4.2 Cargo Hold Preparation, Dunnage & Hold Cleaning

4.2 Cargo Hold Preparation, Dunnage & Hold Cleaning

Preparing cargo holds to receive cargo is one of the most important seamanship duties carried out by deck ratings on bulk carriers and general cargo vessels. Before loading, cargo spaces must be cleaned, washed, dried, and inspected. Failure to prepare holds to required commercial standards can lead to cargo contamination, failed quarantine inspections, severe charterparty disputes, and off-hire claims. General Purpose ratings must master hold cleaning grades, chemical washing tests, bilge preparation, ventilation rules, and hatch cover weathertightness testing.

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1. Cargo Hold Cleaning Standards & Washing Protocols

Commercial dry bulk contracts specify the cleanliness needed for the next cargo. The following common grades guide preparation; follow the actual charterparty and loading-inspection requirements:

Step-by-Step Hold Washing Protocol

1. Dry Sweeping: Prior to introducing any water, ratings must sweep down all dry cargo residues from hatch coamings, under-deck framing, transverse bulkheads, and hopper slope plates. Sweeping residues dry into bags prevents thick sludge from forming, which would instantly choke hold bilges.
2. High-Pressure Salt Water Wash: Washing begins from the top down:
   • Underside of hatch covers and coamings.
   • Under-deck transverse frames and beam knees.
   • Vertical corrugated bulkheads and side shell frames.
   • Lower hopper plating and finally the horizontal tank top.
   • Equipment: High-pressure washguns (such as Combi-jets or Toby guns) connected to the ship's fire and deck wash main, delivering seawater at 6 to 8 bar pressure.
3. Chemical Washing (When Required): For stubborn stains from previous cargoes like coal, petcoke, or oil residues, alkaline emulsifiers or specialized hold cleaners are applied via foaming applicators, left to soak for 15 to 30 minutes, and blasted off with high-pressure water.
4. Fresh Water Rinse (Chlorides Elimination): A comprehensive fresh water washdown from top to bottom must immediately follow all sea water washing.
   • Why Fresh Water Rinsing is Critical: Seawater contains high concentrations of sodium chloride ($NaCl$). If sea water evaporates on hold steel, salt crystals remain behind. Chlorides cause rapid, aggressive pitting corrosion of the ship's steel structure, damage protective epoxy coatings, and contaminate sensitive bulk cargoes such as grain, sugar, and high-grade fertilizers.
5. Drying & Natural/Mechanical Ventilation: Open hatch covers partially or operate hold extraction fans to dry the hold thoroughly until no standing moisture remains.

The Silver Nitrate ($AgNO_3$) Chemical Test

The silver nitrate test is an onboard screening check for residual chlorides in samples from hold surfaces before loading sensitive cargoes.

• Reagent: A 2% to 5% solution of silver nitrate ($AgNO_3$) dissolved in distilled water, kept in an opaque amber dropper bottle.
• Testing Procedure: Collect run-off water droplets from bulkheads or puddles on the tank top into a clean glass beaker (or moisten a clean cotton swab with distilled water, wipe the bulkhead, and place it in the beaker). Add 2 to 3 drops of silver nitrate solution.
• Chemical Reaction:

$AgNO_3 + NaCl \rightarrow AgCl \downarrow + NaNO_3$

Silver ions ($Ag^+$) react with chloride ions ($Cl^-$) to form an insoluble white precipitate of silver chloride ($AgCl$).

• Interpretation:
  • Positive: A milky white precipitate indicates chlorides in the sample. Report it and re-rinse contaminated areas as directed.
  • Negative: No visible reaction means no chlorides were detected by that check. It does not certify the whole hold or authorize loading; the required cleanliness inspection must still be satisfactory.

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2. Cargo Hold Bilges, Drainage & Preparation

A ship's hold drainage system prevents water from accumulating on the tank top and provides early warning of hull leakage.

