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IMO Intact Stability Criterion & Safe Return to Port (SRtP)

The IMO Intact stability criterion must be complied with at all times. Most cargo stowage and stability calculation software provide alarms/highlights for stability conditions not complying with IMO criterion.

Intact stability criteria – Analyzing the data of vessels that behaved well, and especially the data of vessels that did not survive adverse conditions, various researchers and regulatory authorities defined criteria for deciding if the stability of a vessel is satisfactory.

Therefore, it is important to understand that the existing stability regulations are codes of practice that provide reasonable safety margins without giving 100% guaranty that the vessel which meets the requirements can survive all challenges. According to the International Code on Intact stability, 2008, the following criteria are mandatory for passenger and cargo ships constructed on or after 1st January 2010:
  1. The area under the righting lever curve (GZ curve) should not be less than 0.055 metre-radians up to 30° angle of heel.
  2. The area under the righting lever curve (GZ curve) should not be less than 0.09 metreradians up to 40° angle of heel or the angle of downflooding if this is less than 40°.
  3. The area under the righting curve between the angles of heel of 30° and 40° or between 30° and the angle of downflooding if this angle is less than 40°, should not be less than 0.03 metre-radians.
  4. The righting lever GZ should be at least 0.20 m at an angle of heel equal to or greater than 30°.
  5. The maximum righting arm should occur at an angle of heel preferably exceeding 30° but not less than 25°.
  6. The initial metacentric height GMo should not be less than 0.15 m.
  7. Severe wind and rolling criterion (weather criterion) In addition to the criteria described above, ships covered by the 2008 IS Code should meet a weather criterion that considers the effect of strong beam wind and waves applied when the vessel is in dead ship condition.
Further reading: “Ship Stability in Practice”

Safe Return to Port (SRtP)

Capsizing of COSTA CONCORDIA and fire of engine room on board CARNIVAL TRIUMPH show that hundred years after tragic sinking of the ocean liner TITANIC passenger ships are still not sufficiently safe.

“Safe Return to Port” means new SOLAS regulations applicable to new passenger ships having their keel laid on or after 1st July 2010, and having a length of 120m or more, or having 3 or more Main Vertical Zones. As per these regulations, a passenger ship shall be designed so that the essential systems remain operational after a fire casualty which does not exceed casualty threshold or a flooding of any single watertight compartment and the ship is able to proceed to a safe port under their own power. This may sound simple in theory, but in reality poses a real challenge to ship designers.

A casualty threshold includes a loss of space of fire origin up to the nearest “A” class division if the space is protected by a fixed fire-extinguishing system, or a loss of the space of origin and adjacent spaces up to the nearest “A” class divisions which are not part of the space of fire origin if it is not protected by a fixed fire-extinguishing system.

During this “Safe Return to Port” period, all persons onboard are accommodated in a “safe area” where basic services for their safety and health are available. Safe areas are spaces (generally, internally located) where basic services such as food, water, sanitation, alternate medical care, lighting and ventilation are maintained.

If the casualty threshold is exceeded, SOLAS now requires some essential systems to be still operational for three hours in order to support the “orderly evacuation” of the vessel, considering one entire main fire zone lost. However, it is necessary to understand that in case of long shallow side shell damage, ships without double sides in way of machinery spaces will capsize in the same way as COSTA CONCORDIA, even if built according to SRtP rules.

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Related Topics

Draft, Trim and Heel affecting Hull Strength and Stability of containership

Corrected GoM for Hull Strength and Stability of containership

Severe Wind and Rolling criterion affecting Hull Strength and Stability of containership

Visibility from Bridge affecting navigation of containership

Propeller immersion affecting navigation of containership

Shearing forces, Bending moments and Torsional moment affecting Hull Strength and Stability of containership

Other factors affecting Hull Strength and Stability as necessary

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