A major requirement during the carriage of goods at sea is that the carriage should be done in a way that avoids damage to the carrying vessel. Since most solid bulk cargoes are usually heavy high-density cargoes, vessels loading solid bulk cargoes are subject to structural deformation because of uneven weight distribution. Cargo trimming is a measure against the shifting of bulk cargo.
Trimming is the process of shoveling and spreading, within the ship´s hold, dry bulk cargoes, to avoid weight imbalance that might cause structural deformation of the ship.
The carriage of solid bulk cargoes involves serious risks, which must be managed carefully to safeguard the crew and the ship. These risks include damage to ship structures due to poor loading procedures. In this article, we shall discuss some of the key requirements, procedures, and precautions regarding trimming for the safe loading and carriage of solid bulk cargo.
What is the Purpose of Trimming a Cargo Hold?
When certain bulk commodities are transported as cargo on board vessels, the cargo needs to be “trimmed” to ensure safety and to keep the ship’s stability. The purpose of trimming a cargo hold is to ensure that the weight of the cargo is evenly distributed throughout the hold.
This is important because it helps to keep the ship balanced and prevents the shifting of the cargo during transit. Trimming the cargo hold is a crucial part of the loading process and must be done carefully to ensure safety
The International Maritime Organization (IMO), recognizing that one of the major hazards associated with the shipment of solid bulk cargo relates to structural damage due to improper cargo distribution has come out with guidance in the form of a Code – The International Maritime Solid Bulk Cargoes Code (IMSBC-Code).
The code is the main legislation governing the safe carriage of solid bulk cargo. Section 5 of the code, which gives guidance on trimming procedures is as follows:
Trimming a cargo reduces the likelihood of the cargo shifting and minimizes the air entering the cargo. Air entering the cargo could lead to spontaneous heating. To minimize these risks, cargoes shall be trimmed reasonably level, as necessary.IMSBC-Code 5.1.1
Cargo spaces shall be as full as practicable without resulting in excessive loading on the bottom structure or ‘tween deck to prevent sliding of a solid bulk cargo. Due consideration shall be given to the amount of a solid bulk cargo in each cargo space, taking into account the possibility of shifting and longitudinal moments and forces of the ship. Cargo shall be spread as widely as practicable to the boundary of the cargo space. Alternate hold loading restrictions, as required by SOLAS chapter XII, may also need to be taken into account.IMSBC-Code 5.1.2
Observance and application of the provisions of section 5 of the Code harmonize the practices and procedures to be followed, as well as the appropriate measures to be taken, in the loading, trimming, carriage, and discharge of solid bulk cargoes when transported by sea and ensures conformity with the SOLAS Convention’s mandatory provisions.
Bulk Carrier Cargo Trimming Process.
Bulk carrier cargo trimming is the process of equalizing the distribution of the cargo in the hold. This is done by pouring the cargo from one corner of the hold to the opposite corner until the cargo is evenly distributed.
Bulk Cargo Trimming can be done in a variety of ways. The most expensive and time-consuming approach is for a gang of men to physically shift the cargo from the top of the cone to the hold sides with shovels. On economic and/or safety considerations, physically shifting the top of the cone is not always practicable.
Grain, for example, can be spout trimmed, which means that the loading spout or chute is moved across a hold during loading to disperse the cargo more equally. Spout trimming may not be very effective if the ship has a large deck overhang that prevents the spout from reaching the hold extremities.
Most modern ports now use flexible extending grain chutes capable of rotation to reach all parts of a hold. At the final stages of loading, bulldozers can be used to trim the cargo.
Then there is mechanical cargo trimming equipment that can be used to level off the cargo. The use of bulldozers is an example of mechanical trimming. Mechanical cargo trimming may be effective only at the hatchway opening and not behind deck overhangs. The amount of cargo that is trimmed depends on the size of the hold, the type of cargo, and the stowage factor.
Modern bulk carriers are self-trimmers. Under weather decks of modern bulk carriers, have angled upper wing tanks that can be used to trim cargo as it fills the holds, and in such a way most commodities can be spread to fill a ship’s cargo compartments without the need for manual assistance.
The trimming process is important because it helps to prevent cargo shifts. Cargo shift can occur when the cargo is not evenly distributed in the hold, and it can cause the bulk carrier to lose stability and even capsize.
What Factors Should Be Considered When Trimming A Cargo Hold?
To make sure that cargoes are properly distributed throughout the ship’s holds to provide adequate stability and ensure that the ship’s structure is never overstressed, paragraph 1, Appendix 3 of the IMSBC-Code- “Properties of dry bulk cargoes” should be consulted and the appropriate information on the cargo obtained from the shipper. This will mainly be whether the cargo is cohesive or non-cohesive as well as information on the angle of repose.
There are two main types of solid bulk cargo: those that can shift by sliding, and those that can shift by liquefying. Both of these processes can have disastrous consequences, including listing or capsizing, and/or structural damage.
This is where the cargo property- cohesive/non-cohesive and angle of repose come in handy because they are an indication of what sort of hazard will be posed. Solid bulk cargoes can be classed as cohesive or non-cohesive for reasons of trimming.
The angle of repose is an indicator of the stability of non-cohesive bulk cargoes and has been incorporated in the separate schedules for non-cohesive cargoes in the code. Non-cohesive cargo with an angle of repose of less than or equal to 30 degrees indicates the cargo is free-flowing and must be transported in accordance with the requirements governing the stowage of grain cargoes.
Consideration, in this case, must be given to the bulk density of the cargo when determining: The scantlings and securing systems of the compartment and bin bulkheads as well as the effect of free surface on the ship’s stability.
For non-cohesive bulk cargoes having an angle of repose between 30° and 35° inclusive and for those greater than 35° the maximum cargo surface measured as the vertical distance between the highest and lowest levels of the cargo surface has been stipulated. In both cases, trimming is to be done using equipment approved by the competent authority
Ship trimming is a complex operation that requires precise knowledge and experience. It is vital for any vessels carrying bulk cargo, as it helps to create a secure and stable load to take to the sea. Experienced personnel and reliable equipment are key for the successful trimming of cargo during loading operations. Cargo safety must always be prioritized to ensure a safe voyage for the crew and their cargo.