                 CARGO HOLD BILGE WELL CUTAWAY

            TANK TOP PLATING             TANK TOP PLATING
        =============+                         +=============
                     |   [ LIMBER BOARDS ]     |
                     |    (Sealed w/ Burlap)   |
                     |                         |
                     |    +---------------+    |
                     |    |   STRUM BOX   |    |
                     |    |  (ROSE BOX)   |    |
                     |    | Wrapped with  |    |
                     |    | Clean Burlap  |    |
                     |    +-------+-------+    |
                     |            |            |
                     |     BILGE SUCTION       |
                     |         PIPE            |
                     |            |            |
                     |    [ NON-RETURN VALVE ] |
                     +------------+------------+
                         BILGE WELL BOTTOM
                         & STRIKER PLATE

Primary Drainage Components

• Bilge Wells: Rectangular drainage wells recessed into the tank top at the port and starboard after corners of each hold to collect condensation and drainage.
• Bilge Strum Box (Rose Box): A heavy, perforated steel box fitted over the end of the bilge suction pipe inside the well. The perforations (typically 8 to 10 mm in diameter) prevent large debris from entering the suction line while allowing water to pass.
• Non-Return Valve (NRV): A spring-loaded or weighted check valve installed in the bilge suction line. It permits water to be drawn out of the hold by the bilge pump, but physically prevents seawater or ballast water from back-flooding into the cargo hold if valves are misaligned in the engine room.
• Sounding Pipe: A vertical steel pipe extending from the weather deck down into each bilge well, terminating over a reinforced steel striker plate. Sounded daily at sea using a sounding tape and water-finding paste.

Bilge Suction Test

Before loading cargo, a mandatory bilge suction test must be carried out:

1. The bilge well is filled with clean water from a deck wash hose.
2. The engine room watch starts the bilge or general service pump to take suction on that specific hold.
3. The deck officer and rating verify that water draws down rapidly, that suction clears the well, and that the non-return valve seats firmly to prevent back-flow when the pump stops.

Limber Boards and Hessian / Burlap Wrapping

• Limber Boards: Heavy steel or wooden covers fitted flush with the tank top plating over the bilge well.
• Burlap (Hessian) Wrapping: The rose box must be completely wrapped in clean, dry burlap (hessian cloth) and bound tightly with marline or cable ties. When loading fine bulk cargoes (such as grain, bauxite fines, or iron ore fines), burlap is also laid over the limber boards, and the perimeter edges are sealed with cement wash, marine plaster, or heavy adhesive sealing tape.
• Operational Purpose: Burlap functions as a permeable geotextile filter. It allows water to filter through into the bilge well for pumping, but prevents dry cargo grains, powders, or slurry from entering the strum box, which would choke the suction pipe and disable emergency hold drainage.

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3. Dunnage: Functions, Types & Environmental Regulations

Dunnage refers to wooden planks, mats, or inflatable cushions placed beneath and around cargo.

Core Purposes of Dunnage

1. Ventilation: Creates continuous air channels beneath and around cargo units, allowing natural or mechanical hold airflow to dissipate heat and moisture.
2. Moisture Separation: Elevates packaged or breakbulk cargo 50 to 100 mm above the steel tank top, protecting it from sweating steel, condensation, and bilge water.
3. Frictional Grip: Softwood planks laid athwartships increase friction between slick steel decks and heavy cargo units (such as steel coils, pipes, or machinery), preventing cargo shifting during rolling.
4. Load Spreading: Distributes heavy concentrated point loads across multiple double-bottom transverse floors and longitudinal frames, preventing structural denting of the tank top.
5. Chafing & Structure Protection: Prevents sharp cargo edges from gouging tank top protective coatings and protects bagged commodities from chafing against corrugated bulkheads.

Common Dunnage Materials

• Softwood Lumber: Rough-sawn pine, spruce, or fir planks ($50 \times 100\text{ mm}$ or $25 \times 150\text{ mm}$).
• Bamboo Mats & Kraft Paper: Laid over bulkheads and floors to protect bagged commodities (rice, sugar, coffee beans).
• Inflatable Dunnage Bags (Airbags): Heavy woven polymer air bladders placed in void gaps between cargo units and inflated with compressed air to brace cargo against rolling and pitching.

Phytosanitary Regulations (ISPM 15)

Under ISPM 15 (International Standards for Phytosanitary Measures No. 15), solid wood dunnage and packaging within the standard’s scope must be debarked, receive an approved treatment and bear the official IPPC mark identifying the country, authorized producer or treatment provider, and treatment. Conventional heat treatment (HT) requires at least $56^\circ\text{C}$ throughout the wood, including its core, for 30 continuous minutes. Debarking has defined tolerances for small residual bark pieces. Non-compliant wood is dealt with as directed by the importing plant-health authority, which may require treatment, secure disposal or reshipment.

