Ship Stability Calculations & Measures Against Stability Failure
Ship stability is the ability of a ship to float in an upright position and, if inclined
under the action of an external force, to return to this position after the external force has
ceased acting.
Stability is not connected with a defined direction. However, ship inclination in transverse
the direction is most common and most natural to achieve, and in practice, transverse stability is the
most critical to ship safety.
The stability of a loaded ship depends on her shape and dimensions on actual location of her center of gravity. Small ships with low freeboard are more prone to stability
accidents than other seagoing vessels.
After a ship is constructed, an operator does not influence shape, dimensions, as well as mass and location of the center of gravity of an empty vessel. Still, he influences the final mass of the loaded vessel and the location of the center of gravity defining
amount of cargo, stores, and ballast water as well as their locations.
Several ship operations can adversely affect stability. Such effects must be understood and, where possible, mitigated. When a liquid is consumed or removed from tanks, a free surface is created, decreasing stability. When weight is lifted and
suspended, its center of gravity rises to the point of suspension. When a quantity of loose
dry bulk cargo moves transversely across the ship, it will list one side with some loss of
stability. Phenomena such as absorption of moisture by timber or similar deck cargoes,
ice accretion on decks and accumulation of shipped water will rise the vertical center of
gravity (VCG) reducing the righting arm GZ.
"Damage stability" : Defined a ship's stability in the flooded condition. This stability is attained
by installing several watertight compartments. If one of these compartments is
breached, then the watertight bulkheads surrounding it will prevent the inflow of
seawater from spreading to the rest of the ship.
Fig: CMA CGM Lamartine at sea passage
Masters are reminded of the dangers that inaccurate estimating of cargo centers of gravity can present to the vessel's stability. Crew members must advise masters when similar cargoes of differing densities are being loaded so that a proper estimate of weights and positions of centers of gravity can be determined.
They should also provide the Master with accurate stowage factors of different parcels of cargoes. Care must be taken to ensure that stowage factors used for cargoes are accurate. Cargoes such as scrap metal should not be assumed to be homogeneous.
The Master should ensure that terminals are instructed on the correct order of shipment to ensure that the vessel's minimum stability criteria can be exceeded throughout loading and the voyage. Sufficient time must be allowed for the Master to ascertain the vessel's stability and allow for any necessary ballasting operations to be undertaken before proceeding to sea.
Stability calculations and assumptions should err on the side of caution. Incorrect estimates can have serious consequences as the voyage progresses with reducing stability margins.
When ballasting, care must be taken not to stress or overload the vessel unduly. When correcting heel angles, care is necessary to avoid the danger of inappropriate ballasting, especially when the vessel has an angle of 'loll' caused by a negative metacentric height. The free surface and the vertical and transverse movement of the center of gravity must be considered.
The conditions of stability, hull strength, draft and trim of the vessel at sea, and arrival/departure at/from the port and during loading/unloading cargo, bunkering and water ballast exchange, should be worked out, ensuring the safety of the vessel. Safety of the cargo vessel depends on proper GM, stress calculation and other factors as being within appropriate Limits.
Following are the check item confirming stability and hull strength of cargo ship:
Is the GM value within limits specified in the loading manual and compliance with IMO rules to arrival next port?
Are GZ curves of the vessel fully understood, and their characteristics confirmed?
Have expected weather and sea conditions, been taken into consideration when confirming stability & hull strength?
Have free surface effects and any sloshing effects for the planned passage, been taken into consideration?
Are other items mentioned in the loading manual taken into consideration?
Are values of bending moment, shearing force, and torsional stress at sea within acceptable limits up to arrival next port?
If applicable, has the "Auto" mode for the heeling pump been changed over to "Manual" mode prior sailing?
Is the draft within applicable loadline or port/passage limits/restrictions?
Is stack weight/cargo density following maximum permissible values and have precautions as per the loading manual been followed?
Has forward draft limit (per loading manual) to prevent slamming been confirmed?
Is a proper propeller immersion ratio assured?
Have trim and draft changes during the voyage in fresh or brackish water such as rivers, canals, and lakes been considered?
Is squat due to shallow water effect taken into consideration?
Is proper under keel clearance assured as per company policy
Are fuel oil and freshwater consumptions taken into consideration?
Have air draft limitations due to bridges, cargo handling equipment, or other obstructions been assessed as necessary?
Is the navigation bridge visibility restriction in compliance with SOLAS Chapter V and other applicable requirements (Panama Canal, Suez Canal, etc.)?
On tankers, is the Marpol (Reg 18) minimum criteria for draught and trim complied with? And has the departure condition been forwarded to the office for use in case of an emergency?
Our additional pages contain some more useful resources
Ship Encountering Parametric Roll In A Seaway The term parametric roll for a container ship is used to describe the phenomenon of large unstable roll motion suddenly occurring in the head or stern seas. Due to its violent nature, the large accelerations associated with the onset of the parametric roll cause concern for container ships' safety. Possible consequences include loss of containers, machinery failure, structural damage, and even capsize....
Action by vessels navigating in congested water Ships navigation is referred to the voyage practices, focusing on the process of monitoring and controlling the movement of the ship from point "A" to point "B". Choosing the most optimum route while transiting through traffic-congested water is even more challenging. Specifically, due to the presence of many vessels in the vicinity, a repeated risk of collision exists....
Action by vessels navigating in an area of restricted visibility Ship navigation under restricted visibility circumstances is one of the most challenging tasks while accomplishing a safe voyage. The visibility is mentioned as restricted in cases that have been observed fog, heavy rain, or dust storm, all hazardous conditions to navigate. Ship navigation in such conditions doubles the likelihood of a collision or grounding. It calls for the use of specialized equipment and requires some actions to be taken by the time the ship's officer gets information of relevant weather conditions......
