Bulk carrier cargo operations

Bulk carriers load and discharge very large tonnages, often at terminal rates far faster than the ship itself can safely absorb without careful sequencing. The cargo itself is regulated by the IMSBC Code (International Maritime Solid Bulk Cargoes Code), mandatory under SOLAS Chapter VI, while the loading and unloading interface between ship and terminal is covered by SOLAS Chapter VI and Chapter XII (additional safety measures for bulk carriers) together with the BLU Code (Code of Practice for the Safe Loading and Unloading of Bulk Carriers, adopted by the IMO as Assembly resolution A.862(20)).

IMSBC Code cargo groups

A cargo can be scheduled under more than one group (Group A and B together, for example) if it carries both hazards. The IMSBC Code schedule for the named cargo sets out the required certificates, stowage and segregation requirements, and — for Group A cargoes — the moisture-content declaration and testing regime.

Loading plan + hull-girder stress

A dense cargo loaded unevenly can overstress the hull long before the ship reaches its deadweight limit. The loading sequence is planned, and typically executed with a loading computer, to keep shear force and bending moment within the class-approved limits at every stage of loading — not only at the finished condition.

• Alternate-hold loading. For dense cargoes (iron ore, some concentrates) it is common to load cargo into alternate holds only, leaving intervening holds empty, to keep the overall weight distribution and local double-bottom loading within design limits. SOLAS Chapter XII and the ship’s loading manual specify which holds may be loaded in the alternate condition and any restrictions that apply.
• Shear force + bending moment. The loading computer calculates shear force and bending moment along the hull for the current and next loading step, checking each against the class-society-approved limit curve before the next parcel is called forward from the terminal.
• Local (double-bottom) loading limits. Class rules also set a maximum permissible cargo density or weight per hold, independent of the whole-ship shear/bending picture, to prevent local structural overload from a very dense cargo loaded into a single hold.

Terminal loading rates + trimming

Modern bulk terminals can load cargo far faster than a ship’s own stress limits or ballasting capacity can safely absorb, so the loading rate and sequence are agreed with the terminal before cargo starts, and the ship can require the terminal to pause or slow loading into a given hold. Trimming — spreading the cargo evenly across the hold rather than leaving it in a cone under the loading spout — is required both for structural reasons (avoiding a concentrated local load) and, for Group A cargoes, to reduce the risk of a liquefied surface flowing toward one end of the hold. The BLU Code sets out the information exchange (cargo plan, rates, contingency arrangements) that should be agreed between ship and terminal before loading begins.

Hold preparation

Between voyages, holds must be prepared to a cleanliness standard suited to the next cargo — ranging from a simple cargo-clean sweep to a full grain-clean wash, rinse and dry cycle inspected by an independent surveyor before the Notice of Readiness can be tendered. See bulk hold preparation for the grades, procedure and the laytime consequences of a failed inspection.

Group A liquefaction risk

Group A cargoes shipped above their Transportable Moisture Limit can liquefy under the vibration of the voyage, producing a free-surface effect that has caused vessels to capsize. TML is set at a fixed margin below the Flow Moisture Point (FMP) established by laboratory testing, and cargo above TML must not be loaded. See IMSBC Group A for the physics, the testing methods and the cargoes most frequently implicated.

The loading computer

SOLAS Chapter XII requires bulk carriers to carry a loading instrument capable of calculating shear force and bending moment for the loading and unloading condition, approved by the flag administration or a recognised organisation. The loading computer is used to plan the sequence before cargo operations begin and to verify each intermediate step as loading or discharge proceeds, and its output is checked against the ship’s trim and draft as read from a draft survey.

Self-unloaders

Some bulk carriers are fitted with self-unloading gear — typically a system of hold conveyors feeding a boom conveyor that can discharge cargo directly to a shore stockpile or receiving hopper without shore cranes or grabs. Self-unloaders are common in trades (aggregates, coal, some grain) calling at terminals without heavy discharge equipment, and the unloading rate and sequence are managed from the ship rather than by the terminal, though the same hull-stress limits apply in reverse during discharge.

Primary sources

• · IMSBC Code (International Maritime Solid Bulk Cargoes Code) — IMO, mandatory under SOLAS Chapter VI
• · SOLAS Chapter VI (carriage of cargoes) and Chapter XII (additional safety measures for bulk carriers) — IMO
• · BLU Code (Code of Practice for the Safe Loading and Unloading of Bulk Carriers) — IMO

See also

• · IMSBC Code — full group A/B/C framework and schedule layout.
• · IMSBC Group A — liquefaction physics, TML/FMP testing, and notable casualties.
• · Bulk hold preparation — cleanliness grades and the pre-load inspection.
• · Grain Code — the separate stability regime for bulk grain.
• · Draft survey procedure — how loaded quantity is independently verified.

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