MARPOL Annex V Disposal Regulations

Under MARPOL Annex V (Prevention of Pollution by Garbage from Ships), discharging cargo-associated waste, packing materials, and dunnage into the sea is strictly prohibited worldwide:

• Solid timber dunnage that floats cannot be dumped overboard anywhere.
• Collect and sort spent dunnage and packaging under the Garbage Management Plan. Retain it for port reception or use a suitable approved onboard incinerator where MARPOL Annex VI and the operating instructions permit. Record disposal and incineration in the Garbage Record Book where required; retain incinerator ash for shore reception.

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4. Cargo Care, Dew Point & Hold Ventilation

Effective ventilation prevents moisture damage to cargo and ship structures. Mariners use psychrometric principles to control ventilation.

Psychrometric Fundamentals

• Relative Humidity ($RH$): The percentage ratio of water vapor present in air compared to the saturation capacity of air at that temperature.
• Dew Point Temperature ($T_{dp}$): The temperature to which air must be cooled at constant pressure to reach 100% saturation, at which point water vapor condenses into liquid water droplets (sweat or dew).
• Measurement: Monitored using a whirling psychrometer (wet-and-dry bulb hygrometer). The difference between dry-bulb and wet-bulb temperatures is referenced against psychrometric tables to determine the dew point of both outside air and hold air.

Ship Sweat versus Cargo Sweat

                         SHIP SWEAT vs. CARGO SWEAT

        WARM TO COLD CLIMATE                   COLD TO WARM CLIMATE
       [ WARM CARGO IN HOLD ]                 [ COLD CARGO IN HOLD ]
                 |                                      |
                 v                                      v
      Cold Outside Air Chills                Warm Moist Outside Air Enters
     Hull & Deckhead Structure              Strikes Cold Cargo Surface
                 |                                      |
                 v                                      v
      Moisture Condenses ON                  Moisture Condenses ON
     SHIP'S STEEL STRUCTURE                    THE CARGO ITSELF
     Drips Down Onto Cargo                  Causes Rust, Rot, Mold
                 |                                      |
                 v                                      v
      VENTILATE IF CONDITIONS ALLOW        EXCLUDE DAMP OUTSIDE AIR
       (Use dew-point checks)             (While it would wet the cargo)

Ship Sweat (Hold Sweat)

• Voyage Context: Sailing from a Warm climate to a Cold climate (e.g., loading agricultural grain in India or Southeast Asia and sailing to Northern Europe in winter).
• Mechanism: Warm, moist air inside the cargo hold encounters the ship's steel hull, deckhead, and hatch coamings, which are chilled by cold seawater and Arctic air. Moisture condenses directly onto the internal ship steel and drips down onto the cargo.
• Action: Ventilate with outside air of lower dew point when cargo instructions and weather permit. Keep sea spray out of the ventilation openings.

Cargo Sweat

• Voyage Context: Sailing from a Cold climate to a Warm climate (e.g., loading steel coils in Northern Europe or China in winter and sailing to the tropical Persian Gulf or India).
• Mechanism: The cargo has large thermal mass and remains cold. When warm, humid tropical air enters the hold, it strikes the chilled cargo surfaces and drops below its dew point, condensing water directly onto the cargo itself and causing severe rust or rot.
• Action: Exclude outside air while its dew point exceeds the cold cargo’s surface temperature. Reassess measured conditions during the voyage; temperature equalization is not the criterion for reopening ventilation.

The Dew-Point Rule for Moisture Control

Apply these comparisons to moisture-control ventilation, subject to the cargo’s carriage instructions and safe weather conditions. Hazardous-cargo ventilation must follow the individual IMSBC schedule.

$\text{Ventilate ONLY when: } T_{dp(\text{outside})} < T_{dp(\text{hold})}$ $\text{DO NOT Ventilate when: } T_{dp(\text{outside})} \ge T_{dp(\text{hold})} \quad \text{or} \quad T_{dp(\text{outside})} \ge T_{\text{cargo}}$

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Preparing for Refrigerated Cargo

Refrigerated cargo may be chilled, such as fresh fruit, or frozen, such as frozen meat or fish. Its temperature requirements come from the cargo instructions.

