Code of Intact Stability | Intact Stability Criteria for All Ships (Must know for 2MFG Exam)

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The Code for Intact Stability was adopted on 4 November 1993 under IMO Resolution A.749(18). This Intact Stability Code was provided to seafarers to give recommendations related to Intact Stability criteria and other measures for ensuring the safe operations of the ship to minimize risk to ships, the personnel, and the environment. Recommendations mentioned in this code can be found in various codes of IMO, such as the MODU Code or DSC Code.

The Code for Intact Stability applies to:

  • Cargo ships
  • Cargo ships carrying timber cargo
  • Cargo ships carrying grains in bulk
  • Passenger ships
  • Fishing vessels
  • Special purpose vessels (SPVs) (mechanically self-propelled ships, which carries more than 12 personnel on-board)*
  • Mobile Offshore Drilling Vessels (MODU)
  • Pontoons
  • Dynamically Supported Crafts (DSC)
  • Container Ships

Stability Booklet:

  • Stability Booklet gives stability data and plans associated with ships arrangement. The plans should be written in the ship’s official language and the language of the master. If the languages used are neither English nor French then the text should include a translation into one of these languages.
  • Each Ship should be provided with a stability booklet, approved by administrating Flag Society. The Stability Booklet should contain sufficient information which should enable the master to operate the ship in the compliance of this Code.
  • On mobile drilling units, the stability booklet is called Operating Manual
  • the format and the information included may vary depending on the ship’s type and its operation. However, a stability booklet should contain:
    • A general description of the ship;
    • a table of content and index for each booklet;
    • Instruction on use of the booklet;
    • General Arrangement Plans showing watertight compartments, closures, vents, down flooding angles, permanent ballast, allowable deck loading, and freeboard diagrams;
    • Hydrostatic Curves or Tables and Cross Curves of stability calculated on the free-trimming basis, for the ranges of displacement and trims anticipated in normal operating conditions;
    • Tables showing capacities and center of gravity for each cargo stowage space (Capacity Plan);
    • Tables showing capacities, the center of gravity and free surface data of each tank (Tank Sounding Tables);
    • information on loading instructions, such as maximum KG and Minimum GM curve or table which can be used to determine compliance with applicable stability criteria;
    • Standard operating conditions and examples for developing other acceptable loading conditions using the information contained in the stability booklet;
    • a brief description of the stability calculations done, including assumptions;
    • general instructions for preventing unintentional flooding;
    • information concerning the use of any special cross-flooding fittings, with a description of damage conditions which may require cross-flooding;
    • any other necessary guidance for the safe operation of the ship under normal and emergency conditions;
    • inclining test report for the ship; or
      • where the stability data are based on the sister ship, the inclining test report of that sister ship along with the light-ship measurement report for the ship in question, or;
      • where lightship particulars are determined by the other methods than from inclining of the ship or its sister, a summary of methods used to determine those particulars;
  • As an alternative to the stability booklet, a simplified booklet in an approved form containing sufficient information to operate the ship in compliance with the applicable provisions of the Code of Intact Stability may be provided at the discretion of the authority concerned.
  • As a supplement to the approved Stability Booklet, a loading computer may be used to facilitate the stability calculations, including assumptions (loadicator);
    • the input/output form in the computer and the screen representation desired to be similar to the one in the stability booklet so that the operators will be easily familiarized with stability booklet.
    • the language of the manual for the computer to use should be simple and straightforward, written as per sound marine practice and in a language common to all officers on board, and should be provided with the loading computer.
  • In order to validate the proper functioning of the loading computer program, four loading conditions taken from the final stability booklet should run in the computer periodically and the print-outs should be maintained on-board as check conditions for future reference.
  • For Special Purpose Ships, Dynamically Supported Crafts, and Novel Crafts should be provided with additional information in the stability booklet, such as design, limitations, maximum speed, worst intended weather conditions or other information, regarding the handling of the craft that the master needs to handle the ship safely.

