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Safe stability & stress limits of chemical tankers -Loss of stability countermeasures

It is important for a chemical tanker to maintain adequate stability at all times during loading, unloading or ballasting operations. Chief Officer must plan for safe loading or ballasting of the vessel so that stability, stress and trim are acceptable throughout the voyage and that the vessel remains manageable in a seaway without excessive shear forces, bending moments, pounding, or vibration. Damage stability requirements as per IBC code are to be complied with.

The Chief Officer must also ensure that the loading or discharging sequence is such that the vessel’s stability is never compromised and that permitted stress limits are never exceeded. Full and effective use must be made of the ship’s loading computer for both cargo and ballast operations and voyages. During cargo and ballast operation stability and stresses are to be checked hourly and printed records maintained on board.

chemical tanker navigation at sea
At sea the ship must never be loaded to a deeper draft than that permitted by the Load Line Regulations.

Vessel must at all times be ready to sail for sea during cargo operations in port in case of emergencies.

Stress monitoring system

Some vessels may be fitted with strain/stress gauges, which will automatically alarm at pre-set levels. Whilst this equipment provides valuable information it is essential that an accurate cargo/ballast plan is calculated and that stresses will remain within acceptable limits. The vessel must not be loaded/discharged purely on the strain/stress gauges.

If during cargo/ballast operations the stress alarm sounds, all operations must stop until the situation is appraised.

Chemical tanker sea passage

Loss of stability

If loss of stability becomes evident or is suspected at any time during loading or discharging, the following steps are be taken. It should be noted that every vessel is different and the different plans or critical checklists are to be adopted for each vessel.

This is particularly crucial in doublehull ships without centre line bulkheads but nonetheless, is required for all vessels.
  1. Immediately stop cargo and all other operations such as ballast and bunkers.

  2. Advise terminal operator;

  3. Advise office who will declare a contingency

  4. Ensure all mooring ropes are tight;

  5. Carefully check levels in all and verify number of slack tanks (ballast, cargo and bunker);

  6. Determine the cause (e.g. incorrect or deviation from loading/discharging plan or technical cause such as valves or other cause of cargo/ballast internal transfer);

  7. Enter data into loading computer in order to check GM; and check for angle of loll and investigate preventive action accordingly.

  8. The company is to be kept fully advised on the situation. The majority of tankers have contracts with damage stability providers e.g. LR SERs and assistance from them will be sought in most cases. They will require accurate data on the vessels tank status in order to perform these calculations.

  9. Create a draft plan for correcting the situation. No action is to be taken without permission from the company who will be obtaining advice the damage stability provider. The only exception to this is when the master considers it essential to for the purpose of saving the vessel and he considers immediate action is required.

  10. When loss of stability has occurred, on no account is any ballast or cargo to be pumped out. Where ballasting is required only split double bottom tanks are to be filled, starting with the side which the vessel is listed over to, before making the ship upright with double bottom tanks on the opposite side. On no account are DB tanks that run the full width of the ship to be ballasted as this could increase free-surface with disastrous results.

  11. Before attempting to correct the stability, the plan must be carefully checked using the ship’s loading computer in order to check the criteria at every stage of the plan. The plan is to be agreed with the terminal operator before commencing the operation and hoses disconnected.

  12. Once stability is correctly restored, further checks should be made in order to ensure that adequate stability is maintained for the remainder of the cargo operation.

Required documents:

More info pages

Intact stability guideline for double hull chemical tankers
Normally chemical tankers have large number of tanks and this is not an issue. Some older Type III vessels may have this type of configuration. It is important that the stability aspects of these type of double hull vessels is understood as there have been a number of serious incidents involving loss of stability. ....

Cargo calculation -safety factors
Different methods of calculation for various substances can be applied, but they must be similar for loading and discharging. The method must be agreed with the surveyor. The range of substance temperatures (port of loading and port of discharging) must also be taken in to consideration. Necessary data for the expected temperature of a substance should be requested from the surveyor at the loading port. .....

Handling of high vapour pressure cargoes, topping off procedure & use of compressed gas during chemical cargo loading
Loading of various noxious liquid chemicals involved numerous hazards. It is important to exercise safety during all stages of cargo loading. If, at any stage during or immediately after the loading operation, a non-conformance (which may include cargo quantity, quality, temperature and colour etc.) is believed to exist, all operations should be suspended until such time as the situation is resolved. ......

Loading / stress computer for chemical tankers
This instrument is provided to supplement the stability booklet for the vessel. It allows the Officer responsible, to carry out the various complex calculations required to ensure that the ship is not overstressed or damaged during the carriage of the nominated cargoes. It will also permit the assessment of damage stability. The Master and Chief Officer will make themselves aware of the worst case damage stability scenario condition within the stability booklet. ....

Voyage planning and related considerations
Stress, stability, trim and list, Certificate of Fitness (check the list of chemicals attached to the certificate and confirm that the ship is certified to carry the cargo) , IBC/BCH Code to be referenced and the carriage requirements of the cargo noted along with IMO classification etc . Charterer’s requirements for the cargo ....

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