How to confirm stabilty condition? In the northern hemisphere during hurricane season, extreme weather is a common phenomenon. A big storm can run havoc even on the largest containership by tearing off its deck lashings. Most modern ships are designed to survive in harsh conditions and stay on schedule. Nevertheless, facing storms at sea is routinely an unavoidable part of life at sea. Each year substantial weather damages incur huge financial liabilities on ship operators.
......
How to maintain watertight integrity? To maintain Water tightness, Seaworthiness, Fire integrity and Security of the vessel, it is important ships personnel ensure all openings to hull below water line and above waterline ( weathertight & watertight doors etc.) are adequately secured.
......
Guide to watch officer for ships navigation ? Bridge watchkeeping is the most critical activity conducted at sea. Upon the watchkeeper's diligence rests the safety and security of the ship, her entire crew, the cargo, and the environment. It is a demanding activity, requires support, encouragement, motivation, self-discipline and a high standard of professionalism. Ships master must ensure that all watchkeepers understand the use of safety related equipment, prior to them keeping a watch......
Heavy weather countermeasures for prudent navigator Encountering extreme weather conditions at sea along major trade routes is a common phenomenon. Depending upon geographical location and seasonality of revolving tropical storms, a ship, therefore, need to prepare well to survive in harsh conditions. Both heavy weather and tropical storms demand of crew's preparation and immediate response.
How to navigate vessel safely in heavy seas ? Encountering extreme weather conditions at sea along major trade routes is a common phenomenon. Depending upon geographical location and seasonality of revolving tropical storms, a ship, therefore, need to prepare well to survive in harsh conditions. Both heavy weather and tropical storms demand of crew's preparation and immediate response.
Meeting rough sea conditions by containerships In heavy weather conditions where it is unsafe for ship crew to venture out on the deck for purposes of checking deck cargo securing, Master shall consider his ship handling options and heave to if required. The aim should be ensuring the safety of the vessel and its cargo.
.....
Checklist for calculating stability and hull strength for cargo ship
In heavy weather conditions where it is unsafe for ship crew to venture out on the deck for purposes of checking deck cargo securing, Master shall consider his ship handling options and heave to if required. The aim should be ensuring the safety of the vessel and its cargo.
......
Container Ship navigation - passage planning guideline Before proceeding to sea, the Master shall carefully check the Passage Plan, made after receiving the voyage instruction from the Charterer or the Company. Passage plan shall be made from berth to berth acting on the principle of Safety-first, while also taking operating efficiency into consideration.
The passage plan shall be prepared normally by the Second Officer, signed for approval by master and for understanding by all officers, before departure.Based on this Guide, the Master shall collect necessary information and review the Plan including Emergency Contingency Plans.
Navigation in cold districts and countermeasures Ocean water freezes just like freshwater, but at lower temperatures. Freshwater freezes at 32 degrees Fahrenheit, but seawater freezes at about 28.4 degrees Fahrenheit, because of the salt in it. Due to the presence of many hostile conditions, any merchant ship, while entering a freezing sea area, significant challenges are being encountered concerning safety and reliability of navigation.
Safe anchoring - planning and operational guidance for cargo ships
For the safety of the ship, strict anchor watches must be kept when the ship is at anchor. The principal reason for keeping anchor watches by one or more sailors is to maintain the safety and security of the vessel. Anchor watches to be maintained following the Masters's orders. This should include regular inspection of lead and weight on-chain.
How to deal with a damaged anchor?
.... When a part of the anchor chain breaks, it may be due to wear and corrosion or to over-stressing of its weakest part. Typically a ship owner arranges for anchors and chain damage inspection in a dry-dock, full range length, and can take a note on weakest links. A common defect is loose studs that reduce chain strength significantly. In all cases, the class surveyor should be consulted, and defective/ wasted chain be renewed as per surveyors' strict guidelines. It is a ship owner's routine expenditure for anchoring arrangement.
How to recover a lost anchor ?
.... After the anchor and chain are lost, the Master should make an initial report to the management company. After that, from time-to-time, the Master should report further developments. The Master should report the circumstances that led to the loss of the anchor and chain so that the Company can determine whether the general average is affected.
What is stranding ? Investigation of possibility of self-refloating and urgency of danger
.... Stranding means when a vessel has run aground, it is accidental. In consequence, the double bottom area of the vessel will probably suffer considerable damage, especially if the ground is rocky. This is physically the same action as beaching, but with the significant difference that beaching the vessel is an intentional action and under comparatively controlled conditions, whereas stranding is accidental.
What are the emergency procedures for loss of anchor and chain?
.... After the anchor and chain are lost, the Master should make an initial report to the management company. After that, from time-to-time, the Master should report further developments. The Master should report the circumstances that led to the loss of the anchor and chain so that the Company can determine whether the general average is affected.
Securing your vessel for sea passage - when to check and what to check Many maritime accidents are caused by the mistakes of ship personnel for inadequate sailing preparation. To avoid recurrences of the fatality deck and engine department must be well prepared before a vessel’s departure for a voyage at sea. These arrangements may include many complexities, and this is the reason why a bunch of things should be recognized and prepared delicately to ensure a smooth voyage passage and safe navigation.
Shipping industry recognizes environmental protection as one of its highest priorities and that every effort should be made to conserve and protect the environment from marine, atmospheric and other forms of pollution.
Our articles are based on various shipboard activities,prevention of pollution,safe operation & maintenance procedure. We welcome any feedback from our visitors.
For any comments or suggestions please Contact us Site Use and Privacy - Read our privacy policy and site use information. Terms and conditions of use
