Section 10.3 Discharge Sequence and Stability Monitoring

Just as a carefully planned sequence is vital during loading, the order in which cargo is removed from the holds during discharge, and the concurrent ballasting operations, are critical for maintaining the structural integrity and stability of a bulk carrier. As thousands of tonnes of cargo are removed, the vessel’s weight distribution, drafts, trim, stability parameters, and hull stresses change significantly and often rapidly. Without a pre-planned and continuously monitored discharge sequence, the vessel can be subjected to unacceptable bending moments, shear forces, or develop a dangerous list or insufficient stability. The ship’s approved loading instrument (loading computer) remains the indispensable tool for planning this sequence and for ongoing monitoring throughout the discharge process. The Master and Chief Officer must ensure this operation is managed with the highest level of diligence and adherence to safety limits.

1. Why a Detailed Discharge Sequence is Critical:

The objectives are largely the reverse of those during loading, but equally important:

Preventing Overstressing During Intermediate Stages: As cargo is removed from different holds, the weight distribution changes. Discharging heavily from the midship holds first can induce excessive hogging, while discharging heavily from the end holds first can cause excessive sagging if not balanced by appropriate ballasting or cargo in other holds. A planned sequence aims to remove weight progressively, keeping stresses within permissible limits at all times.

Maintaining Adequate Stability Throughout Discharge: Stability (GM) can fluctuate significantly as cargo is removed and ballast is taken on. The sequence must ensure that stability remains positive and meets minimum criteria at all intermediate stages. The free surface effect of partially filled ballast tanks being filled must be carefully managed.

Coordinating with Ballasting Operations: On most bulk carriers, ballasting is carried out concurrently with discharge to maintain adequate draft for stability, manage trim, ensure propeller immersion (especially towards the end of discharge), and control stresses. The discharge sequence must be synchronized with the ballasting plan.

Managing Draft and Trim: The sequence helps control the vessel’s draft and trim as it lightens, ensuring it remains within terminal limits for safe unberthing and departure, and that discharge equipment (grabs, CSUs) can maintain effective operation.

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

Preventing Damage to Discharge Equipment or Ship Structure: Sudden, large lists or trims due to improper sequencing can pose risks to shore-based unloading equipment and to the ship’s structure.

2. Developing the Discharge Sequence:

The Chief Officer, under the Master’s supervision, is primarily responsible for developing the detailed discharge sequence, often in consultation with the terminal if they have specific operational preferences (though ship’s safety always takes precedence).

A. Inputs for Sequence Planning:

Final Loaded Stowage Plan: The distribution of cargo in the holds upon arrival.

Terminal Information:

Number of unloaders (grabs, CSUs) available and their designated working holds.

Maximum discharge rate per unloader and overall.

Unloader travel time between hatches (if applicable).

Any terminal preferences or restrictions on discharge order.

Ship’s Ballasting Plan: Planned sequence and rates for taking on ballast into various tanks.

Loading Instrument: Essential for simulating and verifying each step.

Departure Draft/Trim Requirements: Any specific targets for completion of discharge.

B. General Principles for Sequencing:

Incremental Discharge: Remove cargo in stages or “passes” across multiple holds rather than completely emptying one hold before starting another (unless specifically allowed by the loading manual for certain conditions and carefully calculated). This helps manage stresses more evenly.

Maintain Even Keel or Slight Stern Trim (Generally): Avoid excessive trim by the head as the vessel lightens, which can make discharge from aft holds difficult, affect propeller immersion, and strain the hull.

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

Consider Stress Distribution: Often, discharge may start from midship holds or be spread across several holds to manage initial stress changes, but this depends heavily on the initial loaded condition (e.g., if loaded in alternate holds, the discharge sequence will be specific to that).

Coordinate with Ballasting: Ballasting should ideally keep pace with cargo discharge to manage drafts, trim, and stability. Avoid the vessel becoming too light too quickly or developing excessive trim before sufficient ballast is taken on. The rate of ballasting is often a limiting factor.

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

“Bottoming Out” Holds: Towards the end of discharge in each hold, ensure grabs or other equipment can effectively reach the remaining cargo on the tank top and in corners/wings.

C. Using the Loading Instrument for Sequence Planning:

Initial Condition: Start with the vessel’s fully loaded arrival condition in the loading computer.

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

Simulate Each Step: For each step, input the planned cargo discharged and ballast taken on 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. Free surface moments from slack ballast tanks being filled must be correctly accounted for.

Drafts and Trim: Are as expected and acceptable. Check for adequate propeller immersion towards the end.

