Stability conditions

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Stability Conditions

In order to gauge a vessels stability, as well as to provide crew an operational manual (in regards to ship stability), a stability book is created. The vessels stability is assessed in at least 5 different conditions (lightship, light operating, arrival, departure, and worst operating). Regulatory agencies may require additional conditions, based upon the vessels capabilities and intended operations.

Contents

Standard Conditions

Condition one - Lightship

See Lightship

The vessel is fully outfitted for service at sea. There is no crew or passengers onboard. All fuel, water, and ballast tanks are empty. The lightship value will be used as the ships weight for all further conditions, and is subject to confirmation with an inclining test.

Condition two - Light Operating

Lightship condition (condition one) plus all crew. Full water, fuel, and completely provisioned. The light operating condition does not include passengers or cargo.

Condition three - Departure

Departure condition is lightship condition plus crew and passenger compliment. Full fuel, water and provisions. Departure is usually vessels heaviest condition state, generally the vessel is at it's design waterline.

Condition four - Arrival

Arrival Condition is lightship condition, plus full crew and passenger compliment, 10% fuel, water and stores. Arrival is generally a vessels lightest operating condition, though in the case of fishing vessels, this is often not so (as fishing vessels leave without cargo and return with catch).

Condition five - Worst Operating

Worst designed operating condition, as requested or determined by the naval architect, to be the worst combination of consumables, crew, cargo, and or passengers that the vessel will see in its service life. The worst operating condition generally produces the lowest GM practicable.

Auxiliary Condition - Load Line

see also loadlines

Where a vessels standard conditions do not include the vessel operating at it's load line, an additional load line condition should be considered. (At no point should a vessels standard conditions involve the vessel loaded past the load line). Where absent, the regulatory body may assign a load line as per convention (where the vessel may not be sufficiently stable), or may assign a load line as per the full load departure condition (which may not account for growth over time). The load line condition is the vessel at its most extreme condition of load (all tanks full, full cargo, potentially including additional theoretical loads) and level or near-level trim. The draught will be at or below the convention draught, or the maximum draught at which stability criteria can be achieved. This condition then exists as a recommendation to both the regulatory body and to the master, though dictates neither load line assignment nor loading limitations directly.

Auxiliary Condition - Limited Load

This limited load condition represents the maximum condition of load for which the vessel passes all special conditions (below).

Special Conditions

Special conditions are unique conditions of load, trim, heel, and/or applied forces which the vessel is capable or likely to experience as the result of its capabilities or operations. Such loads may be imposed by the environment (weather, ice, waves), by the vessel (crane, fire monitor, tow loads), or by the the vessels use (aground, ice interactions). Special conditions are applied on top of the standard conditions, with the arrival and full load departure are sufficient (though the limited load condition may be substituted as required). The application of all heeling moments is applied in the direction of any existing heel in the equivalent standard condition.

Aground

Vessels with appropriate reinforcement of bottom can sit aground in ports with sufficient tidal range. An aground condition will be considered, particularly when loading/unloading over the sides.

Crane Heel

see crane heel

Crane heel conditions consider standard conditions, but with the addition of a hook load. The hook load will the be removed (dropped), and the reaction considered. Crane loads should be considered for maximum horizontal extension (maximum heeling moment), and for maximum vertical extension (maximum rise of VCG). This condition will also have trim and heel limits as dictated by the cranes manufacturer.

Escort Heel

Escort heeling conditions consider the arrival and departure conditions, but introduce a heeling moment which represents the maximum force an escorted vessel can exert through the towing equipment. The stability requirements are specially considered, as outlined by classification, see escort heel.

Fire Monitor Heel

Fire monitor heeling conditions consider the standard conditions, but introduce a heeling moment which represents the maximum force exerted by a vessels fire monitors and the opposing force of the vessels thrusters. For a vessel whose firefighting capabilities are secondary, this is the sum of transverse moments. A more complex firefighting system, or a dedicated fire vessel, should include longitudinal moments as well.

Ice Accretion

see Ice Accretion

Standard conditions are considered, with ice buildup on exposed surfaces, as per regulations.

Icebreaking

Icebreaking vessels are considered for stability with the bow hung up on ice (as far as the hull form will allow, i.e., to the ice knife)

Standard Condition (Naval)

Standard condition is a vessel, fully complimented for wartime deployment (full crew, stores, consumables, ammunition, equipment & weapons), however exclusive of fuel or feed water.

Tow Heel

Tow heel conditions mirror the standard conditions, but introduce a heeling moment which represents the maximum sideways force a vessels propellers and towing equipment can induce. The stability requirements are specially considered.

Weather Conditions

Weather conditions consider the effects of wind, wave, or wind and wave on a vessel. For the application of wind, a sail area (the lateral area of the vessel exposed to wind) must be determined. The worst-case scenario is generally taken to be a wave-induced roll with an additional gust of wind applied.

External references

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