Before loading, the responsible officer checks that the hold or locker is clean, dry, free of incompatible residues and odours, and that drains, insulation, door seals, refrigeration and temperature sensors are ready. Pre-cool the space and dunnage as required by the loading plan. Ratings assist under supervision, using clean suitable dunnage and keeping the planned air passages clear.

During loading and carriage, protect the cold space from unnecessary heat entry, preserve airflow around the stow and record compartment temperatures at the required intervals. Report abnormal readings, damaged insulation, blocked airflow or defrost/drainage problems promptly. A record of the temperature trend helps the officer detect loss of cargo-cooling performance.

5. Hatch Cover Weathertightness & Testing Procedures

Weather deck hatch covers prevent sea water from penetrating into cargo holds during heavy weather.

Types of Hatch Covers

• Pontoon Covers: Individual steel box pontoons lifted and positioned by shore gantry cranes or shipboard cranes; standard on cellular container vessels.
• MacGregor Single-Pull Covers: Series of steel panels linked by chains and balance rollers, pulled by winch wire along coaming rails and tilting vertically into a compact stowage bay at the hatch end.
• Hydraulic Folding Covers: Paired panels joined by heavy hinges and lifted into an upright folded position by hydraulic cylinders; standard on modern bulk carriers and general cargo vessels.
• Side-Rolling Covers: Massive two-piece panels rolling athwartships on raised ramps driven by hydraulic rack-and-pinion drives; standard on large Capesize bulk carriers.

Sealing Mechanism

                    HATCH COVER SEALING DETAIL

           HATCH COVER PANEL UNDERSIDE
         +-----------------------------+
         |   [ RUBBER GASKET STRIP ]   |
         +--------------+--------------+
                        |
         ===============v===============  <-- Compression Point
              |   COMPRESSION BAR   |
              |   (Raised Knife-    |
              |    Edge Steel)      |
         +----+---------------------+------------------+
         |     COAMING TOP PLATE                       |
         |                             DRAIN CHANNEL   |
         |                                   |         |
         |                             NON-RETURN VALVE|
         +---------------------------------------------+

• Compression Bar: A raised, clean, continuous vertical steel bar welded along the perimeter of the hatch coaming.
• Rubber Packing Gasket: A resilient synthetic neoprene or EPDM elastomeric channel gasket seated inside a steel retaining channel on the underside of the hatch cover panel. When dogged down, the compression bar embeds into the rubber gasket to form a watertight seal.
• Drainage Channel & Non-Return Drain Valves: The inner drainage channel lies inside the gasket line and collects water that gets past the seal before it can enter the hold. Clear drain passages lead to outlets on the weather deck. Their non-return valves allow drainage out and prevent seawater entering through the drains. Keep channels, outlets and valves clean and serviceable; check any additional outer drainage fitted to the hatch design.

Tightness Testing Methods

1. Chalk Test: Clean the top edge of the compression bar and apply a continuous line of dry white chalk along its entire perimeter. Close the hatch covers, fully dog down all quick-acting cleats, then reopen the covers. A continuous, unbroken chalk line along the center of the rubber confirms proper uniform physical contact. (Verifies physical contact only; does not measure water resistance under pressure).
2. Hose Test: Conducted using a marine fire hose fitted with a nozzle of at least 12 mm diameter, delivering water at a pressure of at least 2.0 bar (0.2 MPa). Direct the water jet perpendicularly at the coaming compression joint and cross-joint panel seams from a distance of 1.0 to 1.5 meters, advancing slowly along the seams at a rate of 1 meter per 10 seconds. An inspector inside the darkened hold uses a high-powered flashlight to detect any water leaks.
3. Ultrasonic Leak Test: An electronic ultrasonic transmitter emitting high-frequency acoustic waves ($40\text{ kHz}$) is activated inside the closed hold. An operator walks the weather deck along all hatch seams using a calibrated ultrasonic receiver wand and headphones. Any acoustic sound leaking through the gasket is converted into a decibel readout. This method pinpoints the exact location of seal failure without using water, eliminates wetting risk, and can be performed with cargo already loaded.

Securing Gear

• Quick-Acting Cleats: Lever-cam mechanisms that hook over coaming snugs to pull the hatch panel down firmly against the compression bar.
• Cross-Joint Wedges: Hardened steel wedges driven into sockets along transverse joints between adjacent panels to maintain uniform compression across the ship's beam.