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 shows acceptable stress and stability values at every stage.

Document the Sequence: The approved sequence should be clearly documented for use by the ship’s officers and for communication to the terminal.

D. Communication of the Sequence:

The agreed discharge sequence should be communicated to the terminal/stevedores before discharge commences.

It must be clearly understood by the ship’s deck officers and engine room staff (for ballasting).

3. Stress and Stability Monitoring During Discharge:

Continuous monitoring during the actual discharge operation is just as vital as during loading.

A. Continuous Use of the Loading Instrument:

The loading instrument must be updated in near real-time as cargo is discharged and ballast is taken on.

The officer on watch (OOW) for cargo operations is responsible for regularly inputting the actual quantities discharged from each hold (obtained from terminal figures, draft changes, or estimates based on grab counts/time) and the actual status of ballast tanks.

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

B. Regular Checks Against Permissible Limits:

At frequent intervals, compare calculated SF, BM, and stability parameters against permissible limits.

Pay close attention as any value approaches its limit. Remember that as the vessel lightens, its displacement decreases, and its capacity to withstand bending moments and shear forces also changes (permissible limits are often draft-dependent).

C. Communication Between Ship and Shore:

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

Which holds are to be discharged next and the approximate quantities.

Any deviations from the planned sequence.

Discharge rates.

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

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

D. Draft Monitoring:

Regularly observe and record the vessel’s drafts and compare them with the drafts predicted by the loading instrument for the amount of cargo discharged and ballast taken on.

Significant discrepancies need immediate investigation (potential errors in discharged quantity reporting, ballast quantities, or loading instrument input).

This also helps ensure the vessel will meet any departure draft restrictions.

E. Visual Inspection:

The OOW should make regular visual inspections of the deck, moorings, gangway, and the discharge operation itself.

Check for any signs of excessive list or trim, undue stress on mooring lines, or problems with the discharge process (e.g., grab damage).

4. Role of the Loading Computer (Recap for Discharge):

Essential for Simulation and Real-time Monitoring: As critical during discharge as it is during loading.

Accounts for Changing Displacement: Correctly calculates how permissible stresses change as the vessel lightens.

Free Surface Moments: Accurately accounts for the increasing impact of free surface moments as ballast tanks are filled.

Accuracy is Paramount (GIGO): Relies on accurate input of discharged cargo quantities (which can be harder to estimate precisely moment-to-moment than loaded quantities) and ballast tank levels.

5. Common Challenges and Pitfalls in Discharge Sequence Management:

Deviation from Planned Sequence by Terminal: Terminals may wish to alter the sequence for their convenience (e.g., to focus on holds that are easier to access or to match their stockpile/transport arrangements). The Master/CGO must ensure any such deviations are safe for the ship.

High Discharge Rates: Modern unloaders can discharge very rapidly. This demands very efficient and well-coordinated ballasting operations by the ship’s crew to keep pace and maintain safety. The ballasting capacity often becomes the limiting factor for the overall discharge speed.

Equipment Breakdown (Ship or Shore): Failure of an unloader or a ship’s ballast pump can disrupt the sequence, requiring prompt recalculation and adjustment of the plan.

Inaccurate Shore Figures for Discharged Quantities: Can lead to errors in updating the loading computer if not cross-checked with draft changes.

Managing Multiple Unloaders: Coordinating the work of several grabs or CSUs working simultaneously in different holds requires careful planning and monitoring to maintain even stress distribution.

Fatigue: Discharge operations can be lengthy. Ensure watch officers are alert and diligently performing their monitoring duties.

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

Master:

Overall responsibility for the safe discharge of the vessel.

Reviews and approves the proposed discharge sequence and ballasting plan.

Ensures officers are competent with the loading instrument and understand all safety limits.

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

Has ultimate authority to slow down or stop discharge if the vessel’s safety is at risk.

Chief Officer (Cargo Officer – CGO):

Develops the detailed discharge sequence and coordinates the ballasting plan.

Supervises the entire discharge operation on deck.

Ensures continuous and accurate updating of the loading instrument.

Monitors stress, stability, drafts, and trim throughout.

Liaises directly with the terminal foreman/unloader operators.

Manages ballasting operations safely and effectively.

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

The safe and efficient discharge of a bulk carrier relies on the same principles of meticulous planning, continuous monitoring, and diligent execution as the loading operation. By carefully managing the discharge sequence and concurrently adjusting ballast, the Master and Chief Officer ensure the vessel remains within all structural and stability safety limits, paving the way for a successful port call and preparation for the next employment.