General Precautions against capsizing:

  • Along with the compliance with the code of intact stability, the master will have to exercise good seamanship in order to ensure the immunity against capsizing. Attention should be paid to the season of the year, weather forecasts, and navigational zones.
  • Make sure that the cargo being carried out on the ship, is within the stowing limits of the ship so that the compliance with the criteria can be achieved.
  • Before voyage commences make sure that all cargo and sizable pieces of equipment are properly lashed and stowed to minimize the lateral and longitudinal shifting, while at sea, under the effect of acceleration caused by rolling and pitching.
  • The number of partially filled or slack tanks to be kept to a minimum because of their adverse effect on stability.
  • If a ship is engaged in towing operations, it should not carry a deck cargo, except if it is of limited amount, properly secured, so that it would not endanger the safe working of the crew on the deck or impede the proper functioning of the towing equipment.
  • Attention to be paid to the possible adverse effect on stability where certain cargoes are carried. In this connection attention to be paid to the Code of Safe Practices for Solid Bulk Cargoes.
  • The stability criteria mentioned in this code of intact stability gives minimum values, not maximum values. Special attention should be paid to values of metacentric height, as excessive values of the same may lead to acceleration forces which could put the ship in danger.
  • Fixed ballast, if used should be installed under the supervision of Administration and in a manner that prevents shifting of position. Fixed ballast should neither be removed from ship nor relocated within ship without the approval of Administration.

Operational Procedures related to Weather Conditions:

  • All the doorways and other openings through which water may enter into the hull or deck-house, forecastle, etc., should be properly closed in adverse weather conditions.
  • Weather-tight and water-tight hatches, doors, etc., should be kept closed during navigation, except when necessarily opened for the working of the ship, and should always be ready for immediate closure and be marked to indicate that these fittings are to be kept closed except for the access.
  • In fishing vessels, hatch covers and flush deck scuttles should be kept properly secured when not in use during fishing operations. All portable deadlights should be maintained in good condition and securely closed in bad weather.
  • Any closing devices provided for vent pipes to fuel tanks should be secured in bad weather.
  • Over-relying on automatic steering may be dangerous as this prevents ready changes to course which may be needed in bad weather.
  • In all conditions of loading necessary care should be taken to maintain a seaworthy freeboard
  • In severe weather, the speed of the ship should be reduced if excessive rolling, propeller emergency, shipping of water on deck or heavy slamming occurs. Six heavy slammings or 25 propeller emergences during 100 pitching motions should be considered dangerous.
  • Special attention should be paid when a ship is sailing in following or quartering seas because dangerous phenomena such as parametric resonance, broaching to, reduction of stability on the wave crest, and excessive rolling may occur singularly, in sequence or simultaneously in multiple combinations, creating a threat of capsizing.
  • Particularly dangerous is the situation when the wavelength is of the order of 1.0 – 1.5 times ship’s length. A ship’s speed and/or course should be altered appropriately to avoid the above-mentioned phenomena.
  • Water trapping in deck wells should be avoided. If freeing ports are not sufficient for the drainage of the well, the speed of the ship should be reduced or course changed, or both. Freeing ports provided with closing appliances should always be capable of functioning and are not to be locked.
  • Masters should be aware that steep or breaking waves may occur in certain areas, or in certain wind and current combinations (river estuaries, shallow water areas, funnel-shaped bays, etc.). These waves are particularly dangerous, especially for small ships.
  • Dynamically supported craft should not be intentionally operated outside the worst intended conditions and limitations specified in the Dynamically Supported Craft Permit to Operate, in the Dynamically Supported Craft Construction and Equipment Certificate, or in documents referred to therein.
  • The use of operational guidelines for avoiding dangerous situations in severe weather conditions or an onboard computer-based system is recommended. The method should be simple to use.
  • Fish should never be carried in bulk without first being sure that the portable divisions in the holds are properly installed.

