How conditions of stability, hull strength, draft, and trim assist a ship endure extreme weather at sea?
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.
The crew on a cargo ship never wants to find themselves amid a hurricane. Therefore, while passage planning, a prudent ship master always considers the probability of facing extreme weather and choose an optimum route that would provide the most favorable weather and also fuel economy for the ship operators. Despite taking the best precautions, a ship still may have to pass through extreme weather.
To avoid dire consequences, taking evasive action in time or making delay to keep away from the direct path of the storm is crucial. Nowadays, many ship owners prefer to use the professional services of a voyage routing provider who uses satellite information with computer-based software to provide almost real-time weather data to optimize the voyage plan. Maersk Line is a pioneer of using such a shore-based technology. Their ships are better informed to avoid harsh weather conditions by emailing updated route plans, weather maps, satellite images, and other information.
Summarized below are some critical points on conditions of stability, hull strength, draft, and trim of a ship that need to be taken into consideration to survive in severe conditions at sea.
A ship with proper weight distribution provides better protection at sea. However, when ships are empty with no cargo, they are most unsafe at extreme conditions. To improve stability during such conditions filling up ballast tanks is considered best.
To ensure cargo ships' safety and efficiency when sailing empty or with low cargo volume, there now available computer-based cargo loading and stability software to adjust the trim, draft, and monitor hull strength.
Shipmasters should ensure the best use of such technology to assess ship stability conditions at sea, and arrival/departure at/from the port and during loading/unloading cargo, bunkering, and water ballast exchanges.
He should confirm the vessel's safety by proper GM, stress and other factors as being within appropriate Limits.
In case of any deviation from the limits, despite best efforts from the vessel (including transfer
of ballast), the Master should contact the vessel operator immediately to rectify the situation, and also, advise his management Company.
Stress and Stability Calculation
The Master should use the classification society approved loading computer for calculating stability, hull strength, draft, trim, and other necessary parameters. However, it is strongly recommended to compare The loading program in use shall be verified regularly by inputting standard loading condition from the Trim & Stability Booklet and comparing results. It can help detect any malfunctioning of the software in use.
The Master should also be familiar with the manual calculations of hull strength and stability, draft, and trim using the Trim & Stability Booklet (Loading Manual).
If computer-loading programs are not functional or reliable, manual calculations are to be done and kept as similar to computer printouts.
The loading computer (approved or otherwise) must be tested at least once every three months by simulated loading condition excerpted from the Loading Manual and results compared.
Stress and Stability Maintenance
At no time, the vessel is to exceed the various restrictions prescribed in the Loading manual. These would relate to the drafts, trim, stability, bending moments, shearing forces, torsional moments (if applicable), and other instructions mentioned in the Loading and related manuals.
It shall be confirmed beforehand that bending and shearing forces during various stages of cargo loading/unloading, ballasting / de-ballasting, and bunkering fall are always within the specified safety range.
Since allowable bending moment and shearing forces in port are set to a higher value than those at sea, values set for in port shall not be used for calculation of respective values at sea
GM & GZ Curves
On containerships, the minimum GM to be maintained as specified in the Loading Manual. When the GM is calculated to be less than 1.0 m, it is recommended to verify it from the rolling period.
On tankers, the maximum GM, as specified in the Loading Manual, is not to be exceeded when part loading cargo in cargo oil tanks.
Excessive GM shall be avoided as far as possible since it can cause intense rolling with possible hull and cargo damage.
Ample stability shall be maintained to withstand any influences caused by weather and sea conditions, tug boat operations, heeling due to helm orders, cargo handling, and other factors.
To check a calculated GM assurance of ample GZ shall be confirmed employing GZ Curves in the loading manual.
Stability failure may lead to disaster
Draft and Trim
At no time shall the mid-ship draft exceed the draft decided under the International Convention of Load Lines ( always taking into account changes in applicable zones and seawater salinity during the voyage)
The forward draft shall be not less than the minimum draft specified in the loading manual to avoid slamming in heavy weather.
It is desirable to have the aft draft sufficient to submerge the propeller.
Drafts during the voyage shall be such that both the voyage and restricted port drafts are not exceeded, and the minimum Under-keel Clearance is maintained.
IMO Visibility shall always be maintained.
Free Surface Effect
Tanks should as far as practicable be full or empty unless they are in use. It helps to minimize the free surface effect.
Aside from reducing the meta-centric height, during rolling, partly filled tanks can cause sloshing and damage to bulkheads or similar structures in tanks.
On certain tankers, care must be paid to the instructions in the loading manual which prohibit cargo tanks carrying cargo with a density greater than 1.0 to be partly loaded, unless the GM is within a certain limit.
Angle of heel – A steady angle of heel created by an external force, such as wind or
waves.
Angle of the list, list – A steady angle of heel created by forces within the ship. For example,
when the ship is inclined due to her asymmetric construction, or by shifting a weight
transversely within the ship. The list reduces of ship's stability. Therefore it is essential to
keep the ship upright by asymmetrical distribution of masses.
Angle of loll – The angle at which a ship with a negative initial metacentric height will
lie at rest in still water. In a seaway, such a ship will oscillate between the angle of loll
on SB and the one on PS. Depending upon external forces such as wind and waves
a ship may suddenly flop over from PS to SB and then back again to PS. Such abrupt
oscillation, different from a continuous roll, is characteristic for negative metacentric
heights.
An angle of loll can be corrected only by lowering the centre of gravity, not by moving
loads transversely. It can be done by moving weight downwards, adding water ballast
in double bottom tanks, or removing weight above the ship's vertical centre of gravity.
Where empty ballast tanks are available, these will afford the simplest means of lowering
the ship's centre of gravity. The correct procedure is to add a ballast on the low side of the
ship. The first effect will be to increase the heel's angle and cause a loss of stability due
to the free surface of the water, but this effect is soon canceled, and the angle of the heel will
rapidly decrease.
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