes into No. 5 Hold while discharging ballast from No. 4 DB Tank”).

Simulate Each Step: For each step, input the planned cargo loaded and ballast discharged into the loading instrument.

Check Results: After each simulated step, verify:

Longitudinal Strength: Shear forces and bending moments are within permissible still water limits.

Stability: GM is adequate, and list is minimal.

Drafts and Trim: Are as expected and acceptable.

Adjust and Iterate: If any parameter exceeds limits at an intermediate stage, the sequence (amount of cargo per step, order of holds, or timing of ballast operations) must be adjusted, and the step re-calculated. This iterative process continues until the entire sequence, from start to finish, shows acceptable stress and stability values at every stage.

Document the Sequence: Once a safe sequence is established, it should be clearly documented (often as a printout from the loading computer or a manually prepared table) showing quantities per hold/tank at each stage, and the corresponding calculated drafts, stability, and stress values.

D. Communication of the Sequence:

The agreed loading sequence should be communicated to the terminal/stevedores before loading commences so they can align their operations.

It should also be clearly understood by the ship’s deck officers and crew involved in monitoring loading and ballast operations.

3. Stress and Stability Monitoring During Loading:

Planning the sequence is only half the battle; continuous monitoring during the actual loading operation is essential to ensure the plan is being followed and that the vessel remains within safe limits.

A. Continuous Use of the Loading Instrument:

The loading instrument should be updated in near real-time as cargo is loaded and ballast is discharged.

The officer on watch (OOW) for cargo operations (usually the Chief Officer or another deck officer) is responsible for regularly inputting the actual quantities loaded into each hold (obtained from terminal figures, draft readings, or automatic weighing systems on some loaders) and the actual status of ballast tanks.

After each significant parcel of cargo is loaded or a ballast tank operation is completed, the calculations should be run to check the current stress and stability condition.

B. Regular Checks Against Permissible Limits:

At frequent intervals (e.g., hourly, or after every few thousand tonnes loaded, or as defined in the company’s SMS), the calculated shear forces, bending moments, and stability parameters must be compared against the permissible limits.

Particular attention should be paid as any value approaches its limit (e.g., 90-95% of permissible).

C. Communication Between Ship and Shore:

Maintain constant communication with the terminal operators/shiploader operators regarding:

Which holds are to be loaded next and the quantities.

Any deviations from the planned sequence.

Loading rates.

Requests to stop or slow down loading if stress or stability limits are being approached, or if de-ballasting cannot keep pace.

The ship always retains the right to control the loading sequence for safety reasons.

D. Draft Monitoring:

Regularly observe and record the vessel’s drafts (forward, aft, and midships port & starboard) and compare them with the drafts predicted by the loading instrument for the amount of cargo loaded.

Significant discrepancies between observed and calculated drafts could indicate:

Errors in declared cargo weight or stowage factor.

Errors in ballast tank soundings.

Incorrect data input into the loading instrument.

Unexpected list or trim due to uneven loading.

(In rare, serious cases) Hull leakage or structural deformation (though this is less likely if stresses are being monitored).

Draft surveys (initial, intermediate, final) are also crucial for determining the actual quantity of cargo loaded (covered in a later section).

E. Visual Inspection:

The OOW should make regular visual inspections of the deck, moorings, gangway, and the cargo being loaded.

Check for any signs of excessive list or trim, undue stress on mooring lines, or problems with the loading process itself.

4. Role of the Loading Computer (Recap and Emphasis):

The loading computer is the central tool for both planning the sequence and monitoring its execution.

Simulation Power: Allows “what-if” scenarios to be tested safely before committing to a physical operation.

Real-time Feedback: Provides immediate calculation of complex stress and stability parameters based on actual inputs.

Alarms and Warnings: Most loading instruments have audible and visual alarms if calculated values exceed pre-set warning levels or permissible limits. These must never be ignored.

Record Keeping: Many systems can log the loading sequence and the calculated values at each step, providing a valuable record.

Accuracy is Paramount: The GIGO (Garbage In, Garbage Out) principle cannot be overstressed. The accuracy of the output depends entirely on:

The integrity of the instrument’s underlying database (hydrostatics, lightship, tank data – verified by Class).

The accuracy of the variable data input by the operator (cargo quantities per hold, tank soundings). Regular cross-checks (e.g., comparing total loaded quantity from ship figures vs. shore figures) are important.

5. Common Challenges and Pitfalls in Loading Sequence Management:

Deviation from Planned Sequence by Terminal: Shore loaders may want to deviate from the ship’s requested sequence for their own operational convenience (e.g., to minimize loader movements or match stockpile availability). The Master/CGO must assess if such deviations are safe. If not, they must insist on adhering to a safe sequence.

High Loading Rates: Modern terminals can load at very high rates (many thousands of tonnes per hour). This leaves less time for the ship’s crew to conduct ballast operations, monitor stresses, and react to deviations. Close coordination and the ability to slow down or stop loading if necessary are vital.

Equipment Breakdown (Ship or Shore): A breakdown of a shiploader or a ship’s ballast pump can disrupt the planned sequence, requiring rapid recalculation and development of a revised, safe plan.

Communication Failures: Misunderstandings between ship and shore regarding which hold to load, quantities, or rates can lead to dangerous situations. Clear, closed-loop communication is essential.

Fatigue: Loading operations can be long and demanding. Fatigue of the OOW responsible for monitoring can lead to errors in data input or delayed recognition of developing problems. Ensure adequate rest and manning.

Over-Reliance on Automation: While the loading computer is a powerful aid, it should not replace sound judgment and seamanship. Officers must understand the principles behind the calculations.

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

Master:

Overall responsibility for the safe loading of the vessel.

Reviews and approves the proposed loading sequence.

Ensures the Chief Officer and other deck officers are competent in using the loading instrument and understand the stress/stability limits.

Is kept informed by the Chief Officer of progress and any deviations or concerns.

Has the ultimate authority to slow down or stop loading if they believe the vessel’s safety is being compromised.

Chief Officer (Cargo Officer – CGO):

Develops the detailed loading sequence in consultation with the Master.

Supervises the entire loading operation on deck.

Ensures continuous and accurate updating of the loading instrument.

Monitors stress, stability, drafts, and trim throughout the operation.

Liaises directly with the terminal foreman/shiploader operators.

Manages de-ballasting operations concurrently and safely.

Alerts the Master immediately if any parameters approach or exceed limits, or if any other problems arise.

“Understanding Your Loading Computer’s Alarms”: What they mean and how to react.

In conclusion, the careful planning of a loading sequence and the diligent monitoring of stresses and stability throughout the operation are fundamental to preventing structural damage and ensuring the safety of the bulk carrier. The loading computer is an essential tool in this process, but it is the knowledge, vigilance, and decisive action of the Master and Chief Officer that ultimately safeguard the vessel during this dynamic and potentially hazardous phase.