General intact stability criteria for all ships:

The following criteria are recommended for passenger and cargo ships:

  • The area under the righting lever curve (GZ Curve) should not be less than 0.055 meter-radians up to θ=30° angle of heel and not less than 0.09 meter-radians up to θ=40° or the angle of flooding θf* if this angle is less than 40°.
  • Additionally, the area under the righting lever curve (GZ curve) between the angles of the heel of 30° and 40° or between 30°and ef, if this angle is less than 40°, should not be less than 0.03 meter-radians.
  • The righting lever GZ should be at least 0.20 m at an angle of heel equal to or greater than 30°.
  • The maximum righting arm should occur at an angle of heel preferably exceeding 30° but not less than 25°.
  • The initial metacentric height GM0 should not be less than 0.15 m.
  • In addition, for passenger ships, the angle of heel on account of crowding of passengers to one side should not exceed 10°.
  • In addition for passenger ships, the angle of heel on account of turning should not exceed 10° when calculated using the following formula:
  • If anti-rolling devices are installed in a ship, the Administration should be satisfied that the above criteria can be maintained when the devices are in operation.
  • A number of influences such as beam wind on ships with large windage area, icing of topsides, water trapped on deck, rolling characteristics, following seas, etc., adversely affect stability and the Administrations are advised to take these into account, so far as is deemed necessary.

θf is the angle of heel at which openings in the hull superstructures or deckhouses which cannot be closed weathertight immerse. In applying this criterion, small openings through which progressive flooding cannot take place need not be considered as open.

Intact Stability Criteria for Tankers

Every oil tanker of 5,000 tonnes deadweight and above delivered on or after 1 February 2002, shall comply with the intact stability criteria of Annex I, Chp. 4/ Reg 27, as appropriate, for any operating draught under the worst possible conditions of cargo and ballast loading, consistent with good operational practice, including intermediate stages of liquid transfer operations. Under all conditions, the ballast tanks shall be assumed slack.

  • At the port, the initial metacentric height GMo, corrected for the free surface measured at 0° heel, shall be not less than 0.15 m;
  • At sea, the following criteria shall be applicable:
    • The area under the righting lever curve (GZ curve) shall be not less than 0.055 m·rad up to θ = 30° angle of heel and not less than 0.09 m·rad up to θ = 40° or another angle of flooding θf, if this angle is less than 40°. Additionally, the area under the righting lever curve (GZ curve) between the angles of the heel of 308 and 40° or between 30° and θf, if this angle is less than 40°, shall be not less than 0.03 m·rad;
    • The righting lever GZ shall be at least 0.20 m at an angle of heel equal to or greater than 30°;
    • The maximum righting arm shall occur at an angle of heel preferably exceeding 30° but not less than 25°; and
      d. The initial metacentric height GMo, corrected for free surface measured at 0° heel, shall be not less than 0.15 m.
  • The requirements of paragraph 1 of this regulation shall be met through design measures.

Intact Stability Criteria for Cargo ships carrying timber deck cargoes:

For ships loaded with timber deck cargoes and provided that the cargo extends longitudinally between superstructures (where there is no limiting superstructure at the after end, the timber deck cargo should extend at least to the after end of the aftermost hatchway) transversely for the full beam of ship after due allowance for a rounded gunwale not exceeding 4% of the breadth of the ship and/or securing the supporting uprights and which remains securely fixed at large angles of heel, the Administration may apply the following criteria which substitute the criteria for all ships:

  • The area under the righting lever curve (GZ curve) should not be less than 0.08 meter-radians up to 0 = 40° or the angle of flooding if this angle is less than 40°.
  • The maximum value of the righting lever (GZ) should be at least 0.25 m.
  • At all times during a voyage, the metacentric height GM0 should be positive after correction for the free surface effects of liquid in tanks and, where appropriate, the absorption of water by the deck cargo and/or ice accretion on the exposed surfaces.
  • Additionally, in the departure condition, the metacentric height should be not less than 0.10 m.

