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Maintaining safe Stability - a brief guide to oil tankers at sea

Safe Stability

The Chief Officer is responsible to the Master for the 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.

The Chief Officer must 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. At sea the ship must never be loaded to a deeper draft than that permitted by the Load Line Regulations.

Stress Monitoring System

Some older 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.

More modern tonnage may be fitted with on-line gauging systems that feed information directly into the loading computer. It is essential that such a system is frequently cross-checked with local readings to ensure all readings are accurate. In the event of a failure of the system, then manual inputs of tank levels are to be regularly fed into the loading computer in order to ensure both stability and stress remain within the desired parameters. If during cargo/ballast operations the stress alarm sounds, all operations must stop until the situation is appraised.

Loading/Stress Computer

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 ensuring 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 condition within the stability booklet.

It must be remembered that a loading computer, as with navigation aids, is only an aid to the operator. It relies on human input of data, and more importantly the human interpretation of the output data. If the input data is incorrect, the output data will also be incorrect. Used correctly it will ensure the safe operation of the ship for all conditions of loading, discharging, ballasting and at all stages of the voyage.

It is a Company requirement that where such equipment is provided to a ship, test conditions must also be supplied for use in verifying the accuracy of the equipment. It is Company policy that test conditions must be run against Class approved cases and records of results maintained:- The test is to be conducted by physically entering the data for each tank into the computer and verifying the result. It is not acceptable to simply retrieve a stored test condition from the computer and compare this against the official conditions

During annual & special survey, class surveyor should be requested to witness the test and endorse the test result or issue a survey report. This record is to be maintained on board.

The frequency and records of such tests are to be recorded in the vessel’s planned maintenance system. Where the running of these reveals significant errors, the Company is to be advised immediately with a request for attention.

Where online gauging of tank contents is not fitted the loading computer must be regularly updated in order that stresses, draft and trim can be monitored throughout the discharging operations.

Stability Of Double Hull Vessels (WITHOUT CENTRE LINE BULKHEADS)

Officers must understand the stability aspects of double hull vessels. In contrast to conventional tankers which are inherently stable, it is now recognised that double hull structures which do not have a centre line bulkhead have potential problems in that large free surface effects may be encountered at various stages of cargo and ballast operations. As a consequence, a loss of GM may result in serious stability problems. It must be stressed that Masters and Officers must be fully aware of this potential problem, and that all cargo and ballast operations are conducted strictly in accordance with the vessel’s approved loading manual. Changes to cargo plans must be fully assessed before the revised plan is undertaken.

The IMO Marine Environment Protection Committee (MEPC) adopted amendments to MARPOL Annex I and the IBC and BCH Codes which will require new and existing oil and chemical tankers to be fitted with an approved stability instrument capable of verifying compliance with the applicable intact and damage stability requirements. These new tankers, constructed on or after 1 January 2016, will need to comply on delivery and existing tankers, constructed before 1 January 2016, will need to comply at the first scheduled renewal survey after 1 January 2016.

Stability data, and loading and unloading instructions, are to be carefully studied and followed. Generally, these instructions will specify a maximum number of tanks, which may be slack at any one time. Sometimes it may be necessary to adjust the quantity of cargo to be loaded in order to avoid slack tanks. Where double bottom ballast tanks extend across the whole width of the vessel, the free surface effect of water in these tanks will be as great as that of full width cargo tanks and account must be taken of this fact.

The Cargo Loading/Discharging Plan must also take into account the problems of free surface and any limitations on the number of tanks which may be slack at any one time. In order to ensure adequate monitoring of the vessel’s stress and stability condition, calculations must be made and recorded, using the vessel loading computer, on an hourly basis during cargo and ballast operations.

If a loss of stability becomes evident during loading or discharging all cargo, ballast and bunker operations must be stopped and a plan prepared to restore positive stability.

On completion of loading oil cargoes, the number of slack tanks must be at a minimum and in any event not more than that specified in the stability information book, or the appendix to the Classification Certificate. If nominated cargo quantities indicate that the vessel, which is restricted in the number of slack holds it may have, will have an excessive number of slack holds after loading, the Master must immediately advise the Company or Charterer.

It is essential that Masters and Officers be aware that partially loading a cargo tank with heavy weather ballast may present a potential problem. The combination of free surface and the flat bottom can result in the generation of wave energy of sufficient power to severely damage internal structure and pipelines. In all cases the Master shall positively verify the maximum number of slack tanks allowed and post a clear notice of instruction in the cargo control room.

Slack Tanks And Sloshing

It is essential that Masters and Officers be aware that the partial loading of a cargo tank with cargo or heavy weather ballast may present a potential problem. The combination of free surface and the flat bottom can result in the generation of wave energy of sufficient power to severely damage internal structures and pipelines.

The movement of liquid within a cargo tank when the vessel is rolling or pitching in a seaway is called “sloshing”. In general, the negative effects of sloshing can be summarised as follows:
In order to eliminate these problems, slack tanks must be avoided wherever possible, and it is imperative that no cargo tank, unless so designed and permitted by Classification, is allowed to remain in a slack condition whilst the vessel is at sea. The Ship’s Trim and Stability manual approved by Classification and/or the Appendix to the Classification Certificate must be referred to on this matter.

Cargo Cooling

On Tankers with single athwartships tanks, and especially those with double hulls, consideration must be taken regarding the effect of the cargo cooling during a long passage, to maintain the stability of the vessel as the free surfaces increase.

In the worst case it may be necessary to commence cargo heating or to transfer cargo from tank to tank to reduce free surface effects; however such operations are subject to the Company’s and or the Charterers approval.

Loss Of Stability

If loss of stability becomes evident or is suspected at any time during loading or discharging, the following steps should 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 double-hull 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 bulker);
  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 to establish GM; and check for angle of loll and investigated preventive action accordingly;


  8. The management 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 vessel’s 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 management Company who will be obtaining advice from the damage stability provider. The only exception to this is when the Master considers it essential, for the purpose of saving the vessel, and when 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 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.


Damage Stability

The vessel must have onboard the approved Stability Information Book (SIB) which is written in the working language of the vessel. The Stability Booklet should include damage stability calculations for different loading conditions in case the vessel suffers side or bottom damage on the voyage.

The stability condition upon departure from a port must be compliant with both intact and damage stability criteria. These departure criteria can be checked against one of the approved cases within the Stability Information Booklet, or by running the loading conditions on the loading program. If the program does not take account of damage stability, the ship must manually evaluate the final loading conditions against the KG limit curves presented in the Damage Stability report. The Master and Chief Officer must make sure they are familiar with this approach.

If the vessel does not have damage stability functionality within the approved software and does not have KG / GM limit curves that incorporate damage stability requirements then the vessel has two options:-
  1. The vessel must load according to the approved loading conditions in the stability information booklet (allowance of 2cm of GM and 1% of deadweight would be considered acceptable and not be treated as an alternate condition).
  2. If the vessel has to load outside the approved conditions then the vessel should apply (well in advance) to have the planned condition approved by the vessel’s Classification Society and Flag





The following reference publications are required for maintaining safe stability : Ship’s “Trim and Stability Data/Manual” (Approved by Class)



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