Stability booklet for Cargo ships carrying timber deck cargoes:

  • The ship should be supplied with comprehensive stability information which takes into account timber deck cargo. Such information should enable the master, rapidly and simply, to obtain accurate guidance as to the stability of the ship under varying conditions of service. Comprehensive rolling period tables or diagrams have proved to be very useful aids in verifying the actual stability conditions.
  • For ships carrying timber deck cargoes, the Administration may deem it necessary that the master is given information setting out the changes in deck cargo from that shown in the loading conditions when the permeability of the deck cargo is significantly different from 25% (see 4.1.6 below).
  • For ships carrying timber deck cargoes, conditions should be shown indicating the maximum permissible amount of deck cargo having regard to the lightest stowage rate likely to be met in service.

Operational measures for Cargo ships carrying timber deck cargoes:

  1. The stability of the ship at all times, including during the process of loading and unloading timber deck cargo, should be positive and to a standard acceptable to the Administration. It should be calculated
    having regard to:
    • the increased weight of the timber deck cargo due to:
      • absorption of water in dried or seasoned timber, and
      • ice accretion, if applicable;
    • variations in consumables;
    • the free surface effect of liquid in tanks; and
    • weight of water trapped in broken spaces within the timber deck cargo and especially logs.
  2. The master should:
    • cease all loading operations if a list develops for which there is no satisfactory explanation and it would be imprudent to continue loading;

Intact Stability Criteria for Containerships greater than 100 m

This section of the intact stability code applies to the container ships greater than 100 m. They may also be applied to other cargo ships with considerable flare or large water-plane areas. The Administration may apply the following criteria instead of intact stability criteria for general cargo ships.

  • The area under the righting lever curve (GZ curve) should not be less than 0.009/C meter-radians up to 0 = 30° angle of heel, and not less than 0.016/C meter-radians up to 0 = 40° or the angle of flooding ef (as defined above) if this angle is less than 40°.
  • Additionally, the area under the righting lever curve (GZ curve) between the angles of heel of 30° and 40° or between 30° and ef, if this angle is less than 40°, should not be less than 0.006/C meter-radians.
  • The righting lever GZ should be at least 0.033/C mat an angle of heel equal or greater than 30°.
  • The maximum righting lever GZ should be at least 0.042/C m.
  • The total area under the righting lever curve (GZ curve) up to the angle of flooding ef should not be less than 0.029/C meter-radians.
  • In the above criteria, the form factor C should be calculated using the formula and figures given below.

Standard conditions of loading to be examined

Loading Conditions:

The standard loading conditions referred to in the text of the present Code are as follows:

For a passenger ship:

  • ship in the fully loaded departure condition with full stores and fuel and with the full number of passengers with their luggage;
  • ship in the fully loaded arrival condition, with the full number of passengers and their luggage but with only 10% stores and fuel remaining;
  • ship without cargo, but with full stores and fuel and the full number of passengers and their luggage;
  • ship in the same condition as at .3 above with only 10% stores and fuel remaining.

For a cargo ship:

  • ship in the fully loaded departure condition, with cargo, homogeneously distributed throughout all cargo spaces and with full stores and fuel;
  • ship in the fully loaded arrival condition with cargo homogeneously distributed throughout all cargo spaces and with 10% stores and fuel remaining;
  • Ship in ballast in the departure condition, without cargo but with full stores and fuel;
  • ship in ballast in the arrival condition, without cargo and with 10% stores and fuel remaining.

For a cargo ship intended to carry deck cargoes:

  • ship in the fully loaded departure condition with cargo homogeneously distributed in the holds and with cargo specified in extension and weight on deck, with full stores and fuel;
  • ship in the fully loaded arrival condition with cargo homogeneously distributed in holds and with a cargo specified in extension and weight on deck, with 10% stores and fuel